【作者】 李媛媛；

【导师】 陈立佳； 李锋；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2014， 博士

【摘要】 铸造低合金钢由于其优良的性能，在许多领域已经逐渐取代了铸造碳钢。随着铸造低合金钢应用范围的越来越广泛，对其力学性能的要求也越来越高。迄今为止，已开发出多种商业化的铸造低合金钢系列，其中ZG20SiMn铸钢因强度高、塑性与韧性好，被广泛用于水压机立柱、横梁、工作缸以及水轮机转轮等构件。疲劳破坏是各种工程构件服役期间的主要失效形式之一，对于铸钢结构件亦不例外。因此，研究ZG20SiMn铸钢的疲劳行为不仅具有实用价值，而且也可以为ZG20SiMn铸钢结构件的抗疲劳设计和安全使用提供可靠的理论依据。本文以用于制作水压机横梁的ZG20SiMn铸钢件为研究材料，针对由实际铸件上切取的疲劳试样在室温下分别进行了应变控制的低周疲劳实验和应力控制的高周疲劳实验，并利用SEM和TEM对疲劳断口形貌和疲劳变形后的微观结构进行了观察与分析，探讨了ZG20SiMn铸钢在室温低周疲劳和高周疲劳加载条件下的变形与断裂机制。室温低周疲劳实验结果表明，ZG20SiMn铸钢在应变控制的疲劳变形期间可以表现为循环硬化和循环稳定，主要取决于外加总应变幅的高低，其中当外加总应变幅为0.25~0.45%时，ZG20SiMn铸钢在疲劳变形前期呈现循环稳定，在疲劳变形的后期则表现为循环应变硬化，直至最终断裂或由于疲劳裂纹出现而导致应力快速下降；当外加应变幅为0.6%时，ZG20SiMn铸钢在整个疲劳变形过程中表现为稳定的循环应力响应行为；当外加应变幅为0.8%时，ZG20SiMn铸钢在整个疲劳变形过程中呈现为循环应变硬化。对于ZG20SiMn铸钢而言，其弹性应变幅、塑性应变幅与疲劳断裂时的载荷反向周次之间均表现为单斜率线性行为，并且分别服从Coffin-Manson和Basquin公式。此外，采用拉伸滞后能作为疲劳损伤参数可以较为准确地预测ZG20SiMn铸钢的低周疲劳寿命。室温高周疲劳实验结果表明，在应力控制的疲劳加载条件下，ZG20SiMn铸钢在0.5的高应力比下的疲劳强度明显高于其在0.1的低应力比下的疲劳强度；在相同的外加应力幅下，ZG20SiMn铸钢在应力比为0.1时的疲劳寿命明显高于其在应力比为0.5时的疲劳寿命，而在相同的最大循环应力下，ZG20SiMn铸钢在应力比为0.5时的疲劳寿命明显高于其在应力比为0.1时的疲劳寿命；在ZG20SiMn铸钢组织中的鱼骨状硫化物以及含锰和钼的夹杂物将使其高周疲劳寿命大大缩短。疲劳断口形貌的扫描电子显微分析结果表明，对于ZG20SiMn铸钢而言，无论是应变控制的低周疲劳加载条件下，还是应力控制的高周疲劳加载条件下，疲劳裂纹均是以穿晶方式萌生于疲劳试样表面，并以穿晶方式扩展，而且在疲劳裂纹扩展区可观察到明显的疲劳条带及疲劳台阶等形貌特征。ZG20SiMn铸钢疲劳变形后微观结构的透射电子显微分析结果表明，低周和高周疲劳变形期间，钢中存在的大量50~80nm的颗粒状碳化物可以对位错产生钉扎作用，阻碍位错滑移，引起强化。ZG20SiMn铸钢低周疲劳变形期间，大量运动位错受到晶界的强烈阻碍，在晶界附近随机分布着位错线、位错缠结及位错带，同时也可形成胞状亚结构。ZG20SiMn铸钢在高周疲劳变形后，部分珠光体在发生变形并开裂，这些开裂的珠光体在后续循环应力作用下发生球化现象，即逐渐转变成短棒状或椭球状，当位错运动至这些珠光体周围时，有弯曲、缠结等现象发生。此外，高周疲劳变形后，ZG20SiMn铸钢中的部分晶界出现了明显的扭折，且在晶界处有大量的位错塞积。更多还原

【Abstract】 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.更多还原