The History of Submarine Steel: How Do The Type-212CD and KSS-III Line Up For Canada?
Opinion
Steel.
Steel is important. Put that on a quote box next to my name. I have talked a bit about steel recently, mostly to tell you all how I was wrong about some of my assumptions on the kinds of steel produced in Canada.
When talking about submarines, the metallurgy of its pressure hull is often overlooked by many general discussions and analyses. While you might get a bone thrown about them from time to time, there isn't really a great breakdown as to why certain manufacturers choose the type of steel they do.
Not all submarines are created equal, and not every hull is created with the same intents in mind. The metallurgy of a submarine’s hull is one of the key factors we can use to judge a submarine’s capabilities, roles, and limits even without knowing all of the equipment inside.
By analyzing what type of steel is used we can get, what is essentially, a forensic profile of the submarine, down into the core of what it's made to do. What kind of environments is it tailored for? What kind of threats does it anticipate it will face? What depths could it reach? What kind of maintenance requirements does it have?
All of this is encoded in the metal. In the modern ocean, there is no single "best" steel—only the choice that best fulfills a nation's strategic needs. It's actually quite fun to get into. You should try it! It's a complex yet fun topic.
Nowhere though is this contrast starker than in the Canadian Patrol Submarine Project. Both the KSS-III and Type-212CD take radically different choices when it comes to what steel makes up their hulls. Both are forged of the trials, limitations, and priorities of the host countries that designed them.
Yet no one has taken the time to get into it, and so I must carry that task myself. Despite my noted limitations in the subject matter of steel and Canada’s steel industry, I have spent several weeks deep diving this topic to the best of my ability.
I even pulled up the assistance of my own mother, a retired underwater welder, to bug her endlessly about this topic. She still doesn't quite know what I do, nor what submarines have to do with it, but shout out to my dear mum for listening to me and trying to help.
Consider this a part one of sorts. This piece will not be diving fully into the maintenance of both subs nor Canadian Industry’s ability to handle it. That will come later on after I have had more time to formulate and ponder.
What this will do, though, is give you the basic rundown you need to understand what both submarines use, their benefits, limitations, and why these grades of steel were chosen for their respective submarine.
From that point, we can then, later on, dive fully into the maintenance requirements, costs, and how prepared Canadian Industry is to take on the task of maintaining both submarines.
I hope you find some joy in this, and thank you all for your patience. I know this is two weeks late, and I apologize. This was absolutely the hardest subject I have ever tackled. Writing this was probably my biggest challenge yet. At several points I wanted to give up.
I yelled. I cried. I got frustrated. It was hard to jump into new territory and have to learn at lightning speed. To those that helped me, thank you. I will forever appreciate your love and support. I also need to thank my wife. She convinced me several times to keep going, pushed me to try and try harder. Even if this failed, I tried.
It is my responsibility as an analyst and educator to tackle the hard, difficult subjects. That is my duty. That is my burden to all of you. If not me, than who else would? That gives me the motivation to do things like this. It is more than a desire, more than content. It is part of my duty to all of you to yackle the hard subjects, to push myself harder and harder into the scary topics I don't know.
For now, though, before I ramble, let's dive into submarine metallurgy and where everyone stands.
The making of a submarine
To understand the steel itself, we need to take a little look at some history and the overall state of things. The first submarines were not constructed of steel but of wood, copper, and iron. Turtle, built by American revolutionary and inventor David Bushnell in 1775, is generally considered the first combat submarine in history.
The Turtle was intended to clandestinely drill into British ships so as to attach a time-delayed mine to its hull. Originally designed to break the blockade of Boston, to which it would not come into service until after, the core idea was that Turtle could sneak past harbour defences, attach its payload, and then scurry out without a soul noticing, thus presenting a way to target Royal Navy vessels in harbour instead of out at sea, where the fledgling Continental Navy founded that same year would struggle.
The Turtle itself was essentially a barrel, built of oak staves bound with wrought iron hoops in the shape of a clam-like sphere. It featured hand-crank propellers and foot pedals for movement, as well as an early innovation of a ballast tank. Again, a motorized barrel, but a very innovative barrel to its credit.
Turtle would sadly never get a chance to taste British blood. An attempt to sink HMS Eagle at New York Harbour in September of 1776 failed after the explosive 'torpedo' could not be properly attached.
Several other attempts would be made to deploy Turtle, all failing due to unfavorable conditions. As you can imagine, Turtle was not designed to operate in deep, challenging conditions. This prevented several attempts to deploy her in combat.
Turtle’s end is somewhat of a fizzle. She, along with the sloop carrying her, would be sunk on the 9th of October 1776 under enemy fire. While she was later recovered, we don't know what happened to her afterwards. Her history ends the night she first sunk.
Turtle, despite my joking, was an innovative vessel. She was the first to introduce the concept of a Ballast Tank as well as the first to use a screw propeller. Despite her failure, she is also still the first recorded combat submarine in service.
For most of the 19th century, submarine development would enter a period of fast-paced development, even if oftentimes limited to single examples. It wasn't until towards the end of the century that we would see proper classes of submarines start to be formulated.
It would be a long road of development, each example slowly building the knowledge base and experimenting with new ways to make a submersible warship work. While no example would solve it all, each would pioneer a solution that would eventually culminate into the wide-scale deployment of submarines.
We won't be covering all of these; however, we will cover a few of the major ones and what they accomplished. From there we can start diving into the modern metallurgy of submarines.
From Turtle we move on to Robert Fulton and the Nautilus. Nautilus is widely considered the first practical submarine, the first truly practical attempt to solve the problems of buoyancy and propulsion in a single vessel.
Operating in France under the patronage of First Consul Napoleon Bonaparte, Fulton designed the Nautilus between 1793 and 1797, envisioning it as a weapon to break the British naval blockade of France. The vessel, constructed at the Perrier boatyard in Rouen and launched in 1800, was an ellipsoid craft measuring 6.48 meters in length with a beam of 1.93 meters. Fulton utilized copper sheets fastened over iron ribs for the hull. In this case, copper offered very good resistance to saltwater corrosion, while the iron ribs provided the necessary structural rigidity to withstand pressure.
The vessel's buoyancy control was achieved through a hollow iron keel, which functioned as a primitive ballast tank. By flooding the keel, Fulton reduced the vessel's positive buoyancy. The Nautilus also featured a small conning tower fitted with a glass-covered porthole!
This provided a limited view of the surface but would be a forerunner to the periscope found in modern submarine design as we get closer to the 20th century. The Nautilus was also most famously a pioneer of dual propulsion.
She was equipped with a fan-shaped, collapsible sail rigged to a mast that could be folded down into the hull before diving. This allowed the vessel to traverse long distances using wind power when surfaced. When submerged, the vessel was driven by a hand-cranked, four-bladed screw propeller. This would also be another forward-looking design choice from Fulton!
Nautilus once again featured a similar concept to Turtle for dealing with enemy vessels: screw a hole into the hull, attach a time-delayed mine, and get the hell out of dodge before she blew. Reportedly, Nautilus did manage to successfully demonstrate this concept in August of 1800. Despite its success, the French Ministry of Marine, dominated by traditionalist officers, viewed the submarine as an "unchivalrous" weapon. Admiral Denis Decrès famously dismissed Fulton, stating that such a mode of warfare was unworthy of France.
So frustrated was Fulton that he decided to defect to Britain, where he demonstrated his system to Prime Minister William Pitt. However, the Royal Navy, recognizing that the submarine threatened their own surface fleet more than it aided it, ultimately paid Fulton to cease his experiments.
The Nautilus was dismantled, and its revolutionary design tossed away to never, ever be heard of again. Fulton would continue to work with the British government for several more years, primarily in the development of early torpedoes, before ultimately moving back to the United States in 1806.
Here Fulton continued to work on torpedoes but would be more famously known for his work on steamboats. He, along with Robert R. Livingston, is credited with designing and constructing the first commercial steamboat in history, the North River Steamboat.
He also went on to design the first-ever steam warship, the Demologos, though sadly died before he could see her come to life. She would be renamed Fulton after his death in his honor. Robert Fulton was no doubt a very small man, and an innovator. While Nautilus failed to capture wider interest, and Fulton himself would eventually find his achievements elsewhere, the work he did for his time was both innovative and inspiring. With several major firsts under his belt, it is safe to call Fulton one of the fathers of the modern submarine. I think that is an honor he deserves.
Sadly though our time with him has passed, but we ain't leaving America just yet. Let's speed this up by starting with Hunley. Financed by Horace Lawson Hunley and built in Mobile, Alabama in 1863, the H.L. Hunley was the third in a series of submersible prototypes (following the Pioneer and American Diver). As the Union blockade strangled Charleston, the Confederate military seized the vessel, hoping to use the Hunley as a blockade breaker. Sound familiar? Its existence was a closely guarded secret, aimed specifically at the formidable steam sloops and ironclads anchoring off the South Carolina coast.
Technically, the Hunley was a testament to adaptation. The hull was constructed from a recycled steam boiler, sliced in half lengthwise and widened with a strip of iron plate riveted between the halves. This resulted in a vessel 40 feet long but only 4 feet wide, built of wrought iron with flush-riveted seams to reduce drag. Propulsion was provided by a hand-cranked shaft running the length of the crew compartment.
The vessel utilized two ballast tanks filled by valves and emptied by hand pumps. Its primary armament was the spar torpedo, a copper canister containing 135 pounds of black powder mounted on a 22-foot iron pole. The intent was to ram the torpedo into the enemy hull, reverse the submarine, and detonate the charge via a lanyard.
Not too dissimilar to previous designs we have discussed, but certainly a step away from drilling into the hull. Sadly, the story of the Hunley is more one of tragic desperation than one of innovation as with Nautilus and Turtle.
The Hunley earned the grim nickname "The Peripatetic Coffin" after sinking twice during training, killing 13 men including Horace Hunley himself. On February 17, 1864, it achieved immortality by sinking the USS Housatonic, becoming the first submarine to sink a warship. However, the victory was pyrrhic as the Hunley never returned to port.
It was discovered in 1995 and raised in 2000. Forensic analysis suggests the crew didn't drown but likely died from the concussive shockwave of their own torpedo. A tragic end to a desperate attempt to tackle the iron grip of the Union blockade. Despite that, Hunley did score at least one sinking, etching its status into history though at a great cost.
While the Confederacy built submarines out of desperation, to the north the Union Navy commissioned the Alligator in 1861. Designed by the French expatriate Brutus de Villeroi, it was initially intended to counter the threat of Confederate ironclads.
The Union Navy, to their credit, viewed Alligator as a multi-mission platform capable of tasks like clearing harbor obstructions and deploying divers. While Alligator was a combatant, there was recognition of both her limitations and other uses, reflecting a more tactical, less suicidal approach to submarine development than their Southern counterparts.
The Alligator was technically superior to the Hunley in almost every way. The 47-foot iron hull was originally equipped with a unique propulsion system of folding mechanical oars, though this was later replaced by a hand-cranked screw propeller to increase speed.
Its most significant innovations were its life support system and diver capabilities. De Villeroi installed an air purification system that used lime to scrub carbon dioxide from the atmosphere, theoretically extending submergence time.
Furthermore, the Alligator featured a forward airlock that allowed a diver to exit the submerged vessel, attach a mine to a target, and return. This would technically be the first example of a lock-out chamber and combat diver capability, though it's debatable given Alligator's actual use, or should I say, lack of.
Despite its advanced design, the Alligator was plagued by delays and the skepticism of naval commanders who preferred ironclad surface ships. It never saw combat.
In April 1863, while being towed south to participate in the attack on Charleston, the Alligator was caught in a severe gale off Cape Hatteras. The towing vessel, endangered by the heavy, plunging submarine, was forced to cut the towline. The Alligator sank into the Atlantic, where it remains undiscovered.
Alligator remains a very cool but uneventful vessel in this list. She certainly had some forward-looking approaches to what a submarine could do, though sadly never really got a chance to demonstrate her full capabilities.
From one side of the Atlantic to the other though, let's speed this up a bit further. We're going to Spain. The Ictineo project is unique in that she was never intended as a warship. Narcís Monturiol, a utopian socialist, witnessed the drowning of a coral diver in 1857 and resolved to build a machine that would allow divers some form of safety when diving.
Ictineo I, built in 1859, was a wooden prototype that proved the concept. While it is mostly unnotable compared to her follow-up, she did pioneer the first example of a double-hull, something that Ictineo II, built in 1864, would further develop upon. Built of olive wood reinforced with copper and oak, the Ictineo II once again featured a double-hull design similar to Ictineo I. This consisted of an inner pressure hull for the crew and an outer hydrodynamic hull for speed, with water ballast tanks located in between.
Its most crowning achievement, though, was the propulsion system. Monturiol realized human power was insufficient and steam engines required oxygen. He invented an anaerobic engine that burned a mixture of zinc, manganese dioxide, and potassium chlorate.
This reaction generated heat for steam and, critically, released pure oxygen into the hull, solving the life-support issue. This would be the first true Air-Independent Propulsion (AIP) system, a technology we'll talk about more later but one that would redefine the capabilities of the conventional submarine almost a century later.
Despite successful trials where Ictineo II stayed submerged for hours and maneuvered under steam power, Monturiol could not secure military or government funding.
The Spanish Navy, like many others, was uninterested, and his funds ran dry. To satisfy creditors, the revolutionary Ictineo II was seized in 1868 and dismantled; its precision-engineered steam engine was sold for scrap metal, and its hull was broken up for firewood.
A technology that could have revolutionized naval warfare was lost for nearly 80 years due to bankruptcy. A common theme as we travel through the 1800s. Innovation was everywhere; people recognized the issues of conventional submarines and tried to tackle them to, sadly, no avail.
To cap off this look at individual submarines, before we shift fully to metallurgy, let's talk about Peral and Gymnote. Let's start with Gymnote.
Championed by Admiral Théophile Aube and designed by Gustave Zédé, the Gymnote was constructed at the Arsenal de Mourillon in Toulon. It was named after the gymnotid to reflect its revolutionary battery system, a first of its kind. The Gymnote was intended less as a proper vessel and more as a proof-of-concept for underwater navigation, tasked with validating the feasibility of the electric motor in a combat environment.
Gymnote utilized a single-hull design constructed of steel plates riveted to circular frames. However, the immense weight of the battery bank forced engineers to use dangerously thin plating, as thin as 4mm in places, to maintain buoyancy.
The propulsion system, designed by Captain Arthur Krebs, featured a 55-horsepower electric motor driving a variable pitch propeller. The original plan called for a massive, 11-tonne bank of 564 Commelin-Bailhache-Desmazures alkaline cells to power her. These proved so heavy (nearly one-third of the ship's displacement) and unreliable that they were replaced in 1891 with a sulfuric acid Laurent-Cély system.
Despite these limitations, the Gymnote pioneered the optical periscope, something not really seen since the early attempts of Nautilus, and the electric gyrocompass, while carrying its armament in external Drzewiecki drop collars to save internal space.
The Gymnote served as a vital test vehicle for the French Navy for nearly twenty years, completing over 2,000 dives and undergoing constant modifications, including the addition of a conning tower and extra rudders for stability.
Its end, however, was accidental. On March 5, 1907, the vessel ran aground. While undergoing repairs in a dry dock at Castigneau, a valve was accidentally left open, flooding the interior with seawater.
The resulting corrosion to the delicate electrical systems was deemed too expensive to repair. The vessel was stricken from the naval list in 1908 and scrapped in 1911.
Alongside Gymnote was another Spanish development in Peral. Unlike Gymnote, the Peral was a warship. Designed by Lieutenant Isaac Peral y Caballero and launched in 1888 at Cadiz, the project was backed by Queen Regent María Cristina but faced stiff opposition from naval traditionalists, as you can imagine.
Unlike the locally sourced French boats, Peral toured the industrial capitals of Europe to source the finest components for his vessel, ensuring that the Peral would be built with the highest quality steel and optics available, a necessary evil in overcoming the limitations of Spain's industrial base.
The Peral was significantly more advanced than its French contemporary. It featured a twin-screw propulsion system powered by two 30-horsepower Immisch motors, offering superior maneuverability and a remarkable surface range of over 350 nautical miles. The interior was designed with ergonomics in mind, featuring white corridors and rubber floor mats to insulate the crew from high-voltage hazards.
Probably its most critical innovation was the "apparatus of depths," an early autopilot system that used servomotors to drive horizontal diving planes, automatically maintaining a specific depth. Furthermore, unlike the Gymnote's external drop collars, the Peral utilized an internal torpedo tube capable of firing Schwarzkopf torpedoes while submerged, allowing the crew to reload under protection.
Peral is an early example of what would become the modern submarine, combining a number of the independently developed technologies that had slowly been reworked and refined over the course of the 19th century. Despite a technically flawless series of sea trials where the Peral demonstrated its ability to navigate, dive, and mock-attack surface cruisers, the project was sabotaged by naval politics.
A new Minister of the Navy, Admiral José María Beránger, viewed the submarine as a threat to traditional hierarchy and declared the vessel unsuitable for combat. The order was given to remove its motors and batteries, and the hull was abandoned at the Arsenal de la Carraca.
It rusted in obscurity for decades until it was finally restored. It is now preserved as a monument in Cartagena if any of you are ever in Spain!
Peral is the final individual submarine we will discuss here in detail. The point of this section is to familiarize you with some of the key developments in submarine technology through the 19th century. Obviously, some platforms have been skipped for time. We could go on and on about it, however, we aren't here for that.
As we enter into the 20th century, we start to see the first true classes of submarines come into service. No longer restricted to singular examples or test beds, submarines come into their own, becoming true parts of the fleet.
It is here that we will start to transition to talking about metallurgy. While I would love to jump into everything, I also recognize that if we were to do that we would be here forever.
The Metallurgy of the World Wars
Heading into the 1900s and World War One we start to see the first classes of submarine enter service. The Holland Class represents the first mass-produced submarine standard. The USS Holland (SS-1) was commissioned by the US Navy on October 12, 1900. It would be the first class of submarines acquired by the American and British navies.
We will be focusing almost exclusively on American and German metallurgical developments as those are the two families we need to know for this discussion. Luckily, for the most part, that is simple leading up to World War Two.
At this time almost all submarines were made using Mild Steel. This material was a low-carbon ferrous alloy, typically containing between 0.15% and 0.25% carbon, with trace amounts of manganese, sulfur, phosphorus, and silicon.
It was the primary material used in almost all shipbuilding, largely replacing the previous wrought iron. It provided a low-cost, readily available source of material that fit the requirements for the time. Navies needed a material that could easily be riveted and shaped into the desired hull form without suffering from cracking.
Mild Steel itself provided a yield strength of around 30,000 to 35,000 psi (roughly 205 to 240 MPa) which provided submarines at the time with a theoretical 100ft depth limit, though for safety reasons many obviously didn't go quite that far.
Yield Strength is one of the figures we will talk significantly about over the course of this. Yield Strength is the most critical number for a submarine. It measures the stress level at which the metal stops acting like a spring and starts behaving like clay (plastic deformation).
The higher the yield strength, the deeper you can theoretically go. That will be more important later, though it is a number you need to watch for. Again most submarines at this time and through to World War Two used some form of Mild Steel or equivalent.
The Germans to this regard actually fared somewhat better than their allied counterparts. They were utilizing their own similar metallurgy called Flussstahl or flow steel. This high-quality Siemens-Martin steel was primarily forged by the Friedrich Krupp AG conglomerate. It would evolve into the more commonly known St 42.
Krupp steel was noted for its homogeneity and consistency. It provided similar yield strengths to the Mild Steel found in the UK, but the open-hearth process generally resulted in a cleaner, more consistent product with better notch toughness.
The primary limitation of the time wasn't necessarily the steel itself but the riveting involved in forming the hulls. Riveting was a mature technology. It was the primary method used in construction at the time. However, rivet holes created a number of ongoing issues.
For example, rivet holes reduced joint strength to only 70–85% of the base steel. This meant that designers were forced to use heavier plating and overlapping joints, adding tons of "parasitic weight" that limited fuel and weapon capacity.
Furthermore, these overlapping lap joints prevented the hull from being a perfect cylinder, introducing eccentricities that significantly lowered the vessel's collapse depth compared to later welded designs.
Most seriously, the static nature of riveted joints proved fatal when subjected to the dynamic hydraulic shock waves of depth charges during combat. While steel plating could momentarily flex under the pressure of an explosion, rigid rivets could not, frequently shearing off or "popping" and turning the pressure hull into a sieve.
What you will find throughout this is that oftentimes the construction, and how we do it, is as much of a limiting factor as the metallurgy itself. Again Mild Steel and its counterparts provided enough capability that few other options really took off in the immediate First World War.
There were attempts at this time. The British most famously tried to get Admiralty High Tensile (AHT) steel to work. Unlike Mild Steel, AHT added a small percentage of nickel to the formula. The goal was to reduce the weight of the hull structure without sacrificing strength, allowing for higher surface speeds.
Of course, this came with tradeoffs. HT steel was harder to drill and rivet effectively without inducing cracking. It was also less ductile than mild steel. This made it more prone to catastrophic failure rather than deforming under depth charge attacks.
And so, for most everyone, despite some experiments, Mild Steel remains the promised steel used both in WW1 and the following postwar years. As we move away from mild steel though, I will once again be shortening our scope.
While I would love to talk about Ducol and S steel, the truth is they matter little to our general discussion. If I talked about every steel we would be here forever. So, for the sake of time, I will be exclusively focused on American and German developments, as these are the families that lead us to the modern day.
In the immediate postwar, the new Reichsmarine, formed from the ashes of the Kaiserliche Marine, was barred from operating submarines under the Treaty of Versailles. This immediate period led to a near twenty-year gap in German submarine development until 1935...
Except it didn't. See, the German navy never truly abandoned the technology. Throughout the 1920s and early 1930s, Germany circumvented these restrictions through a secret front company in the Netherlands called IvS (Ingenieurskantoor voor Scheepsbouw).
By designing and building submarines for other nations, such as Finland, Spain, and Turkey, German engineers kept their skills working and created a pathway for testing new innovations without technically violating the treaty.
When Adolf Hitler formally renounced the disarmament clauses of Versailles in 1935, German engineers already had the experience and technical knowledge to jump back into domestic production.
Seeking to avoid a naval arms race they could not afford, the British signed the Anglo-German Naval Agreement in June 1935. This treaty was a diplomatic triumph for Hitler. While it limited the German surface fleet to 35% of the Royal Navy's tonnage, it finally allowed Germany to build submarines up to 45% of British tonnage.
This agreement effectively legitimized Germany’s violation of Versailles and gave the Kriegsmarine the international legal cover it needed to operate U-boats openly. The Kriegsmarine, now aware it could not match the Royal Navy in tonnage, sought to build submarines that were technically superior. The material foundation of this fleet, from the coastal Type II to the long-range Type IX, was a high-strength low-alloy (HSLA) steel designated as St 52.
St 52 wasn't originally developed for submarines. It was a high-grade structural steel utilized in bridges, civil engineering, and railway construction.
The genius of St 52, though, lay in its simplicity and its reliance on domestic resources. German metallurgists, anticipating a blockade in the event of conflict, formulated St 52 as a Carbon-Manganese steel.
Unlike the complex Nickel-Chromium armor steels of battleships, St 52 achieved its mechanical properties through a precise ratio of Manganese to Carbon, often micro-alloyed with Silicon and Aluminum for grain refinement.
St 52 maintained a yield strength of an astonishing ~51,488 psi (355 MPa), significantly higher than the common Mild Steel in use elsewhere and even the at-the-time new D-steel used in British submarines.
This allowed German submarines to be built to standards that could withstand pressure at depths of 200+ meters, while Allied subs were often limited to 100–120 meters. Crucially, St 52 was highly weldable. This allowed Germany to move away from riveted hulls to fully welded pressure hulls.
For naval applications, the Kriegsmarine established rigid specifications, often referred to as St 52 KM (Kriegsmarine) or St 52 HP (High Performance).
St 52 would be the mainstay of almost every German submarine throughout the Second World War including the Type VII, Type IX, and Type XXI submarines. While some early-war submarines used mild steel, the number is bordering on rounding errors.
As the war progressed and Allied anti-submarine tactics improved, the Kriegsmarine identified a need for deeper diving capabilities to escape sonar and improved depth charges.
The response was the design of the Type VIIC/42, a deeper-diving variant of the standard U-boat. To achieve this without making the hull prohibitively heavy, Krupp developed a new steel grade: CM 351.
CM 351 incorporated small amounts of Vanadium and Chromium, alongside elevated Silicon and Manganese. The addition of Vanadium was a classic method for grain refinement.
This new formula provided a yield strength of a whopping 64,000 psi (440 MPa). This was a roughly 25% increase over St 52, a significant performance increase. However, CM 351 would sadly never truly see the light of day.
Krupp, who was producing the steel, only had the facilities to produce approximately 2,150 tonnes of CM 351 per month. It was also extremely costly and demanding compared to the well-refined, purpose-built St 52.
Ultimately the Type VIIC/42 program was cancelled in 1943 in favor of the radical Type XXI Elektroboot, which itself stuck with the safe option of St 52. It would remain the primary choice of German submarines until the end of the war.
Ultimately German advancements in submarine metallurgy are legendary, and for great reason. Time and time again German industry has proven its capabilities in this field, and despite the setback of the postwar environment once again stripping them of their right to submarines, it would only be a matter of time before they bounced back.
Before we get there though let's talk about the Americans. Unlike the Germans, the Americans had no restrictions in their ability to study and experiment with metallurgy in submarines. They just kinda chose not to.
Okay, a bit dramatic, but point remains. The Americans held on to mild steel for an insanely long time. Again, mild steel was cheap, widely available, and exceptionally easy to work with. It could also be riveted or welded with minimal risk of cracking.
Up until 1942, virtually every U.S. submarine was made with mild steel. It was during the development of the Gato-class that Naval architects Armand Morgan and Andrew McKee recognized that mild steel was not built for the realities of the Pacific War. Japanese depth charges were often set too shallow, but as their tactics improved, the 300-foot limit would become a death sentence.
Like many others, the solution came in the adoption of High-Tensile Steel (HTS). The requirements for the time called for a yield strength of 50,000 psi (345 MPa) along with a theoretical collapse depth of 900 feet (275 m) and a rated test depth of 400 feet (120 m).
The original formulation for HTS was a Chrome-Vanadium alloy. Vanadium is an excellent grain refiner, creating a microstructure that is both strong and tough. However, early in the war, the US faced a critical shortage of Vanadium, which was prioritized for tool steels and aircraft engine components.
So the scientific community went to work, and with them came quite the ingenious solution. American metallurgists at Carnegie-Illinois and Lukens Steel reformulated the HTS specification.
They developed a Titanium-Manganese alloy that was meant to meet the exact same 50,000 psi yield requirement and weldability standards. In reality, the practical, standardized HTS used in the likes of the Balao and Tang-class was limited to 42,000 psi.
Hence, HY-42 was born! In reality, it's quite remarkable how quickly American metallurgists were able to develop a comparable alternative formula; it is a testament to the depth of the US industrial scientific base.
HY-42 would carry the United States to the end of the Cold War. However, immediately after, the realities of the new world were falling upon them. Just as quickly as HY-42 came in, the question over its capabilities were raised.
The Cold War and The HY Family
WW2 was over, and with it? A new threat was rising to the east, one that was quickly putting pressure on American submarines. The Cold War was upon us, and with it came the nuclear age.
As we enter the Cold War we start to see the modern submarine begin to take shape. The 1950s introduced the concept of both Nuclear Propulsion for submarines with the USS Nautilus as well as the classic Teardrop-shaped hull with the USS Albacore.
The Albacore would also introduce another innovation, the use of HY-80 steel for its hull. As we had previously talked about, the U.S. Navy had adopted HTS/HY-42 in the early 1940s to replace the use of Medium/Mild Steel.
Yet despite the leap in performance that HY-42 provided, the introduction of the Cold War continued to expose the limits of the current formulations in use. The increased capabilities of active sonar, as an example, required deeper diving submarines able to reach the thermocline layer, an area of rapid temperature decrease that acts as an acoustic barrier. It refracts active sonar pulses downward, creating a 'shadow zone' where a submarine can hide undetected from surface vessels. Not to be confused with the Thermal Gradient itself.
The thermocline is not a static place. It is a dynamic, shifting space that lives with the currents itself. Its depth fluctuates constantly thanks to things like seasonal heating, internal waves, and the weather itself. For a submarine, this means the 'safe zone' is a moving target that requires constant monitoring.
Vertical Mobility is also important for survivability. While an HY-42-based submarine might have been survivable against early war Japanese Type 95 depth charges and torpedoes, new advances were quickly catching up, able to reach farther and deeper than the generation before them.
The Soviet SAET-50M Homing Torpedo is a prime example of such a weapon coming around that time; an electrically powered, passive acoustic homing torpedo able to operate at depths as deep as, I believe, around 650ft/200m with a 375kg warhead was something that a HY-42-based submarine would struggle to compete against.
In an era where Anti-Submarine technology was rapidly advancing, and advancing fast, the need for a vessel that could dive beyond the 400ft that HY-42 provided was critical. Trying to use HY-42 to increase the maximum depth would require increasingly thicker plates, which, as you know, would make them exceedingly heavy, reduce top speed, and reduce potential payload.
So a new steel was needed, one that could reach depths beyond 1000ft (300m). To achieve this the navy would need to move away from the Carbon-Manganese-Vanadium formula of HY-42, and in truth the legacy of metallurgy that had been built from the first uses of Mild Steel in submarines.
The Bureau of Ships, the predecessor to Naval Sea Systems Command or NAVSEA, was tasked with finding the right composition of high strength, high modulus of elasticity, and low density to provide the navy the hull it wanted. The requirements were fairly demanding for the time.
The new material needed a yield strength specified to be between 80,000 and 95,000 psi. It needed the strength of armor plate, the toughness to resist brittle fracture at freezing ocean temperatures, and, most critically, the weldability to be fabricated in shipyards rather than in laboratory conditions.
The navy almost immediately identified a version of Special Treatment Steel (STS) as a viable candidate to fulfill this requirement. STS was a Krupp-type homogeneous armor developed by Carnegie Steel around 1910 for ballistic protection on surface ships.
Unlike heterogeneous Krupp steel, STS was uniform throughout its thickness. This homogeneity allowed it to deform plastically under impact, absorbing energy through ductility rather than surface hardness. This was a revolutionary advancement in weight efficiency. Instead of bolting armor onto a mild steel hull, the hull itself became the armor.
STS is also referred to by U.S. sources as deck armor, horizontal armor, and Class B armor for those of you with a bit of historical knowledge. For the remainder of this, though, we will be referring to it as STS. STS was originally developed as a nickel-chrome-vanadium alloy steel, though vanadium would eventually be dropped from the formula fairly early on.
STS would be a common inclusion in almost every US warship by the 1930s and through the second world war. Its common usage, the experience of yards with working of it, and high strength made it an ideal base off of which to develop.
At the same time both Norfolk Naval Shipyard and the University of California had been experimenting with a low-carbon Special Treatment Steel that matched the navy's desire for an 80,000 psi steel. To achieve this, the Carbon content in the steel was limited to 0.18%.
While higher carbon levels would have made achieving that required 80,000 psi yield strength easier, it would have rendered the material unweldable in the constrained environments of a shipyard.
To compensate for the lower carbon, the decision was made for high levels of Nickel and Molybdenum to be introduced to the formula, making the steel significantly more expensive but technically superior to any commercial-grade alternatives available.
The reduction in the Carbon had a few add-on effects to how HY-80 performed compared to STS. For example, the lower carbon reduced the peak hardness of the Heat-Affected Zone (HAZ), thereby mitigating the internal stresses that drive cold cracking.
HY-80 is still Quenched and Tempered like STS. This contrasts to HTS, which is a normalized-rolled steel. That basically means it was processed by heating the steel and allowing it to cool in still air, creating a fine-grained, uniform microstructure suitable for demanding applications like structural plates and shipbuilding.
HY-80 on the other hand relies on a cooling process (water quenching) followed by reheating (tempering). This process forces the crystal structure into martensite, which is then softened slightly to gain toughness.
Testing by the University of California showed that low-carbon STS showed ductile behavior at failure and much greater energy absorption at fracture than other commercial alternatives. These initial tests provided enough confidence for the navy to move forward on this version of STS.
On August 15, 1951, with the issuance of Military Specification MIL-S-16216, HY-80 would officially receive its christening. The "HY" stood for "High Yield," and "80" designated the minimum yield strength in ksi.
From here we loop back around to the first tests of HY-80. Technically USS Albacore was ordered before the HY-80 designation was given, so you might find references to it using Low Carbon STS instead. Both here are still the same.
It wasn't just submarines, though, that got a taste of HY-80. USS Forrestal, the world’s first supercarrier, also used HY-80 for its armoured bulkheads, continuing the legacy of STS.
Another fun fact is that HY-80 is also the first steel for which the Explosion Bulge Test (EBT) became a mandatory "go/no-go" qualification standard. This test was developed by W.S. Pellini and his team at the Naval Research Laboratory (NRL) to evaluate existing armour steels.
The Navy needed a way to guarantee that the welded joints of HY-80 would not shatter under the shock of a depth charge, so they turned to EBT to provide that crucial test needed to prove its suitability as a submarine steel. The EBT became the defining qualification test for HY-80 because standard tests, like the Charpy V-notch, couldn't predict how the Heat-Affected Zone (HAZ) would behave under explosive loading.
To explain EBT as best I can without confusing most of you: you basically take a welded steel plate, slap a brick of explosives on it, and detonate it to force the metal to deform rapidly into a bubble. The goal isn't to test strength, but its toughness. More specifically, the goal is proving that the weld will stretch and bend under a shockwave rather than shattering like glass. If the metal bulges, the sub survives the depth charge; if it cracks, it fails. Simple, right? I hope so. Nowadays the EBT has mostly been replaced by newer methods like the Dynamic Tear (DT) Test. However, it is still done on occasion; it isn't fully phased out of existence. That's my little side quest of the day. Let's get back to the main story.
Now, before we continue, we need to talk about the big issue with HY-80 and many High-Yield steels in general. While HY-80 is weldable, it is also notoriously unforgiving. She is not easy on the user, and the worst of that comes from things like Hydrogen-Induced Cracking (HIC).
HIC is a silent killer of HY steels. It is the looming threat that has haunted many in its time. HIC happens when hydrogen—generated from moisture in the welding arc from stuff like humid air, damp flux, or rust—dissolves into the molten weld. As it's cooling, this hydrogen diffuses into the hard, martensitic HAZ. If the hydrogen concentration is high enough, it causes delayed cracking—not immediate cracks, but cracks that may appear hours or days after welding.
You can see on a submarine why that might be very concerning, and almost every sub that has ever dealt with High-Yield steel has run the trial of preventing Hydrogen-Induced Cracking.
For the most part, Albacore and the Skipjack-class managed to avoid these issues; however, lurking underneath all this was a system of complacency and negligence that wouldn't come to light until tragedy struck.
On April 10, 1963, just under a decade after USS Albacore was commissioned, the USS Thresher slipped beneath the waves about 220 miles east of Cape Cod. The first of the Thresher (later Permit)-class nuclear submarines, Thresher was a marvel of engineering for her time, the pinnacle of all the things we had previously mentioned.
Thresher had been sent that day to put her theoretical depth to the test. Escorted by the USS Skylark Submarine Rescue Ship, Thresher descended toward her test depth of 1,300 feet. At 09:13 AM, after a relatively routine descent, the Thresher sends a concerning message to the Skylark:
"Experiencing minor difficulty. Have positive up-angle. Attempting to blow."
It would be the last full message that Skylark would receive from Thresher. In reality, as the following investigations would conclude, a pipe, caused by the immense pressure at 1,400 feet deep, had burst, and a stream of high-pressure seawater began spraying across the engine room of Thresher.
This spray damaged the main electrical switchboards in the engine room, causing the ship's safety systems to trip a reactor scram—an emergency shutdown—to protect the core. Filling with water and without power, Thresher attempted an emergency main ballast blow. This blasts high-pressure air into the ballast tanks to force water out and make the ship buoyant.
Sadly, the air lines were not dry enough, and the excess moisture combined with the high-pressure air rushing in—which, if you are familiar with the concept of the Joule-Thomson effect, rapidly cools as it expands—caused said moisture to rapidly freeze into ice, causing the valves to become plugged.
Within a few moments Thresher had become a rock: her reactor stopped, the air lines in her ballast frozen. Skylark had heard the air rushing to the ballast tanks abruptly stop as they froze. Skylark tried to establish communications with Thresher several times in the following moments. A final, garbled transmission at about 09:17 AM was followed by the sound of Thresher’s pressure hull imploding at 09:18 AM.
129 lives would be lost aboard Thresher that day, still the deadliest submarine disaster in history. The final catalyst was ruled as a failure of the silver-brazed joints used in her construction. Silver-brazed joints are notoriously difficult to test and examine, but it masked a wider problem with submarine construction at the time.
The report of the Court of Inquiry into the Loss of the U.S.S. Thresher (SSN-593) following the disaster paints a bleak picture: a Navy that pressed new technologies they didn't understand far too quickly, combined with a negligent industry that allowed for mistakes and defects to be left through their own lack of care.
A lot of blame in the report is put on the quality control at the Portsmouth Naval Shipyard. Critical inspections were undocumented or foregone altogether; testing was only done on small portions of the hull, potentially missing hundreds of small defects.
Ultrasonic testing in Thresher’s sister ship, the USS Tinosa, showed a 10–14% failure rate in her own silver-brazed joints. The report concluded that Portsmouth skipped hundreds of joint tests because the equipment was slow to arrive or difficult to use in tight spaces. Instead, the team relied on archaic tests like the "hammer test" to determine the quality of the joints, missing hundreds of little time bombs ready to go off at a moment's notice.
And it wasn't like these were unknown issues. Even during her trials, leaking was regularly noted aboard Thresher, as were concerns about the air lines potentially freezing. Both of which were ignored by the Navy at large.
In this case, the HY-80 steel used on Thresher did do its job as intended. She held on down towards 2,400 feet before she imploded. She exceeded expectations in that regard. Yet HY-80 was not spared in this examination. While HY-80 was not the cause of the disaster, the report nonetheless undertook a substantial effort in examining the quality of the HY-80 plates used on Tinosa. What they found was that, hidden from the visible eye, existed hundreds of defects in her hull.
Along with the aforementioned HIC, testing also showed significant slag inclusions and gas pockets in the weld, all of which go against the core requirements when working with HY-80. It wasn't to blame, but the fact remained that the method by which yards like Portsmouth were handling HY-80 presented an extreme risk to future submarines. While it held this time, there was little guarantee that it would in the future.
Coming out of the inquiry, the United States Navy undertook the SUBSAFE program, one of the largest quality control shifts in Navy history. With it came a lot of the standard practices we see used when it comes to working with High-Yield steel. The "big three" mandates coming out of this directed that:
Every plate of HY-80 had to be traceable back to the mill heat and specific chemical analysis.
100% of hull welds required non-destructive testing (NDT) using ultrasonic (UT) and radiographic (RT) methods to detect internal flaws.
Only welders certified for specific HY-80 procedures could work on pressure boundary components.
These practices formed the foundation for how we treat High-Yield steels. It set the framework for quality and ensured that HY-80 remained a valuable contribution to the fleet. Following the Permit-class, almost all U.S. Navy submarines through the Cold War used exclusively HY-80 steel in their construction.
There were some limited exceptions. The USS George Washington, the United States' first nuclear-powered ballistic missile submarine (SSBN), utilized HTS in its missile compartment insert for the Polaris missiles instead of HY-80.
HY-80 did most of the Cold War well; however, the advent of deeper-diving, titanium-hulled submarines like the Alfa and Typhoon-class submarines put a new concern in the mind of Navy planners. Soviet submarines could now dive deeper than their American counterparts, potentially evading the standard Mk-48 torpedo and operating in acoustic zones where the Los Angeles-class could not follow.
This sent Navy planners to the drawing board. While HY-80 had been largely successful, the recognition was that it was no longer enough. The Navy needed to go deeper, and deeper they shall go.
The Seawolf program was the birth of this concern; the next generation of nuclear submarines demanded a lot: a silent speed of 20 knots, a maximum speed of 35 knots, and a test depth of 1,600 feet (490 m)—a significant demand compared to early HY-80 boats.
Once again, the same issues at the transition from HY-42 to HY-80 existed. While possible to get HY-80 to that capability, doing so would have required extremely thick plates that weren't feasible for a submarine. The solution decided was to take HY-80 and shift the formula around just enough to reach the desired capabilities. Chemically, the two steels are very similar to each other, as you can imagine. They both rely on nickel for toughness and chromium and molybdenum for hardenability. The primary difference is that HY-100 maintains a slightly higher nickel content (2.25–3.50% vs. 2.00–3.25%).
The big shift, however, to achieve the higher strength of HY-100 is that it requires slightly more precise alloying and, most importantly, a different heat treatment regiment. HY-100 is typically tempered at a lower temperature than HY-80; this lower tempering temperature retains more of the hardness and strength gained during the quenching phase, though it sacrifices a small margin of ductility.
This process allows HY-100 to achieve its 100,000 psi (690 MPa) yield strength compared to HY-80. A quick, efficient solution to a growing problem. Despite being designed for the Seawolf-class, the initial tests of HY-100 were done on the Los Angeles-class. The USS Albany and USS Topeka both utilized HY-100 partially in their construction. This method was done to directly address the issues with the introduction of HY-80. Utilizing parts of the Los Angeles-class allowed Newport News Shipbuilding and Electric Boat to refine the welding procedures, non-destructive testing techniques, and forming processes required for the harder steel before the Seawolf production line began.
Call it trauma, call it precaution—to prevent Hydrogen-Assisted Cracking (HAC) in HY-100 steel, the Navy mandated rigorous fabrication controls that significantly exceed those for HY-80. These protocols require maintaining high preheat temperatures (often 200°F–300°F) to facilitate hydrogen diffusion and prevent brittle phase formation. Additionally, "interpass temperatures" had to be strictly managed to balance the risk of cracking against the loss of yield strength. Consumables face similar scrutiny. Welding electrodes had to be stored in baking ovens to eliminate moisture and were discarded if exposed to humidity for too long.
This creates a brutal working environment where welders must operate in confined spaces on steel radiating extreme heat, requiring frequent personnel rotation. The difficulty of the process initially led to high defect rates and expensive rework to repair flawed welds. Ultimately, the complexity of working with HY-100 contributed heavily to the astronomical cost of the Seawolf program, with unit costs approaching $3 billion. While the collapse of the Soviet Union did reduce the strategic necessity for such vessels, ultimately the prohibitive labor and energy costs associated with HY-100 construction were a decisive factor in cancelling the class after only three boats.
From the lessons of HY-100, the United States Navy would go on to develop the High-Strength Low-Alloy (HSLA) family of steels during the late 90s and 2000s for the Virginia-class. In U.S. service, HSLA-80 and HSLA-100 have largely replaced the HY-series of steels; however, we won't be discussing those.
Instead, we're going to take this time to pivot across the ocean, because at this same time the Republic of Korea Navy (ROKN) is busy acquiring its first submarine.
In 1987, the ROKN selected the German Type-209 design, a ubiquitous export submarine known for its reliability and cost-effectiveness, as its first ever true submarine. While the ROKN had experimented with midget submarines and such, the future Type-209 represented the Navy's first real sub.
While the first vessel, ROKS Jang Bogo, was built by Howaldtswerke-Deutsche Werft (HDW) in Kiel, Germany, the subsequent eight hulls were constructed domestically by Daewoo Shipbuilding & Marine Engineering (DSME, now Hanwha Ocean). The critical technology transfer during this phase was focused on the welding of HY-80 steel. HY-80 was the primary choice for German export submarines. It was cheap, easy to produce, and provided the capabilities that most countries would demand. The successful domestic production of the Jang Bogo-class demonstrated that Korean shipyards could meet the stringent quality control standards required for pressure hull manufacturing.
This phase laid the groundwork for the ROKN's depot-level maintenance capabilities, allowing Korean industry to eventually service Type-209 submarines for other nations, such as Indonesia, and building the foundations of the Korean submarine industry—something Naval Group apparently will never forgive the Germans for doing.
The follow-up to this in 2000 was the KSS-II program. This followed a similar approach to the Jang Bogo-class in the licensed production of the Type-214 submarine. From a steel perspective, there was little change here. The Type-214, unlike the Type-209, utilizes HY-100 steel. The Type-214 also introduced new technologies that would go into developing Korean industrial expertise, primarily the introduction of Air-Independent Propulsion (AIP). We won't be discussing those, though.
This shift to HY-100 provided the primary thing that Korean industry needed: to slowly and steadily build experience, while at the same time using it as a base to examine future potential in the follow-up class to come. Of course this isnt without issues. The KSS-II suffered from numerous issues and maintenance troubles regard HY-100, among other things, however, those lessons learned proved the testbed needed to prepare for what was next.
When it came time for the KSS-III, the need for a choice of steel was prevalent. Compared to the KSS-II, the KSS-III was designed from the outset as a "blue-water" submarine with nearly twice the displacement of the KSS-II. The primary driver for the KSS-III’s massive size increase was the requirement to house a Vertical Launch System (VLS). This added not only significant weight but also mandated a significant increase in the length and diameter of the hull to accommodate it.
Similar to the United States, Korean officials were left with a pickle. They wanted to maintain a deep-diving submarine able to operate ballistic missiles across the Pacific Ocean, but to do so with HY-80, which industry was most accustomed to, would require significantly thicker plates to accommodate—something that by now you should recognize that no one wants to do.
As such, the need to move up from HY-80 was clear, and for the Koreans, the shift to HY-100 felt like the most practical route. It was already in service, offered the yield strength desired, and had a relatively cheap price tag to produce. Instead of focusing on things like replicating HSLA, the choice was made to focus on the easiest path available and working from there.
However, there was an issue. Korean industry at the time could not produce HY-100 in the quantities and purity that were required for a submarine. Similarly, industry lacked the means to produce a pressure hull to the size and scale of the KSS-III. To do this, the Korean government needed an organized effort from industry to make this work.
This effort was led by POSCO, who served (and still does) as a strategic R&D partner to the Agency for Defense Development (ADD). POSCO’s role was to develop a domestic, ultra-high-strength steel that met the rigorous MIL-S-16216 (or equivalent operational) standards for HY-100, while also addressing the notorious fabrication difficulties associated with high-yield steels.
POSCO’s most significant technical contribution was taming the notoriously unforgiving nature of HY-100. To prevent things like brittle fracture, POSCO utilized advanced vacuum degassing to reduce impurities like sulfur and phosphorus to near-zero levels, ensuring exceptional toughness.
Now, to clarify, POSCO has modified the original formula of HY-100. However, despite my efforts, I could not for the life of me figure out exactly how. I believe they lowered the carbon content? That would be done to reduce the risk of delayed cracking from HIC. However, again, I can't verify what exactly was done.
Beyond metallurgy, POSCO secured South Korea’s domestic sovereignty by investing in the heavy rolling technology required for the KSS-III’s larger hull, which is honestly just as important as the steel itself if we're being serious. The KSS-III's increased displacement necessitated thick, uniform steel plates that were previously only available from specialized Japanese or European mills. POSCO undertook the effort in establishing the capability to produce these heavy plates domestically, eliminating the risks that come from relying on foreign manufacturers.
The KSS-III, to its credit, has never had issues with things like HIC. All the information I can find paints the picture of a generally well-done, top-notch production despite the shift to a new variant of HY-100-based steel. That, of course, also comes from the advances in technology from the 1990s to now.
Modern shipyards like Hanwha's at Okpo rely heavily on things like Digital Twins, robotic welding, and increasingly AI-based supports to assist in the construction process. These provide more accurate, precise support when it comes to the conditions in the yard and the welding process, which significantly reduce the risks of things like HIC. They also benefit from thirty years of American use, and of course the experience of HY-80. It's relatively safe to say that most of the construction risk of HY-100 has been mitigated in modern yards and will only increase as technologies advance to better manage the construction process.
I did want to take the time to at least discuss the Korean development pathway, and to reaffirm quickly what the KSS-III uses. There is some active debate out there, and while it isn't too terribly different, it is still worth noting how far Korean industry has come along.
While there are unknowns, we can mostly rely on HY-100 and its capabilities to judge the KSS-III. While POSCO is working with Hanwha on the next generation of submarine steel, I won't speculate on what that looks like, nor am I expecting our subs to be constructed from this material.
We have now come to the end of the HY family and American developments. That means we can finally move on to...
The Germans: Post-WW2 to Today
To credit them on this front, the development of metallurgy when it comes to German submarines is a lot more simple and straightforward than American developments. Export submarines tend to use HY-80 or similar in their construction.
However, when it comes to domestic submarines? Things get a bit more interesting. Of course, coming out of the Second World War the Germans had successfully developed St 52. We have already been through that.
Following the war, the Kriegsmarine was dissolved, shipyards were stripped, and research was prohibited. It was not until the mid-1950s, with the hardening of the Iron Curtain and the integration of West Germany into NATO, that the prohibition was lifted.
The rebirth of German submarine engineering was not led by the government, which was legally barred from operating arms design bureaus, but by the Ingenieurkontor Lübeck (IKL). Founded by Professor Ulrich Gabler, a veteran U-boat engineer, IKL became the intellectual bridge between the lost expertise of the war years and the new tactical requirements of NATO.
The Soviet Baltic Fleet posed a massive threat to North Atlantic supply lines, and to contain it, the new Bundesmarine needed small, maneuverable submarines capable of lying in wait in the shallow waters of the Kattegat and Skagerrak. In these depths, which average only around 55 meters, a submarine can't deep dive to evade attack like American submarines. It had to be silent, and more importantly, it had to be invisible to the magnetic mines that would inevitably saturate critical chokepoints. This specific requirement birthed the quest for a non-magnetic hull.
However, this road was long and hard, and the attempt from the Bundesmarine to jump into non-magnetic steel would result in one of the most significant engineering disasters in modern naval history: the Type-201.
Early on in the Type-201's development, austenitic steel was identified as the best solution to fulfill the non-magnetic requirement laid out. Unlike standard steel, austenitic steel has a crystal lattice that is inherently non-magnetic. The contract was awarded to an Austrian firm, which proposed a material designated AM 10. On paper, it was perfect. It was chemically inert and virtually transparent to magnetic fields.
However, the validation program was fatally flawed. The material was tested in isolation, not subjected to the dynamic, cyclic loading of a diving submarine in a corrosive saltwater environment. Shortly after the commissioning of the lead boat, U-1, in 1962, inspection teams discovered microscopic cracks riddled through the pressure hull.
You see, AM 10 was a classic austenitic stainless steel. Its fatal flaw was in its carbon management. The alloy contained high levels of Chromium for corrosion resistance, but it lacked sufficient stabilizing elements to control the Carbon content during the thermal shock of shipyard welding. When the hull plates were welded, the heat caused the Carbon to bond aggressively with the Chromium at the grain boundaries. This reaction created Chromium Carbides.
This causes what is known as Intergranular Stress Corrosion Cracking. This reaction depleted the chromium, the very element that prevents rust, leaving the microscopic grain structures defenseless against the chloride in seawater. The operational pressure of diving and surfacing accelerated this corrosion, causing the hull plates to essentially unzip at the atomic level.
The fallout was severe. The first two boats were decommissioned after barely a year. The order for nine more was cancelled, and the Type-201 became a public scandal in West Germany. Industry had to retreat and re-engineer. It became clear that simply buying off-the-shelf "non-magnetic steel" was insufficient.
The interim solution came from the domestic steelmaker Phoenix-Rheinrohr, which developed the alloy PN 18 S2. This was used to build the Type-205 submarines. While the specific recipe is less documented than I would like, the key difference was stabilization. PN 18 S2 acted as a bridge between commercial stainless steel and military-grade material. It introduced the concept of "binding" the carbon, likely by introducing elements that have a higher affinity for carbon than chromium does, and to its credit? It did prove that the concept could work.
The Type-205 were essentially rebuilt versions of the failed Type-201, but with a few minor changes. The hull was slightly lengthened by about 2 meters, and new sensors were adopted for her. She also came with the new steel. Unlike the previous Type-201, the 205 proved structurally sound. The class remained in service for several decades, up until the early 2000s. They even saw some export success, with two being built for the Royal Danish Navy! This success re-validated the concept of the non-magnetic hull.
With the crisis averted, the German Navy standardized on the definitive material for all future non-magnetic hulls: 1.3964 steel. Commercially known as Nitronic 50, and part of the Amanox family, the chemical architecture of 1.3964 is a masterclass in metallurgy. A perfectly refined material meticulously crafted for submarines. It is the crown jewel, in my mind, of the German naval industry.
The defining difference of 1.3964 is the use of Nitrogen. In standard steels (like AM 10), strength comes from Carbon. But Carbon causes corrosion (as seen in the Type-201). 1.3964 replaces Carbon with Nitrogen. The formula for strength here is based on Interstitial Solid Solution Strengthening. A term I still don't fully understand, but let me try to explain: Nitrogen atoms are small enough to fit into the gaps between the larger Iron, Chromium, and Nickel atoms in the crystal lattice.
When the crystal lattice tries to deform under pressure (which would be a dent or a buckle in the hull), the dislocations in the metal structure have to move. The Nitrogen atoms act as "roadblocks" in the lattice, pinning these dislocations in place. Because Nitrogen is a potent austenite stabilizer, it does double duty: it makes the steel stronger and it helps keep it non-magnetic.
Amanox steels also have a high Manganese content, chemically required here because it increases the solubility of Nitrogen in the molten steel; without Manganese, the Nitrogen would bubble out as gas during casting. It is a genius formula. Almost undetectable to magnetic sensors, inherently corrosion resistant, and surprisingly strong for its family.
While it isn't HY-80, it maintains a Yield strength of ~58,000 psi, that is more than suitable for the shallow regions of the Baltic and the Atlantic coasts where the majority of German submarines operate. Now, hypothetically you could make Amanox stronger through things like Cold Working, but as far as I know, it's not something that the likes of TKMS has taken the leap on.
Now, there are issues. The primary challenge is welding. To keep the hull non-magnetic, the welds must be fully austenitic, which makes them prone to "hot cracking" during cooling. Any contamination from ferromagnetic materials creates magnetic signatures, effectively rendering the hull's non-magnetic nature pointless. This necessitated the development of extremely pure filler metals and rigorous process controls. The process requires one to control the heat input precisely to prevent micro-fissuring.
Machining the alloy is just as difficult. The material hardens rapidly when cut, requiring rigid tooling and high power to process. The need for such rigid quality control, as well as the need for essentially new facilities dedicated exclusively to working with austenitic steels to prevent contamination, do raise a significant cost and burden factor.
Despite the difficulties, the payoff was the Type-206 class. These boats could operate safely in water depths as shallow as 20 meters—areas often sown with magnetic mines.
Since then, the Amanox family has been the exclusive steel used on German domestic submarines. While export models continue to rely on the HY family, the Bundesmarine and her select allies got access to the "holy grail" that is the Amanox family.
All subsequent models, including the Type-212A and Type-212CD, use a form of the Amanox family, although these days the formula that they use is a heavily guarded secret. I have tried and tried to find out what exactly the formula is to no avail. I hear there are some changes to increase the yield strength, but I can't confirm.
However, we do know it stays in the Amanox family, with the inner hull being made of this non-magnetic steel and the outer hull being a composite that I believe is fiberglass? Someone once told me that, but I could never get confirmation.
Germans don't like to talk. They aren't social like Koreans. In times like this, I wish I could say more; however, this is as best as we can work with, and truthfully it's good enough. So long as we have a general idea of the families used, we can determine quite a lot about the capabilities of a submarine.
And now you have it: the pinnacle of submarine steel history as it relates to American and German developments. This primer has now set you up to discuss the inherent benefits that both submarines could provide, and how their steel plays a role.
So. How do they compare in reality?
The competition
Now you have the history, you have the background of what it took to get this far, and now we can get into the modern platforms themselves. As we discussed, both the KSS-III and Type-212CD trace their lineages to two very different metallurgical families.
Both are products of their own environment, driven by the needs of two powers that face very different security requirements. In this case, when it comes to the steel used in both submarines, both present their own advantages to Canada.
The use of the Amanox family of steel in the Type-212CD is inherently driven by the need to operate in a magnetically contested environment, as found in the Baltic and North Sea. The Type-212CD is designed with one goal in mind: don't be found in the shallow, littoral depths of the European and Atlantic coasts.
This isn't to say that these aren't ocean-going submarines; however, their primary purpose is to operate in these shallower, highly contested environments. That is why the choice of non-magnetic steel is so important to the Germans, and why so much effort has gone into developing the Amanox family of steels—fine-tuning their formulas over generations, just as we saw with the HY/HSLA families.
The Koreans went with what they knew. They knew HY-80 and learned how to work with HY-100. They knew the family; they could make it at home. HY-100 provided the yield strength that the Koreans needed to operate in the depths of the Pacific Ocean while also being strong enough to host things like a Vertical Launch System (VLS) without compromising the thickness of the plates, as lower-strength steels might require.
The Tiger and the Lynx... I think I used that before. The KSS-III is a beast; the Type-212CD is a silent hunter. Both choices were made to prioritize the environments in which they operate.
The Koreans needed deep-diving, high-strength steel that could descend deep into the Pacific thermocline and hold the VLS needed to carry the 1-ton Hyunmoo-IV-4 without compromising size or weight. The Germans needed a non-magnetic steel to keep them safe from an environment full of platforms like the Ka-27 and Il-38 flying overhead.
But where does that leave us? Where does that leave a navy that needs to operate... everywhere? We're a three-ocean navy, after all. All three of which, evidently, are very different environments where either sub could hold a distinct advantage.
Of course, the Arctic will inevitably come up in this conversation. That's to be expected, and there are a few distinctions one could make about both platforms.
The KSS-III is ferromagnetic. It gives off a magnetic signature. The platform gets around this both by regular deperming and active degaussing, as almost everyone else does. The Koreans, funny enough, take this issue very seriously—like, they really care about this. And not for unobvious reasons, mind you. It is a very important part of a ferromagnetic submarine's regular maintenance and active protection against detection. The KSS-III in this regard employs an increasingly indigenous Active Degaussing System (ADS) combined with a self-developed deperming system to combat the potential magnetic signature.
At the core of this is the KSS-III's Distributed Degaussing System (DDS). Traditional arrangements, such as those found in older Korean submarines, relied on centralized systems backed by massive power supply units. These units would feed heavy copper cables that ran the entire length of the boat. This creates a significant risk with single points of failure and adds significant weight to the hull.
In contrast, the KSS-III's distributed system employs numerous smaller Bipolar Amplifier Units (BPAUs) or Coil Amplifiers located in proximity to the specific coil loops they control. This model significantly reduces the weight required by the degaussing system, provides redundancy by having multiple coils able to overlap in the event one fails, and provides more precise control over the degaussing process.
By utilizing several independent loops, the KSS-III can more precisely control its magnetic signature in specific sections, allowing for greater flexibility in areas where priority is needed. It turns the degaussing process from a singular system into a decentralized network of independent loops working in tandem.
Korean developments have now shifted to focusing on further improving this system and developing it with more indigenous equipment. Hanwha Ocean was awarded a contract in 2023 for the study and development of the next generation of this degaussing system, including the R&D of new demagnetizing coils, control units, power supply units, and magnetic sensor designs.
This is also on top of leveraging machine learning and artificial intelligence to better detect and manage the degaussing process autonomously. Basically, the focus in future developments is on refining the existing systems in place. It isn't making a new system, but further developing this first generation of technologies.
For deperming, the ROKN utilizes Flashing-Degaussing, or the "Flash-D" method, as many others have over the last forty or so years. More specifically, the Koreans utilize their own version dubbed "Flash-KD," developed by the Agency for Defense Development. How this differs from traditional Flash-D (of which there are claims it does)? Well...
Sadly, as you can imagine, the information on this process is very secretive and not really open-source, so we can't necessarily jump into what has changed. However, we can get into the Flash-D process. Basically, for those who don't know, the Navy wraps the submarine in massive cables, or pulls it into a specialized wire cage, and blasts it with huge surges of electricity. These "flashes" scramble the magnetic alignment of the steel atoms so they cancel each other out instead of pulling in one direction. It wipes the submarine's magnetic signature clean.
This is a fairly standard method of deperming. I hear the Koreans, again, are pretty on point with this, but I can't fully say in technical terms how. From what I can tell, the Koreans utilize specific Preisach models to dynamically optimize the process to the specific platform, which saves on time and cost.
The ROKN does have its own facilities near Jinhae for this. It is highly likely such facilities would be replicated here. The Koreans have also found a way to align this process more accurately with usual refit and maintenance schedules by better optimizing the deperming and follow-up degaussing process to ensure that submarines remain in the water as long as possible.
I'm going to be real: I am no Chris Richardson. This stuff is a bit over my head in the science department, however, I hope I explained that well enough, since it is the starkest contrast in the metallurgy of both submarines.
Obviously, the Type-212CD does not need deperming at all, as it is non-magnetic. I should also note that, no matter what, a non-magnetic submarine will always hold the advantage over a ferromagnetic one here. That isn't to say that the deperming process doesn't help significantly, but a KSS-III can never be truly non-magnetic.
Another quick note: non-magnetic submarines like the 212CD also have degaussing systems to compensate for the magnetic signature of the internal systems. Wärtsilä SAM Electronics, I believe, supplies the one on the Type-212A. The hull itself can also generate currents through movement, which are better known as Eddy Currents. That's the signature generated through conductivity.
The Type-212CD, while it does have an outer composite hull, would still generate these currents as it moves. However, the use of non-magnetic steel takes you 90% of the way there, and the use of an active degaussing system on top of that basically takes you the rest of the way—unless we want to get really specific and technical, and I'm not going to do that here. We have already dived past the deep end for me and into the ocean.
Sadly, I have no info on the degaussing system on the Type-212CD. In reality, it is far, far less of a concern than on the KSS-III, so I am not too stressed about figuring it out for this. However, if I ever do? I will certainly let all of you know.
Speaking of that non-magnetic hull, you might hear some technical folks bring up the capabilities of non-magnetic steel in cold and Arctic environments. While there is a lot of focus on the non-magnetic properties of Amanox steel, there are several other unique things it hypothetically brings to the table.
The biggest danger in the Arctic is the extreme cold, which can drop to -60°C. For those of you who have had to deal with me being really technical throughout all this, I'll keep it as simple as possible. While seawater acts as a thermal blanket, keeping at approximately -1.8°C, structures in the waterline (the splash zone) are subjected to the full atmospheric extreme. This creates severe thermal gradients and thermal shock conditions.
That's why we still talk about the cold. A bit of a basic refresher in middle-school science: metal acts differently in the cold. Steels like HY-100 have a "tipping point." At room temperature, they bend if you hit them. But if they get cold enough, they turn brittle and will snap instantly like a ceramic plate. This is called the Ductile-to-Brittle Transition (DBTT).
Naval HY-100 is chemically engineered to suppress the DBTT and is rated for low temperatures, but the risk is still one you will likely hear of. At temperatures approaching -84°C to -196°C, the impact energy of martensitic steels can decrease by nearly 90%. Hypothetically, if the submarine were to impact a multi-year ice keel or experience a collision while the steel is cold-soaked, the risk of brittle fracture in HY-100 is statistically higher than in Amanox steel.
Unlike HY-100, Amanox steels do not exhibit a ductile-to-brittle transition. They retain exceptional ductility and toughness even at temperatures approaching absolute zero. In fact, they actually get tougher in colder temperatures. Although it's highly debatable how this factors into an Arctic environment, the idea is that it provides a massive safety margin for Arctic operations. The hull of the Type-212CD will hypothetically deform (dent) rather than shatter. For a submarine crew operating thousands of nautical miles from rescue, this resilience against brittle fracture is meant as a fail-safe.
One last thing on the Arctic benefit topic: corrosion. Cold water is significantly denser and capable of holding higher concentrations of gas than warmer water. In the Arctic, this creates an oxygen-rich environment perfect for corrosion. Corrosion is fundamentally an oxidation reaction—metal gives electrons to oxygen, and bad things happen to the hull after enough time. Since Arctic water is super-saturated with oxygen, the cathodic reaction (the process behind rusting) is extremely efficient.
This makes the Arctic a highly corrosive, highly conductive environment for a submarine to operate in—something that can't be ignored when discussing the maintenance and sustainment of a submarine over its lifetime.
As you know, the Amanox steel used in the Type-212CD contains high levels of chromium and nickel. This forms a "passive layer" (chromium oxide) on its surface. If the hull is scratched by floating ice or slush (a common occurrence in Arctic littorals), the material "heals" itself by reforming this oxide layer. It does not rust.
Stainless steels like Amanox are naturally corrosion-resistant. I think most people in their adult lives know that. Amanox doesn't require additional coatings to provide a layer of protection, and the composite outer hull obviously doesn't have to worry about this.
The KSS-III and most similar boats don't have this luxury. Carbon steel is naturally reactive and will rust aggressively when exposed to seawater. This means that the KSS-III must employ a layered system of protection: heavy-duty epoxy barrier coatings paired with an Active Impressed Current Cathodic Protection (ICCP) system.
The initial coating, as far as I know, is not publicly known. I don't know who supplies it or exactly what is used. This coating, though, is designed to act as a passive, protective barrier. The ICCP system is the second layer. It's actually a really cool system. Instead of passive, sacrificial anodes, the system on the KSS-III functions as a dynamic feedback loop. As I understand it, zinc reference electrodes continuously measure the hull's potential against seawater, while thyristor control panels inside the pressure hull automatically adjust the current output to match changing environmental conditions, such as salinity or vessel speed.
This drives protective current through flush-mounted Mixed Metal Oxide (MMO) anodes, ensuring the KSS-III hull is protected from corrosion without risking hydrogen embrittlement. I won't pretend to fully understand the system in its entirety, but how it's explained to me makes it seem pretty cool!
It is a regular part of a submarine's maintenance to have to reapply coatings. It is a well-known process, and one we actively do among many others, so I will say it isn't as big a burden as some suggest. It is fairly standard and routine. The primary concern is physical abrasion. Damage to the outer coating can expose the hull to that corrosive brine, and while there is the ICCP system in place as a secondary layer, if the damage is significant or occurs in an area that the electrical current cannot reach effectively, it risks long-term damage to the hull.
That's the last issue specifically tied to the Arctic that I want to raise. While I'm sure someone will point out more, those are the major ones as I've found while asking around and discussing things with people the last few weeks. It highlights not just the inherent capabilities that stainless provides, despite the cost, but also the complex nature of a submarine's metallurgy.
Certainly, I feel I have learned a lot since I began this quest five weeks ago. We have broken down both platforms in as comprehensive a way as I can. You know the history, you know the strengths and weaknesses, and we've tackled the Arctic. But despite all that... does it really matter?
That might be a bit anticlimactic and confusing after I spent so long diving into everything. Yet I think I have to ask: will the metallurgy truly be a factor in the choice?
Does it really matter in the end?
Over the last five weeks, I have listened to people spout both types of steel as superior. And somewhere in the middle of writing it, I came to the conclusion—as controversial as this is—that most of these factors don't actually tip the scale. For the record, this isn't to target the specific, major capabilities. Yes, obviously a non-magnetic hull has its advantages, as does one that allows for greater depths. Those are tangible benefits. However, a lot of the concerns and benefits feel... overhyped.
HY-100 is cheaper, more readily available, and more adept for our existing industry. Modern Amanox steel can get very close to the MPa of HY-100 depending on how it was treated. Amanox provides an easy path in the corrosion department and operates really well in the Arctic. But HY-100 and similar carbon steels have also been operating up there without much issue for the last 70 bloody years. Sure, it's theoretically an issue and provides a safety margin, but is it really a massive "green checkmark" that tips the decision?
Hydrogen-Induced Cracking has long been known and worked with. It isn't an ongoing "mystery" issue. Modern shipbuilding, like we see in Geoje, utilizes digital twins and AI to precise welding with modern robotics. It's an issue, but one we know and have mostly solved.
The benefits that really matter to me are the 212CD's use of a non-magnetic hull—which is a huge deal—and the cheap, efficient nature of utilizing HY-100's natural strength. And that's about it. Turns out the most basic, in-your-face benefits are the ones that matter most, and almost everything else is highly specific or niche.
I don't want to sound dismissive. Obviously, stuff like HIC is a concern, so is reliable access to Amanox steel (which we evidently do produce in one refinery, apparently). But these don't feel like deciding factors. They don't feel like they set one above the other. Korea and Germany had specific, separate requirements that drove their choices.
We operate in a weird space. Again, we are a three-ocean navy that has conflicting demands. No matter what, CPSP will be a sacrifice somewhere. But when it comes to the steel itself? There's no "winner" that drives my choice. Each win comes with a tradeoff. I like the idea of a non-magnetic hull, but it's also exuberantly expensive with debatable applications in all the domains we want to operate in. HY-100 has great natural capabilities, but you take sacrifices in the hull signature.
Is Amanox strong enough for what we need most of the time? Probably. Is HY-100 slightly outdated? Yeah. Does it get the job done? Yeah. Is the increased depth it offers beneficial in the Pacific? Absolutely.
I tried to pick one to hold over the other, but I can't. I don't think the real argument is on the steels themselves. I think the real factor that will set them apart is the maintenance and sustainment side of things.
I have a duty to provide the information for you to make your own assessments. I want to hear your thoughts. I am waiting to be convinced which metallurgic choice is best. Perhaps when I get more into the maintenance side, I will be more readily convinced. For now, if either side wants to sell their steel as a major factor, the only one I think is truly marketable is the non-magnetic property of Amanox. The benefits of HY-100 are really found in the logistics.
Anywho, to cap off: both countries had their choices, and now Canada is reaching the point where we need to decide what platform better matches our needs. Most of all, I don't think the metallurgy will play the major deciding factor, but knowing the histories tells us a lot about what each platform prioritizes.
As I said at the beginning: knowing the metallurgy of a submarine is one of the key factors we can use to judge its capabilities, roles, and limits even without knowing all the classified equipment inside. Look at what we can tell just by knowing the steel? It's actually quite spectacular.
Noah, thanks for putting in the blood sweat and tears into this one. I can see the effort that went into it, and I really appreciate it. I think you struck the right balance of context, technical detail, and non technical overviews. Nice work.
Excellent article Noah. Obviously a lot of research went into this and it is much appreciated.
It does point to something I've been thinking about though. It seems to me that trying to compare these two subs is almost like comparing apples and oranges. They were both designed for different missions. The 212CD being the ambush predator lying in wait for it's prey while the KSS-III is a true blue-water platform.
If Canada wants to chart a course more independent of the US (and from what I read and hear we do) then the KSS-III would be the clear choice. I think it would give the RCN credibility that it hasn't had in a long time (if ever).