Study On Cavitation Resistance Layer On Surface Of CrNiMo Stainless Steel By Laser Cladding And Alloying

0Cr13Ni5Mo stainless steel (CrNiMo) has good structural strength, cavitation resistant, corrosion resistant capabilities and has been widely applied in hydro turbines. However, seriously cracks and cavitation erosion damages were found in CrNiMo large size hydro turbines. In order to decrease its crack tends and to improve its mechanical properties, M-CrNiMo stainless steel was developed by modifying the composition and vacuum refine procedure. In this paper, cavitation erosion damage, cavitation erosion and corrosion interaction damage behaviors of 0Cr13Ni5Mo, M-CrNiMo and 0Cr13Ni4Mo stainless steels were investigated.Supersonic cavitation erosion experiment was tested by supersonic vibration cavitation erosion experiment system. Polarization curves of three kinds of stainless steels in static state and in cavitation erosion condition were obtained by electro-chemistry test system. The erosive wear behaviors were completed by rotating disk equipment.The results show that M-CrNiMo stainless steel has better cavitation resistance property and erosion abrasion resistance property than those of CrNiMo and 0Cr13Ni4Mo stainless steels. During the cavitation erosion, the microstructure phases in stainless steel were plastic deformed and destroyed. At the first incubation period the micro cracks begin in ferrite weak area, then extend to martensite and come into micro-pit. During the static period the micro-pits get wider and deeper and finally the big size cavitaion erosion pits are produced in the entire surface. Corrosion and erosion abrasion strongly accelerate the cavitation erosion damages in three stainless steels. The erosion abrasion resistance properties of M-CrNiMo stainless steel are better than those of CrNiMo and 0Cr13Ni4Mo stainless steels. Micro-structure and hardness were decisive effects for high cavitation resistance properties of M-CrNiMo stainless steel.In order to increase the cavitation erosion of M-CrNiMo stainless steel, complete dense coating with an excellent metallurgical bonding at the interface can be achieved by laser cladding, therefore, the surface treatment technique in hydro turbine can be improvedWC laser alloying layer and NiCrSiB laser cladding layer were made on surface of M-CrNiMo by laser alloying and cladding techniques respectively. Their micro-structure, interface, cavitation erosion mass loss and surface morphologies etc were investigated by optical microscope, Scanning Electron Microscope (SEM and EDAX), Transmission Electron Microscope (TEM), X-ray diffractometer (XRD), supersonic vibration cavitation erosion experiment setup and digital vision Micro-Vickers etc systemly.WC Alloying layer was achieved on surface of M-CrNiMo stainless steel by Nd:YAG pulse laser. The alloying layer was flat, uniform and metallurgical bonded with the substrate, no defect such as crack, air hole was found. The matrix of the WC Alloying layer is Fe-Ni-Cr solid solution. WC alloying layer was strengthened by hard precipitated phases such as W2C, Ni4W, MoNi4, Fe6W6C, Fe7W6, CrC, etc. The surface alloying treatments increase the hardness apparently and strengthen the erosion resistance. Cavitation erosion morphologies of the alloying layer is notably slight compareing to that of the M-CrNiMo stainless steel, with little erosion pits after cavitation erosion test for 6 hours. It is due to the closing metallurgy band with interface and hard phase precipitation as well as the quenching in laser surface alloying treatment.NiCrSiB cladding layer was obtained on the surface of M-CrNiMo stainless steel by Nd:YAG pulse laser and CO2 LASER. The laser cladding layer was uniform, compact and no defect as gas hole, crack and metallurgical bond with the substrate. The substrate of cladding layer are composed by CrNiFe solid soltuion, with precipitated phases such as M23[6P-CB]6 (M=Cr,Fe),CrB,CrSi,Fe2B, etc. The supersonic cavitation erosion test shows that the cumulative mass rate of NiCrSiB cladding layer is 4.2mg/h, which is only 1/3 of the M-CrNiMo stainless steel and lower than that of WC cladding layer. Hardness of cladding layer increased obviously, which means the work hardening occurring on the surface during the test. The cavitation erosion surface damage of NiCrSiB cladding layer was slighter than that of M-CrNiMo stainless steel, with e polished surface after being cavitation eroded for 3 hours and no big cavitation erosion pits after being cavitation eroded for 6 hours.The strengthening mechanism of NiCrSiB coating is a mixture of fine grains strengthening, second phase strengthening and work hardening effect during cavitation erosion process, and the coating has excellent bonding at the interface. In comparison with NiCrSiB coating prepared by CO2 laser cladding, the NiCrSiB cladding layer by LASER shows thicker and more uniform.