This paper describes mineral oil lubricated twin-disc tests with nominal point or line contact at rolling (mean surface) speeds in the range 3-23 m/s and slide-roll ratios in the range 15-80 per cent. The results identify a regime in the sliding/rolling speed domain in which failure of EN36A (750 DPN) and EN24U (350 DPN) is predominantly by scuffing preceded by running-in, which delays scuffing to relatively severe operating conditions. At speed combinations above the identified regime, the steels fail by scuffing in, or close to, mixed lubrication conditions with little or no running-in, so that the conventional failure criterion based on a film thickness to r.m.s. surface roughness value of three is appropriate. At speed combinations below the identified regime, load intensity becomes sufficient so that general plastic deformation intervenes before failure by scuffing can occur and a shakedown-based criterion becomes appropriate. Observations of contact frequency variations suggest that with steel surfaces at 650 and 750 DPN running-in is produced by rubbing of asperity tips and that with steel surfaces at 350 and 450 DPN it is produced by hydrodynamic ripple pressures. For the latter materials at low slide-roll ratios, macro-and micropitting are likely concomitants of plastic deformation.
