ive read that at 500°C or so the you convert some of the retained (or all?) austenite to martensite (in hss and ss), is this instead of multiple cryo? will it give the same effect as cryo (well without the corrosionresistance that is) on hardness and toughness?
When you temper high enough to induce the alloy carbide to precipitate it changes the composition of the austenite and it can thus transform to martensite as it cools. This martensite is tempered when this is repeated which also causes more martensite (but a leser amount) to form in the post temper cooling. This is why the highly alloyed HSS steels have 2-4 tempers usually recommended. Low tempers can also induce austenite to form bainite depending on the composition of the steel, you predict it from the TTT curves. Cold is a different process because it continues the quench directly and thus allows martensite to form without the secondary carbide precipitation. This prevents the decrease in edge stability and corrosion resistance which comes with high temper martensite formation.
I just did a blade at 2000 F, forced air and water and got 62+ and picked up one more pt with the cryo cycle. Final hardness is 60 with a 450 temper with water quench out of the temper oven.
I would be interested in a 300-350C temper and how the edge responds at a higher hardness. The carbide fraction in that steel at that austenization temper is quite low, about 2%, and it should be forming mainly lathe martensite so it should be very tough compared to steels like 154CM. Thus at a given toughness requirement it should be able to be ran harder. This will also enhance ease of sharpening and edge stability as the structure coarsens with tempering.
I would actually be really curious as to how AEB-L at 62/63 HRC compares to 1095 at 65/66 HRC in fine profiles at highly polished edges. Verhoeven notes that AEB-L does better in an wear based comparison, however the 1095 is at 60 HRC which is significantly underhardened. Landes work implies that AEB-L would be higher, he doesn't actually reference 1095 but does describe the performance of similar steels low alloy high carbon steels and they are actually behind AEB-L in those respects though still quite high.
So I may be honing in on the optimum but hard to really know without a X ray diffusion scope to look at RA...
Scattering and crystal structure magnification examination will fail to be productive beyond a certain point, the most sensitive measurements are on the size change due to the conversion which is used to check for small refinements. But of course what you are really insterested in is to see if there is any practical difference in sharpening, maximal sharpness, corrosion resistance, durability, edge retention, etc. . .
I had kind of thought by all of the hype that Cliff has been giving AEB l there would be more of a difference, more like S30v performance, or at least closer to the CPM 154.
Nowhere have I suggested that AEB-L would have extended slicing aggression comparable to S30V, I also noted specifically recently to Larrin that 154CM (P/M or ingot) would not be an alternative to AEB-L as they are two completely different steels. AEB-L is optomized for high edge stability and 154CM for extended slicing aggression. They thus have very different behaviors as I explained in detail in the article on modeling cutting ability. The main benefits of steels with high edge stability is obtaining and maintaining a very high push cutting sharpness at very acute angles. For extended slicing aggression a low edge stability can even be of benefit as I noted many years ago in a commentary on self-sharpening edges.
13C26 (AEB-L) offers an improved hardness and wear resistance at the cost of toughness and corrosion resistance to 12C27 but these are small changes. The dissolved carbon percentages for example are 0.56/0.60 for 12C27/13C26 at 1100C so you are looking at a 1-2 point HRC difference. The retained carbon is 0.04/0.08% so 13C26 would have about twice the carbide fraction but this is a very low fraction as noted previosly. Note as well the size of these changes and the expected elemental composition variances should make it obvious that you would need to look at an average responce. As well if the temper is raised on 13C26 to reduce the hardness closer to 12C27 this will also bring the performance closer because it will coarsen the structure and make it weaker.
-Cliff
