Martensite decay during rolling contact fatigue in ball bearings

During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.     

Application of the Griffith criterion to fracture of boron fibres

Tensile tests were performed on boron fibres and the resulting fracture stresses _f recorded. The fracture surfaces were investigated by SEM, with emphasis on the type of flaw nucleating the fracture. It was found that pre-existing, so-called proximate voids, in the boron mantle near the core-mantle interface, have transverse irregularities, large enough to create stress concentrations. The radial extensions, 2c, of fracture initiating proximate voids in mirror zones were measured from the micrographs. The obtained 2c _f relation was found to be in good agreement with the Griffith criterion for brittle fracture. In addition, the experimental log 2c-log _f plot gives some information on the value of the surface energy,, of boron.     

Tensile strength and crack nucleation in boron fibres

Boron fibres of about 100m diameter have been tested to fracture in a micro-tensile testing machine. The fracture surfaces have been investigated using a scanning electron microscope and classified according to the fracture nucleation type. Comparison with the measured fracture stresses proved a significant correlation with the type of crack nucleation. Transverse cracks, nucleated at the edge of radial cracks along the fibre axis direction, are the most critical weakening feature. Also crack nucleation at the external fibre surface implies a low tensile strength, and is discussed in terms of a notch effect of the well-known nodular surface topography.     

Force and Energy Measurements During Controlled Grooving—A Basic Study of Abrasive Wear

Basic studies of abrasive wear have been performed by controlled grooving in a modified impact tester equipped with a cemented carbide tip. Specimen holders were constructed to permit normal and tangential force measurements during grooving and to enable quick-stop tests. The grooving energy is read directly from the standard pendulum meter or integrated from tangential force curves.

A series of metals were studied by single-tip grooving and the grooving energy was plotted versus weight loss W within a large W interval. Mettallographic studies reveal characteristic friction layers in the groove bottom and walls and also show that the development of these layers is governed by the mechanisms of chip formation.

A particular purpose of this work is to find relations between internal structure and microhardness profiles on one hand and grooving forces/energy and wear resistance on the other. There are indications that the specific grooving energy e = E/W can be used to predict abrasive wear resistance under widely varying conditions from “mild” to “severe” wear.     

Influence of Normal Force and Frequency in Fretting©

The contact conditions for fretting between spherical caps are reviewed. In particular, the relations between critical displacement and tangential force amplitudes for incipient slip on one hand, and normal force and frequency on the other, are analyzed. Experiments are carried, out with a testing apparatus, specially designed for fretting under controlled conditions within wide intervals of testing parameter values. An austenitic stainless steel and a Cu-3%Si alloy are investigated.

It is confirmed that the critical tangential force is proportional to the normal force, and that the critical displacement is proportional to the 213 power of the normal force for two different frequencies, 300 and 800 Hz. The observed amplitude values are significantly higher than the theoretical values for the higher frequency. The frequency dependence is discussed.     

Single-pass pendulum grooving — a technique for abrasive testing

A Charpy impact tester has been modified to perform single-point grooving. The energy consumption during single-pass grooving (the grooving energy) has, for a large number of materials, been found to correlate strongly with the abrasive resistance of these materials in different practical applications. In particular, the grooving energy divided by the material loss (the specific grooving energy) defines a valuable quantity for the classification of materials with respect to resistance against abrasive wear.

The pendulum tester also offers a unique possibility to make instant “quick stops” of the grooving tip at any predetermined position in the groove. This is invaluable for studies of the detailed abrasive mechanisms, knowledge of which is essential for the development of new abrasiveresistant materials.     

Martensite decay during rolling contact fatigue in ball bearings

During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.