Advice ? Messed up heat treatment

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peter123

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Apr 5, 2018
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Long story short I ended up Austenitizing, Quenching, and tempering (x2 tempers) M4 steel. I am currently getting hardness values between 20-30 HRC (obviously way too low). Can this be fixed? Can I do the process again? Do I have to normalize first or can I just bring it back up to Ausenitizing temperature and quench it?
 
We will need to know exactly what process you used and what equipment you have for heat treating to be able to tell you what went wrong and how to fix it.
 
If you did the M4 like it was 1084, then you didn't harden it at all. M4 austenitizes at 2150F to 2200F. It requires a good HT oven and the high-temp type stainless HT foil.



"I didn't know I had to."
This concerns me. You bought a high alloy steel and made a knife from it but did no research on the HT of that steel, .... or HT in general apparently.
Would you be happy with a doctor who bought a scalpel and was going to do surgery on you, but never studied about that surgery or how to do it.
 
I researched the process and did not find anything on a foil wrap. The process I used was first a stress relief stage at 650 C held for an hour, then cranked it up to austenizing temperature of 1200 C for 30 min, then quenched in oil, then executed to tempering stages at 570 C for two hours each.
 
Without foil wrap at those times and temps you will likely have caused massive decarb issues in the steel. It may be hard under the damaged surface, but you will likely have to grind a long way in to find it.
 
That's what I was thinking, I'm going to grind away the surface to hopefully get the hard material underneath. However, I don't know what needs to be done after that (type of foil or the heat treatment after I've tempered) and was looking for some advice. Obviously this knife is not going to great, but I see this as a learning curve in understanding what exactly to do next time.
 
You wrap in the foil to protect the steel during heat treat, not after. The other option beyond most of our means is a vacuum furnace, or a few people plumb inert gas into the kiln. You have to protect the surface from decarburization, which happens quicker as temp rises.

What is your heat source?
 
I have a furnace at school I use. So, after I grind away the surface I'm ok to re - heat treat? This time with a foil wrap
 
peter123 said:
I have a furnace at school I use. So, after I grind away the surface I'm ok to re - heat treat? This time with a foil wrap
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I’m not sure. Depends on how much decarb there is, and how deep it goes. It might be ok, but it might be too damaged.
 
If you mean a gas forge when you said "a furnace", then the blade is likely ruined.
If it is an electric HT oven or a gas muffle oven for HT, then the blade is badly damaged, but possibly reclaimable as a much thinner version.

I apologize for sounding harsh earlier. The info on HT is readily available, including here in Shop Talk, and always mentions that foil wrap, or an inert gas/vacuum oven. It is common knowledge that all stainless and high alloy blades are wrapped in foil packets unless done in a special type oven t that excludes oxygen ( inert gas shield or vacuum). It is sort of like reading a recipe and it saying, " beat eggs briskly for 3 minutes", they don't say to crack the eggs and discard the shells ... because it is common knowledge.

I suggest reading the info in the stickys on knifemaking, HT, metallurgy, etc. It will shorten your learning curve and reduce the error rate.
In the future, what most new makers do is post sketches of their project before starting to get some tips that will save heartache, and topost the HT plan, so those who do it regularly can point out any omissions ... like the foil packet.
 
Get the high temp stainless heat treating foil, I think it’s the 309 alloy.

Hoss
 
At the time and temps used, depending on the steel thickness, you could very well be out of luck on this one. What thickness is your steel? Without something like 309 stainless heat treating foil, I would expect that the damage is DEEP. Decarb is the damaged steel, not just the stuff that’s black. By eye, it will look like normal steel as you grind, but it won’t actually be hard. By the time you grind away all the damaged material your knife will get significantly thinner and maybe narrower as well. I hate throwing things away, but if it were me I would likely end up scrapping the blades and starting over with a much simpler carbon steel. If for some reason the blades didn’t harden under the decarb layer, then you won’t really even be able to tell if or when you have ground through the damage to good steel. Looking at the amount of effort, uncertainty, and the added cost of special heat treating foil, I would probably go a different route.
 
DevinT said:
Get the high temp stainless heat treating foil, I think it’s the 309 alloy.

Hoss
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I’ve seen 321 recently too (fastenal). It’s good up to 2000f. I was looking to buy a larger amount rather than paying the surcharge on repackaged smaller amounts. 2000f leaves a few steels out of the process though.

309 is good to 2240f. I just looked it up. I also saw 1800f in another listing for 321. Go with the 309.
 
Grayzer86 said:
At the time and temps used, depending on the steel thickness, you could very well be out of luck on this one. What thickness is your steel? Without something like 309 stainless heat treating foil, I would expect that the damage is DEEP. Decarb is the damaged steel, not just the stuff that’s black. By eye, it will look like normal steel as you grind, but it won’t actually be hard. By the time you grind away all the damaged material your knife will get significantly thinner and maybe narrower as well. I hate throwing things away, but if it were me I would likely end up scrapping the blades and starting over with a much simpler carbon steel. If for some reason the blades didn’t harden under the decarb layer, then you won’t really even be able to tell if or when you have ground through the damage to good steel. Looking at the amount of effort, uncertainty, and the added cost of special heat treating foil, I would probably go a different route.
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My gut says start over too. It “might” be salvageable, but why chance it?
 
Before you HT again that steel I would check hardness after grinding decarb .......
 
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