Key points for heat treatment of quenched and tempered steel

Quenching and tempering is a heat treatment process used to improve the mechanical properties of metals and alloys. The process involves heating the material to a high temperature, then cooling it rapidly (quenching), followed by reheating it to a lower temperature (tempering).Here are the key points for heat treatment of quenched  and tempered steel:

1.Furnace Loading

When loading the furnace, attention must be given to the loading quantity and method. In a box-type furnace, parts should not be stacked on top of each other. The spacing between parts should be at least half the thickness of the part to ensure uniform heating.

2.Heating and Soaking

The purpose of quenching and heating is to fully austenitize the steel, resulting in fine, uniform austenite grains, which are crucial for the subsequent formation of martensite during cooling. Therefore, selecting the appropriate heating temperature is essential.

For small-section carbon steel parts quenched in oil, the upper limit of the heating temperature should be used to increase hardenability. For low-alloy steel parts quenched in water, the lower limit should be used to prevent deformation and cracking. For parts quenched in salt baths, the heating temperature should be 10–20°C higher than usual, or the upper limit should be used.

heat treatment of quenched and tempered steel

For large parts, the upper limit of the heating temperature should be chosen, with the lower limit of soaking time. For smaller parts, the lower heating temperature should be selected, with the upper limit of soaking time. Parts with varying cross-sections or complex shapes that are prone to deformation should be quenched at lower temperatures.

When tempering large parts with diameters or thicknesses exceeding 150mm, a step-heating method should be used. The initial furnace temperature should not exceed 450°C, with soaking for 1–3 hours before raising the temperature at 30–70°C/h to 600–650°C. After soaking for some time, the temperature should be directly increased to the desired heating temperature.

Alloy elements significantly impact the heating process of quenched and tempered steel. Special care should be taken to avoid overheating manganese steel and decarburization of silicon-containing steel. The heating temperature and duration should be carefully controlled for these materials.

3.Quenching and Cooling

During quenching, it is crucial to strictly control the cooling time to ensure the part fully transforms to the desired martensitic structure and maintains uniformity across the cross-section. Depending on the shape, material, and specific conditions of the part, various quenching methods, such as pre-cooling quenching, dual-medium quenching, or stepped quenching, may be employed.

4.Tempering

To achieve the desired mechanical properties, quenched and tempered parts are typically tempered at high temperatures between 500°C and 650°C. Alloy steel requires tempering at temperatures 30–50°C higher than carbon steel. To prevent cracking, large parts, parts quenched in water, or oil-quenched parts should be tempered immediately after quenching.

To minimize internal and external temperature differences and reduce stress during tempering, large parts should be initially heated to 350–400°C, soaked for 2–5 hours, and then brought to the final tempering temperature. After tempering, large parts should be cooled in the furnace to around 400–500°C before air cooling. For steels prone to secondary temper brittleness, rapid cooling (oil or water) after tempering is necessary, followed by additional tempering at a temperature below 450°C to eliminate residual stresses without inducing brittleness.

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