This thesis studies the influence of austenitization temperature on the hardness of SINOXX 4906 steel, which belongs to the group of steels with 9–12 wt. % Cr and is used mainly at elevated temperatures and pressures. To achieve the appropriate properties of steels, heat treatment of steel is crucial. It begins with austenitization, a process, where temperature and time of austenitization are of great importance.
As part of the thesis, the influence of austenitization temperature on the hardness of SINOXX 4906 steel after the processes of quenching and tempering, as well as monitored the development of its microstructure was studied. The steel has been hardened at three standard austenitization temperatures – 1000, 1050 and 1100 °C – and kept at these temperatures for 45 minutes, after which it was quenched in air. The steel was then tempered for 1 hour at temperatures from 100 °C to 750 °C and after that, Rockwell hardness was measured, tempering diagrams were made, and the microstructure was analyzed on both the light and the scanning electron microscope.
The temperature of 1000 °C was found to be high enough for austenitization, as the steel reaches its maximum hardness after quenching and tempering. It can also be concluded that the austenitization temperature between 1000 and 1100 ° C does not significantly affect the tempering process. On all tempering diagrams the secondary peak appeared at 500 °C. In the microstructure, carbide extracts in the form of small needles are observed inside the martensitic laths and on the boundary surfaces. The highest secondary peak occurred in steel quenched from 1000 °C and the lowest in steel quenched from 1100 °C. The largest difference between the secondary peaks was only 2.1 HRC, indicating that the selected austenitization temperatures did not significantly affect altitude. The lowest hardnesses occurred at the highest tempering temperatures, where coarser spheroidized carbide particles and tempered martensite are present in the microstructure, which loses its morphology and transforms into ferrite.