Will chroming weaken cast steel spindles?

A. Chrome plating will not weaken regular hot rolled or cold rolled steel, but has the potential to embrittle high strength steel if not promptly and properly baked. The parts you are identifying as "cast steel" might be forgings rather than castings.

The landing gear on jumbo jets is chrome plated, so it's not a problem if done right. But that doesn't mean that it is practical to chrome plate cast steel spindles, because I don't know what you mean by spindles and what you are referring to :-)

A. With respect to chrome plating, there can be a trade-off in reduced part life in certain situations. Chrome plating almost always has small pores and microcracks, which under situations of alternating stress in tension can act as crack starters for the underlying steel part. The plating does not reduce the strength under a single stress event, but because it can start a crack the long term fatigue life can be affected. Aircraft landing gear (or shock absorber rods) may be a situation where the coating process is carefully optimized, and where the increase of life from avoiding corrosion or wear is more important. With standard "triple" chrome plate the copper and nickel layers underneath provide an effective corrosion barrier for a while. However on other automobile parts in the drivetrain/ suspension, where millions of stress cycles are expected during driving, I would recommend against chrome plate.

A. Hi, Barrie. I don't know if Paul will be back to reply. The aircraft landing gear is (to my knowledge) shot peened to build a compressive stress into the outer layer of the steel so that the fatigue issue that Paul speaks of is minimized or eliminated.

Are you talking about a couple of bolts on your own show car, or are you an engineer for an OEM who is writing specs a new product line? Anything is possible and I have seen chrome plated hardware on motorcycles, but such things require careful investigation: Can the hardware be big enough that brittle high strength steel is not required? Unhardened steel will "yield" and be less sensitive to fatigue. Shot peened steel will be less sensitive. If you are an OEM it is possible to do a fatigue test of the design.

If it's a show car, older parts tended to be "over designed" because we needed larger factors of safety, not having computers for such techniques as finite element analysis. So, while nobody is going to guarantee that your hardware won't fail if you're doing your own engineering, if it's plated and baked properly, I think the likelihood of failure will be small rather than obvious. Good luck.

Regards,

A. Hi, Barrie. A 1967 MGB, eh? God bless you. I had a 1969, brand new back then, and by 1971 it was a hated monster :-)

There are two things at play with those nuts and bolts. First, many plating processes, including chrome, can cause hydrogen embrittlement -- but it's not usually a major problem if the parts were immediately baked to relieve the hydrogen (which most good chrome plating shops would know about). Second a cracked plating, like chrome, can give rise to stress risers as previously noted, which can promote fatigue failure in a rapidly stressed and unstressed component. I personally doubt that it's an issue with a nut in most circumstances, or with the head of a bolt. Keep the hardware properly torqued, so the component remains under stress at all times, and the variable portion of the stress will be a smaller portion of the load, and a smaller factor toward fatigue failure.

Regards,

Thanks again Ted for your reassuring reply. cheers. Barrie