خدیجه فرجام حاجی آقا

In this study, the kinetics of austenite layer growth on the surface of Fe-23Cr-2.4Mo ferritic stainless steel during solution nitriding and the effects of nitrogen adding on microstructure and hardness of the steel have been investigated. Steel plates of 2 mm thick were solution-nitrided at 1200˚C under nitrogen pressure of 0.25 MPa for 2, 3, 6, 9, 12 hours. Microstructure, the thickness of austenite layer and the hardnes of the nitrided samples, were investigated by using optical microscope, X-ray Diffraction (XRD) and microhardness measurements. The results showed that during solution nitriding, nitrogen diffuses through the lattice and grain boundaries and transforms ferrite to austenite phase, with average nitrogen diffusion coefficient of 6.54×10-5 mm2s-1. The thickness of the austenite layer formed on the samples surfaces increased proportional to the square root of the nitriding time, so that after 12 hours niriding, the whole thickness of the ferritic sample with hardness of 262 HV0.1 transformed to austenite with hardness of 420 HV0.1.

In this study, Influence of formation of Cr-rich precipitates on microstructure and mechanical properties of Fe-23­Cr-2.4­Mo ferritic stainless steel has been investigated. In this regard, samples of this steel were homogenized at 1100°C for 48 hours and then were cooled in furnace. Microstructure, phase composition, and hardness of the samples were investigated using optical and scanning electron microscopes, X-ray diffraction and micro-hardness measurements. Results of microstructural observations and thermodynamic evaluations showed that Cr-rich precipitates are formed in the steel microstructure due to slow cooling after homogenizing treatment. In order to dissolve the Cr-rich precipitates; formed during slow cooling, a number of samples were subjected to solution treatments at 1100°C, 1150°C for 5 hours and then were quenched in water. Results showed that at temperature of 1100°C precipitates are not dissolved entirely but at 1150°C they are dissolved completely. After solution treatment at 1150°C followed by water quenching no longer precipitation is took place due to the fast cooling rate. Results of phase analysis and microhardness measurements showed that solution treatment at 1150°C lead to increase of ferrite lattice parameter from 2.880 Åto 2.893Å and also enhancement hardness from 211 to 317 Vickers despite an increase in ferrite grain size. These results confirm dissolution of Cr-rich precipitates in the ferrite matrix.