Further reading. To discover the full results of this research program, as well as references to other work and a bibliography, please refer to my doctoral thesis:

Steel Lifting Magnet

Dr Steve Ooi, Group Technical Specialist Ovako R&D, outlines how innovative direct-quenching steels can enable forged components to achieve the ideal combination of strength, ductility and toughness without needing secondary heat treatment. It’s an approach that can save energy and carbon emissions while also boosting productivity.

The test was stopped after a predetermined number of cycles and the samples were then evaluated with a variety of advanced characterization techniques.

Magswitch Lifting Magnet

Fatigue failure generally results from the accumulation of microplastic deformation under repeated cyclic conditions. By “microplastic” I mean microscopically small areas of the component where the material is subject to plastic deformation while the bulk retains its elastic properties.

Electric lifting magnet

We used a microprocessor-controlled flat washer test rig, as shown below, to subject samples of 52100 and Hybrid Steel 60 to different RCF loadings. This setup has a 13-ball configuration in which the washer-like sample is positioned over the lower raceway. The 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 the sample.

In recent years, Ovako has introduced Hybrid 60 steel, a novel grade combining secondary hardening and intermetallic precipitates. It was developed to overcome the limitations of existing materials, particularly in bearing applications subject to challenging operational conditions such as in corrosion and hydrogen environments. However, a more comprehensive understanding of how it behaves under RCF is crucial to predict its response to cyclic loading. Investigating this was the basis for my thesis, with the aim of identifying how Hybrid Steel 60 decays under RCF.

Magnetic lifting equipment

Tania Loaiza Uribe is the newest recruit to our R&D team, although her journey with Ovako began back in September 2019 when we sponsored her PhD project in collaboration with the KTH Royal Institute of Technology in Stockholm. We asked her to outline her research findings that show how one of the grades in our new Hybrid Steel family could address the challenges of rolling contact fatigue (RCF) as bearings come under increasingly higher loads in heavy vehicles.

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 two main contributing factors. First, electric motors in large goods vehicles operate at a higher rpm and generate increased torque than cars, so this requires superior fatigue strength to ensure an adequate life for powertrain components. Second, the substantial weight of the traction batteries, crucial for long range, exerts considerable stress on the vehicle’s structural elements, including bearings.

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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.

Magnetic Lifter

One way to address the growing need for improved fatigue resistance could be to increase the thickness of material – and consequently the weight. But this is not feasible, as it would impact the load-carrying capacity of a vehicle that is already challenged by the weight of its batteries. Hence the interest in new materials that can deliver the required fatigue properties with no increase in component size. This is where our Hybrid Steel 60 comes in.

Previous research work has investigated the microstructural decay resulting from RCF, with a focus on the most common bearing steel, grade 52100. The microstructure of 52100 steel comprises tempered carbides, residual cementite (RC), and a martensitic matrix. Forms of microstructural decay identified during RCF include the 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 a subsequent degradation of the bearing’s functionality.

Automotive designers have looked traditionally for materials that offer high strength and crash energy absorption, especially for passenger cars. However, the heavy vehicle industry faces its own distinct challenges, with robust fatigue properties in steel becoming crucial. This is particularly true with the increasing trend towards electrification that imposes additional loads on the powertrain.

This type of failure affects the service life of a wide range of machine components, such as gears, rolling bearings, and camshafts. Rolling bearings, particularly those operating under elastohydrodynamic lubrication, undergo alternating contact stress within a small area. This gives rise to subsurface damage known as rolling contact fatigue (RCF), causing microstructural changes beneath the contact area that ultimately manifests as fatigue damage.

Traditionally, steels were classified into four categories: tool steel, stainless steel, engineering steel and maraging steels. That mold was broken in 2017 with the launch of our Hybrid Steel – a new family of grades with an innovative alloying philosophy. The result is that the key properties of each category are available in one high-performance steel.

The ability of a material to withstand RCF depends largely on its composition and heat treatment. In particular, materials that resist softening can extend the lifespan of bearings.

This difference can be ascribed to the effect of precipitates in Hybrid Steel 60 that enhance its resistance to softening.

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, as illustrated in the graph.

For both steels. the results revealed distinct microstructural alterations in the region of maximum shear stress beneath the raceway surface. These alterations include the presence of ferrite microbands, dissolution of carbides and precipitates, and the formation of nano-ferrite grains.

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The results of this test program suggest that Hybrid Steel 60 is better suited for special bearing applications where corrosion and hydrogen resistance are required, as 52100 does not offer these additional properties. However, future work is needed to clarify the relationship between microstructural decay and fatigue crack initiation.