Due to the uneven wall thickness of castings, effective and structural stresses will occur during heating, cooling and phase transformation. In addition, large parts are easy to produce residual stress after machining, and these internal stresses must be eliminated. The usual heating temperature for stress relief annealing is 500 ~ 550℃, and the holding time is 2 ~ 8h, then furnace cooling (gray cast iron) or air cooling (nodular cast iron). This process can eliminate 90 ~ 95% internal stress of castings, but the structure of cast iron will not change. If the temperature exceeds 550℃ or the holding time is too long, it will cause graphitization and reduce the strength and hardness of the casting.

1. High-temperature graphitization annealing to eliminate white spots.

When the casting is cooled, the surface layer and thin section often produce white spots. The white structure is hard and brittle, with poor processability and easy peeling. Therefore, annealing (or normalizing) must be adopted to eliminate white tissue. The annealing process is: heating to 550-950℃ for 2-5 hours, then cooling to 500-550℃ in the furnace, and then discharging for air cooling. In the process of high temperature insulation, free cementite, * * * crystalline secondary cementite and * * * precipitated cementite also decompose and graphitize. Because cementite improves the mechanical properties of castings. Sometimes normalizing is also the preparation for surface quenching of nodular cast iron. Normalization can be divided into high temperature normalization and low temperature normalization. The high-temperature normalizing temperature generally does not exceed 950 ~ 980℃, and the low-temperature normalizing is generally heated to 820 ~ 860℃ within the folding temperature range. After normalizing, tempering treatment is generally needed to eliminate the internal stress generated during normalizing, so as to realize high-temperature rocky desertification annealing of castings.

2. Quenching and tempering of nodular cast iron

In order to improve the mechanical properties of ductile iron, the castings are generally heated to 30 ~ 50℃ above Afc 1 (AFC1stands for the final temperature of A formation during heating), and the martensite structure is obtained by oil quenching after heat preservation. In order to properly reduce the residual stress after quenching, tempering should generally be carried out after quenching, and the low-temperature tempering structure is tempered martensite+residual bainite+spherical graphite. This structure has good wear resistance and is used for parts requiring high wear resistance and high strength. The intermediate tempering temperature is 350-500℃. After tempering, the microstructure is tempered troostite plus spherical graphite, which is suitable for thick parts with good wear resistance, stability and elasticity. The high temperature tempering temperature is 500-60℃, and the structure after tempering is tempered Soxhlet plus spherical graphite, which has good comprehensive properties of toughness and strength and is widely used in production.

3. Isothermal quenching of ductile iron

After isothermal quenching, ductile iron can obtain high strength, good plasticity and toughness. The selection of multi-temperature quenching heating temperature mainly considers that the original structure is all A-shaped, without F residue, and at the same time avoids the growth of A grains. Generally, the heating temperature is 30 ~ 50℃ above Afc 1, and the isothermal treatment temperature is 0 ~ 350℃ to ensure the lower bainite structure with comprehensive mechanical properties. After isothermal quenching, σ b = 1200 ~ 1400 MPa, α k = 3 ~ 3,6 j/cm2, HRC = 47 ~ 5 1. However, attention should be paid to adding a tempering process after isothermal quenching.

4. Surface quenching

In order to improve the surface hardness, wear resistance and fatigue strength of some castings, surface quenching can be used. Both gray cast iron and nodular cast iron can be surface quenched. Generally, high (medium) frequency induction heating surface quenching and electric contact surface quenching are adopted.

5. Chemical heat treatment

For castings requiring wear resistance, oxidation resistance and corrosion resistance, chemical heat treatment processes similar to those of steel can be adopted, such as gas soft chlorination, chlorination, boronizing and sulphurizing.