Is 42CrMo / 4140 hardened and heat treated suitable for CNC machining or should I get untreated steel? And heat treat afterwards?
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Is 42CrMo / 4140 hardened and heat treated suitable for CNC machining or should I get untreated steel? And heat treat afterwards?
You can check the answer of the people under the question at Quora “4140 steel for knife making“
About Hailey
No, no, no. Unless you have a compelling reason to machine hardened steels, don’t do it!!!
You want, and will most likely buy your 4140 in the ‘normalized’ or solution treated condition. Even in this form it’s not a lot of fun to machine.
Then once it’s machined to specs you have it heat treated. But before that consider the final tolerances and what the part will do during heat treat.
If the tolerances are exceptionally close it will be necessary to leave a little meat on the parts prior to heat treating then finish machine (more typically finish grind) the parts to their final size.
When Solar & Thermal first acquired TM Precision back in 2004, the shop was making aerospace parts, many from 4140, 4350, 8620 and various other hardenable steels, including 15–4PH & 17–4PH, CrMoV, and my personal favourite Waspalloy.
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You’ll be replacing your CNC bit far more often with a hardened/heat-treated piece. Heat build-up will be more as well on a hardened piece, loosing tempering. All said: Heat treat afterword.
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I don’t want to get into an argument, but let me state that fully heat treated AISI 4140 and 4150 have been routinely machined since before WWII, To the best of my recollection those are the steels from which gun barrels were made and they were quenched and tempered to 269–311 HB before being fully machined. CNC machines should have no problem at that hardness level. Of course, normalized material would be easier to machine and cause less tool wear, but then it has to be quenched and tempered, usually accompanied by some distortion.
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The proper answer is, it depends. 4140 is available both ways, annealed and hardened. it can be machined in both states, annealed and hardened. It is much more difficult to machine hardened 4140; both in machine time and tools. 4140 suffers some deformation in heat treating (although how much depends on the shape you machine it into and— less so— how you heat treat it).
If you have a critical application in which deformation is not advisable, perhaps it is worth your time and the hit on your wallet to machine it hard. Gun barrels are a good example. Mostly, though, 4140 is machined to within close tolerances before heat treatment, then hardened, and resolved into the final part by grinding or lapping.
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Fully hardened /heat treated steels are very difficult to machine but distortion free HT finely machined parts require experimentation to ensure distortion free end product
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