what happens when we take out the materials out of the furnace at 550 degree centigrade after completing the tempering cycle and allow them to cool down in the open?
The acicular martensite gets converted in to somewhat circular / spherical in shape; the hardness drops, strength drops a little but ductility improves considerably. It is almost like a stress relief. All these changes can be planned well to get desired end properties.
Depending on the size and geometry of the parts being heat treated (e.g., thick and thin walls in the same part) the increase in the cooling velocity may introduce undesirable amount of stress which may cause distortion or even cracks.
Also, the remaining stress due to heat treatment plus stress introduced during service of the part may lead it to failure.
Nachi has it about right, though I'm not sure most folks would see a visible change in the martensite "rounding" at this temper temperature. But the improvement in ductility etc. is correct. The difference in the martensite will show up in the response to etch.
550 C (1022 F) is just a bit higher than the 950F tempering temperature that will give optimum combination of both hardness and toughness.
The point of the temper is to relieve the stresses from quenching, I'm not at all convinced that pulling parts out of the tempering furnace to cool in still air will accelerate cracking, We're only at 550 C. Its not like its a phase change/ volume change. The original post did state at end of temper cycle.
By the way, you may find cracks from QUENCHING, but they are not attributable to the cooling on tempering.
milo
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Tempering of d2 and Hot Die Steels
Re: Tempering of d2 and Hot Die Steels
