Dana, with that hardness, you had better temper twice for two hours.
Not only do you get some stress relief on the tempering cycles, but especially on the first one you get a benefit that did NOT occur on the hardening cycle.
The austenite was forced to create martensite during the quench - which we WANT - but then cooled down too fast for the complete transformation to take place.
What you have now is what is called "retained austenite". There is a steel formation that is sort of "frozen" in time somewhere between austenite and martensite.
When you do the first tempering cycle, which relieves some of the hardening stresses, as well as removes some of the martensite brittleness, this "retained austenite" proceeds to COMPLETE the transformation form austenite to martensite.
So, you now have BRAND NEW MARTENSITE!
It's brittle, and it's hard and it's under stress.
That's the rreason for two tempering cycles.
That new martensite now needs to be TEMPERED!
It needs stress relief and brittleness reduction.
1 one hour tempering cycle just ain't enough.
If you've got your steel full hard, it needs TWO 2 hour tempering cycles.
There isn't enough gain to be achieved in most situations to do a third.
That's an example of diminishing returns, as there is even new martensite created on the second tempering cycle, but seldom is there enough to demand our attention.
It's great to know you are getting your stuff hard.
Let's just hope that your temp was not so high as to get undue grain growth!
Topic: 01 steel rockwell test (Read 1237 times)
