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Video Version

The general information in this article also exists as a YouTube video for those that prefer to consume their information that way:

Importance of Prior Microstructure

In a previous article I wrote about how to normalize and anneal steel after forging but before the final steps of austenitizing, quenching, and tempering. One of the things that I pointed out in that article was that using a normalized, pearlite microstructure to heat treat from would lead to very rapid response to heat treatment. An annealed structure such as comes from the manufacturer takes more time, and higher temperature, to properly austenitize the steel.  Below shows the resulting as-quenched hardness for 1084 and 52100 comparing normalized and annealed steel. These were all quenched from approximately where they became nonmagnetic (1385°F for 1084 and 1445°F for 52100), relatively low in temperature compared with the typically recommended temperature for heat treatment.

1084 hardness:

52100 hardness:

The high hardness after quenching 52100 from such a low temperature is impressive because of the relatively high chromium (1.5%) of the steel. Higher chromium delays transformations in the steel and typically 52100 is a steel that is thought to require higher temperatures and soak times. However, if starting from a pearlite structure it can be quenched from nonmagnetic and still be heat treated “properly.” This large difference in austenitizing response is also shown in the literature, such as in the study shown below comparing pearlite and spheroidized structures in 52100. You can see that the hardness is at its maximum in less than a minute with the pearlite structure while it takes over 2 hours to reach the same hardness with a spheroidized structure.

52100 steel austenitized at 840°C/1550°F, quenched in oil, and tempered at 175°C/350°F [1]

This occurs because the distance that diffusion occurs is much shorter with a pearlite structure then with a spheroidize annealed structure. See the simplified schematic below where the pearlite (lines indicating the cementite structure) vs spheroidized carbides (black circles) where you can see that the distance between those features is much greater with spheroidized cementite/carbides.

Pearlite (normalized)

Spheroidized carbide (annealed)

Heat Treating in a Forge

This is useful because typically when bladesmiths heat treat knives in a forge they are trying to get to a temperature at some point higher than nonmagnetic and this is a major source of variability. I figured if we use a normalized structure instead I could heat to nonmagnetic and quench without trying to reach a higher temperature that can’t easily be measured with a forge running at high temperature. There are various methods for heat treating in a forge such as using a muffle, or pipe, within the forge to help maintain a more even temperature distribution. However, I wanted to use the simplest method used by many beginning bladesmiths with a simple forge to heat the steel and checking for temperature with nothing but a magnet. If I could successfully heat treat in this way then I would have more confidence in recommending to people methods of how to heat treat with a forge. In the past I have always recommended against heat treating in a forge because a furnace is so much more controllable and consistent. However, I wanted to prove myself wrong in a sense by seeing if I could heat treat in a forge without any past experience with doing it.

I chose a range of different low alloy steels typically used by forging bladesmiths. I wanted to do different steels to make sure that the process worked with different carbon and alloy content. With each I overheated the steel at 2100°F/1150°C for an hour to simulate forging, normalized at 1550-1700°F for 10 minutes and air cooled. The normalizing temperature varied by steel. This was the condition that I used when heating to nonmagnetic in the forge and quenching in Parks 50. These were all 1/8″ pieces of steel.

Because of the different compositions of these steels the hardness of the normalized steel varied though I was able to cut all of them with a bandsaw and drill a small hole. A very high hardenability steel like L6 or thinner steel like 1/16″ may air harden enough where bandsaw cutting or drilling may be difficult.

Hardness of normalized steel

Magnetism and Heating Rate

The steel becomes nonmagnetic when it has transformed to austenite, a nonmagnetic phase of steel. There are certain steels, notably austenitic stainless steels, which are designed to be austenite at room temperature and are therefore nonmagnetic at room temperature. These are not used for knives but I am giving an example for better understanding of magnetic/nonmagnetic behavior. Fortunately for us with normalized pearlitic steel, it becomes nonmagnetic when the pearlite has been replaced with austenite and is ready for quenching. With annealed steel it will also become nonmagnetic when it has transformed to austenite, but the higher required temperature is to get more carbide to dissolve and to put more carbon and alloy in solution by dissolving those carbides. With the pearlite starting condition there is enough carbon in solution immediately after the pearlite has been replaced with austenite (see the previous chart of 52100 hardness vs time for pearlite and spheroidized carbide).

Different steels will transform to austenite at somewhat different temperatures but we don’t have to worry about that because we are checking with a magnet; when the steel has transformed we see that with the magnet. Another potential concern is that the transformation temperature can change based on how rapidly the steel is heated. Faster heating means the transformation is delayed to a somewhat higher temperature. Steel can heat relatively rapidly in a forge so the transformation may be shifted up a few degrees. However, we are checking with a magnet so once the steel has transformed we quench from there regardless of the temperature where that has actually happened. (Note: The dissolution of all of the carbide/cementite is not shown on the diagram.)

52100 transformation temperatures with continuous heating at different rates. Adapted from [2].

However, while with continuous heating the transformation is shifted up, if the steel is held at that temperature for a period of time the transformation will occur at a lower temperature. Therefore if the relatively fast heating of a forge leads to an increased temperature of transformation, attempting to hold the steel at that temperature may lead to the steel being overheated.

52100 transformation temperatures with holding at a constant temperature. Adapted from [2].

Dangers of Overheating

Grain Growth

Bladesmiths are generally familiar with the problem of grain growth with overheating. Larger grains in steel usually means reduced toughness. If the grain size is big enough the steel is brittle and will chip or break easily. This can be evaluated based on fracture appearance if the steel is broken in a brittle condition, ie as-quenched steel that isn’t tempered. Ductile steel that is fractured does not reflect the grain size of the steel, which is why normalized steel cannot be used to evaluate fracture grain. Below shows 1084 which was heated for 10 minutes, quenched, and broken.

1475°F (800°C)

1700°F (925°C)

2000°F (1095°C)

Carbon in Solution

However, grain growth is not the only danger of overheating. When too much carbide is dissolved putting excess carbon in solution toughness is reduced. Above about 0.6% carbon in solution the type of martensite starts to transition from “lath” to “plate” martensite. Plate martensite is brittle and prone to microcracks. A 1% carbon steel does not necessarily have 1% carbon in solution. The steel is generally austenitized at such a temperature where some carbon remains in carbides.

The arrow points to plate martensite within a matrix of primarily lath martensite [3]

Image from [4]

I also compared the toughness of low alloy knife steels in this article where I also used calculated carbon in solution to compare them. The trend was pretty convincing, showing how important this factor is for knife steel toughness, particularly in low alloy knife steel where carbon in solution is not as easily controlled as in high alloy steels.

Results of Forge Heat Treatments

So I heat treated these steels in an Atlas Forge with the propane set to very low pressure. This is still at relatively high temperature, approximately 2000°F/1095°C. I heated them up to a consistent temperature the best I could while occasionally checking with a magnet. When the steel was nonmagnetic I quenched in Parks 50. The samples were then tempered twice for 2 hours at 400°F/205°C. I tested the samples for hardness and also toughness using my standard unnotched subsize charpy specimens.

5160 and 8670

These steels are the lowest in carbon that I tested with these experiments. These steels are also among the highest toughness that I have tested, see the chart below for low alloy knife steels. I have separate articles about the heat treatment of these two steels:

How to Heat Treat 5160

How to Heat Treat 8670

In my forge heat treatments of 5160 and 8670 I measured similar hardness and toughness to the furnace heat treated specimens:

This indicates that the steel was both sufficiently heated (similar hardness to the furnace heat treated specimens) but not overheated (similar toughness). In furnace heat treatments of 5160 we found that when using temperatures of 1550°F or above the toughness was significantly reduced from grain growth. This gives a relatively wide window when forge heat treating between nonmagnetic and excessive grain growth when using normalized steel, as it becomes nonmagnetic at least 100°F below that point.

O1 and 1095

With the relatively low carbon content of 5160 and 8670, excessive carbon in solution is not as much of a potential problem with overheating. Instead we are primarily concerned about grain growth. However, with high carbon steel the effect of carbon in solution is much more significant. This is why the toughness of O1 and 1095 is so much lower than 5160/8670 in the low alloy steel toughness chart. Sometimes using reduced austenitizing temperatures with high carbon steels can result in improved toughness because of the reduction in carbon in solution (less carbide dissolved). See below where austenitizing O1 at 1475 and especially 1425°F resulted in superior toughness when compared with 1550°F even after compensating for hardness.

This was also an important topic when comparing the toughness of low alloy steels with a heat treatment for bainite, as bainite does not have the carbon in solution problem that plate martensite has. This resulted in higher toughness for O1 and 1095 with austempering treatments for bainite because of the elimination of plate martensite.

1475°F is the typical temperature recommended for furnace heat treating in these steels, so this is the comparison I made with the forge heat treated specimens. In this case the hardness was somewhat higher for my forge heat treated coupons but the hardness-toughness balance was similar to the 1475°F furnace heat treated coupons. So this is a good result I think.

52100

With 52100 the furnace heat treated coupons have significantly better toughness than furnace heat treated O1 and 1095. The chromium addition reduces the carbon in solution for a given austenitizing temperature and makes controlling the carbon in solution easier. This gives 52100 excellent toughness and wear resistance when compared with other high carbon low alloy steels. However, this is for furnace heat treating from a spheroidized annealed condition. When heat treating from pearlite you do not have the same control over the carbon in solution and the resulting as-quenched hardness, as shown below.

Chart adapted from [1]

However, 52100 would be the most difficult to heat treat in a forge from the spheroidized annealed condition because of how sluggish the dissolution of carbide is. And even if that was effectively done it would be difficult to target specific temperatures as with a furnace. So like with 1095 and O1 you are limited to relatively high hardness. You could temper higher than 400°F but then you would run into the issue of tempered martensite embrittlement where hardness is reduced but toughness also goes down. This was seen when heat treating 52100 with a 450°F tempering temperature, see this article on heat treating 52100 for more.

The toughness of the 52100 forge heat treated specimens was still good, with higher toughness than O1 and 1095. However, the toughness was somewhat worse than other high hardness furnace heat treated 52100. This may be because of greater carbon in solution from the pearlite starting microstructure. The other high hardness coupons from furnace heat treating used lower tempering temperatures (300°F/150°C) in combination with lower carbon in solution. However, as I said the properties are still reasonable and would do well for thin, high performance knives.

80CrV2 and 1084

I saved these two steels for last because they are in between the medium carbon (5160/8670) and high carbon (1095/O1/52100) categories described above. They therefore have behavior that is basically in between those two categories. 1084 is known as being very easy to heat treat in a forge because of its low alloy content and good hardenability (doesn’t have to be quenched as fast as 1095). Using a pearlite starting structure in all of the steels I have heat treated does somewhat remove the advantage 1084 has from the low alloy content, however, as all of these steels hardened from nonmagnetic just fine. Ironically, 1084 gave me the most trouble in heat treating because two of the coupons had lower toughness than the third. This was because I was having trouble heating the piece evenly and had a hot spot on one side of the steel, as shown in the image below, which is from video footage of me heat treating the specimens:

So to get the entire piece to reach nonmagnetic I ended up overheating those two hotter specimens resulting in grain growth and reduced toughness. This was also visible in the fracture grain of the specimens, though it was not as bad as you might think based on the reduction in toughness (remember these are only 10 mm wide):

This overheated treatment of 1084 would still result in an acceptable knife; there are many steel-heat treatment combinations used in common knives with lower toughness. However, we would of course prefer an optimized heat treatment rather a subpar one.

For 80CrV2 the toughness was somewhat lower than the furnace heat treated specimen. However, the hardness was also higher. So to have some idea of how it compared I overlaid the toughness of the furnace heat treated 1084 coupons since I don’t have a trend for 80CrV2. When viewed against the furnace heat treated 1084 the hardness-toughness-balance of the forge heat treated 80CrV2 looks pretty good.

Overall Trends

Because I heat treated from a pearlite microstructure rather than spheroidized annealed, and all of them were tempered at 400°F, the resulting hardness and toughness was primarily controlled by the carbon content of the steel, as shown below:

And then if we plot the hardness vs toughness balance you can better see where the different steels ended up in their overall properties:

So 5160 and 8670 would be best for knives requiring high toughness like heavy choppers. The medium carbon content gives them good hardness (58-60 Rc with a 400°F temper) without having significant issues with high carbon in solution and plate martensite.

1095, O1, and 52100 would be best for fine cutting knives like kitchen knives. Though 52100 did have better hardness and toughness than 1095 and O1 so it would be my choice. And 52100 has better edge retention than either of those steels.

1084 and 80CrV2 would be best for general purpose knives. Their medium-high carbon content gives them somewhat more hardness/strength and edge retention while still having very good toughness.

Should You Heat Treat in a Forge?

With furnace heat treating, however, you would have somewhat more control over the final properties of the steel and the steels would be more flexible in heat treatment for achieving different properties. 52100, for example, can have toughness as high as 1084/80CrV2 at similar hardness with the added benefit of more carbide for higher wear resistance. So with this type of forge heat treating the steel selection is very important for the type of knife that will be used, without being able to rely as much on changes to heat treatment. However, I did get decent properties with all of these steels with forge heat treatments, and these were the very first coupons I had ever heat treated in a forge. So can you get good results heat treating in a forge? Yes. I attribute this to the use of the normalized, pearlitic microstructure that meant I could quench from nonmagnetic without worrying about how much hotter to go.

The biggest dangers for forge heat treating are overheating for grain growth and excessive carbon in solution, as well as uneven heating leading to spots that are higher/lower in hardness or toughness. The problem of uneven heating can only be fixed with practice which is why I would still recommend that beginner knife makers send out knives to a professional heat treater if a furnace is not yet in the budget. Furnace heat treating is very easy and results in the same properties each time. You can follow recommendations from this website, datasheets, or my book Knife Engineering. Just follow the recommendations and the performance will be good even if you are a novice. The chances of screwing up a forge heat treatment are much higher. One example I gave in my last article was where a knifemaker sent me a whole range of different coupons heat treated in a forge, and he even used the muffle method with a thermocouple to try to have more even heating and more consistent temperature. The samples were obviously overheated according to the fracture appearance, and the toughness was very bad. So there are fewer guarantees with forge heat treatments.

Properly heat treated 52100 on the right and improperly heat treated 52100 on the left

High Alloy Steels and Stainless Steels

All of these examples were with low alloy steels generally used by forging bladesmiths. The method of heat treating normalized steel and checking with a magnet would not work for high alloy tool steels, high speed steels, stainless steels, etc. Basically any steel with 3% chromium or more. I have not attempted to heat treat any of those in a forge and the methods required to do so would be different.

Summary and Conclusions

My goal here was to prove to myself that good forge heat treatments can be done by a novice and that goal was achieved. There are still potential pitfalls to forge heat treating, with overheating and uneven heating being the most common. If you follow my recommendation to use a normalized, pearlitic microstructure, heat to nonmagnet and no hotter. It can be easy to heat significantly above nonmagnetic which will result in reduced toughness. I still recommend furnace heat treating or sending out to a professional heat treater for most people. That way you can have confidence in consistent, repeatable results. And furnace heat treating is just easier.

[1] Stickels, C. A. “Carbide refining heat treatments for 52100 bearing steel.” Metallurgical Transactions 5, no. 4 (1974): 865-874.

[2] Orlich, Jürgen, Adolf Rose, and Paul Wiest. Atlas zur Wärmebehandlung der Stähle;: Band 3: Zeit, Temperatur, Austentisierung, Schaubilder. Matplus GmbH, 1973.

[3] Samuels, Leonard Ernest. Light microscopy of carbon steels. Asm International, 1999.

[4] Krauss, George. Steels: processing, structure, and performance. Asm International, 2015.

The post How to Heat Treat Knife Steel in a Forge appeared first on Knife Steel Nerds.

Thanks to Jake Smith, Erik Mittag-Leffler, Ashley Wagner, David W. Debora Richards, gspam1, James Straub, Tim Ottawa, Flynn Sharp Knives, David Millington, Joseph Baier, Alex Roy, and Noah for becoming Knife Steel Nerds Patreon supporters! These experiments are very expensive and time consuming so your support is always appreciated.

Podcast Appearances

I appeared on two different podcasts since my last article. On Knife Junkie Podcast we discussed MagnaCut and a range of other topics.

On Mark of the Maker we discussed “Metallurgy Mythbusters” where we went over a bunch of different heat treating myths.

Video Version

I have a YouTube version of the following information in a video below. The content is somewhat different between the video and the article so you may like both.

Thermal Cycling

“Thermal cycling” is very non-specific, it just means heating and cooling steel. It probably isn’t specific enough for our purposes, but it is a common phrase used by knifemakers to refer to the steps they perform after forging but before the final heat treatment. The final heat treatment being the austenitize and quench for full hardness. Some knifemakers are also performing very non-specific cycling, however, with no clear purpose to what they are doing. Instead metallurgists perform specific cycles called normalizing and annealing. This sets up the steel to be soft for machining, drilling, cutting, etc. And for good heat treatment response for hardness, wear resistance, and toughness. Typically no “grain refinement” cycles are added though those are also very popular with bladesmiths so I have shown where they should go in the schematic diagrams below:

Grain Size after Forging

One change that occurs during forging is grain growth. Forging is typically done from very high temperature and with low alloy steels grain growth is very rapid in that temperature range. The amount (and speed) of deformation during forging a knife is not typically great enough to compensate for the effects of temperature. So after forging the grain size is typically relatively large. Here are example micrographs of a low carbon steel after being heated to different temperatures [1]. This steel actually has some micro-alloy added to it to help prevent grain growth so these grains are probably smaller than would be found in steels like 1095 after heating to the same temperatures.

920°C (1685°F) [1]

1000°C (1830°F) [1]

1150°C (2100°F) [1]

1300°C (2370°F) [1]

Another method for estimating grain size is looking at the “fracture grain.” If you break a piece of steel in a brittle condition the appearance of the fracture will correlate with its grain size. The steel must be brittle. Tempered steel and especially soft/annealed/normalized steel will break in a ductile fashion and the fracture will not correlate with the grain size. Below are example fractures from 1084 which were heated to different temperatures for 10 minutes, quenched, and then broken.

1475°F (800°C)

1700°F (925°C)

2000°F (1095°C)

A completely smooth specimen is given a fracture grain size rating of 10 and the coarsest possible appearance is given a rating of 1. This also correlates with ASTM grain sizes. A fracture rating of 10 is approximately equivalent to an ASTM grain size of 10. The ASTM grain size can be converted to an average grain diameter using charts that are available everywhere or by using the conversion equation. Below shows fracture grain size vs temperature for a range of different tool steels. The simple carbon steel W1 sees grain growth at relatively low temperature as you can see. Adding some alloy such as in O7 which has W, V, and Cr leads to less grain growth. A high alloy steel like D2 or T1 sees grain growth at much higher temperatures.

Image from [2]

Microstructure After Forging

The microstructure in terms of what phases are present can vary quite a bit after forging. Mixtures of pearlite, ferrite, austenite, martensite, and carbide can be found with different distributions depending on the forging process, steel, and how fast it was cooled after forging. Below shows the inconsistent microstructure found in O1 and L6 steels after forging.

O1 after forging [3]

L6 after forging [3]

Grain Boundary Carbides

Another issue that can happen with steels during forging is the formation of carbides along the grain boundaries. These are detrimental to toughness and we definitely want to eliminate them.

Carbides on grain boundaries [4]

Heating Up During Normalizing

So in normalizing we want to reduce the grain size from forging, achieve a consistent microstructure, and eliminate grain boundary carbide. While using low temperatures is best for fine grains, the most important thing we need to do is dissolve everything so we can eliminate the inconsistent microstructure and grain boundary carbide. The higher the carbon content, beyond 0.75% carbon, the higher the temperature required for dissolving all of the carbide.

Those are temperatures shown on top of the “equilibrium” phase diagram, meaning very long hold times. Since we will hold for 60 minutes or less for normalizing the actual temperatures we use will be higher to ensure the carbide is dissolved. While carbon is the most important element for dictating the normalizing temperature, other alloying elements can also affect the required temperature. One common one is chromium, which shifts up the required temperatures. This is why 52100 requires higher normalizing temperatures than other simple steels with similar carbon content.

Below shows schematic diagrams for what occurs during heating for normalizing, where first the ferrite/pearlite is dissolved and replaced with austenite, and then with higher temperature the last lingering carbides are dissolved.

Normalizing Temperatures

Below shows recommended temperatures for normalizing from my book Knife Engineering. For many of these you can actually normalize up to 100°F higher than this temperature to ensure everything is dissolved. Heat for 10-15 minutes and then air cool to normalize.

Cooling Down During Normalizing

After we have a microstructure of austenite we want to cool in air to form pearlite. This will leave us with a consistent microstructure going in to our next steps. Carbide and ferrite forms simultaneously on the austenite grain boundaries. Ferrite can accommodate almost no carbon, so the carbon leaves austenite by forming carbides. This creates carbon-lean regions where the ferrite can form. Therefore alternating bands of ferrite/carbide form which is a structure called pearlite.

Adapted from [5]

This same process occurs throughout the steel, with new pearlite grains nucleating at grain boundaries and growing to consume the austenite and replace it with pearlite.

Adapted from [6]

Different cooling rates lead to different hardness. A fast cooling rate leads to finer pearlite and higher hardness. A slow cooling rate leads to coarser pearlite and lower hardness. When air cooling you don’t necessarily have control over the cooling rate, but with thicker or thinner knives this will affect the cooling rate and the resulting microstructure and hardness.

Hardness of 1095 after normalizing with different cooling rates (generated from CCT curves)

1080 steel air cooled [7]

1080 steel furnace cooled [7]

With different steels the hardness of the pearlitic structure is also different depending on the hardenability of the steel. High hardenability steel like O1 will have a relatively high hardness and very fine pearlite. Low hardenability steel like 1095 is lower in hardness. If the O1 is very thin then some martensite may form. If the hardenability is very high like L6 or an air hardening steel then even more martensite forms. We prefer pearlite if possible with normalizing. Below are hardness measurements I made after air cooling 1/8″ stock knife steels:

Annealing

There are several different ways of annealing steel which I have previously covered in two articles: Part 1 and Part 2. But here I will give a brief summary of the different types. Generally what we are looking for is a “spheroidized” structure rather than pearlite. The round carbides are better for machining than pearlite. And the heat treating response is somewhat different.

Subcritical Anneal

If you heat the normalized steel and heat it to high temperature below austenite transformation and hold, it will spontaneously spheroidize. Below are images of 1080 steel being held at 1200°F (650°C) for different times [4].

Normalized [4]

4 hours [4]

16 hours [4]

64 hours [4]

240 hours [4]

360 hours (double magnification) [4]

Temper Anneal

A temper anneal is similar to a subcritical anneal in that you hold the steel just below the critical temperature to reduce hardness. The difference is that you heat the steel above critical and quench first. So it becomes a very high temperature temper to lead to low hardness steel and a carbide plus ferrite microstructure. Carbon precipitates out of the martensite and coarsens. Coarser carbide and less carbon in the martensite means lower hardness.

1084 tempering curve (ASM heat treater’s guide)

400°C (750°F) [4]

500°C (930°F) [4]

600°C (1100°F) [4]

700°C (1300°F) [4]

Transformation Anneal (Divorced Eutectoid Transformation)

The most common anneal is called a transformation anneal also called a divorced eutectoid transformation (DET) anneal. You heat above critical but not so high that you dissolve all of the carbides. And then during slow cooling the steel transforms to ferrite while growing the carbides by diffusing carbon to the carbides.

Image from [8]

The temperature of annealing is very important because if you heat to too high a temperature then too much carbide is dissolved. Without carbide it is more favorable for the steel to form pearlite. Below shows 1080 steel annealed from different temperatures.

1080 steel annealed from 1385°F (a), 1450°F (b), 1600°F (c), 1750°F (d). Image from [9].

Here are recommended annealing temperatures from Knife Engineering for a good DET anneal.

The cooling rate of the anneal is important to the final hardness and microstructure. Datasheets and steel manufacturers use very slow cooling rates that lead to a relatively coarse structure. This is the lowest hardness and most machinable structure. However, with a faster cooling rate the structure is finer which can have certain benefits. Below shows 52100 as-received compared with 52100 that I annealed by placing in vermiculite. You can use cooling rates at about 600°F/hr (300°C/hr) for most low alloy steels.

52100 steel as-received from the steel manufacturer (“slow” DET anneal)

52100 with “fast” DET anneal

Forge Heat Treating

One of the things I wanted to see is how much the prior microstructure affects the response to heat treatment. This is particularly important when heat treating without temperature control where you can’t easily do a soak at the appropriate temperature. The general recommendation to new knifemakers heat treating with a forge is to heat 100-150°F (50-80°C) higher than non-magnetic before quenching. Trying to heat some amount over non-magnetic leaves a lot to skill (and chance) and so I don’t like recommending to people that they perform forge heat treatments. Furthermore, one knifemaker once contacted me about testing 52100 that he had heat treated in a forge to look at different parameters and the toughness was horrible. The fracture grain was very coarse confirming they were overheated.

On the left is overaustenitized steel from the knifemaker heat treating with a forge. On the right is furnace heat treated 52100 with an appropriate austenitizing temperature.

The Experiment

But if the steel could be heated to non-magnetic and quenched for the appropriate microstructure without some unknown degree of heating beyond that, then we could remove a lot of the guesswork. The steel becomes non-magnetic when it has transformed to austenite. Read this article for more on checking steel with a magnet and what happens in the steel. I performed experiments with 52100 and 1084 from Alpha Knife Supply.

With both steels I overheated them at 2100°F (1150°C) for one hour and air cooled. Both were normalized, the 52100 at 1700°F (925°C) and the 1084 at 1550°F (845°C) for 15 minutes and air cooled. That is the normalized condition that I tested. I also tested them with a “fast DET” anneal where they were cooled at 600°F/hr (300°C/hr) after holding at 1450°F for 52100 and 1385°F for 1084, both for 15 minutes. Those temperatures are right about at non-magnetic. These were also compared to the as-received condition from the steel company which has a coarser microstructure as I originally stated.

1084 steel as-received (slow DET anneal). Coarse spheroidized carbides.

1084 normalized. Pearlite microstructure. The diagonal streaks are scratches because I struggled to get a good polish on these soft specimens.

1084 with fast DET anneal. There is some evidence of pearlite so it is probably good that the recommended annealing chart from Knife Engineering is at a slightly lower temperature of 1365°F.

Annealed 1084 showing the decarb layer where grains are visible. This isn’t exactly the same grain size as the rest of the structure because the transformations between carbide/pearlite and austenite also affect the grain size. However, it can provide some indication of the grain size of the bulk material. This is around a 9 ASTM grain size which is very fine. So the normalize and anneal was effective in refining the grain size after the high temperature grain growth treatment.

Quenching and Hardness Testing

I heated each steel to non-magnetic, which was about 1385°F for 1084 and 1445°F for 52100. I went up in 20°F increments until each became non-magnetic. I quenched each in Parks 50 and measured the hardness.

1084 had only 1 Rc difference in as-quenched hardness whether it was normalized for pearlite or heat treated from the as-received condition. It is a simple carbon steel so it is primarily carbon with some Mn and Si added. Carbon diffusion is very fast so even with a coarse spheroidized microstructure the final hardness is comparable.

1084 hardness hardness after quenching from just above non-magnetic:

52100, however, showed relatively significant differences between different prior microstructures. From the normalized condition it was a full 67 Rc after quenching. With the as-received microstructure it was only 60.6 Rc. The fast DET was in the middle though closer to the as-received than to the normalized condition. The chromium addition to 52100 delays the “kinetics” (speed) of the transformation.

52100 hardness after quenching from just above non-magnetic:

The coarse spheroidized structure leads to a relatively long distance that diffusion has to occur. Pearlite is a finer structure where transformation can occur more rapidly. Martensite (quenched steel) essentially has perfectly evenly distributed carbon. If you temper high enough and long enough you do get a structure that starts to look more like a fast DET anneal, however.

Spheroidized carbide schematic

Schematic of pearlite

Schematic of quenched martensite

Effect of Prior Microstructure on 52100 Toughness

In a previous experiment with 52100 steel we found a significant difference in toughness between steel heat treated from the as-received condition and the “fast DET” annealed condition. The finer resulting microstructure apparently increased toughness. However, the hardness of the steel from the as-received condition is 1-2 Rc lower than I would expect. Perhaps the steel was particularly coarse from the manufacturer. Read more about these experiments in this article on heat treating 52100.

Effect of Prior Microstructure on CruForgeV Toughness

We also did a series of experiments on a few years ago with CruForgeV which you can read about in this article. To that set of experiments I added a toughness test from the as-received condition to go with it in the past couple weeks for this article. In the original experiment the steel was forged between 1550°F/845°C and 2000°F/1100°C (the effect of forging temperature was one of the tests performed). Each was normalized from 1600°F/870°C. Then they were given three different anneals:

Subcritical anneal: 1250°F/675°C for 2.5 hours. I don’t think this was long enough for significant spheroidization so it was probably mostly pearlite.

Temper anneal: 1450°F (785°C) for 10 minutes, quench, then 1250°F for 2.5 hours.

Fast DET anneal: 1460°F for 30 minutes followed by 670°F/hr (375°C/hr).

The hardness after 1500°F for 10 minutes, Parks 50 quench, and temper at 400°F is shown below:

Then below is the resulting toughness for the different conditions. This also includes other heat treatments with the fast DET condition to show the overall trends of hardness vs toughness:

So the overall toughness-hardness balance was not really affected by the different prior processing but the finer microstructure anneals did lead to higher hardness as expected. However, the prior microstructure does change the optimal austenitizing temperature range. The datasheet for CruForgeV recommends 1500-1550°F. However, using 1450°F with the fast DET led to similar hardness-toughness to using 1500°F with the as-received condition. And using 1550°F with the fast DET prior microstructure led to terrible toughness. So having the finer microstructure of the fast DET led to a drop of about 50°F in the optimal austenitizing temperature range.

Choosing Between Normalized and Annealed Steel for Final Heat Treatment

One reason that I prefer the fast DET rather than a subcritical anneal, temper anneal, or normalized structure is because of the difference in heat treatment response. When you have control over temperature with a furnace there is a range of hardness and microstructure that you can choose between. But with a pearlitic or martensitic starting microstructure you are basically limited to high hardness. Tempering higher to bring the hardness down would lead to issues with tempered martensite embrittlement. However, as I said before when heat treating in a forge the benefits of pearlite not requiring a soak is probably more useful. See the chart below for 52100:

Adapted from [10]

Cycling for Grain Refinement

As discussed previously in this article, grain refining cycles can be performed in between normalizing and annealing. This is like a normalizing cycle but is done from a lower temperature, such as 1400-1450°F. Cycling from low temperature has been shown to lead to finer grain size such as in the 1060 steel below which was cycled from 1490°F (810°C) [11]:

Overheated at 2000°F/1100°C (a), 1-cycle (b), 3-cycles (c), 5 cycles (d), 8 cycles (e). Image from [11].

Effect of Cycling on Carbide Structure

However, grain refinement is not the only change to the microstructure. This cycling treatment with the 1060 also led to spheroidization of the carbide. After several cycles the microstructure looked pretty close to a fast DET structure.

One cycle (top), 5 cycles (middle), 8 cycles (bottom). Images from [11].

Effect of “Grain Refining” Cycles on Heat Treatment Response

So while I think a pearlite structure is best for a forge heat treatment perhaps we would want to add a grain refining cycle or two to help with the grain size. In my previous 52100 samples I also tried a grain refining cycle from 1445°F after normalizing and compared the hardness to see if there was a difference:

The hardness was somewhat reduced when compared with the original normalized steel. To determine why I looked at the microstructure and found that the steel had partially spheroidized even after only one “grain refining” cycle:

Effect of “Grain Refining” Cycles on 1084 Toughness

I did a set of experiments on 1084 with different prior processing to see if grain refining cycles can improve toughness. For each I heated them to 1475°F for 10 minutes, quenched in Parks 50, and tempered at 400°F/205°C. For one condition I used the steel “as-received” from the steel company. In the other two I overheated both at 2100°F for an hour to simulate grain growth from forging. In one I annealed from 1380°F with no normalizing or grain refining. In the other I normalized from 1550°F, then did two grain refining cycles from 1450°F, and finally ended with the same anneal as the other specimen. The hardness was a point higher on the two specimens that I annealed because of the finer microstructure from the fast anneal. However, the toughness was no different whether I did the grain refining cycles or not. The fracture grain of all of the specimens was fine so it could be that there was no difference as long as the final austenitize was done correctly. Or perhaps the anneal leading to a fine distribution of carbides in combination with a fine grain size meant that the prior normalizing and grain refining had no benefit.

1084 steel overheated and annealed prior to austenitizing

1084 steel overheated, normalized, “grain refined” and austenitized

I still recommend normalizing as that is for the purpose of dissolving everything and having a consistent pearlite microstructure. However, it appears that adding extra grain refining cycles is not necessary for a fine grain size, and led to no improvement in toughness.

Stress Relieving

Another type of heat treatment that can be performed prior to the final austenitize is a “stress relief” treatment. This one is commonly done because of stresses induced in the steel during grinding the annealed steel. If the steel is significantly heated in different portions from grinding this can lead to increased chances of warping or cracking during austenitizing and quenching. A stress relief is typically done from 1200°F/650°C for 2 hours.

Thermal Cycling for Stock Removal?

It is relatively common for knifemakers to do various cycling treatments with low alloy steels even for knives that have not been forged (ie stock removal knives). They typically do this with the goal of grain refinement. However, as shown in the above experiments, the changes to the carbide structure and heat treatment response are much more significant than any changes to grain size. If you are forging some knives and doing stock removal with others, I recommend doing the same set of normalizing and annealing treatments to keep the heat treatments consistent. Otherwise some knives will have different hardness, toughness, etc. If you have a furnace where you can control austenitizing temperature and soak time then the steel is probably fine from the manufacturer. If you are heat treating without temperature control in a forge then it may be better to start with a pearlitic structure from normalizing.

Thermal Cycling Stainless?

Stainless steels and high alloy tool steels cannot be normalized. Most of the steels used in knives still have significant carbide all the way up to melting temperature. So the carbides cannot be dissolved as with low alloy steels. And during air cooling you will get martensite rather than pearlite because of the high hardenability. And as shown in charts earlier in this article, grain size stays fine during forging much better in high alloy steels because all of the carbides “pin” the boundaries preventing grain growth.

If you are forging stainless or high alloy tool steels you should simply anneal instead. You can do a standard datasheet anneal which is a slow transformation anneal. I have not seen research on faster anneals on these types of steels and the rates are likely significantly slower than in low alloy steels so I would probably stick with the datasheet rather than attempting a “fast DET” like I recommend with the low alloy steels. However, one alternative is a temper anneal. The required temperatures and times are significantly higher than low alloy steels but experiments on T1, M1, and M2 high speed steels found an improvement in grain size after the final austenitize and quench if done in a specific way. Read more in this article or in Knife Engineering.

How to “Thermal Cycle” Low Alloy Steels in a Forge

1) Normalize by heating to the approximate recommended temperature and air cool.

Determining the exact temperature can be done by color or maybe with a laser thermometer. My cheap laser thermometer is always way off so I don’t know if more expensive ones can do better. Fortunately normalizing can be done from a relatively wide range of temperature. When the steel is fully magnetic again during air cooling it is done transforming. If heat treating in a forge this is the best microstructure going in to austenitizing because no soak is required.

2) Heat to non-magnetic (and no higher) and then place in vermiculite. This step is recommended if your final austenitize and quench will be done in a furnace.

While it might sound funny to do normalizing and annealing with a forge if you have a furnace, the cycles can be done very rapidly in the forge so it can be more convenient.

How to “Thermal Cycle” in a Furnace

1) Normalize by heating to the recommended temperature for 10-15 minutes and air cool.

2) Anneal by heating to the recommended temperature for 15-30 minutes, cool at 600°F/hr (300°C/hr) to 1100°F. After 1100°F the steel can be furnace cooled or air cooled it doesn’t matter.

Summary and Conclusions

There are a lot of different recommendations for different cycles to perform after forging but those recipes sometimes miss what it is we are trying to achieve with normalizing and annealing. Normalizing is to dissolve everything and have a consistent microstructure. Annealing is to give a spheroidized structure which is easily machinable and easily cut and drilled, and to have consistent final heat treatment response. Grain refining cycles don’t seem to provide any benefits beyond a normalize and anneal in the experiments that I performed. When performing a final heat treatment in the forge there are benefits to having a normalized structure rather than annealing because the heat treatment response is so rapid. That means you can heat to non-magnetic without going hotter and quench from there. I will next be doing experiments with forge heat treatments with a normalized structure in a range of steels to see how well this works for a forge heat treating beginner like me.

[1] Souza, Samuel da Silva de, Paulo Sérgio Moreira, and Geraldo Lúcio de Faria. “Austenitizing Temperature and Cooling Rate Effects on the Martensitic Transformation in a Microalloyed-Steel.” Materials Research 23 (2020).

[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

[3] http://www.georgevandervoort.com/images/met_papers/IronandSteel/ToolSteels_Longest.pdf

[4] Samuels, Leonard Ernest. Light microscopy of carbon steels. Asm International, 1999.

[5] Porter, David A., and Kenneth E. Easterling. Phase transformations in metals and alloys (revised reprint). CRC press, 2009.

[6] Brooks, Charlie R. Principles of the heat treatment of plain carbon and low alloy steels. ASM international, 1996.

[7] Toribio, Jesús, Beatriz González, Juan-Carlos Matos, and Francisco-Javier Ayaso. “Influence of microstructure on strength and ductility in fully pearlitic steels.” Metals 6, no. 12 (2016): 318.

[8] Verhoeven, J. D., and E. D. Gibson. “The divorced eutectoid transformation in steel.” Metallurgical and Materials Transactions A 29, no. 4 (1998): 1181-1189.

[9] Payson, Peter. The annealing of steel. Crucible steel company of America, 1943.

[10] Stickels, C. A. “Carbide refining heat treatments for 52100 bearing steel.” Metallurgical Transactions 5, no. 4 (1974): 865-874.

[11] Saha, Atanu, Dipak Kumar Mondal, and Joydeep Maity. “Effect of cyclic heat treatment on microstructure and mechanical properties of 0.6 wt% carbon steel.” Materials Science and Engineering: A 527, no. 16-17 (2010): 4001-4007.

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YouTube

The following information is also available as a YouTube video for those that prefer watching to reading. The video might be more fun though there are more details and more discussion in the article.

Oil

One common rating method for quench oils is the quenchometer “nickel ball” test. A 12 mm nickel ball is heated to 1620°F and then quenched into 200 ml of oil. Nickel reaches the Curie point at 670°F at which point it is attracted to a magnet.

The nickel ball is held on a string and a magnet placed outside of the beaker so that when the nickel ball becomes magnetic it is attracted to the side of the beaker. At that point the test is stopped and the time taken. A general ranking of different quenchants is found below:

Parks 50 and AAA are quite commonly known oils among knifemakers. Parks 50 is a 7-9 second oil, clearly in the “fast oil” category. Parks AAA is a medium-fast oil, taking 9-11 seconds with the nickel ball test. I bought my oils from Maxim but since then DuBois has an easy online store available for these oils.

Quenchfast and Quenchall are offered by McMaster-Carr as “11 second” (Quenchfast) and “28 second” (Quenchall) oils. I asked McMaster-Carr for more information on the oils but the sheet they sent me (hosted here) doesn’t have any more specific information on the nickel ball test ranges. In fact, for some reason the datasheet calls Quenchall a 26 second oil instead. The buckets say Reladyne oil but contacting Reladyne led to nothing; the person I spoke to on the phone didn’t seem to have any information on the products whatsoever and kept asking me for an order number. Of course giving them the McMaster-Carr order number just gave an error in the system.

The Citgo Quenchol 521 came from Jantz. Jantz lists the oil as “14-16 seconds” on their sheet, but the datasheet from Citgo lists the oil as 16.1 seconds, which looks oddly specific when the other products have ranges.

So the ratings of the oils are not quite as simple as I might have liked. There are other products available, of course, notably Houghton makes a range of oils at different speeds. If you have a supplier that regularly sells 5 gallon containers of Houghton oils those are worth looking at as well.

Canola, Motor Oil, and the Inconel Probe Test

Knifemakers looking for oils to use that are cheaper than those available commercially most commonly use canola from the grocery store. However, some will also use motor oil. I found a study on 1045 steel where they found canola to quench more rapidly than motor oil so I am going to stick with canola as my “cheap” quenching option to test.

Data adapted from [1]

Despite quite a few studies that looked at canola oil I did not find any nickel ball measurements. This seems to be because the nickel ball test is outdated and has mostly been replaced by the inconel probe test.

The inconel probe test is similar but the probe can measure the temperature of itself during quenching to generate more information about the quenching process rather than just generating a time in seconds. So you get a curve such as below:

The blue line is the normal time vs temperature curve, and the orange line is the “instantaneous cooling rate” at each position. In other words, the slope of the time vs temperature curve at each position. You can see that the cooling rate is relatively slow at high temperature, then accelerates to a peak cooling rate around 1150°F and the slows down to about 600°F where it becomes more stable. Those three stages are the “vapor blanket,” “nucleate boiling,” and “convection” phases, which are also shown below. I have more information about these stages in this article about hardenability of steel.

Comparing Oils with the Inconel Probe Test

Below shows a range of different oil speeds from Houghton oils, as well as Canola:

Data adapted from Houghton International

The 7-9, 8-10, and 10-12 second oils look relatively similar but the peak cooling rates are slower as the nickel ball time goes up. The 15-22 second oil has a vapor blanket that lasts until a lower temperature, and then the peak cooling rate is significantly lower and at a lower temperature. Canola forms almost no vapor jacket at all and therefore reaches its peak cooling rate at a higher temperature. However, its cooling rate then decreases at higher temperatures, crossing over with the 15-22 second oil at about 1000°F.

However, if we look at water we see that it is much faster than any of the oils, coinciding with the nickel ball results I listed earlier. I have two temperature results listed because water is very sensitive to temperature. The closer the water is to boiling the more tenacious its vapor blanket is.

Water quenching chart from ASM Heat Treater’s Guide

However oil is much less sensitive to temperature. There are small changes in the cooling behavior with temperature but not nearly as extreme as water.

ASM Heat Treater’s Guide

Hardenability of Steel – Jominy

Hardenability of steel also has multiple measures. I first covered hardenability in this article which had an extensive discussion of different CCT curves (continuous cooling transformation) diagrams. Hardenability is how slow you can cool the steel from high temperature and still achieve full hardness. Hardenability is not a measure of how hard a steel can be after quenching, which is controlled by other factors, primarily how much carbon is “in solution” in the martensite. One relatively simple measure of hardenability is the Jominy test where a bar of steel is heated in a furnace and then is placed in a fixture with a water spray that is directed at one end of the bar. So that end is rapidly quenched and the cooling rate is progressively slower toward the other end, which is essentially being air cooled.

Comparing different steels you get a chart that looks something like this:

Adapted from ASM Heat Treater’s Guide

A2 is an air hardening steel so its line is flat; even with slow air cooling it fully hardens. 1095 is a water hardening steel so it only reaches maximum hardness at the position measured directly next to the water quench and the hardness drops rapidly. 5160 and O1 are oil hardening steels though the O1 is significantly more hardenable. 52100 is in between the water hardening and oil hardening steels, and different datasheets will recommend that either can be used depending on the cross section.

Continuous Cooling Transformation Curves

The CCT curves I mentioned above have more information about the behavior of the steel than the Jominy test. It is sort of like the difference between the nickel ball test (limited information) and the inconel probe test (more information). The CCT curve is generated by cooling the steel at different rates and measuring the phase transformations that occur during cooling. This shows the critical cooling rate required to avoid pearlite formation (makes the steel softer) and also the temperatures and times at which different phases will form. There are also certain features that can be different between steels such as some that will form some bainite (labeled B+K) if cooled at an appropriate rate, such as seen with O1 steel below:

Low hardenability steels see pearlite transformation at much shorter times such as can be seen with W2 below:

Different elements added to steel help to suppress the formation of pearlite. These elements are preferentially found in iron carbide so that when the steel tries to form the carbide phase of pearlite it is delayed by the diffusion of those elements. The most effective elements for hardenability are Mo, Mn, and Cr though Ni, Si, C, and V also affect hardenability. W2 has high carbon, low Mn/Si, and a V addition. The vanadium helps to refine the grain size which reduces hardenability (see my hardenability article). O1 has high Mn plus 0.5% Cr so it has relatively high hardenability.

Steels Used in This Study

I chose a range of low alloy steels to test the different oils that I bought. The primary tests I performed were with 1/4″ thick stock. 1/4″ is about as thick as most knives get so if it fully hardens at that cross-section then thinner knives will also work with that particular oil.

The steels above are ranked in order of increasing hardenability based on Jominy data, CCT curves, and otherwise estimated based on the composition using equations found in the hardenability article. W2 has very low Mn and Cr which means its hardenability is quite low. And as mentioned previously the vanadium addition refines the grain and further reduces its hardenability. 1095 has somewhat higher hardenability due to the increase in Mn. 26C3 has very high carbon which reduces hardenability, but the Cr addition helps to overcome that to have somewhat higher hardenability than 1095, at least according to the CCT curves. 26C3 has a similar high carbon+Mn+Cr composition to several steels like the Blue/Aogami series. 1084 is near-eutectoid and higher Mn than the previously mentioned steels which gives it increased hardenability. 80CrV2 has reduced Mn when compared with 1084 but with a significant Cr addition that makes up for it. 15N20 is an interesting case since it has a substantial Ni addition; nickel does not contribute that much to hardenability but 2% certainly has an effect. 52100 is a similar case with relatively low Mn but a substantial Cr addition for hardenability. CruForgeV and O1 combine significant Mn with a 0.5% Cr addition which is why they are the highest hardenability steels on the chart.

The W2, 1095, 1084, 80CrV2, 52100, and O1 were purchased from New Jersey Steel Baron. Most of the steels were produced by Buderus according to the composition sheets, and the O1 was produced by Latrobe. 26C3, 15N20, and CruForgeV came from Alpha Knife Supply. The 26C3 and 15N20 are produced by Uddeholm and CruForgeV is a Crucible product.

Experiment

I tested each of these as 1.5 x 2 inch rectangular specimens. The majority of the tests were with 1/4″ stock though some tests were done from 1/8″. The 1/4″ specimens were held at temperature for 18:30 minutes while the 1/8″ specimens were held for 10 minutes. Parks 50 and water were used at room temperature. The other oils were used at 120-150°F. I ground 1/32″ off the surface and tested the hardness, and then continued in 1/32″ increments checking the hardness each time until the center of the specimen.

1084 and Comparing Oils

It turns out that 1084 was in the sweet spot for comparing the different oils to each other; it shows the clearest differences. This was a little bit of a surprise because I expected 1084 and 80CrV2 to have more similar hardenability since Cr is less effective than Mn for hardenability, so 0.75 Mn should be similar to 0.4 Mn + 0.5 Cr. The results of the different quench media are shown below:

Water led to the highest hardness, as expected being the fastest quenchant. Parks 50 had a drop in hardness at the center of the 1084. Parks AAA had a maximum hardness of 62 Rc near the surface though it dropped to 60 Rc through the rest of the cross-section. Quenchfast (11 second oil) and Quenchol 521 (16 second) had somewhat odd behavior. The Quenchfast started out at higher hardness as expected but then the two oils crossed over at 3/32″ from the surface. I’m not sure what led to that result. The slow oil Quenchall led to low hardness. But the biggest surprise to me was that canola was by far the slowest oil. The overall time for cooling the steel in canola is very similar to the other oils, perhaps even faster. However, the slowing of cooling rate appears to be the deciding factor here. The “pearlite nose” on the CCT curve (the shortest time for transformation) occurs around 1050°F which is where the canola has already slowed down to the cooling rate of a slow oil.

O1 High Hardenability Steel

O1 with its very high hardenability there was no difference between Parks 50 and canola:

There is a small difference in hardness between the two oils shown above, however. This is likely due to “auto-tempering” which is a small amount of tempering that occurs by slow cooling through martensite formation. This explains why the two lines are parallel to each other rather than seeing a drop as would be expected from pearlite formation.

Water Hardening Steels – W2, 1095, and 26C3

I was also somewhat surprised that these water hardening grades did not fully harden in the Parks 50. Parks 50 is often promoted as “approaching the speed of water” but it turns out the word approaching is doing a lot of work in that sentence.

The other surprise here is that the steels are basically in reverse order of hardenability from what was expected on composition. I can’t think of many good reason why W2 with the lowest Mn is showing superior hardenability to the 1095, especially since both are made by the same manufacturer. I would expect the reason for reduced hardenability of 26C3 is because of differences in carbide structure from the manufacturer. Perhaps a follow-up study could be done where I dissolve the carbides and re-anneal them to all have a similar starting microstructure. When I quenched 1095 and W2 in water, however, they fully hardened. I didn’t have the 26C3 yet when I performed this experiment:

Effect of Cross Section with Low Hardenability Steels

Many knives are produced with thinner than 1/4″ stock so I was also interested in how much effect that would have. And some knives have bevels before hardening and so the edges may still harden even if the spine of the knife does not. Of course we expect a faster cooling rate with thinner steel and therefore better hardening.

Canola was unable to harden 26C3, 1095, W2, or 1084, though 1084 was by far the closest of those. I was surprised that canola could not fully harden 1/8″ 1084 since this is such a common oil used by beginning knifemakers. Parks AAA did harden the 1095 and W2 but not the 26C3, it needed Parks 50. Since the W2 and 1095 had higher hardenability than expected in my tests it is possible that other steel from another manufacturer, or the same steel hardened from the normalized condition may require faster than AAA. So for steels in the “water hardening” category I would recommend using water if thicker than 1/8″. Water can be a dangerous quenchant, it is important to avoid stress risers in the knife and keep the grain size small in the steel. Also “hard” water is a faster quenchant than distilled. If 1/8″ or thinner it appears that a very fast oil is effective in these low hardenability steels.

Intermediate Hardenability Steels

80CrV2 and 52100 had somewhat inconsistent hardening results. I believe this is also due to prior microstructure effects. Perhaps if the steel was processed for a finer microstructure in the annealed condition it would show more consistent hardness at the surface with fast oil. A finer microstructure would also reduce the hardenability of the steel so that could be an interesting follow-up as with the 1095/W2. With 80CrV2 the canola did not fully harden the 1/4″ steel and Quenchall was about 2 Rc lower than the other oils throughout. With 52100, canola and Quenchall dropped a couple Rc by the surface which makes it appear that the oils are not fast enough for that cross section. However, with the inconsistent hardening, Parks AAA and Quenchol 521 had somewhat lower hardness at the surface than the center. So it is hard to tell if the canola/Quenchall are insufficiently fast.

With 15N20 the Parks 50 (fast) and Quenchfast (medium) oils behaved similarly, while canola clearly led to softer steel. For CruForgeV all three oils showed similar results, so it seems to be hardenable enough that oil doesn’t matter too much with 1/4″ thickness.

Quenching Too Fast?

A frequent discussion I see on the forums and Facebook groups is whether to use Parks 50 or AAA for a particular steel. These oils are common with knifemakers so it is often a decision between one or the other. Some knifemakers will say that Parks 50 is “too fast” for some steels and you must switch to AAA to avoid reduced toughness. However, this is not as important a decision as is sometimes stated. One thing to remember is that Parks 50 and AAA are not that different from each other; this is not a decision between a very fast and very slow oil. AAA is straddling the line of the fast oil category.

One of the concerns is that the faster oil leads to microcracks and therefore reduced toughness. Microcracks are real but they occur due to plate martensite formation not from overly-fast quenching. Plate martensite comes from very high carbon austenite so this is controlled by austenitizing (how much carbon is in solution) not from quenching.

In a previous study we did on 8670 toughness we looked at toughness differences between Parks 50 and AAA. 8670 has hardenability similar to CruForgeV and is one of the steels that is sometimes said to be a no-no for Parks 50. However, the toughness was the same whether we used Parks 50 or AAA, in fact the Parks 50 was slightly higher but that is probably just due to experimental scatter.

Instead, the concerns with quenching more rapidly are warping and cracking. Cracks are not the same as microcracks. Microcracks are a microstructure-level feature that specific etching techniques are required to reveal. Cracking is on a macro level and are much more obvious. High hardenability steels can be quenched in slower oils to minimize the chances of warping and cracking. However, quenching high hardenability steels in a fast oil is still possible and it doesn’t mean reduced toughness.

Which Oil to Buy? 

Because warping/cracking can be reduced through a slower oil it makes sense to have multiple oils depending on the hardenability of the steel. However, if you buy only one oil it should be a fast oil like Parks 50 so that you can quench the low hardenability steels. If the funds are really so tight that you are considering buying canola I would recommend getting the relatively inexpensive AAA instead.

Which Steels Can be Quenched in Slower Oils?

To know which steels can be used with slower oil to minimize warping/cracking I have the following chart that is from my book Knife Engineering. It is an approximate ranking based on hardenability. The steels at the top are expected to have the lowest hardenability and the bottom has a few air hardening steels as examples.

Summary and Conslusions

The biggest takeaway for me is that canola is not a particularly great quenchant. I would highly recommend buying a commercial quenching oil instead. I was also a bit surprised just how sensitive the water hardening grades were to cross-section and oil. I thought at 1/8″ they would harden no problem. There was a pretty sharp transition where 1084 was a step up in hardenability from 1095/W2/26C3, and then every steel with higher hardenability than that was basically insensitive to oil choice. A few potential follow-up studies were mentioned in this article, such as processing the steels for different prior microstructures. We would expect normalized steel (pearlite) or fine carbide annealed structure to have reduced hardenability, which may be significant for forging bladesmiths. The austenitizing temperature can also affect hardenability, typically low temperatures lead to reduced hardenability. And of course we can look at other steels where we think it will be interesting.

[1] Pérez-Ruiz, Eduardo, Santiago Frye Rocha, and Jorge Freddy Llano Martínez. “Performance of Vegetable Oils on the Hardness and Microstructure of AISI 1045 Steel Quenched.” (2019).

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History of Cronidur 30

It was known that nitrogen could replace carbon to form hard martensite. Both are small “interstitial” elements that create the hard structure necessary for knives. Or in this case, Cronidur 30 was developed for bearings. There have been several steels designed for bearings that also see regular use in knives, such as 52100, 154CM, and BG42. And 440C has a long history of being used in bearings. Nitrogen is also beneficial for corrosion resistance, while carbon is almost always detrimental. Nitrogen improves pitting resistance. It is also less prone to form chromium nitride than carbon is to form chromium carbide. That leaves chromium “in solution” to improve corrosion resistance. You can learn more about how nitrogen works in this article on nitrogen-alloyed knife steels.

Nitrogen contributes somewhat less to hardness than carbon. This creation comes from sources cited in this article.

One of the major reasons nitrogen isn’t used more often is because the nitrogen gas is not very soluble in liquid steel. It tends to bubble out when attempting to add it to steel. So with conventional steel production the nitrogen is often limited to the 0.08-0.12% range. Maybe enough to make a difference but not enough to replace much of the carbon in a knife steel needing to be 60+ Rc. One approach to increasing the amount of nitrogen is to produce the steel under high pressure. In the late 1980s Vereinigte Schmiedewerke of Krupp-Kloeckner-Thyssen (VSG) built a pressurized electroslag remelting furnace (PESR) which was capable of having pressure up to 40 bar with a capacity of 20 tons [1]. In the early 1990s they used this technology was used by researchers at Rurh-Universitat Bochum and FAG Bearing Co. VSG to develop high hardness bearing stainless steels [2][3].

Images of PESR from [4]

The 40 bar pressure limited them to about 0.4% nitrogen, which is not enough to reach hardness of 58 Rc and above. So instead they experimented with different combinations of carbon and nitrogen to achieve the desired hardness levels. These were produced in a 100kg laboratory furnace which was also capable of 40 bar pressure for increased nitrogen. They used a basis of 15% Cr and 1% Mo, using 1.4116/X50Cr15Mo as their base steel to replace carbon with nitrogen.

Through these test heats they found that more carbon was better for higher hardness, with the “X30” composition reaching similar hardness levels to the X50Cr15Mo. This resulted in a similar composition to what would be named Cronidur 30. However, they also tested a composition with increased vanadium. The vanadium addition was to help with hardness when tempered in the high temperature regime (secondary hardening). This grade does not seem to have been pursued, however.

Corrosion testing found greatly improved corrosion resistance when compared with X50Cr15Mo and 440C due to the replacement of carbon with nitrogen, putting nitrogen in solution and also leaving more chromium in solution as opposed to forming chromium carbides. This is shown in a current density-potential curve in sulfuric acid below. The “X30” steel had a significant drop in passive current density, and an increase in breakthrough potential, which demonstrates an improvement in corrosion resistance.

They also found that the reduction in chromium carbide vs other martensitic stainless steels like X50Cr15Mo and 440C led to improved toughness because the carbide size was greatly reduced and the overall volume of carbide/nitride is lower than in those martensitic stainless steels.

Image from [5]

Names of the Steel and Final Composition

So the “X30” grade was released as Cronidur 30, though it is now being sold under several other names such as Zapp LC 200 N, Alpha Knife Supply Z-Finit, and Bohler N360. Knife enthusiasts now may know it better by the name LC200N as sold in knives such as by Spyderco. My supposition is that all of these different versions are ultimately being produced by VSG with the same large PESR facility that they developed. I haven’t been able to confirm this but it seems likely. Perhaps the different companies are buying the ingots and then forging and hot rolling it for sale.

Heat Treating of Cronidur 30/LC200N/Z-Finit/N360

Below is the tempering chart from the Zapp LC200N datasheet [6]:

Those hardness values are roughly equal to those that we have found in our experiments of the steel. These were heat treated either by myself (CATRA) or Warren Krywko (toughness coupons). A summary of the heat treatments we have done are shown in the table below:

It seems that the hardness of the steel tops out around 60 Rc. It may be possible to optimize the heat treatment parameters and achieve 61 Rc or so but using the relatively safe 1900°F austenitize leads to about 59-60 Rc.

Microstructure

Below are micrographs of the steel taken by me, and for comparison I have other low carbide/nitride steels AEB-L and 14C28N, and a large carbide steel 440C. The structure of LC200N is very fine as was reported in the articles about the steel. We would expect this to lead to excellent toughness of the steel.

LC200N (1905°F austenitize)

AEB-L (1975°F)

14C28N (1950°F)

440C (1900°F)

Toughness

As was previewed in the microstructure section, the toughness of Z-Finit/LC200N/Cronidur 30 is excellent, similar to AEB-L and 14C28N. The fine carbide/nitride structure and the low volume of those particles gives it very high toughness.

Edge Retention

The edge retention of LC200N/Cronidur 30/Z-Finit/N360 is pretty good given its relatively low volume of carbide/nitride. When compensating for hardness it did better than AEB-L and Nitro-V, and matches 14C28N. 14C28N can be heat treated to higher hardness, however, giving it potential for better edge retention. These steels are lower in edge retention than higher carbide steels and vanadium-containing steels like 440C, 154CM, S30V, etc. The toughness-edge retention balance of LC200N is similar to those other low carbide grades like AEB-L and 14C28N, but with very excellent corrosion resistance, as will be shown below.

Corrosion Resistance

I have tested LC200N with my standard 1% saltwater test and also the more difficult 3.5% saltwater test, and in both cases I found no rust spots on the steel. This puts it at the top of the charts for corrosion resistance of knife steels, along with other high corrosion resistance grade Vanax. This is better corrosion resistance to AEB-L and 14C28N which I have been comparing it to above. In the images below 24 hours of saltwater spray is on the left and 48 hours on the right, for the 1% saltwater test.

LC200N

14C28N

AEB-L

Conclusions and Summary

Cronidur 30, also sold as LC200N, Z-Finit, N360, and others, has an interesting history. It was developed as a bearing steel using a specialized production process (pressurized electroslag remelting), to increase the nitrogen content of the steel through high pressure. The partial replacement of carbon with nitrogen gives the steel improved corrosion resistance and toughness when compared with the carbon version, X50Cr15Mo. LC200N has a max hardness of about 60 Rc, which is probably its biggest limitation. Its edge retention and wear resistance is similar to other low carbide steels like AEB-L and 14C28N. The steel has excellent corrosion resistance and toughness as well. This gives LC200N a very good combination of properties. If higher hardness is desired at the cost of some corrosion resistance, 14C28N is a good alternative. If you want another high corrosion resistance grade but with higher edge retention, Vanax is also available.

[1] Pant, Paul, Peter Dahlmann, Wolfgang Schlump, and Gerald Stein. “A new nitrogen alloying technique‐a way to distinctly improve the properties of austenitic steel.” Steel research 58, no. 1 (1987): 18-25.

[2] Chin, H. A., R. W. Bursey, D. D. Ehlert, R. Biroscak, E. Streit, and W. Trojahn. “Cronidur 30‐An Advanced Nitrogen Alloyed Stainless Steel For Advanced Corrosion Resistant Fracture Tough Cryogenic Bearings.” In Advanced Earth-to-orbit Propulsion Technology 1994: Proceedings of a Conference Held at NASA George C. Marshall Space Flight Center, Marshall Space Flight Center, May 17-19, 1994, vol. 2, p. 321. National Aeronautics and Space Administration, Marshall Space Flight Center, 1994.

[3] Berns, Hans, and Werner Trojahn. “High-nitrogen Cr-Mo steels for corrosion resistant bearings.” In Creative Use of Bearing Steels. ASTM International, 1993.

[4] https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=461

[5] https://www.progressivealloy.com/pdf/cronidur30.pdf

[6] https://www.zapp.com/fileadmin/_documents/Downloads/materials/powder_metallurgic_tooling_steel/special_material/en/LC200N_Datasheet.pdf

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