Study On Microstructure And Properties Of G115 Martensitic Heat-resistant Cast Steel

G115 steel is a 650?martensitic heat-resistant steel with independent intellectual property rights developed by Central Iron and Steel Research Institute(CISRI)and Suitable for 650?ultra-supercritical coal-fired generating units.At present,its forgings have excellent properties and can be applied to large-caliber boiler tubes in ultra-supercritical power plants.but there is still little research on G115 martensitic cast steel at home and abroad.This paper study on the structure and properties of G115 castings,in order to provide technical support for the heat treatment process of casting components such as steam turbine cylinders and main steam valves of ultra-supercritical power plants.According to the computational analysis and research experience of the thermodynamic software Thermo-Calc G115 steel,the 25kg and 200kg grade cast steel were smelted by vacuum induction furnace.The effects of different heat treatment processes on the microstructure and mechanical properties of castings were studied.The microstructure was characterized by metallographic microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM).The thermodynamic calculation results show that the remelting temperatures of M₂₃C₆ phase,MX phase,Laves phase and Z phase of G115 steel given composition are878?,1171?,815?and 788?under equilibrium conditions respectively;The maximum precipitation of M₂₃C₆ phase increases with the increase of Cr and C content,the Nb C type MX phase increased with the increase of Nb and C content,the VN type MX phase increased with the increase of V and N content,and the Laves phase increased with the increase of W content.The remelting temperature of these phases also increases with increasing maximum content.Study on the influence of normalizing(quenching)temperature and tempering temperature on the microstructure and mechanical properties of G115 cast steel.As for the ingot,as the quenching temperature increases,the strength and hardness decrease while the impact energy increases when quenched at 1030?1050?.the hardness decreases while the strength and impact energy increase when quenched at 1050?1070?.the strength and impact energy decrease and the hardness increases when quenched at1070?1100?.As for the cast pipe is concerned,as the normalizing temperature increases,the strength,hardness and impact energy increase at normal temperature of1040?1100?.When the normal temperature is 1100?1130?,the change of mechanical properties is not obvious.Normalized(quenched)G115 cast steel is tempered between760?and 780?.They all have excellent plasticity and toughness,but the strength and hardness of the latter are lower.Only from the perspective of conventional mechanical properties,the recommended heat treatment process for G115 ingots is:(1070?1100°C)×1h W.Q.+780°C×3h A.C.and for G115 cast pipe is:(1100?1130°C)×1h W.Q.+780°C×3h A.C.Although the casting process conditions have no obvious effect on the strength and hardness of G115 cast steel,have a significant impact on the impact toughness.The impact toughness of the ingot is 17J/cm²,which is much lower than that of the cast tube58J/cm².The effect of furnace cold annealing on the impact properties of the G115 ingot is very significant.After homogenization annealing at 1230°C×6 h,1070°C×1 h F.C.+780°C×3 h A.C.,the impact toughness increased from 17 J/cm ²of the original ingot to 113 J/cm ² of the heat treatment.