ABSTRACT

The effect of microstructure evolution induced by low-cycle fatigue behavior on the corrosion of welded joint in the simulated environment of low pressure nuclear steam turbine was comprehensively investigated. The microstructure evolution was observed through optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Corrosion susceptibility of welded joint was examined by traditional electrochemical measurement and scanning vibrating electrode technique (SVET) measurement. The traditional electrochemical results showed that weld metal was the most susceptive zone with the lowest corrosion potential and the highest corrosion current density in the welded joint, which was in consistence with the results of SVET. In addition, the corrosion resistance of each zone in the post-low-cycle fatigue specimens was higher than that of as- received specimens, which was caused by the lower dislocation density due to low-cycle fatigue behavior.

INTRODUCTION

It is difficult to manufacture the nuclear steam turbine rotor over 1000MW through the total forging process with the increasing power capacity of power plants. As one of the easier and more feasible manufacturing methods, welding not only simplifies forging procedures, but also welded rotors work well in operation. However, the non-uniform distribution of microstructures and chemical elements are introduced by the multi-welding process, which may lead to galvanic corrosion when welded joints are immersed in the corrosive environment.

The corrosion susceptibility of alloy steels depends on their microstructures.1,2 Also the microstructure evolution caused by the deformation could also change the corrosion property of materials. Many researches revealed that plastic deformation during tensile process would accelerate the corrosion activity significantly.3,4 This acceleration of corrosion rate is attributed to the increase of dislocation density.3 On the other hand, the corrosion susceptibility is also affected by shot peening, cold working and grain refinement due to heat treatment. Wang et al.5 studied the effect of shot peening on the corrosion of 1Cr18Ni9Ti stainless steel, and found the corrosion resistance was enhanced by the surface nano-crystallization due to shot peening. Ren et al.6 also discovered the grain refinement induced by shot peening led to better corrosion resistance of ferritic-martensitic steels in the supercritical water solution. Except for grain refinement, the partial increase of corrosion resistance is attributed to the residual compressive stress on the surface after shot peening.

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