heat treating 2

[grouse1]

I have an crank forged from 8620 that gets normalized prior to a rough machine, then carburized for case and finsh ground. Why is it recommended to normalize the part above the temperature of the subsequent carburizing temp ? I have a reference from the ASM handbook Vol 4 "Heat treating" that says for alloy steels normalizing at temps higher than the carburizing temps will/can minimize distortion in carburizing.

I don't understand the underlying metallurgical reason for this. If the steel completely transforms to austenite during the normalizing treatment and all carbides are dissolved what distortion minimizing benefit do you get from temps higher than those the part will see in carburizing ?

Thanks

Mark

 

[metengr]

Mark;

I don't understand the underlying metallurgical reason for this. If the steel completely transforms to austenite during the normalizing treatment and all carbides are dissolved what distortion minimizing benefit do you get from temps higher than those the part will see in carburizing ?

Let's think about what you stated above as a thought process. Distortion during heat treatment (carburizing/quenching) can be caused by improper quenching and inconsistent grain size among other contributing factors. Having an inconsistent grain size (mixture of coarse and fine) along with undissolved carbides affects the formation of transformation products upon cooling from austenitizing (carburizing). Having a consistent, finer grained structure from normalizing sets the stage for more uniform formation of transformation products after quenching (from the carburizing cycle), and less distortion.