Research On Microstructure And Mechanical Properties Of Low Alloy Cr-Mo Deposited Metal

Low alloy Cr-Mo steel is the candidate material for tubes and tube plates in steam generator of sodium cooled fast reactor for its excellent high temperature strength and creep resistance as well as low tempering embrittlement.At present,there are few research data on low alloy Cr-Mo welding consumables for nuclear power in domestic.The welding consumables of low alloy Cr-Mo steel are mainly ER90S-B3 in ASME standard and CB-04X2MA imported from Russia.The content of C in ER90S-B3 was 0.07-0.12%,slightly higher than that in CB-04X2MA(?0.06%).In addition,ER90S-B3 doesn't contain W,while the W content in CB-04X2MA is 0.035%.Martensite-austenite(M-A)constituent distributed along the prior austenite grain boundary are easy to form in low alloy Cr-Mo deposited metal,which deteriorates the impact toughness of deposited metal.Therefore,in order to improve the microstructure and mechanical properties of deposited metal,five kinds of low-alloy Cr-Mo steel wires with different C and W contents were designed according to the thermodynamic calculation by Thermo-Calc software in this paper,the effects of C and W on the microstructure,the types,distribution and precipitation behavior of precipitates and the mechanical properties of deposited metal were investigated as well as the post-weld heat treatment affection.The main research contents and conclusions of this paper are as follows:(1)The microstructure of low alloy Cr-Mo deposited metal was analyzed.It was found that coarse grain heat-affected zone(CGHAZ)was affected by the secondary welding thermal cycle of interlayer deposited metal,resulting in the formation of necklace-type microstructure along the prior austenite grain boundary in deposited metal.Necklace-type microstructure enriched in M-A constituent could deteriorate the toughness of deposited metal.Both C and W could promote the formation of lath bainite in deposited metal,thereby increasing the proportion of large angle grain boundaries in deposited metal.The increase of C content could promote the formation of residual austenite and necklace-type microstructure in deposited metal,thereby deteriorating the toughness of deposited metal;Increasing W content inhibits the formation of retained austenite and necklace-type microstructure in deposited metal,thereby improving the toughness of deposited metal.In addition,C could play a role in solid solution strengthening as well as improve the strength and deteriorate the plasticity of deposited metal.W had little effect on the strength of deposited metal,but deteriorated the plasticity of deposited metal.With temperature rising,nano-sized carbides were precipitated and coarsened in deposited metal,resulting in the strength of deposited metal increasing first and then decreasing,and reaching the maximum at 350?.(2)The post-weld tempering treatment at 650? was applied for the deposited metals with different C and W contents.It was found that the bainite and M-A constituent in necklace-type microstructure of deposited metal were decomposed to form bainite ferrite and precipitate more carbides along the prior austenite grain boundary and bainite lath boundary after tempering treatment.Carbides in tempered deposited metal were mainly M3C,M23C6,M7C3,M6C and M2C.Moreover,M23C6 and M2C were mainly precipitated along the prior austenite grain boundary,M3C and M7C3 were mainly precipitated inside the grain,M6C was mainly precipitated on the surface of M23C6,and there was an evolution relationship between M23C6 and N6C:M23C6?M6C.Increasing C and W content inhibits M7C3 carbide precipitation and promotes M6C carbide precipitation.(3)After tempering treatment,the microhardness and strength of deposited metal decreased but the ductility and toughness of deposited metal were improved due to matrix softening.With temperature increasing,the carbides in tempered deposited metal grew up and coarsened,and the precipitation strengthening effect was weakened,resulting in the decrease of strength on deposited metal.With C content increasing,carbides were coarsened,the inhibition of crack propagation was weakened,resulting in the decrease of impact toughness on deposited metal.With W content increasing,carbides were refined,and a large number of nano-scale spherical carbides were precipitated,the inhibition of crack propagation was strengthened,resulting in the improvement of impact toughness on deposited metal.