The steel pipe is heated to annealing temperature and held for a certain period of time after the furnace slowly cooled to a certain temperature and then out of the cooling of a heat treatment process. Steel pipe annealing process can be divided into: spheroidal annealing, complete annealing, stress relief annealing. The main role of pipe annealing: ① to reduce the hardness of the steel pipe, improve its plasticity to facilitate the subsequent cutting or cold deformation process; ② refine the grain, eliminate organizational defects, uniform internal organization and composition, improve the performance of the steel pipe or to prepare for subsequent processes; ③ to eliminate internal stresses in the steel pipe in order to prevent deformation or cracking.
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Annealing category of seamless steel pipe:
1. Complete annealing
Process: The steel parts are heated to a critical temperature (the critical temperature of different steels is different, generally 710 ~ 750 ° C, individual alloy steel critical temperature can reach 800 ~ 900 ° C) 30 ~ 50 ° C, keep it will be placed for a period of time, and then slowly cooled by the furnace (or buried in the sand to cool).
Purpose: Refinement of the grain, homogenisation of the structure, reduction of the hardness and adequate elimination of internal stresses. Complete annealing is suitable for forgings or steel castings with carbon content (mass fraction) less than 0.8 per cent.
2. Spheroidal annealing
Process: the steel parts heated to 20-30 ° C above the critical temperature, and after insulation will be slowly cooled to below 500 ° C, and then air-cooled from the furnace.
Purpose: To reduce hardness, improve cutting properties and prepare for subsequent hardening to minimise distortion and cracking after hardening. Spheroidal annealing is suitable for carbon and alloy tool steels with carbon content (mass fraction) greater than 0.8%.
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3. Stress relief annealing
Process: Heat the steel to 500 ~ 650 °C, maintain a certain temperature, and then slowly cooled (usually cooled by furnace)
