In selecting a wear-resistant coating for rods, the following factors are important:
• Coating roughness and surface texture (smoothness, porosity),
• Coating hardness,
• Combination of sliding materials (rod coating and packer material),
• Corrosion resistance,
• Adhesion of coating to base metal (bond strength), and
• Previous coating/heat-treating history.
Numerous reconditioning processes are available today for restoring worn rods to their original size and surface conditions. These processes may also be used to provide extended life of rods in new equipment, especially in difficult services. In general, only rods in sound condition should be considered as candidates for reconditioning. Base metal surface condition must be carefully inspected prior to use of any type of coating (discussed later).
Major hardfacing processes include the general categories of flame spraying (metal spray and plasma spray), electroplating (chrome plating) and flame plating (detonation gun). Figure 800-14 summarizes the relative characteristics of these processes and the resultant coatings they produce. The acceptability of each process depends on the service conditions, i.e., lubricated or non-lubricated, sour, corrosive or dirty gas, etc. Only certain coatings applied by each major process are suitable for reconditioning rods to resist adhesive and abrasive sliding-type wear. For the relatively low-service temperatures of interest here (up to 400°F), changes in physical properties and strength of various coatings are of minimal concern.