induction vs flame preheating
05-01-2011, 08:30 PM,
#1
induction vs flame preheating
Hello Gordon

My question to you and everbody else: what do you prefer as preheating source when using HVOF process - induction or flame preheating.
If induction preheating is better (obviously less surface oxdiation), does that mean you don't need to do grit blasting again. We are using Sulzer DJ2700 with propane , but preheating with acetylene which means we need to grit blast again hence contaminate surface .
Do you think we should be using induction heater for this?

Thanks in advance Smile
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05-02-2011, 02:53 AM,
#2
RE: induction vs flame preheating
Hi divanis

Sign0016 to the Surface Engineering Forum.

I must say a strange but interesting question. I have had only direct experience with one process involving induction preheating prior to thermal spray. Very early in my career I work with a valve aluminising machine, which basically in a semi-automated system, preheated valve seats via a induction heating before being combustion wire sprayed with aluminium, followed by further induction heating to finally aluminise the coating with substrate.

With automated, repetitive high production coating processing, in some cases I can see good advantages. Non-contact rapid internal heating, though at the cost of specific inductors to suit each component type and initial capital equipment and development costs. In most cases most sprayers make obvious use of the process gun heating capabilities for preheating. This may not always be the absolute best way, but very often the simplest and most cost effective way.

Quote:We are using Sulzer DJ2700 with propane , but preheating with acetylene which means we need to grit blast again hence contaminate surface .
Do you think we should be using induction heater for this?

Preheating with acetylene flame sounds a little aggressive. Is there a problem with simply preheating with the DJ gun? In certain cases, I would consider preheating substrates before grit blasting (where substrate mass is sufficient not to lose to much temperature) to be an advantage.

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05-02-2011, 07:22 AM,
#3
RE: induction vs flame preheating
Hello Everyone,

We also use, HVOF main flame for preheating as it is quicker and we could continue with the spray pass after the preheating pass with out any delay in between. Is there a downside for using this technique? We normally preheat the substrate to 50Deg C. Is this sufficient?

Are there any other effective methods for preheating that would not promote oxidation of the surface as in comparison to other methods?

Regards

K09
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05-02-2011, 01:15 PM,
#4
RE: induction vs flame preheating
In my work place, we have two preheat method: oven preheat (270F) and HVOF gun preheat.
In oven preheat, the operator have a little hard time to do the job because the heat but it can take the moisture out and keep the dirt away from part.
HVOF gun preheat is the best way to do the job, also can take some remaining grit away from part.
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05-02-2011, 01:36 PM,
#5
RE: induction vs flame preheating
Thanks everyone to your suggestions. Generally we haven't had any problems with the process when using acetylene as a preheating method. Practically we're preheating to 120-140 oC, and then remove surface oxidation by turning. No problems at all

Regards,
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05-02-2011, 01:48 PM,
#6
RE: induction vs flame preheating
(05-01-2011, 08:30 PM)divanis Wrote: Hello Gordon

My question to you and everbody else: what do you prefer as preheating source when using HVOF process - induction or flame preheating.
If induction preheating is better (obviously less surface oxdiation), does that mean you don't need to do grit blasting again. We are using Sulzer DJ2700 with propane , but preheating with acetylene which means we need to grit blast again hence contaminate surface .
Do you think we should be using induction heater for this?

Thanks in advance Smile

Hi
Why heat the opening of the piece before applying any method? For an intensification of diffusion processes between the coating and substrate and provide high adhesion. But in that time span and at relatively low heating temperatures, these processes occur is not enough time. In addition, the heating is important not just parts of bodies, but only a few monolayers of the surface. Therefore, for the technology HVOf preheating parts to several hundred degrees Celsius yields nothing.It is important to carefully dry-clean (chemical) the surface and to give relief by sandblasting.
But the subsequent diffusion annealing parts, ie, after the coating is very helpful.
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05-06-2011, 03:23 PM,
#7
RE: induction vs flame preheating
There isn't really a broad brush-stroke answer to the question. The part in question is probably the variable that is most likely to determine the "best" pre-heat method. As has been said repeatedly by others most of the time simply using the gun itself for pre-heat is usually effective and efficient, but it also ties up the gun for a period of time. If this is an automated, high volume production, then an oven or induction heater make a good bit of sense. I don't like the idea of an acetylene torch in particular, but I suppose if the part soaks a lot of heat and you need the BTU's for that particular part... I can envision a set of circumstances that it may be the best method available. I think most of the time comparing pre-heat methods is likely an issue of splitting hairs unless you have circumstances that make it more important, i.e. high volumes, extremely large parts, parts that loose heat quickly or require a lot of energy for the pre-heat.
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05-13-2011, 05:18 AM,
#8
RE: induction vs flame preheating
I agree with the comments previously. MichiganMan is correct - different jobs mean different methods, and not always is one method significantly better than others.

Like other people here, we predominantly use the gun for preheating for HVOF and HVAF. Our HVOF system is configured to use hydrogen as fuel, which can be expensive, so it is conceivable to switch to propane fuel for preheat. Occasionally parts do need to be preheated prior to blasting, especially if there is large amounts of water or oil in the substrate. Of course a chemical removal of these is often required.

Another heating method not yet mentioned is water heating. We don't do it ourselves, but I've heard of people who are coating large rolls for the paper industry doing this. On very large rolls, even the heat from the gun can be insufficient to maintain an adequate temperature during spraying. As the vast majority of paper machine rolls are hollow it can be feasible to circulate hot water through the roll constantly. Of course this has very obvious downsides, not least of all the complex costly setup for such a system.
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