Recent research indicates that Hybrid Steel® could address the challenges of bearing applications that require higher stress cycles, where microstructural stability is crucial for providing longer service life under rolling contact fatigue (RCF) conditions. Tania Loaiza Uribe of Ovako’s R&D team and Mikael Thunman, Ovako segment leader for bearings, report on the results of a PhD project carried out in collaboration with the KTH Royal Institute of Technology in Stockholm.
As the heavy vehicle industry accelerates towards electrification, significant additional loads are being imposed on powertrains. This is increasing the risk of fatigue failure. The result is a growing emphasis on finding commercially viable new steel grades with enhanced resistance to both high cycle fatigue (HCF) and very high cycle fatigue (VHCF).
A large goods vehicle (LGV) or heavy goods vehicle (HGV) in the European Union (EU) is any lorry with a gross combination mass (GCM) of over 3,500 kg. There are three key factors that make fatigue resistance in this application more important than ever before. First, electric motors in heavy vehicles operate typically at a much wider range of rpm than internal combustion engines. 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 structural elements, and especially bearings.
There is potential to address fatigue concerns by increasing the thickness of material used in critical components. But there is a major downside as it imposes a weight penalty that would impact the load capacity of a vehicle already challenged by the weight of its traction batteries. These concerns are driving the search for 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.
Where fatigue failure occurs, it results generally from the accumulation of microplastic deformation under repeated cyclic conditions. This 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 can be found across a wide range of machine components, such as gears, rolling bearings, and camshafts. Rolling bearings are a special case since they 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.
There is a body of previous research that investigated the microstructural decay resulting from RCF, with a focus on the most common bearing steel, grade 52100. The microstructure of this steel comprises tempered carbides, residual cementite (RC), and a martensitic matrix. During RCF, different forms of microstructural decay have been identified. These include formation of dark etching regions, white etching bands, and carbide dissolution (RC and tempered carbides). This decay becomes apparent after a high number of stress cycles, leading to a decrease in hardness and impacting the bearing’s performance.
The ability of a material to withstand RCF depends on its composition and heat treatment. Using materials that resist softening can extend the lifespan of bearings.
Recently, Ovako has made Hybrid Steel 60 available commercially. This is an innovative grade combining secondary hardening and intermetallic precipitates. It was developed to overcome the limitations of existing materials, particularly in bearing applications subject to challenging corrosion and hydrogen environments. However, to predict its suitability for rolling bearings it is important to understand how it behaves under RCF. This was the theme of Tania’s PhD project.
Physical testing evaluated samples of 52100 and Hybrid Steel 60 under different RCF loadings. This was done with a microprocessor-controlled flat washer test rig, as shown in Figure 1. This setup has a configuration in which 13 silicon-nitride balls are contained in a cup that serves as the outer race of the bearing. These balls rotate to create a circular contact track on the top surface of a washer-like sample.
Each test was stopped after a predetermined number of cycles and the samples were then evaluated with a variety of advanced characterization techniques.
For both steels. the results revealed distinct microstructural alterations in the region of maximum shear stress beneath the raceway surface. For example, the presence of ferrite microbands, dissolution of carbides and precipitates, and formation of nano-ferrite grains.
The most interesting result is that Hybrid Steel 60 had less microstructural decay after the same number of stress cycles (1.0 × 108) compared to 52100 steel. This is illustrated in the graph (Figure 2).
The reason for this difference is believed to be the effect of precipitates in Hybrid Steel 60 that enhance its resistance to softening under cyclic stress.
Conclusions and future work
The implication of this test program is that Hybrid Steel 60 could be better suited for special bearing applications where corrosion and hydrogen resistance are required, as 52100 does not offer these additional properties. However, future research is needed to investigate the mechanisms of crack initiation and propagation, as well as to gain a deeper understanding of RCF in corrosive and hydrogen environments.
Another area of important research will be Tribo-corrosion fatigue testing to evaluate the impact of water-based lubricants that could potentially emerge as the leading environmentally friendly lubricant in a wide range of applications, including electric vehicles (EVs) and offshore applications.
Further reading. To explore the full results of this research program, as well as references to other work and a bibliography, please refer to Tania Loaiza Uribe’s doctoral thesis with the KTH Royal Institute of Technology:
“Microstructural Decay in High-Strength Bearing Steels under Rolling Contact Fatigue”
