Study On Stress Corrosion Cracking Behavior In The Welded Rotor Joint Of Nuclear Steam Turbine

Welded rotors have become a trend of rotor development in high-capacity nuclear steam turbine due to its advantages of compact structure and allowance of manufacturing large components with small forged parts.Nevertheless,for the welded rotor joint of different chemical compositions,heterogeneous microstructures and mechanical properties,the risk of stress corrosion cracking(SCC)will be promoted in the low pressure nuclear turbine with wet steam.The outline of this paper on SCC behavior of nuclear steam turbine Cr-Ni-Mo-V rotor steel welded joint in 3.5wt.%NaCl solution at 180? is in accordance with the three stages of SCC development,i.e.,formation of corrosion pit,pit to crack transition and corrosion crack propagation.The contents of this study are including:mechanisms of pit formation and evolution models of pit distribution in base metal(BM),weld metal(WM)and heat affected zone(HAZ)of welded rotor joint;the most sensitive zone to corrosion crack initiation and the effect of strain and stress on crack initiation mechanisms;SCC growth behavior in HAZ of welded joint.The main conclusions are listed as follows.(1)Pit density of different regions in the rotor steel welded joint is significantly affected by the galvanic effect induced by metallurgical heterogeneity.In addition,different mechanisms of pit initiation and growth in BM and WM are observed.For BM,pits emanate from the pre-existing defects or local deep grooves caused by surface machining and form the irregular shape following growth and coalescence along the grinding direction.By contrast,the majority of pits in WM initiate from the site of inclusion-matrix interface and form the discrete and approximately hemispherical shape.Considering the mechanism difference of pit formation in BM and WM,the models for pit depth evolution applicable to BM and WM are proposed respectively.(2)For rotor steel welded joints,different mechanisms of crack initiation dominated by stress corrosion cracking(SCC)and strain-induced corrosion cracking(SICC)are observed respectively.In terms of SCC,corrosionpit is nucleated,and then crack emanates from the pit where aggressive local environment and stress concentration interact.By contrast,no pit as a precursor for crack is observed when dominated by SICC.Under dynamic straining condition,surface oxide film is directly disrupted by the strain.The corrosive solution passes through the crevice formed on the oxide film and contacts with bare metal before the crack initiates.The fracture locations in the welded joint are also different as determined by SCC and SICC.The region of greatest susceptibility to SCC is HAZ adjacent to fusion line(FL),while the fracture location dominated by SICC is in WM with the lowest strength.(3)In terms of the investigation on HAZ of the highest susceptibility to cracking,SCC crack deflects towards the hard/high strength side of HAZ closing to the fusion line,which is ascribed to the combined effect of the graded mechanical property and electrochemical activity.The effect of crack-tip constraint is firstly introduced to study the crack growth path and the deflection behavior under the environmentally assisted condition.Trigger effect of critical stress triaxiality and out-of-plane constraint at crack tip on the initiation of SCC deflection in HAZ is illustrated.