Multiple temper questions

[Joe Watson]

Hey guys

Ive managed to crank out a handfull of decent knives, and I think Im ready too attempt my own heat treating.

Ive read in several places that 2-3 tempers is better than one, after searching online and reading through a Wayne Goddard book Im still a bit confused. Wont tempering again at the same temp after the 1st temper lower the Rc hardness more? What should be done in-between the tempers, and how long do I wait inbetween?

If it matters I'll be doing this with O-1 in either a toaster or conventional oven, Ive yet to find out what works better. Id also like to try some air cooling steels like ATS-34.

 

[Russ Andrews]

Unclejoe357,
O1 should be tempered twice.
Heating to quench creates austinite which is mostly transformed to
martinsite uppon quenching.
The first tempering cycle tempers the martinsite, but also transforms
more austinite into untempered martincite.
The second tempering cycle tempers the remaining martincite.
A third temper would be of minimal benefit.

A kitchen oven or toaster oven will work, but don't trust the dial.
Use a good oven thermometer.

ATS-34 is a different ball game. Because of it's high chrome content,
it will retain more austinite and require higher tempering temps (960-975F)
than can be achieved in a kitchen oven or toaster oven.

Best of luck to you,
Russ

 

[rlinger]

As Russ says except that I will take difference in his recommended tempering range for ATS-34. Most stainless martensitic steels should, I believe, be temperered lower than that in order to preserve their corrosive resistance.

Russ explained it well and all I can add is a couple things. The smaller the oven cavity (house hold equipment) the more consistant the temperature, so a toaster oven is my preference over the large kitchen oven. You can not temper too much, so if you do not acheive the aim hardness in a couple tempers it will not hurt at all to do a third or more. If you over temper the only way to get higher in steel hardness is to repeat the entire heat treat.

You are correct. The second temper many times will lower the Rockwell hardness reading a bit. It can range but if done reasonably the difference can only be a half point to a point. I usaully, but not always, perform my second temper at about 20 to 30 degrees less than the primary temper. Even at that I usually expect about a half point drop in hardness.

The first temper should be performed immediately upon the steel acheiving hand warm (about 125 to a maximum of about 150 F) after quenching. High alloy steels can go a little cooler before tempering but should also be tempered immediately. After properly tempering the first time you can wait until the next day to proceed with further tempering.

For your O1 and all tool steels, save only a very few, you should always perform the first temper at about no less than 400 F and do NOT temper tool steels between about 500 and 700 F. Always use a trusted oven thermometer because as Russ advised you can not trust the oven dial.

Temper your tool steels for a bare minimum of 1 hour per and I suggest kicking in an extra 10 minutes for the blade to equalize at temperature. ALWAYS have your tempering oven preheated to temperature before quenching the steel.

High alloy steels, such as ATS-34, require minimum 2 hour tempering per.

RL

 

[Joe Watson]

I have the toaster oven, thermometer and a few test blades. I'll give it a whirl soon.

Thanks for the help guys.

 

[mete]

Roger is right.There is a way to make the temperature more uniform especially in a kitchen oven. Put some brick in the oven, this will greatly reduce temperature fluctuations .[works for bread making too ! ] always preheat and check with a thermometer. The 1 or 2 hours is a minimum, longer times don't cause any problems.