Carbon steel hardening and magnet

Juha Perttula

Juha Perttula

Joined
Mar 27, 2013
Messages
209
I just noticed when I tested that AISI 1075 is ready for quenching instantly when it got non-magnetic, but 80CrV2 needs a bit more heat. When do you quench your carbon steels, immediately after disappearance of magnetism, or do you give some extra heat ?
 
I've been doing experiments on this lately because I've been thinking about how to make heat treating as foolproof as possible when using a forge to austenitize. As you noted, adding extra alloy, even the relatively small Cr+V in 80CrV2 is enough to make a significant difference in austenitizing response. 1075/1084 etc. with just C, Mn, Si will heat treat from quite low temperatures basically regardless of the starting structure. Carbon diffusion is so fast that it doesn't make that big of a difference. With alloyed steels like 80CrV2 and especially something like 52100 the starting structure matters a lot. 52100 starting from a spheroidized structure (especially from the manufacturer) needs more temperature than non-magnetic along with a soak. However, if it is pearlitic then you can quench from nonmagnetic and be fully hard. I will have articles/videos coming out over the course of the next month on this topic. The first about "thermal cycling" with normalizing and annealing. The second will be my first attempts at a forge heat treatment on a range of different steels where I see if I can quench them all from non-magnetic using a microstructure optimized for that type of heat treatment. Trying to tell people to go hotter than non-magnetic leaves too much room for error, especially for a beginner.
 
Larrin said:
I've been doing experiments on this lately because I've been thinking about how to make heat treating as foolproof as possible when using a forge to austenitize. As you noted, adding extra alloy, even the relatively small Cr+V in 80CrV2 is enough to make a significant difference in austenitizing response. 1075/1084 etc. with just C, Mn, Si will heat treat from quite low temperatures basically regardless of the starting structure. Carbon diffusion is so fast that it doesn't make that big of a difference. With alloyed steels like 80CrV2 and especially something like 52100 the starting structure matters a lot. 52100 starting from a spheroidized structure (especially from the manufacturer) needs more temperature than non-magnetic along with a soak. However, if it is pearlitic then you can quench from nonmagnetic and be fully hard. I will have articles/videos coming out over the course of the next month on this topic. The first about "thermal cycling" with normalizing and annealing. The second will be my first attempts at a forge heat treatment on a range of different steels where I see if I can quench them all from non-magnetic using a microstructure optimized for that type of heat treatment. Trying to tell people to go hotter than non-magnetic leaves too much room for error, especially for a beginner.
Click to expand...
Very cool Larrin Larrin , thanks for doing the research. I'm looking forward to reading what you come up with!
 
Larrin said:
I've been doing experiments on this lately because I've been thinking about how to make heat treating as foolproof as possible when using a forge to austenitize. As you noted, adding extra alloy, even the relatively small Cr+V in 80CrV2 is enough to make a significant difference in austenitizing response. 1075/1084 etc. with just C, Mn, Si will heat treat from quite low temperatures basically regardless of the starting structure. Carbon diffusion is so fast that it doesn't make that big of a difference. With alloyed steels like 80CrV2 and especially something like 52100 the starting structure matters a lot. 52100 starting from a spheroidized structure (especially from the manufacturer) needs more temperature than non-magnetic along with a soak. However, if it is pearlitic then you can quench from nonmagnetic and be fully hard. I will have articles/videos coming out over the course of the next month on this topic. The first about "thermal cycling" with normalizing and annealing. The second will be my first attempts at a forge heat treatment on a range of different steels where I see if I can quench them all from non-magnetic using a microstructure optimized for that type of heat treatment. Trying to tell people to go hotter than non-magnetic leaves too much room for error, especially for a beginner.
Click to expand...
I have found that pearlitic 80CrV2 gets only 64 HRC when quenched instantly non-magnetic, but a few seconds more heat by gas torch (about 100 F) gives 66 HRC. Spheroidized 80CrV2 is difficult to harden.
 
OK, here is my data on this:
The eutectoid for steel is point where it becomes austenite. 0.83% (1084) carbon reaches this point at around 1350°F/730°C. The curie point, where it stops being magnetic, is around 1420°F/770°C. To harden the steel it needs a bit higher temperature. 1084 is the eutectoid blade steel, and it hardens around 1500°F/725°C. Most simple carbon steels harden in the 1450-1500°F/785-815°C range.1095 hardens around 1475°F/800°C. The higher the carbon content, the lower the hardening point ... but the higher the alloying, the higher the hardening point. This is where a good book on heat treatment is invaluable. Larrin's book has all the HT data as well as the science to explain it.

Doing HT in a forge with a magnet and by eye takes some practice, but here is how it should go.
Forge HT is best done in a darkened location, or at night. The color of the steel looks very different in full daylight.
Use a good strong magnet attached to the side of the forge or near it. A cheap welding magnet from HF is perfect.

Dial down your forge until it is just barely running. A venturi burner may not even get low enough, but at its lowest stable flow, it will work If you have PID control, set it to 1500°F/815°C.

Heat the blade evenly by moving/rotating the blade and pointing the tip and edge toward the sides. Try to get the color to turn red across the whole blade. Pay particular attention to the thinner edge and tip, as these tend to get hot much faster. It is the edge that you want to harden, so pay attention to that area the most.

Once the blade starts to get red, pull it out regularly and touch it to the magnet. When it strops sticking it is around 1350°F/730°C. You want to heat it about 75°F/25°C to 100°/40°F higher. This looks about one shade of red brighter.

Once the color seems right, hold it at that temperature for a minute or so if possible, and then quench. If the forge is too hot to prevent the blade getting hotter, quench immediately when it is at 1450-1500°F/785-815°F.

Temper immediately after the quench at 400°F/200°C for most simple carbon steel blades. Rough use blades should be tempered at 425°F/220°C.
 
i knew a guy that used table salt to check the temp.. was always curious if it is/was a good idea ??
i guess table salt is supposed to go liquid at 1450/1500 !!! seemed like a good idea to test if hot enough..
might have to try it sometime to see what temp it goes liquid....
 
Larrin said:
I've been doing experiments on this lately because I've been thinking about how to make heat treating as foolproof as possible when using a forge to austenitize. As you noted, adding extra alloy, even the relatively small Cr+V in 80CrV2 is enough to make a significant difference in austenitizing response. 1075/1084 etc. with just C, Mn, Si will heat treat from quite low temperatures basically regardless of the starting structure. Carbon diffusion is so fast that it doesn't make that big of a difference. With alloyed steels like 80CrV2 and especially something like 52100 the starting structure matters a lot. 52100 starting from a spheroidized structure (especially from the manufacturer) needs more temperature than non-magnetic along with a soak. However, if it is pearlitic then you can quench from nonmagnetic and be fully hard. I will have articles/videos coming out over the course of the next month on this topic. The first about "thermal cycling" with normalizing and annealing. The second will be my first attempts at a forge heat treatment on a range of different steels where I see if I can quench them all from non-magnetic using a microstructure optimized for that type of heat treatment. Trying to tell people to go hotter than non-magnetic leaves too much room for error, especially for a beginner.
Click to expand...
I appreciate your contributions to the knife community.
 
Juha Perttula said:
I just noticed when I tested that AISI 1075 is ready for quenching instantly when it got non-magnetic, but 80CrV2 needs a bit more heat. When do you quench your carbon steels, immediately after disappearance of magnetism, or do you give some extra heat ?
Click to expand...
If you really know what to look for, it’s the next color after the steel is demagnetized. IIRC, steel becomes demagnetized at 1420, which is red, then you wait until it turns bright red which is around 1500 degrees and quench.
 
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