Hans Berns

Comparison of wear resistant MMC and white cast iron

In this report, the microstructures of conventional white cast irons (WCI) and new metal matrix composites (MMC) are compared. In contrast to casting, the hot isostatic pressing (HIP) of powder mixtures offers more freedom to design specific properties like toughness and wear resistance. Both may be enhanced by a proper MMC composition. Experimental results are so convincing that special industrial applications appear to be feasible as well as cost-effective and some have been initiated. New MMC for high temperature wear, for corrosive environments and for cold forging tools are presented. It is shown how the cost of hard particles in MMC may be reduced by in situ transformation of ferroalloy particles.

ON THE CORRELATION BETWEEN ELECTRON STRUCTURE AND SHORT RANGE ATOMIC ORDER IN IRON-BASED ALLOYS

Abstract The experimental data on the concentration of free electrons in fcc iron-based alloys, results of theoretical calculations on the electronic structure and experimental data of atomic distribution are analysed. The electron structure of iron-based substitutional solid solutions and CrNi austenitic steels alloyed by Mn, Mo, Cu, Si, Al and C, N was studied by means of the measurement of conduction electron spin resonance. The electron exchange in binary fcc Fe–N and Fe–C alloys was also calculated using an ab initio norm-conserving pseudopotential method. It is shown that Ni, Cu, Si and Al increase the concentration of free electrons, whereas Cr, Mn and Mo decrease it. Theoretical calculations as well as experimental data show that nitrogen in fcc iron and iron-based solid solutions increases the state density at the Fermi surface, whereas carbon contributes its electrons to the states below the Fermi surface. Mossbauer spectroscopy was used to study the distribution of carbon and nitrogen in binary fcc Fe–C and Fe–N alloys, while the data on the distribution of d -solutes in multicomponent solid solutions were obtained from the analysis of the contributions of different electronic subsystems, namely free electrons, isolated localized d -electrons (single solute d -atoms) and superparamagnetic clusters (clusters of d -atoms), to the temperature dependence of the magnetic susceptibility. The results of studies concerning the atomic distribution are consistent with the available data on the short range order in iron-based alloys. The following correlation is found: an increase in the concentration of free electrons assists the short range atomic ordering in iron-based alloys, whereas the localization of electrons promotes clustering of solute atoms. The state of atomic order influences properties like austenite stability, corrosion resistance and strength.

Grain boundary strengthening in austenitic nitrogen steels

The effect of nitrogen and carbon on the strengthening of the austenitic steel Cr18Ni16Mn10 by grain boundaries is studied. It is established in accordance with previous results that, in contrast to carbon, nitrogen markedly increases the coefficient k in the Hall-Petch equation. Because of a pronounced planar slip induced by nitrogen and the absence of any noticeable segregation of nitrogen atoms at the grain boundaries, nitrogen austenite presents an excellent object for testing different existing models of grain boundary strengthening (pile-up of dislocations, grain boundary dislocation sources, work hardening). Based on the analysis of available data and measurements of interaction between nitrogen or carbon atoms and dislocations it is shown that the nitrogen effect can be attributed to a strong blocking of dislocation sources in grains adjacent to those where the slip started.

Microstructural properties of wear-resistant alloys

Abstract Microindentation and microscratching are used to gain in situ wear-related properties of hard phases and metal matrices in wear-resistant iron, nickel and cobalt alloys at room and elevated temperature. Vickers indentations allow to measure the hardness, the work of indentation, the modulus of elasticity, and the fracture toughness. Microscratching reveals the coefficient of scratching, the scratch hardness and the specific scratch energy. The results are not meant to predict wear rates but to meet basic requirements of a wear system.

Abrasive wear resistance and microstructure of Fe-Cr-C-B hard surfacing weld deposits☆

Abstract The microstructures of Fe-Cr-C-B hardfacing alloys with chromium contents of 0 to 42% are described and related to the fracture toughness and the abrasive wear resistance, which were measured in laboratory tests. Thereafter, hardfacing alloys were selected for the technical production of commercial filler wires. The microstructures of these newly developed Fe-Cr-C-B hardfacing weld deposits, which were manufactured and welded under commercial conditions, are presented.

Effect of coarse hard particles on high-temperature sliding abrasion of new metal matrix composites

Abstract Iron-base and nickel-base powders as well as several powders of different hard particles were tested by microscratching and microindentation up to 900°C. Those most suited were mixed and hot compacted to give new metal matrix composites(MMC). Up to 50v/o of coarse WC W 2 C particles were dispersed in a high speed steel matrix or in a NiCr20AlSi matrix. The wear rate in three-body sliding abrasion by flint under an argon atmosphere decreases with temperature to about 700 °C due to the formation of a self-protecting flint layer and a change in the hardness ratio of the microstructural constituents and the abrasive. MMC with 30 v/o of hard particles in the steel matrix are suitable for service up to 650 °C and those with a nickel matrix up to 750°C.

Tribological stability of metallic materials at elevated temperatures

Abstract The sliding abrasion wear rates of metallic materials against flint, measured between 25 and 1050 °C under an argon atmosphere, are shown and discussed. Above 550 °C, wear is governed mainly by the properties of the metal matrix and can be correlated with its resistance to plastic deformation. Thus, the hot hardness and work hardening capability affect the wear rate under stationary conditions. Instabilities occur when dynamic recrystallization appears within the metal matrix, bringing about a total loss of its work hardening capability. Hard phases have only a small effect on the tribological behaviour under stationary and non-stationary conditions.

Nitrogen partitioning between matrix, grain boundaries and precipitates in high-alloyed austenitic steels

Nitrogen in austenitic steels is known to increase their resistance to sensitization treatments and to shift TTT diagrams to the right on the time scale, which suppresses the intercrystalline corrosion. It is also shown that nitrogen delays the precipitation of carbides and intermetallic phases, while, at the same time, chromium nitrides precipitate slower as compared to carbides. This paper aims at clarifying the question of possible nitrogen segregation at grain boundaries in austenitic steels. The nitrogen content in the chromium nitride Cr{sub 2}N and the intermetallic Laves phase was also measured as a test of the experimental technique used.

Solution Nitriding of Stainless Steels for Process Engineering

Solution nitriding is a controlled heat treatment of stainless steels carried out at 1100 ± 50 °C in N2 gas. Depending on the alloy content a hard martensitic or a ductile austenitic high nitrogen case of about 2 mm in depth is formed, which reduces wear by sliding, fretting, erosion and cavitation. In contrast to carbon dissolved nitrogen does not impair the resistance to superimposed corrosion, so that solution nitriding is suitable for components subjected to the flow of aggressive fluids containing suspended particles which is often encountered in chemical and process engineering. Randaufsticken nichtrostender Stahle fur die Verfahrenstechnik Randaufsticken ist eine kontrollierte Warmebehandlung fur nichtrostende Stahle, die bei 1100 ± 50 °C in N2 Gas durchgefuhrt wird. In Abhangigkeit von der Legierungszusammensetzung ergibt sich eine hochstickstoffhaltige harte martensitische oder duktile austenitische Randschicht von etwa 2 mm Dicke, die Verschleis durch Gleiten, Fressen, Erosion und Kavitation reduziert. Im Gegensatz zu Kohlenstoff beintrachtigt Stickstoff nicht den Widerstand gegenuber einer uberlagerten Korrosionsbeanspruchung, so das das Randaufsticken fur Komponenten geeignet ist, die der Stromung aggressiver Medien mit gelosten Partikeln ausgesetzt sind, was oft in der Verfahrenstechnik der Fall ist.

Fracture of hot formed ledeburitic chromium steels

Abstract Due to the hot forming procedure the overall mechanical properties of ledeburitic chromium steels become anisotropic. As shown by experiments the fracture toughness K Ic of these steels depends on the hardness of the metal matrix and the orientation of the load with respect to the axis of hot forming. The degree of crack deflection, expressed by the width of the crack path, was found to be the governing quantity for differences in the resistance against crack propagation. The K Ic - tests were modelled by the finite element method using an orthotropic elasto-plastic material law. While the behaviour of the hardened and low tempered specimens could be well predicted by this model, differences between simulation and experiment in the case of a ductile state indicate the occurrence of additional effects acting on a microscopical scale.