First, the establishment of heating operating temperature
The working temperature of galvanized pipe heat treatment process should be standardized to obtain tiny and uniform austenite crystallization, which is beneficial to obtain the martensite structure after heat treatment process. The heat treatment process temperature is important according to the zero boundary point of galvanized pipe, galvanized pipe is usually heated to 30~50℃ above Ac3, eutectoid steel pipe, galvanized pipe heated to 30~50℃ above Ac1. If the temperature of galvanized pipe heat treatment process is in the middle of Ac1 ~ Ac3, in addition to the martensite body, a part of the metallographic structure is stored in the heat treatment process structure, so that the strength and tensile strength of seamless pipe are reduced. But the heat treatment process temperature can not exceed the high Ac3 points to avoid the passivation of austenitic crystalline stainless steel. For carbon steel pipe, high carbon steel pipe, if the temperature is slightly lower Ac3 points of the subhumid heat solution, you can obtain the microstructure plus martensite (5%~20%) two-phase structure, not only can ensure a certain tensile strength of seamless pipe, but also can ensure that seamless pipe has good ductility, plasticity and stamping parts processing formability. The electric heater temperature of the eutectoid galvanized steel pipe is limited to 30 ~ 50℃ above Ac1 because of the small austenite crystallization and storage of a small number of ferritic particles, the heat treatment process to obtain the crystalline martensite and its joint distribution of granular pearlite, not only makes the pipe has a large number of tensile strength, compressive strength and wear resistance, but also has very good plasticity. If the temperature of the hypereutectoid steel pipe exceeds Accm, the pearlite will all melt into austenite, so that the carbon composition of martensite increases, reducing the Ms and Mf points of the pipe fitting, the residual austenite volume increases after heat treatment process, will reduce the strength and wear resistance of the pipe fitting. The high temperature of the heat treatment process will passivate the austenitic crystalline stainless steel. In addition, the carbon composition is high. After the heat treatment process, it is very easy to get small and coarse martensite containing optical microscope cracks, so as to expand the ductility of the seamless tube. In addition, the high temperature heat treatment process of thermal stress, oxidation and nitriding is more serious, will also expand the seamless pipe deformation and cracking tendency.
For galvanized pipe heat treatment process temperature should also be established according to the zero boundary point Ac1 or Ac3, fully consider the role of metal material elements, in order to better accelerate austenitizing, heat treatment process temperature can be higher, generally Ac1 or Ac3 above 50~100℃. High carbon tool steel pipe contains more strong pearlite converted into elements, austenitic crystalline stainless steel passivation temperature is high, higher heat treatment process temperature can be used. For galvanized pipes with higher carbon and manganese content, lower heat treatment process temperature should be used to avoid the passivation of austenitic crystalline stainless steel.
