Corrosion Resistance vs Hardness in Knife Steels

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Video

Here is the video version of the following information:

Misunderstandings about Hardness and Corrosion Resistance of MagnaCut

I have been seeing a disturbing number of comments around the internet saying something like the following: “Larrin says that 60 Rc MagnaCut has better corrosion resistance than 64 Rc MagnaCut, so there are different hardness levels for different applications.” I have never said such a thing. It is true that there are different levels of hardness for various applications, but hardness does not dictate corrosion resistance; therefore, it is not one of the parameters by which they would make this decision. I have a video where I have talked about the pros and cons of higher and lower hardness. How you heat treat a given steel can affect its corrosion resistance but a whole range of hardness values can be achieved with both good or bad heat treatments. I will give a variety of examples for this. A separate but related topic is about how hard different steels can be heat treated to when related to their corrosion resistance. For example, the very corrosion resistant Vanax and LC200N top out around 59-61 Rc. I will explain why they don’t get any harder and why this is partially due to those steels having very high corrosion resistance.

Chromium and Corrosion Resistance

When iron is in a corrosive environment it forms rust – an iron oxide. If you add chromium to iron, the corrosion resistance is increased the more chromium you add. The chromium forms with oxygen to make a passive film at the surface which prevents rust from forming:

The following chart shows an old classic study [1] of corrosion rate of steel in high humidity environments. You will see that the corrosion rate went down with increasing chromium until about 12% Cr. Sometimes the cutoff for a steel being “stainless” is given as 10.5, 11, or 12% chromium. There isn’t any real agreement as far as I can tell. But corrosion resistance will increase with even greater amounts of chromium; it isn’t an on/off after some arbitrary cutoff.

Data adapted from [1]

Heat Treating and Corrosion Resistance

In the annealed (soft) state stainless steel is not yet stainless. When a knifemaker or manufacturer receives steel it is in this annealed condition so it is easy to drill, grind, machine, etc. Most of the chromium is in the form of carbides in the steel. When the chromium is tied up with carbon as a carbide it cannot form the chromium oxide layer at the surface.

There are three major steps to heat treating: austenitizing, quenching, and tempering.

Austenitizing

During austenitizing the steel is heated up to a high temperature and then held, allowing the chromium carbides to be dissolved (or partially dissolved) to put more chromium in solution so that the chromium oxide passive layer can be formed. Below shows the increase in chromium vs austenitizing temperature for Elmax and Vanax:

So one of the key variables for corrosion resistance is the austenitizing temperature because heating it hotter means more chromium in solution. However, the amount of chromium in solution is also controlled in part by the composition. The chromium in solution for Elmax at a very high temperature of 2200°F (the datasheet recommends no higher than 2010°F) is still below Vanax when it is austenitized at 1800°F (well below what the datasheet recommends). Austenitizing at a higher temperature also leads to increased hardness, so in that way higher hardness can sometimes mean improved corrosion resistance.

Quenching

During quenching the steel is rapidly cooled to form the hard phase of steel, martensite. Martensite formation is controlled by temperature rather than time. There is a “martensite start” temperature and a “martensite finish” temperature. In some cases martensite finish can be below room temperature and in that case the steel does not fully transform, meaning some austenite remains in the steel. This is called “retained” austenite. If there is too much retained austenite the hardness is reduced in the steel, and also the edge performance is bad and the knife would be very difficult to sharpen. One of the major factors for martensite start (Ms) and martensite finish (Mf) temperatures is the carbon content, as shown in the chart below.

With higher austenitizing temperatures and more carbide being dissolved, this does not only put more chromium in solution but also more carbon, as seen in this chart of Vanax and Elmax. Or in the case of Vanax, I included the carbon and nitrogen, since nitrogen also contributes to hardness in a similar fashion to carbon.

All of the above happens as long as the quench is sufficiently fast to avoid other competing transformations that will occur with slow cooling. Carbides can precipitate during slow cooling which would also reduce corrosion resistance. This can happen with the relatively common gas quenches that are used in large vacuum furnaces by many knife manufacturers. I covered this in a previous article/video about custom vs production heat treating.

Cryo and Cold Processing

You will notice that the Vanax hardness dropped at 2025°F while the Elmax hardness increased all the way up to 2150°F. These heat treatments included a cryo step in liquid nitrogen after the quench. If cryo had not been used the Vanax would have seen a drop below 2025°F and the maximum hardness would not have been as high. The Elmax likely also would have seen a hardness drop in this tested range. As an example, the following chart shows AEB-L steel when quenched to room temperature vs placed in a freezer or in liquid nitrogen:

AEB-L steel vs austenitizing temperature

The AEB-L achieved its peak hardness around 1975-2000°F just like the Vanax and then dropped in hardness with further increases in austenitizing temperature. It reached a max hardness of around 64 Rc, though when no cold treatment was used it maxed out around 62 Rc. The reason for the drop in hardness above a certain temperature is because of retained austenite. In other words, the martensite finish temperature was below room temperature. Using liquid nitrogen after the quench means cooling the steel to a lower temperature getting closer to martensite finish. However, some retained austenite will stabilize and not be transformed even with the very low temperatures of liquid nitrogen, which is why the hardness still drops with an austenitizing temperature that is too high.

So the use of cryo can allow both higher hardness and corrosion resistance if the austenitizing temperature is increased. For a fixed austenitizing temperature, typically there is a 1-2 Rc increase but the corrosion resistance would not be affected.

Tempering

After quenching, steel is tempered to improve its toughness. The tradeoff is that hardness is also reduced through tempering. Below shows a tempering chart for MagnaCut with different austenitizing temperatures.

Different austenitizing-tempering combinations can be used to achieve the same level of hardness. For example, to achieve ~60 Rc, you could use 1950°F and 300°F, 2000°F and 400°F, or 2100°F and 500°F. Because the higher austenitizing temperature potentially means more chromium in solution we would expect the 2100°F and 500°F combination to achieve the best corrosion resistance for that hardness. A unique aspect of MagnaCut is that all of its chromium carbide is dissolved around 2050°F, as shown in the chart below. So if instead we were heat treating to ~63 Rc, 2050°F and 300°F, 2150°F and 400°F, and 2200°F and 450°F would all have approximately equal corrosion resistance. So austenitizing above 2050°F still leads to an increase in hardness for a fixed tempering temperature but the corrosion resistance would be unaffected.

There is an important aspect to tempering and corrosion when it comes to a different regime of tempering, however. Stainless steels and other high alloy steels see a bump in hardness by tempering above about 750°F, as shown in the following chart for Elmax:

You can see that the hardness sees a peak with tempering around 500°C (930°F). This increase in hardness comes from precipitation of small chromium, molybdenum, tungsten, and vanadium carbides. Because chromium is coming out of solution as a chromium carbide this “secondary hardening” also leads to a decrease in corrosion resistance. Below shows Vanax steel that I tested with a 1% saltwater spray test for 24 hours where one was tempered at 400°F and the other was tempered at 1000°F. The 400°F temper Vanax showed no corrosion while the 1000°F temper led to significant rusting.

Vanax tempered at 400°F (left) or 1000°F (right) before a 1% saltwater spray test

It is relatively common for knifemakers and knife manufacturers to temper at 950-1000°F. They do this for several reasons, though the main one is that the higher tempering temperature means the steel is less sensitive to overheating. This allows them to apply certain coatings to steel (that require higher temperatures to apply than a typical “low” temper of 400°F). It also means they don’t have to be as careful with grinding because the steel can be heated up much higher before approaching the tempering temperature (where the steel would soften).

The high tempering temperature can be used with many stainless steels to achieve similar levels of hardness as the low tempering temperature. In other words, you can have two knives at 60 Rc, 62 Rc, etc. with very different corrosion resistance depending on whether they tempered in the low range or the high range.

Hardness vs Corrosion Resistance for Different Steels

So after all of that we can return to our discussion of what makes one steel more corrosion resistant than another. Below shows Vanax and Elmax after a 1% saltwater spray test for 72 hours:

The Elmax rusted while the Vanax did not. Both were austenitized at 1975°F which is the recommended temperature for both steels according to their datasheets, and both were given a temper in the low range (400°F). As shown in the earlier chart, the difference in corrosion behavior is primarily due to higher chromium in solution for Vanax, which would roughly be 11.5% for Elmax and 14.5% for Vanax. As seen in the hardness chart, Vanax would actually be a bit harder than Elmax for this identical heat treatment. However, Elmax can be austenitized at higher temperature and achieve significantly higher hardness than Vanax which reaches its maximum around 61 Rc. The reason is because chromium also reduces martensite start and finish temperatures. Vanax has more chromium in solution for a given carbon/nitrogen in solution:

This means that for a given austenitizing temperature, Ms and Mf are lower in Vanax than in Elmax. So as seen below with an austenitizing temperature of 2025°F, the Ms was about 150°C. The Mf is sufficiently low that even with liquid nitrogen not all of the austenite is transforming to martensite, and thus hardness was lower with 2025 than 2000°F. Elmax, however, still has a predicted Ms above 150°C all the way to 2200°F, which is why its hardness increased all the way to 2150°F (the highest temperature I tried).

Thus an approximate relationship is created when comparing different steels to each other for their maximum hardness vs their corrosion resistance:

This trendline is usually correlated with chromium in solution. For example, Vanax and LC200N have >14% chromium in solution, which gives them a very high corrosion rating but also limits their potential hardness. On the other end of the spectrum is ZDP-189, which achieves very high hardness and is advertised as a stainless steel, but I discovered a few years ago that it is not, in fact, stainless. I measured only 8.6% chromium in solution with ZDP-189, which is quite low. D2, for example, I measured at 7.7% with a relatively conventional heat treatment, which is famously called a “semi-stainless” steel.

There are other complicating parts to this trendline, however. It does not always perfectly correlate with chromium in solution. Molybdenum, for example, is an element known to improve pitting resistance and to strengthen the chromium oxide passive film. In tests I did with 1% saltwater there was an improvement in corrosion resistance for a given chromium in solution if the Mo was increased:

CPM-154 with its high molybdenum content (4%) had acceptable corrosion resistance even with only 9.5% chromium in solution. (Note: the “corrosion ratings” in this chart are a rating of how much the samples rusted and not the same as the “corrosion rating” in the hardness trend plot which are from my knife steel ratings).

You may have also noticed that S90V, S125V, M390, and S110V are all above the hardness-corrosion resistance trendline. In other words, they have unexpectedly high maximum hardness for their corrosion rating. These steels all have a very high volume of carbide (>20%). Carbides are very hard particles and when the volume of them is reaching these high levels it can affect the bulk hardness that is measured. You can see that for the following two charts comparing non-stainless tool steels with different amounts of carbide (Vanadis 4 Extra, 10V, and 15V):

So while M390 and 14C28N both have similar Cr in solution (depending on austenitizing temperature), the M390 reaches higher hardness because it has so much more carbide:

M390 has a lot of carbide

14C28N not much carbide

The other steel above the trendline for hardness-corrosion resistance is MagnaCut. It does not have particularly high carbide volume, about the same as Vanadis 4 Extra:

MagnaCut also does not have particularly high chromium in solution, around 11%. Instead, MagnaCut has superior corrosion resistance to those other steels because all of its chromium carbides are dissolved in heat treatment. With typical stainless steels they have many chromium carbides, and the chromium in solution is lower around each carbide:

Thus the chromium carbides act as corrosion initiation sites. This is somewhat similar to the phenomena called “sensitization” in low carbon stainless steels that are cooled too slowly.

Image from [2]

Instead, all of the carbides in MagnaCut are vanadium or niobium carbides (after heat treating). Therefore those corrosion initiation sites are missing and so it has superior corrosion resistance for a given level of chromium in solution.

Comparing Steels vs Comparing Heat Treatments

Perhaps it is the above discussion that confuses some people, and they mix the low potential hardness of Vanax and LC200N with heat treating of other steels and assume that lower hardness means better corrosion resistance. However, as I have discussed, hardness and corrosion resistance don’t really correlate for an individual steel. If anything, higher hardness sometimes correlates with higher corrosion resistance. If you use higher austenitizing temperatures in combination with a fast quench and cryo, and temper at a low temperature, you get maximum corrosion resistance. That is also a recipe for high hardness (as long as the austenitizing temperature isn’t too high). However, tempering can be increased without detriment to corrosion resistance to heat treat to lower hardness levels (as long as the >750°F range is avoided).

Summary and Conclusions

Austenitizing temperature, quench rate, and tempering can all affect corrosion resistance. Austenitizing higher leads to both higher hardness and better corrosion resistance. Beyond a certain temperature cryo is necessary to get higher hardness, and even then there is a temperature beyond which hardness decreases. Quenching too slowly, as happens often in industry, can lead to a reduction in corrosion resistance. Tempering in the high temperature range (>750°F), also relatively common, significantly reduces corrosion resistance. Combinations of heat treating variables can be used to achieve a range of hardness values while maintaining high corrosion resistance. In other words, hardness does not correlate with corrosion resistance for a given steel. However, when comparing different steels to each other, there is a trend where more corrosion resistant steels typically have a lower maximum hardness. Molybdenum alloying, avoiding chromium carbides (such as in MagnaCut), and having a high volume of carbide all lead to higher potential hardness for a given level of corrosion resistance when comparing different steels to each other.

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[1] Binder, W. O., and C. M. Brown. “Atmospheric Corrosion Tests on High-Chromium Steels.” In Proceedings of ASTM, vol. 46, pp. 593-606. 1946.

[2] https://newzelindustries.com/what-is-sensitization-in-austenitic-stainless-steel/