Tempering advice

[Zachary Reed]

Question for knife makers. I’ve made a handful of knives with some success but it’s been years. A friend who is interested but has never made knives before asked for my help so we built a small forge and got some 1084. After three normalizing cycles and quenching the steel skates a file nicely. It was pretty late so I tell him to temper in the oven for two rounds of two hours @ 400, which he neglects to do that night. The next day neither of us are available to do it so two days after hardening he does one round of two hours @400. I tell him to do the second round as soon as it’s back to room temp but again he neglects to do it again. Tonight he will do the final temper cycle.

Can someone tell me if delaying the tempering after hardening and also spreading the tempering cycles by 24 hours will be harmful to the heat treatment? Thanks for reading.

 

[velegski]

its a crap shoot. Because of the internal stress caused by heat treat the blade could have cracked during that long wait period. As for the second tempering cycle there should be no trouble. The first set most of the molecular structure in place and relieved the stresses.
Ps. I'm not expert and its just my way of doing things But if I can't complete the process heat treat cycle at least one tempering cycle I dont start it.

 

[Zachary Reed]

its a crap shoot. Because of the internal stress caused by heat treat the blade could have cracked during that long wait period. As for the second tempering cycle there should be no trouble. The first set most of the molecular structure in place and relieved the stresses.
Ps. I'm not expert and its just my way of doing things But if I can't complete the process heat treat cycle at least one tempering cycle I dont start it.

Appreciate the insight. I’ll finish them anyway and report what I find.

 

[Stacy E. Apelt - Bladesmith]

In 1084 you will probably be OK.

Here is the science:

When hardened, the steel is brittle martensite. Internal stress from the quench or a blow to the blade can make it snap in half or crack.
The first tempering cycle relieves the stress and rearranges the atoms a bit to make the steel tougher and less brittle. The sooner you do that, the better.
The second temper makes the blade even tougher.

During the first temper most of any retained austenite is converted to new brittle martensite. In simple carbon steels, retained austenite is usually a very small percentage and not likely to make the blade fail. The second temper will convert this new brittle martensite into tempered martensite and also soften the previous tempered martensite from the first temper a little more. Two tempers is all that are needed for most all blade steels.

A steel like 1084 would likely have almost no retained austenite, so there is really no chance that the delayed second temper will be an issue. The main concern will be the very brittle condition post-quench. The longer it stays in this condition the more risk of blade failure.

If a full temper cannot be done a snap temper is advisable. This is done by heating the blade to around 300°F and holding for 10 to 30 minutes. This will relieve the stress enough to allow a delay of a day or so.

High alloy steels which may have high amounts of retained austenite NEED immediate tempering as soon as te steel reaches the Mf point. If delayed the austenite may become stable and the blade will not have the desired hardness.

 

[Zachary Reed]

In 1084 you will probably be OK.

Here is the science:

When hardened, the steel is brittle martensite. Internal stress from the quench or a blow to the blade can make it snap in half or crack.
The first tempering cycle relieves the stress and rearranges the atoms a bit to make the steel tougher and less brittle. The sooner you do that, the better.
The second temper makes the blade even tougher.

During the first temper most of any retained austenite is converted to new brittle martensite. In simple carbon steels, retained austenite is usually a very small percentage and not likely to make the blade fail. The second temper will convert this new brittle martensite into tempered martensite and also soften the previous tempered martensite from the first temper a little more. Two tempers is all that are needed for most all blade steels.

A steel like 1084 would likely have almost no retained austenite, so there is really no chance that the delayed second temper will be an issue. The main concern will be the very brittle condition post-quench. The longer it stays in this condition the more risk of blade failure.

If a full temper cannot be done a snap temper is advisable. This is done by heating the blade to around 300°F and holding for 10 to 30 minutes. This will relieve the stress enough to allow a delay of a day or so.

High alloy steels which may have high amounts of retained austenite NEED immediate tempering as soon as te steel reaches the Mf point. If delayed the austenite may become stable and the blade will not have the desired hardness.

Very helpful. thanks

 

[Stacy E. Apelt - Bladesmith]

I corrected a typing error in the post where I used the wrong structure. It is "Retained Austenite", not retained martensite.

 

[Zachary Reed]

The knives seem to be just fine. In an attempt to fix the warping I had to bend the blades to some pretty extreme angles. If it were going to shatter or crack that would have been the time.