Citation & Copyright (to be inserted by the publisher ) * corresponding author, awronski@bradford.ac.uk Recent Developments in the Powder Metallurgy Processing of Steels A. S. Wronski 1,* and J. Mascarenhas 2 1 Engineering Materials Unit, University of Bradford, W. Yorks, BD7 1DP, UK 2 INETI, Estrada do Pao do Lumiar, 1649-038 Lisboa, Portugal Keywords : Powder Metallurgy, Structural Steels, Carbothermic Sintering, Metal Injection Moulding, metals Abstract. To compete with wrought steels for demanding applications such as gears in automotive engines, Powder Metallurgy, PM, materials must possess structural reliability. Density has been shown to be the dominant factor in determining fatigue resistance in PM structural components. PM routes which could overcome the 7.4 g/cm 3 barrier in ferrous components include: double press-double sinter, hot isostatic pressing (HIP), sinter- HIP, sinter + HIP, powder forging and (transient and supersolidus) liquid phase sintering. In order to provide high green densities, high pressure (1GPa) cold compaction, warm (~ 140C) compaction and hydrocarbon-coated powders can be employed. Surface densification can be achieved by cold or warm work and includes cold work of hypoeutectoid alloys and ausforming. These processing routes are briefly reviewed, with particular attention to coated powders, warm compaction, high temperature sintering and the novel technique of carbothermic sintering in nitrogen of ferrous powders containing the easily oxidisable chromium and manganese. Introduction Powder Metallurgy, PM, is an evolving production technology to manufacture, frequently net- shape, a growing number of components of increasingly complex shapes from an expanding range of alloys. A mixture of elemental powders, or more usually, a pre-alloyed powder, plus minor additions such as a lubricant and graphite, is compacted to shape to form the green part by cold uniaxial or isostatic pressing, CIP, and then vacuum or atmosphere sintered to produce sinter necks and, in some cases, complete densification. The consolidation process may also include powder forging, extrusion, rolling or hot isostatic pressing, HIP. Metal powder injection moulding, MIM, is a shaping process which must be followed by de-binding before sintering of the green part, generally to near full density. The major advantages of PM are powder compaction into net-shape products, which reduces the costs of material, energy and labour, including those of machining operations, and higher production rates. In terms of raw materials, machining processes utilise 40- 50%, hot drop forging 75-80%, extrusion ~85%, casting ~90% and sintering some 95%. Corresponding energy requirements, in MJ per kg of finished part, are machining 66-82, forging 46- 49, extrusion ~41, casting 30-38 and sintering only 29. PM saves valuable materials through recycling. The global consumption of metal powders in Europe, North America and Japan is estimated to be a million tons, costing the parts makers about 1.5 billion Euros. Processed components raise this value to ~ 4 billion Euros. Iron-base powders account for ~ 75% of the total tonnage, with stainless and tool steels at 4%, aluminium 12%, copper alloys 5%, nickel 3% and tin <1%. A particular advantage of PM is the ability to process materials with compositions, which are not feasible by conventional cast and wrought routes. This is because the resulting alloy microstructures would be so heavily segregated such that, even with thermo-mechanical treatments, it would not be possible to obtain alloys with sufficient homogeneity. A major drawback of die pressing, the inability to manufacture components with re-entrant angles, can be overcome by Materials Science Forum Vols. 455-456 (2004) pp 253-257 online at http://www.scientific.net © (2004) Trans Tech Publications, Switzerland Online available since 2004/May/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 193.137.43.138-17/06/08,11:18:09)