High Pressure Piston Rods HVOF coatings

[HVoilF]

Hello everybody, we guess if a Wc Co Cr JP5000 HVOF coating could do a good service on a Dresser Rand piston Rod compressor manufactured with AISI 4140 steel,working at 7000 PSI.
We have seen the same piston rod 95 MXC (Praxair Tafa hard cored wire) arc spray coated peeled off after few minutes working. Is it due to the rod deflection for the high force found in the piston head? Would the HVOF Wc coating stand such service conditions?if so which should be the reccomended coating thickness?
Further wich could be the reccomended seals for the HVOF coating? Metal seals,Teflon seals or other?

[Gordon]

Hi HVoilF

to the Surface Engineering Forum.

Wc/Co/Cr I think is generally the hardest wearing thermal spray coating for this job.

When I looked into this type of application some years ago when doing some failure investigations I noted the following:

The maximum operating pressure for HVOF coatings on compressor piston rods is not easily answered, as this is not the only factor involved in effective performance. The general feedback on this question is that HVOF and Dgun coatings should have no problems dealing with pressures up to 5,000 psi and probably much higher.

Praxair have indicated that compressor piston rods coated with LW15 or LW1N30 have operated over five years in hydrogen service with carbon filled PTFE packing at 6,000 psi and showed no measurable wear in the packing area. Plungers in hyper-compressors in polyethylene manufacturing have run for 16,000 hours at 20,000 psi and showed only 0.001” wear. Praxair said they were not sure how Dgun coatings compared to HVOF coatings in this type of application, but admitted they were developing HVOF coatings for this type of application.

The lack of service related data on HVOF coatings is probably due to the fact that this is a relatively new and developing process which requires time to yield long term performance data. Plasma sprayed coatings including ceramic coatings have been claimed to be successful at pressures of 3,000 psi. Generally these coatings are lower in bond strength, cohesive strength and density.

Praxair use epoxy resin sealers (UCAR 100) on this type of application. Vacuum or pressure application techniques are apparently not used. I would consider sealing these coatings after finishing with epoxy (ideally vacuum impregnation) or anaerobic type sealers. Phenolic or linseed oil based sealers may be considered for certain less arduous conditions. Sealing just after spraying the coating may not be effective as finishing will probably remove the sealer. I do not think the cause of the coating failure is related in this case to the sealer, but other applications may rely on an effective sealer to guard against corrosion and higher pressures.

Other considerations should include the coating thickness which should be as thin as possible (on new parts 0.003 to 0.005” recommended). As coating thickness increases so does the internal residual coating stress which can alter the effective bond strength and resistance to bending stresses. Corrosion along with component bending stresses and pressure exerted on the coating from the packing seals are other important factors.

Hope that is of some help.

[HVoilF]

Hello Gordon,thanks for your interesting answer. We have been manufacturing a new piston rod with AISI 4140 steel and we have sprayed (JP5000 HVOF gun) with WcCoCr 6/1000" thick.We have superpolished it at 6-8 microinches . My only doubt now is if you really think that should be sealed(considering such a dense JP coating!) and if so what about a Loctite 290 sealer?
Making some calculation on the force found on the piston head working at some 7000PSI we found an axial force in the rod of more than 60.000 pouds!! So the bending consideration on the rod probably is the most important hazard to be taken into account,for this reason I agree with you to keep the coating thickness very thin(below 10/1000"). What about the sealing materials considerations for the Wc coating? Polymers or metals? Do you have any information about?
Thanks for your help.

Hi HVoilF

to the Surface Engineering Forum.

Wc/Co/Cr I think is generally the hardest wearing thermal spray coating for this job.

When I looked into this type of application some years ago when doing some failure investigations I noted the following:

The maximum operating pressure for HVOF coatings on compressor piston rods is not easily answered, as this is not the only factor involved in effective performance. The general feedback on this question is that HVOF and Dgun coatings should have no problems dealing with pressures up to 5,000 psi and probably much higher.

Praxair have indicated that compressor piston rods coated with LW15 or LW1N30 have operated over five years in hydrogen service with carbon filled PTFE packing at 6,000 psi and showed no measurable wear in the packing area. Plungers in hyper-compressors in polyethylene manufacturing have run for 16,000 hours at 20,000 psi and showed only 0.001” wear. Praxair said they were not sure how Dgun coatings compared to HVOF coatings in this type of application, but admitted they were developing HVOF coatings for this type of application.

The lack of service related data on HVOF coatings is probably due to the fact that this is a relatively new and developing process which requires time to yield long term performance data. Plasma sprayed coatings including ceramic coatings have been claimed to be successful at pressures of 3,000 psi. Generally these coatings are lower in bond strength, cohesive strength and density.

Praxair use epoxy resin sealers (UCAR 100) on this type of application. Vacuum or pressure application techniques are apparently not used. I would consider sealing these coatings after finishing with epoxy (ideally vacuum impregnation) or anaerobic type sealers. Phenolic or linseed oil based sealers may be considered for certain less arduous conditions. Sealing just after spraying the coating may not be effective as finishing will probably remove the sealer. I do not think the cause of the coating failure is related in this case to the sealer, but other applications may rely on an effective sealer to guard against corrosion and higher pressures.

Other considerations should include the coating thickness which should be as thin as possible (on new parts 0.003 to 0.005” recommended). As coating thickness increases so does the internal residual coating stress which can alter the effective bond strength and resistance to bending stresses. Corrosion along with component bending stresses and pressure exerted on the coating from the packing seals are other important factors.

Hope that is of some help.

[Gordon]

Hi HVoilF

Sealing probably has little effect with a good HVOF WC/Co/Cr, too dense to allow significant or any penetration. But I know Praxair D-Gun coatings certainly have a much longer history in this type of application and they always use to seal with Ucar 100 which is an epoxy based sealer, so I'm a little nervous to say its a waste of time and money. The loctite sealer is probably worth doing just for belt and braces

Sorry, no info on the packing seal materials other than the mentioned carbon/PTFE, probably a very important consideration, as with assembly of compressor. I have seen a few coating failures with WC/Co/Cr JP5000 coatings on compressor rods. The coatings appeared to of be of high quality with good grit blast quality. Some rods failed in localised spot while others were ok. I put it down to assembly misalignment or something like that I guess, we will probably never know for sure, one side always seems to blame the other

Anyway, good luck, hopefully you will not need.

[hvofhamid]

hello everybody
I think you have to use WC Co Cr 86/10/4 with right parametres of jp 5000.then you will have no problems.
I m used to coat pieces for oil and gas (tail rod,connecting rod,con rod,operating piston,gate......).I use too jp 5000 and never have any problem
thikness recommanded 0.15mm

[HVoilF]

hello everybody
I think you have to use WC Co Cr 86/10/4 with right parametres of jp 5000.then you will have no problems.
I m used to coat pieces for oil and gas (tail rod,connecting rod,con rod,operating piston,gate......).I use too jp 5000 and never have any problem
thikness recommanded 0.15mm

[hvofhamid]

your answer was very interesting ;we learn much with you

[HVoilF]

Sorry hvofhamid, my reply was a mistake....I was going to ask you which is the maximum pressure that you have handled in Wc JP5000 Piston Rods(we are handling some 7000PSI) and if you seal it or not.If so which sealer would you reccomend??

your answer was very interesting ;we learn much with you

[hvofhamid]

Sorry hvofhamid, my reply was a mistake....I was going to ask you which is the maximum pressure that you have handled in Wc JP5000 Piston Rods(we are handling some 7000PSI) and if you seal it or not.If so which sealer would you reccomend??

your answer was very interesting ;we learn much with you

ok it doen t matter
for steel 4140 we use the same steel and I have seen a test reach 13 k psi and more 15 k psi ,if you are really interested send me a private mssage,and I ll give the information you need

[hvofhamid]

Sorry hvofhamid, my reply was a mistake....I was going to ask you which is the maximum pressure that you have handled in Wc JP5000 Piston Rods(we are handling some 7000PSI) and if you seal it or not.If so which sealer would you reccomend??

your answer was very interesting ;we learn much with you

ok it doen t matter
for steel 4140 we use the same steel and I have seen a test reach 13 k psi and more 15 k psi ,if you are really interested send me a private mssage,and I ll give the information you need

in general bellow 7000 psi we use seal O ring (parker) over it s reccomended both "O" ring+back up ring.
but all depend of function and other factories that required from your piston rod
in general there are somes catalogues for sealers;you can find what ever you like

[Frantisek]

Hello everybody,

we have sprayed a hundreds of piston rods with WC/Co/Cr 86/10/4 coating. We use HP/HVOF with JP-5000 gun. Generally is known that the quality is strongly depended on the spraying parameters, so you can on one hand spray very good quality coating, but on the other hand also low quality coating when using "weak" parameters. Our experience is that "starting points" parameters delivered usually by powder suppliers can be improved to get high quality coatings with truly improved mechanical properties of the final coatings. Also the starting powder has a big influence, so only the traditional experienced suppliers are recommended. Of course the final machining is also very important issue and must be also take into consideration.

[Krist3Grant]

Yeah very true just go for good quality of spray coating its really good for HVOF, and when it comes to sealing i heard a lot about carbon seal but not tried this, so just give it a try! I guess it will be better than metal seal.

[joyee]

Helpful post. This is the way to expose your ideas. Weldon and keep it up.
Thanks for sharing this informative post.

[kschewe]

How thick did you spray the tafa 95mxc?

[k09]

I hope you have found a solution to the problem. We too have faced similar problems while repairing Piston Rods. Piston rod is subjected to heavy bending stress and its designed for that however it has to be ensured that coating itself has adequate strength for it to perform satisfactorily under such sever conditions. As per our diagnosis of some failed rods, we arrived at the following conclusion.

1. It was observed that the failure / delamination usually occurs from the Substrate-bond coat interface and in the areas traversed by the packing material. A Wire arc sprayed bond coat for dimensional rebuild in not suitable for piston rod application. The rods usually comes with localised wear wherein the rods have to be re-builded by minimum 0.25mm radially.The bond strength of WAS coatings is considerably lower that HVOF sprayed coatings so even when you have a more reliable HVOF coating on top, the substrate and WAS coat interface acts as the weakest point when subjected to cyclic bending and it tends to crack which later propagates and leads to delamination along the lenght of the rod.

2. Some rods were originally induction hardened (48-55 HRc) with case depth in the range of 2-4mm. This leads to inadequate bonding of sprayed coating (irrespective of the coating process). As per specs for OEM coated piston rods, the substrate is usually 18 to 25 HRc. We observed that when the hardness is lesser than 35HRc, the operational life of a repaired rod is significantly better.

A combination of the above two scenarios is even worse. Sealers play an important role in preventing base material corrosion however in our case base material corrosion was not the culprit.

Hope this helps..

Regards
K09