How soon after hardening do I need to temper?

[zhyla]

The steels I'm working with right now are 1095 and O-1. I've read several pages on heat treating these steels and most of them seem to recommend tempering immediately following the quench. Can someone expound on that? How soon is soon? What is happening in the steel while it's cool but in the hardened state? I think I've read someone recommending tempering before the blade is fully cooled, like catching it when it's around 400 F and then starting the temper there. I haven't fully internalized the chemistry of steel yet (getting there).

For some context, I'm trying to figure out if I can harden a steel one evening and temper it the following morning or evening. The reason is merely practical, my free time is mostly in the evenings and I only have one temperature controlled oven so I have to wait for it to cool down from 1500 F before tempering. My 2nd blade is all ready for heat treatment and I really don't want to wait for the weekend...

Thanks!

 

[samuraistuart]

NO do NOT wait until the next morning. You might/might not find your blade is cracked in the morning. Do NOT temper immediately after quenching. You need to get to the temper once the blade has cooled down to where you can handle it with bare hands. (I wait until the blade is room temp) The martensite is being formed during the cooling from quench down to room temp (for the steels like 1095 the finish point is actually a couple hundred degrees), and that martensite needs to finish converting before you temper. Generally, about 15 minutes after your quench, the blade should be cool enough to temper. If you can hold it with bare hands...you're probably good to go. If you wait too long...like overnight...the stresses that build up may be too great. If nothing else, you can run a "snap" temper....maybe 350F for 30 minutes or so....and then wait to complete the rest of the tempering process the next day. The tempering process you mentioned about "catching" it around 400F is called mar-tempering (I think)...and I wouldn't really recommend it myself.

 

[zhyla]

Ah, that pesky martensite.

Well glad I asked. Sounds like I need to eventually throw a PID controller on that toaster oven that's been collecting dust all this time...

 

[Stacy E. Apelt - Bladesmith]

1095 can easily crack sitting quietly on the workbench while you sleep. I would say to do 1095 and any hypereutectoid steel within 30 minutes. 15 minutes is even safer.

Carbon steels should generally be tempered within half an hour. Stainless within an hour or two.

Metallurgically, the tempers should follow the quench immediately for best results. Practically, it should be as soon as possible after the steel has reached Mf. No need to run to the house fromm the cryo tank, but as soon as you can with the normal delays of cleaning off the blade and heating/cooling ovens, etc.

I turn the tempering oven on when I get ready for hardening, and after quenching and straightening the batch, I wash the blades gently and place in the ready oven. I go wash up, shower, etc. and come back in an hour to dunk the blades in water to cool down to room temp and put right back in the oven for the second temper. I have a beer, snack, meal, etc. and then come back an hour later to take the blades out and cool in water. HT is done!

If you don't have the time to do a full temper on a blade, do a snap temper at 300F for half an hour. Then do the two full tempers later. I would not delay them any more than a day, though. Also, after the first temper, you can wait till tomorrow to do the second.

 

[Gendry]

I've been hardening my 1084 knives in my forge at work, which is about a 15 minute drive from my house. So say, after I have hardened a blade, I have 10 minutes to clean up my forge area, another 15 to get home, still warm to the touch as I leave, cool by the time I get home. I immediately put them into the oven for temper (not great I know, I know man.) And I have never had any stress problems, that I could notice... Then again, I have only followed the instructions for 1084 harden/temper about 5 times, so obviously I really have no idea what I am doing.