Plunger Repair with WC coating

[sal]

Hi guys,
We have coated a 2000 mm plunger with 110 mm diameter with WC/86/10/4 with about 400 micron thickness with HOVF liquid fuel system. but coating was cracked. the plunger was a repair job therefore before coating the previous WC coating was removed via grinding, sand blasted and then coated.
could any body please give us some hint what else we can do to finish the job?
could there be any residual stressed causing this problem or any part of the previous coating left on the surface? can we preheat the surface to reduce the problem? or should we grind deeper to make sure everything on the surface is fresh and then redo the job.
any hint is appreciated.
Regards

[js2know]

I would suggest laser cladding this is a metallurgical bond that will be more robust and less likely to crack.

Hi guys,
We have coated a 2000 mm plunger with 110 mm diameter with WC/86/10/4 with about 400 micron thickness with HOVF liquid fuel system. but coating was cracked. the plunger was a repair job therefore before coating the previous WC coating was removed via grinding, sand blasted and then coated.
could any body please give us some hint what else we can do to finish the job?
could there be any residual stressed causing this problem or any part of the previous coating left on the surface? can we preheat the surface to reduce the problem? or should we grind deeper to make sure everything on the surface is fresh and then redo the job.
any hint is appreciated.
Regards

[djewell]

Did the coating fail while you were spraying or after? Did it fail in tension or compression? What is the base metal?

Spraying solid plungers is always tricky because of the expansion of the metal relative to the coating.

[Vadim Verlotski]

Hi Sal,

The WC-Co layers and WC-Co-Cr can not be risk-free coated with the thickness of about 300 microns (the thicker the layer the more large residual stresses). 400 microns was just too much.
To save the plunger now (to coat to size) You must trick:

1. Grinding all the way up to the base material (- 450 microns of the final dimension in the radius).
2. Corundum blasting with 100-200 microns grain size.
3. Coating with a NiAl 95/5 bond coat about 300 microns (- 150 microns to final size).
4. Preheating the whole plunger evenly to 100-150 ° C.
5. Coating with WC-Co-Cr, the remaining thickness (150-200 microns).

[sal]

Hi Sal,

The WC-Co layers and WC-Co-Cr can not be risk-free coated with the thickness of about 300 microns (the thicker the layer the more large residual stresses). 400 microns was just too much.
To save the plunger now (to coat to size) You must trick:

1. Grinding all the way up to the base material (- 450 microns of the final dimension in the radius).
2. Corundum blasting with 100-200 microns grain size.
3. Coating with a NiAl 95/5 bond coat about 300 microns (- 150 microns to final size).
4. Preheating the whole plunger evenly to 100-150 ° C.
5. Coating with WC-Co-Cr, the remaining thickness (150-200 microns).

Thanks man! I was thinking of using some bond coat but was not sure about it. Now will do it with more confidence.
Sal

---

Hi Sal,

The WC-Co layers and WC-Co-Cr can not be risk-free coated with the thickness of about 300 microns (the thicker the layer the more large residual stresses). 400 microns was just too much.
To save the plunger now (to coat to size) You must trick:

1. Grinding all the way up to the base material (- 450 microns of the final dimension in the radius).
2. Corundum blasting with 100-200 microns grain size.
3. Coating with a NiAl 95/5 bond coat about 300 microns (- 150 microns to final size).
4. Preheating the whole plunger evenly to 100-150 ° C.
5. Coating with WC-Co-Cr, the remaining thickness (150-200 microns).

Thanks man! I was thinking of using some bond coat but was not sure about it. Now will do it with more confidence.
Sal

just one more question can we coat the Ni/Al with twin wire arc on this carbon steel plunger?

---

Did the coating fail while you were spraying or after? Did it fail in tension or compression? What is the base metal?

Spraying solid plungers is always tricky because of the expansion of the metal relative to the coating.

The coating fail during spraying (coating was not finished).
one end of the plunger was free and the base metal is carbon steel.
Sal

[Met.Eng.]

1-Do you check the temperature while spraying?? You have to measure the top of the 5 cm hvof flame, don't measure left or right of the flame if you want to get correct measuring. And if you use 24 grit al2O3 blasting madia and you have to stop while 100-120 C , if you apply NiAl bond coat with Arc spray for ex you can up to 150 160 c may be max 200 C if your coating layers thin.(Up the Travers and surface speed)
2- Coating layers don't exceed 8-10 microns, may be you can down to 8 micron per pass.
3-If you hesitate the coating stress, you can preheat 60C with carburizing oxyacetylene flame and apply max 100 micron Ni5Al arcspray..
4- If this was not enough, you can mix your carbide powder with %10-%20 Metco 15F
5-what is your powder particul size?? I hope its not -25 +5 you know the more particul size the more less stress

[Joris Kraak]

Hi guys,
We have coated a 2000 mm plunger with 110 mm diameter with WC/86/10/4 with about 400 micron thickness with HOVF liquid fuel system. but coating was cracked. the plunger was a repair job therefore before coating the previous WC coating was removed via grinding, sand blasted and then coated.
could any body please give us some hint what else we can do to finish the job?
could there be any residual stressed causing this problem or any part of the previous coating left on the surface? can we preheat the surface to reduce the problem? or should we grind deeper to make sure everything on the surface is fresh and then redo the job.
any hint is appreciated.
Regards

Sal,

Is the problem fixed already?

regards,
joris

[sal]

Well, we grinded and sand blasted the plungers.
coated with Ni/20Cr for about 100 micron.
coated with about 350 micron WC/86/10/4. one of the plungers survived the coating process but the other one start cracking along the length of the plunger after 100 micron Ni/20Cr and 200 micron WC !
an 80 cm crack which can be removed using a knife under it. (length of plunger coating area 2000 mm.
it's very devastating. I don't know if the plunger history has anything to do with it or not. because we have done this on longer plunger but brand new ones without any cracking up to 400 micron even no problem during final grinding.
we controlled the temperature during spraying around 100 C, too.
grinded with alumina grits 24.
powder particle size of -45+15 micron

[pakalano]

Well, we grinded and sand blasted the plungers.
coated with Ni/20Cr for about 100 micron.
coated with about 350 micron WC/86/10/4. one of the plungers survived the coating process but the other one start cracking along the length of the plunger after 100 micron Ni/20Cr and 200 micron WC !
an 80 cm crack which can be removed using a knife under it. (length of plunger coating area 2000 mm.
it's very devastating. I don't know if the plunger history has anything to do with it or not. because we have done this on longer plunger but brand new ones without any cracking up to 400 micron even no problem during final grinding.
we controlled the temperature during spraying around 100 C, too.
grinded with alumina grits 24.
powder particle size of -45+15 micron

How did you prepare the plunger before coating? I don't know the application it's used for, but a field run part in for repair must be handled with care. There will be chemicals and gases trapped in the base metal that must be baked out. You've also been grinding with coolant to remove old coating. We preheat and hold our repair plungers for up to an hour to be sure no contaminant rises to the surface during coating application. Once cooled, blast and spray as you would. I do recommend a bond coat with that thickness, it too can be tricky because of thermal expansion.