Abstract
The Cr-Mo-V-Ti based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperatures and high pressure under continuous service. In the present work conventional heat treatment like normalizing and tempering of the alloys has been performed. The material used in this study was the laboratory prepared experimental low alloy Cr-Mo-V-Ti steel. Samples were austenitized at 980oC for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750oC for 1 hour. Mechanism of bainite transformation has been studied in Fe-C-Cr-Mo-V-Ti steel using high speed dilatometry. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure and the growth of the carbides. The magnitude of these changes depended on the temperature of tempering.
Highlights
The first Cr-Mo steels were used for conventional power-generation applications in the 1920s
The dilatometry results showed that the relative length change during the formation of bainite increases as the isothermal transformation temperature decreases below the Bs temperature, the amount of bainite formed is dependent on the transformation temperature
The austenite to bainite transformation and microstructure evolution have been studied in experimental low alloyed Cr-Mo-V-Ti steel using high speed dilatometry and transmission electron microscope backed by thermodynamic analysis
Summary
The first Cr-Mo steels were used for conventional power-generation applications in the 1920s. Heat-resistant steels for the high-steam parameter of 650°C are being developed. This has put heat-resistant steels such as T/P91, T/P92 and E211 out of consideration because of the loss of the microstructure stability during service at the high temperature [5,6,7]. More advanced steels should be developed to meet this requirement It is well accepted in heat-resistant steels that highly stable microstructure will produce excellent creep strength. The aim of the present investigations was to study the mechanism of bainite transformation and the influence of tempering temperature on microstructure previously normalized steel, the new experimental Cr-Mo-V-Ti heat-resistant steel which contains more alloying contents relative to T22 (21⁄4Cr-1Mo) and other high-quality heat-resistant steels
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