Kevin R. Cashen
Super Moderator
My input regarding marquenching here-Don't drop your freshly quenched 1095 blade!! has spawned a few private questions, but one had so many excellent discussion points that I felt it really should be discussed for the benefit of all. What about steels that are going to be tempered much lower than the interrupt temperature when practicing marquenching? The question was a good one- is the interrupt temperature determined by the tempering temperature desired or is it always around 400F or 420F regardless?
The interrupt temperature is determined by the Ms (martensite start) point of the given steel. The Ms point is the point where martensite begins to form and the actual hardening process is initiated. It is not idea to interrupt the cooling once the steel drops below Ms, as a total hold on cooling could stabilize austenite. But if our final temper will be at 375F, won’t the auto-tempering be in excess of our desired tempering? And what is the Ms, of our steel and how can we get it to match or tempering temperatures.
Ms, is a function of chemistry, it is determined mainly by carbon content but alloying plays a significant role as well. The alloying is a constant, but the carbon content is determined by how much carbon we put into solution during our soak and that is why overheating results in retained austenite, it significantly lowers Ms. But the less carbon we have the higher Ms will be.
Let’s look at two alloys steels, both having a moderate amount of chrome as there only significant alloying- 52100 and 5160. Ms for 52100 can be anywhere from 475F down to 375F based on how high we heat it before the quench, but 5160 only has so much carbon and chrome to work with and so Ms will be above 450F. This sort of leaves us with a dilemma, can hypoeutectoid steels (having less than .8% carbon), that need to be tempered low, be marquenched?
The answer is yes, and no. It all depends on the steel and how picky you are. I once did a consulting job where at a .5% carbon steel was being marquenched, when I arrived to troubleshoot the problem they were having issues with some of the parts, I immediately saw a contradiction. The parts were being quenched below Mf (martensite finish) for that steel, but would not so well for the application in a higher range. This is when it occurred to me that marquenching many hypo-eutectoid steels may not be possible for higher hardness applications. This is just one of the reasons I am not a fan of hypo-eutectoid steels for blades, I like marquenching, I also like to have carbon levels that I can play with to have carbide action as well as toughness.
The truth is that if you want to have totally homogenous outcome, and are really picky, steels with .7 % carbon or lower when, interrupted closer to the desired tempering temperature, which is well below their Mf point, will not reap all of the benefits of the full marquenching process. And indeed, depending on where you interrupt things, it may not even technically be marquenching. But what does it matter? The process of interrupting a quench in normal quenching oil is only an approximation of a real marquench anyhow, so why get hung up on the semantics?
I sometimes do a full and actual marquench (that is I use the salts baths in the industrial definition of marquenching), on 5160 and will get very good numbers with the quench set to 400F. This is close enough to both Ms and the final tempering temperature that the auto-tempering does not have time to detract from the overall hardness; remember that tempering is a function of TIME and temperature. Auto-tempering will often result in 1.5 to 2 points HRC less in the as quenched reading, and the last 5160 blade I did came out with a 61.5 to 62HRC which means I maxed out the hardness possible in that alloy.
The interrupt temperature is determined by the Ms (martensite start) point of the given steel. The Ms point is the point where martensite begins to form and the actual hardening process is initiated. It is not idea to interrupt the cooling once the steel drops below Ms, as a total hold on cooling could stabilize austenite. But if our final temper will be at 375F, won’t the auto-tempering be in excess of our desired tempering? And what is the Ms, of our steel and how can we get it to match or tempering temperatures.
Ms, is a function of chemistry, it is determined mainly by carbon content but alloying plays a significant role as well. The alloying is a constant, but the carbon content is determined by how much carbon we put into solution during our soak and that is why overheating results in retained austenite, it significantly lowers Ms. But the less carbon we have the higher Ms will be.
Let’s look at two alloys steels, both having a moderate amount of chrome as there only significant alloying- 52100 and 5160. Ms for 52100 can be anywhere from 475F down to 375F based on how high we heat it before the quench, but 5160 only has so much carbon and chrome to work with and so Ms will be above 450F. This sort of leaves us with a dilemma, can hypoeutectoid steels (having less than .8% carbon), that need to be tempered low, be marquenched?
The answer is yes, and no. It all depends on the steel and how picky you are. I once did a consulting job where at a .5% carbon steel was being marquenched, when I arrived to troubleshoot the problem they were having issues with some of the parts, I immediately saw a contradiction. The parts were being quenched below Mf (martensite finish) for that steel, but would not so well for the application in a higher range. This is when it occurred to me that marquenching many hypo-eutectoid steels may not be possible for higher hardness applications. This is just one of the reasons I am not a fan of hypo-eutectoid steels for blades, I like marquenching, I also like to have carbon levels that I can play with to have carbide action as well as toughness.
The truth is that if you want to have totally homogenous outcome, and are really picky, steels with .7 % carbon or lower when, interrupted closer to the desired tempering temperature, which is well below their Mf point, will not reap all of the benefits of the full marquenching process. And indeed, depending on where you interrupt things, it may not even technically be marquenching. But what does it matter? The process of interrupting a quench in normal quenching oil is only an approximation of a real marquench anyhow, so why get hung up on the semantics?
I sometimes do a full and actual marquench (that is I use the salts baths in the industrial definition of marquenching), on 5160 and will get very good numbers with the quench set to 400F. This is close enough to both Ms and the final tempering temperature that the auto-tempering does not have time to detract from the overall hardness; remember that tempering is a function of TIME and temperature. Auto-tempering will often result in 1.5 to 2 points HRC less in the as quenched reading, and the last 5160 blade I did came out with a 61.5 to 62HRC which means I maxed out the hardness possible in that alloy.
