High strength medium carbon austenitic stainless steels have been developed through partial and total replacement of nickel by nitrogen. Stainless steels containing 0.4% carbon with different combinations of nickel and nitrogen were produced in 10kg induction furnace under different nitrogen pressures. The produced stainless steels were cast and hot forged and the total nitrogen was determined. Furthermore, the produced forged steels were subjected to either only solution treatment or solution treatment followed by ageing process.

Nonmetallic inclusions such as carbides and nitrides were separated by electrolytic dissolution. Nitrogen as nitrides was determined and soluble nitrogen was calculated. XRD technique was used to investigate the types of nonmetallic inclusions. The microstructure of produced stainless steels was observed and the grain size was measured. The tensile properties at room temperature were determined. The influence of grain size, total nitrogen, insoluble and soluble nitrogen on tensile strength was investigated. All produced stainless steels as-quenched were aged at temperatures range from 450°C to 950°C for different times. Hardness test was carried out for aged stainless steels and the optimum ageing conditions were determined.

After solution treatment of the investigated stainless steels at 1050°C, a great portion of alloy carbides and nitrides is observed to be taken into solution. Nitrogen in solid solution increases both yield and tensile strengths. At optimum ageing temperature, this portion in solution precipitates, mainly as Cr2N, was causing higher precipitation strengthening. The yield strength and ultimate tensile strength of the aged stainless steels were found to increase at average rates of 706 MPa/1 mass % nitrogen and 723 MPa/1 mass % nitrogen, respectively. On the other hand, the increase of nitrogen content deteriorates the steel ductility.

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The factorial design approach can be used to precisely estimate the effect of different parameters on the reduction process of iron oxide. In the current study, a 24 factorial design was used to significantly calculate the magnitude impact of manganese oxide and silica on the reduction yield of iron oxide which was reduced with H2 at 900-1100°C. A regression model was built on the experimental reduction results of pure iron oxide and iron oxide doped with either MnO2 (mass content of 6%) and/or SiO2 (mass content of 7.5%) at 900°C and 1100°C. The developed mathematical model was used to predict the reduction yield as a function of four parameters including MnO2 (mass content of 0-6%), SiO2 (mass content of 0-7.5%), reduction time (1.0-10 min) and temperature (900-1100°C). In addition, the effect of the interaction combination of different parameters (MnO2, SiO2, time, and temperature) on the reduction yield was estimated. The results showed that the reduction time has the highest positive effect on the reduction yield of iron oxide sinter followed by the applied temperature and then SiO2 addition. On the other hand, MnO2 exhibited the highest negative effect on the reduction yield of iron oxide followed by the combination effect of SiO2 with time and temperature. The interaction combination effect of MnO2 with temperature or MnO2-SiO2 with time and temperature on the reduction yield was very small. The regression model was applied to theoretically estimate the reduction yield of pure and doped iron oxide at 900-1100°C. The obtained values of the derived model are in a good agreement with the experimental results under different conditions.

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