Brittle Steel....annealing gone wrong?

[Pairomedicsfish]

The was shaped from a nicholson file....I know....mystery metal! but I had it in the shop and have had good luck on prior knives. Anyway, I heat treated to non magnetic, ran it up a little higher, then two cycles in the oven at 375....the thermostat runs a little high....Back in the shop, the knife was pretty hard, a file skated off of it and I tried to drill the tang with no luck....Well, it snapped....so I took this as a bad sign...duh!. I went ahead and broke the blade and look at the grain....any thoughts as to what may have gone wrong?

[Doug Lester]

I'm a little confused as to what was going on here. I'm not sure why you evidently did two tempering heats after austinizing the blade. How was the blade cooled after austinization? Did you forge it or was it strictly stock removal? I think we're going to need a little more information here to help you out.

BTW, the grain in that steel looks too course. That itself can lead to embrittlement but I'm not sure if that would be enough to account for what happened.

Doug

[Pairomedicsfish]

It WAS forged, some...I almost forgot. The blade was heated in a propane forge to a temperature where I could put a bend in the blade. It was then air cooled. Stock removal followed. It went back in the forge the next day, heated to non-mag plus some and quenced in canola oil which was pre-heated to 120. Once it was quenced, it went into the oven for tempering. The oven was pre-heated to 375, which gets me in the 350 range....I did two cycles, because thats what I have been finding in my research, is one enough? I posted the pic to show the grian, hoping someone could tell me if was too coarse....and you did!

[Kevin R. Cashen]

Overheated in the quenching heat, the grains are huge.

[Eli Jensen]

I agree with Kevin

[Pairomedicsfish]

Sounds good, time for the crayons....

[Ray Rogers]

Anybody would be crazy NOT to agree with Kevin. In addition, I feel like 350 F is too low even if the HT had worked out ...

[Doug Lester]

Nice to have you back as a regular contributor, Kevin.

Pairomedicsfish, somewhere along the line you got that steel too hot for too long. A file with enough carbon in it to make a blade will be made of some type of hypereutectic steel. That is greater than 75-80 points carbon and probably up around 100 points. To austinize for quenching you will need to hold the steel between the Ac1, 1435 degrees F, and Accm, 1600 degrees, for no more than 5-10 minutes. I would actually try to keep that upper temperature to about 1500 degrees just to give some wiggle room on the soak. The temperatures quoted was from a carbon phase diagram for 1% carbon with the critical points being shifted to reflect that the steel is being heated.

Also, you did not anneal the steel, which, if you've read Kevin's post on annealing, is a good thing with that probable carbon content. That tends to grow large plates of carbides that will give you problems machining later. If all you did was heat the blade and let it air cool then you normalized the steel which will still reduce hardness enough to machine easily. Soak no longer than is necessary to make sure that the steel is heated all the way through. Annealing is austinizing the steel and then cooling it slowly in an oven or insulating material.

If you ever suspect that you have over heated the steel at any time you can normalize the steel for three cycles, making sure that you don't overheat the steel again. That will cause the crystals to reform at an ever decreasing size.

Go back and try again watching your temperature more closely.

Doug

[Kevin R. Cashen]

Originally Posted by Doug Lester

Nice to have you back as a regular contributor, Kevin...

..., if you've read Kevin's post on annealing, is a good thing with that probable carbon content...

Doug, I think your are like me in that you visit enough forums that you forget which one is which, I do it quite a bit. My annealing thread is over on Knifedogs

[Pairomedicsfish]

Thank you gentlemen....Ordering steel from Sheffields on Monday!

[Doug Lester]

OOPS!! My bad.

Doug