Development of a new bearing steel with improved life for service environments near ambient temperature (BESTSEAT) : final report

The project objective was to develop an enhanced bearing steel with associated heat treatment processes that will facilitate a 30% increase in the load carrying capacity of bearings for marine applications. The Mermaid Pod and UUC Azimuth thruster nose bearings are potential applications. Service data were reviewed and a specification drawn up by Rolls-Royce and SKF. Application modelling demonstrated that, in addition to increased hardness, it is important that excellent levels of cleanness are retained with no deterioration in fatigue crack growth threshold stress intensity relative to the existing steel. Three concepts were assessed; low alloy through hardening steels where all carbides can be dissolved during processing, high alloy through hardening steels where the carbides cannot be dissolved but can be refined in size and a case hardening steel. Thermodynamic modelling indicated that 10-20% of hard carbides could be sufficient to achieve the target hardness. Hot ductility of the high alloy compositions was poor and hot yield strength high, leading to surface cracking. Heat treatments were developed to obtain different hardness levels and carbide volume fractions but although the target hardness could be achieved in dilatometry none achieved the aim hardness level at the slow cooling rates experienced by the heavy section part used for the chosen application. Increased volume fractions of coarse carbide led to higher elastic modulus and increased wear resistance but lower rotating bending fatigue strength and toughness. Microstructures have been assessed using XRD combined with optical, scanning electron and transmission electron microscopy.