A concentrated downcomer stub of a boiler drum with 13MnNiMoR steel in a power plant

which was replaced due to cracks and non-metallic inclusions after 14 years in service

was dissected

and the behavior characteristics and influence of the non-metallic inclusion were analyzed through chemical composition analysis

mechanical property testing

microstructure and defect morphology observation. The results show that under service load

non-metallic inclusions become crack sources and microcracks by means of self cracking

interface separation from matrix or hole formation at the end

and microcracks converge to form macro cracks. When tensile and impact tests on the serviced materials

the area without inclusions was cracked and expanded in plastic mode. While innonmetallic inclusions area

the cracks nucleated with non-metallic inclusions and expanded in a brittle mode

resulting in a significant reduction in the strength and plastic toughness of the materials compared with that before service.