Time between Hardening and tempering 440C?

L

LCoop

Joined
May 5, 2007
Messages
628
I was wondering is it critical to take a freshly hardened knife made out of 440C and temper it quickly as some other metals to relieve stress.

The reason I ask is I do my own HT of 440C but don't have a RH tester, however my father-in law has access to one where he works in a machine shop. So if it's not detremental I'd love to RW test a blade as it comes off the plate quench before I temper to see where I'm at and then of course after tempering. But this wouldn't be able to be done quickly. I'm thinking like 24hrs. Is this something do able or should I just do sample pieces of cut off pieces?
 
I don't think it's likely to rip itself apart, but I wouldn't leave my blades 24 hours without at least a minimum stress relieving. I'd do the test piece instead.
 
24 hrs is surely not best practice. I wouldn't do it to a customer blade and I wouldn't recommend it for every blade.

Having said that, I've only seen one stainless blade crack and that one was surface ground too aggressively. I'd be shocked if you had a problem with it - but don't tempt fate too often. ;)
 
I would give it an immediate temper at the lowest range for 440C then do RC testing when able. If it shows to hard for the use intended for the knife, temper higher to get down to the RC you want.
 
I'm with Eric, I would do a snap temper at 200F, and cryo, then test the blade. As soon as the blade is tested, do the two normal tempers.

The snap temper will not reduce the as quenched hardness much at all, but will stabilize things a bit. To keep the RA from getting too cozy, do the temper cycles as soon as possible, but 24 hours sounds OK to me.
 
I would be happier with a 300 F snap temper .The cracking from delaying temper may not be immedately noticeable . There may be internal microcracks which cause blade failure some time later in use.
 
Well thanks I knew I could get some straight answers here. One of the reasons I asked is I've searched several HT recipes and none I've seen mentions a snap temper. Here's cut and pasted from Evenheats web page for HT 440C:
Heat Treating Information
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Special Thanks to Rob Ridley of Ranger Original for this Information

We have had a number of people ask us for our �secrets� about heat treating knife blades. While we appreciate the compliment, there really are no secrets � and what works for us may not be your favoured solutions. Here are our recipes. You may copy them, publish them or use them as you please. We have drawn from various sources, including Crucible data sheets, other steel suppliers and postings on Blade Forums as well as a good dose of personal experience.

All of our stainless blades get double wrapped in high temperature, 309SS foil envelopes � with double folded seams pressed down firmly. Always put them in the envelope the same way � so you can put them in spine down and pull them out by the handle, instead of the tip. We use brown paper (no idea why brown � just what we use) in pieces about 2 x 2 inches. 2 pieces for an average hunter and four for a big bowie. The trick is to use enough paper to eat up the Oxygen in the envelope � without blowing up the envelope like a balloon.

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[BACK TO INDEX]154CM, CPM154, ATS34

All three of these, heat treat the same. After a full speed ramp up to temperature, they soak 45 minutes to an hour in the Evenheat Oven at 1950F degrees. Then they get placed � still in the foil � spine down � then flat - on a 1� aluminum plate. The second aluminum plate is placed on top and pressure is applied. We used to use weights for the pressure, but now we use clamps. You are looking for good firm contact. If you are thinking hydraulics you are thinking WAY too much pressure. After 2 minutes (or less) they will be hand cool, and ready to remove from the foil for cryogenics. They don�t have to go straight into cryo, but aim for something less than an hour from plate quenching. We use liquid nitrogen for cryo but dry ice in acetone will also do. The household freezer is no use at all. They probably only need a couple hours in cryo, but we leave them overnight.

The next morning, they will be in the range of RHC63+. After warming to room temperature, they get tempered. We temper twice at 500F degrees for two hours each time � to get about RHC61. You can experiment for other hardness's but 61 is a very good target for these steels.

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[BACK TO INDEX]CPMS30V

Same wrap � same temperature (1950F) - same soak time � same plate quench � same cryo as 154CM above. This steel gets double tempered at 400F degree for about RHC60

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[BACK TO INDEX]440C

Same wrap � Harden at 1900F degrees and only about 15 minutes at temperature � then plate quench and cryo as above. Hardness out of cryo will be about 61. Temper (twice for 2 hours) at 275F for RHC60 � 325F for RHC59 � and 375F for an excellent RHC57-58.

Notice they don't mention a "snap" temper.
 
A snap temper is not used by everyone .This temper is to make a blade safer to handle between quench and proper temper such as before it goes into cryo.At 300 F it tempers but not to the point where it stabilizes the retained austenite. When we use cryo it's to remove as much RA a possible so we don't want to stabilize the RA.
After cryo you must temper ,and this is done to get your desired hardness.
 
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