This applies to roller chain, and should carry through to bike chain:

Link plate wear is uncommon as link plates are usually made out of 1040 or 1045, and through hardened. This means the hardness is full depth, and appreciable wear is very slow. Rollers are also full hard.

Pins and bushings are case hardened 1018 or 1020, with a fairly shallow case (.006-.008" on #40 chain, which is closest to bicycle). This shallow case, with a soft under layer (think nuggat), wears quickly and shows developed wear more quickly. With bike chain having a combined roller/bushing, I would expect it to be case hardened.