Super304h Austenitic Heat Resistant Steel Weld Microstructure And Properties Research

In order to localize the weldig rod used to weld Super304H austenitic heat resistant steels, basing on the metallurgy, strengthening mechanisms and demanded properties of the base metal, this desertation investigated the effect of rods’ composition on the microstructures and properties of the deposited metals considering the weldability of the base metal. Meanwhile, great attention was paid to the transition of deposited metal microstructures during long-term creep test, as well as the microstructures and properties of the joints.Based on Thermo-calc and designing experiences in welding rods for austenitic heat resistant steel, three experimental welding rods were developed. Mechanical tests, phase analysis and microstructure observation were employed to investigate three deposited metals, all of which were found to solidify in austenite modes. Niobium phases normaly distributed in the solidification subgrain boundary and solidification grain boundary, and the size of which were as much as10μm. The weight percent of niobium phases in the deposited metal with0.6%niobium and0.12%nitrogen was0.452%, two times the amount of the one with0.28%niobium whose niobium phases was0.207%in weight percent, as the driving force of Nb(C,N) phase was different in two alloy systerms. Form the results of the phase analysis, about60%niobium went into niobium phases for both alloy systems. And the amount of niobium consumed increaced with the increace of Nb/(C+N) ratio, it was also concluded that the amount of niobium consumed in the deposited metals with about0.6%niobium was propotional to that in the one with0.28%niobium, and the proportion was approximated to be the ratio of the Nb/(C+N) ratio for both systems. Better impact toughness and plasticity could be achieved in the deposited metal with0.28%niobium with less niobium phases which were small in size.After being long-term creep tested under the condition of650℃and200MPa stress for more than8960hours, the deposited metal with0.28%niobium hasn’t broken down. Analysis on the transition of microstructure of deposited metal from the as-welded to the long-term creep tested was performed. Most of solidification subgrain boundaries became granular after long-term creep test, and niobium phases granulized in the shape of chain, band and block. Besides, the niobium phases partly dissolved in certain regions of second welding heat affected zone. The corrosion resistance of the creep tested specimen significantly decreased, and chromium and molybdenum clustered in solidification grain boundary. The corrosion resistance of which was however relatively high, as the chromium content was as much as40%. Moreover, the secondary phases in deposited metal with0.28%niobium under different stesses varied. Extra secondary phases such as M23C6and NbCrN precipitated during creep test in the deposited metal under200MPa, and some a phase precipated in the deposited metal under78MPa. It was M23C6phase that precipitated with serious detriment to the corrosion resistance, especially to the solidification subgrain boundary.Solution treated Super304H tubes were GTAW welded with14kJ/cm heat input using experimental welding rod with0.28%niobium in its deposited metal. The yield strength and the reduction area percent of the joints were308MPa and40%, respectively. Toughness of the heat affected zone and fusion zone were higher than that of the weld metal whose toughness was very close to that of the deposited metal, for the composition of weld metal and deposited metal were similar. The weld metal exhibited a microstructure of austenite, and the quantity, size, shape, and the distribution of the secondary phases were similar to these of the phases in the deposited metal. Niobium phases on the solidification subgrain boundary partly dissolved after the weld metal was reheated by the following welding heat. Moreover, the width of grain-coarsed zone, the grains of which didn’t coarsen much, was700μm around with this heat input. There were changes in the size, shape and distribution of niobium phases in the heat affected zone, and close to the fusion zone grain boundary were found melted.