TMCP Processing Of 800-MPa-Grade Steel Used For Hydropower Penstocks And Microstructure And Properties Of Welding Heat Affected Zone

At present,800-MPa-grade steel used for hydropower penstocks is delivered only in a quenched and tempered condition.Development of 800-MPa-grade non-quenched and tempered steel used for hydropower penstocks is an effective way to enhance production efficiency and lower cost.For this purpose,thermomechanical simulation of TMCP was performed for a self-designed non-quenched and tempered steel.The effect of TMCP processing parameter on microstructure and mechanical properties was studied and TMCP processing parameter was optimized.In addition,the microstructure and mechanical properties in simulated welding HAZ of the TMCP samples was examined.This could be expected to provide the references for the development of 800-MPa-grade non-quenched and tempered steel used for hydropower penstocks.The results of TMCP simulation show that during the controlled rolling the appropriate rough rolling temperature and reduction are selected to be 1050 oC and 30%,respectively,which can refine grains.During the controlled cooling stage,start cooling temperature is closer to the finish rolling temperature,the finer the microstructure after rapid cooling is.The optimum start cooling temperature is ranged in 780~800 oC.The increase in cooling rate can refine the microstructure,but reduce the impact toughness;therefore,the suitable cooling rate is 10~20 °C/s.The finish cooling temperature(FCT)has the greatest influence on microstructure and mechanical properties of the TMCP steel.As the FCT decreases from 550 °C to 350 °C,the mechanical properties of simulated TMCP specimens are non-monotonic functions of the FCT.The minimum values of yield and tensile strengths and hardness and the maximum value of impact absorbed energy occur at the FCT of 450 °C.The microstructures are primarily composed of granular bainite(GB)and lathy bainite(LB)in samples with different FCTs.The decrease in the FCT can result in the increase in the volume fraction of LB and the decrease in the volume fraction of GB,and the decrease in the amount and the size and more dispersive distribution of M/A constituents.The optimum FCT of the TMCP for the steel is around 400 °C,which results in the room-temperature yield strength of 820-830 MPa and the-20 °C impact absorbed energy of 52-66 J.The SHCCT curve of the TMCP steel was achieved using the welding thermal simulation.The results indicate that the CGHAZ microstructures are composed of LB and GB in a large range of cooling rates(1~30 °C/s).Reducing the cooling rate is beneficial to the formation of GB,but increasing the cooling rate is beneficial to the formation of LB.Based on Johnson-Mehl-Avrami-Kolmogorov model,the phase transformation kinetics of HAZ in the steel was obtained,which show the two-stage phase transformation,that is,diffusion-controlled growth and interface-controlled growth.As the cooling rate increases,the diffusion-controlled phase transformation is promoted,and the interface-controlled phase transformation is retarded.Thermal simulation study on the welding HAZ in the TMCP steel shows that the CGHAZ microstructures are composed of LB and GB in various heat inputs.With increasing the heat input,the prior austenite grain size and the volume fractions of GB and M/A constituent increase,whereas the volume fraction of LB decreases;meanwhile the volume fractions of HAGBs and LAGBs decrease and increase,respectively.Compared with the base metal,the CGHAZ is not seriously softened for the low heat input,but seriously softened for the high heat input.When the heat inputs are 18 kJ/cm and 25 kJ/cm,the-20 °C impact absorbed energy of CGHAZ increases by 359.3% and 367.8% compared to the base metal,respectively,and the 50% FATT is lower than-50 °C.When the heat input is more than 44 kJ/cm,the-20 °C impact absorbed energy of CGHAZ decreases rapidly.When the heat input is in the range of 18~25 kJ/cm,the CGHAZ has high impact toughness and is not softened seriously.Accordingly,the optimum heat input is between 18 kJ/cm and 25 kJ/cm.For the heat input of 25 kJ/cm,the microstructure in the subcritical heat affected zone(SCHAZ)is similar to the base metal,which is composed of LB and GB and a large amount of carbides.In the intercritical heat affected zone(ICHAZ),the finer LB+GB is formed by phase transformation from austenite and quasi-polygonal ferrite is formed by recovery and recrystallization of prior GB,and others remain the original morphology.Fine and coarse grain zones are composed of fine and coarse LB+GB,respectively,which are not softened.A certain degree of softening occurs in both SCHAZ and ICHAZ.The best impact toughness region is FGHAZ.The impact toughness in ICHAZ and SCHAZ is relatively lower;however,compared to the base metal,embrittlement phenomenon does not occur in the HAZ in the tested steel.