(05-03-2022, 08:14 PM)mrobie Wrote: [img]file:///C:/Users/webuser2/Pictures/Tungsten%20Pit.jpg[/img]
Hello All,
I am having issues with small pits becoming visible in my HVOF Tungsten Carbide coating durring our Final Diamond Grinding process. I have attached a photo as an example. I am using a Metco Diamnond Jet 2700 gun with water cooled hardware. I have a 9MP-DJ Powder feeder. I have oxygen fed in from a bulk tank system and Natural gas as the fuel fed from our G-TEC gas boosters. The gun also uses hydrogen but only to ignite the flame before fully switching over to Nat. Gas. We are coating hydraulic cylinder rods with Praxair 1350VM/WC-731-1, 86-10-4 tungsten carbide. The coating is being done roboticly with a part rpm of 380 and a robot traverse speed of 95 IPM on a 3'' dia shaft. We monitor the heat of the part at all times and never exceed 300 F. We use orange HVOF tape and a two part white and blue masking compound to mask off what is not to be coated. Before coating we Grit Blast the part with a 16 grit white virgin sand. Our coating room has ventilation set up behind the hydraulic cylinders to collect all excess powder and other partials that may be in the air. We continue to see these pits in our coatings time after time causing a lot of reworking of these shafts. If anyone could give us some helpful advise on what may be the cause of this that would be greatly apppreciated. It would be nice to get this headache behind us.
Hello,
I have seen similar issues with our DJ 2700 water cooled unit. While there is no way to 100% eliminate the chance of a pit popping up in the final grinding stage, there are ways I reduce the chance.
Obviously, some pits may be opened up during the grit blast prep, and your operator may not see them. That would probably be seen in the pre-grind inspection however, but I thought it worth mentioning.
I use a 60 grit aluminum oxide VIRGIN blast material and carefully inspect the part for any grit that may have embedded itself into the base material prior to applying the Wc. Other possible contaminants that can embed themselves in the freshly blasted base material include rubber from the ID of the blast hose, ceramic bits from the nozzle, shielding metal from a vacuum grit attachment pipe, etc.
Another possibility is the deterioration of the torch components themselves. The long nozzle that is used (DJ2701 for most Wc powders ) is coated with Industrial Hard Chrome on the I.D. The chrome can wear out and allow the high velocity Wc particles to wear at the surface, galling it, building up globs, or spitting it at the part right away. The warning sign to look for is the spray flame will get offshoots, and not look right. Another area is the feed tube itself can wear, introducing contaminants into the flame. The torch tip can also build up with bits ( possible debris in supply lines, or by-product of combustion ) and need cleaning or replacement.
Monitoring the spray process carefully can give you clues as to when its needing some maintenance. Often, you can see the contaminants fire from the torch and partially bounce off the part, often leaving some behind that gets covered by the subsequent passes, to possibly be opened up in the grinding stage. Catching that happening, stopping the application, removing the contaminant, and re-starting the application sequence is a time-consuming process, and the choice to do so may be based on where you are in the process. Spits in early stages of a heavy build can be nothing, if they are small enough, spits once past the finish grind size may be ignored, but anything near the finish size ( hard to determine with the part at high temp ) will bite you during the finish grind.
Do you use any kind of bond, or build-up coating prior to applying the Wc?
Regards,
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Pitting in HVOF Tungsten Carbide
Pitting in HVOF Tungsten Carbide 
