Effect of porosity on the electrical properties of Fe-based specimens fabricated by powder metallurgy

Abstract

Important efforts have been made to develop simple and nondestructive methods of assessing microstructural characteristics, so that the causal factor of defects can be detected and corrected before they cause additional defects and failure. Specifically in products made by the powder metallurgy process, the inherent porosity, which in turn affects the mechanical properties of the components, must be reduced as much as possible. The powder metallurgy is complicated by numerous variables involved in the process and it is not always easy to identify a problem when it arises. The lack of nondestructive techniques may complicate the detection of defects in the appropriate time frame, so that corrective tasks can be incorporated into the production line process to reduce the number of bad components. As a result of this, during the last decade, in conventional casting metallurgy, a technique based on the determination of either electrical conductivity or resistivity has been shown to be effective in the determination of modification in Al-Si and the degree of grain refinement of aluminum melts.[71 Also, it has been useful to detect the presence of finer constituents (second-phase particles) due to fast solidification The purpose of this article is to use electrical measurements to analyze the microstructural characteristics, mainly porosity in an automobile component made by the powder metallurgy technique. The component studied in the present work was an automobile gear made by the powder metallurgy process. For this purpose, powder of the 1030 steel with an average particle size of 50 p m was used. Two processing steps were used in the gear: a cold isostatic press followed by sintering in a conveyor furnace at a temperature between 742 "C and 1120 "C. To avoid oxidation of the automobile gears, the thermal treatment was camed out under inert gas atmosphere (nitrogen). The external diameter of the gear was 145 mm (Figure 1); the thickness of the internal part studied was t = 4 mm. In the present work, 35 gears were studied; they were randomly picked on different days from the production line during 1 week. Local resistivity measurements in different parts of a sample were made with the standard four-probe technique, using the linear probe configuration, with an interprobe spacing (s) of 3 mm. To calculate the resistivity p from the measured voltage V between the two internal probes induced by current I passing through the external probes, the standard express i ~ n [ ~ . ' ~ ] was used: