H. Danninger

Dry sliding wear behavior of molybdenum alloyed sintered steels

Abstract Powder metallurgy (PM) steels of the composition Fe–3.5% Mo–1.0% C were prepared by pressing and sintering, and the wear behavior of the steels was investigated. Through wear test combined with SEM surface analysis and metallurgical analysis of the specimens showed that the wear behavior of these steels depends on the PM processing parameters and the heat treatment as well as on the wear conditions. Specimens quenched and tempered at 500°C have high wear coefficient compared with specimens as-sintered. At sliding speeds of 1 to 5 m s−1 and normal loads of 10 to 30 N, the PM steels have low and stable wear rate. The wear mechanisms are oxidation wear, adhesive wear, abrasive wear and melt wear depending on the wear conditions.

Processes in PM steel compacts during the initial stages of sintering

Abstract In classical sintering procedures of ferrous precision part, important processes occur during the heating period and/or during presintering stages. Apart from the de-waxing process, which is not discussed here, changes in surface chemistry, formation of metallic bridges, and dissolution of alloy elements added as powders take place. In this work, desorption of gases, reduction of surface oxides, and neck formation have been studied as well as the dissolution of carbon and of metallic alloy elements. It showed that surface chemistry is changed within fairly narrow temperature intervals the position of which depends on the composition of the powder particles and in part on pretreatment. Dissolution of carbon in iron and steel matrices is a slower process than might be anticipated from the thermodynamic viewpoint, in plain Fe complete dissolution occurring only at T >900°C and the atmosphere also playing a major role. Formation of stable necks in steel–graphite compacts is retarded until dissolution of graphite has occurred to a significant extent. Finally, dissolution of metallic alloying elements, also if promoted by transient liquid phase, starts at still higher temperatures, in the case of Mo even T >1200°C being necessary, and full homogenization is complete only after extended isothermal sintering.

Effects of porosity on delamination wear behaviour of sintered plain iron

Abstract The role of porosity on the wear behaviour of sintered iron was investigated. Sintered iron was used in preference to steel because the various alloy additions and matrix heterogeneity in the latter might affect the microwear behaviour and the role of porosity. The wear tests were performed in a pin on disc test bed under 10-40 N loads at 0·56 m s-1 sliding speed in air. For the given tribological conditions, it was found that the wear mechanism is delamination or mechanical wear, which is basically similar to that of wrought materials according to subsurface crack generation and crack propagation processes. However, the open pores on the surface act as a site for generation and collection of wear debris. Furthermore, the pores affect the stress distribution and the metal deformation of subsurface layers during dry sliding. Accumulation of the plastic shearing on the subsurface results in nucleation of cracks that further grow by shearing: the fracture process. The propagation of subsurface cracks at a critical depth parallel to and eventually up to the sliding surface results in the formation of flakelike debris. This paper presents the role of porosity, which is the peculiar property of PM materials, on the dry sliding behaviour of sintered iron as a model material. The implication of Archards law for describing the results obtained is presented. The shortcomings in evaluation of the wear behaviour of sintered materials are addressed and compared with non-porous materials.

Comparison of Mn, Cr and Mo alloyed sintered steels prepared from elemental powders

Abstract Molybdenum, chromium and manganese offer considerable potential as alloy elements in sintered steels, especially for PM precision parts used, for example, in automotive engines and transmissions. This holds in particular for recycling and health/safety aspects. Within this work, the influence of these elements as admixed alloy metals on the sintering behaviour and the properties of structural PM steels is discussed. The considerable differences in the homogenisation behaviour during sintering are described as well as the respective advantages and drawbacks. It is shown that for high density PM steels, Cr and Mo are better suited while for conventionally produced mass products Mn is attractive, its affinity to oxygen being less of a problem today than commonly assumed and since Mn addition promotes swelling, this element might be a replacement for Cu. For Mn, admixing is the more attractive route, for Cr, in contrast, prealloying offers advantages, while for Mo both alloying techniques are feasible.

The influence of defects on high cycle fatigue of metallic materials

Abstract Structural ceramics are known to be particularly sensitive to singular defects such as inclusions, secondary pores, pore clusters, etc. Within this work, it is shown that similar behavior can be found also with high strength/low ductility metallic materials in the case of high cycle fatigue loading. Various types of such defects are described in different metallic materials, those produced by powder metallurgy being particularly suited for these investigations. It is shown that the influence of the singular defects can be described by a modified Kitagawa diagram based on the fatigue limit of the defect-free material and the threshold stress intensity factor. Realistic predictions are however only possible if both the stress field around the defects and possible interaction between adjacent defects are taken into account.

Impact notch toughness of high-strength porous steels

Abstract Instrumented impact testing was used to investigate the effect of both notch severity and matrix strength on the impact fracture behaviour of different Fe–Mo–C high-strength steels produced by powder metallurgy, in the as-sintered as well as in the heat-treated condition. As the matrix strength and/or the notch severity are increased, two fracture mode transitions were observed: from a ductile to a very localized (but still ductile) type of fracture, and from a very localized to a mixed type of fracture. The fracture behaviour was found to be directly correlated with the shape of the impact curves. In the case of the ductile type of fracture, two types of load-deflection curves were encountered. The first type shows the presence of a distinct yield point and a strain hardening stage up to the fracture load. In the second type (encountered in specimens with increased matrix strength and/or notch severity) the distinct yield point disappears and the load–deflection curve shows a continuous yielding up to fracture. In the case of the ductile but very localized and the mixed type of fracture, the load–deflection curve is linear up to fracture. The deformation and fracture behaviour of the specimens which display this type of impact curve is explained using fracture mechanics arguments and it is concluded that the surface pores act as crack precursors.

Gigacycle fatigue of ultra high density sintered alloy steels

Abstract Sintered steel specimens with density levels of up to 7·6 g cm−3 have been prepared from Cr–Mo and Mo prealloyed powders. The fatigue response has been studied using an ultrasonic resonance testing device that enabled testing up to 109 cycles. It showed that the fatigue endurance strength can be drastically increased by raising the density and that the sintering conditions are effective, though less than the density. The existence of a true fatigue limit was disproved up to 109 cycles for all materials tested, with sintered steels thus being similar to wrought ones. Cr–Mo steels was shown to be superior to Mo alloyed grades due to the markedly finer as sintered microstructure and higher sintering activity. Fatigue crack initiation was found to originate from pores at first at multiple sites, with microstructural orientation being dominant compared to the direction of stress; with progressive loading, some cracks join to form a propagating macrocrack from which the final failure then starts.

Defining the pores in PM components

Porosity levels are important for powder metallurgists. Liquids or gases can penetrate pores during the manufacturing process and have a negative influence on properties. An Austro-Swedish team looked at the transition from open to closed porosity and pore connectivity at different densities and temperatures…

New concept in analysis of supersolidus liquid phase sintering of alpha brass

Abstract The present study focuses on modelling, optimisation and experimental investigation into properties and microstructure of supersolidus liquid phase sintered Cu–28Zn brass from prealloyed powder. The experiments are designed using response surface methodology based on central composite rotatable design to evaluate the effect of process variables on liquid phase sintering of prealloyed brass powder. Three variables namely temperature, time and atmosphere were changed during sintering. The mathematical equations were derived to predict densification and impact energy using second order regression analysis. The optimum condition was predicted when the sintering variables were set at about 876°C, 43 min and N2 atmosphere. Selecting optimum sintering parameters is an important factor for achieving improved properties and relatively homogeneous microstructure. Gravitational force has a detrimental influence on homogeneity which is reflected by a graded structure that is formed especially at higher sintering temperature and extended time. Also structural coarsening occurs at higher sintering temperatures and longer times. It is concluded that both gravitational effect and structural coarsening should be considered in manufacturing of Cu–28Zn alloy parts. Furthermore, a combination of modelling and experimental investigation provide a new concept for better understanding and analysing the sintering process of brass and related structures.

Electrical conductivity and microstructure of sintered ferrous materials: iron–graphite compacts

Abstract The dissolution of graphite during sintering of PM steels prepared from iron and graphite powder mixes was studied using electrical conductivity measurement. The effect of graphite content and grade on the carbon transport rate and related mechanisms was investigated by sintering in the range 500 to 1200°C in different atmospheres, including hydrogen, nitrogen, and vacuum. The electrical conductivity measurement method was accompanied by quantitative metallography and microfractography techniques for determining the extent of carbon dissolution and its influence on the formation of sintered contacts. Weight loss measurement and the dilatometric method were also performed. The effect of manufacturing parameters on the mechanical properties of sintered parts was studied to find a correlation between mechanical behaviour, microstructure, and the conductivity/resistivity in order to develop a nondestructive testing method. The results show that changes in the material structure occurring during processing of sintered materials can be precisely detected via electrical conductivity measurement. The application of the method for iron–graphite powder compacts is shown.