WC10Co4Cr HVOF coating leaves some lumps on the surface

[tobjung]

Dear All

Lately we sprayed WC10Co4Cr using TAFA JP-5000 on paper industry rollers. We only have 2 sets of parameter which are 98g/min and 55g/min. Normally we would use 98g/min parameters with an actual feed rate of 70g/min. The thing is this time we use 98g/min parameter with an actual feed rate of 98g/min as well. After spraying and grinding, there were some metal lumps all over the roller's surface. Now we are looking at two main reasons.

1. We didn't dry off the powder by heat before using it.
2. We used too high feed rate even the Bulletin suggested so. And that caused unmelts parameters due to insufficient heat.

Please find pictures here

Also, find parameters that I mentioned as attached files.

Any reply would be useful. Thanks guys

[Vadim Verlotski]

There are only two causes of lumps in thermal spraying, which are very clear and always the same:

1. Material first sticks to the inner surface of the nozzle and then periodically pops off this surface forming large particles (also known as "spitting").
2. Spray powder contains lumps that remain throughout the conveyance up to the nozzle.

Why does material stick to the inner surface of the nozzle? There are various reasons, but in your case the most obvious is: too much powder feeding.

The second cause of lump formation can also occur in your case: a powder that has been stored for a long time can collect moisture from the air and form lumps. Countermeasures include drying, keeping warm and sieving, which is why preheating the powder is important.