Sintering bonds Danninger to the Skaupy Prize

PM teacher and sintering expert Professor Herbert Danninger from the Technical University Vienna was presented with the prestigious Skaupy Prize during the 2006 Hagen Symposium. Bernard Williams summarises Herbert Danninger’s many contributions to PM and questions him on his approach to good performance in PM materials...

Professor Herbert Danninger, head of the Institute of Chemical Technology of Inorganic Materials at the Technical University (TU) Vienna, Austria, received the prestigious Skaupy Prize at the 2006 Hagen Symposium for his many contributions to powder metallurgy through teaching, research, and writing over a period spanning nearly 30 years.
The eulogy for the 2006 Skaupy Prize winner was given by Professor Winfried Huppmann who said that Herbert Danninger had graduated from the TU Vienna as a ‘Diplom Ingenieur’ in technical chemistry in 1979, having been a student under Professor Gerhard Jangg, a well known powder metallurgist who invented dispersion strengthened carbide containing Al alloys. Herbert Danninger received his doctorate in technical sciences a year later in 1980 for research on the sintering of tungsten heavy alloys, and continued working under Professor Jangg focusing on sintering phenomena and processes. He developed close contact in the 1980s with sintering guru Professor Werner Schatt at the Technical University Dresden, and he found that removal of oxides from particle surfaces and degassing were critical to successful sintering. In 1990 he took on the responsibility for teaching PM at his Institute in Vienna where he also established a research team which covered a broad spectrum of PM materials processing and testing but with a strong focus on sintering processes, in particular sintering PM steels. He was appointed to full Professor at the TU Vienna in 2003, and was recently appointed head of his Institute of Chemical Technology of Inorganic Materials with responsibility for some 140 academics, researchers and administrative staff.
Professor Danninger has been a prolific writer of technical papers with some 250 papers published over the past 25 or so years, and has also co-authored a number of books on PM including : Powder Metallurgy of Steels (with H E Exner), Fatigue of PM Steels, and Machinability of PM Steels (with A Salak and M Selecka). He was co-chairman for the 2004 PM World Congress held in Vienna helping to make it one of the best organised and most successful EPMA events.
Professor Danninger has not only been active in promoting the teaching of PM in his own department at TU Vienna, but also at a wider European level with lectures at the EPMA PM Summer Schools and at other institutions in Europe and abroad. He has been an active member and supporter of the EPMA PM Education Working Group since its inception in 1991. His PM department at the TU Vienna, sponsors some 5-10 PhD students annually, and is also one of the key partners in the Höganäs PM Chair which brings together several European universities’ efforts in PM research. Research topics include: tungsten alloys, iron-based magnetic materials, high-density high strength PM steels and fatigue of PM steels, sintering and post-sintering treatments and phenomena, and correlating the microstructure and mechanical properties of porous iron.
Professor Huppmann said that Professor Danninger was often eager to provide support for PM teaching and research in countries where there was a lack of facilities by inviting and sponsoring overseas students to study in his department, and by supporting PM seminars or conferences in developing countries thereby helping to spread the word of powder metallurgy. He has admitted to having contracted the strange disease "sinteritis" – said to be incurable.
Sintering of Modern Alloy Steels
Professor Danninger chose the subject of sintering of modern PM alloy steel systems as the topic for his Skaupy Lecture. He stated that PM has to fulfil a specific function – namely to meet the properties demanded by the customer in the most cost effective way. One of the most important functions in achieving the desired properties is the sintering process which allows for a build-up of sintering contacts between powder particles and the dissolution of alloying elements in the mix leading to a homogeneous mixture and sintered structure giving the desired strength, hardness, etc. In modern PM alloy steels such as Cr-Mo steels and Mn steels, the homogenisation process during sintering is widely different, he said.
Professor Danninger stated that the sintering process involves the removal of the pressing lubricant and adsorbed gases plus the removal of non-metallics (oxide) from the surface of powder particles as well as internal oxides within the powder particles. He stated that modern thermoanalytical equipment such as mass spectrometers allows the monitoring of the sintering process, for example loss of mass, gas phase transport and transient liquid phase sintering.
Using Cr-Mo and Mo prealloyed steel powder as examples, Professor Danninger said that surface oxides reduction occurs in the temperature range of 700°C - 1000°C with the final reduction of internal oxides at the higher temperature of 1200°C. High temperature sintering is essential for high oxygen affinity steel powders to achieve good homogeneous microstructures, and desired strength and ductility.
Professor Danninger also referred to the use of Mn as a "modern" alloying element in PM steels. He stated that Mn admixed in amounts up to 2 per cent offers an attractive combination of strength and ductility in moderate density PM parts with sintering in conveyor belt furnaces at 1120°C. Mn’s affinity to oxygen is thought to be less of a problem today than commonly assumed especially if a reasonably pure sintering atmosphere is used, and as Mn promotes swelling (Figure 3) it has the potential to substitute Cu in PM steels without loss of properties or significant changes to the production process. In higher density Mn steel PM parts or at higher Mn content, the oxides would need to be reduced at higher sintering temperatures.
Professor Danninger also discussed the sintering of model Fe-1%V-0.7%C, Fe-0.6%B and Fe-0.3%B-0.8C powders when V and B are added to water atomised steel powders to act as sintering activators. Sintering of the Fe-B-C powders is carried out in a hydrogen atmosphere because of the strong interaction between boron and nitrogen.
BW. Herbert, first I would like to congratulate you on behalf of all Metal Powder Report readers on being awarded the Skaupy Prize for 2006. Design engineers have good knowledge of the compositions of wrought steels and are often conservative in their choice of materials for highly loaded components. How can PM convince them that “modern” PM steels such as Cr-Mo and Mn alloyed materials will do the job just as well as wrought steels?

HD. It is of critical importance to supply reliable data for PM steels, which also implies that the respective manufacturing parameters are clearly defined (since with PM steels there are many more parameters to consider than in wrought steels). The EPMA-MPIF-JPMA database is a promising approach in that respect. Bringing mechanical engineers into contact with PM in schools or at universities should also be an effective strategy for avoiding prejudice.

BW. Common alloying elements such as Ni and Cu used in PM steels are facing substitution by alternative elements such as Cr, Mo and Mn? Why is this and have we exhausted our search for material combinations for PM steels?
HD. Ni is being progressively banned in many countries, being regarded as carcinogenic. Cu is a nuisance during steel recycling, being enriched in the steel, and therefore the OEMs try to limit its use. Further, the price of Ni and Cu has gone up very much recently. On the other hand, the Cr-Mo steels which hitherto have been regarded as tricky to sinter due to the high oxygen affinity of Cr, can now be sintered to good effect as a result of progress in furnace technology and atmosphere control.

BW. Another question related to wrought steels is their stable microstructure. Do you think PM will ever be able to achieve the same stable microstructures and mechanical properties as their wrought counterparts?

HD. The thermodynamically stable, ie homogeneous, microstructure of wrought steels is not necessarily always the optimum. In my opinion, the non-equilibrium, heterogeneous microstructures that are achievable by PM but not by ingot metallurgy are also highly attractive. Regarding mechanical properties we should distinguish between “data sheet properties”, such as tensile strength or elongation, and “application properties”, such as complex fatigue or wear loads. In real applications, PM parts frequently perform much better than would be expected from the data sheet properties.

BW. What are factors you believe play the most important roles in the fatigue strength of PM steels?
HD. Total porosity definitely plays a major role in fatigue behaviour; in the case of high density/low porosity components also pore distribution and morphology as well as presence of singularities – large pores, inclusion etc – have to be considered. At high density levels, optimising the matrix microstructure becomes increasingly important. From the designer’s viewpoint, also the geometrical effects, as for example notches in components, have to be taken into account.

BW. What would be your favoured route to improving machinability of PM steels?

HD. According to our experience, higher density is beneficial, as is avoiding Ni as alloy element, in particular if heterogeneously distributed. Adding machining aids such as MnS is helpful but should be done in a carefully balanced way in order to limit adverse effects on the mechanical properties.

BW. You have been using Continuous-Cooling-Transformation (CCT) diagrams in the study of sinter hardening PM steels. What does CCT actually tell us?

HD. When applying sinter hardening, the CCT diagrams tell us which effective cooling rates are necessary to obtain e.g. a fully martensitic microstructure. Taking several CCT diagrams may be helpful to show e.g. the effect of the carbon content on sinter hardening which may be considerable, necessitating careful carbon control during sintering.

BW. You have been involved in the development of PM Aluminium alloy materials. What do you think is holding back applications of such materials?

HD. Fully dense Al alloys such as the carbide dispersion strengthened grade Dispal C offer excellent properties but suffer from the very expensive processing routes. The classical pressed and sintered Al materials have the drawback that the oxide layers covering the powder particles can be cracked during sintering but are not removed, remaining within the sintering contacts and resulting in fairly low strength and ductility compared to similar wrought alloys. Furthermore, sintering of Al precision parts is markedly trickier than that of ferrous components. Better prospects are found with applications where for example wear and corrosion resistance, diamagnetism or a given CTE are required, not just mechanical strength.

BW. You have devoted your career to teaching and research in powder metallurgy. What would you say to students who are considering making a career in this industry?

HD. PM is a fascinating technology also in industrial practice. Industry gives the chance to transform one’s ideas and experience into marketable products, but it is essential to remain in contact with new developments in PM from the scientific viewpoint; this is most easily done by keeping contact with academic institutions, ideally their own alma mater.

BW. What areas of research do you think are still needed to be covered in PM in general and sintering in particular?

HD. In general, attaining high density – overall or local - at reasonable cost is a major goal. In the long-term perspective, however, PM applications that do not depend on the classical combustion engine and transmission have to be found; PM functional materials could be an attractive topic. Regarding sintering, the combination of non-conventional alloy systems with high sintering temperatures and clean atmospheres is very interesting and demanding.

BW. What do you think has been your most important contribution to powder metallurgy?

HD. Showing the importance of chemical reactions during sintering of PM steels, in particular of degassing and reduction effects in the early stages of sintering. These effects are not practically relevant for standard carbon or alloy steels but become increasingly important for more sophisticated alloy systems involving elements with high oxygen affinity.