Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Chrome Over Nickel - Hydrogen Embrittlement
Q. We have a two step finishing process, QQ-N-290A Class I, Grade G, SB nickle strike on 1070 carbon heat treated to C45 per AMS 6875. This is followed by a chrome plate per QQ-C-320 Class I, Type I. We now understand the importance of hydrogen de-embrittlement bake after the nickle strike. Since we are depositing chrome on the nickle, not the high strength steel, is a secondary 190C bake required after the chrome deposition? Do we still risk hydrogen embrittlement if we do not bake post chrome?
A. I think I see your point, but unless otherwise specified, I think the thought of omitting the bake after chrome is more like “suicidal” than risky!. To me there’s no gray area in QQ-C-320 Class I, Type I, the alloy you describe, “spec” calls for an 8 hour minimum bake after chrome plating at 191oC +/- 14oC, Para. 3.2.6., AMS-QQ-C-320 and Table 1, AMS 2460 (AMS 2406 issued July 2007 replaces AMS-QQ-C-320 Issued July 2000)
I don’t find any reference to bake requirements (if any) after the initial application of nickel or “nickel strike,” before chrome, but I would suggest, if you now bake between nickel & chrome, you have my sympathy, but most definitely continue the practice. Either way, you might do best to ask for clarification from your customer, the purchaser or cognizant engineering authority and let them provide a technical interpretation. AMS-QQ-N-290 also doesn’t reveal many clues about a bake between the nickel & chrome.
Just in case, here’s another think to think about, unless otherwise specified, when the steel base metal is 34 RC in hardness or higher, the spec requires a 3 hour stress relief bake per AMS 2759/11 “before plating.”