Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Hydrogen Embrittlement of Decorative Chrome on Chromoly
Q. Is hydrogen embrittlement still a concern if 4130 chromoly steel is copper plated then nickle plated, then chrome plated? I'm building a helicopter and I want the steel to look nice without compromising it's strength. My parts are sitting at the plating shop but I'm having second thoughts...
A. I'm not qualified to advise you about your subject parts and a multi-coating of copper-nickel-chrome on aircraft parts, but I can tell you what is considered standard practice when it comes to the subject of hydrogen embrittlement.
One of the most important properties of metals is ductility. This is defined as the ability of the metal to deform under stress. Hydrogen embrittlement can cause metals to become brittle and inhibit its ability to deform or stretch. Baking after plating is a method of driving out the hydrogen and stress relieving the metal.
Just as important as knowing the type of metal alloy (in your case 4130), is to know the hardness of that material and the "tensile strength". These values are necessary in order to establish the need for baking and if so, for how long and what temperature.
Hardness is defined as the hardness value of the metal as obtained by a heat treatment, presumably based on a hardness test value in Rockwell, Vickers, Knoop or Brinnel. "
Tensile strength" in thousands of pounds per square inch is usually determined by sustained load testing of parts or samples made from the same material or heat treat lot and that of the actual aircraft parts. Tensile strength directly correlates to part hardness.
Once you have confirmed the type, hardness and tensile strength of your components, you can determine if a bake is required after plating to remove hydrogen and therefore reduce the affects of hydrogen embrittlement
General rule of thumb is to bake within 2 hours of removal from plating tank, in a certified oven, temperature range 375 - 400 degrees Fahrenheit, bake cycle times that range between 3 and 24 hours, depending on the "heat treatment" or "tensile strength" of the part(s). Charts are available.
If metal parts are below 180 KSI (180,000 PSI) tensile strength or a hardness of 40 Rockwell "C" or less, baking after plating isn't required.
The aforementioned information is based on metal plating on ferrous alloys. It doesn't take into account the potential for added complications using multi coatings of copper, nickel then chrome, or the affects of hydrogen embrittlement and "decorative" metal finishing.