Screws threaded into blind tapped holes are commonly affected by galling, and components made of stainless steel are especially susceptible. Stainless steel fasteners are widely used in corrosive environments for corrosion resistance, but often suffer from galling and thread wear issues.
Galling is a type of mechanical wear caused by a combination of friction and adhesion between sliding surfaces typically under a compressive load, resulting in damaged threads, broken fasteners, weakened joints, and seized bolts.
Several factors can lead to galling, such as low surface hardness, rough surface finishes, the absence of lubricants, and using finely-threaded fasteners or locking fasteners. But some of these features may be essential to fastener performance in certain applications. Therefore, it is vital to balance a fastener’s properties accordingly. Generally, once a fastener has seized up from galling it is impossible to remove without cutting the bolt or splitting the nut.
Galling resistance in stainless steel metal–on–metal applications can be achieved through Bodycote’s Specialty Stainless Steel Processes (S³P) while preserving the corrosion resistance properties of the base material, it says.
Kolsterising is a process developed to alleviate galling and thread wear issues by significantly increasing the surface hardness (typically greater than 1,000 HV), thereby improving the mechanical performance of such alloy systems whilst maintaining their natural corrosion resistance. After treatment with the Kolsterising process, fasteners exhibit dramatically improved galling resistance, wear resistance, and fatigue strength, according to the company.
Bodycote points out that as Kolsterising is a low-temperature diffusion process, not a coating, it allows for treated components to show no changes in dimensions or surface finish. The process is appropriate for a wide range of materials, including austenitic and duplex stainless steels, as well as nickel and cobalt-based alloys. The process produces a hardened layer under the surface of the material, which it says improves surface integrity and provides more consistent fastener performance.
