The Effect Of Heat Treatment System On The Microstructure And Properties Of G115 Steel Castings

At present,the actual generating capacity of China’s thermal power is more than60%,and it is still the main force to ensure China’s power safety and power supply.However,supercritical and subcritical units below 600,000 account for more than 55%.These small units have small capacity,low parameters and high coal consumption,which seriously affect the overall carbon emissions of the thermal power industry.Therefore,ultra-supercritical power stations with large capacity,high parameters and low energy consumption are one of the important ways to clean and efficient utilization of coal,which will strongly support the national strategy of energy conservation and emission reduction and carbon peak carbon neutrality.G115steel is a new type of martensitic heat resistant steel independently developed in China.It is the only optional material for boiler assembly box and main steam pipeline of the national 630℃ultra-supercritical secondary reheat power demonstration project.It has a batch and stable supply capacity.On the turbine side,G115 steel and its improved type is one of the candidate materials for turbine rotor,cylinder and other forgings and castings of 630℃ultra supercritical unit.In this paper,the industrial scale 10 ton G115 steel castings as the research object,focus on the study of its heat treatment system,to investigate the influence of different heat treatment process on the microstructure and mechanical properties of G115 steel castings,preliminary to obtain the temperature parameters affecting the microstructure and properties.The whole heat treatment verification and comprehensive performance evaluation of 10T G115 large casting were carried out.The main conclusions are as follows:Thermodynamic calculation results show that the main equilibrium precipitates of G115 martensitic heat resistant steel are Mx phase（FCC-A1#2:Nb C;FCC-A1#3:VN）,M₂₃C₆,Laves phase and Z phase,and this calculation result was also verified in the experiment.The resolution temperature of MX type Nb C（FCC-A1#2）is1130℃.The resolution temperature of MX-type VN（FCC-A1#3）is 942℃.The dissolution temperature of Laves phase is about 817℃.The resolution temperature of M₂₃C₆is 878℃.The resolution temperature of Cu-rich phase is 734℃.With the increase of C content,the precipitation amount of Laves phase decreases,while the precipitation amount of M₂₃C₆increases.Cr content has an effect on the precipitation of VN type MX phase,but the effect is not significant.With the increase of W content,the precipitation amount of Laves phase will increase accordingly.Nb content has a significant effect on the precipitation of Nb C-type MX phase.However,B had no significant effect on the precipitated phase.It is recommended that the content of W should remain about 3%above the upper limit of G115 steel.Under the optimal heat treatment condition,the tensile strength of G115 steel castings is increased from 772.5MPa to 778.5MPa at room temperature,the tensile strength is increased from 328MPa to 372.5MPa at 650℃,and the impact energy is increased from 55J to 122J after homogenization annealing.Compared with the unhomogenized samples,the precipitated phase distribution of the samples after homogenized annealing is more uniform,and the mechanical properties of the samples are significantly improved.It is recommended that the G115steel casting be treated by homogenizing annealing first,followed by heat treatment:the process parameters are normalizing 1070±10℃,air cooling;The tempering temperature is 780±10℃,the holding time depends on the wall thickness,air cooling.The tensile strength at room temperature≥660MPa,the yield strength≥560MPa,and the average impact energy at room temperature≥30J of the industrial scale 10T G115 steel castings after the industrial integral heat treatment.650℃high temperature tensile strength≥300MPa,yield strength≥250MPa.In terms of microstructure,there are many regions of Laves phase segregation in the microstructure.Compared with laboratory G115 steel castings,the precipitation phase distribution of 10t grade G115steel castings after industrial heat treatment is more dispersed,the number density is significantly increased,the size is significantly decreased,both martensite slats（block）and martensite slats（lath）are refined,and the dislocation has obvious rearrangement and annihilation.