OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 30], and the signifi cant deterioration of IC-CGHAZ is associated with the presence of a blocky component of the martensite-austenite grain boundary (M-A). It is well known that the HAZ is the weakest part of the welded joint and determines the safety of pipeline operation. In particular, the lowest impact toughness was obtained in the coarse-grained microstructure of the HAZ [16–19], which is adjacent to the fusion line of the weld. Low impact toughness of the HAZ at low temperatures is the main problem limiting the use of highquality steels for pipelines [2–4, 11–29]. Description of microstructures It should be recognized that the problems of welding high-strength steels are far from being solved. For example, it is known that non-metallic inclusions formed in the cast weld metal have two opposite eff ects on impact strength [30]. Firstly, inclusions act as initiation sites for both ductile and shear fractures [29–31], and secondly, it can promote the formation of acicular ferrite, which is recognized as the most optimal microstructure [31–42]. One of the basic requirements for pipeline joints is to obtain weld metal of equal or higher strength than the base material to avoid localized deformation or failure of the weld under load. However, suffi cient strength is also required, which is usually verifi ed using Charpy impact tests. A common solution is to develop the weld metal to produce acicular ferrite (AF) in the metal structure, which provides a balance between strength and toughness [28, 29]. This fact has stimulated extensive research into the mechanisms of AF formation in weld metals and the determination of what factors control its formation [11–39]. The key factor in the formation of AF is the chemical composition of the consumable welding wire, both from the point of view of the isolated eff ect of each element and the combined eff ect of the general composition [29–38]. In a review [32] on the formation of acicular ferrite in carbon-manganese surfacing, it was reported that the following elements infl uence the formation of acicular ferrite: C, Mn, Si, Ni, Ti, Al, Mo and Nb. The eff ect of austenitization temperature in the range of 850–1,000 °C on acicular ferritic transformation in Cr65 HSLA pipeline steel was investigated [31]. As shown in fi gure 5, the initial and fi nal phase transformation temperatures during continuous cooling, namely Ar1 and Ar3, respectively, decreased with increasing austenitization temperature. This result suggests that increasing austenitization improves the stability of austenite during cooling and thus delays the decomposition of austenite. The decomposition products of austenite in Cr65 steel consist mainly of polygonal ferrite, pearlite, acicular ferrite, etc. [30]. Increasing the austenitization temperature promotes the formation of acicular ferrite and prevents the formation of pearlite and polygonal ferrite (see fi gure 6). A higher austenitization temperature leads to more suffi cient dissolution of carbide-forming elements such as Nb, V and Ti, as well as more suffi cient homogenization in the austenite [30]. The authors of [31] believe that dissolved alloy elements improve the stability of metastable austenite. Thus, according to the authors, the decomposition of austenite is delayed to a lower temperature, which is also confi rmed by fi g. 3. As a diff usionless reaction [32], the acicular ferrite transformation is more likely to occur at a relatively low temperature than the diff usion-controlled pearlite or polygonal ferrite transformation [33], since the rate of atomic diff usion decreases with decreasing temperature. Fig. 5. The fractions of phase transformation determined by dilatometric measurements as a function of temperature during continuous cooling in X65 pipeline steel specimens austenitized at diff erent temperatures from 850 °C to 1,000 °C [30]
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