Study On Phase Parameters And Properties Of T/P91-92 Heat-resistant Steels

In order to improve the efficiency of power generation and reduce CO2 emissions,(ultra)supercritical power units are currently widely used in coal-fired power plants from all over the world.The increase in steam temperature and pressure has brought higher performance requirements for boiler steels.T/P91-92 heat resistant steels are the main materials of the boiler steels.A large number of operation situations at home and abroad show that the hardness values of the P91 components have been evidently reduced after a relatively short term(tens of thousands of hours)of service exposures.Same phenomenon has begun to occur in P92 components.Failures of T91 tubes and Type IV crackings of T91 welded joints often happen in service.In addition,threadlike structures are often found in quality inspections of P91/P92 steel welded joints.All of these indicate that safety risks may exist in the(ultra)supercritical power units,leading to a worldwide concern in the related fields.However,there is still a lack of relevant scientific evidences or technical standard/method to find out the cause and assess safety of P91 components.Therefore,it is important and necessary to carry out the relevant work.Phase parameter analysis(1)The effect of phase parameter variation on hardness of P91 components subject to a series of power plant service exposures at 530?550?/3.53?17.4MPa/57,000?63,000h was investigated.Equations describing a quantitative relation between total amount of carbon in matrix(Pc-matrx)and hardness value(HV0.1)with volume fractions and equivalent diameters of M23C6 carbides and Laves phase(VM23C6,VLaves dM23C6 and dLaves)of the P91 components were established,respectively.The reason why the microstructures of the components degraded prematurely was revealed.Aberrant thermoforming,heat treatment and service conditions of the components were main cause(s)for an over amount precipitation of M23C6 accompanied by decarburization of martensite and formation of ferrite.The interaction and coarsening mechanisms of M23C6 carbide and Laves phase were analyzed.The elemental exchanges of Cr,Mo and Si between phases and the interactions between precipitates and boundaries were also discussed.(2)Creep rupture tests were performed at 625 ? and 150MPa,and rupture times,1354 and 167 hrs,were obtained on the P92 samples from two manufacture sources.The effect of phase parameter variations of M23C6,Laves phase and MX on creep rupture life were investigated.The results revealed the mechanism of effect of alloy design and heat treatment process on the phase stabilities and properties of the steel.The precipitation behavior of Laves phase precipitated in and around ?-ferrite was also discussed.(3)Phase parameter analysis of two T91 superheater tubes which bursted after service exposures was carried out.The results revealed the main reason and mechanism for the tube failures.The experimental and theoretical evidences for the existence of defects and over-heating in straight segment of the T91 tubes were provided,which will facilitate the on-site failure analysis of this steel.(4)Phase parameter variations of typical regions(CGHAZ,BM,FGHAZ)of a T91 welded joint taken from a high temperature reheater with type IV cracking were analyzed.A simulation test was carried out to find out the effect of grain size on M23C6 precipitation and hardness of the steel.The results indicated that fine grain structure promotes the property degradation.(5)Threadlike structures of P91 and P92 welded joints were investigated by phase parameter analysis and thermodynamic calculations.The phase constitution and formation mechanism of the threadlike structure were revealed.The 650? ageing treatment shows that the threadlike structures are detrimental to the long term performance of the welded joints.Creep rupture strength and safety assessmentA function set is established to predict/extrapolate 570?/105h creep-rupture strengths of Grade 91 components with a Brinell hardness value between 145 and 205 after service exposures at 530?610?.RT,550 and 600? instant tensile behavious of service exposed Grade 91 component specimens with a variety of hardness values were investigated.A reversed change of yield ratio with hardness reduction indicates a critical change in safety of the service exposed components after tensile tests at 550 and 600?.An approach was proposed to assess the component safety due to lack of relevant technical standard/method.The results of this work can be referenced to analysize the causes of premature failure and assess the properties and safety of this class of steel components after service exposures.