Since this subject has popped up in several threads lately:
(information provided is approximate for the eutectic - 1075/1080)
When steel is heated to the AUSTENITIC point (approx. 1350F) the structure changes to AUSTENITE Upon cooling it goes through several possible forms and structures, such as PEARLITE, FERRITE, CEMENTITE, BAINITE, AND MARTENSITE. The cooling rate and time control all these .Knifemakers usually want to end with martensite.
For knifemakers, once the steel has cooled at a quick enough rate to around 450F, the microstructure changes to MARTENSITE. It continues to convert from austenite to martensite over a range of temperature from 450F to around 200F. This is called the MARTENSITIC RANGE, The beginning is the Ms (martensitic start) and the end is the Mf (martensitic finish). Any temperature below 200F is low enough to move on to the temper cycle. The rule of thumb is that if you can barely hold it in your hand , it must be around 120-150F.
Now, when the steel continues to cool below 100F the new martensite can create great stress in the blade. It can literally tear itself apart. This is what causes cracking on cooling. Any sudden shock can cause the steel to shear and the blade may snap in half or shatter like glass (Anyone ever drop an untempered blade on the concrete floor about a day after quench?)
The martensite is untempered, so you reheat it to a temperature above the Mf and usually near the Ms, to relieve the stresses and allow the structures to reach an equilibrium. During this revisiting to the martensitic range, some of the unconverted austenite will convert to martensite. When the temperature cools below 200F again, the new martensite (which is untempered) is under stress.Since this is now a small percentage (between 1% and 5%, typically), it won't affect the stability of the blade as far as cracking, but could make the steel easier to chip. So we re-temper again around 400F. Now virtually all the austenite is converted (all that will) and the martensite is all tempered.
BUT WHAT ABOUT TEMPERING ABOVE 400F?
When we temper below 400F, we are trying to retain most of the martensitic structure. Above 400F, the structure starts to change. Since these new structures can have some advantage over the very brittle martensite, this is a good thing. At 500F it starts to greatly affect the hardness of the steel. So as knifemakers we try to temper as close to 400F as we can, and still get a tough blade with lots of hardness. Usually this is between 425F and 450F, depending on the use of the blade. This can be determined by testing and experimentation. Charts are only totally accurate for the lab that did the tests that made up the charts. Most of us use the charts as a guideline and adjust the temperatures as our procedures and equipment dictate.
WHAT IS A SNAP TEMPER?
Snap tempering is giving the quenched steel a quick temper at the Mf point, 200F. This is done immediately after cooling to 50-100F below the Mf, which translates to 100-150F.
This "snap" temper is to prevent the blade from snapping in handling and from the internal stress causing a crack. When some steels are to be cryogenicly treated, a snap temper can be important to avoid problems.
Hope this helps - Stacy
