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[rkepler]

Soak time. There are limits to how much carbon you can get into the case but it's not that hard to get it up where it quenches surface hard while the...

[rkepler]

For abrasion resistance. 41xx is my go to for "tough" but sometimes (like in a bullet swaging die) you need both tough and surface hard....

[rkepler]

You can case 4140 same as 8620, just needs a longer soak for the same depth of case. But I'd be happy with 4140PH by itself as chuck jaws.
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[rkepler]

If those haven't actually failed by now (and it sounds like the starting one has for sure) they will soon. Just replace them.