.316 L And 420 Stainless Steel Powder Temperature And Pressure Process Research

The resistance to chemical corrosion and the mechanical property of powder stainless steels is poor compared to the conventional wrought stainless steels because of their lower densities, which limits their wide applications. In order to increase the density, warm-pressing was used for powder stainless steel in this research. During the present experiment, density was examined by Archimedes' method, optical microscope was used to observe the micro-structure,SEM was used to observe the particle morphology and fracture morphology, EDS was used to analyse the component in microregions. Effects of the contents of lubricant, compacting pressure, temperature of powder and die, contents and types of fine powders on green density were studied systematically. And the comparison of green density and spring back between warm-pressing and conventional pressing were also carried out in the experiment. Samples were sintered in cracked ammonia at 1130℃.Results show that, the optimum condition to product stainless steel is that, the maximum green density are obtained at 784MPa as they are, lubricant contents is 0.7% for 420 powder stainless steel and 0.8% for 316L powder stainless steel, temperatures of powder and die are 90℃and 120℃respectively. The green density of 420 powder stainless steel obtained by this process is 6.86g/cm³, which is 0.18g/cm³ higher than conventional pressing. The hardness is 33HRC and the density is 6.83g/cm³ which is 0.03g/cm³ lower than the green density after sintering. The green density of 316L stainless steel obtained by the optimum process is 6.92 g/cm³, which is 0.26g/cm³ higher than conventional pressing. The hardness is 76HRB and the density is 7.02g/cm³ which is 0.10g/cm³ higher than the green density after sintering.The green density exhibits bimodal distribution, when the fine powder contents is 2% and 30% , on the curve which is about the relation between the green density and the fine powder contents. When adding spherical fine powders( < 20μm) , the peak values of green density are 7.00 g/cm³ and 7.09g/cm³ respectively at 784MPa. When adding water atomized fine powders( < 43μm) ,the peak values of green density are 6.97 g/cm³ and 7.03g/cm³ respectively at 784MPa.For Samples which were added with fine powders, the density are improved after sintering. The extent of density improvement, which is greater when adding spherical fine powders than water atomized fine powders and is almost irrelated with the pressing pressure, increases with the contents of fine powders. The hardness is 95HRB when the sample's density is 7.21 g/cm³. Comparing with samples with no fine powders, the hardness is improved even when the density is almost the same. The tensile strength, which increases with the density, especially when the density is bigger than 7.0g/cm³, is 486MPa while the density is 7.14 g/cm³. The b in formulaσ_b=σ₀exp(-bθ)is 6-7 which matches with the conclusion of that b is 4-7.