Study On Heat Treatment For Low Pressure Rotors Of30Cr2Ni4Mov Steel

Large-sized low pressure (LP) rotor of30Cr2Ni4MoV steel is one of the keycomponents in AP1000(1000MW-grade advanced passive pressurized water reactor)nuclear power unit. This study was accomplished around the establishment of heattreatment process for the LP rotors. TTT curves, CCT curves and phase transformationkinetics were investigated using dilatometer, and the detailed analysis of themicrostructure evolution under different holding temperatures and cooling rates werecarried out by optical microscopy (OM), scanning electron microscopy (SEM), X-raydiffraction (XRD), thermal simulation and mechanical property tests to demonstrate thecomplexity involved in the transformation in30Cr2Ni4MoV steel. The material database,including transformation kinetics, thermophysical properties and mechanical properties,was then set up by thermal expansion, drainage and laser pulse methods, and thedifferential scanning calorimetry as well. To investigate the effect and feasibility of grainrefinement in large-sized LP rotors, the grain evolution at the rotor center during variousstages of a multi-normalizing treatment was traced by combining numerical and physicalsimulations. As a novel grain refinement method, the isothermal pretreatment was thenproposed based on the partial decomposition of austenite into pearlite. A numerical modelcoupling temperature, transformation and stress/strain was established to predict theirevolution during quenching process of LP rotors. The heat treatment processes wereestablished for the typical LP rotors with diameter of1768mm and2826mm respectively,and the suggestions on the corresponding equipment improvement and engineeringimplement were proposed based on the results of trial production as well.The austenization kinetics of30Cr2Ni4MoV steel was investigated to guide theformulation of the heating process. The dilatometric curves with the heating rates from0.008to20°C/s were measured, and the austenization kinetics was then analyzed usingKissinger method based on the non-isothermal Johnson-Mehl-Avrami (J-M-A) model. The parameters in the model were obtained as the austenization activation energy Q,pre-exponential factorln k0and J-M-A exponent n being about2.367×106J/mol,0.2448and270.5, respectively. The isothermal austenization kinetics curves could thus bedetermined by setting the temperature in the J-M-A equation as different values. Thisindicates that the isothermal transformation kinetics curves could be derived by extractinguseful information directly from continuous transformation kinetics, in the case ofextremely short incubation period causing it difficult to be accurately measured.TTT and CCT curves were measured and analyzed in detail to guide the formulationof the quenching process for LP rotors. The critical cooling rate to form pearlite is about3.3°C/h and that for mixture of matensite and bainite is1°C/s. Based on the carefulcomparison of microstructures from isothermal transformation and continuous coolingtransformation, the morphological characters and microstructure evolution uponcontinuous cooling of different rates were obtained. When the cooling rate is between5-10°C/s, only martensite forms and the amount of self-tempered martensite graduallyincreases as cooling is slowed down. When the cooling rate is between1-2°C/s, thetransformation products are martensite and a certain amount of lower bainite, and thecarbide particles becomes more and larger as for slower cooling. When the cooling rate isbetween0.2-0.5°C/s, martensite and low temperature upper bainite are the maintransformation products. When the cooling rate is between0.01-0.1°C/s, thetransformation products are mainly massive ferrite, granular bainite and coarse upperbainite.The relationship of cooling rate, microstructure and hardness on different positions ofLP rotor was investigated by microstructure observation and quenching simulation. It isobvious that the cooling rates at different positions of LP rotors are significantly different,leading to great differences in microstructure and mechanical properties. At the surfacepoints the average cooling rate in the medium temperature transformation region could behigher than10°C/s, and thus the hardness could reach up to500HV due to formedmartensite. The average cooling rate at the central points was nearly0.01°C/s, and granular bainite and massive ferrite having a negative impact on mechanical propertiesformed and resulted in the hardness below370HV. Therefore, when the cooling rate variesfrom10to0.01°C/s, the hardness of transformation products gradually decreases as thecooling rate decreases, and a turning point at cooling rate of0.2°C/s exists on the curvebetween cooling rate and hardness. It is demonstrated that various kinds of bainite havesignificant differences in morphology and mechanical properties, and the cooling rate atwhich no coarse upper bainite or massive ferrite (0.2°C/s) would form should be regardedas the criteria, rather than that of no proeutectoid ferrite or pearlite (3.3°C/h), for thedevelopment of quenching process for the LP rotors in the practical production.The material database, including thermophysical and mechanical properties, wasestablished by systematical measurements, providing the input data for heat treatmentprocess simulation later. Thermal expansion coefficient, density, heat capacity, latent heat,thermal conductivity, elastic modulus, yield strength and plastic modulus were tested andformulated into functions of temperature. The relationship of temperature, equivalentstress and modeling parameters were set up experimentally. The parameters wereextracted and formulated into the functions of temperature and equivalent stress based onGreenwood-Johnson phase transformation plasticity model and thermal-mechanicalsimulator experiments. The results showed that the effect of applied stress onM sand thecoefficient could be neglected for martensitic transformation. However, for bainitictransformation, the incubation periodt sincreases with increasing temperature, while theparameter n decreases as temperature and equivalent stress increase, and the parameterb is almost a constant. The transformation plasticity parameter K for martensite andbainite were found as8.771MPa-1and8.9488MPa^-1, respectively.The main purpose of the heat treatment after forging was to eliminate the coarsemicrostructure and refine the grains. The effect of different heat treatment processes ongrain refinement was comparatively investigated. The grain size evolution at the centerpart of the2900mm LP rotor during multi-normalizing was studied by the combinationof FEM numerical and physical simulations. The multi-normalizing process to meet the requirements of grain refinement was therefore recommended. A novel grain refinementmethod through isothermal pretreatment based on partial decomposition of austenite intopearlite was proposed and verified experimentally. The results indicate that the proposedmethod has the same effect as the four times normalizing process but exhibits clearadvantage of timesaving and energy efficiency. Then, the heat treatment processes afterforging based on multi-normalizing and isothermal pretreatment were formulated for both1800mm and2900mm LP rotors.A numerical model coupling temperature, transformation and stress/strain wasestablished to analyze the distribution and evolution of temperature, phase transformationand stress during quenching process of LP rotors. The simulated results showed thattransformation strain and transformation plasticity play important roles on the evolutionand distribution of the stress. The results also indicated that the change of water sprayingintensity in a range of10¹00(l/m2s) could hardly affect the cooling time and the averagecooling rate at the center part of LP rotors in the bainitic phase transformation zone(300⁵00C), and the spraying time needed was mainly determined by the sectionaldiameter of the rotor, providing a basis for the design and improvement of quenchingequipment. Besides, the heat treatment processes were developed for the typical LP rotorswith both diameters of1768mm and2826mm, and the former had been validated by trialproduction, in which the mechanical properties could comprehensively meet therequirements.