Cooling rate effect on the microstructure of Fe-Cu-C alloy during continuous furnace sintering and their mechanical behaviour

Abstract

60% of the total volume of structural components manufactured for the automotive industry, with the press and sinter technology, are composed of Fe-Cu-C. The Fe-Cu-C alloys provide desirable properties while being cost-effective and easily sinterable in continuous belt furnaces. The resulting microstructure consists of equilibrium structures: pro-eutectoid ferrite and pearlite, proportion dependent on the amount of carbon alloyed. Among the many advantages the sintering technology offers, the ability to obtain a complex shape, before the sintering, reduces the need for post-sintering machining operations, sometimes even eliminating them. Fe-Cu-C components that were sintered under the same conditions, in different furnaces with slightly different cooling rates, presented similar properties and have indistinguishable microstructures, however, they were found to behave differently in the machining processes, increasing the cost and the time inverted in these operations. A study of the machining process would expend a significant amount of components, consuming a large amount of resources and time, for this reason, the following study was proposed. Three compositions of the iron copper carbon alloy, Fe-1.5%Cu-0.2%C, Fe-1.5%Cu-0.5%C and Fe-2%Cu-0.8%C; were sintered and cooled at four different rates, with the objective to carry out measurements of their mechanical properties and observe the microstructures, to find the possible variances that the cooling rates could cause before the machining process. Cooling rates are given by the speed, in revolutions per minute, of the convectors used in the Rapid Cooling section of the sintering furnaces. The cooling rates proposed for this study are 300, 600, 900 and 1200 rpm. Mechanical properties, such as tensile and yield strength, were measured by tensile testing. For a set of samples, the transverse rupture strength was measured. The macroscopic or bulk hardness of all samples was measured by indentation in the HRB scale. Furthermore, the sintered densities were calculated and the carbon contents of the alloys were checked by using a Carbon & Sulphur analyser. The microindentation hardness, or Vickers indentation hardness, was measured for the three compositions. In the Fe-1.5%Cu-0.5%C compositions, it was possible to differentiate between the two structures involved, pro-eutectoid ferrite and pearlite, which allowed to see the influence of the cooling rate in each. The microstructure was observed through optical and electron microscopy, after cutting the test bars to mount, polish and etch metallographic samples. The micrographs were used to conduct image analyses for the Fe-1.5%Cu-0.5%C alloy, to find the amount of ferrite and pearlite formed at the slowest and the fastest cooling rates. The conclusions of this study will determine how the cooling rate affects the behaviour of the alloys and could serve as the basis for a larger study involving machining operations