Subject: Re: Tool Steel - Grain refinement

From: [email-masked]
Date: Thu, 11 Jul 1996 20:35:04 -0400

To All,
In the process of hardening steel, the material is heated to a temperature
above the critical point, held until the transformation from ferrite and iron
carbide to austenite is complete and then the material is quenched in the
appropriate media.
What is really going on here?
The transformation from ferrite and carbide to austenite is a nucleation and
growth process. The new austenite grains nucleate at the prior austenite
grain boundaries. Upon cooling the products of austenite decomposition again
nucleate at the austenite grain boundaries. 
Refinement of the austenite grains is best accomplished through
recrystallization of the structure as the result of hot work. To maximize the
effectiveness of this process the entire section must be worked to nearly the
same degree. Great care is taken when commercially forging large sections to
insure that the work reaches the center of the forging. The more work that is
put into the structure the more nucleation sites there are and the finer the
resulting structure. Lightly working the structure can result in "explosive"
grain growth. Finer structures are tougher but are less hardenable.
Cycling at a low temperature may refine the ferrite and carbide but the
original austenite grain boundaries remain. When you heat the piece up to the
critical point, the prior boundaries take over as the nucleation sites.

Get it hot, hit it hard is very good advice.

Hochewa