Investigation On Fatigue Behavior Of ZG20SiMn Cast Steel

Because of the excellent properties, low alloy cast steels have been gradually insteadof cast carbon steels in many fields. Since the application ranges of low alloy cast steelsbecome wider, the requirements in the mechanical properties of low alloy cast steels arehigher. So far many series of commercial low alloy cast steel have been developed.ZG20SiMn cast steel, which is characterized by high strength, excellent plasticity andtoughness, has been widely used for manufacturing such structural components as thecolumn, beam and working cylinder of hydraulic forging press as well as the runner ofhydraulic turbine. The fatigue damage is one of main failure modes in the working periodfor various engineering components including cast steels components. Thus, theinvestigations on the fatigue behavior of ZG20SiMn cast steel not only have practicalvalues, but also can provide a reliable theory basis for the fatigue resistant design andsafety usage of ZG20SiMn cast steel components.In this dissertation, the ZG20SiMn steel casting used for manufacturing the beam ofhydraulic forging press is chosen as the experimental material, the strain-controlled lowcycle fatigue tests and stress-controlled high cycle fatigue tests are respectively conductedwith the fatigue specimens cutting from the practical beam castings. The fatigue fracturesurfaces and microstructures after the fatigue deformation are observed and analyzed usingSEM and TEM. The deformation and fracture mechanisms of ZG20SiMn cast steel underlow cycle fatigue and high cycle fatigue loading conditions are discussed.The results of low cycle fatigue tests at room temperature indicate that during thestrain-controlled fatigue deformation, the ZG20SiMn cast steel can show the cyclichardening and cyclic stability, depending on the imposed total strain amplitude. At the totalstrain amplitudes ranging from0.25%to0.45%, the ZG20SiMn cast steel shows the cyclicstability in the early stage of fatigue deformation, and then the cyclic hardening occurs inthe late stage of fatigue deformation till the finally fracture or fast stress falling induced bythe formation of fatigue cracks. When the total strain amplitude is0.6%, the ZG20SiMncast steel shows the cyclic stability during whole fatigue deformation. When the total strainamplitude is0.8%, ZG20SiMn cast steel shows cyclic hardening during the whole fatigue deformation. For the ZG20SiMn cast steel, the relationship between elastic strainamplitude and reversals to failure and the relationship between plastic strain amplitude andreversals to failure show the single-slope linear behavior, and obey the Coffin-Manson andBasquin equations, respectively. In addition, the tensile hysteresis energy can be taken asthe fatigue damage parameter to predict accurately the low cycle fatigue life of ZG20SiMncast steel.The results of high cycle fatigue tests at room temperature show that understress-controlled fatigue loading condition, the fatigue strength of ZG20SiMn cast steel atstress ratio of0.5is remarkably higher than that at the stress ratio of0.1. At the same stressamplitude, the fatigue life of ZG20SiMn cast steel at the stress ratio of0.1is significantlyhigher than that at stress ratio of0.5, while at the same maximum cyclic stress, the fatiguelife of ZG20SiMn cast steel at the stress ratio of0.5is significantly higher than that atstress ratio of0.1. For the ZG20SiMn cast steel, the fishbone-shaped sulphide as well asthe spherical inclusion containing Mn and Mo will lead to a great reduction of high cyclefatigue life.The SEM analysis results on the morphology of fatigue fracture surface reveal that forZG20SiMn cast steel, under strain-controlled low cycle fatigue or stress-controlled highcycle fatigue loading conditions, the fatigue cracks transgranularly initiate on the freesurface of fatigue specimens and propagate in transgranular mode. In the fatigue crackpropagation area, the fatigue striation and fatigue step can be observed.The TEM analysis results on the microstructures after fatigue deformation indicatethat during low cycle and high cycle fatigue deformation, a large number of granularcarbides with the size of50to80nm can pin the dislocations and impede the dislocationslip, and finally cause the strengthening. During the low cycle fatigue deformation, a greatamount of mobile dislocations are heavily impeded by grain boundaries. The dislocationline, dislocation tangle and dislocation band randomly distribute near the grain boundaries.After the high cycle fatigue deformation, some pearlites deform and crack. Thespheroidization of these cracked pearlites occurs under the action of cyclic stress. Themorphologies of these cracked pearlites gradually change into rod-shaped andellipsoid-shaped. When the dislocations move around these pearlites, the dislocation bend and tangle occur. After the high cycle fatigue deformation, some grain boundaries arefound to be obviously wrested, and a great amount of dislocations are piled up at the grainboundaries.