Research On Nondestructive Test Technique For Detecting Temper Embrittlement Of Steam Turbine Rotor

The principle and character of the nondestructive test method for steam turbine rotor steel were reviewed. A kind of improved nondestructive test method based on electrochemical potential technique was proposed. The test of mechanical properties, metallography analysis and electrochemical potential was carried out on lab-charged phosphorus -doped 30Cr2MoV rotor steels. and then a predicting model for detecting nondestructively the fracture appearance transition temperature (FATT50) of the steels was constructed. The predicting model was verified on two simulate temper embrittled rotor steels and a retired turbine rotor, and was proved to be feasible and effective.The chemical composition, microstructure, granularity, strength, ductility, fracture appearance transition temperature (FATT50), hardness and electrochemical parameter for lab-charged materials were measured. It was found that phosphorus can strengthen the steels by solid-solution. However, an increase in phosphorus content resulted in an increase in the fracture appearance transition temperature (FATT50), and a decrease in the ductility and toughness.The step cooling heat treatment can increases the fracture appearance transition temperature (FATT50) by 13 to 57℃. It indicates that the step cooling heat treatment can be used to embrittle rotor steels and to simulate the temper embrittlement treatment during the long-term service of turbine rotor.Three kinds of electrochemical polarization test were carried out, such as potentiodynamic anodic polarization test, single loop electrochemical polarization reactivation (EPR) test and double loop electrochemical polarization reactivation (EPR) test, and then electrochemical parameters of the steels were obtained in different electrolyte at different temperature. The solution of sodium molybdate was firstly used as electrolyte in electrochemical polarization test for rotor steels, and electrochemical parameter was easy to get from the polarization curve by this method. The effect of electrolyte temperature on electrochemical parameter was studied, the result showed that it was necessary to add electrolyte temperature as a variable to the model for estimating FATT50of rotor steels.The models for predicted temper embrittlement of rotor steels were obtained by multiple linear regression, BP (Back-Propagation) neural network and genetic programming. The result showed that the prediction error of the models obtained by...