Novel Steel Grade Resists Fatigue as Life Becomes Harder for Bearings

Finding new steel grades with robust fatigue properties is becoming increasingly important for heavy vehicles. A key driver is the accelerating trend towards electrification that imposes additional loads on the powertrain. Specifically, the electrification of large goods vehicles requires improved fatigue properties under both high cycle fatigue (HCF) and very high cycle fatigue (VHCF). There are three main contributing factors. First, electric motors in large goods vehicles operate typically at a much wider range of rpm than internal combustion engines and second, they generate increased torque compared with cars. This requires superior fatigue strength to ensure an adequate life for powertrain components. Third, the substantial weight of the traction batteries, crucial for long range, exerts considerable additional stress on the vehicle’s structure, and especially bearings. It is possible to improve fatigue resistance by increasing the thickness of material used in critical components. However, this imposes a weight penalty that would impact the load-carrying capacity of a vehicle already challenged by the weight of its traction batteries. This is creating interest in novel steel grades that can deliver enhanced fatigue properties with no increase in component size. This is where Ovako’s Hybrid Steel 60 shows particular promise. Fatigue failure generally results from the accumulation of microplastic deformation under repeated cyclic conditions. The term microplastic refers to the microscopically small areas of the component where the material is subject to plastic deformation while the bulk retains its elastic properties. This type of failure affects the service life of a wide range of machine components, such as gears, rolling bearings, and camshafts. Rolling bearings often operate under elastohydrodynamic lubrication (EHL). This is a regime where significant elastic deformation of the surfaces takes place, with a considerable effect on the shape and thickness of the lubricant film. This leads to alternating contact stresses within a small area that can cause subsurface damage known as rolling contact fatigue (RCF). The result is microstructural changes in the contact areas that ultimately manifest as fatigue damage.