Sintering Equipment and Atmospheres used in Powder Metallurgy

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Sintering Equipment and Atmospheres used in Powder Metallurgy

IPMD 15th Edition 2012-2013, 9 pages, 3867 words

Author: Bernard Williams, PM Consultant, Shrewsbury, United Kingdom

                                                   


sintering_equipmentSintering is critical in powder metallurgy (PM) processing giving components their microstructures, density, and mechanical properties. Bernard Williams reviews the wide range of sintering furnaces now being used in the PM industry which have contributed to improvements in both the quality of finished parts and the industry’s competitiveness.


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 basically have 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. 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 and densification to take place.

The various heated zones in the furnace will also each require their specific atmospheres (see section ‘Sintering Atmospheres’). For conventional continuous mesh belt furnaces, the ideal atmosphere distribution consists of 20% N2 introduced into the pre-heat zone, 60% N2+H2 into the high temperature zone and 20% dry N2 into the cooling zone. 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 the predominantly closed porosity compared with compacts of about 7.0g/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’ (RBO) zone. Various furnace equipment manufacturers have developed their own variants of RBO.......

Further sections of this article include:

  • What happens in sintering furnaces?
    - Pre-heat/dewaxing zone
    - Sintering zone
    - Cooling zone/sinter hardening
  • Continuous furnaces
    - Continuous conveyor furnace
    - Hybrid conveyor furnaces
    - Walking beam, roller hearth and rotary hearth furnaces
    - Modular roller hearth furnaces
    - Pusher furnaces
    - Furnace Muffles/Refractories
  • Batch furnaces
    - Vacuum sintering
    - Sinter-HIP furnaces
    - Bell furnaces
  • Other sintering processes
    - Hot pressing
    - Spark plasma sintering
    - Gas pressure sintering (GPS)
    - Laser sintering
  • Heating media
  • Sintering atmospheres
    - Synthetic atmospheres
    - Atmosphere sampling and control
  • Troubleshooting in sintering
    - Frosted parts
    - Sooting
    - Black spots
    - Oxidation
    - Decarburisation

Figures and Tables:

Fig. 1 The zones of a continuous mesh belt sintering furnace (Courtesy of Cremer Thermoprozessanlagen GmbH)
Fig. 2 Typical zoned nitrogen-hydrogen atmosphere system in a continuous mesh belt furnace. From Paper presented by T. Philips, et al (Air Products & Chemicals Inc) at PowderMet2011, San Francisco. (Courtesy MPIF) 
Fig. 3  2 x 600mm twin-belt furnace boosts productivity for sintering PM parts at 1150°C. (Courtesy Sarnes Ingeniere OHG, Germany) 
Fig. 4 Principle of a walking beam transport mechanism (Courtesy Cremer Thermoprozessanlagen GmbH, Germany)
Fig. 5 High capacity, high temperature (1300°C) roller hearth sintering furnace (Courtesy Mahler GmbH, Germany)
Fig. 6 Integrated debinding and sintering pusher furnace (Courtesy Elino Industrie-Ofenbau GmbH, Germany)  
Fig. 7 MIM-Master debinding and sintering furnace (Courtesy Cremer Thermoprozessanlagen GmbH, Germany)
Fig. 8 A series of batch furnaces for MIM. These furnaces can either be used for a combined debind and sinter cycle, or for sintering only (Courtesy Elnik Systems Inc, USA)
Fig. 9 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. 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.12 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.

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