Preheating

[powerkiv]

Hi,

I am trying to analyse what harm could preheating cause in Arc Spraying, When it seems to help (uniform micro hardness)in HVOF spraying (wire feedstock).

Thanks
PV.

[Simon Kim]

Hi, PV.

Do you mean the case of complex spray, Arc spraying and HVOF spaying?

Because I can not catch your point.

I have some experience both of them.

Please explain again easily step by step.

Simon

[powerkiv]

Hi Simon,
I am comparing two different coating sprayed from a wire feed stock. One HVOF coating which required preheating (substrate)for adhesion and uniform micro hardness(vickers)

Other coating was Arc Sprayed. I have a good as sprayed deposit.

To get an apples to apples comparison, I preheated the substrate. The resultant arc sprayed coating had cracks and was poor hardness consistancy.

I know arc sprayed coating in genral do not require preheating, but why would it be deleterious?

Thanks
PV

[LEN WOOD]

'Flame torch' preheating directly onto prepared surface will cause surface oxidation that will be detrimental to bonding. Therefore in arc spray it is not usually used. However if preheat is required (certain applications and substrates) then indirect heating can be performed - see post "Coating freak Off From Aluminium". The initial arc spray bond layer should be applied with a 'burn pass'.

Preheating the substrate may cause (material dependent) post spray shrink cracking within but shouldn't affect coating hardness unless it was extreme. One purpose of preheating is to minimise the difference in the coefficient of expansion between the substrate and the heated particles. Also to promote metallurgical bonding, remove surface moisture and stop condensation forming.

HVOF processed wire coatings will be superior/more consistent than regular arc spray, but it is a 'horses for courses' situation.

Hope this helps

[Gordon]

Hi PV

to the Surface Engineering Forum.

Pre-heating in general should be beneficial for all thermal spray processes if done properly. Pre-heating in most cases is not employed with arc spray. Mainly, because the gun itself can not be used to pre-heat substrate. Also the arc spray process does not give off water as a bye-product of combustion so water condensation on cold substrates is far less of a problem. Assuming no contamination/oxidation of substrate surface - hotter substrates generally give better bonding than colder ones. Also, in some cases pre-expanding the substrate prior to spraying can have benefits in reducing stresses in the coating.

What is the substrate?
What is the coating?
How are you pre-heating and to what temperature?

One the whole I agree with Len's comments, but

'Flame torch' preheating directly onto prepared surface will cause surface oxidation that will be detrimental to bonding.

well this depends very much on how you do it

[powerkiv]

Hi PV

to the Surface Engineering Forum.

Pre-heating in general should be beneficial for all thermal spray processes if done properly. Pre-heating in most cases is not employed with arc spray. Mainly, because the gun itself can not be used to pre-heat substrate. Also the arc spray process does not give off water as a bye-product of combustion so water condensation on cold substrates is far less of a problem. Assuming no contamination/oxidation of substrate surface - hotter substrates generally give better bonding than colder ones. Also, in some cases pre-expanding the substrate prior to spraying can have benefits in reducing stresses in the coating.

What is the substrate?
What is the coating?
How are you pre-heating and to what temperature?

One the whole I agree with Len's comments, but

'Flame torch' preheating directly onto prepared surface will cause surface oxidation that will be detrimental to bonding.

well this depends very much on how you do it

Hi Gordon,

You have a great Forum going on here. !!

I am using a 1/8X2X3 in steel coupon.
My coating material is a amorphous steel wire.
I am preheating with a induction heater coupled with a PLC. I have found that 250degC works best with the HVOF process.

Thanks
PV

[Gordon]

Hi PV

I would consider 250degC too high normally, typically ~100degC is considered best. Steel tends to oxidise more and more rapidly as you go above 200degC. HVOF will probably be a little more tolerant to slight surface oxidation than arc spray due to its more aggressive particle impacts disrupting the surface.

Generally, the benefits of preheating for arc spray very often are not thought significant enough to outweigh the bother of an extra process.

My coating material is a amorphous steel wire.

Not sure on this one, but higher substrate/coating temperatures with reduce particle cooling rates may effect the amorphousness of the coating.