Effect Of QPQ Process Parameters On Microstructures And Properties Of SAF2906 Duplex Stainless Steel

In comparison with the third generation duplex stainless steels, SAF2906 duplex stainless steel has higher strength and better corrosion resistance. But its low hardness and poor wear resistance limit application in the field of high requirement of wear resistance(e.g. corrosion, wear and overloading). Therefore, it is necessary to take measures to improve its wear resistance to expand its application. In recent years, some researchers have used QPQ treatment into improving wear resistance of stainless steels and achieved good results. However, QPQ treatment is mainly used for single phase steels such as austenite and martensite, few literatures report its application to SAF2906 duplex stainless steel instead. In this paper, QPQ treatment is used for surface modification of SAF2906 duplex stainless steel, and the effect of QPQ process parameters(nitriding temperature, nitriding time and the concentration of CNO-)on micro structures and properties of SAF2906 duplex stainless steel is investigated by means of OM, SEM, XRD, micro-hardness and wear resistance test.Microstructure observation showes that the surface layer of the samples after ferritic nitrocarburized is composed of oxide layer, compact compound layer and diffusion layer from the outmost surface to the core. And the surface layer of the samples after austenitic nitrocarburized is composed of oxide layer, loose compound layer, compact compound layer and diffusion layer. The oxide layer is composed of Fe3O4. The main phases of the loose compound layer are CrN, Fe4N and Fe3O4. The compact compound layer are composed of CrN and Fe2-3N. The main phases of the diffusion layer are CrN, αN and S.When nitriding temperature is increased from 550℃ to 610℃, the compound layer thickness increases dramatically, in which the nitriding temperature reaches 590℃, there is some loose microstructure appearing in the compound layer, and the the temperature is furtherly increased to 610℃, the loose microstructure is more severe, the surface layer porosity increases and its density dropes, and the adhesion strength of surface layer-substrate also declines. With the increase of nitriding temperature from 550℃ to 610℃, the surface hardness, is over 3 times of the substrate, first increases and then decreases, and the maximum hardness is 1283HV0.2 at 570℃The samples were nitrided at 570℃ and 610℃ for 60min,90min,120min, 150min and 180min respectively. The results show that the growth rate of the compound layer of two temperature-treated samples gradually slowes down with nitriding time increasing. When nitriding temperature is 570℃, the samples have good density during 120-150min, with the time extension to 180min, the properties of the surface layer drop. When nitriding temperature is 610℃, the samples have good density during 60-90min. With the increase of time, the surface layer porosity increases and its density dropes, the adhesion strength of surface layer-substrate. The samples nitrided at 570℃ reach maximum of 1283HVo.2 at 150min, the samples nitrided at 610℃ reach maximum of 1164HVo.2 at 120min. The surface hardness of the two temperature-treated samples with different time is more than l000HV0.2.The samples were nitrided at different concentration of CNO"(30%,33%,36% and 39%) with nitriding temperature of 570℃ and nitriding time of 150min. The results show that with the increase of CNO-, there is almost a linear increase in the compound layer thickness. When the concentration of CNO- was 30% and 33%, the surface layer has good density. The concentration of CNO- is 36% or more, there is some loose microstructure appearing in the compound layer, the surface layer porosity increases, and the adhesion strength of surface layer-substrate also declines. With the increase of CNO- concentration, the surface hardness first increases and then decreases, and the maximum hardness is 1283 HV0.2 at 33%.The substrate, ferritic nitrocarburized sample(570℃/150min/33%) and austenitic nitrocarburized sample(610℃/150min/33%) were selected for dry-sliding test. The results show that the mass loss of the QPQ treated samples is much lower than the substrate, and its wear resistance gets a significant improvement. Furthermore, the mass loss of the samples nitrited at 610℃ is higher than that of the samples nitrited at 590℃ during the first 29h. When test time is exceeded 29h, the former is lower than the latter. Overall, the wear resistance of samples nitrited at 610℃ is better than that of the samples nitrited at 590℃. The worn surface morphology observation showed the wear form of the substrate is plough cutting after a short test time(1h), while the wear form of QPQ treated samples is micro cutting. The wear mechanism of the substrate is adhesive wear after a long test time(38h), the wear mechanism of QPQ treated samples is abrasive wear.