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Author: gearloose
Time: 04/07/08 19:45 PST
This is a reply to message no. 19654 by Robert Thompson
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Heat treating 4340
Yes, that's certainly what you should expect.  See, these steels are hardened by a transformation mechanism, when the high temperature austenite changes to the hard, room temperature martensite only when it's cooled fast enough.  How fast you have to cool depends on the alloy composition.  You also have to remember that the cooling rate is faster at the surface, where the oil hits it, than in the center: so the hardness in the center will not be as high as on the outside.  Now, 4340 might have enough hardenability to properly harden in 4 inch diameters, possibly even in 6 inch diameters, but 4140 and 4130 do not.  So for them, a drop off in the hardness when turning down to 2 inches or 1 inch is to be expected.
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