Abstract
When the weldability of high strength steels is analyzed, it is the softening in the heat-affected zone (HAZ) that is mostly investigated, and the reduction of toughness properties is generally less considered. The outstanding toughness properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the unfavorable microstructural changes in the HAZ. Relevant technological variants (t8/5 = 2.5–100 s) for arc welding technologies were applied during the HAZ simulation of S960QL steel (EN 10025-6) in a Gleeble 3500 physical simulator, and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analyzed. Thermal cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by scanning electron microscope, macro and micro hardness tests and instrumented Charpy V-notch pendulum impact tests. The examined HAZ subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Due to the observed brittle behavior of all subzones in the whole t8/5 range, the possible lowest welding heat input should be applied in order to minimize the volume of HAZ that does not put fulfillment of the allowed maximal (450 HV10) hardness at risk and does not lead to the formation of cold cracks.
Highlights
High strength steels have an increasingly important role in engineering applications, especially in the vehicle and transportation industry, where the producers are faced with the challenges of strict requirements for lower CO2 emission, and there is customer demand for lower operational costs
By the combination of alloying elements, rolling and heat-treating processes, various high strength steel grades were developed in the past few decades, and nowadays high strength steels are available in thin sheets and thicker plates
The weldability of S960QL according to EN 10025-6 is investigated in the aspect of microstructural changes in the heat-affected zone (HAZ)
Summary
High strength steels have an increasingly important role in engineering applications, especially in the vehicle and transportation industry, where the producers are faced with the challenges of strict requirements for lower CO2 emission, and there is customer demand for lower operational costs.By the combination of alloying elements, rolling and heat-treating processes, various high strength steel grades were developed in the past few decades, and nowadays high strength steels are available in thin sheets and thicker plates. Besides increasing the strength and toughness properties, the development of weldability characteristics plays an important role in steel producers. Quenched and tempered (Q + T) steels are considered the highest strength structural steels [2,5]. Due to their outstanding mechanical properties, especially strength, significant weight reduction can be achieved by their application. Besides their high yield strength, they have good toughness properties thanks to the fine-grained tempered martensitic and bainitic microstructure [7]. Due to water cooling during quenching and to the application of high temperature tempering (HTT), this grade generally has a non-equilibrium
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