Abstract: The chains of the slag extractor of a certain ultra supercritical boiler repeatedly broke. The causes of chain fracture were studied using methods such as macroscopic observation, chemical composition analysis, mechanical property testing, metallographic examination and scanning electron microscopy analysis. The results show that the content of Cr and Mo elements in the chain was low, and the chain was not subjected to tempering treatment, resulting in excessive brittleness of the material and susceptibility to brittle fracture under external impact. After long－term service, the surface hardening layer of the chain exhibited severe intergranular cracking, and there were too many inclusions inside the chain, causing the crack to propagate inward and ultimately led to chain fracture.