There are several commonly used heat treatment methods for ductile iron.
In the structure of ductile iron, the graphite is spherical, and its weakening and damaging effect on the matrix is weaker than that of flake graphite. The performance of ductile iron mainly depends on the matrix structure, and the influence of graphite is secondary. Improving the matrix structure of ductile iron through various heat treatments can improve its mechanical properties to varying degrees. Due to the influence of chemical composition, cooling rate, spheroidizing agent and other factors, a mixed structure of ferrite + pearlite + cementite + graphite often appears in the as-cast structure, especially at the thin wall of the casting. The purpose of heat treatment is to obtain the required structure and thereby improve the mechanical properties.
Commonly used heat treatment methods for ductile iron are as follows.
(1) Low-temperature graphitization annealing heating temperature 720~760℃. It is cooled in the furnace to below 500℃ and then air-cooled. Decompose the eutectoid cementite to obtain ductile iron with a ferrite matrix to improve toughness.
(2) High-temperature graphitization annealing at 880~930℃, then transferred to 720~760℃ for heat preservation, and then cooled with the furnace to below 500℃ and air-cooled out of the furnace. Eliminate the white structure and obtain ductile iron with ferrite matrix, which improves plasticity, reduces hardness and increases toughness.
(3) Complete austenitization and normalizing at 880~930℃, cooling method: mist cooling, air cooling or air cooling. In order to reduce stress, add tempering process: 500~600℃ to obtain pearlite + a small amount of ferrite + spherical shape Graphite, which increases strength, hardness and wear resistance.
(4) Incomplete austenitization, normalizing and heating at 820~860℃, cooling method: mist cooling, air cooling or air cooling. In order to reduce stress, add tempering process: 500~600℃ to obtain pearlite + a small amount of dispersed iron The body structure achieves better comprehensive mechanical properties.
(5) Quenching and tempering treatment: heating at 840~880°C, cooling method: oil or water cooling, tempering temperature after quenching: 550~600°C, to obtain tempered sorbite structure and improve comprehensive mechanical properties.
(6) Isothermal quenching: Heating at 840~880℃ and quenching in a salt bath at 250~350℃ to obtain comprehensive mechanical properties, especially to improve strength, toughness and wear resistance.
During heat treatment and heating, the temperature of the casting entering the furnace is generally less than 350°C. The heating speed depends on the size and complexity of the casting, and is selected between 30~120°C/h. The furnace entry temperature for large and complex parts should be lower and the heating rate should be slower. The heating temperature depends on the matrix structure and chemical composition. The holding time depends on the wall thickness of the casting.
In addition, ductile iron castings can also be surface quenched using high frequency, medium frequency, flame and other methods to obtain high hardness, wear resistance and fatigue resistance. It can also be treated with soft nitriding to improve the wear resistance of castings.
1.Quenching and tempering treatment of ductile iron
Ductile castings require higher hardness as bearings, and cast iron parts are often quenched and tempered at low temperatures. The process is: heating the casting to a temperature of 860-900°C, insulating it to allow all the original matrix to austenitize, then cooling it in oil or molten salt to achieve quenching, and then heating and maintaining it at 250-350°C for tempering, and the original matrix is converted into Fire martensite and retained austenite structure, the original spherical graphite shape remains unchanged. The treated castings have high hardness and certain toughness, retain the lubrication properties of graphite, and have improved wear resistance.
Ductile iron castings, as shaft parts, such as crankshafts and connecting rods of diesel engines, require comprehensive mechanical properties with high strength and good toughness. The cast iron parts must be quenched and tempered. The process is: the cast iron is heated to a temperature of 860-900°C and insulated to austenitize the matrix, then cooled in oil or molten salt to achieve quenching, and then tempered at a high temperature of 500-600°C to obtain a tempered troostite structure. (Generally there is still a small amount of pure massive ferrite), and the shape of the original spherical graphite remains unchanged. After treatment, the strength and toughness match well and are suitable for the working conditions of shaft parts.
2. Annealing of ductile iron to improve toughness
During the casting process of ductile iron, ordinary gray cast iron has a large whitening tendency and large internal stress. It is difficult to obtain a pure ferrite or pearlite matrix for cast iron parts. In order to improve the ductility or toughness of cast iron parts, cast iron is often The parts are reheated to 900-950°C and kept warm for sufficient time to perform high-temperature annealing, and then cooled to 600°C and cooled out of the furnace. During the process, the cementite in the matrix decomposes into graphite, and graphite is precipitated from austenite. These graphites gather around the original spherical graphite, and the matrix is completely converted into ferrite.
If the as-cast structure is composed of (ferrite + pearlite) matrix and spherical graphite, in order to improve the toughness, the cementite in the pearlite only needs to be decomposed and converted into ferrite and spherical graphite. For this purpose, the cast iron part must be reheated. After being insulated up and down the eutectoid temperature of 700-760℃, the furnace is cooled to 600℃ and then cooled out of the furnace.
3. Normalizing to improve the strength of ductile iron
The purpose of normalizing ductile iron is to convert the matrix structure into fine pearlite structure. The process is to reheat the ductile iron casting with a matrix of ferrite and pearlite to a temperature of 850-900°C. The original ferrite and pearlite are converted into austenite, and some spherical graphite is dissolved in the austenite. After heat preservation, the air-cooled austenite transforms into fine pearlite, so the strength of the ductile casting increases.
Post time: May-08-2024
