View Full Version : 440C heat treat


gthomas
10-21-2001, 05:40 PM
Well I seem to have successfully heat treated a 440C fillet knife I am making in a propane forge. We just eyeballed the colors at around 1900 degrees F and I let it soak for about 10 minutes and then held it in front of a fan. Seemed to harden fine and after a 375 degree temper I was able to straighten the blade which had warped a bit. However, when I went to clean the blade back up with 320 grit paper I discovered that I more than likely will have to go back to a belt grinder to remove the strange ridges and flowlike formations all over the surface of the blade. Anyone have any experience with this?

Guy Thomas

Raymond Richard
10-21-2001, 08:36 PM
Guy, Heat treating any of the stainless alloys usually requires a heat treating oven and the blades are wrapped in a special heat treating foil to stop the decarb. Hard to really judge the heat by eye and do the requiried soak. You'd be better off if your doing a number of 440C blades is to send them off for heat treat. Ray

AchimW
10-21-2001, 11:58 PM
Raymond, i don't agree at all. Heat treating all sorts of stainless and high alloy tool steel can easily be done in a gas furnace if you have a bit of experience. I have access to electronically controlled heat treating equipment, cryo stuff and all. And i always read the technical sheets the steel mills provide whenever i use a new steel. Anyway, i have consistantly good results using gas furnaces and had good results with charcoal fires, too.

Please read the thread "smooth surface", too.

Achim

Raymond Richard
10-22-2001, 05:57 AM
Achim, I didn't say you couldn't. Need to get off to work, talk more later. Ray

Raymond Richard
10-22-2001, 08:39 PM
Achim, Read your thread on smooth surfaces. Tell us how you do it with out getting any decarb. Guy Thomas said he was using his propane forge to get to hardening temp. You said you use a gas furnace. Are we talking the same thing, forge = furnace. Is your furnace controlled atmosphere? I can't understand how you can use charcoal to heat treat and not get any decarb. You need to release your secret. Ray

AchimW
10-23-2001, 02:52 AM
Ray, there simply is no secret. Maybe i don't understand you. Or i don't understand the terms. So here we go.

1. forge / furnace

I am talking about a propane forge with controlled air flow. You may adjust the air/propane mix to suit your needs with a choke device.

2. decarb

To me this means decarburization of the outer layer of the blade steel. When i say steel, i mean steel and not slag. You have a thin layer of slag building up during the heat treating process in both gas forge or charcoal forge. This slag has to be ground off after heat treating IF it doesn't come down during the hardening in oil. You don't have such a layer if you are working with electric heat treating ovens using inert gas or if using heat treating foil to keep the oxygen away. But i finish my blades after heat treating and not before, so it doesn't matter to me. We have access to a spark spectrometers and sophisticated hardness testing devices in a University. We tested the steel and found no carbon or hardness loss. Besides, both are not very important, if the grain size of the steel and the edge holding are at an optimum.

When you are using charcoal it is pretty easy not to get any decarburization at all by paying close attention where you hold the blade during the heating. If you hold it in the carburizing layer of the fire, there simply is no oxygen left in the heat stream that would touch the blade. You may even go a step further and use a charcoal dust filled tube in the fire to heat the blade. The process you are using this way is called cementation and was used in ancient times to make steel from soft iron. What you do is putting carbon INTO the blade, but not pulling it OUT OF it.

Achim

Dana Acker
10-23-2001, 11:59 AM
Let's talk about cryogenics. I've always heard that Stainless steel blades must be first quenched in oil, then submerged in liquid nitrogen or hydrogen to obtain maximum hardness. Many of us do not have the capabilities to do this easily, and find it would not be worth the investment to get set up to do it. However, I read someplace that if one mixes dry ice and acetone (both cheap and readily available) one could achieve a temperature of minus 75 degrees below zero, Farenheit. Now while that is only about a third of the coldness provided by the liquid gasses mentioned above, I wonder if soaking a blade in this mixture for a time might help provide a degree of hardness. WARNING: I don't remember which goes in the tank first, the dry ice or the acetone. FIND OUT BEFORE ATTEMPTING. Mixing it the right way causes coldness. Mixing it the wrong way causes explosions. Any thoughts?

Don Cowles
10-23-2001, 05:08 PM
Dana, kerosene works as well as acetone, and either will get you into the necessary range (about 100 below). The key is to leave them there for several hours.

I do this *after* the first temper, and before the second.

In centuries past, knifemakers in Finland used to put their forged blades in a snowbank for the winter to accomplish the same purpose, though they probably didn't know much about metallurgy.

Raymond Richard
10-23-2001, 07:01 PM
Dana, I used to use acitone but I changed over to kerosene. The kerosene that I got has a pink tint to it which helps out us old guys to see it. I've had a Rockwell test done a few years ago, with and without the freeze and did get a couple more points hardness doing it. That was with ATS-34.
Don, are you quenching in oil or air hardening? What temp are your draws? Ray

AchimW
10-24-2001, 12:17 AM
Well, the cryo quench is nothing other than a prolongation of the normal quench.

Technical background:
Because of the strong solution of alloy elements during austenitization the saturation of the material with austenite-stabilizing elements is increased (expansion of the gamma-zone). This leads to the fact that during the quench no longer all formed austenite can be transformed. >> residual austenite (RA)
At the same time the temperatures for the martensite start (Ms) and the martensite finish (Mf) are lowered.

RA is a so-called metastable phase, which after a certain time, about two hours after quenching to room temperature, stabilizes. The more time passes the more stable is the RA and the less RA can be transformed to martensite by cryo quenching.

The changing of RA to martensite during cryo quench takes place with sound speed (Mach 1) once the optimum temperature of the material is reached. Strangely enough this optimum temperatur for the most effective transformation is at about -100? Celsius (-148? F). Lower temperatures don't give better transformation, but the danger of stress cracks rises.

Cryo quenching after tempering diminishes the possibility of good results because the RA begins to stabilize itself through the tempering in low temperatures.

Recommendation:
- if possible, cryo quench directly after the quenching

- be sure that the entire blade is cooled down over the total cross-section.

- avoid longer holding times at cryo temperature because of the danger of stress cracks. Longer holding times don't get you better results once the whole cross section is cold.

- the cycle: hardening/cryo/tempering/cryo/tempering is the best for our purposes.

- cryo obviously only makes sense when using alloys that tend to build up high rates of RA (high carbon, high chromium, high other alloy element content)

Hope this wasn't too long.

Achim

Dana Acker
10-24-2001, 07:20 AM
How long of a time at -100F is optimum? How long is too much?

ghostdog
10-24-2001, 08:47 AM
My question is: is the performance gain in a standard forging steel enough to go to the added expense and trouble? Raymond says a couple of points of hardness gained, which is not necessary at all for most knives made from a standard forging steel.


ghostdog

Don Cowles
10-24-2001, 08:51 AM
Ray, remember I am not forging- with stock-removal ATS34, I air quench, and temper at 425F.

AchimW
10-25-2001, 12:28 AM
Dana this is NOT -100? Fahrenheit, it's CELSIUS! I live in Europe! Metric system rules over here!

The freezing time depends on the diameter of the workpiece, but for a knife of up to, say, 10 mm thickness (ohhh, Metric system again!!!), in a quenching medium with direct contact (liquid, no gas!) about 20 to 30 minutes are fully sufficient.

Performance gain through cryo quench doesn't necessarily mean more HRc points. What you really want is the finer grain you get. This means tougher steel with finer edge and higher abrasion resistance. Hence better sharpness, longer edge holding and easier sharpening.

Achim

Dana Acker
05-14-2002, 09:59 AM
Didn't want this one to disappear into dataland. Good stuff.

Raymond Richard
05-14-2002, 12:40 PM
That is an old one, I had a job then.......