4140 and 4340 don't have quite enough carbon to get to 60HRC - I think after a low-temperature temper, you can expect 50-53, maybe 55. I've never seen 4150 alloy in anything other than oversize rounds.

Is chrome plating normally applied to a steel that is that hard?

1040 and 1095 can both be hardened above 60 HRC and are readily available in those dimensions.

My source says 1040's as-quenched hardness is 52HRC. I don't think either 1040 or 1095 would through-harden reliably (I don't know if that is important to USMech6).

If you Cr plate something that hard there is a real risk of hydrogen cracking.
Stress relive first and then back directly after plating.
Another option would be electoless-nickle, then you bake afterwards to increase the hardness.
I would just use 1060 and not worry about through hardening, this is a thin piece so the loads must not be much.

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Ed arose a good point on H2 embrittlement, H2 pickup in Cr electroplating is inevitable. However, stress relief needs temp 150-400C, which is higher than Mf temp for 1060. outgas temp (170-250C) may also higher than Mf. Both of them can decrease hardness.
You may try 4340, and may get max hardening 60RC (good luck).

Cr plating is reported to get over 60RC, Ni plating is only 20RC.

One question is why you wanted to provide a surface to 440C, which is hard and good corrosion compared with hardened carbon steel. and how to cover the 440C roller?

Ben, the electroless Ni-P platting will be RC60 as applied, and if your base steel can tolerate being tempered at 700-750F then you can jack the hardness much higher (900 HV).
How about using a piece of M2 tool steel? Harden & quench, temper at 1000F, plate, age at 750F and you are good to go.

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Thanks, all. Either case-hardening or thru hardening is fine - are there any thickness ranges you would recommend to prevent distortion? To answer MagBen's question, I am stuck with 440c due to OEM design.
Tensile strength is not a concern as this will just be a wear strip.

Tool steel is a good idea, too. Is there a plating/chrome you would recommend?

You could use an "A" or "D" tool steel and air cool, these steels have very low distortion and are very tough.
You could still use M2 and air cool it when hardening. It may not through harden but it will be pretty close.
I would then give it a grind after to get a good surface and go for low or med phos electroless Ni plating. Then bake after plating, using 700F for 1 hr should give you the hardest plating.

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Ed, do the A-series or D-series see any temper embrittlement when they are tempered at 700F?

My experience is that the tool steels in general show very little of this effect.
https://www.hudsontoolsteel.com/technical-data/ste...
https://www.crucible.com/eselector/prodbyapp/plast...
The high temp steels which are a bit softer (H13) don't show any of this effect at all.

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Thanks everyone for your help.