Study On Structural Heredity Of 30Cr2Ni4MoV Steel For Steam Turbine LP Rotore Of Heavy Forgings

Steam turbine LP rotor forging is the most important part of power equipment and 30Cr2Ni4MoV steel is one of the most widly used material for LP rotors. In this paper, the phenomenon of structural heredity of 30Cr2Ni4MoV steel and heat treatment technique for eliminating structure heredity were studied. On the basis of several times'high-temperature normalizing with disadvantages of high normalizing temperature, long production cycle, large power consuming and heavy pollution, two kinds of new after-forged heat treatment techniques, "prior high-temperature temper, critical and routine normalizing" and "prior high-temperature temper and normalizing", with high efficiency and short process were proposed. Compared with that of several times'normalizing process, mechanism of eliminating structural heredity and effect of grain refinement were studied after treatment of new techniques. Samples'microstructures were analyzed by modern analysis and testing technologies, such as optical microscope(OM), scanning electron microscope(SEM), X-ray diffraction(XRD) and transmission electron microscope(TEM). Finally, impact toughness, tensile property and hardness were tested.The experimental results are showed as follows. Prior high-temperature temper can promote a-phase recovery and recrystallization, critical normalizing is beneficial to spherical austenitic nucleation and growth and routine normalizing boosts a-y recrystallization. The process of "prior high-temperature temper, critical normalizing and routine normalizing" utilized the above three styles synthetically to refine coarse austenite grains to grade 8.0-9.0 which is higher than that of several times'high-temperature normalizing technique. Prior high-temperature temper can transform the morphology of lath martensite and needled bainite and promote dispersion of tempered structure, which is good for postponing formation of flake austenitic. The longer holding time of prior high-temperature temper increased, the better effect of structural heredity was put off. Different grain size grade and contents of retained austenite were obtained after processes of "prior high-temperature temper and normalizing". The lowest grain size grade 7.5 and minimus content of retained austenite 0.73% were obtained by of 690℃×8h prior temper and 810℃×4h normalizing. Maximum contents of retained austenite achieve 2.06% after 690℃×8h and 790℃×4h treatment. Retained austenite, the nucleation core of needled austenite, keeps K-S relationship with original a-phase, which will lead to structure heredity. Mechanical properties of three after-forged heat treatment samples are all meet or exceed the demands of performance index after hardening and tempering. In short, the techinques of "prior high-temperature temper, critical and routine normalizing" and "prior high-temperature temper and normalizing" decreased normalizing times, lowered normalizing temperature and greatly reduced energy consumption and pollution. This two kinds of techniques have very important practical value and can sever as the evidence of actual production.