5160, 80CrV2, W2 (NJSB W2) are all roughly eutectoid or hypoeutectoid steels. 52100 and O1 are hyper eutectoid steels. They have extra carbon past the eutectoid point. The goal is maximum post quench hardness (65HRC+), and there are 2 ways to go about dealing with that extra carbon. Higher temps and no soak time or lower temps plus the soak time. Using the higher temps causes a few undesirable issues that when using the lower temps and extended soak times doesn't happen. But it also depends on what carbide forming elements the steel has, and in what %. The higher the alloy % and the higher the carbon content past the eutectoid point, you need to free that carbon up somehow that is tied up in carbides as received from the supplier, usually this is a fine spheroidized state.
If the steel has equal to or less than ~.8% carbon, extended soak times (past 5-10 minutes) aren't all that necessary. Bring up to temp, equalize and short soak if able, quench. But if the steel is hypereutectoid, carbon greater than roughly .8%, it's better to use the lower hardening temperature and extended soak times. So like for O1, if you were to simply bring to 1475f/1500f and quench, the result would be a lower than expected hardness, because you didn't allow time to do it's thing. If you were to use a hotter temp (rather than low temp and time), you could bring in the necessary carbon for the desired hardness (65HRC+), but you also bring in more retained austenite and higher percentage of plate martensite. The better route being to use the 1475F temperature and extended soak times in order to get the necessary .8% carbon in solution for the desired maximum post quench hardness.
Interestingly, I often hear O1 needs only a 10 minute soak, and this may be enough. I often see the data sheets saying 30 minutes up to the first 1/2" (or 1" whatever it is I don't recall) thickness, which means that even if the piece is only 1/8", it still should get the 30 minutes of soak. If the steel exceeds a certain thickness, time is added. I know some makers do 20-30 minute soak times with O1, and I don't think this would cause any issues concerning grain growth, due to the tungsten and vanadium present, but at some point in time during an extended soak, too much carbon can come into solution, and thus bring the same problems as too high of a temp.
Then there is the issue of higher % of chromium as in the case with 52100, and exactly what hardening temperature is best suited for that steel, and I have always been taught that this was heavily dependent upon the previous thermal treatment the 52100 underwent, and thus the resulting carbide structure. No normalizing was done? Higher temps are needed to get that carbon in solution. But if the normalizing WAS done, the carbon is ready for solution by using the lower austenitizing temps and ~10 minute soak. While O1 does have a chromium content, it isn't quite as high as the 1.5% in 52100.
Good question, no doubt about it. What helped me understand better was that when steel is received in the fine spheroidized state, the carbon you need to get the hardness you're after is tied up in the carbides, and must be freed in order to harden the steel. Annealed steel has more carbides in it than hardened steel does.
Larrin
this is for you and the other metallurgy experts.
