| It has already been widely used in offshore platform structures,bridge structures.In these applications,these structures may bear reciprocating loads which may cause fatigue problems.In this paper,the corrosion behavior and corrosion fatigue perfermance of G20Mn5 QT cast steel base metal and its butt welds with Q345 D hot-rolled steel in wt.3.5% Na Cl solution are carried out by the corrosion experiment and corrosion fatigue experiment.Corrosion failure mechanism and corrosion fatigue failure mechanism are obtained.Finally,the corrosion fatigue performance is evaluated by the Weibull three-parameter model S-N curve.(1)The corrosion behavior of G20Mn5 QT cast steel base metal and its butt welds with Q345 D is studied.The corrosion test of G20Mn5 QT cast steel base metal and its butt weld with Q345 D hot rolled steel in wt.3.5% Na Cl solution is carried out,and the corrosion rate,corrosion morphology,corrosion failure mechanism and pitting evolution simplified model are obtained.The corrosion rate of the G20Mn5 QT cast steel base metal sample and the butt weld sample at the initial stage of corrosion is the fastest.After 3 days of corrosion,the corrosion rate of the cast steel base metal sample is the lowest,and then has a small increase and then slowly reduces and tend to be stable.The butt weld sample reach the minimum after 7 days and tend to be stable.The corrosion morphology of cast steel base metal is dominated by pitting corrosion,and the corrosion morphology of butt weld is dominated by uniform corrosion accompanied by pitting corrosion.G20Mn5 QT cast steel and its butt welds undergo oxygen depolarization reaction in the corrosion solution to form a rust layer on the surface,forming pitting corrosion during the continuous erosion of chloride ions,forming a slightly acidic environment to accelerate the pitting deep digging and expansion.For the pitting evolution,the irregular pits can be divided into several approximate circular corrosion pits,and the maximum radius of the irregular corrosion pit can be defined as the largest approximate circular pit radius.(2)Corrosion fatigue behavior of G20Mn5 QT cast steel base metal is studied.The corrosion fatigue test of G20Mn5 QT cast steel base metal in wt.3.5% Na Cl solution is carried out,the stress ratio is-1.The corrosion fatigue stress-life(S-N)curve is obtained.Fracture analysis of the corrosion fatigue failure specimens under different stress amplitudes is carried out respectively,and the failure morphology of the fractures samples are obtained.It is found that the fatigue failure process of G20Mn5 QT cast steel base metal in wt.3.5% Na Cl solution is affected by the corrosion solution.It belongs to the mixed fracture mode of ductile cracking and transgranular cracking.The fatigue source is the internal casting defect close to the surface and the surface pitting evolution forms irregular pits.The fatigue striation and the secondary crack parallel to the crack propagation direction can be seen on the crack propagation region,and the obvious dimples are found in instantaneous fracture region.Comparing with the fatigue performance in the air,it can be found that under the influence of the corrosion solution,the fatigue life of G20Mn5 QT cast steel is significantly reduced,and there is no fatigue limit.The corrosion fatigue failure mechanism of G20Mn5 QT cast steel base metal in wt.3.5% Na Cl solution is revealed.When the stress amplitude is greater than the fatigue ultimate strength in air,the stress concentration cause by the casting defects inside the cast steel is the main factor causing crack nucleation.When the stress amplitude is less than the fatigue ultimate strength in air,the pitting corrosion of the surface affected by chloride ions in the corrosive solution forms irregular craters,which cause stress concentration and leads to crack nucleation.After the crack nucleation,the corrosion products embed in the crack tip accelerates the crack propagates and eventually leads to fatigue failure.(3)The corrosion fatigue behavior of butt weld of G20Mn5 QT cast steel and Q345 D hot roll steel in wt.3.5% Na Cl solution is studied.The corrosion fatigue test of the butt welds of G20Mn5 QT cast steel and Q345 D hot rolled steel in wt.3.5% Na Cl solution is carried out,the stress ratio is-1,and the corrosion fatigue stress-life(S-N)curve is obtained.Fracture analysis of the corrosion fatigue failure specimens under different stress amplitudes is carried out,and the failure morphology of the fractures is obtained.Casting defects and welding defects were observed in the fractured samples of the G20Mn5 QT cast steel heat affected zone and the weld zone,while Q345 D There are no obvious internal defects in the samples in the heat-affected zone fracture;the fatigue steps formed by the fatigue sources of different horizontal plane can be seen in the fractures;there are obvious shear lips in the instantaneous break zone.Comparing with the fatigue failure fracture and fatigue failure mechanism in the air,the fatigue performance is significantly reduced,and there is no fatigue limit.For the samples fracture in the heat affected zone of G20Mn5 QT cast steel and weld zone,the main reason for crack nucleation under the larger stress amplitude is the stress concentration cause by internal casting defects and welding defects near the surface,under the small stress amplitude,the main reason for crack nucleation is that the pitting corrosion cause by chloride ion evolution evolves into irregular crater formation stress concentration.The main cause of fatigue crack nucleation of samples fracture in the heat affected zone of G20Mn5 QT cast steel is the dislocation and stress concentration cause by chloride ion erosion.The main reason for the fatigue crack nucleation of the fractured specimen on the side of the heat affected zone of Q345 D hot rolled steel is the dislocation and stress concentration cause by pitting corrosion caused by chloride ion erosion.After the crack nucleation,the corrosion product embed in the crack tip accelerates the crack propagates and eventually leads to fatigue failure.(4)Cast steel and its butt welding corrosion fatigue life evaluation method based on three-parameter Weibull model.The stress-life(S-N)curve of the G20Mn5 QT cast steel base metal and its butt weld with Q345 D hot-rolled steel is modified based on the three-parameter Weibull distribution model,and the S-N curves expressions under different failure probabilities are obtained.The two general solutions of the Weibull parameters are obtained by the Probability Weight Moment Method(PWM).The fitting results are close and the curves are the same form.No complicated iterative process is needed,which can reduce the calculation amount.Comparing with the fitting curve of Basquin formula,the fitting precision of S-N curve based on three-parameter Weibull distribution model is higher,and the curve has better bending shape,which is helpful for evaluating corrosion fatigue life. |
PDF Full Text Request