Nondestructive characterization of microstructural changes in powder metallurgy parts subjected to various conditions of sintering and steaming treatments

Sintering is an important step in production of powder metallurgy parts which is affected by variations in temperature. It is a well-known fact that enhancement in temperature increases the rate of sintering and the density of the parts as positive effects, and increase in grain size of the final microstructure and cost of production as negative influences. Therefore, performing sintering treatment at an optimum range of temperature can create desirable mechanical and microstructural properties. Steam treatment as a one of the secondary operations is a thermal process that creates a controlled thin oxide layer (Fe3O4) on the surface of an iron based metal component to increase corrosion resistance, hardness and density, and improve the wear characteristics. As in most thermal treatments, time and temperature are controlled to provide the optimal conditions for the expected finish. However, the improper conditions reduce the desired thickness of the Fe3O4 layer. In the present paper, to evaluate the microstructural changes, two groups of the sample were prepared. The first one was sintered in the range 1000-1150 °C (for 30 min) and the second group were placed in a steam treatment unit and heated to approximately 550 °C for different times varying in the range 0.5 to 3.0 h. Finally, magnetic hysteresis loop method was applied on all samples to characterize the microstructural features. The results show that the proposed nondestructive method can be used as a reliable technique to separate the improper microstructures due to the inappropriate conditions of sintering and steam treatments.