The Effect of Flame Straightening Thermal Cycles on S1100M Properties

The use of high-strength steels in the vehicle industry is increasing. In many cases, the use of flame straightening is unavoidable to reduce deformation after welding. Due to the not very concentrated but relatively high temperature heat source, the process can cause significant changes in the microstructure and mechanical properties which can endanger the safe use of these steels. This can be particularly true for ultra-high-strength (UHSS) steels, for which we have limited experience and concrete measurement results. Due to the different thermal physical properties of the flammable gases, the resulting heat effect varies depending on the gas and technology. Nowadays, there is a lack of studies that analyse the effect of flame straightening thermal cycles on high-strength steel properties. The changes of the microstructure and mechanical properties were experimentally investigated on S1100M high strength structural steel. A Gleeble 3500 thermomechanical physical simulator was used to perform thermal cycles previously measured by thermocouples during real flame straightening experiments. The effect of two heating flames (acetylene/oxygen, propane/oxygen), three characteristic peak temperatures (1000 °C, 800 °C and 675 °C) and two types of cooling conditions (air cooling and intensive water cooling) were studied. Softening occurred at all 675 °C peak temperatures for both cooling conditions and flammable gases, and softening was also observed at 800 °C peak temperature with air cooling for acetylene and propane heating.