| Energy is an important base for economic and social development.As the rapid development of national economy energy need has been increased in China.Hydro-power is very abundant in China and takes Nol position in the world.However,the effective development is lower.Therefore,great effort to develop hydro-electricity can not only promote whole professional technology progress of hydro-power equipment but also adjust and optimize the electricity ratio.It plays a very important role in building environmental friend society.As the building and successful operation of Three Gorges hydro-power station,taking the Francis turbine runners as representi ve the unit capacity and the structure size of a hydro-turbine have been increased.Therefore,designers,manufactures and power station owners pay great attentions to the manufacturing quality and using reliability of a hydro-turbine,especially to the quality of a super thickness welds.The orientation is that microstructures and properties of welds become more complicated,and the opportunity for various welding defects increases because lager welds size will be as the larger turbine runners.Meanwhile,the weld residual stress distribution transforms into 3D from 2D.Therefore,the material properties for hydro-turbine runner have to be further improved,and the matched welding consumables and welding techniques must be optimized as well.On the basis of analysis above mentioned,many investigations closely related to the welds toughness are conducted in dissertation.On view of steel metallurgy,the effects of middle frequency induction melting method under nature atmosphere and vacuum high frequency melting method on the chemical compositions and mechanical properties of 00Cr13Ni5Mo martensitic stainless steel welding wire deposited metal are comparatively investigated.It is verificated that vacuum high frequency melting technology can effectively control the harmful gases into welding wire such as[N]?[O]?[H]et.al,typically[N]<200ppm?[O]<50ppm?[H]<2ppm.At the same time,a metal and flux cored wire with same chemical composition system is developed to prevent the welds surface wrinkles and non-fusion from oxides often occurred with solid wire.Experimental investigation shows stable arc and,good bead appearance and less surface oxides.The mechanical properties of deposited weld metal reach that of imported similar wire.Five welding processes,Commonly used MAG welding process with high purified Ar+ a small amount of CO2 as shielded gases,automatic TIG welding with high purified Ar shielded,high speed MAG welding process with high purified Ar+ a small amount of CO2 as shielded gases,laser-are hybrid welding with high purified Ar+ a small amount of CO2 as shielded gases and laser-CMT hybrid welding with high purified Ar shielded,are comparatively investigated.The welding characters and mechanical properties of deposited weld metal with different welding processes are studied.The applied investigation of laser-CMT hybrid welding on the martensitic stainless steel is made to greatly improve the impact toughness and fracture toughness of the deposited weld metal.The experimental results show Charpy absorbed energy of the deposited weld metal is over 120J,which has reached the base metal level.The max.CTODs for both the deposited weld metal and weld metal are 0.18mm,which is nearly the level of base metal.These research results will be a new solution for high efficient and good quality welding of super thickness martensitic stainless steel.On the basis of welding processes investigations above mentioned,the effects of welding parameters for different welding process including main welding parameters,temperature between welds,post welding heat treatment and especially the CO2 added into shielded gases on chemical compositions and mechanical properties of deposited weld metal are investigated respectively.Meanwhile,a small amount of CO2?N2?O2 is respectively added into the high purified Ar as shielded gases for TIG welding to investigate the influences of C?[N]?[O]on chemical compositions and mechanical properties of the deposited weld metal.The results indicate[O]content in the deposited weld metal plays the most important role in the impact toughness.Therefore,no matter of what kinds of welding consumables and welding processes is control of[O]content essential and first solution to improve the toughness of deposited weld metal.Based on the above mentioned experimental investigations,the fracture characters and mechanisms of deposited weld metal are analyzed.As the[O]content in deposited weld metal is lower than 200ppm the initial cracks occurred by the martensite cleavages at welded condition,and then propagated by partial cleavages and dimples.After the PWHT the initial cracks occurred and propagated by dimples as the toughness improvement contributed by the reverse austenite transforms.While the[O]content in deposited weld metal is higher than 200ppm,the crack brittle nucleated at the spherical Al2O3 oxides,and propagated along Al2O3 oxides nucleated cracks around some small dimple.Such fracture characters indicate lower toughness.Through investigation results proposed in this dissertation,the manufacturing technology for 00Cr13Ni5Mo martensitic stainless steel solid welding wire has been optimized into vacuum high frequency melting method +electric slag remelting + clean forging and drawing processes from middle frequency induction melting method under nature atmosphere.Laser-CMT has applied in the repair of hydro-turbine blades,and will used in the welding of key positions of hydro-turbine runners. |
PDF Full Text Request