Abstract: Molybdenum piercing mandrel matched with 2Cr13 martensite stainless steel beam section was used by a steel pipe plant for hot-rolling 13Cr steel tubes, and it was found that the beam sections fractured frequently at the connecting top rod position during use. The fracture reasons were analyzed by means of macro observation, chemical composition analysis, metallographic examination and so on. The results show that the main reason for the facture was overheating which resulted in coarse grain and a great deal of δ-ferrite on the grain boundary. It was not only reduced the toughness and plasticity of the material, but also destroyed the continuity of the matrix, so intergranular birttle fracture occurred to the beam sections.