In one machine shop in which I put in betweeen 11 & 12 years 4130 & 4340 were our two main alloy steels. I would likely be utterly amazed if I had some way of knowing just how many pounds of each of these alloys I machined on & virtually all of it was subsequently heat treated. In all that time I never saw a part made of either case hardened. We normally did not case harden a part having a carbon content higher than 0.2%. 4130 we considered a water hardening steel & 4340 an oil hardening steel. Both of these alloys we heated in a neutral salt bath to hold down surface oxidation. Although we did not use near as much of it, we did occasionally use 4140 which due to the extra carbon over the 4130 was also oil quenched.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra