I need to clarify what I said a bit. We would specify 1050 material, not 1020 as I said earlier. The problem with the higher content carbon material is, when the casting is poured and starts to cool, it looses surface carbon in the casting. And for the product we used it for, we had to have the surface carbon so it would respond to heat treat using induction heat and quench. Without the carbon restoration, there would be a depth of about .01-.02" on the casting that would not respond to heat treatment because of the diffusion of the carbon loss in the cooling process of the casting. So what the casting house would do was put it into a carbon atmosphere, the same one as used for carburizing, but no quench. Once the carbon restoration was done, the temperature in the furnace would be brought down gradually until it got to a safe level then the parts were removed from the furnace an air cooled. It worked 95% of the time. Years pasted, new company owners tool over, this was thrown out the window and went to the simpler process of standard carburizing 1020 castings. Dropped our processing cost by almost half!
I like to add a little to this. What people call 4140 pre-hard is really 4140 Q & T (Quenched & Tempered) to 28-36 HRC. And in rounds up to 3" in diameter, it's fairly consistent hardness to the center of the bar, not just the outer area of a bar. And really, the higher heat treated material in my opinion cuts much nicer. It's tough drilling, but with good cutting oils/fluids, it's no different than cutting 1018/1020 material dry.
Todays job was shoulder bolts, 1 1/8" 4140 hex at 35 -40 RC. 1.000" +.000 - .002 on the shoulder (finish turn after hardening), 3/4-16 thread. I left the shoulder .020" big for finish hard turning after they are cased to 55-60 RC on Monday. The Boss did not want to grind them otherwise I would have left .002 per side for grinding. This material turns fairly well at 35-40 hardness., I suspect that the relief at the end of the thread was not long enough and ripped the corner off of the first threading tool yet it measured in with the ring gauges, I made the undercut longer and had no problems with the rest of the parts.