Press-hardened boron steel (22MnB5) is an ultra high-strength steel (UHSS) consisting of a significant proportion of martensite, with an ultimate tensile strength (UTS) of up to 1500 MPa. It is used across a variety of industries, with a particularly important application in the automotive industry. A large proportion of car manufacturers use boron steel for structural components and anti-intrusion systems—such as B-pillars, bumper reinforcements, roof, and side rails—in automobiles, as it provides high strength and weight-saving potential, allowing for stronger yet lighter cars, with increased passenger safety.

In the automotive industry, a major joining method for boron steel components is “resistance spot welding”, with several thousand welds being made on a single car. Spot welding exposes the boron steel sheet directly underneath the electrodes to very high temperatures, causing the metal to exceed melting temperature and then rapidly solidify upon cooling. This results in a soft heat-affected zone, which is surrounded by a hard nugget and outlying base material. This soft zone reduces the strength of the weld and makes it susceptible to failure.

Automotive manufacturers and designers want to understand the exact effects spot welding has on boron steel, as the heat-affected zones can exhibit reduced hardness, which can in turn reduce the components strength. However most conventional scanning methods will struggle to penetrate such a strong and challenging material so we decided to seek out a partnership with a research facility that could give us access to more powerful means of conducting non-destructive testing—a directed beam from neutrons beams generated by a nuclear reactor.

—Dr Darren Hughes one of the WMG researchers on the project

The researchers made three perpendicular strain measurements along the radial, hoop, and axial directions. The residual strains were mapped, and the residual stresses were subsequently calculated as a function of position from the weld center.

Our study 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 on mechanical properties as spot welding, especially because there is nothing that can be done to avoid tempering when spot welding is used on boron steel.

Our study has shown the need to apply alternative welding methods that can lengthen the lifetime of the widely-used boron steel to its full potential. With several thousand welds being made on a single car, future work on minimal-heat input welding techniques and post-spot welding treatments will enable the boron steel components of cars to maintain their hardness and avoid residual stress. Importantly, this will ultimately provide top-tier passenger safety in stronger yet lighter vehicles.

The next step is to use the same technology to develop methods that can evade this issue. This will include 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. This will be of particular importance to industries that use boron steel such as the automotive industry.

—WMG Research Fellow Dr Neill Raath, the lead researcher on the project

ILL and SALSA. The ILL is a service facility dedicated to providing scientists with a very high flux of neutrons feeding some 40 state-of-the-art instruments, which are constantly being developed and upgraded. Based in Grenoble, ILL is funded and managed by France, Germany and the United Kingdom, in partnership with 10 other countries.

SALSA is the ILL strain imager dedicated to the determination of residual stresses in a broad range of applications in terms of components and materials. It is designed for diffraction measurements in “real” engineering components and optimized for stress determination in metallic components.

SALSA layout. Click to enlarge.

The SALSA beamline is a well-suited instrument for this study, as it specialises in determining residual stresses in a broad range of engineering materials, including steels. It also allows 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 analysed effectively, as hardness profiles could be determined on the same weld following the neutron diffraction tests for residual stress.

—Dr Thilo Pirling, the ILL scientist leading ILL’s SALSA team

Resources

• Raath et al. (2017) “Effect of Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds,” Metallurgical and Materials Transactions Volume 48, Issue 6, pp 2900–2914. doi: 10.1007/s11661-017-4079-9