INFLUENCE OF AUSTENITIZING AND TEMPERING TEMPERATURE ON MICROSTRUCTURE, SUBSTRUCTURE AND MECHANICAL PROPERTIES OF CREEP RESISTANT 9CrNb STEEL

This paper deals with influence of austenitizing and tempering temperature on microstructure, substructure and mechanical properties of creep resistant 9CrNB steel. Austenitizing temperature of 1050 degrees C was used with holding time of 15 min (state N1) and 45 min (state N3). Subsequently, the samples in these conditions were tempered at 790 degrees C/60 min (state S1 and S3). Microstructure after austenitization is formed by lath martensite and low bainite. The microstructure after tempering was formed by tempered martensite and lower bainite, with darker and lighter areas. This is due to the non-homogeneous distribution of precipitates. It can also be seen, that fine particles of precipitates were found at the martensite and low bainite lattice interfaces. It is likely to be a carbide particles, whereby excluded this particles occurred during the tempering on the original grain boundaries of austenite. The substructure was analysed in the following austenitizing conditions: 1050 degrees C/15 min and 1050 degrees C/45 min and their subsequent tempering at 790 degrees C/60 min. The precipitates were identified by selective electron diffraction analysis. The mechanical properties were evaluated after austenitization and tempering conditions.