Papers by Author: Bin Zhong

Abstract: The Plasma transferred arc welding (PTAW) is widely used for hardfacing components exposed to severe conditions. Without post welding heat treatments, large tensile residual stresses remain in the hardfacing coating, which is detrimental. In this paper, a set of post welding heat treatments was evaluated for the heat-resistant steel substrate – Co-based alloy hardfacing coating system. Microstructural and mechanical properties, including the chemical phases of coating surface, the microstructure of coating surface, the Vickers hardness and the residual welding stress, were investigated before and after the heat treatments. Results revealed that during the heat treatments, some elements reprecipitated and the secondary carbide Cr23C6 was formed. After the treatments, a more regular structure and a higher Vickers hardness were obtained. Moreover, the tensile residual stresses in the coating decreased significantly. Therefore, it can be inferred that the post welding heat treatments employed in this paper were proper for this material system.

Abstract: The main task of this paper was to evaluate the influence of hardfacing technique and service temperature on the fatigue properties of heat-resistant steel X45CrSi9-3 coated with Co-based alloy Stellite 12. The results of rotating bending fatigue tests showed that at room temperature (RT), the fatigue strength of specimens welded by the acetylene gas welding (AGW) was lower than that of specimens welded by the plasma transferred arc welding (PTAW). For PTAW specimens, the fatigue strength at 500oC was much higher than that at RT. Two failure modes were presented, one was termed as the coating failure mode at RT and the other was termed as coating-interface failure mode at 500oC. The fatigue life prediction was conducted by using a modified Murakami’s model.

Abstract: The fatigue behaviors of a directionally solidified (DS) nickel base superalloy, coated with a MCrAlY coating (NiCrAlYSi) were studied. Two kinds of tests were performed. One kind of tests are low cycle fatigue (LCF) test under strain control at different temperatures, another kind of tests are stress controlled LCF test with SEM-servo hydraulic testing machine for in situ cracking observation. The results show that the effect of coating on LCF life of coating/substrate system was rather different according to different strain levels and temperatures. The coating has no or less effect on LCF life under high strain range and the LCF life is governed by fatigue behavior of substrate in spite of the difference of temperature. However, when strain range is smaller, crack initiation and propagation are observably affected by temperature, which leads to a shorter LCF life of coating/substrate system at 500°C and a longer LCF life at 760°C or 980°C. This means the failure of coating/substrate system is dominated by the cracking of surface coating under low strain range. The brittleness at 500°C lower than DBTT results in rapid stage II crack propagation. The crack initiation from coating surface was in situ observed at room temperature and 700 °C and it was found that cracks usually initiated from the surface roughness of coating and then propagate to failure. The brittleness and surface roughness are the basic acceptable causes leading to the early damage of a coating/substrate system.

Abstract: The isothermal low cycle fatigue (LCF) behaviors of a directionally solidified (DS) nickel base superalloy, coated with a NiCrAlYSi coating were studied. The study concerned NiCrAlYSi coating formed by an arc-discharged physical vapor deposition (PVD) process for protection against high-temperature corrosion and oxidation of gas turbine blades. The effect of protective coating on LCF life of coating/substrate system was investigated at high temperatures and compared with uncoated alloy. The test results show that coating has no or less effect on LCF life under high strain range and the LCF life is governed by the fatigue behavior of substrate at different temperatures. However, when strain range is smaller, crack initiation and propagation are observably affected by temperature, which leads to a shorter LCF life of coating/substrate system at 500°C and a longer LCF life at 760°C or 980°C.