Recrystallization in Welded Joints of Steamers During Long-Term Operation

Abstract

It has been established that due to the peculiarities of the recrystallization process, the structure of welded joints of steam lines made of heat-resistant pearlite steels 15Kh1M1F and 12Kh1MF under creep conditions of more than 270 thousand hours undergoes recovery processes. The process of resting with an increasing service life of welded joints accelerates after their service life exceeds 270 thousand hours and begins to transition to recrystallization. During the recrystallization process, individual grain boundaries gradually crumble, which leads to an increase in their size and a change in shape. The speed of recrystallization processes in welded joints’ heat-affected zone (HAZ) differs significantly. The recrystallization processes in the heat-affected zone areas’ structure are more intense compared to the base metal of welded joints that have not been exposed to welding heat. When a common boundary between two contacting grains is crumbled, a single grain is formed, eventually acquiring the same crystallographic orientation. In the area of overheating of the zone of thermal influence of welded joints made of 12Kh1MF steel, the average initial grain size corresponded to the 9th point (DSTU 8972:2019). After 280 thousand hours of operation (about 40 years), it is the 7th. Such processes are accompanied by a decrease in the density of dislocations in a-phase grains and along their boundaries, which does not significantly affect the decrease in mechanical properties. The dependence of the recrystallization on the self-diffusion of alloying elements, the distribution of dislocations in the grains of the a-phase, and the coagulating carbides was established. In this case, the recrystallization process results in a decrease in the mechanical properties of welded joints. Recrystallization in incomplete recrystallization, overheating, and fusion of the filler material is more intense than in the weld metal and the base metal of the welded joints. Recrystallization contributes to damage to welded joints by creep and fatigue mechanisms.