| Heat-resistant steel is an alloy steel with high mechanical properties and chemical stability under high temperature conditions.It has a wide range of applications in thermal power generation,ship power,chemical refining and other industries.The welding results of heat-resistant steel indicated: the ultimate tensile strength keeps stable at the multi-layer and multi-pass weld metals with the same welding variables and the same chemical compositions.However,the impact toughness significantly varies in different regions.Worse still,the toughness in some regions of weld metal(WM)even can not reach the lowest standard set for the practical engineering.Therefore,it is very important to control and improve the toughness stability of WM in the research field of weld metal.In this study,submerged arc welding and manual arc welding were adopted,the reason and factors affecting both the lower value and the variation of low temperature impact toughness in the multi-layer and multi-pass weld metals of heat-resistant thick-plate steel were carefully investigated.Advanced study method and technologies,including optical microscope(OM),scanning electron microscope(SEM),energy spectrum analysis(EDS),and X-ray fluorescence(XRF)etc.,were used to observe the fracture surface,to characterize microstructures and to identify chemical composition of WM.The relationship between impact toughness and microstructures was revealed,in addition,the reason affecting the lower value and the variation of low temperature impact toughness were investigated.Finally,differential scanning calorimetry(DSC)curves of WM were measured by synchronous thermal analyzer(STA)to obtain corresponding phase transformation temperatures,which are beneficial to optimize heat-treatment parameters and then to decrease the variation of impact toughness.The main results are shown as followes:The significant variation of the impact toughness in the as-welded WM by manual arc welding can be attributed to the inhomogeneity of microstructure in WM.Coarse massive ferrite,coarse boundary ferrite were found in the specimen with lower impact toughness.Nevertheless,the variation of the impact toughness in the specimen under post tempering heat treatment(PWDT)ascribes from the coarse and uneven grain in columnar grain zone.Post heat treatment process of tempering after normalizing(PWNT)make the microstructures of the weld metals homogenize.By comparing the microstructures and microhardness at the weld metals under PWNT and PWDT,the low-temperature toughness of WM under PWNT process is greatly improved.One possible reason affecting the huge variation of impact toughness among the three different zones(i.e.,top zone A,middle zone B and bottom zone C)in the submerged arc welding specimen is attributed to different volume fraction and different sizes of coarse ?-ferrite.The coarse ?-ferrite can reach the size of several hundred micrometers(300?m).The impact toughness of the weld metals is directly related to the volume fraction of coarse ?-ferrite,and decrease with the increase of the volume fraction of coarse ?-ferrite.Furthermore,the variation of the impact toughness was also influenced by the number of coarse ferrite grains,in other words,the average size and the distribution of grains.The above research on the correlation between microstructure and impact toughness of heat-resistant steel weld metal in low temperature environment has important theoretical value for the low temperature toughness of weld metal and its stability.At the same time,the solution of related problems will have important engineering significance for the safe and stable operation of heat-resistant steel weldments and the improvement of service life. |
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