Annealing process commonly used in steel pipe heat treatment equipment

Complete annealing and isothermal annealing in heat treatment annealing!

1. Complete annealing

Process: heating the steel pipe to 20 ~ 30 ℃ above AC3, holding for a period of time, then slowly cooling (with furnace) to obtain the heat treatment process (complete austenitization) which is close to the equilibrium structure.

Complete annealing is mainly used for hypoeutectoid steel (WC = 0.3 ~ 0.6%), generally for medium carbon steel and low and medium carbon alloy steel castings, forgings and hot rolled profiles, and sometimes for their weldments. The results show that the hardness of low carbon steel after complete annealing is low, which is not conducive to cutting; when hypereutectoid steel is heated to austenite state above ACCM and annealed slowly, Fe3C Ⅱ will precipitate along grain boundary in network form, which significantly reduces the strength, hardness, plasticity and toughness of steel, leaving hidden trouble for final heat treatment.

Objective: to refine grain, uniform structure, eliminate internal stress, reduce hardness and improve machinability of steel tube. The microstructure of hypoeutectoid steel after complete annealing is F + P.

In actual production, in order to improve productivity, annealing and cooling to 500 ℃ or so is out of furnace air cooling.

2. Isothermal annealing

Complete annealing takes a long time, especially for the alloy steel with stable undercooling austenization. If the austenitized steel is rapidly cooled to a temperature slightly lower than AR1, then a is transformed into P, and then air cooled to room temperature, the annealing time can be greatly shortened. This annealing method is called isothermal annealing.

Process: heating the steel pipe to a temperature higher than AC3 (or AC1), holding for an appropriate time, cooling quickly to a certain temperature in pearlite area, and holding isothermal, so as to transform austenite into pearlite, and then air cooling to room temperature.

Objective: as the same as complete annealing, the transformation is easy to control.

Suitable for a stable steel: high carbon steel (WC > 0.6%), alloy tool steel, high alloy steel (total alloy elements > 10%). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. But it is not suitable for large cross-section steel parts and large batch of furnace charge, because isothermal annealing is not easy to make the internal parts or batch parts reach isothermal temperature.