Master alloys in powder metallurgy: the challenge of exploring new alloying compositions

Abstract

ABSTRACTSintering of steels containing oxidation-sensitive elements such as Cr, Mn and Si is a big challenge for the powder metallurgy (PM) industry but also a chance that could open the door to a new variety of compositions, properties and prices. However, even when very small amounts of these elements are mixed with an iron base powder, the chemical reactions taking place during sintering can be changed significantly. Application of high purity sintering atmospheres is not sufficient to avoid the formation of stable oxides on the surface of the alloying particles, since the source of oxygen can be – and in general is – the base powder itself. This is because the gaseous reaction products from the reduction of the oxides covering the base powder particles act as oxidising agents for elements with higher oxygen affinity. In this study, thermoanalytical techniques have been applied in a systematic study about the influence of different alloying additions either as elemental powders (Cr, Mn, Si) or as master alloys. While Si shows to be relatively inert up to 900°C, Cr and especially Mn present high tendency to act as ‘oxygen-getters’ already at 400–500°C. When these elements are added in the form of a master alloy, their reactivity decreases, alleviating considerably the gettering effect. Moreover, reduction of the iron oxide layers with H2 at 400°C shows positive results, however, from 400°C Mn shows an important tendency to oxidation by reaction with the sintering atmosphere.This paper is part of a special issue on the Advances in Materials and Processing Technologies (AMPT) 2015 and has subsequently been revised and extended before publication in Powder Metallurgy.