Hot Workability Of Austenitic Heat Resistant Steel For Ultra-Supercritical Thermal Power Units

Super304H austenitic heat-resistant steel is widely selected for superheater and reheater pipe of ultra-supercritical thermal power units due to its excellent microstructure,properties and economic advantages.The higher parameters of global thermal power units require better superheaters and reheater pipes materials.CHDG-A austenite heat-resisting steel was designed and developed through improving the composition of Super304H austenite heat-resisting steel.Our previous studies showed that the microstructure,strength,creep property and oxidation resistance of CHDG-A austenitic steels at high temperature are in the same level or even better than Super304H austenitic heat-resistant steel due to the lower C and higher N.This paper studies the hot processing property of these two materials to provide scientific guidance for the optimization of the hot processing technology.The hot compressions experiments of two kinds of austenitic heat resistant steel（Super304H/CHDG-A）were carried out at 950-1100℃/0.01-1 s^-1 on Gleeble–3500to study the hot workabilities.The flow stress curve was obtained,and the relationship between hot processing parameters and the flow stress curve was discussed;The relation between hot processing parameters and microstructure was established;The high temperature constitutive equations is constructed considering strain compensation;The DMM hot deformation map was constructed.We can draw these conclusions:（1）The flow stress curve can be divided into three types:WH,DRV and DRX.The curve type of the two materials under the same hot deformation parameters is consistent,while the value of stress has no definite relation.The strain rate,temperature and strain are three main factors that affect the hot processing behavior of austenitic heat-resistant steel.The flow stress curve under low strain rate and higher temperature is more likely to be DRX-type characteristics with lower stress value,and dynamic recrystallization happens in this condition.The stress is lower with larger strain value under a given temperature and strain rate.（2）The Arrhenius constitutive model considering the influence of strain is developed by considering the material constants as a sixth polynomial function of strain.The constitutive model is used to predict flow stress values.The comparation between predict values and tested ones shows that the predictable efficiency of the high temperature constitutive model is high.（3）When the strain increases,the region of flow instability is smaller,while the distribution of power dissipation coefficient is assemble.The hot processing map based on the dynamic material model（DMM）shows the proposed heat processing parameter range of these two austenitic heat resistant steel.The best hot processing parameter of Super304H is:1100℃、0.01 s^-1.The optimal hot processing range:1060-1100℃、0.01-1.8 s^-1;The best hot processing parameter of CHDG-A is:1050℃、0.01 s^-1,and the optimal hot processing range is:1000-1070℃、0.01-0.18 s^-1 and 1070-1100℃、0.1-5 s^-1.