| Marine engineering steel and manufacturing technology are the foundation of marine industry development.Currently,marine engineering steel is rapidly progressing toward high strength and high toughness.However,the mismatch of strength and toughness between the weld metal and the base metal limits the application of marine engineering steel with high strength and high toughness.Therefore,how to reasonably design the composition of the matching welding consumables,how to optimize the post-weld heat treatment process of the weld metal,and how to improve its mechanical properties are problems that need to be solved urgently.Chemical composition of welding consumables and post-weld heat treatment process are important for the improvement of the weld metal microstructure and mechanical properties.In this context,based on the Fe-Cr-Ni-Mo series high-strength steel welding consumables,this paper systematically studied the influences of V and Cu on the microstructure and mechanical properties of the weld metals in different post-weld heat treatments.The analysis focused on the evolution of the types,composition,distribution,and structure of the precipitates under different components and different post-weld heat treatment processes.Meanwhile,the effects of their evolution on the strength and toughness of deposited metal also were analysed.The main research content and conclusions of the paper include:(1)Based on Thermal-calc thermodynamic calculation,three different Fe-Cr-Ni-Mo series welding consumables with different V content and four different Fe-Cr-Ni-Mo series welding consumables with different Cu content were designed.The corresponding deposited metals were prepared by multi-layer and multi-pass gas metal arc welding.The post-weld heat treatments were set to 550?,600? and 640? for 2h durations,followed by air cooling,respectively.The mechanical properties of the deposited metal were evaluated by uniaxial tensile test and impact test,and the microstructure and precipitates of the intermediate welding in the deposited metal were systematically studied using conventional equipment and techniques.The results indicated that the slight change in the V content and Cu content had no appreciable influence on the matrix microstructure under the same condition,but the change of their content mainly affected the precipitation behavior of the precipitates.(2)The electron probe microanalyser analysis(EPMA)indicated that the nonuniform distribution of elements in the V-containing as-welded metal,with Cr,Mo,Ni and Mn enriched at inter-dendritic regions,which increased the hardenability of the matrix,leading to the formation of martensite and retained austenite.However,the microstructure of the dendrite core region was dominated by bainite,and coalesced bainite appeared in the local areas.The distribution of the C element in the as-welded metal was relatively uniform as a result of its higher diffusion coefficient,which was confirmed by Fick's second law model.In addition,a small amount of nano-scale MC carbides appeared in the V-containing deposited metal owing to the reheating effect of the subsequent weld bead,which presented some degree of precipitation strengthening.Thus,the strength of the deposited metal increased with the increase of V content owing to the synergistic effect of precipitation strengthening and solid solution strengthening,and the impact toughness decreased.(3)The distribution of the precipitates of the deposited metal after the post-weld heat treatment was inhomogeneous.The segregation of strong carbide forming elements(V and Mo)at inter-dendritic region,which was produced by solute redistribution during solidification of the deposited metal,exhibited a major contributor to the continuous diffusion of C element from dendrite core region to inter-dendritic region during post-weld heat treatment,so-called uphill diffusion,leading to the enrichment of the C element.Also,the nano-scale M2C or MC carbides were mainly precipitated in inter-dendritic region whilst large M3C carbides were developed in dendrite core region for the post-weld heat treatment deposited metal owing to the V and Mo elemental segregation as well as the higher density dislocation at inter-dendritic segregation.Comparing with dendrite core region,nano-scale MC carbides at inter-dendritic region was smaller.Simultaneously,the area fraction of nano-scale MC carbides decreased significantly from inter-dendritic region to dendrite core region.In addition,it is reasonable that increasing the V content leaded to an increase in the volume fraction of nano-scale MC carbides.It is believed that MC carbides and dislocations interacted by either MC nucleating on dislocations or through MC carbides pinning dislocations.Thus,it is evident that the dislocation density increased significantly with an increase in the V content for the post-weld heat treatment deposited metal.(4)Based on atom probe tomography(APT)and transmission electron microscopy(TEM)results,it is evident that the nano-sized MC carbides with an FCC structure was mainly rich in V elements in the initial stage of post-weld heat treatment.The Mo atoms dissolved in the matrix continuously diffused into nano-scale MC carbides with the extension of holding time to promote the growth and coarsening of them,resulting in a higher content of Mo element in the large size nano-scale MC carbides than V element.(5)The filmy retained austenite at inter-dendritic region could decompose into nano-scale MC carbides during the post-weld heat treatment owing to a redistribution of Cr,V,and Mo between the martensite and the filmy retained austenite.(6)The strength of Cu-containing deposited metal was higher than that of Cu-free deposited metal.However,when Cu content in the welding consumable changed between 0.5wt.%and 1.5wt%,no visible changes in mechanical properties occurred for the deposited metal.In addition,the size of the nano-scale Cu precipitates during the post-weld heat treatment was very sensitive to temperature,which led to the size increased significantly with the increase of temperature. |
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