Heat treat AISI 4142 material

[QAFitz]

I have to heat treat AISI 4142 material and meet a minimum tensile of 143,000 and (here's the kicker) we need to run charpy tests at -40 and obtain 25-mils lateral expansion. In the AISI 4142 range of chemistry allowed by ASTM A-540 B22, what elements will give me a better chance of meeting this (High C, low P & S, High Mo etc.) ?
The finished material specification A-540 requires a minimum tempering temp of 850 F.
If any one has performed a heat treat and met this I'd like to correspond w/you. Thanks in advance for your time.

 

[metengr]

QAFitz;
I would not change a thing regarding chemistry ( one exception; lower carbon is better because of the direct affect on notch toughness), expect make sure you have a tempered martensite structure throughout. Quench in oil and temper at 1100 deg F. The tempered martensite structure of 4142 should meet the impact requirements.

For 4140 Q&T

1100 deg F
148 UTS Ksi
132 YS Ksi
19.0 % E (2" gage)
58% RA

311 BHN
69 ft-lbs at RT

 

[Metalguy]

How thick? I'd try to get a heat with the Cr, Mn and Mo near the high limits if it's over ~1" thick, and as Metengr says, as low C as possible. You should also check the grain size before you HT--you want it as small as possible. Assuming it's a thick piece, quench from 1525 deg max. into cool oil or polymer with as much circulation as possible. You may find an 1125 deg. temper will be right on.

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