Neutrons point the way to optimized crash-tolerant vehicles

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A large proportion of car manufacturers use boron steel for structural components and anti-intrusion systems in vehicles. Spot welding exposes the boron steel sheet directly underneath to very high temperatures, causing the metal to exceed melting temperature and then rapidly solidify upon cooling. This results in a heat-affected zone, where surrounding material contracts and microstructures are altered.

A recent collaboration between the ILL (Institut Laue-Langevin in France); the Warwick Manufacturing Group (WMG) at the University of Warwick in the UK; Tata Steel; and the Engineering and Physical Science Research Council (EPSRC) conducted a study to investigate the correlation between boron steel hardness and residual stress.

Tata Steel provided the boron steel sheets for the experiments, which were carried out by WMG researchers on the ILL’s SALSA (Stress Analyser for Large-Scale Applications) beamline. Neutron diffraction was chosen as the method for measuring residual stress in this study due its ability to penetrate heavy materials such as boron steel, and the finer resolution it provides. Hardness distributions were measured on the same welds.

This study experimentally determined for the first time a strong correlation between reduced hardness in heat-affected zones of boron steel spot welds and increased residual stress. The findings have indicated the need to develop new welding methods that do not have the same damaging impact as spot welding, especially because there is nothing that can be done to avoid the reaction of reduced hardness when spot welding is used on boron steel.

Dr Neill Raath, research fellow at WMG, and principal researcher of this study, said, “Our future work will look into two methods that can evade this issue: magnetic pulse welding, which does not use heat and as such does not cause a heat-affected zone, and post-welding heat treatment, which reverses the reduction in hardness caused by spot welding.”

Dr Thilo Pirling, ILL scientist leading the SALSA team, said, “The SALSA beamline is a well-suited instrument for this study as it specializes in determining residual stresses in a broad range of engineering materials, including steels.

“It also enables larger structures to be placed within the beamline. In this case, the non-destructive nature of the technique allowed the correlation of interest to be analyzed effectively, as hardness profiles could be determined on the same weld following the neutron diffraction tests for residual stress.”

August 2, 2017

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John joined UKi Media & Events in 2012 and has worked across a range of B2B titles within the company's automotive, marine and entertainment divisions. Currently editor of Automotive Testing Technology International, Crash Test Technology International and Electric & Hybrid Marine Technology International, John co-ordinates the day-the-day operations of each magazine, from commissioning and writing to editing and signing-off, as well managing web content. Aside from the magazines, John also serves as co-chairman of the annual Electric & Hybrid Marine Awards and can be found sniffing out stories throughout the halls of several of UKI's industry-leading expo events.

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