Letter 5287

Compressive stress 

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I'd like to have some help with understanding stress in a parts area and stress in chromium deposits. Can one part which is shotpeeened to make compressive stress in the part counteract macrocracks in chromium deposit if the compressive stress is higher than tension stress in the chromium deposit.

I think a part with a Rockwell C 55 shall be shot peened with a steel shot with Rockwell C 55 or higher to make the compressive stress to propagate into the part more than that will be if I shot peen with a lower Rockwell C. It is my reasoning the macrocracks arise because when we grind the part and get a temperature over 220 C the compressive stress arrives. the part we have a problem with is steel with a high hardness. Best regards to you Anders S

Anders Sundman     surface finishing engineer Sweden

As you probably know, a tiny crack in the surface of a part is a stress concentration point, such that cracks propagate (sometimes to failure) in a part with fatigue loading. Shot peening is indeed done both on plated and on unplated parts to compress the skin to mitigate this stress concentration problem.

Sorry, I don't know the exact formulas to use to give a quantitative answer to your question, but in principle you are exactly on track. Regards.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Hi people.

I like to have more information about this. Is it any problem with the translating to English let me know. Best Regards,

Anders Sundman     surface finishing engineer Sweden

Dear Anders, If you have a problem with cracks in the chromium after grinding, I would look at the control of the finish of the part before grinding, the pre-cleaning/etch of the steel, the plating bath conditions, and the grinding conditions, I don't see how it is practical to offset a stress in the surface of a part, in anticipation of another stress from the plating. Regards, Tom

Tom Pullizzi Platronica.com Falls Township, PA

Hi Anders,
All metal has some stress in it. It comes from the manufacture of the part and the follow on operations. Bad design and abuse of the metal leads to stress risers which can lead to microcracking which can lead to cracking and failure. A stress riser is any flaw in the metal that causes an accumulation of stresses in that area. Post plating and post machining heat treatment has been a standard mitigating remedy.

Stress in chrome is always or virtually always compressive. EN on the other hand can be compressive or tensile as can sulfamate nickel. Because chrome is so highly compressively (internal to the coating) stressed, it is actively looking for places to relieve itself. Hello cracking. Micro cracking in chrome is actually beneficial as it is so tiny and so weak that it does not have a significant effect on the base metal. As the cracking becomes larger and larger, it is still attached to the part and attempts to crack the part, which it can succeed in doing in really bad cases. When you machine chrome you are actually relieving some of the stresses and abusive machining can aggravate or increase the stress. Micro cracking that you see after grinding would give some indication of abusive grinding. Pratt and Whitney standard practices manual has pictures of what is acceptable and what is unacceptable as far as the cracking. Basically, it allows some "mudflat" cracking and prohibits linear cracking. Shotpeening appears to do a couple of things. (very non scientific) it increases the density of the metal in a very thin layer of the surface making it stronger. It makes that skin compressive which tends to hold any cracking caused by the chrome to that layer rather than letting it migrate into the sub layers of the metal.

The solution to the problem is to maintain plating parameters as closely as possible to the specification you are plating to. Then, the most difficult part is to convince managers and machinists that they should use the proper grinding wheel and operate it at the optimum speed . Cutting depth is very very small and this takes time. This costs money and that makes it very susceptible to cost cutting stupidity. The cost of machining out all of a bad plate caused by bad grinding and the replate and regrind operation, is far more than a couple of hours longer in the grind operation. As a plater, you also have an opportunity to go to conforming anodes that will put the chrome where it needs to be and not in gross excesses that you get from tank anodes. This will save hours and hours of grind time. Over a period of time you can get the chrome closer and closer to actual size needed. A killer of this is anodes tend to lose there efficiency after several hours of use which changes the plate rate.

Knowing the reasons of why things work the way they do is nice. The more important thing is to get everybody on the same end of the rope and perform all of the operations to the letter and the intent of the governing specification.

Good luck, because too many managers are too interested in getting one more part out of the shop this accounting period.

James Watts
- Navarre, Florida

Thanks Ted,Tom and especially to Jim.
Jim you get the answer I hope to have because the that what you write down is the same theoretical problem I try to explain to the management at my company.

Best Regards to you gentlemen.

Anders Sundman     surface finishing engineer Sweden

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