Home » Surface Hardening for Austenitic Stainless Steels
Despite some inherent deficiencies such as low base hardness and susceptibility to galling, austenitic stainless steels – through conventional heat treating such as carburizing – have been able to answer the demand in most environments and for most applications.
The issue surrounding conventional heat treatments of austenitic stainless steels lies in the reality that standard treatments generally reduce the material’s corrosion-resistant capabilities. In turn, this causes a bit of a trade off when looking for both a harder surface and the sustainable level of corrosion resistance needed for a particular application. The net result of this trade off has been a restriction on the range of applications for these materials.
A newly introduced heat treatment in North America, however, may open some doors that have previously been closed to the austenitic stainless steel family. The name of the treatment is Kolsterising®, and it has recently become available to North American markets after more than 15 years of commercial availability in Europe.
This low-temperature process has the ability to create a surface hardness on a given component in the range of 68-70 HRc without creating dimensional change to the component. It is also worth noting that the corrosion-resistant capabilities of certain high-nickel austenitic stainless steels – such as AISI 316 or AISI 316L – are not adversely affected and may in certain instances actually be improved through use of this treatment process.
The process creates what is commonly referred to as a “super-saturated” carbon or Kolsterised layer from the surface of the component inward (Fig. 1). This layer is formed through diffusion of large amounts of carbon on the order of 3-5 wt% into the base material at a temperature that precludes the formation of chromium carbides. Chromium carbides formed during the higher temperatures associated with conventional heat treatments result in the material losing some of its corrosion-resistant capabilities.
The treatment process takes its ability to maintain its corrosion-resistant capabilities from the fact that the carbon is diffused interstitially into the austenitic lattice causing residual stresses. It is diffused at a low enough temperature to ensure that the formation of chromium carbides is not possible.