work hardening?

[miner49r]

Everything has gone well until Murphy's Law showed his ugly head.

I was drilling some unknown steel out of the scrap box and all of a sudden the bit just wouldn't cut anymore. Upon inspection, the 5/16" HSS looked like a brad point. Another bit promptly dulled. So, I annealed the piece with a propane torch and tried a third bit. Not a scratch. Where did I go wrong?

Let's see. It turned in the lathe, surfaced in the mill and the 1/8" holes drilled without pause. After annealing the surface had a brittle carbon shell. Maybe the propane torch isn't hot enough to anneal this steel. Must be some high carbon stuff.

Guido

 

[Ken I]

The torch is probably hot enough but it needs to cool down really slowly.
Try annealing it in a charcoal fire and let it burn out slowly - unless of course you have access to a furnace.

Ken

 

[miner49r]

Thx Ken,
I am sure you are right about having the piece cool slower than in free air. I'll try that tonight and see what happens in the morning. Time to fire up the hibachi.
Alan

 

[Omnimill]

I had the same thing happen some years back with some stainless steel plate. I'd already sawn a piece to size and milled a slot in it, also drilled a 5mm hole in it then tried to open the hole out and it wrecked the drill. I heated it cherry red and tried again with a new bit and it went through fine.

 

[miner49r]

Ken,
I learn something every day. Thanks to you I learned that air cooling is too fast for some metals. My Hibachi grill made a decent little furnace. I will remember that.

I have only run into this problem once before trying to turn a bushing out of a piece of 440 stainless. IIRC, it needed annealing about every third pass.

Alan

 

[Ken I]

Yeah - work hardening materials can be a right SOB - plenty of power big feeds and carbide tips with feed rates higher than the work hardenening depth will keep you out of trouble.

Its a case of he who hesitates is lost - if you allow the tool to rub - just once - you are stymied.

You can't "feel" the tool (drill / whatever) but must charge in at feed rate - you just can't muck about.

I had a similar problem screw cutting some taper threads in a weird stainless grade (mystery metal supplied by the customer) for the petrochemical industry - I never want to do that job again.

Ken

 

[miner49r]

You nailed it on the head Ken. I had stopped just long enough to grab the next handle on the down feed. That's all it took. I'll have to remember to lift slightly pulling the bit off the cut before switching handles.
Alan

 

[Ken I]

Sorry to hear that - if you must interupt the cutting (as in peck drilling) do so abruptly - back out - resume at feed rate - go at it aggresively.
The meek might inherit the earth - but they're not going to make it through work hardening steels.

Ken

 

[Springbok]

Throw it away and try another bit of scrap from your scrap box and not damage any more tooling

Bob