Highly strengthened heat-resistant steels for boiler tubing are required to develop for higher efficiency of coal-fired power plants. Creep rupture test and impact test after long-term thermal aging have been carried out on austenitic steels to reveal the influence of alloying elements on the properties which are required for boiler materials. Creep rupture strength at 700 ℃ for 105 h of a highly-alloyed steel B was estimated to be higher than 100 MPa. However, aging toughness of the steels studied became lower than 30 J/cm2 after aging for 1000 h. It was clarified that precipitation and growth of M23C6, Laves and W-rich phases at grain boundaries lower the toughness of the steels. Therefore, optimization of contents of alloying elements such as C, Cr, Mo and W is needed to suppress the precipitation and growth.
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