| NiCrMoV steel is widely used in pressure vessels,piping and steam turbines.The fatigue strength and reliability of the welded joints made from NiCrMoV steel is an important aspect for the safety of these equipment.In this paper,a new type of steam turbine rotor welded joint was used to investigate the fracture and fatigue failure mechanisms of welded joint and the micro-zones by combining digital image correlation technique and scanning electron microscopy.Due to the limitation of traditional speckles in optical microscope,the techniques associated with reliable preparation of micro-speckles and micro-strain measurement were firstly studied.Suitable speckles for micro strain measurement in NiCrMoV steel and its welded joint were proposed after an error analysis of three kinds of speckles.Furthermore,strain heterogeneous distribution associated with micro structures around fusion zone of welded joint and within multi-pass weld metal were investigated based on the developed micro speckles.In addition,the near-tip damage accumulation behavior in cases of constant amplitude and overload were discussed.The crack initiation and micro-crack micro-defect interaction behaviors were finally analyzed.The main contents and conclusions are listed as follows:(1)The relationship between micro-strain measurement error and speckle types and magnifications were clarified,and the nano-speckle matched with in-situ tensile testing in scanning electron microscopy was developed.It was found that the strain error level of Al2O3 speckle was very high when the magnification was larger than 1000X,while the error of nano-Au based speckle varied little with the increasing of magnification.It was interesting to find a relatively stable measurement results when using NiCrMoV steel micro structure as speckles.There existed a relationship between suitable speckle size D and measuring magnification M:D=-1000/M μm,which was helpful in selecting suitable speckle size.(2)The relationship between microstructures and strain localization in NiCrMoV steel welded joint was firstly found.Results from in-situ SEM tensile test showed micro-strain localization in block б-ferrite area.Alternative distribution of columnar crystals and equiaxial crystals were observed within the multi-pass weld metal,and micro-strain was often localized in columnar microstructures with larger grain size.Macro-strain localization around fusion zone of the welded joint was transferred from base metal to weld metal under tensile loading,which was induced by a varied work-hardening ability of local microstructures.(3)Crack tip strain evolution tests in NiCrMoV steel and 304 stainless steel were conducted by combining DIC and in-situ SEM fatigue device.It was found that strain ratcheting existed in fatigue crack tip under constant amplitude stress,which was independent of crystal type.Crack tip strain obtained by DIC revealed that absolute strain increasing rate varied but relative strain increasing rate were similar at different measurement points in front of crack tip.The ratio of relative strain increasing rate and crack growth rate for different materials and at different stress levels was almost stable.In case of overload,linear relationship existed between crack growth rate and crack tip plastic zone size,indicating the plastic zone size was the driving force of fatigue crack growth in overloading.(4)Micro-crack initiation behavior and crack-defect interaction in NiCrMoV welded joint were investigated.Results from in-situ SEM fatigue tests found that strain accumulation occurred around micro-defect leading to crack initiation,and a critical strain value was obtained.The interaction between crack and micro-defect could be divided into four stages,i.e.,free from effect,crack growth increasing,crack re-initiation,and recovery.Crack acceleration stage appeared when the distance between crack and micro-defect to initial distance ratio d/a0 reaches 0.52,accompanied by the decreasing of plastic zone size. |
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