Geno I think you are right...20-30 thousandths of hard case is not going to make any appreciable difference to the tensile strength of a bar of steel 1" thick. It does prevent wear on moving parts, protect hammers from deformation, prevent scratches on cosmetic surfaces, and reduce the propensity of the steel to corrode. It also looks pretty and cuts down on reflection..a practical consideration on a waterfowl gun or a rifle.
Low carbon steels are prone to growth in grain size when heated much above 1600F from what I have read. So if one were to recolor a receiver repeatedly at high temps, grain size would grow making the receiver more brittle.
