Ice Age But Rated R
Marthe Bernskoetter
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Pilkington Pyrostop is a fire-rated and impact safety-rated glazing material that blocks radiant heat, protecting people and valuables on the non-fire side of the glass where radiant heat transfer might be a concern. It is listed for use in doors, sidelites, transoms, borrowed lites and wall applications.
Thanks to Bill Smutz, Alex Topfer, Florian Bachler, Brunhard, Art, Rod H, Sach, Jinny Koh, Jon Duda, Cory Henderson, and UPKnife for becoming Knife Steeel Nerds Patreon supporters! And Michael Fitzgerald, Tim Marais, and Head VI for increasing their contributions. All of the experiments shown below are possible thanks to supporters.
I also have a video that summarizes some of the information below while also showing how some of the experiments work. Lots of information is still specific to this article, however. I think they are complementary and you should watch/read both.
I have a (relatively) short introduction before getting into the ratings with a few important things to put them into context. That way you can get into the steel ratings quickly. Most of the discussion of how the ratings were generated, various caveats and details, etc. are after the ratings. If you want to learn more than keep reading past the ratings.
Another important caveat before we get to the ratings are that these are for the steel only. This does not predict which knife will cut longer or be more resistant to chipping. The reason is because sharpening and edge geometry will also greatly control properties. For example, see the chart below for how much edge retention can change with edge geometry for a single steel (in this case 154CM and CPM-154). Using 10 dps sharpening (20 degrees inclusive on the chart) leads to about 5x the edge retention of 25 dps.
High alloy tool steels are designed to be air hardening, so they can be cooled even slower than the oil hardening steels found above. This is good for ease in heat treating in large batches and for even cooling that greatly reduces warping and size changes. High Speed steels are a subset that have significant additions of Mo and/or W that makes them resist softening when they are used for machining operations. The big difference in properties vs the low alloy steels, however, are the harder carbides that are found in these steels. Vanadium carbides are among the hardest that form in steel, and chromium carbides are in between iron carbide and vanadium carbide. Steels with very high vanadium content like Vanadis 8, CPM-10V, K390, CPM-15V, etc. have extremely high edge retention. Maxamet and Rex 121 are so extreme in terms of wear resistance and edge retention that I rated them higher than 10 because otherwise it throws off the ratings for everything else. Powder metallurgy steels with low vanadium content like CPM-1V and Z-Tuff/CD#1 have extremely high toughness. The best steels with balanced properties include 4V/Vanadis4E, CPM-CruWear, and CPM-M4. My favorites of the high edge retention group are Vanadis 8 and CPM-10V.
You can read about my CATRA edge retention testing in this article. Each steel was tested with a knife that was produced just for the test, and then sharpened the same way for each test (15 dps 400 grit CBN sharpening). A few steels have been added since such as MagnaCut and M398. I also added a few more steels in this study. The studies confirmed that the primary controlling factors are hardness of the steel, volume of carbides, and hardness of the carbides. The highest edge retention steel was Rex 121 which was at 70 Rc in combination with lots of high hardness vanadium carbides. We can predict edge retention of a steel within a relatively narrow band based on hardness and carbide volume. We should be suspicious of anyone who is claiming very high edge retention with a steel at low hardness and a small amount of carbide. The chart below has dotted lines which indicate the average effect of hardness for any given steel. So you can estimate how much a change in hardness would affect edge retention by following the slope of those lines.
In some previous articles I have shown the balance between my toughness and edge retention measurements such as in the following chart, where the high alloy non-stainless are in orange and the blue are stainless:
However, one issue with these charts are that difference in toughness is that a linear scale for toughness is a bit misleading for visualizing practical toughness differences. If you look at the chart you may notice that at high toughness levels if you increase edge retention by only a relatively small amount you get very big drops in toughness. For example, increasing edge retention from Z-Tuff to 3V (100 mm in the CATRA test) led to a drop in over 10 ft-lbs, a similar drop is seen by going from 3V to CPM-CruWear. But then if you look at an increase of 100 mm in the CATRA test from Maxamet to Rex 121 the toughness only drops 1-2 ft-lbs. However, the relative difference in toughness between these different examples are similar. When we plot toughness vs edge retention on a log scale instead we get a straight line that is a better visualization of toughness differences. This is the basis on which I do the ratings rather than a linear scale.
Powder metallurgy is a technology designed to maintain a small carbide size. Read more about how it works here. It is most useful for steels with large amounts of carbide but also helps to be able to add certain carbide types. Vanadium carbides are very large with conventional production of steels but are very small with powder metallurgy. With conventional steels this limited vanadium additions to about 4-5%, and this was greatly expanded when powder metallurgy was developed. The biggest change that is seen with powder metallurgy in measured properties is in regards to toughness. Below shows a comparison of carbide structure between D2 and CPM-D2, and then toughness measurements between the conventional and PM versions of CruWear, D2, and 154CM.
With steels that have a small amount of carbide the size of the carbides can be kept small through processing (see the AEB-L micrograph earlier in the article). Most low alloy tool steels and carbon steels also have fine carbide structures without powder metallurgy processing. Therefore powder metallurgy is not necessary for certain steels, or could even be slightly detrimental. As wear resistance is increased the differences between conventional and powder metallurgy steels become greater.
Corrosion is not just about cosmetics and rusting, however, but can also affect edge performance. I did a test with knives in 440A (stainless), D2 (high alloy steel with some corrosion resistance), and 1095 (no corrosion resistance). I dipped each in lemon juice and left in open air and tested after 30, 100, and 300 minutes, dipping in lemon juice again each time. There was significant sharpness loss with 1095, almost none with 440A, and D2 was in between.
Austenitizing is the process where the steel is heated to high temperature prior to quenching (rapid cooling) to harden the steel. If the steel is overheated in austenitizing, very large reductions in toughness are possible. See the chart below showing 52100 steel that was overaustenitized (unintentionally) by a knifemaker that sent me specimens for toughness testing. Using controlled furnace heat treating resulted in toughness around 23-28 ft-lbs at 61-62 Rc, while the knifemaker heat treated specimens were 7 ft-lbs or below.
This high temperature tempering can be done for several reasons, such as better resistance to overheating during grinding, or because a coating will be applied to the knife that requires a high temperature. However, in our testing there is a reduction in toughness by using the high temperature range rather than the low temperature range, such as was found with CPM-CruWear (Z-Wear) or CPM-10V. The 10V specimens tempered at 1000F were 4-5 ft-lbs while the specimens tempered at 4-500F were 7-8 ft-lbs.
Typically an increase in corrosion resistance means a reduction in potential hardness for a given steel. This was described in this article on Vanax heat treating. Non-stainless steels can be heat treated to 66 Rc or even higher depending on the particular steel. Stainless steels usually top out around 64 Rc and may require careful heat treating to get there. The ultra high corrosion resistance steels Vanax or LC200N max out around 60-61 Rc instead. A cryo treatment and close temperature control is necessary to achieve those hardness levels. The majority of knives target 63 Rc or below so this limitation of stainless steels does not always come into play but can be an important factor for certain knives targeting high performance and thin edges. Below shows approximate maximum hardness vs stainless rating for several stainless knife steels. This is about comparing steels to each other rather than a limitation of an individual steel. In other words, heat treating a steel to its maximum hardness does not necessarily mean reduced corrosion resistance.
The biggest factor for cost of knife steel is whether it is produced with conventional ingot technology or powder metallurgy. However, there are other factors. Some steel companies charge more than others. Some steels are more difficult to manufacture for the steel company or have more expensive alloying elements so the cost is increased. Importing steel from Europe to the USA, or vice versa, generally increases the cost. Steel produced in China is generally less expensive. Poor availability may effectively increase cost of steel. In many cases the cost of working with the steel for the knife companies is more significant than the cost of the steel itself. In a pocket knife the total amount of steel is rather small. However, high wear resistance means that abrasives are used up more rapidly, more careful grinding is necessary to avoid overheating, finishing and polishing is much more time consuming, etc. High toughness steels can be produced without powder metallurgy and also have low wear resistance for lower manufacturing costs. High wear resistance steels are more expensive to buy and to process, especially since many require powder metallurgy. You can read an article I wrote on budget steels here.
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