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

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.

Austenite Transformation Behavior and Mechanical Properties of Constructional V, Nb-Alloyed TRIP-Assisted Steel

The present article is aimed at studying the austenite transformation kinetics and tensile properties of constructional 0.2 wt%C-Si2Mn2CrMoVNb TRIP-assisted steel subjected to isothermal holding in the subcritical temperature range (350-650 °C with the step of 50 °C) after intercritical annealing at 770 °C. The study was fulfilled using optical microscopy (OLYMPUS GX-71), electron scanning microscopy (JEOL JSM-), dilatometric analysis, tensile testing, Vickers hardness measurements. The critical temperatures of the steel were found to be Ac1=750-760 °C and Ac3=930 °C. The results showed that austenite demonstrated increased stability to pearlite and bainite transformations with an incubation period of decades of seconds at any of the mentioned temperatures. The bainitizing treatment at 400 °C with holding of 300-600 s resulted in ferrite/bainite/retained austenite structure with precipitates of nanosized carbide (V,Nb)C providing an improved combination of mechanical properties as compared to direct quenching (YS=548-555 MPa, UTS=908-1000 MPa, total elongation=16-18 %, PSE index=14.6-18.0 GPa%, YS/UTS ratio=0.55-0.60). The contributions of different strengthening components were estimated in order to reveal the benefits of a multi-phase microstructure for constructional applications.