Reactions between ferrous powder compacts and atmospheres during sintering – an overview

Abstract

This overview paper describes the interaction of powder metallurgical iron-base alloys with the atmosphere during sintering. The methods of thermal analysis serve to clarify the processes that take place especially during the heating stage of the sintering cycle. After a discussion of the physical and chemical fundamentals of the sintering process, the methods of thermal analysis are explained. The differences between plain iron and alloyed systems are discussed in detail. Classical PM low alloy steels with alloying elements, such as Cu, Ni and Mo, react in a similar way as unalloyed carbon steels. The situation changes dramatically, when oxygen sensitive elements as chromium, manganese and even more silicon come into play. The removal of the surface oxygen is much more crucial, and there are several competing reactions, which have to be considered when these systems should be sintered in industrial scale to reach the desired mechanical and dimensional properties.

The impacts of post‐processing treatments done under reactive and non‐reactive atmospheres on the densification of binder jetting parts

“…In contrast, molybdenum used as an alloying element hardly affects deoxidation and is comparable to iron-carbon systems with deoxidation of the more stable oxides appearing at 640-730 C in inert atmospheres. [23][24][25] Another interesting material system is the WC-Co composite where sintering under a mixture of 95% nitrogen and 5% hydrogen exhibits better mechanical properties than under vacuum. [26] Lastly, and as an example of a ceramic parts' manufacturing, alumina compacts are best sintered under nitrogen as it promoted shrinkage and higher densification.…”

“…For example, parts made from plain iron absolutely need hydrogen atmospheres to remove oxides from their surfaces, as sintering in an inert atmosphere such as argon can causes serious embrittlement. [ 23 ] Alloy elements mixed with iron can also play a major role in the effect of atmosphere during sintering. During sintering of a chromium‐alloyed steel under argon, for instance, deoxidation does not start until 900–950°C and is not completely finished at 1300°C, as the critical temperature depends on the chromium content, which ultimately modifies the final mechanical properties.…”

Microstructure and mechanical properties of high carbon M2 powder metallurgy high-speed steel prepared by the carbide addition

“…Compared with atomised iron powder, the fine particle size of the carbonyl powder promotes sintering which in turn helps increase the density [16]. Another noteworthy problem is the existence of stable oxides on the surface of atomised pre-alloyed HSS powder due to the higher oxygen affinity elements such as Cr, V [5,17]. These stable oxides may strongly inhibit the formation of stable metallic sintering bridges and increase the amount of residual oxygen.…”

Site-specific control of alloy property through binder jet 3D printing

“…pressureless sintering). The sintering temperature was selected based on the published literature [133,134], and the duration prolonged to achieve the maximum possible density. The sintered density was plotted against the duration of sintering as shown in Figure 44.…”