Corrosion resistant stainless steel is widely used in industry and trade due to its outstanding properties. Despite its numerous advantages, particularly its resistance to corrosion in numerous media, stainless steel also has considerable weaknesses that sometimes severely limit its application.
One of these weaknesses is its susceptibility to cold welding or galling, which can occur particularly with screw connections. Surface hardening offers a solution to this problem.
Improving Galling Resistance on Stainless Steel
Galling can be favoured by various aspects, such as insufficient surface hardness, excessive roughness or lack of lubricants. Nevertheless, fasteners are essential in many applications to allow for particularly frequent assembly and disassembly, or high tightening torques. It is therefore important to carefully tailor the properties of such elements.
When fasteners are jammed due to galling, their removal can turn out to be an enormous challenge, especially if it needs to be done without damaging the affected bolt or destroying the associated nut. Surface hardening, a process in which only the surface layer (few to tens of microns) of a workpiece is hardened, has been proven to significantly improve the seizure behaviour of stainless steel screws. The diffusion-based process increases the hardness of the surface through solid solution hardening and subsequently reduces its susceptibility to plastic deformation. Depending on the material, the corrosion resistance is maintained or even improved, and the stiffness of the base material is not impaired
The process
Surface hardening is not an applied coating, but a diffusion-based thermo-chemical surface hardening process. Although classic processes for the surface hardening of corrosion resistant stainless steels have been available to the industry for some time, their application has two main disadvantages. Firstly, all classic processes such as salt bath processes are known to reduce corrosion resistance. On the other hand, the hardness values of stainless steels usually hardened with nitrogen or carbon drop very quickly, i.e the hardening depth is only a few micrometers and the underlying base material is very soft, which can potentially lead to the ‘eggshell’ effect.
In contrast, Expanite has developed its proprietary technology SuperExpanite®, having solved those shortcomings as a priority. With this technological process not only the outermost layer but also the underlying material is hardened to a greater depth, which also contributes to an improvement in corrosion resistance in many cases.
A two stage process is used in Expanite’s treatment for stainless steels, in which nitrogen is introduced deep into the surface area in the first step called ExpaniteHigh-T (high temperature process), whereby the material is hardened to approximately 300HV for austenitic materials and 850HV for alloys with martensitic structure to a depth of up to 1mm. In the second process step, ExpaniteLow-T, the workpiece is heated to temperatures below 500°C and the surface layer is hardened to 1,100HV – 1,300HV at a hardening depth of 5μm – 30μm (depending on the solution). The hardening is achieved by incorporating large amounts of interstitial carbon and nitrogen.
By combining the two process steps, the so-called ‘eggshell’ effect, which occurs in most surface hardening processes, is avoided. This offers a decisive advantage for numerous applications that require improved load-bearing capacity.
