Erik Sundelöf

entrepreneur, thinker and Swede

PhD thesis

150 150 eriks

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.

the light in the tunnel…

150 150 eriks

At last it seems like there is a light at the end of the tunnel and the thesis will be finished. This Friday I printed the thesis and realised that it acctually keeps the argumentation together. For three weeks I have been buried in writing, simulations and just been locked inside the room really shutting everything else out.

Yesterday I went to the coast with my parents and realised I have not been in touch with the world lately. There is a World Championship in swimming, things are happening in the world all the time and especially I have no idea that I am really leaving the country for 9 months. This is especially one thing I realised after my mother asked me numerous questions and all was answered by "I do not know".

The really scary part is the last – leaving the country for 9 months. I have absolutely no idea what I am heading for, and according to me for the first time in my life I will be in collection of people that all of them really feels like extremely competent. The other day, I sat down and read their academic background and merits, and first I thought: "How come I ended up in this company?" That thing puzzled me for some weeks…

Just sitting down with my parents barbequeing (probably misspelled now) my mother said: "You are as competent as they are." She is right as usual (or at least in most cases). My personal motto is quite simple: "If somebody else have done it, you can too." Sometimes you just need to use that as a mantra to keep yourself going. The thing that scares people as well as me is the unknown. What we cannot understand seems to frighten us.

Everytime I feel like I do not have the guts to face something I think of just a short moment when I was about to go to the north of Sweden fishing with some friends of mine. My mother probably sensed some kind of anxiety from my part and just said in short: "Life is always about facing the unknown, but you have to face it." That moment was not as sentimental as the comment might seem, but it did come back time after the other. Looking back I do not know what I was afraid of, and that is the usual outcome. This can be summarized by my fathers mothers saying: "You always get disappointed by the unnecassary problems". Usually we spend to much time worrying about what to happen than seeing what acctually will happen.

Before Tom will tell me once again that I should try keep one topic in each blog item and not mix three or four topics. Some other not named persons will say I am rambling and I probably are. I just felt like letting you all know about the intricate little mind ghosts I have had. I really feel calm and things can acctually work out alright. The thesis will be as good as I like it to be and the time in California will be awesome. I have no idea what I have to face both academically and personally, but I will grow. That is for sure.