OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Consumables with a richer chemical composition (C, Ni and Ti) showed higher strength and hardness due to the fi ner microstructure of the fi nal weld metal; however, Charpy impact test results showed that the depleted chemical wire had higher impact strength at low temperature. Since both weld metals had a similar acicular ferrite structure, the lower toughness of the richer weld was attributed to the presence of titanium inclusions, which could become crack initiation sites. The eff ect of welding method and preheating on the weld metals of pipeline steels was investigated in the work of HSLA steel. It is known that post-welding treatment reduces the strength characteristics of the metal of the welded joint [2, 4]. The results of studies [4] assessing the preheating up to 200 °C and welding treatment of welded joints showed a tendency towards a decrease in mechanical strength and an increase in impact strength as a consequence of some important aspects, such as a lower percentage of martensite, coarsening of the microstructure and a higher proportion of high-angle boundaries (> 15 %). Longer cooling times (time spent in the temperature range of 800–500 °C) show a tendency to improve the impact toughness and reduce the mechanical strength of deposited metals in high-strength steels. Microstructure features aff ecting the impact strength of weld metals Multi-pass welding is required to connect the main pipes. This leads to overheating of the HAZ. This creates its own peculiarities of thermal eff ects on the metal and, as a result, non-classical phase and structural transformations with sharp temperature and stress gradients. The HAZ zone can be divided into coarse-grained HAZ (CGHAZ), fi ne-grained HAZ (FGHAZ), intercritical HAZ (ICHAZ) and subcritical HAZ (SCHAZ), when a single thermal cycle is applied to weld the material [43]. When a second weld bead is applied over an existing one, it results in the formation of a plurality of reheated HAZ structures that are characterized by corresponding second peak temperatures and include supercritical, intercritical and subcritical structures. The strength and toughness of HSLA pipeline steel can degrade signifi cantly after one or two welding thermal cycles, so CGHAZ with intercritical reheating (ICR) are often considered to be the weakest link or most fragile area of the weld joint. A schematic representation of a weld with diff erent heat-aff ected zones is shown in fi gure 7 [38]. Various metallurgical factors such as austenite grain size and bainite stack size, as well as the size, shape and distribution of any second phase (carbide or martensitic-austenitic) can infl uence fracture toughness. In particular, the presence of so-called martensitic-austenitic (MA) constituents formed in ICR CGHAZ plays a decisive role in the fracture toughness at low temperatures. Fig. 7. Schematic representation of microstructures in the heat-aff ected zone of multi-pass welds [43]
RkJQdWJsaXNoZXIy MTk0ODM1