| Through solution treatment, long-term aging test and mechanical test, solution treatment technology of S31042 is optimized; the evolution regularities of structure and its effect on the mechanical properties in high temperature were systematically studied, the strengthen mechanism is discussed in the paper with the aid of optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction, and the optimized component range is obtained according to the effect of strengthen mechanism on the component. The results show that:The solution treating temperature has more significant influences on microstructure and mechanical properties of S31042 steel than that of solution holding time. The size, amount of a Nb-rich precipitated phase are reduced, the strength is decreased gradually, the ductility is increased with the increasing of solution temperature. After different kinds of solution treatment technology and long-term aging test of S31042 at 700 ℃, the amount and strength of tiny second phase in the aging steel is increasing apparently with the rising of solution treatment temperature and prolonging of holding time, which show that rising the solution treatment temperature and prolonging the holding time is benefited for the long-term aging property of S321042.The undissolved Z phase and MX phase, whose size are ranged from 100 to 800 nm, are in the solid solution state S31042 steel. The M23C6, MX, a and M6C are precipitated during the high temperature long-term aging process, and the precipitated sequence of S31042 steel was got in the process of aging at 650℃,700℃ and 750℃.M23C6 are mainly precipitated on the grain boundary, few are distributed on the twin boundary and in the crystal after high temperature aging. The M23C6 phases on the grain boundary are gradually growing from scattered tiny granulation to continuous chain, and evolved to large amounts of discontinuous granulation with the increasing the aging time. The growth discipline of M23C6 phase on the grain boundary meets the curing mechanism, and dynamic growth equation is got at 650℃,700℃ and 750℃. The Z phase is precipitated in the crystal, the changing of size and morphology is slight, whose size is kept under 100nm and existed steadily in the steel all the time. The form of Z phase is mainly controlled by the diffusing of Nb. σ phase is precipitated on the M23C6 phase in the crystal, and it is also precipitated in the crystal separately; the compound precipitated phase of σ and M23C6 are found in the crystal when aging time is exceeded 3,000h at 700℃, and the similar phenomena is appeared on aging 300h at 750℃, which is show that the rising of aging temperature promote the precipitating of σ phase. The reason that M6C is detected after aging on 6,000h and 10,000h at750℃ is because of the transformation of M23C6.The effect of the aging temperature on the precipitating behavior of precipitated phase is precipitated. The precipitating and coarsening rate of M236 phase is increasing and the evolution of morphology is accelerating apparently. Aging temperature has less effect on the coarsening rate, and the change of size and morphology is slight, but the precipitating rate is accelerating with the increasing of the aging temperature significantly.The effect of high temperature stress on M23C6 is indistinctive; whose evolution process is the same as unstressed aged state. Stress has significant effect on the Z phase, dislocation motion is blocked by the Z phase, and formed dislocation pile-up around Z phase, but the new Z phase is easily precipitated with agglomerated state on the dislocation and tangled with dislocation.The change of performance is bound up with the evolution of precipitated phase during the aging of S31042. Over aging is appeared in the process of aging at 700℃ and 750 ℃, the high temperature yield strength is decreased apparently after aging on 300h at 700℃, and decreased slightly on 6,000h at 700 ℃. The peak value of hardness and high temperature yield strength is emerged earlier with the rising of aging temperature; the reason caused over aging is because of the rising the temperature, the coarsening rate of M23C6 particle is speeded up, and the strength effect is decreased, sometimes lost. When aging time is over 1000h, in the beginning of aging the impact toughness is dropped rapidly. Because of the influence of M23C6 morphology, the change of impact toughness is slightly different under different aging temperature, impact toughness was falling all the time at 650℃, keeping stable at 700 ℃,and appearing rising trend at 750 ℃The creep fracture life of S31042 steel is not monotonically increase with niobium increase.The test steel which contain 0.40% niobium element have the best creep properties. Through optimizing the component, decrease the content of C and Si, add few vanadium and trace amount of B, the endurance strength is increased, the extrapolated valuation of endurance strength is 80. IMpa on 100,000 at 700℃, which is about 30% higher than before optimization. |
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