preheating before HVOF

[vevecaz]

Dear all,

I'm very new on this forum, then please excuse me if i make some mistake.
I would like to know how to evaluate the impact of the preheating the steel substract before HVOF spraying.
What are the principal effect on coating ?

Regards.

[Gordon]

Hi Vevecaz

to the Surface Engineering Forum.

The effect of substrate temperature on the thermal spray process (not just HVOF) and reasons for preheating:

* Increasing substrate temperature normally increases bonding properties. This needs balancing with the negative effects of surface oxidation. Hence typical preheat for steel is ~ 100 C, going above 200 C will cause surface oxidation (yellowing or bluing surface) which will have a greater detrimental effect than the beneficial effects of higher temperature. See this link here for a little more info on bonding.

* Raising the substrate temperature well above the dew point is important. Water is a bye product of many thermal spray processes, and a good preheat will stop the condensation of water on the substrate.

* Substrate temperature at start of spraying and just as importantly during the whole spraying process will influence balance of stress between coating and substrate due to expansion/contraction effects.

Ideal situation is to preheat ( ~100C for steel) well above dew point temperature, but below temperature point leading to rapid surface oxidation. Just as important is to try and maintain this temperature of substrate and coating evenly throughout spraying process.

[hvofhamid]

Gordon Wrote:Hi Vevecaz

to the Surface Engineering Forum.

The effect of substrate temperature on the thermal spray process (not just HVOF) and reasons for preheating:

* Increasing substrate temperature normally increases bonding properties. This needs balancing with the negative effects of surface oxidation. Hence typical preheat for steel is ~ 100 C, going above 200 C will cause surface oxidation (yellowing or bluing surface) which will have a greater detrimental effect than the beneficial effects of higher temperature. See this link here for a little more info on bonding.

* Raising the substrate temperature well above the dew point is important. Water is a bye product of many thermal spray processes, and a good preheat will stop the condensation of water on the substrate.

* Substrate temperature at start of spraying and just as importantly during the whole spraying process will influence balance of stress between coating and substrate due to expansion/contraction effects.

Ideal situation is to preheat ( ~100C for steel) well above dew point temperature, but below temperature point leading to rapid surface oxidation. Just as important is to try and maintain this temperature of substrate and coating evenly throughout spraying process.

hi gordon and all
I think that technical coating specially on hvof spraying have known some evolutions that we don t need preheat,maybe because i live in hot country
franckly I work for a very important and exigent international firm(cameron)and never preheat steel for hvof spraying carbide.and my coating is controlled for all mechanical caracteristiques:porosity,hardnesse....ect
may be it s my chance not to lose time
my substrat temperature reach 160 but doesnt cause any visuel or physical problem .
sometimes ,i wonder if theories about coating correspond to reality.
best regards

[TurbineRepair]

Hi guys,

Are there any problems with using a standard blow torch to achieve the preheat? As opposed to oxy-acetylene or the gun. We have problems with burning the masking tapes during preheating using the gun.

Jim

[vevecaz]

Hi,
Thanks for the reply.
I have to make tests with 2 kind of coating.

Regards.

[Gordon]

Hi All

hvofhamid Wrote:

Gordon Wrote:Hi Vevecaz

to the Surface Engineering Forum.

The effect of substrate temperature on the thermal spray process (not just HVOF) and reasons for preheating:

* Increasing substrate temperature normally increases bonding properties. This needs balancing with the negative effects of surface oxidation. Hence typical preheat for steel is ~ 100 C, going above 200 C will cause surface oxidation (yellowing or bluing surface) which will have a greater detrimental effect than the beneficial effects of higher temperature. See this link here for a little more info on bonding.

* Raising the substrate temperature well above the dew point is important. Water is a bye product of many thermal spray processes, and a good preheat will stop the condensation of water on the substrate.

* Substrate temperature at start of spraying and just as importantly during the whole spraying process will influence balance of stress between coating and substrate due to expansion/contraction effects.

Ideal situation is to preheat ( ~100C for steel) well above dew point temperature, but below temperature point leading to rapid surface oxidation. Just as important is to try and maintain this temperature of substrate and coating evenly throughout spraying process.

hi gordon and all
I think that technical coating specially on hvof spraying have known some evolutions that we don t need preheat,maybe because i live in hot country
franckly I work for a very important and exigent international firm(cameron)and never preheat steel for hvof spraying carbide.and my coating is controlled for all mechanical caracteristiques:porosity,hardnesse....ect
may be it s my chance not to lose time
my substrat temperature reach 160 but doesnt cause any visuel or physical problem .
sometimes ,i wonder if theories about coating correspond to reality.
best regards

I'm sure in many situations, preheating may not show a significant benefit. If you test out your procedure and get consistent results meeting with your expectations, then that is fine. I certainly would not claim "no preheating" to be good practice". I know from experience (not theory) that some situations, preheating can make the diffrence between a coating falling off and being sound .

Another benefit (particularly with high velocity processes) when preheating with the spray gun, is the blast cleaning of the substrate surface.

TurbineRepair Wrote:Hi guys,

Are there any problems with using a standard blow torch to achieve the preheat? As opposed to oxy-acetylene or the gun. We have problems with burning the masking tapes during preheating using the gun.

Jim

There are situations when direct preheating with the spray gun is not ideal. Other means of heating can be beneficial as long as they do not contaminate or oxidise the surface to be sprayed. In some cases (bulky parts) oven heating or thorough heating of parts, followed by grit blasting while hot and imediately spraying has advantages in removing any surface contamination caused from preheating.

Specific to Jim's case, the problem may be more a problem of choice of masking tape or trying to cool down the process (faster gun speed or more auxillary cooling).

vevecaz Wrote:Hi,
Thanks for the reply.
I have to make tests with 2 kind of coating.

Regards.

At the end of the day, doing your own tests will always be better than just following theory and others recommendations Good luck.

[hvofhamid]

Gordon Wrote:Hi All

hvofhamid Wrote:

Gordon Wrote:Hi Vevecaz

to the Surface Engineering Forum.

The effect of substrate temperature on the thermal spray process (not just HVOF) and reasons for preheating:

* Increasing substrate temperature normally increases bonding properties. This needs balancing with the negative effects of surface oxidation. Hence typical preheat for steel is ~ 100 C, going above 200 C will cause surface oxidation (yellowing or bluing surface) which will have a greater detrimental effect than the beneficial effects of higher temperature. See this link here for a little more info on bonding.

* Raising the substrate temperature well above the dew point is important. Water is a bye product of many thermal spray processes, and a good preheat will stop the condensation of water on the substrate.

* Substrate temperature at start of spraying and just as importantly during the whole spraying process will influence balance of stress between coating and substrate due to expansion/contraction effects.

Ideal situation is to preheat ( ~100C for steel) well above dew point temperature, but below temperature point leading to rapid surface oxidation. Just as important is to try and maintain this temperature of substrate and coating evenly throughout spraying process.

hi gordon and all
I think that technical coating specially on hvof spraying have known some evolutions that we don t need preheat,maybe because i live in hot country
franckly I work for a very important and exigent international firm(cameron)and never preheat steel for hvof spraying carbide.and my coating is controlled for all mechanical caracteristiques:porosity,hardnesse....ect
may be it s my chance not to lose time
my substrat temperature reach 160 but doesnt cause any visuel or physical problem .
sometimes ,i wonder if theories about coating correspond to reality.
best regards

I'm sure in many situations, preheating may not show a significant benefit. If you test out your procedure and get consistent results meeting with your expectations, then that is fine. I certainly would not claim "no preheating" to be good practice". I know from experience (not theory) that some situations, preheating can make the diffrence between a coating falling off and being sound .

Another benefit (particularly with high velocity processes) when preheating with the spray gun, is the blast cleaning of the substrate surface.

TurbineRepair Wrote:Hi guys,

Are there any problems with using a standard blow torch to achieve the preheat? As opposed to oxy-acetylene or the gun. We have problems with burning the masking tapes during preheating using the gun.

Jim

There are situations when direct preheating with the spray gun is not ideal. Other means of heating can be beneficial as long as they do not contaminate or oxidise the surface to be sprayed. In some cases (bulky parts) oven heating or thorough heating of parts, followed by grit blasting while hot and imediately spraying has advantages in removing any surface contamination caused from preheating.

Specific to Jim's case, the problem may be more a problem of choice of masking tape or trying to cool down the process (faster gun speed or more auxillary cooling).

vevecaz Wrote:Hi,
Thanks for the reply.
I have to make tests with 2 kind of coating.

Regards.

At the end of the day, doing your own tests will always be better than just following theory and others recommendations Good luck.

hi gordon
FOR HVOF I think that preheat can be benefic for few substrats,which have large porosity .
FOR PLASMA I agree with you that preheat is important
for people who want to preheat and don t like to loose time,I recommand a simple way:
we all know that heat goes up and not down that s why we make our cooking on fire in normal situation
so we can start coating substrat from down and go up ,in this way we preheat it

[Gordon]

Hi Hvofhamid

Quote:FOR HVOF I think that preheat can be benefic for few substrats,which have large porosity .

Not sure I understand this statement.

Quote:FOR PLASMA I agree with you that preheat is important

I think preheating can be very important for all processes, particularly those processes producing water as a bye-product of combustion. Arc spray is one process that commonly does not employ preheating, mainly because of convenience as preheating with the gun is not possible and also the process does not produce water as a bye-product (that is not say that preheating does not have its benefits here though).

Quote:for people who want to preheat and don t like to loose time,I recommand a simple way:
we all know that heat goes up and not down that s why we make our cooking on fire in normal situation
so we can start coating substrat from down and go up ,in this way we preheat it

Sorry, I have to disagree with you here in principle. One very important part of the thermal spray process is the control of temperature. Ideally, temperature should remain constant throughout the spraying operation. Raising the temperature from cold during spraying is not the equivalent of preheating. The first initial layer of coating is bonding to a cold substrate, so there will be no benefit to substrate/coating bonding properties from the temperature rising later on. I know we don't live in an ideal world and we very often have to compromise with temperature control (particularly with HVOF on smaller jobs), I suspect your method is a compromise due to difficulties in controlling temperature. Although, you are successful with your process method in your situation (as I said before testing out your process method is all important), I would certainly not recommend this generally for obtaining the best in coatings.

[hvofhamid]

Gordon Wrote:Hi Hvofhamid

Quote:FOR HVOF I think that preheat can be benefic for few substrats,which have large porosity .

Not sure I understand this statement.

Quote:FOR PLASMA I agree with you that preheat is important

I think preheating can be very important for all processes, particularly those processes producing water as a bye-product of combustion. Arc spray is one process that commonly does not employ preheating, mainly because of convenience as preheating with the gun is not possible and also the process does not produce water as a bye-product (that is not say that preheating does not have its benefits here though).

Quote:for people who want to preheat and don t like to loose time,I recommand a simple way:
we all know that heat goes up and not down that s why we make our cooking on fire in normal situation
so we can start coating substrat from down and go up ,in this way we preheat it

Sorry, I have to disagree with you here in principle. One very important part of the thermal spray process is the control of temperature. Ideally, temperature should remain constant throughout the spraying operation. Raising the temperature from cold during spraying is not the equivalent of preheating. The first initial layer of coating is bonding to a cold substrate, so there will be no benefit to substrate/coating bonding properties from the temperature rising later on. I know we don't live in an ideal world and we very often have to compromise with temperature control (particularly with HVOF on smaller jobs), I suspect your method is a compromise due to difficulties in controlling temperature. Although, you are successful with your process method in your situation (as I said before testing out your process method is all important), I would certainly not recommend this generally for obtaining the best in coatings.

hi gordon
the important is the result ,no mind how you coat ,it s true that we have to follow some principal lows of coating ,but we can always think to earn time and time is money
testing gave me assurance that nothing is perfect,and develope new manner to work
best regards

[erick212]

I have found benefit in both preheating and not preheating... All depends on coating type and equipment used. Take for instance copper/nickle on inconel 718(low energy plasma) you can't get a good bond without preheat(in my experience)

[Gordon]

erick212 Wrote:I have found benefit in both preheating and not preheating... All depends on coating type and equipment used. Take for instance copper/nickle on inconel 718(low energy plasma) you can't get a good bond without preheat(in my experience)

Yes, agreed. The substrate material is also a big consideration. Materials like paper, wood, plastics, aluminium and magnesium certainly do not respond well to direct (heating surface area to be coated with naked flame or gun) preheating or any preheating. Even so, I have found moderate lower temperature indirect preheating beneficial on some of these

[hvofhamid]

Gordon Wrote:

erick212 Wrote:I have found benefit in both preheating and not preheating... All depends on coating type and equipment used. Take for instance copper/nickle on inconel 718(low energy plasma) you can't get a good bond without preheat(in my experience)

Yes, agreed. The substrate material is also a big consideration. Materials like paper, wood, plastics, aluminium and magnesium certainly do not respond well to direct (heating surface area to be coated with naked flame or gun) preheating or any preheating. Even so, I have found moderate lower temperature indirect preheating beneficial on some of these

hi all
you are right gordon paper,wood,plastics etc don t need preheat,you only forget steel
steel also doesn t need preheat,if you like ,i invite you to see it ,and you can see the control result
best regards

[Gordon]

Hi hvofhamid

I have no doubt that in your situation you may benefit from not preheating, saving process time, while still achieving acceptable quality. But, I have experienced many situations where preheating is beneficial and in some cases makes the difference of a good coating or a coating that literally just falls off.