Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Coating for A2 Tool Steel
Q. I have been asked by a friend to inquire of others options as to a good coating/plating to put on an A2 Tool Steel spring. Needs moderate corrosion protection. They have used Zinc but are finding H2 embrittlement and post bake blistering. Do not know the zinc; Cyanide, acid or ANC. They were thinking Cad but, (ouch) Please share thoughts black oxide with an organic over coat?, Electroless nickel? Zinc should work? Any feedback welcome.
Thank you,
Jon Quirt- Minneapolis, Minnesota
2003
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A. Jon, Anything will work. The question is what additional properties do they want? We used a cold black oxide with a dry to the touch oil on tooling fixture parts with no major problems. A hot black oxide would be superior. I am a believer in EN for tools that get a lot of handling and the numerous resulting fingerprints are not a problem. A very high probability reason for the zinc blisters is they stayed in the acid activator too long. Which, is really bad if H2 embrittlement is a problem. James Watts- Navarre, Florida 2003 A. I don't think it as much the type of plating you put on as it is how the part is pretreated. Find a plater who is comfortable plating on that type of steel.A2 is not easy to plate. Todd Osmolski- Charlotte, North Carolina, USA 2003 |
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A. While I'm a "wet" plater, I don't think I'm all wet on this response: MECHANICAL PLATING. Millions of pounds of fasteners for the auto industry and others are mechanically plated weekly to avoid embrittlement and problems such as this. There are several sources surrounding Detroit, but it's available nationwide. Todd is correct (usually is!) on A2 being a difficult to activate and plate alloy; air hardened steels tend to be! Too long in the acid to get the resulting scale off is one of long list of potential sources of hydrogen leading to embrittlement. What was the post-plate bake? Probably should have been at least 24 hours and started within 1 hour of plating. Not soon enough or long enough are two standard causes. ASTM B850-98 has several remarks on this post-plate bake. Lastly, I'm also a CAD plater, but would steer you away from this coating. The Euros and EPA are doing their best to regulate this coating into oblivion despite its unique and unmatched physical and chemical advantages. Good luck  Milt Stevenson, Jr. Syracuse, New York 2003 A. Jon, if your part is a spring, I'd assume that it will undergo some amount of flexing- and hence I'd avoid coatings that are too thick or too brittle. The black oxide that you and James mentioned would likely be my first choice.  Lee Gearhart metallurgist - E. Aurora, New York 2003 |
Coating A2 Steel Blades
Q. Our company made a number of blades from A2 tool steel, since we were able to achieve high hardness (HRC 58-60) and high impact toughness with this material. We now want to improve the corrosion resistance on the parts that have already been completed including the final sharpening. Can anyone recommend a process to improve corrosion resistance with minimal added thickness that won't change the sharp edge or impact toughness?
Bill VanDesign Engineer - Colorado USA
September 11, 2016
I ran some tests years ago on the effect of ZrN PVD on corrosion resistance of steel. It works reasonably well -- we went from full rust in well under one hour in a neutral salt fog on a mild steel sample to a small amount of rust after 24 hours with the PVD coating. The coating was not a barrier -- it was pretty porous, having been deposited by cathodic arc -- so the improvement was due to creating many small areas for corrosion, spreading the potential about the surface. Problem you are going to have is the sharp edge. PVD coatings are in the micron range, but the tip "diameter" of a sharp edge is less than 0.5 micron. Of course, the PVD coating will preferentially deposit on the tip where the bias electric field concentrates...
Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover
San Diego, California
September 12, 2016
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