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chrome plating questions on engine valves

chrome plating questions on engine valves

(OP)
When a supplier is saying their valves are chrome plated, would the chrome be thick enough to be classified as Hard chrome?  What chrome specs are used in that industry?  What exactly is 'flash chrome' and is that enough for sufficient wear abatement?

After a few hours of running on a new engine (engine assembled by shop), I noticed blue smoke.  One cylinder was fouling a plug.  I cut open the oil filter and noticed many chrome flakes and the only thing chromed in the engine was the vavles.  I pulled the head where the offending, fouled plug was and noticed immense wear in its cylinder.  I disassembled that head and the intake valves had very abraded areas (no chrome) the valve stems had a nice step where they were riding in the guides.  
The heads were rebuilt, not new using the original guides as the shop said the clearances were "on the loose side but still within spec."  
Not all the valves were as bad as the intakes but wear was evident - remember, the engine had only hours on it.
I cross sectioned some of the valves (and some extra I had) to look at the micro-hardness of the chrome and the substrate.  Not enough chrome on any to do a micro-hardness test, about 0.1 mils in average thickness though.  The substrate micro-hardness had a gradient of ~47 Rc on the edges to ~36 Rc in the center - multiple indentations from the center out.  Also, on the extra un-run valves I had, under 200X there were pits of chrome missing - an indication of poor adhesion?
This is outside of my element so I seek your advice here.  It seems that the heat treat was bad and the chrome process was bad also.  What do you guys think?

 

RE: chrome plating questions on engine valves

I use ASTM B 650 for a hard chrome spec.  You can also use SAE AMS 2406 but it's a bit overkill.  It's up to you to decide how much you need.  For my applications I usually use 5 to 8 microns, 2 to 3 times what you measured.

RE: chrome plating questions on engine valves

Poppet valves for internal combustion engines are hard chromium plated, which is also known as industrial chromium plating.  The thickness for this process in general ranges from ~ 3 to more than 100 micrometers, but for engines valves it is usually in the range of 3 to 35 micrometers.  Flash chrome usually means that the thickness is less than 1 micrometer. It sounds like the thickness of your parts was quite low, and possibly the adhesion was poor as well.  Here are some links on valve materials, coatings, and processes:


http://www.enginebuildermag.com/Article/1171/valve_selection_hot_valve_materials_for_hot_engines.aspx

http://www.mahle.com/C1256F7900537A47/vwContentByKey/W26QPJLS933STULEN/$FILE/Motorenteileprogramm_en.pdf

http://www.fkm.utm.my/~zulkepli/examples_templates/Final%20Report%20_GP5_autoValve.pdf

RE: chrome plating questions on engine valves

(OP)
dgallup, and TVP,
Thanks for your replies and I'll be studying your references.  It's great to have a community of engineers willing to help.

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