Research Of Duplex Stainless Steel Applying Plasma Harden Treatment At Low Temperature

Duplex stainless steel was widely used in the food machinery,the chemical machinery manufacturing because of its many advantages such as high strength, excellent corrosion resistance, etc. However, its low hardness limits its application. How to maintain its good corrosion resistance, but also improve the surface hardness, wear resistance, fatigue resistance, is of great significance to promote its industrial application, and domestic and international research in this area was rarely reported.In this paper, taking duplex stainless steel applying low temperature plasma nitriding as the breakthrough point, plasma nitriding, plasma nitrocarburizing were studied. The difference between these three treatments were also studied. The surface hardness, hardness gradient, surface brittleness of harden layer was tested by Vickers hardness tester, the cross-section’s matrix microstructure of harden layer was observed by optical microscope, the phase structure of harden layer was tested by X-ray diffraction analysis.The results showed that structure and properties of surface harden layer (surface hardness, thickness, microstructure and phase structure) applying low temperature plasma nitriding, carburizing and nitrocarburizing was approximately the same change rule:with the improvement of the processing temperature or the extension of processing time, surface hardness and thickness increased, but in the end would be stable,surface brittleness remain unchanged or slightly increased.When treated at low temperature, there was no precipitate phase appearing in diffraction pattern, harden layer was white light under the metallurgical microscope,while processing temperature exceeded a critical value, precipitated phase such as nitride, carbide would appear in diffraction pattern,harden layer was grey black.The treatment temperature of duplex stainless steel separating out the precipitated phase respectively as followed:plasma nitriding 470 ℃, plasma carburizing 510 ℃, plasma nitrocarburizing 460 ℃. Duplex stainless steel applying plasma hardening treatment without loss of corrosion resistance, processing temperature should be lower than the above temperature. Low temperature long term treat process can be used to obtain thick harden layer with good corrosion resistance.The study also found that under the same processing parameters of plasma harden treatment plasma nitrocarburizing was twice as fast as plasma nitriding and carburizing. So compared with low temperature plasma nitriding and carburizing, duplex stainless steel applying plasma nitrocarburizing has more practical application value.