Investigation of Thermal Effects of Flame Straightening on High-Strength Steels

Abstract

The use of high-strength steels in the automotive industry is increasing. In many cases, the use of flame straightening to reduce deformation after welding is unavoidable in the manufacture of trailers, semitrailers, heavy vehicles, earthmoving machinery, military bridges etc. Due to the not very concentrated but relatively high temperature heat source, the process can cause significant changes in the microstructure which can endanger the safe use of these steels. This may be particularly true for the high-strength steels tested, for which we have very little 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 used. Nowadays, there is a lack of studies that analyse the effect of these types of heat cycles. During our experiments, we investigate the changes of the microstructure and mechanical properties caused by heat effect on unalloyed structural and high strength steels (S355J2+N, S690QL). The situation is complicated by the fact that manual technology typically also carries a high risk of local overheating, which can cause heat effects that are too long in time and/or too high temperature. In addition to the direct thermal effect study, a Gleeble 3500 thermomechanical physical simulator was used to perform thermal cycles measured during the technology. 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. Both the real direct thermal effect study and physical simulation showed a clear negative effect of overheating and intensive water cooling for the exanimated steels.