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What Factors Should be Considered in the Design of Large-scale Quenching and Tempering Heat Treatment Process

During the quenching and tempering heat treatment of large parts (diameter ≥100mm), due to the limitation of hardenability, heat dissipation conditions and quenching medium, the core is allowed to have lower bainite, fine pearlite or ferrite and upper bainite; at the same time, Large parts have more internal defects, which are likely to cause greater internal stress, so the tendency to crack is greater. The main considerations are as follows.

1. Heating temperature

1) Heating temperature. The upper limit of the quenching heating temperature is often used to increase the hardenability of the steel.

2) Heating method

① In general, low-temperature furnaces are used. For carbon steel and low-alloy steel workpieces with a simple shape and a diameter of less than 400mm, the temperature can be raised with the furnace, but the heating speed in the low temperature stage should not be too fast, generally controlled at 30-70°C/h.

② For medium alloy steel workpieces with more complex shapes, heating should be done in sections. The heating rate in the low temperature stage is controlled at 30～70℃/h. Keep it at 600～650℃ for a period of time, and then increase to the quenching heating rate.

3) Uniform temperature. The temperature equalization time generally takes 1 to 2 hours.

4) Holding time. For low hardenability steels, the thermal insulation coefficient is 0.6～0.8h/100mm; for medium alloy steels, 0.8～1.0 h/100mm.

2. Quenching and Cooling

1) Cooling method. Generally, water cooling, water quenching and oil cooling, water→air→water cooling methods can be used; when water quenching tends to crack, oil→air→oil, polymer quenching methods can be used.

2) Cooling time. The cooling time can be calculated as follows:

τ=αD

Where τ-cooling time, s; α-effective thickness of the workpiece, mm; D-factor, 1.5-2 for water quenching, 9-13 for oil quenching, and 0.8-1.0 for water quenching and oil cooling. Oil cold takes 7-9.

3. Tempering

1) Control the time interval between tempering and quenching. The general requirements are as follows: carbon steel and alloy structural steel, when the diameter is <700mm, the time interval should be <3h; when the diameter is >700mm, the time interval is <2h; for medium alloy steel, the time interval is ≤2h. Water-quenched, water-quenched and oil-cooled workpieces should be tempered immediately.

2) Enter the tempering furnace temperature. All furnaces must be installed at low temperatures.

3) Heat up. The heating rate is generally controlled at 30-100°C/mm.

4) Tempering temperature. Generally, it should be lower than the tempering temperature of small parts.

5) Uniform temperature and holding time. The temperature equalization time is estimated as 1 h/100mm, and the holding time is estimated as 2h/100mm. When the cross-section of the workpiece is small, the holding time should be appropriately extended. Generally, the holding time can be increased by 30% to 50%.

6) Cooling after tempering. For steel with temper brittleness, it must be cooled quickly after tempering. In order to eliminate stress, supplementary tempering can be performed below 450℃; for steel without temper brittleness, the furnace is cooled to 400～450℃ after tempering ( (Alloy steel 500～600℃) After isothermal for a period of time, it is discharged from the furnace and air-cooled.