Influences Of Deformation And Cooling Rate On The Microstructure Evolution Of T91 Heat-resistant Steels

As a kind of sophisticated ferrite heat-resist steel, T91 steel has been produced and widely applied for ten years. However, main parts in superior critical power plants, such as boiler pipes, have to be imported for lacking in basic knowledge on microstructure formation of T91. Therefore, to obtain fundamental rule in T91's microstructure formation process, through analysis of the phase transformation mechanism of completely-austenited T91 sample, the effect of cooling speed, small applied stress and large deformation on phase transformation has been investigated here. According to the above experimental research and theoretical analysis, it can be concluded that:After completing austenization, the sample was deformed with 200MPa at 650℃and deformed with 200MPa at 750℃in the isochronal cooling process and then cooled to ambient temperature. As the deformation temperature decreases, the starting temperature of austenite→martensite transformation increases, and the corresponding microstructure is more refined in low deformation temperature than that at the high one. Also, after complete austenization, the sample was deformed with 100MPa, 150MPa and 200MPa at 750℃in the cooling process and then cooled to ambient temperature. As the applied stress increases, the martensite lath becomes inhomogeneous, the grain size decreases and the grain boundary becomes irregular.After being heated to austenite zone, the sample was deformed by a reduction of 50%, 60% and 70% at 750℃respectively in the following cooling process, and then cooled to ambient temperature. The result shows that the microstructure is obviously refined with the accentuation of deformation extent. At the same time, after being heated to austenite zone, the sample was deformed by a reduction of 60% at 650℃and 750℃respectively and then cooled to ambient temperature. Under the same deformation extent, more refined microstructure was found in the specimen with higher deformation temperature.After being heated to austenite zone, the sample was cooled to ambient temperature at different cooling rates of 10, 30, 50, 100 and 200℃/min. As the cooling rate increases, the starting transformation temperature decreases and the microstructure obtained varies. Both bulk-shaped ferrite and lath-shaped martensite was found in those with cooling rates of 10 and 30℃/min. Only single lath-shaped martensite was found in those with cooling rates of 50℃/min and 100℃/min. In the specimen cooled with a rate of 200℃/min, a mixture of lath-shaped and lamellar martensite was obtained.