Abstract: Effect of high-temperature aging at 675 ℃ on microstructure evolution and room temperature mechanical properties of large-diameter thick-walled pipes of G115 steel was investigated by using optical microscope, field-emission scanning electron microscope, field-emission transmission electron microscope, room temperature tensile and impact tests. The results show that the matrix structure of the G115 steel in the original normalized and tempered state is tempered martensite, and the precipitated phases are M₂₃C₆ carbide and MX carbonitrides. During the long-term aging process at 675 ℃, the matrix structure remains tempered martensite, however, a slow recovery of the martensite laths occurs. The M₂₃C₆ and MX phases grow slowly, and the Laves phase precipitates rapidly and coarsens. Compared with the properties in the original state, both the tensile strength and yield strength of the G115 steel show a monotonous decreasing trend, while the elongation first increases and then decreases. And the impact property of the G115 steel decreases in the initial stage of aging, but as the aging progresses, the impact property as a whole shows a trend of first increasing and then decreasing.

Key words: G115 steel, long-term aging, microstructure, mechanical properties