Low Cr-Mo Steel Research Articles

Sources of variability and improvement in impact toughness of a low Cr–Mo steel weld metal

For the heat resistance steel weld metal, special post-weld heat treatments need to be performed to maintain the creep resistance, improve the impact toughness, and relieve the welding residual stress. In this paper, firstly, the three factors affecting variability and lower values of toughness were revealed. Secondly, two different post-weld heat treatment (PWHT) methods have been used to improve the impact toughness of a heat resistance steel weld metal. Two PWHT methods include post-weld direct tempering (PWDT) process at 690 °C/26 h/air cooling and post-weld normalized tempering (PWNT) process with re-austenitization at 960 °C/1.5 h/air cooling + tempering at 690 °C/26 h/air cooling. For three type specimens with as-welded (AW), PWDT, PWNT processes, the impact toughness and the hardness were measured, and the fracture surface, microstructures were observed and analyzed by scanning electron microscope (SEM). Results indicated that PWDT heat treatment can improve the impact toughness of the weld metals from the original 79 J - 169 J to 82 J–290 J because the microstructure is changed from granular bainite to block ferrite and granular bainite. PWNT heat treatment can remarkably improve the impact toughness of the weld metals from the original 79 J- 169 J to 148 J–325 J. It is attributed to that PWNT heat treatment process can remove the boundaries of the columnar grain zone and reheated zone, make the difference of the microstructure between the columnar grain zone and reheated zone decrease, and improve the homogeneity of the microstructure.

Massive Ferrite Causes Toughness Decrease in Low Cr-Mo Steel Weld

Massive ferrite belts were observed in a low Cr-Mo steel multilayer weldment, which consisted of numerous massive ferrite lath blocks (200 μm × 200 μm × 800 μm). The massive ferrite lath blocks were brittle, which initiated and provided a path to propagate cleavage cracking and reduced the steel weldment toughness significantly. In the low Cr-Mo steel multilayer weldment, local low toughness regions were caused by the massive ferrite belts.

Comparative Study of Co-based Alloys in Repairing Low Cr-Mo steel Components by Laser Cladding

Refurbishment of damaged components is nowadays a useful solution to save maintenance costs. Laser cladding is one of the most advantageous solutions to fulfill this kind of repair. With the aim of reusing deteriorated steam circuit parts of thermal power stations, a laser cladding process has been developed. Different Co-based alloys of Stellite® and Tribaloy® families, which offer good resistance against impact, corrosion and erosive wear at high temperatures, were used as coating materials to improve the durability of this repair. A comparative study of morphology, structure and hardness of the deposited layers with optimized parameters has been carried out.

SP-Creep 시험의 응력 및 변형률 환산에 의한 초임계압 발전설비용 9Cr1Mo강의 크리프 특성 평가

Due to the need of increasing thermal efficiency, supercritical pressure and temperature have been utilized in power plants. It is well known that 9Cr1Mo steel is suitable for use in power plants operating at supercritical conditions. Therefore, to ensure the safety and the soundness of the power plant, creep characterization of the steel is important. In this study, the creep characterization of the 9Cr1Mo steel using small punch creep(SP-Creep) test has been described. The applied load and the central displacement of the specimen in SP-Creep test have been converted to bearing stress and strain of arc, respectively. The converted SP-Creep curves clearly showed the typical three-stage behavior of creep. The steady-state creep rate and the rupture time of the steel logarithmically changed with the bearing stress and satisfied the power law relationship. Furthermore, the Larson-Miller parameter of the SP-Creep test agreed with that of the tensile creep test. From the comparison with low Cr-Mo steels, the creep characteristics of 9Cr1Mo steel proved to be superior. Thus, it can be confirmed that the 9Cr1Mo steel is suitable for supercritical power plant.