A large area cracking and leakage occurred in weld seam of water wall pipe ring of a supercritical 350 MW unit at the early stage of deep peak shaving operation. To find out the reasons

multiple methods such as macro inspection

chemical composition analysis

tensile test at room temperature

hardness test

metallographic structure analysis and fracture analysis were carried out. The results show that

the chemical composition

hardness

room temperature tensile properties and metallographic structure of the tube samples met the requirements

and the tube samples were deformed from the girth weld as the inflection point to the furnace

the backfire surface of the girth cross section was cracked from the inner wall to the outer wall

and the cracking property of the water wall tube was fatigue cracking. According to the degree of spheroidization of metallographic structure of different tube samples

it is determined that the wall temperature between tubes was uneven. The main reason for the cracking of water wall tube is that the water wall tube is frequently overheated in deep peak-shaving operation of the unit

so the circumferential alternating thermal stress is produced in the inner wall of the tube.