TWI Industrial Member Report Summary 932/2009
S Westgate and S Shi
The need to reduce vehicle body weight, by reducing material thicknesses, for fuel economy, and to increase safety standards, in terms of passenger protection, has led to the development and application of a range of high and ultra-high strength (UHS) steels over the past 20 years. Generally there is little problem with resistance spot welding, which accounts for the majority of welding in vehicle bodies, for the automotive grade sheet steels up to about 600MPa tensile strength. However, the higher strength dual phase, complex phase and transformation induced plasticity (TRIP) steels, can present welding problems because of their hardenability under the fast quench rates associated with the spot welding process.
The very high strength (1500MPa), hot pressed boron steels are one of the latest material types to be applied in automotive applications, typically as reinforcement sections in components such as bumper beams and A or B-posts, for improving safety performance and weight saving. These steels have low ductility in the hardened condition at ambient temperature. Consequently, they would require high press loads and exhibit substantial spring-back when pressed at ambient temperatures. In order to avoid these problems, these steels are pressed at high temperature, typically 900°C in the austenite range, where ductility is high. The water-cooled press tools then provide the quenching effect, transforming the high temperature austenite to martensite, to achieve fully hardened components. The high hardenability of these steels, by virtue of the boron addition, and the high resulting hardness associated particularly with the carbon content of the steels, present some difficulties in welding, similar to other high and (UHS) steels.
This report covers the examination of resistance spot welding schedules for 1500MPa tensile strength boron steels of 1.2 and 1.6mm thickness. Weld hardening behaviour, fracture mode and weld properties were examined, and compared with results for other steels and guideline weld strengths from BS EN ISO 14373.
- Study the resistance welding of coated and uncoated hot pressed boron steels, of 1.2 and 1.6mm thickness, typical of automotive use.
- Develop resistance spot welding procedures for these steels.
- Examine the effect of process parameters on spot weldability.
- Examine the hardening and fracture mode behaviour of the spot welds.
- Provide static mechanical test data for welds in these steels.