SURFACE HARDENING OF AUSTENITIC STAINLESS STEEL

AUK university has developed an improved and relatively cost-effectivemethod for the treatment of austenitic stainless steel that precludes or diminishes the problems faced with existing methods such as the formation of the oxide scale and loss of chromium depletion resulting in reduction of corrosion resistance. The university is now lookingfor licensees to further develop this process.

This university has developed a process for the treatment of austenitic stainless steel. The process comprises plasma heat treatment at a temperature in therange of 300°C to 600°C for 0.1 to 100 hours and at a pres­sure in the range of 100 to 1500 Pa in a carbon-containing treat­ment atmosphere so as to intro­duce carbon interstitially into the austenite phase in a surface layer. The treatment process is

fig.60. Steel products

relatively cost-effective and capable of forming, at a relative­ly low temperature, a corrosion-resistant hardened surface layer with high ductility and uniform thickness on austenitic stainless steel articles. It enhances their wear resistance without adverse­ly affecting the corrosion resist­ance to an undue extent.

In a test of this process the apparatus comprised a sealed vessel, a vacuum system, a DC power supply and control unit, a gas supply system, a tempera­ture measurement and control system. The articles were 316 type austenitic stainless steel discs of 25 mm in diameter and 8 mm in thickness. After the process the discs were subjected to X-ray diffraction analysis, which showed that the process had produced a "white" (corro­sion resistant) layer enriched with carbon. They had a high surface hardness and a diffuse-type hardness profile compris­ing an expanded austenite with a possible nanocrystalline/ amorphous structure. Inden­tation, scratch and simple bend­ing tests were conducted that showed no cracks or debonding of the layer and the hardened layer was found to deform with the substrate, thus confirming that the layer had good ductility.

These are the main advantages:

* a relatively cost-effective process that is capable of form­ing, at a relatively low tempera­ture, a corrosion-resistant hard­ened surface layer with high ductility and uniform thickness on stainless steel articles;

* enhanced wear-resistance with­out affecting the corrosion-resist­ance adversely;

* it can be applied as a final proce­dure without causing deterioration of the properties of the substrate or dimensional distortion.

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