Can one of you masters explain tempering?

Leu Custom

Leu Custom

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Jun 2, 2007
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What is the difference between tempering a blade for 2 hours - 3 times with cooling to room temperature between each tempering, vs. tempering a blade for 6 hours all the way through just once? Mete or Kevin Cashen or Bladsmth or any of the rest of you got an answer for me? Thanks in advance !
 
It's awfully early in the morning to answer a question like that . It has to do with the more complex steels -simple steels 1 temper, complex 2 tempers, most complex 3 tempers !! The reason is that the complex steels can form more martensite from the retained austenite on cooling after tempering and that new martensite has to be in turn tempered. A 1080 would do fine with one temper, S30V should have three tempers !
 
As mete said, when you raise the temperature of the blade form below the Mf point (approx. 200 for most simple steels, but as much as below -100F for complex steels), to between the Mf and the Ms point, some of the retained Austenite can convert to Martensite. This doesn't happen on the up swing , but when the metal cools back down from the temper (falling back through the Mf point) . That means that the new Martensite is untempered. A second temper (or third) will be required to finally have the blade get as much tempered martensitc structure as possible. More than 3 tempers is a case of diminishing returns in any steel we will deal with.
The more alloy ingredients, the harder it is to get all the Austenite to convert. With a simple steel ,like 1080/1095, all the austenite usually converts with the quench (hopefully to Martensite), thus a single temper will give you a tempered martensite blade.A double temper is a good rule to follow, though.

One thing I regularly notice is newer smiths trying to temper at a very low point. This is counter productive to what the temper does.The blade will not suddenly become soft as lead at 350F (or some imagined point). It has to go well above the Ms point to start getting a rapid decrease in hardness.A look at the HvsTT charts will show you that most carbon steels would be fine in excess of 500F. 1080, and O-1 both will be Rc60 at 500F. What the blade does gain when you temper at the right temp, is an increase in toughness.
Stacy
 
So if a blade has been cryoed after quenching, is there still likely to be more martensite after the first and second tempers, or are we bumping even more up against the law of diminishing returns.

I guess it also begs the question; If cooling to room temp or hand warm between tempers, converts more austenite to martensite, maybe we should be going all the way to Mf between tempers...?

Rob!
 
Retained austenite is not all bad as some think.You'll always have some and it adds toughness. Proper hardening temperature minimizes the problem. Excess hardening temperature gives you more and of course adds grain growth to the problem !! ..Tempering stabilizes RA too ! Careful control of temperatures and knowing the proerties of that particular steel gives the best results ....Stacy , thanks for filling in the blanks which I had in the early hours !
 
Rob,
After the second temper cycle on a cryo'ed blade, it would gain a slight advantage with a second cryo and third temper. After that it would be gaining only fractional percentages of martensite, and not worth the investment in time and energy.
An example is:
"SuperXX steel with CrV" will benefit in martensite conversion at -200F .
You snap temper after quench, then cryo in LN.
Next you temper. Now you have (fictitiously) 97% martensite
A second trip through the Ln and back to the oven will increase the martensite to ,maybe,98%. A third would add (at most) another 0.5%, a fourth 0.25% ,......(diminishing returns).Cycles three through ten would not give you an additional 1% of martensite,total.
Stacy
 
well then that clears everything up with my question basically :D Thanks. Or enough that I understand what the point is.
 
so stacy you say that tempering has the affect on the return to room temp below the Mf point. so does that mean that time at tempering temp is not importint but that the blade be compley and evenley soaked. that said, does that also mean that a blade can soak extreamley long with no addvirs affects.
 
The martensite transformation is a shear type transformation taking an instant . Tempering is a diffusion type transformation taking some time .We use two hour tempering for blades to give a thorough, uniform structure throughout the blade.If you forget to take the blade out of the furnace and leave it overnight there won't be a problem .
 
JT,
Two things are happening in the temper. The most important it the evening out of the structures, allowing the stress of quench to be relieved, and the metal to become less brittle and more tough. That is what the temperature and time do. The second is the transformation of retained austenite. That happens, as mete said, instantaneously when it crosses the Mf point. Of the two things the structural adjustment is far more important than any gain in martensite.Thus the two hours at a target temperature done twice is recommended for good blade quality.
Stacy
 
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