PSI - Issue 68

D. Tomerlin et al. / Procedia Structural Integrity 68 (2025) 1237–1244 D. Tomerlin et al./ Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction When employing the fusion welding process, several characteristic zones can be distinguished, namely the base metal (BM), weld metal (WM), and heat affected zone (HAZ). Therefore, welded joints are significantly heterogeneous structures. During the investigation of the welded joints' heterogeneous properties, mechanical testing of certain HAZ regions using conventional test specimens is difficult or sometimes even impossible. The inability to precisely position and extract mechanical test specimens from the narrow HAZ regions is a limiting factor during conventional mechanical testing. Therefore, researchers such as Kumar et al. (2014), Kim et al. (2015) and Tomerlin et al. (2023 and 2024) have already investigated some alternative methods. For example, the HAZ physically simulated, at the laboratory scale result in samples with homogeneous and reproducible microstructures ready for mechanical testing. The use of thermo-mechanical simulations has been previously investigated by various researchers: Bertolo et al. (2023), Gáspár (2019) and Neubert et al. (2018). The authors of this paper conducted some previous research of the topic using a Gleeble ® 3800 simulator, Tomerlin et al. (2023), laying the foundations for this work and further investigation. Thermal cycles that occur during welding significantly influence the microstructure and mechanical properties of the welded joints, which is especially evident in HAZ. The HAZ consists of several characteristic regions with specific microstructures and mechanical properties. In single-pass welds, those HAZ regions as arranged in the direction from weld metal to base metal, are as follows: the coarse-grained zone (CGHAZ), fine-grained zone (FGHAZ), inter-critical zone (ICHAZ), and sub-critical zone (SCHAZ). In two-pass welds, 1 st pass CGHAZ is reheated by 2 nd pass, and the formation of new sub-regions takes place: sub-critical (SC CGHAZ), intercritical (IC CGHAZ), supercritical (S CGHAZ) and unaltered (U CGHAZ), proposed by Guillal et al. (2018), shown in Fig. 1.

Fig. 1. Two-pass welding process HAZ regions and corresponding thermal cycles

Welding of HSS grades in general requires establishing and following of welding procedure to achieve the least degradation of mechanical properties in the heat-affected zone (HAZ). Maintaining the low heat input during the welding process, with optimal Δ t 8/5 HAZ cooling time between 800°C and 500°C, narrows the soft zone, thus improving the overall welded joint mechanical properties, Hochhauser et al. (2012). This work investigates the influence of Metal Active Gas (MAG) fusion welding process parameters in multi-pass configuration on the microstructure and mechanical properties using the physical thermo-mechanical simulations. The monotonic tensile and hardness properties were previously determined on S690QL bar specimens subjected to two-pass welding on Gleeble simulator, Tomerlin et al. (2023). The current work focuses on the fracture behavior of simulated HAZ regions using the experimental testing methods: ASTM E1820 fracture toughness and ASTM E647 fatigue crack growth.