Effect of aging treatment on structure and mechanical properties of new heat-resistant steels
The new alumina-forming austenitic heat-resistant steel(AFA)has become a candidate material for ultra-supercritical thermal power units because of its stable microstructure,excellent high-temperature performance,and low preparation cost in the temperature range of 650-700 ℃.The new alumina-forming austenitic heat-resistant steel can be used for the thermal power units of ultra-supercritical thermal power plants.In order to systematically eluci-date the short-time precipitation behavior of the second phase of the new alumina-forming austenitic heat-resistant steel at 700 ℃ and the relationship with the mechanical properties,the effect of short-time(≤500 h)aging at 700 ℃on the microstructure and mechanical properties of the new alumina-forming austenitic heat-resistant steel Fe-18.4Cr-31Ni-1Nb-2.9Al(mass percent,％)has been analyzed using a combination of aging heat treatment,the second-phase precipitation model of coarsening,and the comprehensive mechanical test.The results show that the size of Laves and B2-NiAl phases increases gradually with the aging time.When aging to 20 h,the Laves phase starts to precipitate along the grain boundaries,and its size coarsens and covers the grain boundaries with the exten-sion of aging time.In contrast,at 100 h,the B2-NiAl phase precipitates within the grain and grows from a short bar to a needle.The average size of the Laves and B2-NiAl phases increased from 449 nm to 706.9 nm with the exten-sion of the aging time from 100 h to 500 h.The relationship between the coarsening kinetics and isothermal aging time of the Laves and B2-NiAl phases was quantitatively analyzed using the Lifshitz-Slyozov-Wanger(LSW)coars-ening model theory.The coarsening rate k of the precipitated phases was calculated to be 1.925 8 × 10-27 m3/s,indi-cating that the growth rates of the Laves and B2-NiAl phases are controlled by the diffusion of solute atoms.The precipitation of Laves and B2-NiAl phases in the alloys leads to enhancement of hardness and high temperature ten-sile properties.The hardness increases continuously from 0 to 500 h,while the tensile strength increases and then decreases.This is due to the precipitation of Laves and diffuse B2-NiAl phases from 0 h to 200 h,whereas from 200 h to 500 h,the B2-NiAl phase grows,resulting in a decrease in tensile strength and an increase in elongation.The ten-sile strength increased to 579 MPa with 52％elongation after 500 h aging compared to 528 MPa with 76％elonga-tion in the unaged condition.
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