Please explain mechanical of tempering

[shqxk]

I think it would be good for people to understand the basic of how tempering process effect on structure of steel.

1. Why tempering reduce hardness and increase toughness? Where's carbon goes?

2. How does tempering affect on retained austenite? I heard secondary hardening range tempering is almost eliminate all RA, but how?

3. I heard (again..) steel will lose some toughness over period of time after HT. Why this happen? Could process like cryo able to prevent this?

 

[Nathan the Machinist]

2. I heard secondary hardening range tempering is almost eliminate all RA?

Yes to some extent, but not as much as folks once thought. A piece of high speed steel, even after three ~1000 tempers, is still ~20% retained austenite. That's a finished cutting tool that is 1/5th RA, if done with a standard heat treat.


* Tool Steels, 5th edition

 

[mete]

Nathan , so much for those who think that RA is all evil !
Tempering removes some of the carbon from the martensite and that carbon forms carbides. In the process the dimensions of the martensite crystal change. The tempering process also makes some of the RA
convert to martensite .That martensite then converts then has to be tempered -thus the second tempering . Some of the tempering also stabilizes the RA so it resists conversion.
Sub-zero and cryo convert more RA and further stabilize the RA .1,2, or 3 temper cycles and sub-zero and cryo minimize the remaining RA and stabilize it .That in turn is all a concern for long term dimensional stability .

 

[Stacy E. Apelt - Bladesmith]

I will try a basic simple language explanation.


When the steel is quenched it becomes brittle martensite. This is under a lot of stress and will break or chip easily. The steel may also have some amount of retained austenite that did not convert to martensite. The first temper will convert the brittle martensite into tempered martensite. The structure of the martensite becomes a little more like pearlite. This can move some of the carbon to combine with alloy elements in the grain boundaries. The new tempered martensite is tougher, but less hard. Upon cooling from the first temper, some of the retained martensite will convert into new martensite, which is brittle. The second temper will convert this new martensite into tempered martensite, and upon cooling some more retained austenite will convert into new martensite. This new martensite is usually such a small proportion of the mix that it does not need a third temper except on high alloy tool steels.

In the old days of very simple HT and short tempers, the steel was sometimes referred to as needing to "age". This was to allow some of the brittle martensite still in the metal after the short temper to finish converting. At room temperature it is much slower than at 400F. Today with better HT, that is not an issue.

The stories of steel loosing toughness and/or growing harder was most likely due to the steel being of a much lower hardness in days past. Most knives were hardened in the low Rc50s. The blade was very tough, but not all that hard. Through sharpening and use the steel slightly hardened more. This decreased the toughness.