Heat Treatment - questions regarding tempering and cryo treatment theory 1

[MatthewMachinist]

I have done some heat treatment at my workplace, but not enough to have any expertise, but recently I was queried on the theory and let’s just say the “Socratic method” revealed a few (big) gaps in my knowledge, I would appreciate some help to understand the theory behind the processes.

My first question is regarding tempering; I understand that during heating to the correct temperature the steel becomes Austenite and that upon quenching it becomes Martensite (with some retained Austenite).
When you temper steel, what exactly is happening to the steel? I have heard tempered steel referred to as tempered Martensite; but I thought (and I am probably wrong) that tempered steel becomes a fine grained pearlite.
By far the largest gap in my knowledge is this, what is the difference in the steel exactly between dead hard Martensite and steel at say 60 Rockwell C, 40 Rockwell C and 20 Rockwell C, is it the ratio of Martensite to Pearlite? I really have no idea. To put my question another way, when you draw the temper what change is happening to make the steel softer? I realize this question probably opens a can of worms because steel is not uniformly pearlite unless it is euctiodal steel with a 0.83% carbon content, if the steel has higher carbon it will have some cementite, if it is lower it will have some ferrite?

My second and separate question regards cryo treatment. My understanding is that if you were to cyro treat steel after quenching it would probably break due to stress, so the steel must be tempered first.
Once tempered the steel can then be cryo treated, the idea is to turn the retained austenite into Martensite.
If this is correct then cryo treating is pointless unless the steel is then tempered a second time to “refine” the Martensite that has developed, if you didn’t re temper the steel the Martensite that has been created could potentially crack. Once tempered the second time the steel should then have a uniform structure of “tempered Martensite”?

Thanks,
Matt

 

[metengr]

I would suggest you perform a search on this web site first, using steel tempering as key words, followed by an Internet search.

 

[CoryPad]

Quick answers:

1) During tempering, the non-equilibrium martensite phase converts to ferrite plus uniformly distributed cementite. This is not pearlite (alternating lamellae).

2) Cryogenic treatment can follow the quenching process without a prior tempering. If cryogenic cooling is used to transform retained austenite, it must be followed by tempering since untempered martensite can be brittle.

 

[redpicker]

CoryPad's answers are spot on.

It appears, by your questions, that you would be interested in an introduction to heat treating course. They are online courses, and ASM has a "Metallurgy for the Non-Metallurgist" coruse that may suit you. Some of the courses are math-intensive, so you may have to look around to find one that is aimed at your skill level. But, I encourage you to find one as you definitely show an interest and increased knowledge is never rarely a bad thing.

rp