Fracture Features of Low-Alloy Steel Produced by Metal Injection Molding

In the manufacture of sintered steels by metal injection molding (MIM), typical microstructural defects, such as pores and pore agglomerates, phase structure heterogeneities, and boundaries between different phases, are hard to avoid. Such heterogeneities cause crack origination, growth, and propagation when sintered materials are subjected to mechanical loads. The crack propagation path and fracture resistance are associated with the complex heterogeneous structure including ferrites, cementites, martensites, pores, and weak interfaces. With increasing sintering times, metal grains grow rapidly, leading to brittle fracture of the samples. Subsequent heat treatment substantially decreases the grain size and changes brittle fracture to ductile one. Multicycle sintering of the Catamold 8740 low-alloy steel greatly increases the impact strength of V-notched samples (from 7.55 to 13.95 J/cm²). Greater density of the samples and fewer stress concentrators favorably influence the material’s capability to withstand impact loads. Thus when density of the billets following six sintering cycles increases by 2.5%, their impact strength becomes 1.8 times higher. With a greater number of sintering cycles, the ductile dimples become significantly larger, while the increase in shock impact and density of the sintered material gradually slows down. The grain size substantially increases (in turn, suppressing pore healing) and density of the samples becomes greater over the total sintering time. X-ray diffraction and spectral analysis revealed additional phases after sintering and heat treatment. Additional fine-crystalline carbide and oxide phases become more distinguished with further increase in the sintering temperature and heat treatment. Brittle inclusions, along with residual porosity, present in sintered steel decrease the dynamic properties of the material.