5160, tempered for 4 hours. . . Oops!

me2 said:
Stacey's got it. An exponential increase in time (from 1 hr to 2 hr to 4 hr) might result in the loss of a point or 2 of hardness, depending on the target hardness.
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Read the underlined part again. Sorry to the OP for turning his thread into an episode of Big Bang Theory.
 
I think he probably made a decimal error. He may have meant .1 to .2 Rc lower.

In any case, I think we all agree that the knife in question is fine, and normal knife tempering in a well regulated oven is good at 2 or 4 hours.
 
Stupid question alert!

So is it the cool down cycle that shaves Rc points off the steel? If I temper at 400 for 2 hrs, cool, then 400 for 2 hours again I'm going to end up with a considerably (2 or 3 Rc) softer blade, but if I just leave it at 400 for 4 hours I will only achieve at most a point reduction??
I've only made a couple knives and this thread is kind of changing some things I thought I understood.
 
Binalith said:
Stupid question alert!

So is it the cool down cycle that shaves Rc points off the steel? If I temper at 400 for 2 hrs, cool, then 400 for 2 hours again I'm going to end up with a considerably (2 or 3 Rc) softer blade, but if I just leave it at 400 for 4 hours I will only achieve at most a point reduction??
I've only made a couple knives and this thread is kind of changing some things I thought I understood.
Click to expand...

Not from what I've read. It is the time at temperature.
 
The first temper converts the martensite ( created in the quench) into tempered martensite, which is tougher and less brittle. It is a little softer than the As-Quenched hardness, but much more durable. This takes at least an hour to happen. Most recommendations are to do it for two hours to assure a complete change. Longer will accomplish nothing. As the steel cools from 350-400F ( you can just cool it in water), any retained austenite hanging around will convert to martensite. The cool down to room temp between the tempers is necessary for this conversion to happen. This is new martensite and is brittle. The second temper at 350-400F will transform that new brittle martensite into tempered martensite. The blade looses no real amount of hardness after the first temper. There are metallurgical reasons to quickly cool the blade from the temper to room temperature. Slow cooling can allow some stabilization of RA and a few other things. A blade will not crack or warp when cooled from 350-400F with water.

There are a whole lot of Ms and Mf and other funny letters that could be used, but the above tells what is happening and why you need two separate tempers.
 
bladsmth said:
There are metallurgical reasons to quickly cool the blade from the temper to room temperature. Slow cooling can allow some stabilization of RA and a few other things.
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Could you elaborate, or point me to where this is discussed?
 
Roman Landes has done research on this. His book is Messerklingen und Stahl. He is a regular in hypefreeblade.com ( a metallurgical knife site). Cooling from black heat and from temper has been discussed there a good bit.
I think Verhoeven discussed it, too.

From black heat, quicker cooling rate avoids the formation of fine pearlite. From temper, it makes any RA drop toward the Mf more rapidly, and not stay stabilized as austenite.
This is a metallurgical reason more than a real world problem. The difference is detected in a lab by metallurgists who care about things like %RA, but you couldn't tell one blade from the other in the field.
 
bladsmth said:
Roman Landes has done research on this. His book is Messerklingen und Stahl. He is a regular in hypefreeblade.com ( a metallurgical knife site). Cooling from black heat and from temper has been discussed there a good bit.
I think Verhoeven discussed it, too.

From black heat, quicker cooling rate avoids the formation of fine pearlite. From temper, it makes any RA drop toward the Mf more rapidly, and not stay stabilized as austenite.
This is a metallurgical reason more than a real world problem. The difference is detected in a lab by metallurgists who care about things like %RA, but you couldn't tell one blade from the other in the field.
Click to expand...

Thank you.
 
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