Steel choice and you
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There have been a couple of posts lately about the variety of steels available and their use in cutlery. I've participated in a couple of these discussions and Larrin Thomas always come up (ok usually I'm bringing him up). If you don't know who Larrin Thomas, he's the owner of [Knife Steel Nerds - Metallurgy and Testing of Knives and Steel](https://knifesteelnerds.com/) , he's a metallurgical engineer who has been studying knife steel metallurgy for a long time. I own his book, Knife Engineering, available on Amazon, and have been reading his site ever since I got interested in buying a better kitchen knife. I want to post a few of the charts he's known for and give you a bit of interpretation. I will only be looking at steel and stainless steel, and leave out high-alloy or semi-stainless steel.
First 2 charts
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What can we tell from these 2 charts? First, pretty much every low-alloy steel has relatively low toughness at nearly any hardness, and they are all so close together it's not even worth separating them out. The sole exception in regards to kitchen knives is 52100, because of it's alloying it has significantly higher toughness than any other low-alloy steel, in fact it's also tougher than any stainless steel other than AEB-L and 14C28N (which is a Nitrogen modified version of AEB-L). The only makers I know of using 52100 are Zwilling Kramer and Shihan, or various custom makers. So if looking at a low-alloy steel Japanese gyuto, toughness is nearly the same among any of them at a given hardness. Obviously as hardness increases, we can see from the graph that toughness will trend down. Now I'm interpreting some of the Hitachi paper steels here, Shirogami #2 isn't represented, but it's close enough in composition to 1095 and O1 to use those as stand ins. Shirogami #1 is very close to 26C3 in composition, and Aogami #2 is similar to V-Toku 2 (which is a Takefu steel). 1.2519/1.2442 are close to Aogami #1, so that can act as a stand in. Aogami Super is shown already. SK-5 tool steel as used in the Fujiwara FKH is equivalent to 1095, and we can see here, even at an HRC 57, it's not particularly tough.
As far as stainless steels, almost all of them have even less toughness than the low-alloy steels. If you read Larrin's other articles, you know this is a result of large chromium carbides being formed in high Cr alloy steels. These large carbides are hard, but fairly brittle. What about powdered metallurgy steels?
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Well now, we see here that even SG2 isn't much better than VG10, and none of them come close to AEB-L, even the vaunted M390... A really surprising find here is 1.4116, or as we know it X50, used in German knives like Wusthof and Messermeister. See what it's toughness is at HRC 57? Yeah, doesn't look good...
What about edge retention? You want a knife that will hold it's edge forever, right?
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Now, I've said multiple times that Larrin Thomas's CATRA test doesn't really apply to kitchen knives, the test medium is too abrasive because it contains silica. Here we can see that none of the low-allow steels beat any stainless steel, not even AEB-L which has the lowest edge retention of any stainless steel. Not even Aogami Super beats it. Every single stainless steel scores much higher than any low-alloy steel. However, among the stainless steels, there's not a lot of separation. Look at where 154CM and VG10 lie. 154CM has a Hitachi clone in ATS-34, which is used by Manaka and Hinoura (I believe), it barely beats VG10, and I'd gather that the difference is so close, being less than 10%, that you wouldn't notice it in practice. Let's look at some edge retention tests for SG2.
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Now here Larrin uses percent of edge retention in relation to 440C, which is the baseline at 100%. SG2 is around 130% of 440C, while CPM-154 is around 120%, CPM-154 being the PM version of 154CM. M390 is 170% of 440C, but as we saw earlier you get that at the penalty of very low toughness. How do these relate to low-alloy steels?
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Oh my, even 52100 which has a good balance of traits is rated at only 50% of 440C in edge retention, so does O1.
So what's my point in all of this? When we talk about steels, in practice, there aren't a lot of performance differences between low-alloy steels at the same hardness and between stainless steels at the same hardness. As I've learned more and tried more knives it really does come down to 2 basic decisions. First, stainless or not? If not, carry on and buy based on other factors besides steel choice. If stainless, ease of sharpening or not? If yes, pick a Swedish stainless steel or similar (AEB-L, 19C27, Ginsan), or a powdered steel in the same class as SG2 (SG2, SRS15, Elmax, CPM-154, S35VN). If the answer is no, you are probably going to end up with VG10, 440x, AUS-8/10, or similar. Actually the latter is probably more driven by budget than steel choice anyways.
For all practical purposes, as it was told to me, most people should probably pick a stainless steel knife. Those of us that own carbon steel knives have to realize that the steel's performance in edge retention and toughness are a very small part of the decision making process.
Thanks to u/fiskedyret for always answering my dumb steel questions. Thanks to u/ironballoon for the meme, I totally stole it. Also thanks to the other members of r/chefknives here, I've learned a lot, and spent way too much money since I've been here.
Edit: Since u/Duukt asked about HAP40, I decided to add to the post here. There isn't anything in Larrin Thomas's chart that's exactly like HAP40, the closest I could find is Crucible CPM-4V.
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If HAP40 follows 4V, it's got equivalent toughness to CPM-154, and only around 20% better edge retention in relation to 440C. This comes at the cost of not being fully stainless though. In general the semi-stainless steels like A2/SKD-12, D2/SKD-11 will fall somewhere in between low-alloy steels and stainless steels. The more Chromium you trade away, you tend to gain toughness but lose out on corrosion resistance. D2 is almost fully stainless, and has about the same level of toughness of other stainless steels (not very good), while A2 is similar to Aogami Super in performance and toughness. Takamura Chromax is also a variant of A2, at least compositionally. Might as well add in this chart.
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Edit 2: I'm wrong, Larrin Thomas is a metallurgical engineer, not a mechanical engineer. My apologies, and fixed.