OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Copper has been reported to contribute to the formation of AF when using manual arc welding [32]. Many elements will combine with the oxygen present in the weld metal, which can be controlled by shielding gas and/or the composition of the weld metal. The oxygen reaction infl uences AF formation by either promoting or inhibiting the formation of nonmetallic inclusions such as oxides. Some authors argue [29–48] that oxides act as nucleation sites for AF, so an increase in oxygen content favors the formation of AF. For example, it was reported [33, 34] that increasing oxygen content to 300 ppm changed the weld metal of Widmanstätten side plates to AF microstructure [33, 34]. The formation of AF is also facilitated by coarse austenite grains with a large number of inclusions with a diameter of more than 0.2 μm. The details of the formation of AF are now well described as a variant of the bainite structure in numerous works by Bhadeshia and his students [48], where it is shown that it is a special variant that depends on intragranular formation [33]. Thus, it is necessary to achieve suffi cient preliminary austenite grain size and number density of non-metallic inclusions of favorable chemical composition, especially based on titanium oxides. However, it has also been noted in many works that if the amount of non-metallic inclusions reaches a certain level depending on the oxygen content, it has a detrimental eff ect on the toughness as the crack initiation sites outweigh the benefi ts of achieving a fi ne AF structure. Two welding materials suitable for joining Cr80 steel pipes are compared in terms of weld metal microstructure, hardness, toughness and tensile properties [35]. The chemical composition of the consumables was similar: one of the consumables had a rich chemical composition of the wire and contained higher alloying additives C, Ni, Ti compared to the depleted wire. Deposit welding was performed using a gas arc welding (GMAW) process system to achieve the same heat input of 0.66 kJ/mm. The results showed that for both wires, the microstructure of the weld metal was mainly composed of acicular ferrite. a b c d Fig. 6. Optical micrographs of X65 pipeline steel specimens after continuous cooling with diff erent austenitization temperatures: 850 °C (a), 900 °C (b), 950 °C (c) and 1,000 °C (d) [30]
RkJQdWJsaXNoZXIy MTk0ODM1