Influence of welding thermal cycles on properties of TMCP and Q&T steels evaluated by thermo-physical simulation

Abstract

The welding process represents an unintentional, but unavoidable, heat treatment in the form of soft annealing or tempering, but also hardening during the cooling of the melt. Particularly in the case of high-strength fine-grained structural steels, this leads to critical states of the mechanical-technological properties of the base material. In order to investigate the influence of the heat input during welding on the resulting properties of the heat-affected areas, a thermo-physical simulation was carried out on a quenching and forming dilatometer Bähr DIL 805 A/D, considering low-alloyed quenched and tempered (Q&T) and low-alloyed thermo-mechanically controlled processed steels (TMCP) with yield strengths in the range of 500 to 960 MPa (S500MC, S700MC, S770QL, and S960QL). For this purpose, time–temperature cycles based on gas metal arc welding (GMAW) were simulated with different maximum temperatures (1200 °C; 1000 °C; 800 °C), representing the typical microstructural regions of the heat-affected zones (HAZ), and cooling times t8/5 (5 s; 12 s; 20 s; 25 s) on miniature tensile specimens. To evaluate the property changes of the characteristic HAZ, tensile tests, hardness measurements, and microstructural investigations were analyzed. The investigations illustrate the significant influence of heat input during the welding process on the resulting mechanical-technological properties and microstructure for both kinds of steel. It was demonstrated that all the steels investigated tend to soften with increasing cooling times. The investigated Q&T steels have a lower risk of falling below the strength of the untreated base material than the investigated TMCP fine-grained structural steels. The considerably pronounced softening also resulted in the minimum strength values not being achieved for certain cooling time ranges.