The fracture surface was ex-amined by using the macroscopic and micro scopic methods the results showed that small holes caused by bubble in raw material and excessive non-metallic indusions leakd to cracking in forging process and subsequendly fracturing in straightening proless.

The fracture process of metallic materials is affected by void growth and coalescence directly, most of the researchers supposed that metal is isotropic, but they ignored the factors of crystallographic orientation, inter- and intra-granular inhomogeneous deformation.