Characteristics and Sintering of Fine Water-atomized and Carbonyl Iron Powder
Licentiate thesis, 2018
Various types of iron powder are extensively used in the powder metallurgy (PM) industry. Water-atomized iron powder is commonly used as the base in many low-alloyed steel products manufactured through the conventional 'press and sinter' route, while carbonyl iron powder is often used for metal injection moulded (MIM) components. The two manufacturing methods share many similarities but differ in the powder shaping step where water-atomized iron powder is typically shaped by uniaxial die-compaction whereas the large fraction of organic material in MIM allows for a moulding step that enables higher component complexity.
The initial powder characteristics, such as the particle size, size distribution and morphology will largely determine the sintering behaviour of the powder grades. A fine, spherical powder has inherently higher sintering driving forces whereas coarse powder sinters to less extent. The chemical composition of the powder, especially the surface chemical state, will also play a role in sintering due to the enlarged surface area and reactivity of a powder. All iron powder particles are covered by a thin surface iron oxide layer that comprises the major fraction of the total surface area. The oxide layer contains a large amount of the total oxygen in the powder and furthermore acts as a barrier to diffusion that can hamper interparticle neck formation during the early stages of sintering. Consequently, the properties of the oxide layer are important for all subsequent processing of the powder. In addition to iron oxides, the powder contains oxidation-sensitive elements that are particularly vulnerable and tend to form oxide particulates that can deteriorate the mechanical properties of the components. Compaction of powder introduces deformation on a particle level, with heavily deformed regions localized to particle edges and where small sections of particles have a large fraction of their volume deformed. This is another factor that also influences sintering.
The scope of this study covers a number of methods to analyse the initial states of a given powder and compact in order to provide a foundation for its subsequent processing. Surface analytical techniques were used to determine some of the characteristics of the different powder grades in terms of the presence, composition and thickness of the surface iron oxide layer and oxide particulates. Additionally, the properties of the compacted green states were determined. Thermal analysis techniques were then used to provide information about how the powder interacts with the sintering atmosphere. The activation energy of the reduction of the surface iron oxide layer could be determined for both water-atomized and carbonyl powder, and nitrogen- and carbon-removal processes could also be shown for carbonyl iron. Furthermore, removal of internal oxygen in water-atomized powder by diffusion and subsequent reduction was identified. The sintering behaviour of compacted water-atomized iron powder and injection moulded carbonyl iron powder was then studied with dilatometry using a number of different processing parameters like initial compaction pressures, heating rates, and atmosphere composition. It was found that sintering is strongly affected by prior compaction, especially in the high-diffusivity ferrite region. High heating rates affect sintering of carbonyl iron powder to large extent, with a density increase at high heating rates. However, the heating rate also influences the chemical composition due to the altered interaction with the atmosphere.