After austenitic stainless steel is sensitized, intergranular corrosion is likely to occur in the corrosive medium, intergranular cracking, grain shedding, and even accelerated overall corrosion. Therefore, the two lower heads have no meaning for repair and need to be remanufactured and replaced. There are two solutions for remanufacturing the head to solve the sensitization problem.

　　(1) Changing spinning forming to split forming, avoiding heat treatment, avoiding the sensitization temperature zone of austenitic stainless steel, and maintaining the solid solution state of stainless steel is the most effective way to prevent sensitization and intergranular corrosion.

　　(2) Replace 0Cr18Ni9 with ultra-low carbon austenitic stainless steel 00Cr19Ni10; still use the original forming process, only modify the final heat treatment system to reduce or eliminate the sensitization of the head.

　　Actually adopted the second scheme to manufacture the new head. The key to this plan is that the final heat treatment needs to balance two aspects: First, in order to eliminate the sensitization generated during the forming process, the heat treatment temperature should be high enough, and it is best to reach the solution treatment temperature of austenitic stainless steel, which is combined in the grain boundary. The carbide needs a high enough temperature to start to decompose, and the time should be long enough, and the carbon element in the grain boundary needs time to diffuse and dissolve into the austenite; second, in order to ensure the strength and impact toughness of the base carbon steel, the heat treatment temperature is The time cannot be too high or too long, and it is best to control the normalizing temperature range of carbon steel materials.

　　The sampling simulation test proved that the heat treatment system of holding 980°C for 50 minutes and cooling to 400°C within 22 minutes after the furnace can eliminate or avoid the sensitization of the multi-layer austenitic stainless steel. Although the strength and toughness of the base layer carbon steel are reduced, it can still be Meet the requirements of the corresponding material standards. Therefore, the revised final heat treatment plan is: the holding temperature is still 980°C, and the temperature measuring thermocouple is bolted to the inner surface of the head to ensure that all temperature points on the inner surface of the head reach 980°C; extend the holding time to 50min; The head is out of the furnace and is cooled by water mist to ensure that the temperature of the head is cooled to 400 ℃ within 22 minutes. After the final heat treatment of the new head was completed, the metallographic coating inspection showed that no sensitization phenomenon was found; the structure is austenite, the austenite grain boundary is not obvious, and the small rectangular twins are visible, which is a solid solution structure.