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We make automotive parts of 4140 grade. Heat treat lot size max. is appx. 950 lbs. An average part weight is 0.2 lbs. Quenching temp. is 160 F. My question is 

Is there a publish literature that states the maximum acceptable time delay between quench and temper to prevent quench cracks e.g. 1/2 hr., 1 hr. or 2 hr. etc.? Most of the existing literature states temper as soon as the parts cool enough to feel comfortable to the bare hands (100 - 120F).  

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Bhaskar Chatrath
Plant metallurgist
McGard
Orchard Park NY
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I don't think time is a factor. As long as the parts are kept warm, I believe they could be held indefinitely without cracking.

I would add that in my experience, 4140 is not particularly prone to quench cracking, either immediately upon quench or delayed cracking near room temperature. Perhaps your parts have some geometry that makes them very sensitive, but if not, I would not anticipate immediate or delayed quench cracking with 4140.

Of course, I understand taking a conservative approach if cracking the parts would have unacceptable consequences. Don't take any risks you aren't comfortable with based on my advice.

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Ken Kirby
Snap-on, Inc.
Kenosha WI
(262) 748-3836
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Bhaskar,

Get a copy of AMS2759/1.  There are conservative 'snap' tempering and tempering guidelines in that document that work well.  Although 4140 is not an alloy one associates with post-quench, pre-temper cracking, there are circumstances where it could occur.  The usual culprits are complex and thick cross sections adjacent to one another, re-entrant angles, corners with little or no radius on them, decarb, and castings.

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Ed Engelhard
Solar Atmospheres
Souderton PA
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