Study Of Laser Transformation Hardening On Droplet Erosion Resistence Of Sturbine Blades

In this paper, surface laser transformation hardening technology (LTH) was applied on the blade material 17-4PH. By means of optical microscopy (OM), SEM, XRD, TEM and Mossbauer spectroscopy, the microstructure of the hardened layer and matrix was analyzed. The hardness, distribution of surface residual stress and corrosion resistance were also studied on the surface after laser hardening with the micro-hardness tester, X-ray analyzer and electrochemical testing. After laser quenching, the surface hardened layer was aged at 500℃. The solid transformation process of hardened layer was studied by TEM and mossbauer spectroscopy.Results show that, after laser quenching, the cross of specimens was divided into three parts:hardened layer, transition zone and substrate. The hardened layer was primarily hardening martensite, and microstructure close to the substrate was mainly tempered martensite. The ferrite on the surface was dissolved and refined, while NbC was not dissolved. The austenite grains of hardened layer were significantly refined to level 11-12. After laser hardening, the austenite and the rich copper phase almost dissolved. Results of TEM and mossbauer spectroscopy show that after laser hardening treatment, atoms diffused hardly, new paramagnetic phase of austenitic was formed after the copper phase dissolution, the austenitic had the high copper content, and the austenite was translated into martensite.Section hardness distribution shows that the hardness in the hardened layer increased by 40-50HV, which was resulted from the fine-grain strengthening, surface defects, the transformation of the original austenitic and microstructure. Potentiodynamic polarization experiment indicated that the corrosion resistance was improved after laser quenching. The distribution figure surface residual stress shows that, the surface residual stress was compressive stress after laser hardening, the average axial residual stress was about-112 MPa, the average tangential residual stress was about-83 MPa, the range of the surface pressure stress was over 1 mm, after aging at 500℃for 2 h, the compressive stress was more even, the size did not change a lot. These three factors will enhance the resistance of water erosion performance.The micro-hardness of the hardened layer reached the maximum as about 457 HV after ageing at 500℃for 4 h. But the hardness did not increase largely, also the aged peak time was delayed. After ageing for 2 h, rich copper phase did not significantly precipitate; After ageing for 4 h, the rich copper was in a mixed forms, where some rich copper phase had already coarsened; After ageing for 8 h, lots of rich copper phase in martensite matrix was precipitated and coarsened, and the precipitations existed a relation with the substrate: (332)M//(001)Cu, [110]M//[111]Cu, which was different from usual K-S relationship. The paramagnetic phase aged at 500℃constantly reduced, and rich copper phase formed gradually.