Preheating heat sources

[powerman]

To preheat substrate before thermal spray, any problem if use combustion heat sources (eg. HVOF uses kerosene+oxygen, flame spray uses acetylene+oxygen or use LPG) to heat the substrate? I think if there is uncomplete burning, the kerosene, acetylene or LPG will contaminate the substrate surface. Then what should we do?

[Gordon]

Hi Powerman

Actually, a very good question. Though carbon sooting could be a problem with the liquid kerosene HVOF systems, I think one of the major problems is with one of the products of combustion of most fuels, that being water. This is primarily why preheating substrates is so important. Initial contact with a cold surface will produce water condensation.
An experience from a company trying to preheat a very large roll using HVOF guns. The conditions were very cold (eastern european in winter, little heating). With an initial grit blast preparation, preheating commenced, only to find after a short duration that the roll was turning to an orange colour with rust, long before they had even achieved anywhere near the desired preheat temperature. Having no other means of heating, without combustion flames, they continued heating to slightly above the desired preheat temperature and repeated the grit blasting procedure to remove the newly form rust and then continued the process without further problems. Carbon sooting problems are due to poor equipment design or incorrect fuel/oxygen ratios, whereas water is a natural bye-product of combustion of hydrogen and hydrocarbon based fuels.

[powerman]

Hi Gordon,

That means as long as we get the correct ratio of fuel & oxygen (pressure and/or flow), or put oxygen ratio slightly more, then we do not need to worry about the contamination of incomplete burning fuel? Only worry on the condensation of water on worpiece?


Regards,

powerman