Computer Simulation And Experimental Investigation Of Laser Hardening And Shot Peening Process Of Martensitic Stainless Steel

Laser hardening treatment as a surface treatment is widely used in industry. However, it is inevitable that the large tensile stresses appear on the surface between laser hardened area and matrix area. The large tensile stresses existing in transitional area are harmful to fatigue life of material property, which leads to decrement of component operation reliability. Shot peening is an efficient cold working method to improve the residual stress distribution in transitional area of laser hardened component. In this dissertation, laser hardening treatment plus shot peening treatment were investigated on 17-4PH and AISI 4140 steel via experiment and simulation analysis, and the residual stress as well as microstructures induced by this two surface treatment were studied systematically. Furthermore, the 3D shot peening model for laser hardened material was build via FEM software ABAQUS. The influence of shot peening on residual stress distribution in different areas of laser hardened material were discussed under different shot peening conditions.The results of 17-4PH laser hardening experiment revealed that in the condition of 5 kW CO2 laser beam power, 6 mm×6 mm laser beam dimension and 2 mm/s laser beam feed velocity, large tensile stresses appeared in transitional area in longitudinal direction with the value of about 125 MPa. Furthermore, large tensile stresses existed in transitional area from surface to the depth of 500μm at least. Under 0.5 mmA + 0.1 mmA peening intensity, the residual stress value in transitional area was -711 MPa in longitudinal direction and the depth with compressive residual stresses was 500μm, which proved shot peening surely can improve the residual stress distribution of laser hardened 17-4PH. In the following research, X-ray line profile and quantitative X-ray diffraction phase analysis were carried out on laser hardened 17-4PH in order to investigate microstructure and retained austenite content before and after shot peening treatment. Results showed domain size refined and microstrain as well as dislocation density increased after shot peening treatment on treated surface, no matter in laser hardened area or matrix area. Specifically, the dimension of domain size on treated surface was in nanoscale, the dislocation density on treated surface increased an order of magnitude at least and retained austenite on treated surface nearly disappeared after shot peening treatment with 0.5 mmA + 0.1 mmA peening intensity. The fatigue life of laser hardened 17-4PH (107) decreased one order of magnitude by comparing with fatigue life of matrix 17-4PH (4.98×105) in the load condition of 492 MPa. The average fatigue lives of 17-4PH after 0.2 mmA + 0.1 mmA and 0.5 mmA + 0.1 mmA intensities shot peening treatments were 1.33×10~7 and 1.77×10~7. Fatigue tests showed shot peening was an efficient method to increase the fatigue life of laser hardened 17-4PH. Relative lager peening intensity could create lager maximum compressive residual stress and deeper plastic deformation zone, so the increment of fatigue life of laser hardend 17-4PH was more dramatic in larger peening intensity condition.By considering the interaction between temperature, phase, hardness and residual stress, 3D laser hardening simulation model consisting of coupled temperature-displacement and kinetics of phase transformation was proposed in this dissertation. This model predicted temperature, hardness, microstructure of phase transformation and residual stress development during laser hardening treatment and the simulated results were validated by experimental specimen which was exactly the same as simulated model condition. Results showed that a good correlation between simulated and experimental results in the fields of phase transformation, temperature development and the hardness distribution. In terms of surface residual stress, it could be seen that the simulated residual stress state qualitatively agreed well with the experiment but the simulated values of maximum compressive stresses were overestimated. One possible reason for this overestimation is the transformation plasticity parameter using in laser hardening process is smaller than the real value. Another possible reason for this overestimation is due to no consideration of transformation plasticity during the austenitization in this model. According to the simulated results about temperature field and phase structure with different laser beam velocities processes, it could be seen that the maximum temperature position on laser hardening treated surface did not appear at the center of laser beam but 2mm behind the center and the maximum temperature position did not change with increasing of laser moving velocity. In the condition of keeping maximum temperature as a constant, the change of laser moving velocity influenced heating rate more significantly than cooling rate and the cooling rate was relative stable when laser moving velocity change. The main factor affecting the cooling rate was the property of the material itself and the thermal boundary condition and the secondary factor was laser moving velocity.Considering initial stresses and initial phase distribution of laser hardened component, a three-dimensional finite element (FE) modeling of shot peening process were carried out in different typical areas of laser hardened AISI4140. In order to take into account a highly irregular elasto-plastic cyclic loading with high strain rates near the surface of shot peened material, an adequate description of material model was necessary to describe the viscous effects due to strain-rate and temperature changes and the initial as well as the cyclic deformation behavior. After verifying residual stress profile in depth via X-ray diffraction measurements, a systematic study was conducted using this shot peening model to investigate the effect of different shot peening parameters on the residual stress profiles of different typical areas. The result revealed that no matter in laser hardened area or transitional area, the depth of shot peening direct influence zone (δd) and the depth of maximum compressive residual stresses (δmax) increased significantly by growing shot velocity or shot diameter when other shot parameters were equal. However, the surface residual stresses (σs) and the maximum compressive residual stresses (σmax) maintained nearly a constant value with increment of shot velocity or shot diameter when the kinematic energy of shots was higher enough to produce saturated surface compressive residual stresses and saturated maximum compressive residual stresses. Initial residual stresses from laser hardening process had no influence on the resulting residual stresses in shot peening direct influence zone, which meant the influence of initial residual stress produced by upstream manufacturing processes on final residual stress after shot peening treatment can be neglected. With increasing coverage, the surface residual stresses (σs), the maximum residual stresses (σmax), the depth of shot peening direct influence zone (δd) and the depth of maximum compressive residual stresses (δmax) increased pronouncedly. However, these four values did not increase when the coverage was lager than 200%. This phenomenon could be attributed surface material hardening after shots impact.