Abstract: The typical inactiviation forms of retained-mandrel of the Diescher pipe mill are thermal wear and thermal fatigue. Thermal wear mainly includes particle wear and sticking wear,while thermal fatigue mainly includes net-like cracking and ring-like cracking, both promote the wear development. To enhance thermal-wear resistance of the mandrel, it is required to use mandrel material which has dispersed carbide, linear expansion factor of the base metal close to that of the chide film, minimum elastic modulus and maximum thermal conductivity factor. The high-temperature strength and alloy carbide quantity and morphology of the material are key elements for adjustment of all the above said characteristics. It is a very cost-effective way to make the retained mandrel with proper plug material and make the die-forged piercing plug with scrapped mandrels.