Blade hardness

[gnique]

There is something that I do that I have been told that I should NOT do. I make blades by the batch. I first do all of the shaping that I am going to do while the blade steel is as it came from my supplier. The next thing I do is apply an anti oxidant compound to all of the blades. The next thing that I do is harden all of the blades. I have read that a blade should be tempered very soon after hardening. I don't do that. It takes just as long for the oven to cool down as it does to heat up so I let the internal temperature drop to about 1000 or 1100, pop in a blade and run the instruction set that I have programmed into the controller. I pile up the hardened blades until I have hardened them all. I then temper them all once (as many as I can get in my oven) and then straighten and further temper them one at a time. The straightening part is important to me because I use thin (3/32" and 1/16") steel for kitchen knives.The only method that I have to check for hardness is with a set of hardness files that I got from Grizzly Tools. Here is my question and a finding that I think I have perceived: Am I doing any real harm in waiting up to a week or two weeks to temper hardened blades? Also I seem to have found with my files that the blades are one hardness just after quench and usually a higher hardness at sometime an hour or day later. Is this right? I have never been given any dependable instruction on how to correctly use hardness testing files - I just kinda do what seems right to me. But I swear that my blades sure SEEM to get harder after some time has passed after hardening. I am a numbers person so I really don't even LIKE the word "seems" but, as I said, all I have are those files. One more thing, 15N20 steel is the steel that seems (that word again) have the most noticeable change in hardness. The hardness never seems to go down; only up. I don't even know if this is even worth wasting time on but both these items have piqued my butterfly like curiosity. Nicholas Jasper

 

[akey]

Seems like I remember a post recently about 1095 hardening well after quench but I think it was a day or something not a week. One concern with waiting so long is there is more of a chance of blade getting shattered.

 

[GHEzell]

I had one blade crack about 5 minutes after hardening but before tempering just setting on a cold anvil.

I have another blade I hardened 8 years ago and never tempered (lost interest in it), and it ain't cracked yet...

For safety's sake I like to get them in the tempering oven ASAP, I'm not saying you have to, but you never know...

 

[Kevin R. Cashen]

I can give you a quick answer with one word- Nickel. Due to the comparative size of the distortion caused in the lattice by nickel atoms it is one of the more powerful austenite stabilizers. If you want to make a steel stay completely austenitic at room temp, as in "austenitic stainless" you alloy it with nickel. But even moderate amounts of nickel will have some effect on how quickly austenite converts, and I often see the very thing you are observing in simpler nickel bearing alloys. I had a friend show up one time with a sword blade made from tootie fruity (an eclectic mix of steels) damascus that he wanted to heat treat, some of those steels had nickel and the carbon content was not accounted for. We ran it through a cycle but it was still soft, I was blade to bend the blade and the hardness readings were lower that I preferred, but it was late and we went in to have a couple of beers. The next day, after a cool Michigan night, I went to bend the darned thing straight and couldn't, a quick check showed that it had gained at least two Rockwell points overnight.

What I would do is either marquench the blades or give them a quick snap temper and then put them in cold water to be sure they are entirely cool before testing, and then get to you temper as soon as possible. Marquenching or snap tempering buys you some time when waiting for a temper, but a blade quenched all the way to cold with no flash temper is just waiting to come apart, you may get lucky but eventually the odds will catch up to you if you are actually fully hardening the blade.

I hope this helps to make head or tails of things.

 

[gnique]

So it could be the nickel. I wonder if by extension the same thing would happen with L6. I used L6 (could have been L6 - Ha!) when I was cutting up saw blades that I got at the scrap yard. I was happy to just get the scale rivets to lineup in those days. I may order some L6 and see what happens. Thank you all. At least I know that I am no crazier than I am used to being (one gets comfortable with ones usual insanity over time). I BELIEVE that I am getting my steel near to a fully hardened state. They files skate and they hold a reasonably good edge. Thank you all again. Nicholas Jasper