The Effect of Multiple Flame Straightening on High-Strength Steels Applied in Vehicle Industry

High strength structural steels (HSSS) in medium plate thickness are frequently used as the structural elements of trucks, railway wagons, cranes and earthmoving machines. Since large, welded components are produced in many cases, the use of flame straightening is unavoidable after welding to comply with the strict dimensional tolerances. Due to the not very concentrated heat source, the process can cause significant changes in the microstructure. Among the flammable gases (hydrocarbons), acetylene is typically used for flame straightening. To compensate for the welding deformations, heat source may pass through the part multiple times. Therefore, there is a relatively high risk of the repeated thermal effect reducing the heated area’s strength and toughness. In the present experimental work, the effects of multiple flame straightening were compared between S355J2 + N mild steel and an S960QL HSSS. The thermal cycles were determined by using thermocouples arrangement during real experimental circumstances. Three characteristic peak temperatures were selected for the investigations with the Gleeble 3500 thermophysical simulator: 675 ℃, 800 ℃, and 1000 ℃. The applied specimen geometry was 10 × 10 × 70 mm. The simulated heated zones were analysed by optical microscopic analysis, hardness tests, and instrumented Charpy V-notch pendulum impact tests. Based on the results, using a maximum temperature above A₁ during flame straightening is not recommended since a significant toughness reduction can occur in the intercritical temperature range. However, multiple heating in the same location typically does not cause further negative changes and may even slightly improve toughness.