Hvof spraying on top of arc spray

[velocity]

Hello
In repair jobs generally wear occurs more then hvof limit. So i use 420SS 1 to 3 mm thicknesses. Then i grind arc spray layer to adjust diameter.

Many times i have crack issue on top of 420SS.
Is there anything special i have to take care between layers?
After machining arc spray, i sandblast and apply buffer hvof too...

[loriolo]

Hi Velocity
Quite all martensitic steels like 420SS has a low temperature eutectic point with volume increment, that could cause coating cracks, so you must take care of the temperature during coating process, arc spray has high interchange temperature with base material, take care that grinding must be done dry, and is better machining by ladle, ever dry, some customer use to do arc spray coatings and after chrome plating without any problem, with HVOF there is not problem if you grit blast and has a surface without any contaminant.

[velocity]

Hi Velocity
Quite all martensitic steels like 420SS has a low temperature eutectic point with volume increment, that could cause coating cracks, so you must take care of the temperature during coating process, arc spray has high interchange temperature with base material, take care that grinding must be done dry, and is better machining by ladle, ever dry, some customer use to do arc spray coatings and after chrome plating without any problem, with HVOF there is not problem if you grit blast and has a surface without any contaminant.

Hi Luigi
Thank you for comment.
Is there any better wire for od restoration before hvof?

Should i keep temp. lower then 100? My normal is 100 to 150 C

[loriolo]

Hi
Lower than 150 °C could be ok, over 200 degrees C begin some transformation on the metallurgic martensitic morphology, with volume modification. the material to use it depend by the specific use, anticorrosion, wear resistance, generally a 13 Cr steel could be ok because has low shrinkage or 316SS or low carbon steel for low cost material.

[Red Baron]

Hello Velocity

Build up coating under HVOF applied materials can be tricky. First issue is the potential for porosity in the base coat. That can be minimized by a number of factors including material choice, type of equipment used, nozzle / aircap selection, voltage, amperage, air pressure, gun stand-off distance, rotating speed of part, angle of application, dust collection during spray operations, and I'm sure I'm missing others.

I struggled with the 420SS under Wc HVOF applied coatings, and found the 420 to show signs of microcracking as viewed after the finish grind of the HVOF applied coating.

My solution was to apply a more expensive twin arc wire as a base, then grind it to a size that will allow for the proper amount of HVOF applied coating. The material is metco 8280 wire, formerly sold as Vecalloy B. There is even a material spool that can be blended with the Metco 8280 that allows for reduced porosity as well. The Metco 8280 sprays on base metals after a good 24 grit aluminum oxide blast with no need for bond wire. Application temperature rarely sees above 200 degrees F. Application rate is about .001" - .0015 thickness per pass. ( as controlled by speed/feed of manipulator ) Wet DIAMOND grind to pre-HVOF size. ( Conventional Aluminum oxide grinding wheels will not cut the metco material )

Once ready for HVOF topcoat, I inspect for any pits, and if found, I fill using a process best explained another time. Then Blast the surface with 24 grit aluminum oxide. This blast is just to take the grind sheen off the part, blasting too hard opens up can of worms sometime.

Finally, PRE-HEAT the part to be sprayed to a minimum of 140 degrees F with the HVOF gun. Only turn on the feeder after that temperature minimum has been attained. Apply the HVOF material at a rate not to exceed .0005" thickness per pass. 86-10-4 Wc is most common.

I hope that helps. And yes, material and grinding costs increase significantly with this process. The benefit however is a eye pleasing surface, that performs well, and has a "security layer" between the soft base metal and the HVOF applied coating. In severe wear environments, once the HVOF layer is compromised, 420SS will have no chance to hold up for very long. The metco 8280 is tough, and offers significant wear resistance itself, therefore increasing life, reducing costly downtime, and reducing the damage to the base metal.

Regards, I hope this makes sense.

[velocity]

Thank you so much Luigi and "Red Baron".
Very valuable comments for me. About 8280 issue, i heard same suggestion from Polymet agent too. But it makes my cost very high and seems still too tricky. But i really wonder how you fix pores in arc spray layer.
I am still making trials to have a successful 86 10 4 layer on top of 420SS arc spray...

[loriolo]

Hi Velocity
I could suppose that you maybe have an old generation arc spray system, because in the new generation of arc spray porosity is very low, after 13Cr arc spray coating the chrome plating do not has residual porosity, maybe your problem is the HVOF coating

[velocity]

Yes you might right. So i added a dc motor to drive. Please see my arc flow attached. Can you comment with video?
https://youtu.be/XFGHmpDHTwQ