Isothermal annealing process of steel tube heat treatment

    It takes a long time to complete annealing, especially for alloy steels that are relatively cold under austenite. If the austenitized steel is cooled to a temperature slightly lower than the temperature of Ar1, the temperature is changed from A to P and then air-cooled to room temperature, which can greatly shorten the annealing time. This annealing method is called isothermal annealing.

    Process: The steel is heated to a temperature higher than Ac3 (or Ac1), and after being held for a proper time, it is cooled to a certain temperature in the pearlite region quickly, and maintained isothermally, so that the austenite is transformed into pearlite, and then air-cooled to room temperature Heat treatment process.

    Purpose: As with full annealing, the transition is easier to control.

    Suitable for A more stable steel: high carbon steel (wc> 0.6%), alloy tool steel, high alloy steel (total amount of alloy elements> 10%). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. However, it is not suitable for large-section steel parts and large batches of charge, because isothermal annealing is not easy to make the workpiece or batch workpieces reach isothermal temperature.

    Process: The steel is heated to 20 ~ 30 ° C above Ac3, and heated for a period of time (using a furnace) to slowly cool down to obtain a heat treatment process (complete austenitization) close to the equilibrium structure.

    Full annealing is mainly used for hypoeutectoid steel (wc = 0.3 ~ 0.6%), usually used for medium carbon steel and medium and low carbon alloy steel castings, forgings and hot-rolled profiles, and sometimes its welded parts. Low carbon steel has a lower hardness after being fully annealed, which is not conducive to cutting. When the supereutectoid steel is heated above Accm and slowly cooled and annealed, Fe3CII will precipitate in a network shape along the grain boundaries, thereby making the steel strong, hard, and malleable. And the toughness is significantly reduced, leaving a hidden danger to the final heat treatment.

    China Steel Pipe Information Port Investigation Team was informed: Purpose: To refine grains, make the structure uniform, eliminate internal stress, reduce hardness and improve the machinability of steel. The microstructure of the fully annealed hypoeutectoid steel is F + P.

    In actual production, in order to improve productivity, it is air-cooled to cool the annealing to about 500 ° C.

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