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P91耐热钢(10Cr9Mo VNb)是应用于超(超)临界发电机组的必选钢种。鉴于机组的主蒸汽管道工作温度为600℃,试验考察了P91耐热钢在该温度下的蠕变行为。在应力140 MPa下进行试验,蠕变断裂时间为5 534 h,蠕变速率为0.2μm/h,随后在同样的条件下蠕变100 h和1 800 h,分别对应于减速蠕变和稳态蠕变阶段,与蠕变前和蠕变断裂后的样品进行对比。采用透射电子显微镜分析蠕变不同阶段的微观组织,重点考察持久蠕变过程中的亚微米级析出相。结果表明:蠕变过程中P91耐热钢微观组织发生了退化,即马氏体板条宽化、位错密度降低、析出相略有粗化,并有新相形成。P91供货态析出相主要是100~200 nm的M23C6型析出相和10~50 nm的MX型析出相,这两种析出相在蠕变过程中变化不大。蠕变至1 800 h时,发现了少量含Mo的Laves相,尺寸从50~150 nm,主要分布于马氏体板条界和亚晶界上,特别是多个亚晶粒或板条交接的位置。Laves相成分较稳定,但极易粗化,长时间蠕变(5 534 h)的断裂试样中Laves相最大Feret直径超过600 nm。细小Laves相具有析出强化作用,但粗大的Laves相将减弱其强化作用,加之基体强度的降低,使蠕变加速进而导致裂纹产生甚至断裂。
P91 heat-resistant steel (10Cr9Mo VNb) is the steel of choice for supercritical (super) critical generating units. In view of the main steam pipe working temperature of the unit is 600 ℃, the creep behavior of P91 heat-resistant steel at this temperature is investigated. Under the stress of 140 MPa, the creep rupture time is 5 534 h and the creep rate is 0.2 μm / h, then creep for 100 h and 1 800 h under the same conditions, corresponding to the deceleration creep and steady state Creep phase, compared to creep and creep rupture samples. The microstructures of different phases of creep were analyzed by transmission electron microscopy, and the submicron precipitates in the process of permanent creep were investigated. The results show that the microstructure of P91 heat-resisting steel degenerates during the creep process. That is martensite lath is widened, the dislocation density is reduced, the precipitated phase is slightly roughened and a new phase is formed. The precipitated phases of P91 are mainly M23C6 precipitates of 100 ~ 200 nm and MX precipitates of 10 ~ 50 nm, and the two precipitates have little change during the creep process. When creeping to 1800 h, a few Mo-containing Laves phases were found, with sizes ranging from 50-150 nm, mainly distributed in the martensite lath and subgrain boundaries, especially the subgrains or laths s position. The composition of Laves phase is stable, but extremely coarsening. The maximum Feret diameter of Laves phase in long-time creep (534 h) fracture exceeds 600 nm. The fine Laves phase has the effect of precipitation strengthening, but the coarse Laves phase will weaken its strengthening effect, and with the decrease of the matrix strength, the creep acceleration will lead to the crack even being cracked.