Grain reduction on air hardening steel

Cody Hofsommer

Cody Hofsommer

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Dec 2, 2011
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I have a ? on grain reduction post forging on air hardening steel, in particular s7 or h13. I make hot work tools like punches and pritchel and am wondering does the reducing heats thermocycling that we do in, say 1084, or whatever,do the same thing in air hardening steel ?
 
cody

Generaly speaking any martenistic steel will experience grain size reduction by thermal cycling from approximately 100 degrees F above critical to below critical. This can be accomplished by heating and then cooling in still air or by heating and quenching in an appropriate media. This being said I suggest you try two or three thermal cycles on the steels you are using and see what happens.

Jim Arbuckle
 
Thanks Jim, so in this case you are cooling slow in still air and in proper quench media. That was my guess, but wanted to hear some one with more knowledge then me way in.


Poop, what can I say....lol:)
 
I'm sorry, but there's not always a simple answer to this question. It is dependent on the alloy, the particular chemistry of your lot of steel, and its internal condition (previous heats).

The default answer is if you follow the prescribed heats to anneal your steel it will have the industry accepted grain. A lot of complex steels this is very simple, you heat to 1600 F and cool at no more than 20F per hour to below 800 and you're good to go. Normalizing not recommended (although spheroidizing is).

However, what the majority of texts fail to mention is that often times a prequench (quenching from some temperature below the normal austenitization temperature before your final heat) will significantly reduce grain size in high alloy and high temperature steels. The risk being the potential to drastically increase grain size.

Repeated descending heats like used for grain refinement on simple steels can reduce the grain size to a point, and then suddenly grow it enormously during final austenitization. This is because the steel relies on undissolved carbides to retard grain growth at the higher austenitization temperatures. As your grains and grain boundaries get finer, you run the risk of the boundaries simply disappearing during your final hardening heat.

Sorry if this wasn't particularly helpful.
 
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