Heat treatment of cast iron
The heat treatment of cast iron is similar to that of steel, but also different. Generally, the heat treatment of cast iron can not improve the morphology and distribution of graphite in the original structure. For gray cast iron, the stress concentration effect caused by flake graphite is the leading factor for the properties of gray cast iron, so the strengthening effect of heat treatment on gray cast iron is far less obvious than that of steel and ductile iron. Therefore, the heat treatment process of gray cast iron is mainly annealing and normalizing.
For nodular cast iron, because the graphite is spherical, the splitting effect on the matrix is greatly reduced. Through heat treatment, the matrix structure can give full play to its role, so that the mechanical properties of the nodular cast iron can be significantly improved. Therefore, like steel, the heat treatment process of ductile iron includes annealing, normalizing, quenching and tempering, multi temperature quenching, induction heating quenching and surface chemical heat treatment.
Heat treatment process of cast iron:
Stress relief annealing
Due to the non-uniform wall thickness of castings, the effective stress and microstructure stress will be produced during heating, cooling and phase transformation. In addition, large parts are easy to residual stress after machining, all these internal stresses must be eliminated. Stress relief annealing is usually heated at 500 ~ 550 ℃ for 2 ~ 8h, then furnace cooled (gray iron) or air cooled (ductile iron). 90-95% of the internal stress of castings can be eliminated by this process, but the structure of cast iron does 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.
High temperature graphitization annealing to eliminate white spots in castings
When the casting is cooled, white spots often appear on the surface and thin section. The microstructure is hard and brittle with poor processability and easy to peel off. Therefore, annealing (or normalizing) must be used to eliminate the white mouth structure. The annealing process is as follows: heating to 550-950 ℃ for 2-5h, then cooling to 500-550 ℃ and then air cooling. During the high temperature holding period, the eutectoid cementite and the eutectoid cementite decompose into graphite and a, and the secondary cementite and eutectoid cementite also decompose during the subsequent cold protection process, resulting in the graphitization process. Due to the decomposition of cementite, the hardness decreases, so the machinability is improved.
Normalizing of ductile iron
The purpose of normalizing ductile iron is to obtain pearlite matrix structure, refine grain size and uniform structure, so as to improve the mechanical properties of castings. Sometimes, normalizing is also the preparation for the structure of ductile iron surface quenching. Normalizing can be divided into high temperature normalizing and low temperature normalizing. The normalizing temperature of high temperature is generally no more than 950 ~ 980 ℃, and the normalizing temperature of low temperature is generally 820 ~ 860 ℃. After normalizing, it needs to be treated to eliminate the internal stress.
Quenching and tempering of ductile iron
In order to improve the mechanical properties of nodular cast iron, the casting is generally heated to 30-50 ℃ above Afc1 (Afc1 represents the end temperature of a formation during heating), and then quenched into oil after heat preservation to obtain martensite structure. In order to reduce the residual stress after quenching properly, tempering should be carried out after quenching. The microstructure of low temperature tempering is tempered martensite plus residual bainite plus spherical graphite. This kind of structure has good wear resistance and is used for parts requiring high wear resistance and high strength. The medium temperature 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, effective stability and elasticity. High temperature tempering temperature is 500-60d ℃. After tempering, the structure is tempered sorbite plus spherical graphite. It has good comprehensive properties of toughness and strength, so it is widely used in production.
Multi temperature quenching of ductile iron
High strength, good plasticity and toughness can be obtained after austempering. The selection of heating temperature for multi temperature quenching is mainly considered to make the original structure a completely without residual F and to avoid a grain growth. Generally, the heating temperature is 30-50 ℃ above Afc1, and the isothermal treatment temperature is 0-350 ℃ to obtain lower bainite with comprehensive mechanical properties. After austempering of RE Mg al ductile iron σ b=1200～1400MPa， α k=3～3.6J/cm2，HRC=47～51. However, attention should be paid to adding a tempering process after austempering.
Surface hardening
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 ductile iron castings can be surface quenched. Generally, high (medium) frequency induction heating and electric contact surface quenching are used.
Chemical heat treatment
The chemical heat treatment process similar to steel, such as gas soft chlorination, chlorination, boronizing and sulphurization, can be used for the castings requiring surface wear resistance, oxidation resistance and corrosion resistance.
The heat treatment of cast iron is similar to that of steel, but also different. Generally, the heat treatment of cast iron can not improve the morphology and distribution of graphite in the original structure. For gray cast iron, the stress concentration effect caused by flake graphite is the leading factor for the properties of gray cast iron, so the strengthening effect of heat treatment on gray cast iron is far less obvious than that of steel and ductile iron. Therefore, the heat treatment process of gray cast iron is mainly annealing and normalizing. For nodular cast iron, because the graphite is spherical, the splitting effect on the matrix is greatly reduced. Through heat treatment, the matrix structure can give full play to its role, so that the mechanical properties of the nodular cast iron can be significantly improved. Therefore, like steel, the heat treatment process of ductile iron includes annealing, normalizing, quenching and tempering, multi temperature quenching, induction heating quenching and surface chemical heat treatment.
Source: China Fittings Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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