Abstract: Investigation is made into the causes for early stage failure of cracking or even breaking of the H13 fixed-mandrel of the horizontal steel tube extruding mill as occurring during extruding stainless steel tubes,involving such aspects as crack morphology,hardness,residual austenite,microstructure and operation,etc. As a result,it is identified that the major cause for the said failure is that the H13 fixed-mandrel is not well cooled during the tube extrusion operation,and thus the mandrel’s surface gets secondarily-quenched. Therefore,it is proposed that actions like improving the cooling system of the fixed-mandrel and relevant heat treatment process be taken to prolong the service life of the fixed-mandrel.