High - Strength steel hollow spheres

Metal hollow sphere structures (MHS) feature excellent properties for functional applications [1]. But, since their mechanical properties are still too weak for an use in lightweight constructions. Automotive industry demands a significant increase in strength and a decrease of density. In the present work, a study on the increase in the strength of carbon steel hollow spheres by additional heat treatment was conducted. Such additional heat treatments for improving the steel properties are quite usual. But, in contrast to the hardening of bulk material, metal hollow spheres show an extreme low thermal conductivity and high internal surfaces. Thus, the additional heat treatment of carbon steel hollow spheres gives rise to three main challenges, which are closely related. First the carbon content must be adjusted at an amount of 0.6 -0.8 wt.-percent and this amount must be held without a loss during austenitisation. This carbon content is optimal for high strength combined with high ductility. The second challenge is how to avoid the oxidation. Metal Hollow spheres show high internal surfaces. Hence, oxidation is promoted. Since massive oxidation of the shells of the MHS will harm the mechanical properties, it can hide the effect of the increasing strength of the residual shell due to hardening. Oxidation can occur during austenitisation by the atmosphere, during cooling by water or air and after the process, due to remaining water between the MHS. The third challenge is the fast cooling of MHS. Low alloyed and unalloyed carbon steel requires fast cooling in order to obtain phase changes from austenite to martensite. On the one hand water as cooling fluid allows the maximal cooling rate due to its high heat capacity. On the other hand, cooling in water is challenging owing to the corrosiveness of remaining water. Furthermore, packages and structures of MHS are closed-cell foams with low thermal conductivity. This lowers the cooling rale in the centre of structure or packages. Consequently, incomplete phase transition may occur, causing a gradient in the strength of such structures. Entnommen aus <a_href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a>