| P92 heat-resistant steel is commonly used in the main steam pipelines,reheat steam pipelines and high-temperature headers of ultra-supercritical thermal power plants.The maximum steam parameters for service are 620?,30MPa.The creep rupture failure of high temperature and high pressure components will directly affect the safety and production of thermal power plants.Therefore,reliable creep constitutive models and life prediction methods are needed to ensure the safe operation of components.The engineering mainly uses the extrapolation method such as isotherm extrapolation method or time-temperature parameter method to predict the creep properties of heat-resistant steel under long-term service.However,if the high-stress or high-temperature short-term creep properties of P92 heat-resistant steel are used to extrapolate the long-term creep properties under low-stress service conditions,excessive evaluation will be made,that is,the creep properties of the low-stress regions are degraded.The degradation of creep properties in low stress regions is related to the evolution of micro structure under high temperature thermal aging.However,the micro scopic influence factors that cause the degradation of creep properties in low stress areas are still unclear,and on the other hand,the drawbacks based on empirical extrapolation are reflected.Therefore,it is of great practical significance to analyze the tissue degeneration factor that affect the low-stress creep properties of P92 heat-resistant steel and to explore the constitutive laws of creep behavior under single conditions.This article systematically analyzes the micro structure changes caused by thermal aging and its influence on creep properties,and attempts to reveal the tissue degradation factors affecting low stress creep properties and establish a new creep constitutive model.Main research contents and conclusions as follows:(1)The micro structure of P92 heat-resistant steel at 650? for 0?8000 h was characterized by optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results show:the original P92 heat-resistant steel is a tempered martensite lath structure,M23C6 carbide and MX phase are dispersed on the prior austenite grain boundary,subgrain boundary and matrix;as the thermal aging time prolongs,the martensite lath has a certain coarsening,but the tempered martensite lath structure is basically maintained;the Laves phase precipitates after thermal aging and the coarsening rate is significantly higher than the M23C6 carbide and MX phase.(2)The creep properties of P92 heat-resistant steel with different thermal aging time at 20MPa,35MPa,65MPa and 75MPa were tested at 650?.The creep test under high stress(65 MPa,75 MPa)was performed by uniaxial rectangular creep test method.The creep experiment under low stress(20 MPa,35 MPa)uses a multi-axis helical spring creep test method.The results show:the low-stress creep properties of the samples with thermal aging for 3000 h were strengthened,and the lower the stress,the more obvious the strengthening effect;the micro structure degradation factor affecting the creep properties of P92 heat-resistant steel at 650?,20MPa and 35MPa is mainly Laves phase,while at higher stresses of 65MPa and 75MPa is mainly M23C6 carbide.(3)The creep rupture test of 170MPa for P92 heat-resistant steel with different thermal aging time was carried out at 650?.The results show:the creep strength and creep rupture life of P92 heat-resistant steel gradually decrease with the increase in aging time,but the creep rupture strain is slightly improved;the creep fracture morphology of P92 heat-resistant steels with different aging time are dimples of different sizes,which are typical ductile fractures;the stress concentration caused by the coarsening of the precipitated phase causes many subgrains to be formed after the martensite strips in the heat resistant steel are decomposed.(4)The short-term and long-term creep behaviors of P92 heat-resistant steel were fitted and analyzed by using multiple sets of constitutive equations(four groups in this paper).The results show:the primary creep strain-time curve of P92 heat resistant steel conforms to Blackburn eq.;the primary creep rate-time relationship is in accordance with Power law',and the tertiary creep rate-time relationship is in accordance with the Logarithmic law';Power law' and Logarithmic law' can form two composite models to describe the overall creep behavior,and based on the composite constitutive model the K-parameter method was proposed for residual life assessment. |
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