The Phase Transition,Creep Property And Microstructure Evolution Of High Nitrogen 9Cr Martensitic Heat Resistant Cast Steel

At present,600?ultra-supercritical power generation technology is the most advanced commercial coal-fired power station technology in the world.The development of ultra-supercritical coal-fired power station technology with a grade above 630?will establish China's leading position in the world in thermal power technology and have important strategic significance for realizing China's"energy conservation and emission reduction"and"dual carbon"goals.Therefore,it is necessary to develop heat-resistant materials with better mechanical properties at high temperature as structural guarantee.A newly-developed high nitrogen(0.3%(mass fraction))martensitic heat-resistant steel was designed and prepared based on 9Cr heat resistant steel,aiming at improving its high temperature performance by precipitation strengthening of nitrides.The heat treatment process was optimized,the phase transformation process and strengthening mechanism were analyzed,and the microstructure evolution behavior under persistent stress was explored.It provides theoretical and experimental support for the research and development of new martensitic heat resistant steel and the subsequent industrialization of high nitrogen martensitic heat resistant cast steel.Based on Thermo-Calc simulated phase diagram and DTA measured results,a variety of heat treatment processes including homogenization,normalizing and tempering were developed.With the increase of homogenization and normalizing temperature,the microstructure of the experimental steel changes from complete martensite to two-phase martensite and?-ferrite.The tempering temperature only affects the width of martensitic lath,and the width increases with the increase of tempering temperature.Finally,the heat treatment process of homogenizing 1030?-normalizing 1050?-tempering 760?was developed.TEM results show that the microstructure is composed of fine tempered martensite,M₂N phase,MN phase,Laves phase and?phase,with a width of 220 nm and a dislocation density of 1.82±0.1×10¹⁴/m².The tensile strength,yield strength,elongation and area shrinkage of high nitrogen martensitic heat resistant cast steel at 650?are 409 MPa,295 MPa,30.2%and 76%,respectively.Its durable fracture time at 650?,140 MPa,155 MPa,160 MPa and 170MPa is 2425 h,986 h,787 h and 601 h,respectively.At 650?,81.4 MPa,620?,111.5MPa and 600?,139.8 MPa,the creep life can reach the target life of 100,000 h.During high temperature creep,the tempered martensite deforms significantly and the dynamic recrystallized grains and the original structure grow by strain-induced boundary migration mechanism.The degree of dynamic recrystallization increases with the increase of persistent stress,and the proportion of high-angle grain boundary increases obviously when the stress exceeds 155 MPa.Under low stress conditions,a large amount of martensitic lath is retained to delay the lasting damage.Under high stress conditions,the martensite lath,as an irregular dislocation network structure,recovers and forms high-angle martensite blocks and martensite bundles,resulting in the reduction of dislocation density and strength degradation.At the same time,under the action of persistent stress,the independent nucleated Laves phase coarsed and coalesced into a chain.In contrast,The MN type precipitates show strong thermal stability,and the fine MN phase has strong pinning effect on the boundary and dislocation.On the other hand,it is the core of some Laves phase nuclei,which limits the growth of Laves phase.Therefore,the new high nitrogen martensitic heat resistant cast steel exhibits better mechanical properties at high temperature than T/P91,T/P92and CB2 heat resistant steels.