Improve the overall performance of the commonly used cast iron heat treatment

Improve the overall performance of the commonly usedcast iron heat treatment
To improve overall performance often eliminate chilling iron annealing, improve the toughness of ductile cast iron annealing, increased the strength of ductile cast iron of normalizing, quenching, etc.
1. Eliminate white cast chilling annealing
Ordinary gray cast iron or ductile casting surface for cooling speed too fast or thin walled in casting process appear white cast, iron can not machining. To eliminate chilling reheated reduce hardness often will this kind of cast iron parts above the eutectoid temperature (generally 880 ~ 900 ℃), insulation and 1 ~ 2 h (if cast iron high Si content, time is short) on annealing, cementite decomposition into graphite, then iron slowly cooled to 400 ℃ to 500 ℃ from air cooling. In 700-780 ℃ temperature, namely the eutectoid temperature near the unfavorable cold speed too slow, too much so that cementite into graphite, reduces the strength of cast iron parts.
2. Improve the toughness of ductile cast iron annealing
Nodular cast iron in the process of casting the chilling tendency of ordinary gray cast iron, internal stress is also bigger , cast iron or hard to get pure ferrite pearlite matrix, in order to improve the ductility or toughness of cast iron parts, often used the iron to heat to 900-950 ℃ high temperature annealing and heat preservation time, furnace cooling to 600 ℃ after getting cold again. Cementite decomposition of matrix in the process of the graphite, the precipitation in the austenite, graphite, gathered in the graphite spheroidal graphite surrounding, matrix into ferrite.
If as-cast organization by ferrite + pearlite matrix, and the composition of nodular graphite, to improve the toughness, just put the pearlite transformation of ferrite and cementite decomposition of spheroidal graphite, so it will soon be iron to eutectoid temperature and heated to 700-760 ℃ after heat oven to 600 ℃ cold from getting cold.
3. Improve the strength of ductile cast iron is fire
Ductile cast iron is the purpose of normalizing will be converted into fine pearlite matrix organization. Process is the substrate of ferrite and pearlite ductile iron pieces to heated to 850-900 ℃ temperature, the original ferrite and pearlite transformation to austenite, and a portion of the spheroidal graphite dissolves in austenite, air cooling after heat preservation austenitic transformation of fine pearlite, thus the strength of the castings.
4. Nodular cast iron quenching and tempering treatment
As bearing ductile casting need higher hardness, often will cast iron quenching and tempering treatment at low temperature. Process: casting temperature, heated to 860-900 ℃ heat preservation to the original matrix all austenitizing after cooling achieve quenching in oil or molten salt, after 250-350 ℃ heat insulation tempering, the original matrix into tempered martensite and retained austenite, the spheroidal graphite morphology is constant. After processing of castings with high hardness and a certain toughness, retained the lubrication performance of the graphite, wear-resisting performance is improved.
Ductile cast iron as the axle parts, such as diesel engine crankshaft, connecting rod, high strength and toughness good comprehensive mechanical machinery performance, conditioning treatment on cast iron pieces. Process is: cast iron heating to 860-900 ℃ austenitizing temperature thermal insulation for matrix, and then implement quenching in oil or cooling in the molten salt, after 500-600 ℃ high temperature tempering, obtain tempering sorbite structure (usually has a small amount of essence blocky ferrite), spheroidal graphite morphology is constant. After processing intensity, good toughness match, adapted to the working conditions of shaft parts.
5. The isothermal quenching process of ductile iron
Isothermal quenching process of ductile iron to make cast iron pieces of lower bainite matrix organization into a strong organization, ultimate strength can be more than 1100 mpa, impact toughness AK 32 p j. Process is: ductile iron is heated to 830-870 ℃ after austenitizing temperature insulation matrix, 280-350 ℃ heat preservation in the molten salt, let austenite bainite under part into, the spheroidal graphite remains the same. High strength ductile iron.
Heat treatment shows that the cast iron: iron heat treatment can change the matrix organization, can't change the morphology and distribution of the graphite and matrix was caused by the change of the change in mechanical properties. Ordinary gray cast iron (including meehanite) flake graphite on the mechanical properties (strength and ductility) influence is very big, grey cast iron after heat treatment to improve the mechanical performance is not significant. Also need to be aware of is the thermal conductivity of cast iron steel is poor, the existence of graphite lead to high notch sensitivity is steel, so the cast iron cooling speed during heat treatment (quenching) especially should be controlled strictly.