Circumferential Cracking Reason of 15CrMoG Steel Connecting Tube for Boiler Transition-Section Water Wall during Installation Stage

The circumferential cracking reason of 15CrMoG steel connecting tube of transition-section water wall during installation stage had been analyzed by macroscopic examination, chemical composition analysis, mechanical property testing, microstructure observation and fracture appearance analysis. The results show that the great stress concentration occurred in incomplete penetration of fillet weld at tail of fin after butt-weld, microstructure contained martensite+bainite hardening structure and hardness was extremely high. The pipeline restraint stress and welding residual stress caused the crack initiation from this location. Base metal of the area had high hardness and low plasticity and the crack expanded along the tube circumference under the transverse tension stress, which resulted in the brittle cracking of tube at the circumferential direction.