Heat treatment process of quenched and tempered steel

Quenched and tempered steel undergoes a heat treatment process that includes both preliminary and final heat treatments. The objective is to achieve steel with a balanced combination of strength, toughness, and hardness, making it ideal for mechanical parts subjected to complex stresses.

(1) Preliminary Heat Treatment

Preliminary heat treatment is performed on quenched and tempered steel after forging and rolling but before cold working. The purpose of this stage is to reduce hardness, refine grain structure, eliminate undesirable microstructures, and prevent defects such as white spots. This treatment also prepares the steel’s microstructure for the final quenching and tempering treatment.

Heat Treatment Process of Quenched and Tempered Steel

For carbon steel and low-alloy steel, normalizing or annealing can be used for preliminary treatment.

For high-alloy steels with greater hardenability, normalizing plus high-temperature tempering or full annealing is recommended.

Large parts should always undergo preliminary heat treatment (normalizing or annealing), followed by inspection to ensure no internal defects (such as white spots or cracks) before moving on to the final heat treatment. For boron steels, the preliminary treatment should involve normalizing and high-temperature tempering. Slow cooling at around 750°C should be avoided, as it may lead to “boron embrittlement”; hence, annealing is not recommended.

(2) Final Heat Treatment

The final heat treatment for quenched and tempered steel typically involves quenching and tempering. The aim is to produce tempered sorbite (or a mixture of tempered sorbite and troostite), which offers excellent mechanical properties like strength, ductility, and toughness.

1.Quenching and Tempering Process

① Quenching Heating Temperature

The quenching heating temperature for quenched and tempered steel is set at Ac3 + 30-50°C. Since quenched and tempered steels are hypoeutectoid steels, heating at too low a temperature will leave undissolved proeutectoid ferrite in the microstructure, reducing hardness and strength. Overheating will cause coarsening of austenite grains, deteriorating the steel’s mechanical properties. Manganese steel and boron steel are particularly prone to overheating, requiring careful temperature control during quenching.

② Quenching Heating Soak Time

The soak time for quenching must be sufficient to allow enough carbide to dissolve into the austenite and to ensure uniformity.

For air resistance furnaces, the soak time is 1-1.2 min/mm for carbon steel and 1.2-1.5 min/mm for low-alloy steel.

In salt bath furnaces, the soak time is reduced to 0.3-0.5 min/mm for carbon steel and 0.5-0.8 min/mm for low-alloy steel.

③ Quenching Cooling

Quenching cooling is a critical step in the heat treatment of quenched and tempered steel. The choice of quenching medium and cooling method should match the steel’s hardenability and the size of the part, ensuring complete hardening.

Carbon steel generally requires water quenching or a combination of water quenching followed by oil cooling.

Low-alloy steels can be quenched in oil.

For alloyed quenched and tempered steels with more alloying elements, slower quenching media and gentler cooling methods may be used.

For large alloy steel parts, water quenching followed by oil cooling can be employed to enhance the depth of the hardened layer.

④ Tempering

The tempering temperature is selected based on the required hardness of the part. Tempering is usually carried out at 500-650°C to achieve tempered sorbite with a good balance of strength and toughness.

The heat treatment process for quenched and tempered steel is meticulously designed to optimize mechanical properties, ensuring steel components meet their performance demands. The combination of quenching and tempering allows engineers to finely tune the hardness, strength, and toughness of steel, making it suitable for applications requiring high reliability under complex stress conditions.

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