Study Of Nb On Microstructures And Properties Of Heat-Affected Zones In HSLA Steel With Large Heat Input Welding

In this thesis, the microstructures and properties of heat affected zones (HAZ) in two different Nb content steels melted in laboratory were investigated by actual electro-gas welding and welding simulation, the welding properties of Nb-containing high strength low alloy steel (HSLA) produced in industry were also studyied.The results of actual welding showed that the microstructures of welding joints consisted of weld metal, coarse-grained region, fine-grained region and incomplete quenched region. At large heat input of 80kJ/cm, the width of coarse-grained heat affected zone (CGHAZ) increased approximately 1.5mm when the content of Nb increased from 0.014% to 0.17%, the whole HAZ width varied from 7.5mm to 8mm. with low Nb content steel, the microstructures of CGHAZ were AF and GF (grain boundary ferrite), which was granular bainite (GB) when Nb content increased. Correspondingly, the microstructures of fine-grained region, incomplete quenched region and base metal varied from polygonal ferrite (PF) and pearlite to ferrite and bainite. These changes of microstructures can be attributed to the Nb in solid solution, which suppressed GF nucleation, decreased transformation start temperature and promoted the formation of GB.Results of welding simulation with different peak temperature indicated that the microstructures of HAZ were coarser than that of actual welding, but the type of microstructures was in a good accordance. Welding simulation results showed that the microstructures of CGHAZ were obvious different with different heat input. With low Nb content, the microstructures varied from predominately GB and AF to PF and AF, finally they completely changed to PF and P with large heat input. While the microstructures of high Nb content steel varied from GB and a little amount of martensite to bainite and ferrite, then transformed to PF.Vickers-hardness tests showed that the weld metal had highest hardness because of a large amount of acicular ferrite, both the coarse-grain region near fusion line and incomplete quench region softened after welding. The hardness of steel with high Nb content was a little higher than that of the steel with low Nb content. Welding simulation also showed that the vickers-hardness decrease with heat input increasing.Results of Nb-containing HSLA steel produced in industry showed that the steel had good low temperature ductility with a heat input of 42kJ/cm and 55kJ/cm welding.