Coatings vs Foil – Heat Treating Steel

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YouTube Video

I have a video version of the following information:

Why Protect the Surface?

There are two things we are trying to prevent by protecting the surface during heat treating: scale and decarburization. I frequently see knifemakers confusing these two things. Scale is not decarburization, and removing scale does not mean that the layer of decarburization is removed. Scale is the black iron oxide that forms on steel at high temperature in the presence of oxygen. During forging of steel it is common to see this scale come off in flakes.

A bar of 4340 steel with black scale on it

However, the oxygen doesn’t only interact with the iron, but also carbon. The carbon leaves the steel at the surface after forming carbon dioxide or carbon monoxide. This leads to a layer of steel with very low carbon, called a “decarburization layer.”

Problems from Decarburization

This decarb layer doesn’t harden because of the lack of carbon. However, unlike scale, a decarburization layer is still steel so it can be difficult to see without etching in acid first. So sometimes knifemakers are measuring low hardness or easily scratching with a file because they are unaware of this decarburization layer. That soft layer will also obviously affect performance, if an edge is sharpened within the “decarb” layer it will be soft and the knife won’t hold an edge. It can also affect aesthetics because the soft steel will polish differently. Another common issue can be warping and distortion from the decarb layer. This layer being free from carbon means that it will transform to other phases at a different temperature than the rest of the steel. The uneven transformation leads to different size changes within the steel leading to warping.

Protective Atmosphere and Salt Pots

If no oxygen is present, scale will not form and decarburization will not occur. So if the steel is heat treated in a vacuum furnace or within a protective atmosphere like argon then scale and decarb are not issues. With salt pots the steel is within molten salt without an atmosphere, so again there is no scale or decarb. However, these solutions are not always practical so other ways of protecting steel are often used by individual knifemakers with simple heat treating equipment.

Foil

A common method for preventing scale and decarb is to use a heat treating foil. The steel is wrapped in the foil and then each exposed side is folded over at least twice to prevent air from entering the packet. The foil is thin and on the outside so it heats up first and oxidizes which further eliminates oxygen left within the packet.

There are two major types of heat treating foil: 321 and 309 stainless steels. The 321 foil is rated up to 2000°F and the 309 is rated up to 2240°F. The 309 is more expensive than 321. The 321 stainless also has a small titanium addition which is also claimed to help with reacting with oxygen inside the packet [1]. Foil is very sharp so gloves should be worn when handling it. Some people recommend adding paper or oil to burn up inside the packet but this is not necessary. Some people recommend talcum powder to prevent sticking though I haven’t tried it. Reusing foil is not recommended because the foil has already oxidized and also becomes brittle.

The main limitation of foil is for oil or water hardening steels because removing the foil prior to quenching is very difficult within a reasonable time. With air hardening steels the foil works well with a “plate quench” because when the plates are held tight the heat will steel conduct through the foil.

Coatings

Various coatings are also available that are designed to protect steel from scale and decarb during heat treating. These are especially useful for oil and water hardening steels since no foil needs to be removed. I used several different coatings for my recent experiments:

ATP-641 [2] – a water based ceramic coating that is rated up to 2300°F. It can be sprayed, dipped, or brushed.

NoScale2000 [3] – another water based coating that is rated up to 2000°F. The coating is made by Daniel O’Connor for knives. It can be sprayed or brushed. O’Connor also makes a version made for developing a hamon called Hamon1800.

Turco Pretreat [4] – This product is also named Bonderite L-FM Pretreat Aero. It is a paint, so the texture is quite different than the two above. The primary recommendation is to spray this coating though I used it by dipping. The datasheet does not list a maximum temperature.

Condursal Z1100 [5] – Another paint coating like Turco. Rated up to 1100°C (2012°F). Can by applied by brushing, dipping, or spraying.

Experiments

I did three experiments with the different coatings. Each used a 1/8 x 3/4 x 1-1/2 inch (19 x 38 mm) coupon. They were finished to 120 grit and cleaned with soap and water prior to dipping in one of the coatings, followed by letting them dry for 24 hours. The 1095 coupons were only finished to 60 grit and the Condursal Z1100 coupons were only finished to 60 grit. I also tried brushing the NoScale2000 which I will explain in that section. A “bare” sample was also tested with each without a coating.

1. 1095 steel heated to 1475°F for 10 minutes, quenched in Parks 50 oil. This is to test the effect of each coating on the quench since 1095 is a water hardening steel and needs a fast quench.

2. 52100 steel heated to 1550°F for thirty minutes, quenched in Parks 50 oil. This is a relatively standard datasheet heat treatment for 52100. It is on the high end for temperature and time for oil/water hardening steels so it serves as a good test of how much decarb happens for these types of steels.

3. AEB-L steel heated to 1975°F for 30 minutes, then quenched in Parks 50, then placed in liquid nitrogen for an hour. For the AEB-L I also added a sample that was  wrapped in foil prior to a plate quench and then liquid nitrogen. This was a test to see how the coatings did near their temperature limit. Also to see how much decarb happens at this relatively high temperature.

For each coupon I took pictures after cleaning them to see the amount of scale from each. I did microscopy of the cross-section for ATP-641, NoScale2000, and Turco but I didn’t get the Condursal until later so no microscopy for that coating. I also measured hardness for all of them.

Uncoated Coupons

The 1095 coupon formed only a small amount of scale because it was heated to a relatively low temperature of 1475°F (800°C) for a short time of 10 minutes.

1095 coupon heat treated without a coating

The 52100 coupon heat treated at 1550°F for 30 minutes had significantly more scale and it was very dark in color. Some came off when cleaning the oil off the coupon.

52100 coupon heat treated without a coating

The AEB-L coupon heat treated at 1975°F for 30 minutes was not as dark as the 52100 coupon and the scale was a bit less even.

AEB-L coupon heat treated without a coating

Looking at the cross section, the 52100 has a thin decarb layer and thin layer of scale. The AEB-L, however, had a very thick decarb layer measuring over 0.2 mm (~0.01 inches).

52100 uncoated coupon cross section

AEB-L uncoated coupon cross section

1095 Quenching Experiment

I decided to do the quenching experiment based on an earlier experience. A knifemaker contacted me about difficulty with heat treating 80CrV2; it wasn’t hardening. We talked through the process and he told me he was coating each blade with ATP-641. I recommended he try it without the coating and he didn’t have any problems after that.

All of the 1095 specimens fully hardened to 66-67 Rc and this was true at the surface as well as after grinding 1 mm into the specimen. The one exception was the NoScale2000, which was about 36 Rc. I repeated the experiment to make sure it wasn’t a fluke and got 33 Rc. In both cases the coating did not come off during quenching and I had to clean it off to be able to test hardness. I saw that O’Connell recommended brushing on the NoScale2000 rather than dipping so I tried it again by brushing it on. In this case the steel fully hardened.

1095 NoScale2000 coupon after quenching in Parks 50 oil. The coating remained on the steel.

I did not find an issue with hardening the 1095 with ATP-641 despite the prior issue the knifemaker experienced with 80CrV2. However, I do think that there is significant “danger” to quenching slowly with coatings applied. Ensuring the coating is not too thick helps with quenching as expected.

NoScale2000

Another puzzling thing was that the NoScale2000 appeared to lead to even more scale than when heat treating it without any coating. The repeat experiment of 1095 also led to a lot of scale. When I brushed on the NoScale2000 there was still scale that formed it was just more “spotty.”

1095 “bare” coupon (heat treated without a coating)

1095 NoScale2000 dipped coupon #1

1095 NoScale2000 dipped coupon #2

1095 NoScale2000 brushed coupon

The 52100 coupon coated with NoScale200 also had a somewhat spotty appearance similar to the 1095 brushed coupon. The AEB-L NoScale2000 coupon looked at least as bad as the uncoated coupon, perhaps worse.

52100 NoScale2000 coupon

AEB-L NoScale2000 coupon

Looking at the cross-sections, the 52100 coupon looks very similar to the uncoated coupon. The AEB-L coupon has a decarb layer of approximately the same thickness as the uncoated coupon.

52100 NoScale2000 cross-section

AEB-L NoScale2000 cross-section

I let Daniel O’Connor know about my subpar results and he said that, “[Y]our results are not typical of my user base.” He told me about good experiences many knifemakers have had using it. If you have had better experiences with the NoScale2000 let us know in the comments.

ATP-641 Coupons

Surprisingly the 1095 coupon coated with ATP-641 had some spotty scale that formed, not looking too different than the uncoated coupon. The 52100 coupon, however, didn’t have any obvious scale just some discoloration. But the AEB-L coupon formed scale and didn’t look very different than the corresponding uncoated coupon.

1095 ATP-641 coupon

52100 ATP-641 coupon

AEB-L ATP-641 coupon

Looking at the cross-sections with metallography there is no obvious decarburization in either the 52100 or the AEB-L. This is somewhat surprising given the rough scale that formed on the AEB-L. Maybe the ATP-641 affected the surface in some way on its own while still preventing decarb.

52100 ATP-641 cross-section

AEB-L ATP-641 cross-section

Turco Coupons

The 1095 Turco coupon had some scale that formed, lining up with the grinding marks. Perhaps this was because of the coarser 60 grit finish on the 1095 coupon vs the 120 grit on the 52100 and AEB-L coupons. The 52100 coupon had some discoloration and it is hard to tell if it was scale or not but otherwise looked good. The AEB-L perhaps had some light scale that formed but otherwise looked very good.

1095 Turco coupon

52100 Turco coupon

AEB-L Turco coupon

There was no apparent decarburization visible in the metallography analysis of the 52100 coupon. The AEB-L coupon also looked good apart from one corner where presumably the Turco had rubbed off prior to heat treatment.

52100 Turco cross-section

AEB-L Turco cross-section

Condursal Z1100 Coupons

The Condursal seemed to be very similar to the Turco apart from color (green vs orange paint). The resulting coupons also looked pretty similar to the Turco coupons. For some reason there was more scale on the 52100 coupon. And the rougher 60 grit finish I did with these followup coupons also seemed to affect the amount of scale that formed somewhat. I did not do metallography with these coupons.

1095 Condursal Z1100 coupon

52100 Condursal Z1100 coupon

AEB-L Condursal Z1100 coupon

Foil

I only used foil with AEB-L. As expected the steel came out completely clean apart from some rainbow coloration from the small amount of oxygen left in the packet.

AEB-L foil coupon

Surprisingly there is what looks like a thin decarb layer visible in the cross-section. Perhaps the little bit of oxygen that interacts with the steel is enough for a small amount of decarburization.

AEB-L foil coupon cross-section

User Error?

This is my first time using many of these coatings so there can certainly be criticism of my technique. Dipping into the coating did not seem like the very best way to apply most of them. The one I thought had the best texture for dipping was the NoScale2000 which was also the one that didn’t come off when quenching. The Turco and Condursal recommend very thin coatings in the datasheets but I was concerned some of it might have been running off while drying. The coating thickness did not appear to be even. Perhaps spraying would have been better for some/all of these coatings. I attempted brushing the coatings on but didn’t like it; even letting them dry in between coats seemed to lead to me brushing off as much as I was brushing on. The surface finish also appears to matter with these coatings, a finer finish is better. And they need to be clean of course.

Scale-X

One coating I found about too late is Thermodur Scale-X. It is a paint-based coating that comes in a spray can so you don’t need a dedicated sprayer. Based on my experience with the coatings in this article I think spraying might be better so this sounds very convenient. And the two paint-based coatings I tried worked well. So I will be getting some to test it.

Summary and Conclusions

Coatings can be useful in certain instances for oil and water hardening steels. I had somewhat inconsistent results with them. I also found that the amount of decarb is relatively small at temperatures typical of water and oil hardening steels (less than 1600°F/870°C) so it depends on how much of a hassle you think applying the coatings is. If you plan to remove material through grinding after the coatings may not be necessary for low alloy steels. Thoroughly cleaning the steel and having a fine finish is helpful for the coatings to work at their potential. Learning to effectively apply them likely requires some trial and error. I wasn’t totally happy with dipping or brushing them on, may spraying is better. For steels that require high temperature, in this case AEB-L, I was able to prevent decarb with the coatings except for NoScale2000. Decarburization is quite significant without any type of protection and should definitely be avoided. The Turco and Condursal coatings led to a more scale-free surface than the ATP-641 and NoScale2000. I greatly prefer foil for heat treating air hardening steels.

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[1] https://www.toolwrap.com/pages/all-about-tool-wrap?srsltid=AfmBOoqG9o8GgfgNq8u2WT2Eh9IeyYGLYITqxhqOkq-qoc1ENjuQ2joR

[2] https://atp-europe.com/wp-content/uploads/2023/08/ATP-641.pdf

[3] https://nuclayer.twinoaksforge.com/product/noscale-2000-16oz/

[4] https://webaps.ellsworth.com/edl/Actions/GetLibraryFile.aspx?document=12319&language=en

[5] https://www.theduffycompany.com/condursal-z1100/