Laser Compound Strengthening Mechanism On Precipitation Hardening Martensite Stainless Steel Steam Turbine Blades

As the precipitation hardening stainless steel, the traditional strengthening methods can not both harden the surface and hold the integrated performance at the same time. In order to solve the fault, laser strengthening technique is introduced to hardening the 17-4PH blades used on Ultra-supercritical steam turbine. During the one time scanning, the laser alloying and solution was completed to obtain the high hardness and anti-cavitation layer.In this paper, the different laser strengthening techniques were compared and the surface macroscope pattern, the microstructure, the elements distribution, hardness distribution and phases of laser strengthening layer were analyzed. After aging treatment, the hardness distribution, the phases change, the pattern and distribution of precipitated phases in all zones of surface layer were compared. Therefore the different strengthening mechanisms of both alloying zone and solution layer were compared. The compound laser strengthening mechanism after aging treatment was discussed. The anti-cavitation performance of laser strengthening layer was also simulated and evaluated.The results showed the surface layer was smooth after laser processing at the laser parameters of 2200W, 400mm/min and1800W, 300mm/min. The high hardness intermetallic compound(Fe-Cr、Al_0.28Co_0.4Cr_0.24W_0.08) and diffused distribution hard phase（W2C）, thus the average hardness of this zone was HV_0.2361. After aging treatment, although the alloyed zone hardness decreases, the solution zone hardness was improved because of the large quantity tiny precipitation phases on the martensite substrate. The average hardness of solution zone was HV_0.2455, compared with the solution zone without aging, the hardness was increased about 26%. After the simulation of cavitaion experiment, the weight loss of substrate was twice than that of the laser strengthening sample.