- Joined
- Dec 21, 2006
- Messages
- 3,158
52100 is a low alloy high carbon steel often used in the production of ball bearings, but makes for a very good blade steel. The basic composition consists of...
Carbon: 1.05%
Manganese: 0.30%
Chromium: 1.5%
Silicon: 0.30%
When heat treating this steel, the chromium content must be taken into consideration, as 52100 is all about the carbides. It has been said before here on BladeForums, but just for the sake of this thread and clarification, there are no chromium carbides in 52100. Rather, the cementite carbide is "enriched" by the chromium. In order for there to be chromium carbides, the chromium content needs to be higher than just 1.5%. There probably has been more talk about 52100 heat treating than any other knife steel because of that chromium content, and how it changes the heat treat protocol.
Larrin Thomas has done a lot of magnificent work in collating data on different steels, their history and use, their heat treatment, knife edge stability, and various other subjects related to our field. Recently he discussed 52100 and got me to thinking more about the "best" way to go about heat treating this steel. One of the images included with his post, which can be found here on BladeForums and also on his website: https://knifesteelnerds.com/2019/01/28/history-and-properties-of-52100-steel/ shows how the chromium addition to a steel requires higher hardening temperatures than simple 1% carbon steels like 1095, O1, or W2. And it makes sense just by looking at the recommended austenitizing (hardening) temperature of A2 tool steel (1750°F), which is also a ~1% carbon steel, but has a full 5% chromium. And the more chromium in the mix, the higher the austentizing temperature needs to be. Many stainless steels (~13% chromium) are hardened at 1950°F and higher.
So what temperature should we recommend for austenitizing 52100? The answer is not so simple, and depends on a few factors, and we need to consider the source of our steel and more importantly, what condition it is in prior to hardening. Is it coarse spheroidite, or fine spheroidite, or fine pearlite, or even martensite (previously quenched)? It also can depend on the steel itself, not just it's microstructure going into hardening. The advice that has been given here in the past by various makers is very true, not all 52100 is the same and not all 52100 from the same supplier is the same. The "best" thing to do is buy a LOT of one particular batch of steel and learn it's quirks. Ideally a lifetime's supply! And even better yet, if you can buy steel that has it's certifications included with it, even better. If there are ever any issues or problems, with the steel's certifications you have a record telling you who/what/when/where.
The two main sources for 52100 in annealed bar stock form that I know of are Aldo Bruno (the New Jersey Steel Baron) and Chuck (Alpha Knife Supply). I think the folks over at Midwest Knife Maker's Supply (USA Knifemaker) also carry 52100, but I think they source it from either Aldo or Chuck (if anyone has different or further info, feel free to comment). Aldo's 52100 is notorious for being heavily spheroidized (coarse spheroidized) from the mill he receives it from. This makes the steel butter soft and nice for grinding, drilling, machining, filing, but requires extra steps to get it to harden properly. Namely, a normalizing heat of 1650°F-1700°F, and subsequent thermal cycling, then to austenitizing/quenching. Chuck has said that his 52100 comes in a fine spheroidized state, and thus does not need to be normalized or cycled prior to hardening, and can simply be hardened as received.
Back to austenitizing temperature for 52100. With basic 1% carbon steels like 1095, W2, O1, the optimum temperature is ~1475°F with a short soak of 5-10 minutes (O1 is probably best with a slightly longer soak, due to it's alloying). Using this temperature will result in putting just enough of the available carbon into solution to attain the maximum hardness post quench, but also keeps retained austenite low, and a better balance of lathe vs plate martensite. Going below this temperature will result in lower than expected hardness. Going above this temperature also reduces hardness because the retained austenite % is higher, and results in more brittle plate martensite vs the tougher lathe martensite. 1475°F is a ballpark temperature for these steels, and should be adjusted accordingly to your equipment, the actual steel chemistry, etc.
But 52100 has that 1.5% chromium content, and thus should be hardened at a higher temperature than 1475°F in order to bring that necessary carbon in solution. So why, in the past, have we seen recommended hardening temps of 1475°F for 52100, and not 1525°F-1550°F? This was because of the condition the steel was in (microstructure of coarse spheroidite) and more importantly the condition the steel was in after our normalizing and thermal cycling (fine pearlite). Once the steel was normalized, cycled, hardened, quenched, the final HRC was 66-67, an excellent post quench hardness. The only way to attain that hardness with 1475°F (plus the recommended 10 minute soak) was because we had changed the microstructure from spheroidite to pearlite, which allows carbon to come into solution easier/quicker. If, after normalizing and cycling, we still used the recommended 1550°F temperature, testing has shown that the post quench hardness is actually lower, the retained austenite % is higher, and there is more plate martensite vs the preferred lathe martensite.
Now onto Chuck's 52100, which is said to be fine spheroidite and ready to harden. I have previously publicly stated that the hardening temperature for his steel should also be 1475°F, but after using it more (especially this past week), and thinking about what Larrin mentioned in his 52100 discussion, I think there are 2 ways to approach Chuck's 52100. First, you can go straight to austenitizng/quenching, but I would use 1525°F-1550°F. The spheroidite takes a higher temperature to get the necessary carbon in solution for max hardness. Just this week I did a heat treat on 4 thin paring knives using 1475°F and an extended soak, but the post quench hardness was not as high as I think it could be. Without a hardness tester, my educated guess with files of known hardness was ~63-64HRC. Not bad, but not optimum. Using the higher temperature Larrin mentioned (1525°F-1550°F), I strongly believe (and will be further testing) it will bump that HRC to where it should be. Now the 2nd approach, treat Chuck's 52100 just like you would Aldo's 52100 by normalizing and thermal cycling a few times in order to change it from spheroidite to fine pearlite. Then you can use the lower hardening temperature of 1475°F and attain that maximum post quench hardness. There is some debate (I think) over that last sentence, that it doesn't matter what condition the 52100 is in prior to hardening, that it will not achieve maximum post quench hardness using 1475°F.
Carbon: 1.05%
Manganese: 0.30%
Chromium: 1.5%
Silicon: 0.30%
When heat treating this steel, the chromium content must be taken into consideration, as 52100 is all about the carbides. It has been said before here on BladeForums, but just for the sake of this thread and clarification, there are no chromium carbides in 52100. Rather, the cementite carbide is "enriched" by the chromium. In order for there to be chromium carbides, the chromium content needs to be higher than just 1.5%. There probably has been more talk about 52100 heat treating than any other knife steel because of that chromium content, and how it changes the heat treat protocol.
Larrin Thomas has done a lot of magnificent work in collating data on different steels, their history and use, their heat treatment, knife edge stability, and various other subjects related to our field. Recently he discussed 52100 and got me to thinking more about the "best" way to go about heat treating this steel. One of the images included with his post, which can be found here on BladeForums and also on his website: https://knifesteelnerds.com/2019/01/28/history-and-properties-of-52100-steel/ shows how the chromium addition to a steel requires higher hardening temperatures than simple 1% carbon steels like 1095, O1, or W2. And it makes sense just by looking at the recommended austenitizing (hardening) temperature of A2 tool steel (1750°F), which is also a ~1% carbon steel, but has a full 5% chromium. And the more chromium in the mix, the higher the austentizing temperature needs to be. Many stainless steels (~13% chromium) are hardened at 1950°F and higher.
So what temperature should we recommend for austenitizing 52100? The answer is not so simple, and depends on a few factors, and we need to consider the source of our steel and more importantly, what condition it is in prior to hardening. Is it coarse spheroidite, or fine spheroidite, or fine pearlite, or even martensite (previously quenched)? It also can depend on the steel itself, not just it's microstructure going into hardening. The advice that has been given here in the past by various makers is very true, not all 52100 is the same and not all 52100 from the same supplier is the same. The "best" thing to do is buy a LOT of one particular batch of steel and learn it's quirks. Ideally a lifetime's supply! And even better yet, if you can buy steel that has it's certifications included with it, even better. If there are ever any issues or problems, with the steel's certifications you have a record telling you who/what/when/where.
The two main sources for 52100 in annealed bar stock form that I know of are Aldo Bruno (the New Jersey Steel Baron) and Chuck (Alpha Knife Supply). I think the folks over at Midwest Knife Maker's Supply (USA Knifemaker) also carry 52100, but I think they source it from either Aldo or Chuck (if anyone has different or further info, feel free to comment). Aldo's 52100 is notorious for being heavily spheroidized (coarse spheroidized) from the mill he receives it from. This makes the steel butter soft and nice for grinding, drilling, machining, filing, but requires extra steps to get it to harden properly. Namely, a normalizing heat of 1650°F-1700°F, and subsequent thermal cycling, then to austenitizing/quenching. Chuck has said that his 52100 comes in a fine spheroidized state, and thus does not need to be normalized or cycled prior to hardening, and can simply be hardened as received.
Back to austenitizing temperature for 52100. With basic 1% carbon steels like 1095, W2, O1, the optimum temperature is ~1475°F with a short soak of 5-10 minutes (O1 is probably best with a slightly longer soak, due to it's alloying). Using this temperature will result in putting just enough of the available carbon into solution to attain the maximum hardness post quench, but also keeps retained austenite low, and a better balance of lathe vs plate martensite. Going below this temperature will result in lower than expected hardness. Going above this temperature also reduces hardness because the retained austenite % is higher, and results in more brittle plate martensite vs the tougher lathe martensite. 1475°F is a ballpark temperature for these steels, and should be adjusted accordingly to your equipment, the actual steel chemistry, etc.
But 52100 has that 1.5% chromium content, and thus should be hardened at a higher temperature than 1475°F in order to bring that necessary carbon in solution. So why, in the past, have we seen recommended hardening temps of 1475°F for 52100, and not 1525°F-1550°F? This was because of the condition the steel was in (microstructure of coarse spheroidite) and more importantly the condition the steel was in after our normalizing and thermal cycling (fine pearlite). Once the steel was normalized, cycled, hardened, quenched, the final HRC was 66-67, an excellent post quench hardness. The only way to attain that hardness with 1475°F (plus the recommended 10 minute soak) was because we had changed the microstructure from spheroidite to pearlite, which allows carbon to come into solution easier/quicker. If, after normalizing and cycling, we still used the recommended 1550°F temperature, testing has shown that the post quench hardness is actually lower, the retained austenite % is higher, and there is more plate martensite vs the preferred lathe martensite.
Now onto Chuck's 52100, which is said to be fine spheroidite and ready to harden. I have previously publicly stated that the hardening temperature for his steel should also be 1475°F, but after using it more (especially this past week), and thinking about what Larrin mentioned in his 52100 discussion, I think there are 2 ways to approach Chuck's 52100. First, you can go straight to austenitizng/quenching, but I would use 1525°F-1550°F. The spheroidite takes a higher temperature to get the necessary carbon in solution for max hardness. Just this week I did a heat treat on 4 thin paring knives using 1475°F and an extended soak, but the post quench hardness was not as high as I think it could be. Without a hardness tester, my educated guess with files of known hardness was ~63-64HRC. Not bad, but not optimum. Using the higher temperature Larrin mentioned (1525°F-1550°F), I strongly believe (and will be further testing) it will bump that HRC to where it should be. Now the 2nd approach, treat Chuck's 52100 just like you would Aldo's 52100 by normalizing and thermal cycling a few times in order to change it from spheroidite to fine pearlite. Then you can use the lower hardening temperature of 1475°F and attain that maximum post quench hardness. There is some debate (I think) over that last sentence, that it doesn't matter what condition the 52100 is in prior to hardening, that it will not achieve maximum post quench hardness using 1475°F.
