In situ observation of sintered iron and carbon steel compacts using a low-temperature acoustic microscope

Sintered iron compacts and carbon steel compacts (0.3 mass% carbon) were observed in situ using a low-temperature acoustic microscope (LTSAM). The effects of structure, pore distribution, tensile load and testing temperature were evaluated by using the acoustic images. Before the tensile test was conducted, initial inner defects, such as pores and cracks, existed in the specimens and the conditions of the defect distribution were different for each specimen. As the tensile load increased, the newly generated defect area was extended where expansion, deformation and connection of pores occurred. As the testing temperature was lowered, the area became more localized. In different structures, for example, in ferrite and in ferrite-pearlite, the trend of the acoustic images also differed, and the defect area of the latter was sharper than that of the former. These results are consistent with elongation data obtained by tensile tests and observations using an optical microscope. Therefore, the LTSAM is useful for understanding the low-temperature brittleness of sintered iron and carbon steel compacts.