The characteristics and classification of quenching medium

1.1 Characteristics of quenching medium

1.1.1 Appropriate cooling characteristics

 Before carbon steel and low-alloy steel are quenched and cooled to 650°C, the austenite is relatively stable, and it is allowed to cool at a slower rate to reduce the thermal stress caused by the temperature difference between the inside and outside of the workpiece. In the range of 650~450°C, it is required to have a sufficiently fast cooling rate (above the critical cooling rate, avoiding the tip of the C curve nose), below 400°C, especially below the Ms point, slow cooling is required to reduce tissue stress. Prevent excessive distortion and quench cracking.

Different steel grades have different temperature ranges for the most unstable austenite. Therefore, the ideal cooling curve varies with different austenite transformation kinetic curves. It is practically impossible to obtain a quenching medium suitable for various steels and different workpieces. Therefore, it is necessary to understand the cooling characteristics of various quenching media, so as to select suitable quenching media and reasonable quenching operation methods according to the specific parts of different steel grades.

1.1.2 Good stability

The medium has stable performance during use, is not easy to decompose, and does not deteriorate due to aging. Various quenching oils and organic aqueous solutions have varying degrees of aging tendency. Slow-aging and easy-to-maintain varieties should be used as much as possible.

1.1.3 Uniformity of cooling

The cooling of different surfaces and parts of the workpiece cannot be uniform. The degree of this unevenness is related to the type, variety and stirring method of the quenching medium.

1.1.4 It can keep the workpiece clean after quenching and does not corrode the workpiece.

1.1.5 No large amount of smoke is produced during quenching, no toxic and irritating gas can be produced, and the waste liquid brought out does not pollute the environment and meets environmental protection requirements.

1.1.6 Non-flammable and explosive, safe to use.

1.2 Classification of quenching media

1.2.1 Classification according to physical state:

(1) Gas

Air: includes still air, flowing air (natural wind, fan wind), and compressed air.

Reducing gas: such as hydrogen.

Inert gas: such as argon, helium, etc.

(2) Liquid

Water: including clear water, inorganic salt solution, aqueous solution, and organic aqueous solution.

Oil: including vegetable oil, machine oil, special quenching oil (additives in mineral oil)

Molten hot bath: including molten salt bath, molten metal bath.

(3) Solid

 Fluidized bed: including gas-solid fluidized bed, gas-liquid solid fluidized bed.

 Metal plate.

1.2.2 According to whether the state of the medium changes during quenching, there are the following categories:

  (1) Medium with changes in physical state:

  Such as water and various aqueous solutions, various quenching oils.

  (2) Medium without changes in the state of matter

  Such as various gases, molten metals, various salt baths, copper plates, steel plates, gas-solid fluidized beds.