Specifying hardness of chrome plating 

Letter 10037

Hello,

We produce plastic and rubber laboratory processing machines in Bangkok, Thailand. We are experiencing problems with our range of two-roll mills where the rolls have been hard chromed. Unfortunately the surface is not hard enough. The rolls are equipped with side guides made of Teflon which touches the chromed surface with a slight pressure. The Teflon will scratch the chrome only after a few hours use and eventually make deep marks after a few months. The rolls are running slowly, only around 30 RPM for a roll diameter of 6 inch.

We use an outside shop for the chrome plating and the rolls are also ground and polished to a mirror surface after the plating is done.

I would appreciate very much to get some help on what we could recommend the plating shop to do in order to improve the hardness of the chrome plating.

Peter Jurgensen
- Bangkok, Thailand

I would put specification of minimum 50 microns of hard chrome plating. The hardness can be checked with hardness testers and should be around 1000 VPN. Thickness also can be checked.

Payal
- Charlotte, North Carolina

Dear Pete:

First, I would find out if it really is chrome or something else (like electro or electroless nickel). A droplet of hydrochloric acid will reveal it by attacking chrome and not nickel. Then, I would also look at the hardness of the substrate steel. Chrome is generally a thin layer and it should resist very superficial scratches but not deep ones unless the rolls are hardened to a minimum of 50 Rc and a depth of at least 1 millimeter.

Good luck,

Guillermo Marrufo
Monterrey, NL, Mexico

Many possible factors.

If you have a very soft metal underneath and a very heavy load, it will fail. Hard chrome is by generally accepted definition, over one mill thick. Some aircraft specifications require over 1.5 mill per side. It just plain works better if you have at least 3 mill per side. This has enough strength to resist a lot of wear and not crack and fail from the base metal yielding.

My pet peeve and a source of great problems--Machinists that want to hog grind chrome or use the wrong wheel with inadequate cooling and frequently the wrong speeds. You can soften the chrome to in essence "mush". The typical is to burn it and then kiss grind the burn indication off. I will put a cup of coffee that this is your problem, if you have enough thickness.

The plater can cause it also, by using the wrong amount of amps per sq inch for the temperature of the bath. This typically gives you a cloudy or milky look before grinding.

James Watts
- Navarre, Florida