H2 Steel – H1 the sequel

Background

A recent Spyderco Byte has announced a replacement for H1 steel – H2. H1 is known for being a very high corrosion resistance steel used primarily in Spyderco’s Salt line of knives. I have a previous article about the design of H1 and how it “works” which you can read here. I am somewhat tempted to re-write large chunks of that article here because H1 is one of the most misunderstood knife steels but I am going to try to hit a few of the highlights without much explanation and I hope you will read the earlier article to understand what I am referring to:

H1 is an “austenitic” stainless steel. This category of steels includes grades like 301, 302, and 304 and are best known for being used in non-knife applications like pots and pans. Though they are used in butter knives, I suppose. They have very high corrosion resistance and excellent formability, and are non-magnetic. The grades are not known for their high hardness, which is generally a requirement for knife steels. Austenitic stainless steels will transform to hard martensite with cold working, however. The degree to which it transforms to martensite is dependent on its composition and the temperature at which the steel is cold rolled. Colder temperatures will lead to more transformation to martensite with less cold working. There are many models and equations for predicting how readily the steel will transform to martensite but no real consensus about how to predict it because of the difficulty of modeling the behavior. One measurement is the temperature at which 50% martensite will form from 30% strain. Here is an equation to predict that temperature:

M_D30 (°C) = 413 – 462*(C+N) – 9.2*Si – 8.1*Mn – 13.7*Cr – 9.5*Ni – 18.5*Mo

So for a steel that is designed to be transformed to martensite through cold rolling we want that M_D30 temperature to be relatively high so that when it is cold rolled at room temperature it will transform nearly 100% to martensite (without cracking). That achieves the relatively high hardness of H1 steel in finished knives, which has been measured in the 55-58 Rc range.

I have the composition of H1 as reported by Spyderco as well as an actual measurement of the steel independently performed by Sandvik:

H1 composition compared with two other somewhat similar austenitic stainless steels:

Sandvik’s composition measurement of H1:

This process of cold rolling an austenitic stainless steel is quite different than the majority of knife steels which are typically heated to a high temperature to transform to austenite and then rapidly quenched to transform to martensite, rather than cold rolled. The temperatures at which the steels transform to martensite are above room temperature, whereas if an austenitic stainless steel is quenched to room temperature it remains austenitic because martensite would only form at temperatures below room temperature.

H2 Steel

So with all of that background (hopefully you read the H1 article I linked) we can look at what Spyderco has reported about the change to H2. I will be quoting from the Spyderco Byte and then explaining what they are talking about.

Nitrogen

“The key ingredient of the Pacific Salt—and all Spyderco’s original Salt Series knives—is
H-1 steel. Developed by Myodo Corporation in Japan, H-1 steel is a radically different alloy that combines the extreme corrosion resistance and toughness of a conventional austenitic stainless steel with enhanced levels of hardness that allow it to perform well as a knife blade material. Unlike traditional carbon-based steels, H-1 uses nitrogen to help create its steel-like properties. Nitrogen, like carbon, is a small atom and has a similar ability to influence changes to iron’s ferrite structure. Unlike carbon, however, it does not increase the steel’s vulnerability to corrosion.”

Here they describe the nature of H1 as an austenitic stainless steel with higher than typical hardness. However, they then provide the reason for this as because H1 has a nitrogen addition. However, this is not entirely accurate. H1 has a nitrogen content listed as 0.1%. If this were indeed the nitrogen content it would make a small contribution to hardness. However, this is a rather low amount of nitrogen regardless. A steel with 0.1% nitrogen would not reach knife steel levels of hardness. Even the maximum amounts of carbon (0.15%) and nitrogen (0.1%) would make H1 essentially a “mild” steel not a tool steel. Instead it is the cold working that converts the structure to martensite which gives it the higher hardness and not nitrogen. In fact there are austenitic stainless steels which have elevated levels of nitrogen to partially replace the nickel content for stabilizing austenite. These are the “200 series” of austenitic stainless steels. The nitrogen is not to increase hardness and these steels in their typical form are austenitic and not high in hardness. But also it is typical for austenitic stainless steels to have maximum carbon and nitrogen contents allowable, particularly carbon because the element can lead to carbide formation and reduced corrosion resistance. But somewhere along the line the “max” part of the H1 spec for C, N, and Mn was lost which appears to have led to some confusion about the nature of the steel.

Phosphorus, Sulfur, and Age Hardening

“Besides nitrogen, H-1’s unique alloy composition also includes chromium, manganese, molybdenum, nickel, phosphorus, and sulfur. This finely tuned recipe of elements allows it to be precipitation hardened during production. This process, also called age hardening, uses controlled heat to increase the tensile and yield strength of the material. The real secret to H-1’s hardness, however, is the transformation of its retained austenite to hard martensite through both the rolling of the steel and the subsequent working of the material to create finished blades.”

Here the article does mention that it is the cold rolling which leads to the higher hardness of H1 so I am not implying that Spyderco completely misunderstands the nature of the steel. This is another case of composition “maximums” however, as phosphorus and sulfur are mentioned as being part of the “finely tuned recipe.” Phosphorus and sulfur are impurities in steel. There are certain steels where sulfur is added for machinability but H1 is not one of them. So the steel company is reporting a phosphorus and sulfur number to Spyderco but that is just to show the degree to which the company guarantees a maximum of those elements. They aren’t added for precipitation hardening/age hardening. Age hardening is done on the steel after its cold rolling has been completed. This is done by heating the steel to an intermediate temperature and allowing certain small precipitates to form which increases strength.

Cold Working and Grinding

“H-1 begins as a 7mm-thick sheet and is progressively rolled down to the required finished thickness—typically 3.0mm or 2.5mm, depending upon the Spyderco model. The extreme compressive forces of this rolling process transform some of the steel’s austenite to hard martensite, increasing the overall hardness of the material without sacrificing its toughness or corrosion resistance. The mechanical processes used to shape and grind a finished blade further enhance its hardness through the selective transformation of additional austenite to martensite. The result is a blade that holds an excellent edge, is incredibly strong and flexible, and offers extreme resistance to corrosion.”

It is interesting that Spyderco reports the degree of cold reduction that H1 sees, which is 57-64% depending on the final stock thickness. The article also mentions further martensite formation during grinding of the knives, something that I have expressed skepticism about previously. When Sandvik measured microhardness of an H1 knife they found slightly lower hardness near the edge (~55 Rc) where more grinding had occurred.

A drop in hardness was found by Sandvik near the edge of an H1 knife

Logistical Issues with H1

“It is no secret that the COVID-19 pandemic has had far-reaching impact on all aspects of the world’s industry and economy. As the third largest steel producer in the world, Japan has been particularly hard hit and has been forced to restructure its steel industry. One of the consequences of this change is that H-1 will no longer be produced. Where there is challenge, however, there is also opportunity. Through close cooperation with our manufacturing partners in Japan, Spyderco has helped drive the development of a new ultra-corrosion-resistant steel, appropriately named H-2.
In simple terms, H-2 takes the time-tested alloy composition and manufacturing processes of H-1 and refines them even further. The result is a steel that retains all the extraordinary qualities of the original, while being readily manufacturable.”

They are not completely specific here as to whether H2 was developed to be easier to manufacture by the original steel company (Myodo) or whether the new development was made along with a new manufacturer because Myodo was no longer able to produce more H1. The mention of “manufacturing partners” makes me think that a new manufacturer was necessary to replace Myodo.

Element by Element Analysis

The article then has a breakdown of the elemental changes between H1 and H2 which we can discuss:

• Carbon – reduced from 0.15 to 0.09 percent: Carbon in steel promotes the formation of carbides and increases the steel’s vulnerability to corrosion. Reducing the amount of carbon in H-2 further reduces this risk and therefore helps enhance its corrosion resistance.
• Chromium – reduced from 14.00-16.00 to 13.73 percent: Chromium in solution in a steel increases its corrosion resistance by allowing it to form a protective chromium oxide layer on the surface. The slight reduction of chromium was possible due to the reduction in carbon, but still ensures a high amount of chromium in solution.
• Copper – 0.17 percent added: The addition of copper in austenitic steel enhances its precipitation hardening properties and increases its corrosion resistance, especially in seawater environments and against exposure to sulfuric acid.
• Manganese – reduced from 2.00 to 0.32 percent:  Manganese improves the hot-working properties of steel and increases its strength, toughness and hardenability. Like nickel, it is also an austenite-forming element. The reduction in manganese is largely balanced by H-2’s increase in nickel and its substantial increase in molybdenum.
• Molybdenum – increased from 0.50-1.50 to 2.24 percent: Molybdenum increases a steel’s strength, hardness, hardenability, and toughness. It also improves its machinability and resistance to corrosion. When added to chromium-nickel austenitic steels, molybdenum improves resistance to pitting and crevice corrosion, particularly in chlorides and environments containing sulfur—like seawater.
• Nickel – increased from 6.00-8.00 to 8.25 percent: Nickel’s ability to form austenite gives austenitic steels great toughness and strength. It also greatly improves resistance to oxidation and corrosion.
• Nitrogen – reduced from 0.10 to 0.06 percent:  Like nickel, nitrogen is an austenite-forming element. It increases the austenite stability of stainless steel, improves its yield strength, and enhances its resistance to pitting corrosion. The adjustment to H-2’s nitrogen content is balanced by its increase in nickel content.
• Phosphorous – reduced from 0.04 to 0.027 percent: Phosphorus improves machinability and increases the strength of austenitic steels; however, it can also have a detrimental effect on corrosion resistance. Reducing H-2’s phosphorous content helps enhance its corrosion-resistant qualities.
• Silicon – reduced from 3.00-4.50 to 2.63 percent: Silicon is the most common alloying element in steel. It helps purify the iron ore during the smelting process by deoxidizing it and removing impurities. The fine-tuning of the volume of this element is based on the needs of the smelting process.
• Sulfur – reduced from 0.03 to 0.001 percent: Small amounts of sulfur improve a steel’s machinability; however, like phosphorous, it is detrimental to corrosion resistance. Reducing H-2’s sulfur content further contributes to its corrosion-resistant properties.

The comments above are what Spydero reported about the changes to the composition. Some of these descriptions of what the elements do in the steel are somewhat unrelated to the steel but I don’t want to go through all of them. For example, sulfur is listed as being added for higher machinability, but also that the content is 0.001% which is an extremely low level that the steel company would have to use specific manufacturing processes to achieve. They certainly didn’t add it on purpose. So there isn’t any reason to mention improvement in machinability as none is being added for that purpose. Sometimes I think that talking about effects of different elements can add confusion rather than simplifying things which is why I am somewhat critical of this. I have written about the danger of discussing steel elements in this way in this article about steel rating articles. But this is a common way of discussing elements of steel even by many metallurgists so I can’t criticize too much.

Carbon and Nitrogen

Another issue with the elements described above are that ranges are given for H1 while a very specific percentage is given for H2. There is no way that the steel manufacturer is guaranteeing 13.73% chromium and 2.63% silicon in every single heat of H2. They have some internal range of composition for the alloy. Sometimes the public composition range is wider than the “actual” range acceptable by the manufacturer. So the article says that carbon was reduced from 0.15 to 0.09%, but the Sandvik measurement of H1 was 0.10% carbon. So it is certainly possible that the carbon content hasn’t changed at all. They are using similar practices to H1 to achieve a relatively low carbon content to avoid carbide formation for good corrosion resistance. Same with nitrogen which is described as being reduced from 0.10% to 0.06%, but again this is similar to the Sandvik measurement of 0.05% with H1. Those are maximum tolerance values for H1 (max 0.15% carbon and max 0.1% nitrogen) and specific measured values for H2 (0.09% carbon and 0.06> nitrogen). So I don’t think the nitrogen content has actually been reduced. At the very least I think we can say that carbon and nitrogen is very similar between H1 and H2.

Chromium

The range of H1 is given as 14-16% with H2 as 13.73%. Probably it is most likely that the chromium content is 13.5 or 14% for H2 with some acceptable bracketing around that, such as 13-15% chromium. This may slightly reduce the corrosion resistance of H2 but it is probably made up for with the increased molybdenum. Interestingly when Sandvik measured H1 chromium they measured only 13.8%. I am not sure if this was a particularly chromium-lean heat of H1, a measurement error of some kind, or both.

Copper

An addition of 0.17% copper is described. However, that is a relatively low level of copper and it isn’t clear if that is actually an intentional addition. Stainless steels and tool steels are typically made from recycled steel that contains some amount of copper in it. It is relatively likely that the steel company simply reported the copper level that resulted rather than claiming an addition.

Manganese

H1 is listed as having 2% Mn, however a 2% Mn maximum is typical for many austenitic stainless steels. And Sandvik measured the Mn of H1 as around 1% which makes sense. However, that still means that H2 has lower Mn at around 0.3%, a more typical level used for taking care of sulfur. Manganese sulfide is less detrimental to steel than iron sulfide. Manganese can partially replace the more expensive element nickel for stabilizing austenite in these types of steels.

Molybdenum

The Mo content has been increased from ~1% in H1 to ~2.25% in H2. Molybdenum is particularly good at improving “pitting resistance” in saltwater. It strengthens the chromium oxide layer in stainless steels for improved corrosion resistance. This helps maintain corrosion resistance with the reduction in chromium and helps with saltwater environments.

Nickel

The nickel content was also increased from ~7% to ~8.25%. This is a more typical amount for austenitic stainless steels, though this is somewhat interesting since nickel was likely reduced in H1 relative to other steels to ensure it transformed to martensite during cold rolling.

Silicon

The silicon content was reduced from ~3.75% to ~2.5%. I noted in the H1 article that this is an oddly high value for silicon, and was perhaps added because silicon has a high strengthening effect. So it will be interesting to see if this reduction in silicon leads to a reduction in hardness of H2 when compared with H1. Silicon is also a strong “ferrite stabilizer” and H1 micrographs taken by Sandvik were found to have some ferrite in it. Ferrite is a soft phase in steel (room temperature iron is all ferrite) and not really desirable in H1. The reduction in Cr and Si and the increase in Ni may very well have been made to avoid ferrite.

 

There are light grey ferrite “streaks” in H1

M_D30 Temperature

All of these changes in composition also affect the stability of the austenite and therefore how readily it transforms to martensite during cold rolling. The M_D30 temperature is somewhat lower with H2 according to the equation, but somewhat higher according to JMatPro, a software package that predicts steel behavior. It appears that they balanced the composition to maintain similar transformation behavior between H2 and H1 during cold rolling.

How do the properties of H1 and H2 compare?

Without measuring the properties of the two steels we can only speculate, of course. But we can make some general statements.

Corrosion Resistance

I think the reduction of Cr with increase in Mo likely leads to similar corrosion resistance of the two but perhaps an improvement with H2. This depends on the exact environment; Mo is particularly good at helping with saltwater, for example.

Toughness

The toughness of H1 is high because of the relatively low hardness and complete lack of carbides. Carbides are hard, brittle particles that contribute to wear resistance but reduce toughness. So H2 would be expected to be similar because it lacks carbides just like H1. Most other knife steels have carbides for enhanced wear resistance but they are detrimental to those steels’ toughness.

Edge Wear

When it comes to edge retention from wear, the lack of carbides in H1 and H2 means that this is largely dependent on the hardness of the steel. H1 has low wear resistance because there are no carbides and H2 would remain the same.

Hardness and Edge Deformation

Resistance to edge deformation is controlled by strength, which strongly correlates with hardness. Higher hardness also contributes to wear resistance as mentioned above. So this will be the big question with H2 as to whether it achieves a similar hardness level to H1. I would imagine they wouldn’t use H2 if it hadn’t achieved similar hardness levels. But perhaps a reduction by 1-2 Rc would still be considered acceptable I do not know. As I mentioned in the section on the M_D30 temperature, I expect the martensite transformation during cold rolling to be similar between H1 and H2 but the reduction in Si may lead to a reduction in hardness. I am interested to see where the hardness ends up with H2 knives.

Conclusions and Summary

The development of H2 seems to have occurred by necessity because of availability rather than a need for improving its properties, though that doesn’t mean that the properties can’t be improved, of course. Overall I expect the properties between H1 and H2 to be similar. H2 will maintain the very high levels of corrosion resistance and toughness of H1, and also its relatively low hardness and wear resistance. The most interesting question to me is whether H2 achieves the same level of hardness as H1 or if it has dropped by a point or two on the Rc scale. We will see as knives are released.