Effect Of Heat Treatment Process On Second Phase Redissolution And Precipitation Behavior Of COST E Rotor Steel

At present,China’s energy structure is still dominated by thermal power,and the energy consumption and environmental pollution problems caused by thermal power generation are becoming increasingly serious.High energy efficiency and low pollution ultra supercritical coal-fired power generation technology has become one of the important measures to achieve coal consumption upgrading and energy-saving and emission reduction strategic goals.The development of ultra supercritical coal-fired power generation technology is constrained by the performance of the rotor material of the unit.Using different heat treatment processes to regulate the second phase type,morphology,and distribution of the rotor material is the main way to improve the performance of the rotor material.Therefore,studying the evolution law of the second phase under different heat treatment process conditions is beneficial for improving the comprehensive mechanical properties of rotor materials,and laying a theoretical foundation for improving and optimizing existing heat treatment process parameters.This article studies the microstructure and second phase transformation of forged and annealed COST E steel under different solution temperatures,solution holding times,and solution cooling rates through solid solution treatment experiments.The results show that the precipitated phases in the quenched COST E steel after forging mainly include M₂₃C₆phase and MX phase;After 4 hours of solution treatment at 920℃,the M₂₃C₆ phase begins to dissolve back in the matrix,and completely dissolved back at 950℃.After solution treatment at 920℃～1160℃,the content of MX phase gradually decreases and the size remains basically unchanged;During the insulation period of 0.5～1h at 1070℃,the M₂₃C₆ phase begins to dissolve back,while the quantity and size of MX remains basically unchanged.The insulation time is longer than 2h,and the M₂₃C₆ phase completely dissolves back,and needle like M₃C phase precipitates during the air cooling process;After 4 hours of solid solution insulation at 1070℃,a small amount of spherical MX phase is mainly observed in the water cooled and oil cooled tissues,with no M₂₃C₆ or needle like M₃C phase observed.However,a large amount of spherical MX phase,needle like M₃C phase,and M₂₃C₆ phase are mainly observed in the air cooled and furnace cooled tissues.Through the first tempering treatment experiment,the precipitation pattern of the second phase in the solid solution COST E steel is studied under different first tempering temperatures and holding times.The results showed that the first tempering of COST E steel is carried out at 490～690℃,and the transformation pattern of precipitated phases at different temperatures is as follows:M₃C+M₇C₃+MX（490～530℃）→M₂X+M₇C₃+MX（530～610℃）→M₂₃C₆+MX（610～690℃）;The transformation pattern of precipitated phases within 36h of the first tempering at 570℃:M₃C+M₇C₃+MX（<1h）→M₂₃C₆+MX（≥18h）;The transformation pattern of precipitated phases within 18h of the first tempering at 690℃is as follows:M₃C+M₇C₃+M₂₃C₆+MX（<0.5h）→M₂₃C₆+MX（≥1h）.Through secondary tempering treatment experiments,the second phase transformation law of COST E steel in the primary tempering state is studied at different secondary tempering temperatures and times.The results show that the precipitated phases in the microstructure of COST E steel after secondary tempering are mainly M₂₃C₆ phase and MX phase.During the process of secondary tempering temperature increasing from660℃to 720℃,the number of precipitated phases decreases and the distribution tends to become uniform;Within 48 hours of insulation at 690℃,the number and size of M₂₃C₆phases gradually increases,and their distribution tends to become uniform.