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I'm trying to find out what the differences are between SAE4140 & SAE 4142 steels.  Is 4142 just a prehardened form of 4140?
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RE: SAE4140

According to SAE J404 Chemical Compositions of SAE Alloy Steels, alloy 4140 has an allowable carbon concentration (in mass percent) of 0.38-0.43 while alloy 4142 has an allowable carbon concentration of 0.40-0.45.

According to SAE J1268 Hardenability Bands for Carbon and Alloy H Steels, alloy 4142 has better hardenability.

4142 is not a prehardened version of 4140.  They both require the same thermomechanical processing to develop their properties.

RE: SAE4140

The 'code' for all these types of alloys is that the last two numbers designate the nominal carbon content - 4130 has a nominal 0.30%C, 4340 a nominal 0.40%C, 4145 a nominal 0.45%C etc

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