What is the heat treatment process of seamless steel pipe?

What are the specific processes for the heat treatment of seamless steel pipe? What precautions should be taken?

[1] Preheating According to the deformation requirements of the die, in order to reduce the deformation of the heat treatment, the large seamless steel pipe is preheated twice to ensure the consistency of the internal and external temperatures, thereby reducing the effect of thermal stress. The first preheating temperature is 400-500°C, and the holding time is 1min/mm; the second preheating is carried out in a fully deoxidized salt bath furnace, the preheating temperature is 840-860°C, and the holding time is 40-60s/mm.

[2] Quenching heating and cooling The selected quenching heating temperature is 980-1000°C. Since the seamless steel pipe is fully preheated, the internal and external temperatures are consistent, so the holding time is calculated as 15-20s/mm. In order to reduce quenching deformation, sufficient pre-cooling should be carried out. When the seamless steel pipe is air-cooled to a light two-color (about 830°C), it is quenched in hot oil at 80-1109°C and cooled in the oil. When it is cooled to 180-200°C in the oil (it smokes but does not catch fire when taken out of the oil), it is taken out and air-cooled.

[3] Low-temperature tempering: Keep it in a nitrate bath at 180-220°C for 1-3 hours, and air-cool it after it is taken out of the furnace. It should be noted that the tempering temperature and holding time should be determined according to the hardness after quenching. When the hardness after quenching is 63-65HRC, tempering at 200TC for 2h can obtain the required hardness of 60-62HRC, and its deformation meets the technical requirements.