Questions on a failed 3V heat treat

S

Steve_p

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Apr 9, 2019
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I did a few 3v blades over the weekend. Normally I use stainless foil wrap, but I was out so I used anti scale compound instead. I definitely noticed way more decarb than I normally get, then when I started grinding it away I noticed tons of small cracks all over the blade.

I broke both, to my novice eye the grain structure looks ok but what I’m curious/worried about is the dark ring close to the surface. Any thoughts about what I might have done wrong? I’m thinking I didn’t apply the anti scale correctly and I burned up all the carbon.
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The second blade looked the same Once broken, just a way more jagged break so it was difficult to get a good picture.
 
1975f for 30 min
Plate quench
Dry ice/denatured alcohol overnight
Temper at 400f 2hrs x2

after the plate quench they were still around 200-250f so I cooled in water briefly before going into the sub zero
 
Just a comment, but there is no need of advantage to the overnight soak in the dry ice bath. Five or ten minutes is sufficient. Once the blade reaches the -90°F temp, the Mf is done. It probably won't hurt to leave it, but isn't necessary. The best method of HT is to do the first temper immediately after the blade reaches the Mf.

A dry ice bath is to reach the Mf point.
Cryo, in liquid nitrogen at -320°F requires a longer time to create changes in the structures and form eta carbides. Overnight won't hurt in cryo, but it really only needs an hour or so.
 
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Theoretically it is better to go straight to tempering after cold treatment to Mf, rather than waiting over night yes? Or am I remebering wrong?
 
There can be a minor danger to long cold treatments for the same reason many datasheets recommend tempering right away (the steel is brittle and may crack without tempering). That said, I’ve never heard of a knife cracking in cryo overnight apart from laminated blades, probably because of the thin cross-section used in knives. I often leave knives in cryo overnight to let my furnace cool down to temper in.

Leaving in liquid nitrogen for extended periods of time does not lead to any benefits either, I have written about that extensively in the past.
 
If you left them in dry ice slurry over night and the reach room temperature they may crack from the stress waiting to be tempered.
 
Steve_p said:
after the plate quench they were still around 200-250f so I cooled in water briefly before going into the sub zero
Click to expand...

This is a problem. This steel is forming most of its martensite in this temperature range. You want a relatively rapid quench at first that gradually slows down a little during the martensite formation temperature range. It is usually okay to dunk hot steel into water most of the time, but never during the primary quench.
 
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