Study On Room Temperature Mechanical Properties And Thermal Fatigue Property Of New Austenitic Stainless Steel Pipe

The steam generator heat transfer pipe is a key component of ultra-supercritical thermal power units.Its operating temperature is more than 600?,and the steels of high demands are proposed,such as the creep performance,corrosion resistance,thermal fatigue performance,hot and cold forming performance and welding performance.Ferritic heat-resistant steel has a lower cost,but its operating temperature is lower than 600?.Nickel-based alloys have excellent high-temperature properties but the price is expensive.They are generally used only in units with temperatures above 700?.Based on the study experience of austenitic steels for many years,our research group used the principle of multiple composite reinforcement to optimize the content of Ti,N,Cu or Al,Nb,and Ni on the basis of Cr-Ni austenitic stainless steel,and exploited two series of austenitic heat-resistant steels.H1 and H2 steels with excellent high-temperature properties were selected in this study,and the normal-temperature mechanical properties and thermal fatigue properties of the pipes were studied and compared with 800H steel.The following conclusions are obtained through research:?1?Mechanical properties analysis of the experimental steel after perforating tube at room temperature,H1,H2,and 800H steels are still single austenite tissue.The grains of the H1 experimental steel are small and contain twins.The grains of the800H experimental steel are coarse and also contain the twins,and the grains of the H2 experimental steel are the largest.The hardness of the H1,H2,and 800H steels at room temperature was 206.1 HV,169.1 HV,and 194.7 HV,respectively;The tensile fractures of H1 and H2 experimental steels are ductile fractures and 800H experimental steel is shear fractures.The H1 fracture did not find inclusions,and the H2 and 800H fractures found inclusions.The tensile strengths were 646.5MPa,560.7MPa,and 547.5MPa,respectively;Under the same impact conditions,H1experimental steel is not disconnected.H2 and 800H experimental steels are disconnected.and the energy that absorbed from impact was 231.5 J,171.2 J,and140.7 J,respectively.The experimental data show that the comprehensive mechanical properties of H1 are relatively better and 800H steel is relatively weaker.?2?In the thermal fatigue test,with the increase of the experimental temperature,the crack initiation is shortened,the time of crack initiation of H1 is the longest,and the time of initiation of 800H is the shortest;The cracks preferentially occured at the V-notch,and the grain boundary also becomes a crack source with the increase of temperature.At the same time,the propagation of cracks is mainly along the grain boundary.When the experimental temperature is low,some cracks will also propagate through the crystal;the crack propagation speed will increase with the increase of temperature,and the propagation speed of H1 is the smallest,while the propagation speed of 800H is the largest.?3?After analyzing the precipitates and oxides at grain boundaries in the three experimental steels,the precipitates are mainly?Cr,Fe?₂₃C₆ carbides,and the oxides are mainly Cr₂O₃.These precipitates will affect the toughness and binding force of grain boundaries,and cause cracks to easily initiate and propagate at grain boundaries.After the thermal fatigue test,their microstructure was still single austenite tissue.The microhardness of the surface was higher than the initial hardness by more than 11%.The lower the experimental temperature is,and the better hardening effect will be.At the same time,under the condition of high temperature recovery,the hardness of the steel decrease with the experimental temperature increases.