Changes of the Fracture Toughness of ZG40Cr25Ni20 Furnace Tube after Service and the Corresponding Change of the Microstructure of the Fracture Surface

According to the principle of fracture mechanics, the fracture toughness of the furnace tube after 30000 hours service and new furnace tube were tested with the MTS810 tester. The microstructure of fracture surface were analysed by the SEM. The results show that the fracture toughness of the furnace tube after service decreases greatly compared with that of the new furnace tube. The microstructure of fracture surface of the furnace tube changes significantly corresponding to the change of the fracture toughness.