Research On Mechanical Properties Of New Austenitic Heat-resistant Stainless Steel C-HRA-5

The new austenitic heat-resistant steel C-HRA-5 is a new generation of stainless steel for thermal power units independently researched and developed in China.A complete production line for this steel from raw materials to use has been developed in China,and it has the capacity for mass production.Compared with traditional austenitic heat-resistant steel,C-HRA-5 has more excellent high temperature resistance and oxidation resistance.It can be used for a long time in harsh environments such as high temperature and pressure with flue gas corrosion.It is the preferred material for critical high-temperature components(superheater,reheater)for ultra-supercritical(USC)units.The performance is comparable to the Sanicro25 developed by the Swedish Sandvik company,and it has the ability to compete with similar materials at home and abroad.However,because limited research on mechanical properties and high temperature corrosion resistance of C-HRA-5 steel,and it has not been used for a long time in China,it has not been recognized by the industry.Therefore,it is necessary to accumulate experimental data to prove its outstanding mechanical properties under service conditions and the corrosion resistance superior to traditional steel grades.This article mainly studies the basic mechanical properties,fracture mechanism and fatigue resistance of C-HRA-5 steel under service environment.A uniaxial tensile test was performed on C-HRA-5 steel at room temperature and 700?.At room temperature,the strength and toughness of heat-resistant steel decrease with increasing loading rate,while the strength and toughness increase at high temperature and high loading rate.In addition,although the high-temperature strength is lower than room temperature,it the material shows superior overall performance than other similar austenitic heat-resistant steels.At low displacement rates,the hardness of heat-resistant steel at high temperatures is lower than at room temperature;while at high displacement rates,the hardness at high temperatures is significantly greater than room temperature.At room temperature and 700?,C-HRA-5 steel exhibited the characteristics of transgranular fracture.At room temperature,more dispersed MX phases are precipitated in the crystals,while M₂₃C₆ is relatively less precipitated at the grain boundaries.At high temperatures,M₂₃C₆precipitates at the grain boundaries increase,and the MX phases are analyzed inside the crystals to dissolve into the matrix at high temperatures.Spherical inclusions were found at the bottom of a few ductile dimples,mainly spinel and alumina,and the brittle phase MX was easy to nucleate on the surface of the inclusions.Therefore,controlling the inclusion elements has a great promotion effect on the mechanical properties of heat-resistant steel.C-HRA-5 steel was subjected to an in-situ tensile test at room temperature and 700?.The in-situ tensile crack initiation and propagation process of C-HRA-5 steel can be divided into three parts:microcrack nucleation,microcrack propagation,and crack connection.At the stage of microcrack initiation,under the action of normal stress?,microcracks were generated at the trigeminal boundaries where the precipitated phase Cr₂₃C₆ agglomerated at both temperatures.In addition,the MX phase precipitates inside the grains of the room temperature specimen also produced brittle cracks.During the crack propagation stage,the grain boundary microcracks of the two temperature specimens are extended and connected to each other.During the crack connectivity failure stage,under the action of shear stress?,the grain boundary microcracks and precipitated phase microcracks of the room temperature specimens rapidly expand and finally reach each other.The micro-cracks in the local area of the high-temperature specimen propagate into the crystal along the shear direction and form transgranular cracks.Low-cycle fatigue tests were performed on C-HRA-5 steel at room temperature and 700?.At room temperature,with strain amplitudes of 0.3 and0.35,the specimen undergoes a rapid softening-stability-rapid softening stress change process during low cycle fatigue;at 700?,the sample steel undergoes low cycle fatigue Stress change process after rapid hardening-stability-softening.At high temperatures,as the strain amplitude increases,the degree of cyclic hardening of heat-resistant steel during fatigue deformation will increase.The different strain amplitude hysteresis rings of the heat-resistant steel at room temperature and 700?basically keep coincide,showing good Masing characteristics,indicating that the elastic part of C-HRA-5 steel has not changed or changed slightly during loading.Heat-resistant steel shows better mechanical stability under the dual effects of force and temperature.Under both temperature environments,the irreversible deformation work absorbed by C-HRA-5 steel under low-cycle fatigue test conditions increased significantly with the increase of the total strain amplitude,indicating that the cyclic toughness of the heat-resistant steel gradually increases during the loading process.Fatigue cracks originate from the surface of the specimen to form the main crack source,and then the main crack source propagates radially into the specimen.C-HRA-5steel has relatively many MX phases in the low-cycle fatigue structure at room temperature,which is one of the main reasons for its high strength and poor toughness at room temperature.And the fracture surface has obvious cleavage fault characteristics in the transient fault area.The high-temperature transient fault zone is a typical ductile fracture,and the toughness of C-HRA-5 steel is relatively good under high temperature conditions.