石墨化碳素钢室温压缩过程中的不均匀变形行为

Inhomogeneous deformation behavior in compressive deformation process at room temperature of graphitized carbon steel

  • 摘要: 将0. 46%含碳量(质量分数) 的石墨化碳素钢在万能材料试验机上进行室温压缩变形, 试验钢表现出良好的压缩变形性能.根据载荷-位移曲线的变化特点, 试验钢的压缩变形过程以位移7. 0 mm (对应相对压下量为58. 3%) 为节点分为两个阶段: 在位移≤7. 0 mm的压缩阶段, 载荷呈线性增加, 压缩试样的鼓度值逐渐增加而达到一个极大值(14. 6%), 压缩试样中心位置的维氏硬度增幅最大, 为38. 1 HV, 至位移7. 0 mm时试样端面径向伸长率的增幅为34%;而在位移 > 7. 0 mm的压缩阶段, 载荷呈指数增加, 压缩试样的鼓度值从极大值开始逐渐减小, 至位移为10. 72 mm时(相对压下量为89. 3%), 试样端面的径向伸长率相比于位移7. 0 mm时增加了83. 1%, 压缩试样的中心位置的维氏硬度增幅最小, 为32. 7 HV.上述试验数据表明, 在位移≤7. 0 mm的压缩过程中, 压缩试样内的三个不均匀变形区的位置与传统压缩模型一致, 但是当压缩变形进入位移 > 7. 0 mm的压缩过程中, 试样中心位置已不再是传统压缩模中变形程度最大的变形区了, 即在这个阶段试样中的3个不均匀变形区的变形程度发生了改变.正因这种不均匀变形区变形程度的改变导致了变形过程中载荷的急剧增加和鼓度值的减低.另外, 在压缩变形过程中, 三个不均匀变形区中石墨粒子的微观变形量总是高于铁素体基体, 其原因之一可以归结为石墨粒子中层与层之间容易于滑动的结果.

     

    Abstract: Based on the development trends, graphitized carbon steel has been proposed as a low-sulfur and Pb-free free-cutting steel. This steel has attracted considerable attention because of its excellent cutting performance and good cold forging performance.This study investigates graphitized carbon steel containing 0. 46% C with ferrite and graphite. In particular, its compression deformation at room temperature was studied using a universal testing machine. The load-displacement curve was fitted, the drum shape and radial elongation of the end face of the compression specimens were calculated, the surface quality and microstructure of the compression specimens were observed using optical microscopy and field-emission scanning electron microscopy, and the micro-deformation of graphite particles and the ferritic matrix in the compression specimens was statistically analyzed using Image-Pro 6. 0. The results show that the tested steel exhibits good compression deformation performance. According to the varying characteristics of the load with respect to displacement, the compression deformation process of the tested steel is divided into two stages with a displacement of 7 mm (corresponding to 58. 3% reduction) : at the compression stage with displacement ≤7. 0 mm, the load increases linearly with displacement.The value of the drum shape increases with increasing displacement, reaching a maximum value of 14. 6%, the radial elongation of the end face of the compression sample increases 34%, and the Vickers hardness at the center of the compression sample reaches its maximum value of 38. 1 HV. At the compression stage with displacement > 7. 0 mm, the load increases exponentially, the value of the drum shape gradually decreases from its maximum value, the radial elongation of the end face of compression sample increases by 83. 1%compared with that at 7. 0 mm displacement, and the Vickers hardness at the center of the compression sample reaches its minimum value of 32. 7 HV. The aforementioned experimental data show that, in the compression process with displacement ≤7. 0 mm, the three non-uniform deformation zones within the compression sample are consistent with the traditional compression model; however, in the compression process with displacement > 7. 0 mm, the center of the sample is no longer the deformation zone with the largest deformation degree in the traditional compression model. That is, the deformation degree of the three nonuniform deformation zones changes at this stage. This change leads to sharp increase in the load and to a decrease in the drum shape. In addition, during the compression deformation process, the micro-deformation degree of the graphite particles is greater than that of the ferritic matrix in the three inhomogeneous deformation zones. This is attributed to the crystal structure of graphite. In particular, graphite has a layered, planar structure in which bonding between layers occurs via weak van der Waals interactions, which enables layers of graphite to be easily separated or to slide past each other.

     

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