Anyone venture on chroming piston rods?

Had the piston rods on the 150D compressors and rod on the Worthington hot water pump hard chromed and ground. So far the results are good.

Also chromed rods on compound air pumps. Working very well.

steamrr@mashell.com

I seem to recall a string about this last year and some responded that there could/are cracking in the base metal due to the plating or its mechanical properties.

Anybody remember anything like this, or know if it is an issue?

Ray

I have always been shy of the idea of chroming piston rods because of possible fatigue problems.

According to National Bureau of Standards information published in the 1940's, "chromium coatings reduced the endurance limit of SAE 6130 steel by amounts varying from 3 percent to 41 percent, depending upon the hardness of the steel, the thickness of the coating, and the method of fatigue testing."

I doubt that it would be a problem with lower speed, lower stress, rods as in air and water pumps, but it would be interesting to hear from someone with some metallurgical expertise.

> Anyone venture on chroming piston rods?

Yeah, on highly cycled parts, chrome is really detrimental. Chrome is hard, which makes it ideal for wear resistance. However, like most hard materials, its very brittle. Therefore, under repetitive loading, it tends to crack. These cracks then serve as "stress concentrations," which increases local stress in the base metal. This increase in stress may exceed the yield strength of the base metal, which causes the crack to propogate into the material, eventually failing the part. Shot peening the part after chroming will set up favorable residual compressive stress in the part, which will solve or at least mitigate the problem. At least, I'm pretty sure thats how that works. I've, er, drank since machine design class.
-Chris

The problem with hard chrome plating is hydrogen embrittlement.

After a plating process, a part which is subject to a fatigue loading or a loading at a temperature below the upper shelf energy temperature should be baked at 200 to 300F for one hour per inch of thickness. The exact temperature depends upon the material. This treatment should be done immediately after removal from the plating bath. Caution should be taken to ensure that the baking temperature is not at a temperature which may cause temper embrittlement of a previously heat treated part.

Another factor is the heat treatment condition of the part. Ideally, any heat treatment to increase the tensile strength should be done after the plating process, unless appearance is important.

Hydrogen embrittlement becomes more of a factor as the tensile strength of the material increases as generally the toughness of the material will decrease as the tensile strength increases.

The only effect that speed will have upon the loadings and subsequent stresses, is the dynamic loads that are imposed on the part; i.e., F=ma.

Basically, fatigue life is solely dependent on the maximum and minimum stress sprectrum on the part. As the maximum stress and delta stress on the part decrease the fatigue life will increase.

Usual disclaimers and limitations do apply.

Your results may vary.

Hope that this helps.

PKurilecz

> I have always been shy of the idea of
> chroming piston rods because of possible
> fatigue problems.

> According to National Bureau of Standards
> information published in the 1940's,
> "chromium coatings reduced the
> endurance limit of SAE 6130 steel by amounts
> varying from 3 percent to 41 percent,
> depending upon the hardness of the steel,
> the thickness of the coating, and the method
> of fatigue testing."

> I doubt that it would be a problem with
> lower speed, lower stress, rods as in air
> and water pumps, but it would be interesting
> to hear from someone with some metallurgical
> expertise.


pkurilecz@yahoo.com

One of the major problems you will run into when you chrome plate anything is microscopic surface defects. These will cause voids in the plating leading to cracking, flaking and loss of the "plate". There are millions of hydraulic cylinders in service today with plated rods with no problems ...but these are carefully prepared by : 1. specified clean steel practices in the manufacture ie. vacuum degassing, clean alloy additions and extra fluxing 2. removal of material in the semifinished state (billet) to eliminate areas known to contain these defects. 3. grinding and polishing the bar to finished size. 4. Magnaflux testing where practicable of the bar before plating.

One of the toughest applications we could never make work right was your plain old automotive shock absorber rod. We would jump thru the clean steel hoops, vacuum degas the steel to remove hydrogen, remove almost 1/2" of surface from a 6x6 billet ... remagnaflux it to insure removal of all longitudinal defects and still end up with an unacceptable rejection rate for "pits" in the plating. When we finally got the process down pat the steel was too expensive to make any money on the order.

Hydrogen embrittlement: Unless you vacuum degas steel or "slo-cool" the billets/blooms, hydrogen is present dissolved in the product. Upon cooling the hydrogen leaves voids in the matrix that range from tiny bubbles to huge cracks called flakes. If your reduction ratio for the finished part is not heavy enough to roll out these defects (they will reweld at rolling temperatures) you are left with the voids. Migration of hydrogen in steel just is too slow to measure at temps below 1500F.
Dirt: Is present in varying amounts in all steel. This can take various forms ranging from rocks to particles that can only be seen under a microscope.
If the particle is on the finished surface it will also cause a void in the plating.

In summary, when you plate a part that has not been predesigned for it, there is no guarantee that it will perform to expectations. If you are remanufacturing a part that has been redesigned for chrome plating you will not get a satisfactory product off the shelf from a steel warehouse unless the supplier can certify "chrome plating quality".

CAVEAT EMPTOR


lamontdc@adelphia.net

And if the parts are metal sprayed and then ground instead of plating how much of a difference is there?

> And if the parts are metal sprayed and then
> ground instead of plating how much of a
> difference is there?

I have no experience in the metal spray field. The reports I have seen on the web give mixed reviews. The only advice I can give is to get some kind of guarantee from your vendor.


lamontdc@adelphia.net

All good points....

It just points out that the design of any point is not as easy as it appears.

With respect to hydrogen embrittlement, there are several sources for this.

With respect to steel produced from electric arc furnaces, it can be a problem and it is solved through good melt practices.

Plating processes can be a source of hydrogen embrittlement. Due to the time in the plating tank, the migration of hydrogen is very shallow and can be treated as I have described. With respect to diffusion rates, this is well modelled through the Arhenius equation.

Billet quality is a concern for the production of any part that is subsequently rolled. Whether the quality concern is nonmetallic inclusions, porosity or TCCs, they all can be handled through appropriate QC and melt practices. As with most processes, the quality of the billet can vary and the mill can produce to your specifications for cleanliness, et al. It just depends on what you are willing to pay.

Triple vacuum degassing certainly does remove a lot of problem issues, particularly in Ti alloys.

I would have to say that the machining process for the manufacture of a rod to be chrome plated will have more of an impact upon the quality of the part. Certainly, polishing and cleaning prior to the plating process will have a great effect upon the quality and durability of the finished coating.

Regards;

PKurilecz

Just make new piston rods and leave the chrome plating to smaller compressor rods, valve stems, etc. From all the reponses it should be easy to see plating old piston rods is not worth the risk. Remember, piston rods see double the deadcenter loading and are the most highly stressed part in the drive train.

Skinner Engine Company standardized on 4140 or 4340 alloy steel for piston rods. It was not heat treated so much that it couldn't be machined. I have made several rods out of this material and the jobs turned out well.

Bill Petitjean


petitinc@nwlink.com