The Effect Of Hot Isostatic Pressing Temperature On Microstructure And Mechanical Properties Of 14Cr Oxide Dispersion Strengthened Ferritic Steel

Nano-structured ODS steels are the leading candidate structural materials for first wall application in future fusion reactors because of their excellent radiation tolerance, helium embrittlement resistance and improved high-temperature properties. The outstanding properties of nano-structure ODS steels come from the characteristic microstructure, depending strongly on the composition and manufracture technology. In this paper, the effect of hot isostatic pressing temperature on the microstructure and mechanical properties of 14Cr-ODS steel was studied. The supersaturated alloy powders with composition Fe-14Cr-2W-0.3Ti-0.3Y2O3 (wt%) were prepared by mechanical alloying (MA), and then consolidated by hot isostatic pressing (HIP) at 900℃ and 1200℃, respectively. The following results were obtained:(1) The relative density of 14Cr-ODS ferritic steels prepared by HIP at 900℃ and 1200 ℃ are all above 99%. The ultrahigh-density nano-size precipitates are formed in both steels.(2) The microstructure of 14Cr-ODS ferritic steel HIPed at 900℃ is consisted of a great number of nano-grains with the size of 50-100 nm and a few coarse grains with the size of 1-2 μm. In 14Cr-ODS ferritic steel HIPed at 1200℃, there are more coarse grains of ～2 μm and some ultra-fine grains of 100-200 μm. The average grain size of the sample HIPed at 900 ℃ is smaller than that at 1200 ℃.(3) Y-Ti-O-enriched nano-clusters and Y2Ti2O7 complex precipitates are formed in 14Cr-ODS ferritic steels consolidated at 900℃ and 1200℃. The density of precipitates in 900℃ HIP sample is 2.8×1023/m3 with average size of 2.24μm; and the density of precipitates in 1200℃ HIP sample is 4.2×1022/m3 with average size of 3.62nm. Some Cr-Ti-O-enriched precipitates are observed at grain boundary in both 14Cr-ODS ferritic steels. The density of Cr-Ti-O-enriched precipitates is 2.6×1020/m3 with average size of 44 nm,6.5×1019/m3 with average size of 99μm, respectively, in 900℃ and 1200℃ HIPed steels;(4) The hardness of the 14Cr-ODS ferritic steel consolidated at 900℃ is 599 HV, which is slight higher than 1200℃ HIPed sample with an hardness of 580 HV.(5) The 14Cr-ODS ferritic steels show a good strength and plasticity at room and high temperature tensile test. The tensile strength decreases and the toughness increases with the increasing test temperature. The fracture mechanism changes from brittle fracture to quasi-cleavage fracture. The tensile strengths of the sample HIPed at 900℃ is higher than the sample HIPed at 1200℃ at all test temperature and the tensile strength of 900℃ HIP sample at 600℃ and 700℃ are 590 MPa and 369 MPa, respectively. The elongations of the sample HIPed at 1200℃ are higher than the sample HIPed at 900℃ at all test temperature. It indicates that the higher density, the smaller size of nano-scale precipitates and the finer grain have positive effect on the tensile strength. The lower density, the larger size of nano-scale precipitates and the coarser grains have an active effect on plastic deformation.