Investigation on brittle fracture mechanism of a grade E cast steel knuckle

• The fracture of this knuckle made of grade E cast steel is brittle fracture.
• Knuckle's matrix is consisted of developed coarse dendrites and shrinkage porosity.
• The shrinkage porosity separated the metallic matrix and resulted in the unsound solidification microstructure of the knuckle.
• Dendrites and shrinkage porosity decreases the knuckle plasticity and toughness.
• Improper welding repair will initiate the formation of micro-cracks and propagate.

This paper investigated the brittle fracture mechanism of a grade E cast steel knuckle that is one of the key components of the coupler for railway wagon by using scanning electronic microscope, optical microscope, energy dispersive spectroscopy as well as mechanical property testing methods. It is found that large amounts of developed coarse dendrites and shrinkage porosity existed in the knuckle's microstructure, the plasticity and impact toughness of the knuckle entity are very bad and far below that of the Keel block represented this knuckle. Apparently, the shrinkage porosity resulted in the knuckle unsound metallic matrix and poor mechanical properties. The fracture morphology mainly shows the characteristic of cleavage, quasi-cleavage fracture as well as a little ductile fracture, so the failure of the knuckle can be attributed to brittle fracture distinctly. Moreover, a welding repair area with high hardness is also found in the fracture region, which might initiate micro-cracks on the surface of the knuckle. As a result, it can be inferred that under an impact load that did not exceed material fracture limit, the micro-cracks initiated from weld repair zone propagated rapidly into the inner matrix by continuously fracturing the dendrites along shrinkage porosity regions until the knuckle failed instantly.