Components with critical dimensions and tight tolerances are more difficult to repair back to size without affecting the part. When it comes to worn, scratched and scored parts such as Shafts, Stems, Impellers, Seats the most popular process has historically been Hard Chrome plating, but could HVOF now replace this?
What is Hard Chrome?
Although many manufacturers often specify Chrome for repair jobs, they are actually specifying Hard Chrome. The difference between these processes is that Chrome Plating will most often be carried out over the top of another process (most commonly Nickel Plating) and will only add a nominal thickness of Chrome between 1-2 microns. Hard Chrome Plating is a much thicker deposit of Chrome often over 100um and is purely used as a functional coating where Chrome Plating is used for decoration. (Most noticeably Car Bumpers)
What is HVOF?
HVOF is a thermal spray process in which powdered materials are heated up and fired out at extreme velocities to form a coating that is hard and dense in nature with a low porosity. This is an additive process and as such can be used to build up worn areas.
One of the common misconceptions of electro-plating is that it will fill in cracks in the vein of polyfiller on a wall. This is not the case, in electro-plating the same amount of thickness is added to the entirety of a surface meaning that where you have a dint, you will still have a dint only now with a layer of chrome. This means that often in repair jobs masking is required so as to not affect the rest of the component, and even with masking an amount of grinding is needed. HVOF has an advantage in that it is additive in nature and will fill in those dented areas. Here at B&B we have a KUKA robotic arm which ensures accuracy and many jobs may not require masking on many items, saving time and money. Where there is over-spill we can easily grind it back to tolerance.
If your component is going for repair, then it’s quite likely to have experienced heavy service. For effective electro-plating the surface of the part needs to be completely clean of oils, dust and imperfections and this means your part may be subject to a number pre-plating processes. Even with this preparation many platers will struggle to plate on super alloys such as Inconel and Stellite. Alloys made of these components are expensive to produce and where the most cost savings could be made with repair. HVOF has advantages here, as we can tailor our powder to suit the base material and can actually spray with Inconel and Stellite equivalents. The very nature of the HVOF process is High Velocity and the material is applied at speeds faster than a jumbo jet, often impregnating the metal as it impacts creating an extremely strong bond with less preparation steps beforehand.
Last but not least, although Hard Chrome can be built up quite thick in electro-plating terms, above 200 microns, the thicker you build it up the softer the Chrome will be. We can spray metals using our HVOF process up to 1mm thick whilst remaining hard and dense in nature with a high level of wear resistance.
Next time you have a repair job for Stems, Seats, Shafts, Seals or any other component likely to encounter a high level of wear, why not send us a drawing and see if HVOF might be right for your application? B&B Precision are based in West Yorkshire and are fully ISO certified.