Heat treatment methods for ductile iron

Ductile iron, known for its excellent strength and toughness, undergoes various heat treatment methods to enhance its mechanical properties. Common techniques include annealing, normalizing, and tempering. Annealing helps to reduce stress and improve ductility by decomposing cementite, while normalizing refines the microstructure by controlling cooling rates. Tempering is often applied after quenching to relieve internal stresses and increase toughness. High and low-temperature annealing are used depending on the matrix structure, such as free cementite or ferrite + pearlite + graphite. These heat treatments are crucial in optimizing ductile iron for various industrial applications, ensuring it meets the desired balance of strength, hardness, and machinability.

The heat treatment methods for ductile iron mainly include the following:

1. Annealing: Purpose:

Annealing eliminates casting stress and improves cutting performance. It decomposes eutectoid cementite to form ferrite. This enhances plasticity and toughness. The annealing temperature is 720-760℃ or 880-930℃, based on casting structure and state. After annealing, the casting will be cooled to below 500℃ with the furnace and air-cooled out of the furnace. High temperature annealing targets free cementite castings. Low temperature annealing is for ferrite + pearlite + graphite castings.

2. Normalizing:

Purpose: Normalizing treatment aims to refine the matrix structure and improve the hardness and strength of ductile iron. Through normalizing treatment, a pearlite matrix (accounting for more than 75% of the matrix) can be obtained, and the strength of the cast iron may be further improved by air cooling, spray cooling, etc. Process: Normalizing temperature is usually in the range of 820-860℃ or 880-930℃. To increase pearlite content, accelerate cooling with air or spray cooling. After normalizing, castings can air-cool. To reduce stress, temper at 500-600℃.

3. Quenching and tempering treatment:

Purpose: Quenching and tempering treatment is to heat the casting to a specific temperature, quench it after keeping it warm, and then temper it at a lower temperature to obtain a tempered troostite structure, thereby improving the comprehensive mechanical properties of the casting.

Process: Quenching and tempering treatment usually heats the casting to 840-880℃ or 860-920℃, keeps it warm for a certain period of time, quench it in oil or water, and then temper it at a temperature of 550-600℃.

4. Isothermal quenching: Purpose:

The purpose of isothermal quenching is to obtain the best comprehensive mechanical properties. By obtaining the structure of lower bainite matrix plus spheroidal graphite, the casting has the comprehensive properties of high strength, high hardness and high toughness. Process: Isothermal quenching heats the casting to 850-900℃, keeps it warm and quickly puts it into a salt bath at 250-350℃ for 60-90 minutes, then takes it out of the furnace and air cools it. Due to the limited cooling capacity of the salt bath, this method is generally only used for parts with small cross-sections.

There are many ways to heat treat ductile iron, and the choice of which method depends on the specific requirements and application scenarios of the casting. These heat treatment methods can effectively improve the performance of castings and increase their service life and reliability.