OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 a cast structure that cools quickly and have a large number of oxide inclusions. These characteristics cause high levels of segregation and constant changes in solidification behavior even within the same columnar region [11–16], which makes understanding the microstructure and mechanical properties challenging. The effect of the cooling rate on pipe welding The higher the cooling rate, the higher the mechanical strength. The cooling rate depends on several factors such as: physical properties of the material, preheating, interpass temperature, pipe thickness, welding energy and joint geometry [1, 24]. Preheating is used to reduce the cooling rate. The preheat temperature can be determined based on the carbon equivalent calculation. Fig. 6 shows a graph of preheat temperature versus carbon equivalent for API 5L X100 steels. Fig. 6. The dependences of the preheating temperature on the carbon equivalent for steels and Seferian metal thickness [24] When welding API 5L X80 steel, the preheating values used range from 100 to 150. The author [24] considers the risk of cracking as a function of the preheating temperature and carbon equivalent when using cellulose-coated electrodes. In pipes with thicker walls, the heat transfer to the rest of the base metal is higher, which increases the cooling rate. Consequently, the greater the thickness of the pipe, the higher the cooling rate and, consequently, the hardening obtained in the HAZ. Pipes with thicker walls are also subjected to greater compression during welding, which leads to higher residual stresses [24]. The diameter of the pipe also affects weldability, since large diameter pipes tend to increase the time between passes, causing the weld to cool faster, which can lead to cracking [1]. The influence of structural parameters on the micromechanism of the fracture of a welded joint made of traditional low-carbon low-alloy pipe steels has been the subject of significant work [11–23]. It is shown that the destruction of the metal of the HAZ section of the welded joint of steels of this class occurs by two mechanisms: brittle transcrystalline and viscous. In [36, 37], the influence of bainite structure parameters on the micromechanism of fracture during welding of low-carbon low-alloy high-strength steels (strength categories K65 and K70) was investigated. It is shown that a predominantly bainite structure is formed, which differs from the morphology of traditional pipe steels (as a bainite structure with a granular microstructure, i.e. globular bainite ferrite (GBF), as well as lath bainite ferrite (LBF), consisting of thin long rails combined into large packages of relatively equiaxed shape).
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