| As the leading power technology, Ultra-supercritical generating unit willachieve a significant improvement in the environment. While highly efficient andenvironmentally protective technology need more stable materials to ensuresecurity. The new type of heat resistant steel is becoming the focus of research. Toguarantee safety operation of pipelines, we need high-precision prediction oflong-term life of heat resistant steel. At present, the accuracy of extrapolationmethod is too unfaithful to meet the requirements of practical engineering. In theprocess of long-time running, the internal structure of heat resistant steel evolvesintricately, and the mechanical properties changed significantly. In order to makereasonable prediction, we need to make a more accurate description of long-termaging microstructure. Based on this, the optimization method of Larson-Millerconstant C was studied, furthermore the typical microstructures of heat-resistantferritic steel and austenitic steel in long time aging treatment were analyzed, thefollowing observations can be made.(1)When applying the method of Larson-Miller, the constant C is a greatinfluence on the accuracy of the creep life prediction. Optimized C improved theprediction precision a lot. Optimized C was used to predict the creep rupture lifeof T92steel, we got more exact and reasonable prediction.14kinds ofheat-resistant steel creep data which provided by NIMS were analyzed, differentcontent of alloy elements, heat treatment process and hardness means differentactivation energy of self-diffusion, so the constant C are also different.16kinds offorms of polynomial fitting function were analyzed,lgσ=a3P3+a2P2+a1P+a0wasthe best form and was recommended for the life prediction.(2)The typical microstructures of one kind ferritic heat-resistant steel after20000h aging treatment at620℃and its hardness were analyzed. Martensiticpackets became indistinct and wider, Grain boundary became bent, dislocationdensity decreased and precipitated phases coarsened with increasing the agingtime. Laves phases were found after4320h, and they were geologically stableduring4320h～11376h,the content of W element decreased gradually，while thecontent of Mo element Increased gradually. Z phases precipitated after 20000h.The dislocation density decreases with the increase of aging time, whilethe grain boundary misorientation distribution function of the specimens showlittle change, misorientations of1.5°contribute the large proportion. Thehardness decrease with increasing the aging time, it descended quickly before11376h and descended slowly after11376h.(3)The typical microstructures of HR3C austenite heat-resistant steel after20000h aging treatment at650℃and its hardness were analyzed. The grainboundary became coarsen, a larger number of precipitates precipitated out andbecame coarsen, also a larger number of twin boundary appeared. M23C6appearedon the grain boundary and twin boundary, while in the grains after5000h agingtreatment at650℃, little rod-like M23C6attach on surface of sphere-like CrNbNto nucleate and to grow. Mode and fishbone like grain boundary appeared in thesteel after20000h aging treatment. The backbone was CrNbN, the deputy bonewas M23C6, the CrNbN made an58.3°angle with M23C6.The grain size increasedwith the increase of aging time, the frequency of low Σ-CSL boundaries and thedislocation density decreased. Misorientations tends to be polarized, ultimately,conversion to1.5°and59.5°. The hardness increased with increasing the agingtime, it related to the formation of precipitated phases. |
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