Complete Annealing Process For Steel

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Complete annealing is to heat the steel or steel to Ac3 or more, and hold it for a long time enough to make the structure completely austenitized and then slowly cool it to obtain a heat treatment process that is close to the equilibrium structure. It is mainly used for hypoeutectoid steel (wc=0.3%~0.6%), and its purpose is to refine the grain, uniform structure, eliminate internal stress, reduce hardness and improve the workability of steel. Low carbon steel and hypereutectoid steel should not be fully annealed. The hardness of mild steel is low after being fully annealed, which is not conducive to cutting. When the hypereutectoid steel is heated to an austenite state above Accm and slowly cooled and annealed, a network of secondary cementite is precipitated, which significantly reduces the strength, plasticity and impact toughness of the steel.

In medium carbon structural steel castings and forged (rolled) parts, common defect structures include Widmanstatten structure, coarse grains and banded structures. The structure at the weld seam of the welding workpiece is not uniform, and the heat-affected zone has overheated structure and Widmanstatten structure, resulting in a large internal stress. Widmanstatten structure and coarse grains will significantly reduce the plasticity and impact toughness of steel, while the banded structure makes the mechanical properties of steel anisotropic, with low reduction of area, especially low transverse impact toughness. Through complete annealing, the grains of the steel are refined and the structure is uniform, the Widmanstatten structure is difficult to form, and the band structure can be eliminated.

Full annealing adopts slow cooling along the furnace, which can ensure the precipitation of proeutectoid ferrite and the transformation of subcooled austenite into pearlite in the higher temperature range below Ar1, so as to eliminate internal stress, reduce hardness and improve workability the goal of.

Determination of complete annealing process parameters:

1. Heating temperature. The complete annealing temperature must be appropriately higher than the Ac3 point. In principle, the carbon steel is Ac3+30~50℃, and the alloy steel is Ac3+50~70℃.

2. Holding time. The holding time of the workpiece at the annealing temperature must not only ensure that the workpiece is burnt through, that is, the core of the workpiece reaches the required heating temperature, but also ensure that all homogenized austenite is obtained. The holding time of complete annealing is related to factors such as steel composition, workpiece thickness, furnace loading capacity and furnace loading method. Generally, the heating time is calculated based on the effective thickness of the workpiece. For general carbon steel or low-alloy steel workpieces, when the furnace load is not large, the holding time for annealing in the box furnace can be calculated as follows:

t=KD

t- holding time (min);D-The effective thickness of the workpiece (mm);K-heating coefficient, generally K=1.5~2.0min/mm.

3. Cooling rate. The cooling rate depends on the steel grade and performance requirements. The general principle is to transform the structure in the pearlite region. If the cooling is too fast, the resulting pearlite flakes will be too thin and the hardness will be too high, which is not conducive to cutting; if the cooling rate is too slow, the productivity will be reduced, and coarse massive ferrite will appear. Cooling can be roughly controlled in this way, 100~200℃/h for carbon steel and 50~100℃/h for alloy steel. In short, the cooling after annealing should be sufficiently slow to ensure that the austenite undergoes pearlite transformation under the condition of low undercooling below the A1 temperature, otherwise the pearlite structure with large dispersion will be obtained due to the large undercooling. , Make the hardness higher. When the temperature drops to about 600°C, the transformation of austenite to pearlite has been completed. In order to improve productivity, air cooling can be carried out at this time.

Editing by Baccata

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