Results in Materials (Jun 2021)
Comparative study on the densification of chromium pre-alloyed powder metallurgy steel through nanopowder addition using design of experiments
Abstract
There is a constant demand for high density press and sinter powder metallurgical components for automotive applications. Steel powder pre-alloyed with chromium is an attractive material for such applications, but new ways to further increase the sinter density are required for successful processing of these powders to high density. Nanopowder could be used as a potential sintering aid in order to boost the densification of the steel powder compact. In this study, steel powder pre-alloyed with chromium, without and with admixed nickel, is used as base powder, to which nanopowder was added. Surface oxide removal, crucial for successful sintering of such materials, was studied by thermogravimetry analysis in order to understand the influence of nanopowder addition on the oxide reduction. Powder compacts containing nanopowder showed higher mass loss in comparison to the ones without nanopowder. Linear shrinkage obtained from dilatometric curves increased with the addition of nanopowder. To depict the influence of the critical parameters; sintering temperature, powder size, addition of nanopowder and composition (with or without nickel), a design of experiment approach was applied. The critical parameters were then adjusted at 2 different values (categorical parameters) and a ‘full factorial design model’ was used involving 16 experiments, with sinter density and hardness as output measures of the experiments determined. The results were analyzed using polynomial fit to determine which of the parameter exerts the maximum influence. Presence of nickel increased the hardness whereas sintering temperature and presence of nanopowder enhanced the sinter density. This led to the tentative design of optimum conditions that resulted in increase in sinter density from 7.25 g/cm3 (92.5% of the theoretical density) to 7.4 g/cm3 (94% of the theoretical density) with an addition of 5% nanopowder to Ni-containing grade when sintered at 1350 °C instead of 1250 °C.
