Cracks when Grinding hard chromium plating

A. Dear Anders,

The most important thing about grinding hard chrome is not to develop any heat.

Use a grinding wheel with an open structure to make good cooling possible. Keep your grinding wheel sharp by dressing it more often than you normally would do.

The grinding speed should preferably be about 35 mtrs/sec.

Consequently, the number of r.p.m. depends on the diameter of the grinding wheel.

We have more detailed information on this subject available.

Kind regards,

A. Not that simple. It is the tiny area under the grinding wheel where the chrome reaches a high temperature and then immediately cools off as soon as the wheel moves.

Causes:

• The grinders are taking too deep a cut per pass
• They are using the wrong resin in the wheel
• They are using the wrong grit size
• They are using too high a RPM for the size of the wheel or are using too slow a part speed.
• Or all of the above.

My experience is reason one and two, 99% of the time.

The machine shop gets paid for getting the parts out and they rush grinding. The fact that it recycles is the platers union problem, not the machinist union.

There is about 1 in a thousand chip cutter that will listen to a plater, even when presented articles stating fact. After all, machinists are highly talented and trained people. If you do not think so, just ask one.

Been there and done that. Talk with straight tongue. Every time we had a problem, I would check the grinders and sure enough, they forgot to order wheels and were using unsuitable substitutes. They also thought that a 0.005 grinding cut was fine, because it "worked" (for about 100 revolutions).

A. Excellent reply Mr. Watts! You've hit the nail squarely on the head. Improper grinding costs this country billions of dollars a year in excess tool wear and component failure, but woe to the engineer who tells the grinder operator that there is a problem.

A little additional information: I have personally measured temperature under a grinding wheel of up to 1200 °F, and suspect that my temperature measurements are low. These high temperatures can cause the metal to expand so much that it plastically deforms because it kept from expanding by the cool metal around it. Then when it cools, it tries to contract but cannot, and a tensile residual stress is the result. Since chrome is more brittle than most steels, it stands to reason that the chrome will crack long before the steel.

In the aircraft industry, great effort is often made to achieve low stress grinding. I know of one company that will deeply investigate for heat damage if they see a spark while grinding. Yes, that is "A" spark.

It might be worth your time to investigate CBN [cubic boron nitride] wheels in addition to taking Mr. Watts advice to heart very seriously. Good Luck

A. Mr. Diekman,

You mentioned CBN wheels and it just rings the bell with me. We have had a very positive experience grinding Molybdenum plasma-sprayed component by a vitrified CBN wheel. We used to apply a set of two wheel for this operation - an electroplated one to remove the bulk of the material and alumina wheels for finishing. We have been able to replace them both by a single vitrified CBN wheel with induced porosity. It grinds very cool and puts mush less stress on the component. It does not inflict metal burns, that were common before.

A. In my experience the mud cracking is due to poor adhesion, If the grinder has a incorrect wheel or is taking to much off on his grinding passes, you can get mud cracking but there is often a discoloration along with the mud cracking.

A. Hard Chrome is under very high tensile stress for almost all plating conditions and chemistries and has less than 1% elongation. This is responsible for the tendency to alleviate by either cracking or spalling. The grinding adds more stresses through cutting forces and heat. Some substrates also differ too much from chrome in thermal expansion coefficient, which adds another factor.

A. I had many a war over this subject. The problem is in the machine shop management. The machinists are doing one or more things wrong.

1. The wrong wheel. I cannot count the times that they ran out of the correct wheel and put on a wrong type, and then bitched like mad because the plating failed. There is an optimum grit material, grit size and binder to be used!
2. The wrong feed or speed, Faster is not better!
3. Too deep a cut. They would literally crush the surface of the wheel and the plating by trying to take a 0.002" cut.
4. Inadequate or the wrong cutting fluid.
5. Not dressing the wheel often enough.

Besides, everyone knows that if plating fails, it has to be the platers fault. It certainly could not be an engineering, or management problem! If you do not believe that, just ask them. (just like it was the platers fault when the part had chrome on it, but was 0.020 smaller than it started out)

I loved the parenthetical comment, James; you created an entire colorful scene for me in 25 words or less.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Miro,

Read thread 12398, "Hard Chromium Plating of Nickel-Based Alloys" and 25160 "Hard nickel won't adhere to Inconel" about plating on Inconel 718. Be very careful when starting grinding on the high current density areas on the part.

Regards,

A. I have to agree with all of the responses. Working in the plating field for close to thirty years, especially in hard chrome, I have yet to see any of the Machinist Foreman that I have to deal with, say it was their fault. I was even told by the Director of Operations, that I didn't know what I was doing, must of been absent during that course. Slow is better, and use a lot of coolant, best of luck.

A. What activation process are you using for Inconel? Are you going in the bath live? What amps per square inch are you plating at? How much grind stock are you putting on part? How much chrome are you putting on per hour?

A. Hi Chris,

Have you measured the hardness on the part before the grinding?

How is your stress relive is that by baking in a oven or shoot peening? Or is it together in the same operation? (remember that it is different temperature in same operation or not).

How is the trivalent chrome content in your chrome solution.

How deep gets the grinding wheel for every cut? (remember when starting grinding that most take carefully not more than .0002 per cut.

Regards,

A. I think you've answered your own question -- inappropriate grinding procedure.

A. Chris,
I think you have your answer there by the grinding.

Regards,

A. The chrome plating could have become more stressed, but that is not likely unless the plater made a huge change in parameters, really huge change.
Cracking in chrome during grinding is normally-virtually always- caused by the wrong wheel or the grinder taking off too much at a pass. 0.0005 is about the max that you should take on a pass. You might also look at the coolant. Did it change in any way-point of application or flow rate or?

A. Dan,

Ask the plating engineering how hard is the material. I suppose the hardness in the material is 40-55 HRC. Have the part shoots peening before chrome plating, heat treatments before chrome plating, the sulphate content's in the chrome solution. Many thing can gets cracks in a hard chrome layer in a grinding operation.

Regards,

A. Chromium is a very good conductor of heat and therefore it is possible to create grinding burns, Tempered or untempered Martensite (white layer) creates cracks in base material and also in the chromium layer.
Regards,
Jos

A. We use I to K grade wheels with 45 to 60 grit, and grind 0.0002" max cut per pass, and only feed on at one end usually at tail stock. We also dress at 0.001" above final size and have [ed. note: maybe "don't have"?] any problems with etch inspection.

A. You absolutely cannot take 0.003 per pass on chrome, especially on aircraft parts.
What it is telling you is that when the grind dimension is within 0.003 of the final dimension that you must dress the wheel.
The correct grit and resin of the wheel makes a major difference in the finish. The wrong wheel and too aggressive grinding is begging for failed plating and it is not the plater's fault.

A. I haven't read the Specification, and the context of that paragraph could shed more light, but to my understanding it does not imply any specific amount of material removal but to treat the last 3 mils gently. Good luck,

A. Hi Tiger,

I am not going to second-guess Boeing's meaning, you need to talk to your Boeing technical representative to get a technical decision on interpretation.

A. The spec is saying that the wheel must be dressed .003" max above finished size. My company grinds chromed landing gear pistons and we dress .001" max above finished size, and we use .0002" max per cut.

A. What is the parent metal? Hardness? Prebaked. Shot peened?
Plate thickness? Post plate bake cycle??

My cracking problems normally rested with the grinder operator. Their first passes were normally excessive. Feed and speed were sometimes cheated on. Depth of cut after the first pass normally was cheated on. Failure to true the wheel often enough.
The absolute biggest problem was the choice of the disk-grit size and resin. The last year, the failure was always related to using an unsuitable substitute wheel when they could not find the correct one.

Some companies allow a bit of mud flat cracking on certain areas.

A. Hi cousin Sharath. The grinding process is not being done correctly. Please send good photos to mooney@finishing.com for posting here, describe what parameters are used in grinding, and give us some synonyms for "unclear" so we can understand exactly what the issue is. In the meanwhile, here is a huge amount of helpful info on this thread, and we have at least a dozen long threads on chrome grinding problems & solutions which you can review by searching the site. Good luck!

Regards,

A. Hi Sharath,
Your question is not clear; I guess you are not able to finish grind to your required size, so it may be a problem of less/non uniform deposit. As Ted says ... a picture says a thousand words. If you want a clear detailed answer let your question be the same.
Wish you good luck mate..

A. Any parts hardened above 44RC need to be De-Hydrogen Embrittlement done by baking the part to 180 °C, for 4 hours after hard chrome plating.
Also, reduce the cathodic degreasing time by half,