Study On Evolution Regulation Of Microstructure And Properties Of Foreign P92 Steel During The Heat Treatment

P92 steel is a new type of ferritic heat-resistant alloy steel,which is widely used in ultra-supercritical thermal power unit steam boilers and steam pipes due to its excellent high temperature performance.In recent years,China is in the high-speed development period of ultra-supercritical units.And P92 steel has great application prospects.Although China has initially possessed the production capacity of P92 steel,some properties gaps between domestic materials and foreign materials still exist.The properties of the materials are closely related to the heat treatment process.Therefore,the study of the microstructure evolution of foreign P92 steel in different heat treatment processes and the exploration of the final heat treatment process of foreign P92 steel are of great significance to optimize the heat treatment process of domestic P92 steel and improve the final performance of domestic P92 steel products.In this paper,the phase transition time and transition temperature of foreign P92steel at different isothermal temperatures and different cooling rates were studied using thermal expansion instrument.The observation of different cooling rates and microstructures at different isothermal temperatures was carried out by OM and SEM,combined microhardness determination,the supercooled austenite continuous cooling transition curve?CCT diagram?and isothermal transformation curve?TTT diagram?of foreign P92 steel are drawed.Secondly,the different heat treatment of foreign P92 steel was carried out.The microstructure evolution of foreign P92 steel under different heat treatment processes was analyzed by OM and SEM.And foreign P92 steel samples after different temperature quenching were corroded using electrolytic corrosion method and the original austenite grain size was determined by means of OM.Combined with the heat treatment process of domestic P92 steel,the foreign P92 steel which is quenched at1060?and tempered at 760?is subjected to normal temperature stretching and room temperature impact and short-term high temperature stretching at 450?,500?,550?,600?and 650?.Experiments were carried out to determine its tensile properties at normal temperature,impact properties and short-term high-temperature tensile mechanical properties,and observation and analysis of fracture morphology by SEM.The results show that the critical point of phase transformation of foreign P92 steel is:A_c11 is 792.4?,A_c33 is 879.8?,M_s is 372.3?,M_f is 233.3?.The continuous cooling curve?CCT diagram?and the isothermal transformation curve?TTT diagram?both include proeutectoid ferrite,pearlite,medium-temperature bainite,and low-temperature martensite in the high temperature zone,and the martensite transition zone partially overlaps with the bainite transformation zone.After the quenching of foreign P92 steel at 1040?¹080?,lath martensite appears.As the quenching temperature increases,the martensite structure becomes coarser,the second phase undissolved material decreases,the hardness gradually decreases,and grain size increased from 9 to 7 grade.Foreign P92 steel after 1060?quenching+740?,760?,780?isothermal tempering 7h heat treatment,as the tempering temperature increases,the structure is transformed from tempered martensite+tempered torsite to tempered torsite,tempered tortite+tempered sorbite.Foreign P92 steel is quenched at 1060?+760?isothermal for 1h⁷h after tempering heat treatment,with tempering isothermal time prolonged,the tissue is tempered Martensite evolved into tempering torsite.After tempering for 5h,the microstructure transformation was basically completed.At the same time,the second phase particles decreased with increasing tempering temperature,the particle size became larger and tends to be uniform,and the second time with tempering time was extended.The number of phase particles increases,the particle size becomes larger and tends to be uniform.After foreign P92 steel is quenched at 1060?+760?tempering heat treatment,the yield strength of the material at normal temperature is reduced by 25 MPa,the tensile strength is basically unchanged,the elongation is increased by 3.5%,and the section is improved.The shrinkage rate increased by 10%,and the impact energy KV₂increased by 13.1J.In the short-time high-temperature tensile test,the tensile strength increased,the strength of the sample decreased,and the plastic toughness increased.At the same stretching temperature,the heat-treated state was abroad.The strength of P92steel at high temperature is lower than that of the use state,but the plasticity and toughness are improved.