Some automotive OEMs are reluctant to use recycled steel for crankshafts as they believe potential problems in grinding outweigh its significant environmental benefits. However, recent research shows that both recycled and ore-based steel can perform equally well, as Thomas Björk, PhD, group technical specialist at Ovako in Sweden explains.

Recycled steel, produced using a high proportion of scrap material, has a much lower environmental footprint than ore-based steel. However, a number of automotive OEMs are reluctant to use it for their forged crankshafts as they believe the material is more susceptible to ‘grinding burns’ during the production of high-quality bearing surfaces. This is a type of thermal damage that decreases surface hardness, introduces residual tensile stresses and shortens fatigue life.

Recently, a number of major players in the industry came together as part of a three-year project to challenge this mythology by benchmarking the performance of recycled and ore-based steels. With financial support from the Swedish Government, the project was coordinated by Chalmers University of Technology, with members including Volvo Group, Volvo Cars, Scania, Bharat Forge, Ovako and RISE IVF – a Swedish state owned research institute.

Two test groups of bar material in a grade used regularly for forging were drawn from different steel makers:

Hardened tubular workpieces were fabricated as shown in Figure 1. Both materials are nominally identical, however some small differences were observed that influenced the test results, even though they remained within the specified parameters.

The test pieces were hardened by austenitisation at 870°C for 1 hour followed by water quenching and tempering.

To assess the grindability of the two materials, three main criteria were used:

The tests employed cylindrical plunge grinding with one-third of the CBN (cubic boron nitride) wheel width:

Figure 2 shows the specific energy determined during the tests. This was broadly similar for both materials.

The geometrical changes in the grinding wheel, as increased amounts of steel were ground away, were used to determine wheel wear, as shown in Figure 3.

There was a larger radius change and hence wear for grinding recycled material. However, it should be noted that the observed changes in the radius are smaller than the CBN grit size (181µm grit diameter).

There could be a number of possible reasons for the different wheel wear rates. Clearly, the mechanical and thermal loads on both materials are similar – as indicated by the similar specific grinding energies and residual stress profiles (as discussed in the next section).

Therefore, it was most likely that hard non-metallic inclusions had influenced wheel wear. This was confirmed by metallurgical tests that showed a slightly higher proportion of both soft magnesium sulphide (MnS) inclusions as well as hard aluminium-rich oxides in this particular batch of recycled steel.

The surface integrity tests were carried out using the same grinding parameters as the wheel wear tests. However, the workpieces were switched from time to time to focus on the effect of the workpiece material and exclude the potential effect of changing wheel topography.

Surface integrity was assessed using Barkhausen Noise (BN) measurement. This is a non-destructive method involving the measurement of a noise-like signal induced in a ferromagnetic material by an applied magnetic field. There are two main material characteristics that directly affect the intensity of the Barkhausen noise signal: hardness and stress.

An increase in BN is usually connected to more residual tensile stresses and/or decrease in hardness (tempering) – which result from grinding burns. In contrast, a low BN is associated with compressive residual stresses and/or high hardness.

The results shown in Figure 4 show a clear difference between the recycled and ore-based steels, indicating different surface integrities and suggesting worse grindability of recycled material.

To investigate further, X-ray diffraction measurements of surface residual stress was carried out. These did not confirm the indications given by BN and the stress profiles were very similar for almost all measurements.

It has been reported that – apart from residual stresses and hardness – BN is affected by material microstructure. Therefore additional work was carried out to compare the microstructures of the two materials.

These tests showed a slightly lower bulk hardness for the recycled batch and similar martensite morphologies for both materials. However, there was a difference in the average size of the prior-austenite grains (when it was austenitised before quenching and tempering) – these were larger in the recycled steel and studies report that larger grains result in higher BN. The reason for this difference is believed to be varying amounts of carbon nitrides when the materials were austenitised, which resulted in different grain growth rates.

The next step was to carry out BN measurements on metallographic samples taken from the workpieces. The goal was to minimise the influence of surface deformation/stresses and instead measure the BN response of the bulk material. The results showed, as in the grinding tests, a higher BN for the recycled steel batch. The conclusion is that the increased BN level is an artifact of grain size rather than resulting from the grinding process.

No reason not to use recycled steel

The project has enabled a number of conclusions to be drawn:

The positive results of this benchmarking process should help remove the barriers to automotive OEMs using recycled steel. The implications for sustainability could be significant. It is hard to put an exact figure on how much steel is used each year in the manufacture of crankshafts but we do know that some 90 million internal combustion engine vehicles are produced each year. Therefore a conservative estimate is that the annual use of steel for crankshafts is in the order of several million tonnes, most of it currently ore-based.

Ovako’s investigations have shown that the carbon footprint of ore-based hot-rolled bar is well over 3,000kg of carbon dioxide (CO2) per tonne. In contrast, one tonne of our recycled bar steel generates just 389kg of CO2. Values from other suppliers may vary but even so, an industrywide switch to recycled steel for crankshafts could save millions of tonnes of CO2 a year.

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