Temper question

Gliden07

Gliden07

Well-Known Member
This is really kinda a Newb question really. Why do we temper blade as soon as we can out of quench?? I never questioned it just did it so now I'm asking!! LOL!! Other than it being brittle and the chance of the blade breaking what is the reason. Thanks!
 
That's a good question. I believe that the reason is if any potentially catastrophic latent stresses are imparted during quench, getting into temper may relieve those latent stresses before time and/or circumstance allows those stresses to manifest in cracks or deformations. In other words, I think it's just best practice to minimize potential by shortening the window of opportunity for disaster.
 
Bühlmann said:
That's a good question. I believe that the reason is if any potentially catastrophic latent stresses are imparted during quench, getting into temper may relieve those latent stresses before time and/or circumstance allows those stresses to manifest in cracks or deformations. In other words, I think it's just best practice to minimize potential by shortening the window of opportunity for disaster.
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Yeah, like I said I've always just done it. But leave me alone with nothing to do and I start thinking about random stuff! So here we are!! LOL!!
 
My not scientific understanding is that certain steels will continue to harden (not sure right word) even after the quench and it may lead to cracking or warping. By tempering you eliminate those stresses and reduce the risk of such reaction. Again, my unscientific explanation...

I have quenched several knives over the years and not tempered immediately. They come out of the quench flat and the next day have warped. I hate that. I never had any crack or break. Looking forward to that, someday.
 
It’s a great question. I asked a couple years back and all the feedback was sooner rather than later.
 
Another reason is retained austenite. Tempering, or in the case of stainless steels, cryo treatment, can convert retained austenite to untempered martensite. This brings the reason for 2 cycles of tempering, to temper the untempered martensite converted in the first tempering cycle. The longer the time between quenching and tempering or cryo, the less retained austenite will be converted.
 
Gene Kimmi said:
Another reason is retained austenite. Tempering, or in the case of stainless steels, cryo treatment, can convert retained austenite to untempered martensite. This brings the reason for 2 cycles of tempering, to temper the untempered martensite converted in the first tempering cycle. The longer the time between quenching and tempering or cryo, the less retained austenite will be converted.
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Is the retained austenite conversion time between quench & temper a function on minutes, hours, or days?
 
Bühlmann said:
Is the retained austenite conversion time between quench & temper a function on minutes, hours, or days?
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Going off Larrin Thomas' articles on cryo for stainless steel, I believe he said even a wait time of 15 minutes could make a difference. You want to have a continuous cooling of the blade from quench to cryo. From my understanding, the longer retained austenite is at a steady temperature before the process that converts it to untempered martensite, the less will be converted.
 
In addition to the comments about retained austenite and the need for brevity regarding the time between quench and temper:
Freshly crystallized martensite has pointy corners. That sets up huge stresses and allows for easier propagation lines in micro- and macro-cracking. Tempering rounds the points on the crystals a bit and both reduces stresses and creates geometry that doesn't favor long cracks to propagate.

Ask this question in the Heat Treating forum and perhaps Kevin will pick it up.
 
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