Austenitic materials with high sensitization resistance and high temperature strength are required for furnace and reaction tower of desulfurizing plants in the petroleum refinery industry. For these requirements, a new steel (LowC-18Cr-11Ni-3Cu-Mo-Nb-B-N) has been developed. The steel shows no intergranular stress corrosion cracking in polythionic acid environment after aging in the temperature range from 565 to 700 °C for up to 10,000 hours. This excellent PTA-SCC resistance is attributed to the prevention of M23C6 carbide precipitation along grain boundary due to extra low carbon content with high ratio of niobium to carbon. The maximum allowable tensile stress of this steel is estimated to be more than 30% higher than that of ASME SA213 Type347H. This excellent strength is based on the precipitation strengthening effect due to fine precipitates of a copper rich phase which are coherent with the austenite matrix in addition to Z-phase (NbCrN). Moreover, boron addition improves creep strength and creep ductility of the steel. From these results, it is concluded that the newly developed steel is a promising material not only for refinery processes but also for other elevated temperature usages.

The susceptibility to hydrogen embrittlement and the fatigue properties of several high nitrogen stainless steels were evaluated by SSRT (Slow Strain Rate Test) and external fatigue test. The tensile properties were evaluated by SSRT in gaseous hydrogen pressurized up to 90MPa in the temperature range from −40 degree C to room temperature. Despite the increase of nitrogen content, high nitrogen stainless steels showed no degradation by hydrogen. The susceptibility to hydrogen embrittlement depended on the chemical compositions. Fatigue properties in high pressure gaseous hydrogen were evaluated by the external cyclic pressurization test using tubular specimens. In this test the tubular specimen was filled with high pressure hydrogen gas filled with hydrogen pressurized up to 90MPa, and the outside of the specimen was cyclically pressurized with water pressurized up to 90MPa. The difference of fatigue life between hydrogen and inert gas was extremely small. Susceptibility to hydrogen embrittlement was discussed based on stability of an austenitic structure.

To achieve high efficiency and cost reduction, large heat input welding such as submerged arc welding (SAW) and metal inert gas arc welding (MIG) using large currents are employed. However, the large heat input given to the weld may produce coarse grains in the HAZ and a reduction of toughness. To avoid these problems, narrow gap welding (NGW) which is effective in reducing heat input and achieving high productivity is often employed. However, the pear-shaped bead cracking may form according to the welding condition in the case of narrow gap welding. Therefore, the welding conditions must be carefully selected to prevent the cracks. In this study, the influences of the heat input parameters, such as heat input width, heat input depth and the total amount of heat input on the pear-shaped bead crack formation under narrow gap welding are investigated using the Thermal-elastic-plastic FEM with temperature dependent interface element. Furthermore, to find the optimum values of the heat input parameters, the optimization system which can consider the formation of pear-shaped bead cracking is developed. As optimization methods, Complex Method (CM) and Genetic Algorithm (GA) are adopted. Using these methods, the ideal heat input distribution to achieve deeper penetration with small heat input and without cracking can be computed. As a result, the superiority of the welding using narrow gap can be shown in the viewpoint of heat input efficiency.