How to fix a wonky 1095 heat treat?

J

Jason Fry

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Jun 5, 2008
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I'm bringing along a group of knifemakers who are relatively inexperienced. One of them brought me a batch of knives to rockwell test last night. One of them tested around 59, but the others were all still soft. The steel was 1095.

The reported heat treat was to heat it to "nonmagnetic" in a charcoal forge, then quench in warm/hot motor oil. All of the blades had some of the bubbly look that I typically think of as signs of over-heating. Just for kicks, I also ground one down a bit to ensure I wasn't just testing surface de-carb.

My theory is that the HT failed because he overheated the steel, then used too slow a quench. If I understand right, that leaves grain growth and mixed pearlite, both of which contribute to a soft blade.

If I'm right, and the steel is currently mixed structures with large grain, what needs to be done to get things back to good? I assume the steel is enough of an unknown mess so that another heat/quench alone won't do much good.

I pointed him toward less heat and a faster quench, but what else? Normalize in descending heats? Sub-critical anneal? He doesn't have the temp control to do these steps well, but I could let him use my oven.

How do I best help this guy? I offered him a couple feet of 1084 already.
 
He may or may not have overheated the blades. Soft usually indicates that the blades went into the quench too cool. Also, warm motor oil won't harden 1095 worth a darn.
The fire might have been too hot and/or had a very oxidizing atmosphere.....and he burned/blistered the steel while getting it up to heat. Also, if he used coal, not charcoal,the coal may have been sulfurous.

Overheating won't necessarily give a soft blade, but it can make a brittle blade if excessive grain growth occurred.

He should clean up the blades, do a three or four step normalization series ( Something like - 1650, 1550, 1450,1300...or 1650 1400,1300) and quench from 1475F into fast oil.
 
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thanks Stacy. I was corrected on another forum as well regarding grain growth vs. hardness. I guarantee he used too slow of oil. I believe he was using briquette charcoal, but he does have a brick enclosure if I remember right, so he might not have had too much of an atmosphere problem.

I pointed him toward less heat and a faster quench. I'll pass along the info on normalizing. That was what I suggested, but I wasn't sure if that was the best answer.
 
Charcoal briquettes are full of all sorts of impurities. You can get along with forging on them, but HT will be nearly impossible for anything but 1084. He would do far better with lump hardwood charcoal.....or forging coal ( Pocahontas #3). He should be able to find it at the big box hardware stores as well as restaurant suppliers. Tell him to lower the air blast ( probably a lot lower) and try for the lowest glow that will take it above non-magnetic. Time is his friend, not his enemy. If it takes two or three minutes to get to non-magnetic that is far better than getting there in 30 seconds. With the slow heat up, he can keep it slightly above non-magnetic easily for a few minutes. In the overly hot colas, it may well be 1800F in 30 more seconds.
 
Usually theblade has to be quenched from about 75F above non-magnetic.
 
Yes, I was just referring to getting it hot enough, but not overheating. Once at non-magnetic, it needs to be heated about 50-75°F hotter, which is about one shade of red more.. That is also a problem with a charcoal/coal forge, as you can't see the blade color if it is buried in the coals.

I know there are hundreds of coal forge guys who will jump up and yell at their computer screens when they see this, but I consider a coal/charcoal forge in the hands of a novice as one of the worst HT devices. They rarely can control the heat, usually have a very bad atmosphere, and have no idea what temp the blade is at....then, when the HT doesn't turn out right....... they ask what is wrong with the steel.
 
You could lower the grain size by normalizing it many times, but you will face problems with decarburation...

What can you do for the guy?
Get him different steel.
Because there is nothing wrong with 10xx why couldn't he start with some easy, grain-enlargment resistant AIR hardening steel?
Or even better, some super easy hypoeutectoid tool steel?
 
I think with a thin edge canola oil would go a long ways as far as improving hardness on a budget. I use brine and a thicker edge...
 
You might not be able to help him. You have offered a more appropriate steel, but did he accept it? I would suggest asking him to choose between setting up a forge with pyrometer at minimum, and proper oil, or the much simpler solution of different steel. The other option with quenchants might be a brine interrupted quench then finish the quench in heated canola. That is a lot of additional variables though.
 
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