Study On Hot Deformation Behavior Of New Austenitic Heat Resistant Steel For Ultra-supercritical Thermal Power

Super304H austenitic heat resistant steel has become the first-choice material for boiler superheater and reheater of ultra-supercritical(USC)thermal power plant because of its excellent high temperature creep strength and good resistance to steam oxidation and flue gas corrosion.Though China has realized the localization of Super304 H and met the ASME standard,there is still a large gap in the production quality in terms of microstructure stability between China and foreign countries.In order to realize the independent innovation of key materials for thermal power plant and develop materials with excellent high temperature performance,the research group has developed a new austenitic heat resistant steel CHDG-A by composition optimization of reducing C and increasing N based on Super304 H.The preliminary results show that the new steel has superior service properties than Super304 H such as high temperature strength,creep resistance,oxidation resistance,corrosion resistance,etc.However,no research concerning hot-working properties of this new steel has been done so far.And USC boiler pipes need to experience some processing technology like hot punching before equipping,so studying the high temperature plastic deformation behavior provides references for hot-working process optimization in order to control the microstructure evolution and properties of the final products.In this paper,single pass hot compression tests of CHDG-A steel are performed on Gleeble 3500 thermal-mechanical simulator in the temperature range of 950~1100? and strain rate range of 0.01~1s-1 to investigate its hot deformation behavior systematically from different aspects,such as flow stress curves,high temperature deformation structures,dynamic recrystallization critical model,high temperature constitutive relationship and hot processing map.The main conclusions and innovation points are as follows:The flow stress curves of new CHDG-A steel can be divided into three types: work hardening type,dynamic recovery type and dynamic recrystallization type,of which dynamic recrystallization is the main softening behavior.The effects of hot deformation parameters on high temperature flow stress and microstructure evolution of CHDG-A steel are studied based on flow stress curves and the OM and TEM technology.The results show that when deformation temperature increases and strain rate decreases,flow stress decreases and dynamic recrystallization tends to complete,with grain size being larger.The dynamic recrystallization nucleation mechanism of CHDG-A steel is revealed to be the bulging of grain boundary for the primary and the growth of substructure as a supplement by observing the substructure morphology under different strain.The critical condition of dynamic recrystallization of CHDG-A steel is confirmed by using three polynomials to fit the relationship curve of ln??-and the critical strain model is established;the Arrhenius high temperature constitutive equation is constructed to predict the flow stress during hot deformation process;and the hot processing map based on dynamic material model(DMM)theory of CHDG-A steel is established to attain the machinable safety zone and rheological instability zone at strain of 0.6 so as to get the optimum hot-working parameter range of CHDG-A steel,which is finally determined to be: 1000~1075??0.01~0.1s-1.The numerical models above can provide theoretical basis for the final hot-working process of this new steel.