The effect of flame straightening on the microstructure and mechanical properties of different strength steels

Abstract

In many cases, flame straightening is unavoidable after welding for the reduction of deformation. Due to the not very concentrated heat source, the process can cause significant changes in the microstructure, especially in high strength and wear-resistant steels. Due to their different physical properties, the effects vary depending on the flammable gases (acetylene, propane). The situation is complicated by the fact that the manual technology carries a risk of overheating, which can have detrimental effects on the mechanical properties. During our experiments, three steels are investigated (S355J2 + N, XAR400, S960QL). The thermal cycles for the physical simulations were determined by thermocouple measurement during real experimental conditions. Three peak temperatures (1000 °C, 800 °C and 675 °C) and two types of industrial cooling conditions (air and water cooling) were studied. The samples were examined by optical microscopy tests, hardness testing and Charpy V-notch impact tests. During straightening the XAR400 showed high sensitivity to softening even in the lower temperature range, while hardening occurred in the S960QL steel at a higher peak temperature values during water cooling. The inter- and supercritical temperature should be avoided in all steels; however, the subcritical temperature can be beneficial to the toughness properties of the S960QL and XAR400.

Post-Treatment of Welding Joints of High Strength Steels II.:Improving Residual Stress Condition – Overview

“…Water cooling is even more dangerous as the higher the strength group. Based on practical investigations, Gunnert's method is not recommended as high strength steels fatigue strength improvement process (Gyura et al, 2021).…”

“…The high strength steel spot heating has influence on the microstructure and crack propagation of the welding joints. Researchers (Gyura et al, 2021) found that in high strength steels intentional or accidental heating in the intercritical A 1 -A 3 results in significant damage to the original structure of the smoothed steel and martensite-austenite (M-A) islands causes a significant decrease of toughness, due to the mixed structure formed. The results of the instrumented impact tests have shown that in such case, due to the mixed fabric formed, the crack propagation energy is drastically reduced with nearly unchanged crack initiation energy (Gyura, 2021).…”