Microscopic Mechanism Of 17-4PH Stainless Steel By Laser Solid Solution And Laser Alloying Hybrid Strengthening

17-4PH stainless steel, featured with high strength capacity and toughness, as well as excellent corrosion resistance and easy to the intensity level adjustment, has been widely used in large-capacity turbine components, nuclear reactor parts and equipments. Due to the reason that conventional strengthening means cannot cover the requirements of either excellent surface property or overall strength, an attempt has been made to strengthen 17-4PH steel used in large-capacity steam turbine blade by laser hybrid strengthening technique. A diode laser utilized, through two laser beams exerting on the specimens, with firstly laser solid solution and secondly laser alloying treatment, the hybrid strengthened layer was achieved. The hybrid strengthened layer, grouped up with laser alloyed layer and laser solid solution layer in subsurface, behaves high surface hardness and superior anti-cavitation performance.To start with, the processed material was subjected to microscope observation and microhardness test so that the premium processing parameters were found. After TEM sample preparation, the laser solid solution area was characterized to STEM, EDX and SAED analysis, in order that the laser hybrid strengthening mechanism could be investigated. The results are as follows:(1) The hybrid strengthened layer was prepared on the surface of 17-4PH stainless steel by the laser hybrid strengthening treatment, constituted with laser alloyed layer, transition zone, laser solid solution zone, heat affected zone and the substrate from the top to inner in the cross direction. The depth of the hybrid strengthened layer and the laser alloyed layer are 1.5mm and 0.2mm, respectively. The average microhardness of the laser alloyed layer reaches 625HVo.2, while that of the laser solid solution zone is 497HVo.2.(2) Pretty fine grains forms in the laser alloyed layer and the grain size is about 20nm, which leads to high strength. Highly dispersed distributed W2C particles are found which are testified to be another cause of strengthening the laser alloyed layer.(3) Precipitation hardening phases, ε-Cu, NbC, M23C6 particles, are found in laser solid solution zone after aging, which resulted in the increase of the microhardness. ε-Cu and NbC particles harden the matrix by cutting mechanism interactively exerted by dislocations, while M23C6 particles by circumvention mechanism.(4) On the other hand, the formation of the reversed austenite, appearing 0.5μm, and spinodal decomposition are the main causes of toughening the solid solution zone.The experimental research of this paper provides theoretical supports for further comprehension of laser hybrid strengthening technique and its improvement for industrialization.