Steel is an exciting and versatile material. I still think that after working with it and studying it for 42 years! Yes, even in these days of nano-miracles, steel is still useful and interesting ... and we don’t know everything about it.
When I was in school, I learned that if a piece of steel is cooled too rapidly during the quench portion of a hardening heat treatment, it might crack. Sharp notch features, square corners, large differences in section mass next to each other, high alloy content and high carbon content all play their part to make a component more prone to quench cracking.
I also learned that while the martensitic transformation that results in the material getting harder than it started happens nominally instantly, sometimes there’s quite a bit of austenite – the high-temperature phase – left in the microstructure. Sometimes this “retained austenite,” for various reasons, remains in a metastable condition in the part. Many different things can cause it to transform to martensite at a later time. Mechanical stresses can allow the remaining austenite to transform or temperature changes. Sometimes simply time will allow the transformation to happen. That is why the old Swiss watchmakers reportedly left their parts in a cave in the Alps for seven years before final shaping and sizing. The microstructure transformation is accompanied by a volume change, and waiting will let that happen. Today, we don’t have time for such a strategy!
Sometimes multiple tempers will be used to minimize retained austenite. The part may be cryogenically treated to try to “convince” the steel to transform to austenite through the higher driving force that results from a very low temperature. Then another temper is performed to temper the new martensite.
But if whatever methods are used to minimize the chances of quench cracks do not work, the part may crack. Usually, quench cracks are intergranular. They look like “rock candy” when viewed with a high enough magnification microscope. It’s a distinctive look.
We will finish our discussion next time.