Industry is continuously looking for reduced costs and in some cases also reduced weights of their components at the same time as quality must be maintained or increased. One way of achieving this is to manufacture the components via powder metallurgy (P/M) and such components play an increasing role in various mechanical constructions. Two important properties of P/M components are contact fatigue strength and wear resistance, which mainly depend on the physical and chemical properties in the surface region of the component. The production process of such components involves both a form compression of the metal grains and a sintering step at high temperatures. Even if the sintering is performed in an almost inert atmosphere oxides are easily formed on the grain surface and due to the large surface to volume ratio this may severely affect the material properties. Thus a reduction step must be included during production. Although of basic importance for the product quality this chemical reduction step is by no means understood in all details and the present study is an atteempt to improve our understanding especially with respect to the kinetics of the process and the surface oxides of importance.
The dominating P/M-based process is uniaxial pressing and sintering of low-alloy ferrous powders. The sintering takes place in the temperature interval 1120-1350°C in specially designed furnaces under gas protection. It is of major importance to protect the metal powder from oxidation, and at the same time avoid that the surface is depleted in carbon. In many cases carbon is introduced into the surface region via a carburization step in the temperature region 800-1000°C in order to improve the mechanical strength and introduce compressive stresses.
The oxidation behavior is strongly dependent on the concentration level of oxygen-containing gases such as oxygen gas or water vapor, and on the composition of the alloy. In order to keep the cost of alloying elements down, there is a wish to move to the more oxidation-prone alloying elements chromium and manganese. This increases the importance of understanding the kinetics of the oxidation and reduction reactions during the various processing steps.
It has also been observed that nitrogen may enter the solid during sintering, and this may have an influence on the mechanical properties. The mechanism of nitrogen uptake is similar to carburization.
There are many parameters that influence carburization/decarburization, oxidation/reduction and nitrogen uptake, e.g. gas composition, temperature, time and prehistory, total porosity and pore geometry, alloy composition and microstructure.
Sintering is a fundamental step in P/M technology, which in principle is the unification of small, separate solid grains to large, three-dimensional microporous structures. Evidently, the strength of the final material will depend on the resulting bond area and bond strength between the grains, the grain properties, and the microporosity of the particles. An essential prerequisite to obtain good material properties is therefore to be able to control the surface properties of the grains during sintering. The small grains are very reactive and oxides form easily on the surface.
Since a typical P/M (sintering) process is performed at elevated temperatures in a streaming gas, a model must take into account both the composition and flow of the gas and the chemical (reduction) reactions at the gas/solid interface. Even if the primary processes occur in the immediate vicinity of the surface also some transport within the solid itself must be considered – especially so since the temperature is high and the grain size is small. Furthermore, the small grain size makes the total surface to volume ratio very high, and thus the influence of surface reactions is large.
Several studies of the reduction of surface oxides during sintering have been published and some will be presented and discussed later on. At this point, however, two important earlier studies will be mentioned focusing on the gas production during sintering of metal components – Danninger et al and Grabke. The study by Danninger has mainly influenced the experimental part of this work, whilst the study by Grabke has been of central importance for the modelling work.
Danninger used a combination of mass spectrometry and thermogravimetry, and the latter makes it possible to link the mass losses to a certain gas production peak. Danninger studied the granulate specimens after reduction of varioous powder compositions. The measured gas production of Astaloy CrM did show three gas peaks, which could be correlated to a certain reduction path. The peaks indicated the relative amounts of the different oxides reduced at certain temperatures. Water was produced at around 300°C, but as the mass loss was negligible, the amount of ferrous oxides had also to be negligble. The production of carbonoxides was located in two temperature regions around 1100 and 1250°C, respectively.
The works of Grabke focused on the problem of defining an appropriate kinetic model of the surface reactions for different types of gas atmospheres. Grabke has studied the effect of several gas atmospheres, and also processes other than carburization using electrical conductivity measurements on thin foils..
The sintering is performed in a temperature range up to 1250°C and in an atmosphere composed of a nitrogen-hydrogen mixture. Based on these facts – also taking into account some featues of the technical process – a simulation model is developed that describes the reaction kinetics of the reduction of surface oxides, and of the gas transport via diffusion and convection in the porous structure.
