R&D

SURFACE HARDENING
OF STAINLESS STEELS

Stainless steels have a very wide range of applications, wherever direct contact with corrosive media is present. However, the main disadvantage of these steels is the relatively low hardness and strength.

The materials, which are used by BHDT GmbH for valves in urea or ammonia plants, are very often austenitic stainless steels or duplex steels. Special components in these valves such as stems and seats are very often exposed to wear, cavitation and erosion beside to the corrosive influence, so that hardened surfaces are mandatory.

Thermomechanical diffusion methods

Basically, surfaces can be hardened by thermomechanical diffusion methods, applied energy or thermal methods or by surface coating. As for austenitic stainless steels no transformation hardening is possible, applied energy or thermal methods are not effective. A direct coating of the surface with a thin wear layer would quickly lead to spalling of the hard layer due to the low strength of the base material. For this reason only thermomechanical diffusion methods are purposeful, where for example, carbon diffuses into the surface. The disadvantage of this method is that it requires by default such high temperatures that there is grain boundary diffusion of chromium where the chromium bonds with the diffusing carbon. This formation of chromium carbides leads to the local chromium depletion, which destroys the corrosion resistance of the material.

Innovative solutions

To successfully increase the hardness of austenitic stainless steels, duplex steels and also nickel-based alloys without losing corrosion resistance a very innovative method is available at BHDT GmbH. Here the near surface lattice is strained at substantially lower temperatures than usual (< 300 °C) with diffusing carbon atoms, resulting in a high hardness. Numerous experiments have shown that this hardness can be substantially increased, while no adverse effect on the corrosion resistance is detectable. An additional positive side effect is that the fatigue strength is increased by the formation of compressive residual stresses in the surface region.

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In the left picture, the layer thickness of a hardened duplex steel surface using Kolsterising® can be seen, while the right picture shows the course of hardness of this layer.