The effect of austenitizing temperature on tempering of SINOXX 4906 steel

V diplomski nalogi smo preiskovali vpliv temperature avstenitizacije na trdoto jekla SINOXX 4906, ki spada v skupino jekel z 9-12 mas% Cr in se uporablja predvsem pri povišanih temperaturah in tlakih. Za doseganje ustreznih lastnosti jekel je ključna toplotna obdelava jekla. Ta se začne z avstenitizacijo, pri čemer sta pomembna temperatura in čas avstenitizacije. V okviru diplomskega dela smo preučevali vpliv temperature avstenitizacije na trdoto jekla SINOXX 4906 po kaljenju in popuščanju ter spremljali razvoj mikrostrukture. Jeklo smo kalili iz treh standardnih temperatur avstenitizacije, in sicer iz 1000, 1050 in 1100 °C ter ga na tej temperaturi zadrževali 45 minpotem smo ga kalili na zraku. Nato smo jeklo popuščali 1 uro pri temperaturah od 100 °C do 750 °C ter izmerili trdote po Rockwellu, izdelali popuščne diagrame in analizirali mikrostrukturo na svetlobnem in vrstičnem elektronskem mikroskopu. Ugotovljeno je bilo, da je temperatura 1000 °C dovolj visoka za avstenitizacijo, saj jeklo po kaljenju in popuščanju doseže najvišjo trdoto. Prav tako lahko sklenemo, da temperatura avstenitizacije med 1000 in 1100 °C bistveno ne vpliva na popuščanje. Na vseh popuščnih diagramih se je sekundarni vrh pojavil pri 500 °C. V mikrostrukturi so opaženi karbidni izločki v obliki drobnih iglic znotraj martenzitnih letev in na mejnih površinah. Najvišji sekundarni vrh se je pojavil pri jeklu, ki je bil kaljen iz 1000 °C, najmanjši pa pri jeklu kaljenem iz 1100 °C. Največja razlika med sekundarnimi vrhovi je bila le 2,1 HRC, kar naznanja, da izbrane temperature avstenitizacije niso pomembno vplivale na višino. Najnižje trdote so se pojavile pri najvišjih temperaturah popuščanja, kjer so v mikrostrukturi prisotni bolj grobi sfereoidizirani karbidni delci in popuščeni martenzit, ki izgublja svojo morfologijo in se spreminja v ferit.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