Category: Steel and Knife Properties
2 thoughts on “Steel and Knife Properties”
Leave a Reply
Cryogenic Processing of Steel Part 3 – Wear Resistance and Edge Retention
Thanks to Ian Rogers for becoming a Knife Steel Nerds Patreon supporter!
Intro to Cryo and Wear Resistance
In Cryogenic Processing Part 1 I covered the effects of cryo on retained austenite and hardness. In Cryogenic Processing Part 2 I looked at the studies on cryo and toughness. Wear resistance is the most controversial aspect of cryogenic processing of steel. In particular there are claims that the use of cryogenic processing (liquid nitrogen) leads to an improvement in wear resistance that is not found with subzero processing (dry ice). Sometimes it is claimed that cryo can lead to massive increases in wear resistance [1]:
Cryogenic Processing of Steel Part 2 – Toughness and Strength
Thanks to Gator, Russell Dodd, and Matt de Clercq for becoming Knife Steel Nerds Patreon supporters!
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
Cryogenic Processing of Steel Part 1 – Maximizing Hardness
Thanks to Hiroaki Misono and Robert Venable for becoming Knife Steel Nerds Patreon supporters!
Heat Treating and Austenitizing
During heat treatment of steel, the steel is heated to a high temperature called the “austenitizing” temperature where a phase called austenite is formed. Steel has different phases which refer to different arrangements of iron atoms within the steel. Austenite has a different set of properties from the typical room temperature phase of steel. One example of the different properties of austenite is that it is non-magnetic unlike the room temperature ferrite or martensite.
Room temperature iron/steel – Ferrite – Body Centered Cubic Atom Arrangement
High temperature iron/steel – Austenite – Face Centered Cubic Atom Arrangement
After holding the steel at the high austenitizing temperature, the steel is then rapidly quenched which transforms the steel to a phase called martensite which has high hardness. It gains its high hardness because carbon is trapped in between the atoms which makes the room temperature phase martensite as opposed to the soft ferrite.
Normal soft room temperature ferrite on the left and hard martensite on the right
Which Steel Has the Best Edge Retention? Part 2
Thanks to Chad Kelly, Isaiah Schroeder, and mflgrmp for becoming Knife Steel Nerds Patreon supporters!
Background Information and CATRA Curves
Make sure you read Part 1 first so that you understand all of the background information for this article.
Below I have another Youtube video of CATRA testing so that you can see how the curves are generated during testing.
You can get a feel for how differently these steels cut by plotting a few of them together with the same edge angle. The top curve is a high wear resistance steel which cut 835 mm of cardstock after 60 cuts, which is the TCC value (Total Cardstock Cut). The 244 mm is a medium-low wear resistance steel which shows much more sharpness loss than the higher wear resistance steels.
Edge Radius During the CATRA Test
I’m not sure where in the test the average person would decide that the knife needs to be resharpened, but I would guess that it is before cut 60 because the CATRA test wears the edge pretty significantly. In CATRA tests performed by Verhoeven [1] the edge radius was reduced from ~0.5 micron all the way to 3-5 microns after only the second cycle (note he calls the cycles strokes):
The tests performed by Verhoeven were with low wear resistance steels (AEB-L, 52100, 1086, and Wootz) but it still shows the relatively significant wear that occurs with the CATRA test. In the CATRA article on 154CM, it was found that the edge width (rather than radius measured by Verhoeven) was increased to 23 microns with a 20° edge angle and the 50° edge to 17 microns, which is quite dull. A ten micron edge width has been reported previously as a dull edge that needs sharpening [2].
Regression Factors Analysis
In part 1 I described the process by which we calculated the relative factors that affect edge retention which resulted in the equation below. CrC is a general term to refer to either Cr7C3 or Cr23C6 chromium carbides. CrVC is a general term to refer to M7C3 where M can be either Cr or V; when vanadium is added to a high chromium steel the chromium carbides are enriched with vanadium which increases the hardness of the Cr carbides. MC can refer to either vanadium carbides (VC) or niobium carbides (NbC). MN can refer to either vanadium nitrides (VN) or niobium nitrides (NbN). CrN refers to chromium nitrides. The formation of these particles is controlled primarily by the composition of the steel and secondarily by processing and heat treating. The equation below and the tables in Part 1 and here in Part 2 come from journal articles and books that have reported the carbide fractions after heat treatment.
TCC (mm) = -157 + 15.8*Hardness (Rc) – 17.8*EdgeAngle(°) + 11.2*CrC(%) + 14.6*CrVC(%) + 26.2*MC(%) + 9.5*M6C(%) + 20.9*MN(%) + 19.4*CrN(%)
Carbide Hardness
Taking an average value of the hardness of each carbide type we can compare between carbide hardness [3][4] and the calculated coefficient. Below I have plotted the carbide hardness in vickers (Hv) on the x-axis vs our calculated coefficient in the equation above for the relative contribution to edge retention for each carbide type. We get a very good correlation, demonstrating that carbide hardness strongly controls the effect of a carbide on slicing edge retention:
The coefficient for VC (listed as MC in the equation) is somewhat higher than the nitrogen version (VN or MN) despite their reported similar hardness [4]. Either the VN is actually somewhat softer or this is due to the VN coefficient being based on only two CATRA tests on one steel (Vanax 35). Either way it appears to be qualitatively accurate. Another possibility is that there may be some formation of V2N or chromium carbide/nitride which is lower in hardness than VN. The value for CrN also falls off the trend line of the others but that value comes from only one steel, Cronidur 30, and would likely change if further tests were performed. The M6C value is based on only two steels, CPM-M4 and M2, which both get wear resistance from VC so the accuracy of the M6C coefficient could definitely be improved with other high speed steels. No simple carbon steels with Fe3C cementite were tested which would be nice to add to the regression. Due to the low hardness of cementite it would be expected to have a relatively low value. This is confirmed by the Verhoeven study comparing 52100, 1086, and AEB-L where AEB-L with chromium carbides had superior edge retention to 52100 and 1086 with cementite [1]. If we extend the trendline in the plot above to the hardness of cementite we would estimate a coefficient of 5, or about half of chromium carbide. Experiments would be necessary to confirm that. Another interesting set of tests would be on the low-alloy tungsten steels such as the Blue series, V-Toku series, F2, O7, etc. The tungsten carbides in those steels are reported to be the very hard WC so it would be nice to know if that carbide improves edge retention to the same extent as VC. They are relatively niche steels so they have not received as much study as many tool steels and stainless steels. I wrote about these low-alloy tungsten steels
in an earlier article on this site
Which Steel Has the Best Edge Retention? Part 1
Thanks to Mark McKinley and Alfredo Faccipieri for becoming
Knife Steel Nerds Patreon
Rockwell Hardness is the Megapixels of Knife Steel Specs
Thanks to Michael Drinkwine for becoming a Knife Steel Nerds Patreon supporter!
Hardness and Megapixels
In the early-to-mid 2000’s with digital cameras and somewhat more recently with smartphone cameras we had the battle of megapixels. The number of megapixels is simply the number of pixels that are captured by a digital camera. When we had 0.3 megapixel cameras the pictures were quite blurry and jumping up to 2 or 3 megapixels made a big difference. However, when comparing 5 to 7 megapixels the quality of the image was much more likely to be controlled by the quality of the lens and sensor than simply the number of megapixels. Despite that, megapixels became an easy marketing point because it is a simple number to present to the public. We haven’t seen rockwell hardness climbing for no reason other than marketing, but it is one of the few simple numbers that are used to advertise for a knife. Therefore it is often misunderstood by knife buyers, and yes, even some knife makers. In this article I cover some simple reasons why hardness is not as important as other factors for predicting most steel properties. And then we get into the nitty gritty with why hardness is not always the same as strength and how heat treatment can affect strength independent of hardness.
Intro to Rockwell Hardness
Rockwell hardness is a simple test for checking the relative strength of materials. It works by indenting steel with a fixed load and measuring the distance that the indenter travels into the steel. It is commonly used by knifemakers, heat treaters, and knife companies. Often the hardness value or a range of hardness is given along with a knife, ie 58-60 Rc or 59 Rc. Hardness correlates well with strength, which tells us how resistant the material is to permanently deforming. With thin edges on knives, resistance to deformation is important to avoid rolled edges. Higher hardness also correlates with higher wear resistance and lower toughness. However, there are times when this hardness value can be misleading. I have summarized a few of those cases below:
Toughness
Higher hardness/strength reduces the toughness of steel. Toughness is the resistance to fracture or chipping which you can read more about in this article. However, there are more factors that control steel toughness than just hardness. For one major example, the amount and size of carbides present in steel greatly controls the toughness of steel, as carbides are very hard brittle particles that promote fracture [1]. Therefore, hardness cannot be used as a proxy for toughness. A 62 Rc steel of one type may be tougher than another at 58 Rc.
Even if we focus on only one steel type, different heat treatments can lead to different levels of toughness at the same hardness. If a steel is austenitized at too-high a temperature, toughness can be greatly reduced. You can read more in this article on austenitizing. In our toughness testing of CruForgeV, we even found a rather sharp drop in toughness even when using an austenitizing temperature recommended by the datasheet, from 14 ft-lbs with 1500°F austenitize to less than 2 ft-lbs with 1550°F:
Wear Resistance
Higher hardness increases wear resistance [2][3][4][5]. However, just like toughness the carbides cannot be ignored. The high hardness of carbides leads to improved wear resistance even when the steel may be at a lower hardness. Wear resistance correlates with slicing edge retention. Therefore, hardness should not be used as a proxy for wear resistance. L6 is a low alloy steel with relatively soft iron carbides (cementite). A2 and D2 contain harder chromium carbides but D2 with its 15% carbide is more wear resistant than A2 which has about 1/3 as much carbide. 10V and 9V have a similar amount of carbide as D2 but have the much harder vanadium carbide for better wear resistance. Cementite is about 1000 on the Vickers hardness scale, chromium carbide 1500 Hv and vanadium carbide 2800 Hv [6]. Note that in the plot below lower is better.
Tensile and Compression Testing
Hardness correlates well with strength. A more complete strength test is called the tensile test where a bar of steel is pulled until it breaks. A compression test is similar but usually uses a cylinder and the steel is compressed until it breaks. The two stress-strain “curves” end up looking similar except for brittle materials which fail prematurely in a tensile test.
There are three things I want to point out in the tensile test: elastic deformation, yield point, and ultimate stress/strength. Elastic deformation is the period where flexing or pulling a piece of steel and letting go allows it to return to its original position. I wrote all about this behavior in Why Doesn’t Heat Treating Affect Steel Flex? At the yield point the steel permanently, or plastically deforms, so that when you let go the steel doesn’t return to its original shape but stays deformed. The stress (load divided by cross section) required to reach the yield point is the yield stress. The ultimate stress is the stress required to break the material. If we think about this in terms of a knife edge being pressed into a rod, we see the same three regions:
- Pressing into the rod and letting go leads to the edge returning to its original shape (elastic deformation)
- Pressing into the rod and letting go leaves a roll in the edge (exceeded the yield stress and permanently deformed the edge)
- Pressing into the rod until the edge chips (exceeded the ultimate strength)
This is all important because we need to understand what the hardness test is showing us. Hardness of steel usually correlates very well with both yield stress and ultimate stress. However, there are certain cases where yield stress and ultimate stress do not correlate with each other. A steel may have a low yield stress so it is relatively easy to deform but a high ultimate stress. In that case which is the hardness test measuring?
Retained Austenite
When steel is heat treated it is heated up to a high temperature where a phase called austenite is formed, followed by quenching the steel rapidly to form the hard phase called martensite. In certain cases a certain amount of austenite is “retained” after quenching.
Learn more about quenching and retained austenite in this article
All About D2 Steel – Development, Use in Knives, and Properties
Thanks to Robert Erickson, knifeandgear_swiss, Dale Bushness, and Paul Hart for becoming Knife Steel Nerds Patreon supporters!
Update 10/22/2020: I now have an article with how to heat treat D2, PSF27, and CPM-D2 and it also includes toughness testing of each steel and edge retention testing of D2. https://knifesteelnerds.com/2020/08/31/how-to-heat-treat-d2-psf27-and-cpm-d2/
=&0=&
D2 is a common tool steel and knife steel. It is also known by other names such as the Japanese designation SKD11, German designation 1.2379, Hitachi SLD, Uddeholm Sverker 21, and many others. How long has it been around? Where did it come from? Who started using it in knives? How do its properties compare to other steels? Find your answers here!
Early Chromium Steels
The development of D2 steel coincides in part with the invention of stainless steel as well as high speed steel. You can read
an article about the history of stainless steel here
The Sharpest Youtube Channel in the World
Thanks to Rusty Craig, Dylan Curtis-Reeve, Brendan Porter, and Ian Cox for becoming Knife Steel Nerds Patreon supporters!
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
Thanks to Daniel Jackson for becoming a Knife Steel Nerds Patreon supporter!
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
What is Edge Stability? Part 2 – The Experiments
Thanks to Gary Creely and Sans Jeux for becoming Knife Steel Nerds Patreon supporters!
Background
Read Part 1 before this article as it covers the ideas behind the Edge Stability theory and how things like hardness of steel, carbide volume, and carbide size are thought to affect knife edges. Then you will have an understanding of what we are looking for in the experiments described below.
Test Setup
Roman developed his edge stability test described below for work done for his thesis: Schneidkantenstabilität von Messerchneiden, or Stability of Knife Edges. The test was designed to measure the resistance of an edge to deformation or chipping from an applied load. The test setup was as follows [1]:
The sample used is seen on the right and the test setup is on the left. He used a Vickers hardness test where rather than an indenter to measure hardness, a Titanium Nitride coated 2mm diameter steel rod was used instead. The small test specimen was held vertically as shown and then the indenter was pressed into the edge with a load of 1kg for 10s. Ten indentations were made at 1mm increments along the edge so that an average could be taken. The length of the half-circle was measured and then converted into a height using the following equation:
In the equation, r is the radius of the rod used to make the indentation. Then edge stability is 1/h using the average arc length, s, of the 10 measurements from each edge. An example of 10 measurements on an ATS-34 knife are shown below:
Edge Stability Tests
Roman tested a series of steels for his Masters thesis and also performed several tests on steel 1.4111 with different heat treatments. Here are the compositions of the steels tested:
For a few of the steels there were edge stability tests performed with multiple heat treatments, others were only given one heat treatment for edge stability testing. That makes it somewhat difficult to separate the effects of steel and heat treatment or hardness. Here are the heat treatments, hardness, and corresponding 1/h edge stability values (higher is better):
Effect of Hardness
Summarizing the tests from the results shown in the table below shows a clear trend is with hardness, where higher hardness meant a higher value of edge stability. This is in line with what was described in Part 1 where higher hardness means higher yield stress and therefore better resistance to edge deformation or rolling:
Effect of Microstructure
As described in Part 1, the carbide structure is thought to be important for edge stability (hard particles throughout the steel that add to wear resistance but detract from toughness). One potential outlier was one of the four ATS-34 tests that had a much higher value than the others (0.037 vs 0.022-0.027) which perhaps shows the difficulty in performing tests on a very small scale. Each test, after all, is deforming only a small portion of a thin edge. Roman speculates that perhaps the edge of the outlier ATS-34 value was in a section of the steel that avoided segregated regions full of large carbides [2]. Most of the steels were tested with only one hardness which makes it difficult to separate the effects of steel and microstructure vs hardness. There is a clear difference between the two 64.5 Rc steels, where one was a 1.1740 (0.64C low alloy steel) and the other 1.2838 (a 1.45C-2.91V alloy steel). The 1.1740 had little or no carbide present while the 1.2838 had some larger vanadium carbides, so one might speculate that the vanadium carbides led to a reduction in edge stability. However, the edge stability of 1.2562 at 63.2 Rc had a high stability value and it has a relatively large volume of tungsten carbides, as seen in the micrograph below [1]:
Micrograph of 1.2562 steel
Plotting carbide volume vs edge stability there is a slight negative trend, but the higher carbide volume steels also tended to have lower hardness. Therefore when the effect of hardness is adjusted for, carbide volume did not have a clear effect on edge stability in this dataset:
A 20° single bevel edge (which was tested) is approximately equal to a 30-40° double bevel edge in terms of cutting ability and may also be similar in terms of edge stability. Therefore it may be that the 20° single bevel edge was too obtuse to clearly reveal differences in carbide volume or size. Roman tells me that other tests were performed on specimens with a 10° edge bevel and that those showed a greater difference between low carbide and high carbide volume steels, with brittle fracture failures with the high carbide steels [2]. However, those tests were not presented in the thesis. A 10° angle with the same load applied would lead to a greater stress applied to the material, and a smaller volume of the edge, which would be more likely to show the effects of carbides.
Roman also reports that while the low carbide steels deformed plastically and had a round indentation, the high carbide steels fractured in a brittle manner indicative of chipping due to carbides. Therefore even if the quantitative measure of edge stability presented here did not indicate clear trends with carbide volume that may be because it is not a perfect representation of the response to the load applied to the edge.
Effects of Heat Treatment
Four tests were performed with ATS-34 with different heat treatments. Two of the heat treatments resulted in ~61 Rc while the other two resulted in ~57.5 Rc, and as expected the higher hardness steel showed superior behavior, even without considering the potential outlier value (0.037 seems unrealistically high). One interesting comparison, however is the two 57.5 Rc conditions, where one was given a low temper at 190°C (374°F) and the other a high temper at 530°C (986°F). The low temper condition showed slightly superior edge stability though it is within the noise of the test so I’m not sure we can conclude that a low temperature temper is superior in terms of edge stability based on that result.
Tests on Steel 1.4111
A broader set of tests and heat treatments were performed on 1.4111 which is a 1% carbon, 15% chromium stainless steel. The heat treatments performed along with the measured hardness and edge stability are shown below:
One surprising result of the series of heat treatments performed with 1.4111 was that no clear trend with hardness was observed in the edge stability tests:
Looking at the specific heat treatments it is apparent that part of the reason for this unexpected behavior is that higher hardening (austenitizing) temperatures led to reduced edge stability. Higher austenitizing temperature generally means higher hardness, so if a higher austenitizing temperature leads to a reduction in edge stability that may lead to a flat or negative trend with hardness. Here is the effect of austenitizing temperature on edge stability:
Higher austenitizing temperatures usually leads to higher hardness but lower toughness (see Austenitizing Part 2) because of grain growth and higher carbon in solution, which may indicate that the reason that higher hardness 1.4111 did not show superior edge stability was because of a reduction in toughness. Another potential effect on edge stability is retained austenite, as a drop in hardness was observed at the highest austenitizing temperature, which is generally because of excess retained austenite. So despite the use of cold treatment at -80°C, retained austenite was present in the microstructure of at least the 1100°C heat treatments, and likely a somewhat smaller fraction in the lower temperature conditions. Retained austenite lowers the yield stress even when at the same hardness [3] which could possibly lead to more deformation in the edge stability test. The yield stress is the stress at which a material begins to permanently deform, and the tensile stress is the peak stress a material reaches prior to fracture. Hardness typically correlates most strongly with tensile stress, but perhaps the more localized nature of the edge stability test leads to more sensitivity to yield stress.
The 1000°C austenitizing conditions were actually a double hardening treatment, where they were first austenitized at 1100°C and quenched followed by a second at 1000°C. Double austenitization treatments like this sometimes show superior toughness because the first high temperature austenitize can dissolve some more carbide (less carbide means better toughness) but the second austenitize gives lower carbon in solution. You can read more about these types of heat treatments in Austenitizing Part 3. However, the double austenitizing heat treatments still fit the trend of the effect of austenitizing temperature if the value of 1000°C is used in the earlier plot. In other words, the initial high temperature treatment doesn’t seem to have made a difference.
One effect of heat treatment that appears to have a little simpler behavior is different tempering temperatures. Reducing the tempering temperature increased both hardness and edge stability. Perhaps the reduction in tempering temperature was not as deleterious to toughness as the higher austenitizing temperature and therefore the higher hardness effectively increased edge stability. Alternatively, changing the tempering temperature within this range likely had no effect on retained austenite, so if it was the retained austenite that led to poor behavior with high austenitizing temperatures, that high retained austenite problem wouldn’t have been present with the change in tempering temperature.
Conclusions and Summary
The edge stability test is interesting in that it is a test of an edge rather than a bulk toughness or strength test. Knife edges are very small relative to bulk specimens and therefore behave differently. Adding more steels and heat treatments to edge stability testing would certainly add more to our understanding. The effect of different applied loads and different edge angles are also good areas for future study, if someone chooses to take up the test method and carry it further. Roman reports that many more tests were performed that were lost in a hard drive crash, which is unfortunate. Hardness appears to be the strongest controlling factor for edge stability. There was not a clear trend with carbide volume though that may be because of the stability of 20° single bevel edges relative to double bevel edges, or perhaps because the quantitative measurement method does not capture brittle vs ductile behavior. Austenitizing at temperatures that were too high led to poor edge stability either because of reduced toughness or high content of retained austenite. Tempering at lower temperatures increased both hardness and edge stability, at least within the narrow range that was studied.
In order to ensure that this valuable research is not lost, I have uploaded copies of both the thesis and paper on 1.4111 (with permission) and you can download them below:
Thesis Part 1 – Schneidkantenstabilität von Messerchneiden
Those photos and drawings will help people understand ! Orientation – I was curious about what Crucible does with that .Their CPM goes to Niagra Metals for rolling and they cross roll the steel. Cut into squares and rolled alternating direction to minimize directional properties. I think Matt Gregory found that in his visit.
It would be interesting to test the transverse toughness of those steels for sure.