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Old 12-18-2016, 04:14 PM
samuraistuart samuraistuart is offline
Steel Addict
 
Join Date: Dec 2012
Location: San Antonio Texas
Posts: 163
Excellent question. I'll cite some material from the Verhoeven PDF, as he is a metallurgist.

1. There are a few things going on during tempering. The first would be strain relief caused by the quench.
2. The first temper also will allow the formation of super small carbides, carbides so small you need an electron microscope to see them.
3. The 2nd temper decomposes retained austenite into ferrite and carbides.
4. The 3rd temper is really for the higher alloy steels, in the higher temperature range, where those super small carbides from round 1 are transformed into cementite.

Most low alloy steels like O1, 52100, 1095 etc are only tempered with 2 cycles, whereas higher alloy like D2 and stainless steels benefit from 3 temper cycles.
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