Category: Toughness
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How to Heat Treat 52100
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52100 Steel
I previously wrote about the history and properties of 52100 in this article. The steel has been around since 1905, has been known as 52100 since 1919, and has been used in knives since at least the 1940’s. It was developed for bearings and its common use in bearings meant it has been a ready source for knife steel for decades. It is known for its fine carbide size and good toughness. The chromium addition compared with the chromium-free 1095 means that it has somewhat higher hardenability so it is easier to harden in oil and obtain full hardness. The chromium also helps keep the carbide size small. The chromium also shifts up the temperatures required for hardening.
A2 Steel – History and Properties
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History
A2 steel is quite old, though determining the exact year it was released is a bit difficult. A2 steel was developed in the early 20th century during the explosion of tool steels that occurred after the discovery of high speed steel which was first presented in 1900. You can read about that history in this article: The History of the First Tool Steel. During the development of the first high speed steel included the switch from manganese to chromium as the primary hardenability element, and most high speed steels had about 4% Cr. That high chromium content was primarily for “hardenability” which is the degree of cooling required to achieve full hardness. A “water quenching” steel has low hardenability and must be quenched very rapidly from high temperature, and an “air hardening” steel can be left in air and it will fully harden. You can read more about hardenability in this article on quenching. The first high speed steel came to be known as T1, which had 4% Cr and 18% W (tungsten). The earliest record I have found of a precursor A2-type steel is in a summary of tool steels in 1925 [1], while summaries of tool steels from 1910 [2] and 1915 [3] do not have any similar steels. Therefore these types of steels probably arose sometime between 1915 and 1925.
How to Heat Treat 5160 – Optimizing Toughness
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5160
5160 is a low alloy steel known for its excellent toughness. It has been used by many forging bladesmiths due to its good properties and also wide availability, especially in the form of leaf springs. However, information on how to maximize the steel’s properties with heat treating is scant. So along with my father, Devin Thomas, we did a small study on the toughness of 5160.
Toughness Database
Why Cold Steel Is Brittle
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Effect of Temperature on Strength
Steels become stronger at lower temperatures. This can be measured with the “yield strength” which is the load to permanently deform the steel. This deformation is in the form of a shape change, ie. if you are bending steel it stays bent, when hammering hot steel it dents, when flexing an edge it rolls. This is perhaps easier to visualize for those that have forged steel because hot steel is easier to forge, and is progressively more difficult to forge as it cools down. This increase in strength at lower temperatures continues below room temperature, so steel at cryogenic temperatures is stronger than at room temperature. Here are values for yield stress for 410 stainless steel heat treated to 39 Rc [1]:
Cryogenic Processing of Steel Part 2 – Toughness and Strength
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Introduction
Part 1 of the Cryogenic Processing series covered the transformation of retained austenite to martensite and the increase in hardness that occurs. That is the least controversial aspect of cryogenic processing of steel. The other two primary properties of steel affected by cryo processing are toughness and wear resistance. Both of these properties can be difficult to pin down as they have high variability. Tool steels are known for their relatively poor toughness which means we are often comparing small numbers.
Detour – Tempering
One important interrelation to keep in mind with subzero and cryo studies is the transformation of retained austenite in tempering. With sufficiently high tempering temperatures all/most of the retained austenite is transformed without any cold treatment. This depends on the alloy content, as low-alloy 52100 will have lost its retained austenite with a 500-600°F temper while high alloy steels need over 900°F. You can read more in the article on tempering. With high alloy steels the loss of retained austenite also coincides with “secondary hardening” which is a high temperature tempering treatment that increases hardness [1]:
Above is a tempering chart for Caldie steel (0.7C-5.0Cr-2.3Mo) which shows both hardness vs hardening temperature and also retained austenite. You can see that at low tempering temperatures (<400°C) the retained austenite is basically constant. You can also see that the hardness decreases with higher tempering temperatures up to about 350°C and then it increases to a peak at around 520°C (950°F). Therefore tempering in the secondary hardening region above 400°C can lead to both high hardness and also the elimination of retained austenite.
Subzero or cryo processing prior to tempering also shifts the tempering-hardness curve to lower temperatures when using the secondary hardening range of tempering [2]:
This means that in general, a lower tempering temperature is required to achieve the same hardness level with secondary hardening. Using the same tempering temperature as without a subzero treatment will lead to a greater degree of tempering. More tempering can be good or bad depending on the situation. Excessive tempering can lead to coarsening of tempering carbides which can reduce toughness. However, if the tempering was insufficient without subzero, the use of subzero processing may increase toughness due to shifting the “optimal toughness” range.
Toughness
In an earlier article where we tested the effects of heat treatment on Z-Wear toughness
The Sharpest Youtube Channel in the World
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Kiwami Japan
A popular Youtube channel called “kiwami japan” includes several videos of making knives out of unusual materials such as jello, pasta, chocolate, etc. The video on making a knife out of cardboard has over 20 million views which means that these videos have reached a broader audience than just knife makers or enthusiasts. As a materials engineer I find the videos interesting from a materials perspective, but they are entertaining in other ways as well. The videos are a bit quirky so I decided to take a dive into these videos and try to figure out what is going on. I also e-mailed the person who makes the videos and he answered a few of my questions. I will refer to him as “Kiwami” for the rest of this article though I know that is not his name. Kiwami means extreme in Japanese.
The person making these YouTube videos hides his identity. I think for the true fans there might be some small reflections to use to piece together what he looks like (I don’t have that much time), but in general he either doesn’t show his face, or covers it up. He also rarely speaks and many of the videos include only the sounds of whatever he is working on:
That GIF of him holding a sickle is perhaps more menacing than the channel appears, in general. He often uses subtle humor in the videos such as being expressive with his hands:
Or by adding anthropomorphic elements to materials he is working on:
Or by showing how much time has lapsed in a project which is taking a particularly long time:
His videos started out with making various things with cheap materials, like “I made a karambit knife with a sickle” or “I made a Nunchaku with 4 dollars.” The first knife video was the karambit, and he followed it up with a video of making a butterfly knife. The making of knife videos started to ramp up with “
Manually repair very rusty Japan’s $500 kitchen knife
Super Steels vs Regular Knife Steels
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Super Steel
I see frequent references to “super steel” online, and I was curious about how long that terminology has been around. I did searches on bladeforums as it is one of the oldest knife forums. The number of references to “super steel” has increased over time, but so have the number of posts on bladeforums. I saw how many references to “super steel” there were in each year, and then as a proxy to how many posts there were on bladeforums I did a search for “154” and saw how many references there were each year. Google tops out at 200 results but at that point the dataset was big enough to get an idea:
So referring to steels as “super steel” or the category of “super steels” is at least as old as still-existent knife forums on the internet. Reading through the descriptions of “super steel” now and for as long as bladeforums has existed, they are typically defined as one or more of the following [1][2][3][4]:
- New (relatively)
- Excellent edge retention
- Difficult to sharpen
- Stainless
Not all of those qualities are universally used. Sometimes non-stainless steels such as 3V or Infi have been called “super steels” [4]. I am not sure if a steel must have high edge retention to be called super but in general the “new” steels that come out have high wear resistance and edge retention. Super steels are often described as having high edge retention but greater difficulty in sharpening, however. Different steels slowly lose the title of “super” over time. In the early bladeforums era, VG-10 was sometimes called a “super steel” [5] but I don’t see it called super much anymore [6]. This confirms the “new” part of the definition. I’m not sure why edge retention or wear resistance became synonymous with super rather than other properties like toughness, but this is where we have ended up.
Pre-Internet History
Unfortunately, searching through magazines and books that predate the internet is not as easy as searching through bladeforums. However, I did find one reference from Outdoor Oklahoma 1978, where a very modern sounding description of “super steel” is found:
“Some hunters are a bit reluctant to opt for super steels because these have a reputation for being hard to sharpen. It’s true, good edge holding qualities go hand in glove with hard steels and hard sharpening. Some steels, especially stainless…”
And that’s where my free view through Google Books ends. Reading through the descriptions of super steels on bladeforums I find similar descriptions to this day. In 1978 basically none of the current “super steels” were even in existence, meaning that the article was likely referring to 154CM, 440C, or both as super steels. Those steels are not called “super” any more which again confirms that steels tend to lose their super title over time.
Edit 6/27/2022: I found an even earlier reference to the term “super steel” though the above quotes are still great because they reflect a similar mentality to today. This new reference I found is from the first issue of The American Blade Magazine (now Blade Magazine) from 1973 in an article by John Wootters called “Blades for Game”: “In this day of super-steels, there is no reason why a hunting knife shouldn’t have a hollow-ground blade. Such a grind offers less drag in meat-slicing and is easy to keep razor-sharp. the higher the hollow-grind bevel lies on the blade, however, the less “spine” or strength the blade will have, and the less abuse the knife can be expected to stand. If the steel is not absolutely top quality, however, a flat bevel offers more resistance to edge-chipping.”
The second issue of American Blade Magazine in an interview with Ted Dowell mentions that “He is still field testing the new ‘super stainless,’ 154-CM, and remains unconvinced although he offers it as an option to those who want it.”
Is “Super” a Positive or Negative?
Even in that 1978 article it was stated that some don’t want super steels because of difficulty in sharpening. The sometimes negative connotation of super steels as being nothing more than a “flavor of the month” or being too difficult to sharpen continues to this day. Therefore, it is not clear to me if the term super steel was originally coined as a negative or positive description. Many discussions on bladeforums about super steels continue to be about whether we need the so-called super steels or whether the old classics are good enough or even superior [7][8][9].
Current Views of Super Steels
While many decry the super steels as being unnecessary, the conflation of “high wear resistance” and super, or premium steel, continues. For example, in the Knife Informer article rating knife steels [10], the steels are categorized from “Super Premium” down to “Low End” with the differentiating property being edge retention. See this article for more information on articles that rate and rank steels. Because these steels are viewed as being superior, they are often perceived as also having high toughness despite their high wear resistance. In the linked article on steel ratings I pointed out that M390 is often given high scores for toughness despite Bohler not providing any toughness data on the steel. Toughness testing here at Knife Steel Nerds has also found unspectacular toughness values for M390, though it has only been tested at relatively high hardness:
The reason why almost any steel will have relatively low toughness that is designed for very high wear resistance is the large amount of carbide that is present in the microstructure. You can read more about the effect of carbides on toughness in the article I wrote on microchipping and in t
he summary of edge stability theory
Tests of Knife Edge Toughness
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In an earlier article I wrote about the microscopic mechanisms by which chipping and micro-chipping occurs in edges. However, that article did not cover specific tests of edge toughness. Correlating conventional toughness tests with edge toughness is difficult for many reasons:
- Fracture toughness tests do not incorporate the effect of crack initiation which is the primary mechanism by which micro-chipping occurs, and is part of the energy required for macro chipping. Unnotched and to some extent c-notch or u-notch impact tests do include crack initiation, however.
- Toughness test specimens are generally much larger than the tip of a cutting edge. Therefore, the statistical occurrence of large carbides or inclusions that act as crack initiation sites is very different [1]. Other small microstructure features such as retained austenite may have different effects in such a small volume.
- The small cross section of a thin edge means it is likely to deform rather than chip even at relatively high hardness (see the article on bending of steel for more information).
- The loss of sharpness through micro-chipping and gross fracture through chipping likely occur through somewhat different mechanisms (see my article on chipping for more information). For micro-chipping it may be useful to perform actual sharpness tests to measure the degradation of the edge.
- Measuring the effect of edge thickness, edge angle, shape, etc. is obviously not possible without a test of actual edges.
Factors that Affect Toughness
Higher hardness, impurities, retained austenite, larger grain size, greater carbide volume, larger carbides, and smaller spacing between carbides all reduce toughness. Thicker edges and more obtuse edges require more energy for deformation or fracture. However, determining the extent of influence of each of these different variables requires edge toughness testing.
Tests of Edges
Tool Steel Simplified
In Tool Steel Simplified they describe a process for impact testing of edges [2]. They used a 3/8″ piece of M2 with a 45° single bevel edge:
They then impacted the edge with a pendulum at different heights. As shown below the knife was held at an angle relative to the impact. At lower impact energies the edge would deform, they increased the energy until the edge would deform sufficiently to fracture. Greater height meant higher impact measured in inch-lbs:
They then produced a plot comparing edge toughness against tempering tempreature for the M2 steel austenitized at 2225°F:
Highest toughness correlated with the lowest hardness, and showed a great decrease in the initial secondary hardening range (800-1200°F) due to increase in hardness and the transformation of retained austenite (see my article on tempering). They compared the knife edge toughness against several other measures of mechanical properties:
The comparisons were made with torsion tests which are simply twisting a bar of steel until it fractures. The strength required to break is the torsional strength. The strength required to yield, or begin plastic deformation, is the elastic limit. Torsion impact toughness is a rapid twist of the steel so that toughness is measured. Poor correlation was found between the edge toughness test and torsional strength, torsional elastic limit, and torsional impact toughness, with a strong correlation with torsional ductility. Torsional ductility is the amount of twist a bar undergoes prior to fracture, and is similar to strain in a tensile test. However, there are other tests that they did not compare with, such as pendulum impact tests or fracture toughness. Fracture toughness, for example, shows a peak in toughness at a similar tempering temperature [3]:
I don’t know if they ever continued the use of this toughness test. It was removed from the 4th edition of Tool Steel Simplified, probably in part because all references to torsion testing were removed from the 4th edition. No other steels, edge angles, etc. are reported in the book.
Agricultural Blades
Impact tests have also been reported for agricultural blades [4][5] which are single bevel as well:
A test was performed with a pendulum impact tester using simple single bevel blades to mimic the cutting edges of the agriculture blades. The test overall is very similar to that described in Tool Steel Simplified. The hammer was dropped from an unspecified height and the degree of deformation was measured. I emailed the primary author of the study for more information such as the energy used of the impact tester, what type of impact hammer was used, orientation of the sample relative to the hammer, etc. but he wouldn’t tell me anything. However, it appears that the amount of deformation and/or chipping of the edges was relatively significant:
When comparing different steels it is apparent that hardness is an important parameter, presumably because higher strength would reduce the amount of deformation that occurs in the edge. However, there are still differences between steels independent of hardness:
Almost all of the steels that were tested are medium carbon steels meaning they are unlikely to have much in terms of carbides while 1.3243 is a high speed steel which does have some primary carbide making fracture initiation easier. Perhaps that is why XAR650 showed superior behavior to 1.3243 despite its lower hardness; the high speed steel fractures more easily due to carbides. However, none of that is confirmed or described in the referenced paper.
The researchers also tested a range of different edge angles where, as expected, a more acute edge led to more deformation during the impact testing.
Edge Stability
Edge stability tests were designed by Roman Landes and reported in different publications but most prominently in his book [6]. The test was conducted by pressing a 2 mm Titanium Nitride coated rod into the knife edge with a load of 1 kg for 10 seconds. The initial tests were performed with 20° single bevel edges:
A series of 10 indentations were performed on each edge and the extent of deformation or chipping was measured. The tests were performed on a range of steels and heat treatments to test the effect of steel properties on “edge stability,” or the resistance to deformation and chipping. In the future I am going to write a couple articles on edge stability so I’m not going to go in depth on the edge stability test in this article. However, I think it was important to mention it because it would be strange not to bring it up.
Future Research
I think the impact tests of edges show promise for quantitatively characterizing edge toughness. I think doing the impacts in combination with a sharpness test would help to characterize the effect of impacts on the micro-scale. Perhaps tests could be separated into different regimes such as sharpness loss, deformation, chipping, etc. Tests would be necessary to determine where those dividing lines might be. With a well designed test it would be possible to explore what steel property and edge geometry parameters are most significant for edge toughness. Some have asked what is the point of toughness testing if the knives are designed simply for cutting and not for any heavy chopping. However, edges also lose sharpness due to micro-chipping or rolling, particularly in thin edges which are desirable for superior cutting ability. The combination of an impact test with a sharpness tester is appealing for measuring the effect of small impacts on sharpness. Also multiple low energy impacts could be used to simulate sustained use. I’m currently exploring my options for purchasing a small impact tester to start on edge toughness tests. With such tests we can determine what correlation there is, if any, between charpy impact tests and edge toughness testing. The number of knifemakers I have convinced to make charpy toughness specimens for me to test is still relatively small but maybe it would be easier to convince them to make simple knives since that is what they are used to producing. To see some of the charpy impact testing we have done you can read the article on Cruwear toughness and
the article on CruForgeV toughness
Toughness testing – Cru-Wear, Z-Wear, Upper vs Lower temper, Cryo vs No Cryo
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I recently completed some toughness tests on samples that were heat treated by knifemaker Warren Krywko. The steel was donated by Chuck Bybee of Alpha Knife Supply. The samples are subsize unnotched charpy specimens with dimensions as specified on the bottom of this page: http://knifesteelnerds.com/how-you-can-help/ If we can get more people to make toughness specimens we can have more comparisons between steels, hardness points, heat treatment parameters, etc. Patreon dollars are for the purpose of paying for machining, shipping, testing, etc. for tests like toughness and CATRA edge retention, so if you are able to contribute that way please visit the Knife Steel Nerds Patreon page.
Warren likes to use CruWear and PM Z-wear in his knives because he likes its combination of good toughness and wear resistance along with high hardness. Both are copies of the older VascoWear developed by Vasco, the company that James Gill worked for who I wrote about here: The Development of High Vanadium Steels, M4 and the First Tool Steels Book. James Gill didn’t develop VascoWear but I won’t be writing about the history of VascoWear in this article. Maybe another time. It is also sold by Carpenter as PD-1 and probably other companies under other names.
Warren and I discussed comparing low vs upper tempers (400 vs 1000°F), with and without cryo, and the ingot version vs the powder metallurgy (PM) version of the steel. Crucible shows a significant increase in toughness for the PM version [1][2] so we wanted to see if our toughness testing shows similar behavior. We also saw a big difference between them:
This makes sense because of the great refinement of the carbide structure from the powder metallurgy process. You can see a comparison between micrographs of ingot and PM 154CM in this article:
Micrographs of Niolox, CPM-154, and AEB-L
You should make a Facebook page
Just out of curiosity, did you ever test ztuff with a high temper. At one time I did some testing, and found it to be much tougher if tempered at around 1000 degrees. This was compared to tempering at 400 degrees!
I haven’t tested the high temper on Z-Tuff.
I think it would score quite a bit higher on you’re charts! Maybe in the future I could heat treat a few samples and send them to you. I’m interested personally!
Why is there a huge discrepancy with 14c28n toughness results. Older charts Say it’s a 3. I’ve abused my Kershaw blur since 2013 and never had issues. You have it at a 9 I think. Which is fantastic
Is DC53 tougher than 14c28N? Google ai says that it is. But it explains that DC53 is twice as tough as D2. Which D2 is not that tough. As far as I understand, 1.4116 is tougher, and I found a chart that says it’s rated as a 2.5 out of ten. When 14C28N is a 9 out of ten.
I have not tested DC53 but it would be unlikely to be tougher than 14C28N. I wouldn’t bother asking any AI these types of questions. My tests for A2 were around 15 ft-lbs. A2 has the same carbon but 3% lower chromium, which would likely give it an advantage in toughness over DC53.