Investigation On Fatigue Damage Mechanism Of An Ultra-high-strength Maraging Stainless Steel

With the progressing of modern science and technology, maraging stainless steels, as a class of integrated excellent performance steels, used in the aerospace, marine engineering, land transportation and the nuclear industry have been challenged a lot. Not only the service conditions are more severe and their mechanical properties need to be much higher, but also new challenges are raised to their energy-saving, environmental protection and other social benefits. However since investigations on fatigue properties of maraging stainless steels are relatively rare, which are urgently neededfor practical applications, study on the fatigue properties of maraging steels is quite important.In this thesis which was focused on maraging stainless steel, the optimal heat treatment process of a 1900 MPa grade maraging stainless steel was obtained through the improvement of heat treatment process and adjustment of relative parameters: solid solution at 1050 ℃ for 1h + cryogenic treatment at liquid nitrogen for 8h + aging at 500℃ for 12 h. Meanwhile, the distribution and morphology of second-phase particles were observed and analyzed by using three dimensional atom probe and transmission electron microscopy on the peak-aged maraging stainless steel. Moreover, analyses and studies were conducted to deeply understand the effect of three precipitates(Cr-rich phase, Mo-rich phase and Ni(Ti,Al) phase) in peak-aged maraging stainless steel on the mechanical properties.Fatigue properties of the peak-aged maraging stainless steel were evaluated by using an ultrasonic fatigue testing machine. According to the S-N data, fatigue strength coefficient and exponent in the Basquin formula were caculated under different failure conditions, and the relative curves were also fitted. The fractographs of the specimens were observed and analyzed by using scanning electron microscope and confocal laser scanning microscope. A new model on ultra-high-cycle crack initiation at internal matrix was proposed based on the observation and analysis on fatigue fractures. Meanwhile, the results of statistics and calculation of fatigue fracture surface feature size also verified the reliability of the model. Finally, some studies were performed on the thermal effect generated during the ultrasonic fatigue tests. It was found that the ablation phenomenon in the surface cracking specimens was more serious than the internal matrix cracking specimen, and the starting points of ablation position were nearly symmetrical on both sides of the fracture. The ablation phenomenon in the double surface cracking specimen was much more serious in the crack interchange location, and two ablation lines perpendicular to the crack propagation direction in a single cracking fracture surface were observed. After calculation and analysis, it can be preliminarily concluded that the thermal effect on fatigue life of specimen whose crack initiation sites were the inner matrix during ultrasonic fatigue test was insensitive.