Decarb with 1200 F Stress Relief Cycle

[MBB]

Howdy,

I've read conflicting reports on how much decarb occurs with a 1-2 hour 1200 F stress relief cycle. I tried it recently with a 1075 blade that had developed a bend, and I feel like it was pretty significant, as I seemed to have a lot of metal to remove prior to having anything that would etch. It did fix the bend nicely, though. Are people using stainless steel foil or decarb surface treatments for this? Or are you making it standard practice to stress relieve your barstock before even profiling?

Thanks,

Mike

 

[jdm61]

You can get a funky grey coating. I use foil.

 

[stezann]

I don't understand...you assumed you got decarb because you couldn't etch the barstock after your stress relieving at 1200 °F?
Did you try to etch before the stress relieving? Maybe there were significant decarb straight from the mill.
Did you remove mill scale and any oil trace?

 

[MBB]

It was a fully ground knife that hadn't been heat treated but was warped. Afterwards, there was a ton of decarb.

 

[MBB]

My question is at what temperature does decarb start? I was hoping that Larrin would pop in on this one.

 

[stezann]

I'll be tuned in, i have also always heard that stress relieving, sub critical temperatures were not enough to cause significant decarburation.
As often the 2 things are confused, maybe people referred to scaling, and at that temperature you don't get scaling, but maybe decarburation if the athmosphere is oxydising. Moisture would accentuate decarburation, whereas dryier air has less effect. Anyway, if the carbon can move, it can also leave the steel even though at a slower pace than at higher temperatures.
Let's see if Larrin and others would like to give their insight.

 

[Karl B. Andersen]

..... Afterwards, there was a ton of decarb.

You really can't SEE decarb. Decarb is a loss of carbon.
But you can see scaling. Scale is iron oxide, which is iron combined with oxygen in the environment.
Which are you referring to?

 

[Stacy E. Apelt - Bladesmith]

Decarb is the process of oxygen combining with carbon and gassing of as CO2. Carbon converts to a gas (CO2) at 1250F, IIRC. That is not to say that there isn't significant damage to the surface by combining with the oxygen in a solid form at 1200F for an extended time.

The two processes are different:
When carbon reaches 1250F it will start to vaporize. It combines with the oxygen in the hot air around it and becomes CO2. If you put a piece of charcoal in the oven, 1250 is when it will disappear as gas. Since the carbon is bonded with the iron, this is a slow process, and usually only affects the surface and a few thousandths in.

When the hot steel is exposed to oxygen in the air, besides the carbon being oxidized, the now elemental iron also combines with oxygen. This creates iron oxide in the form of magnetite, which we call scale. This is a layer of destroyed steel. The higher the temperature and the longer the exposure to the air, the more the scale.

Both are pretty easy to deal with. Reducing the exposure to oxygen by coating the steel with a layer of something will slow down decarb, and greatly reduce scale. Having the forge chamber at a neutral atmosphere also help alot with scale. Wapping the blade in HT foil is necessary when the temps get above 1600F or exposure gets longer than 10-15 minutes. The most efficient way is to have no oxygen in the chamber by replacing it with an inert or non-reactive gas ... like argon, or nitrogen

In all cases of HT, the entire blade surface will need to be ground away to some degree. With a gas filled chamber in a HT oven, or foil wrap, the amount is very small. In an ope forge and with extended HT times, there can be several thousandths needing removal. Until all this steel is removed, you aren't getting to the hard steel, and etching will be blotchy or not work at all.

 

[MBB]

Thanks guys for your responses.

What I was referring to was the amount of surface material I had to remove before the knife in question would etch in ferric chloride. I am a relative newbie but it seems that a carbon depleted surface layer does not etch. With the knife in question (1075 steel I was trying to get a hamon from), post-stress relief cycling and heat treat (1500 F with 5 min soak, Parks 50 quench 375 F temper x2), I had to extensively regrind it to get to the point where I could successfully etch the surface. Thus, my question regarding how much decarb occurs with stress relief. (As an aside, the knife in question turned out just fine once it was ground to a more narrow profile.)

Knowing that decarb starts at ~1250 F, it makes sense to protect the steel from oxygen exposure during stress relief cycling. Part of my curiosity is because I would like to try AEB-L next, and some posts I have read have recommended stress relieving it to remove the memory of when it was coiled. From what I've read, the steel develops memory from internal matrix dislocations which can recrystallize/renucleate at 1200 F to relieve stress/remove memory. So I was planning on giving the chunk of AEB-L I have a 1200 F x 1 hour cycle prior to profiling the knife.

I hope that makes sense.

Mike