Review: IPMD 14th Edition 2010-2011, 8 pages, 3603 words

Author: Bernard Williams, Consultant, Shrewsbury, UK

Sintering is critical in Powder Metallurgy (PM), giving a part its microstructure and the required mechanical properties. Bernard Williams takes a look at the wide variety of sintering furnace types now available to the industry and reviews the advances that have been made in recent years that have contributed to improvements in both the quality of finished parts and the industry’s competitiveness.

Introduction

Sintering plays a key role in powder metallurgy (PM) because it is at this stage that the powder compact acquires its microstructure and required mechanical properties. As in other parts of the PM manufacturing process significant advances have been made in furnace equipment in recent years which have contributed to improving both the quality and competitiveness of a large variety of PM products. These are for the most part continuous furnaces, the PM parts being loaded at one end and transported at a predetermined speed and temperature profile through the various zones to emerge more or less cold at the other end. However, there is also the need for batch furnaces such as those used in the sintering of hardmetal (cemented carbide) cutting tools and parts produced by powder injection moulding (PIM).

What happens in sintering furnaces?

Sintering furnaces have basically three heated zones each of which can be controlled independently; (a) pre-heat where the dewaxing, or removal of the pressing lubricant takes place, (b) the high temperature sintering zone where surface oxide layers on the powder particles are removed to facilitate interparticle diffusion and (c) the cooling zone (Fig. 1). The dwell time at the sintering temperature is just long enough to allow all parts to reach a uniform temperature and in the case of powders of different compositions, for the necessary alloying to take place. We will take a closer look at these zones before reviewing the different furnaces available to the PM industry

Pre-heat/dewaxing zone: In the pre-heat/dewaxing zone the lubricant must be removed completely before sintering of the surface can take place. In higher density ferrous compacts, ie exceeding 7.25 g/cm3 green density, dewaxing will take longer because of predominantly closed porosity compared with compacts of say 7.0 g/cm3 or lower density. Advances in the removal of lubricants from compacts include the introduction of some source of oxidant, such as water vapour, oxygen and carbon dioxide, into the pre-heat section of the furnace, the so-called ‘rapid burn off’ zone. The oxidant facilitates the burning off of the lubricant as it decomposes at temperatures below 600°C. The heat generated in this combustion can be used to heat the parts and furnace belt, thereby reducing total energy requirements and increasing productivity. Most of the applications for the above are for large parts and heavy loads, but the use of auxiliary combustion is now also being applied for smaller parts and lighter loads due to better control of the combustion process. Another innovation is the extension of the pre-heat muffle which allows the hot gases exiting the furnace to pre-heat the parts and provides more time for the lubricants to be removed.

Sintering zone: In the sintering zone of the furnace the oxide layers on the particle surfaces are reduced by a reaction with the atmosphere. This allows sinter necks to be formed with some macroscopic shrinkage. The temperature gradients must be minimised to avoid distortion, especially in the case of asymmetrical parts..............

Further sections of this article include:

Figures and Tables:

Fig. 1 Basic design of a pusher furnace (Courtesy EPMA)

Fig. 2 The three stages of solid state sintering (Courtesy EPMA)

Fig. 3 MIM-Master debinding and sintering furnace for MIM components (Courtesy Cremer GmbH)

Fig. 4 Principle of a walking beam transport mechanism (Courtesy Cremer GmbH)

Fig. 5 Cross-section of a twin-runner walking-beam furnace (Courtesy Cremer GmbH)

Fig. 6 Pusher furnace for MIM production at Polymer Technologies Inc., (PTI), USA, as published in PIM International, Vol 1 No 3

Fig. 7 Rear view of a large sinter-low pressure HIP furnace with 3m long payload space capable of processing 2.5 tonnes of hardmetal with rapid cooling times of ca. 4h. (Courtesy PVA TePla AG, Germany)

Fig. 8 A full size MIM-VacTM furnace from Centorr, designed for the production needs of the medical MIM market (Courtesy Centorr Vacuum Industries, USA)

Fig. 9 Inside view of a B53TH vacuum furnace for MIM components (Courtesy BMI Fours Industriels, France)

Fig. 10 Cross section of vacuum sintering furnace for dewaxing, pre and final sintering. (From H. Kolaska, etal, Hard Metal Lecture Series, Lecture 5, published by EPMA, 1995)

Fig. 11 SINTERFLEX atmosphere control system used for the monitoring of sintering atmospheres in furnaces (Courtesy Linde AG, Germany)

Table 1 Operating characteristics and capital cost of sintering furnaces (Source: Fundamentals of Powder Metallurgy, by L. F. Pease III, W. G West. MPIF, Princeton, NJ, USA)

Table 2 Typical sintering temperatures used in PM.