Relationship Between Thermal Processes in Welds and Their Morphology and Promising Types of Welds for Thick-Walled Electric-Welded Pipes

The increase being seen in the thickness of the wall of large-diameter pipes is requiring an increase in heat input to form the longitudinal weld on these pipes. Increasing the heat input in turn slows the cooling of the heat-affected zone (HAZ) and reduces the reliability of the weld. A study of the morphology of the structure of the HAZ showed that it is nonuniform. The most critical section of the HAZ is located at 1/2 of the height of the external weld, which is related to its minimal cooling rate. Theoretical formulas were derived to determine cooling rates in different sections of the HAZ. Scanning measurements of the hardness of a weld revealed the existence of a region in the HAZ in which hardness decreases. The decrease in hardness is due to the fact that part of the HAZ is heated within the intercritical temperature range. Factory tests were performed with the use of laser-arc welding following by multi-electrode submerged-arc welding. It is shown that the use of a hybrid welding technology makes it possible to significantly reduce heat input in the welding of thick-walled pipes.