The purpose of the cast iron heat treatment

In addition to annealing malleable cast iron nodular cast iron cementite is decomposed into flocculent mass graphite, cast iron heat treatment purpose lies in two aspects: one is to change the matrix organization, improve the performance of cast iron, the second is to eliminate casting stress. It is worth noting that cast iron is: heat treatment of castings can't change the casting graphite morphology and distribution of the original, the original is flake or spheroidal graphite after heat treatment is still the flake or globular, at the same time it will not change, the size of the distribution will not change.
Cooling rate of castings were situated in the process of casting, form of casting stress, if not eliminate, in the process of machining and using it will make parts deformation and cracking. For the release of the stress often USES the artificial aging and natural aging two ways. Casting is heated to about 500 ~ 560 ℃ heat preservation time, and then with air cooling furnace cooling to take out the castings, the limitation for the artificial aging; Natural ageing is to cast iron pieces in outdoor 6 ~ 18 months, release stress nature, this limitation can be will release some stress, but also for a long time, low efficiency, has not used.
 improve iron integrity for heat treatment
1. Eliminate chilling annealing
Ordinary grey cast iron or ductile casting surface or thin walled in casting process for cooling speed too quickly appear white, iron can not machining. To eliminate chilling lower hardness often will this kind of castings to heated to above the eutectoid temperature (typically 880 ~ 900 ℃), insulation and 1 ~ 2 h (if cast iron high Si content, time is short) on annealing, cementite is decomposed into graphite, cast iron pieces, slow cooling to 400 ℃ to 500 ℃ from air cooling. In 700-780 ℃ temperature, namely the eutectoid temperature near the unfavorable cold speed too slow, so that cementite too much into graphite, reduces the strength of cast iron parts.
2. Improve the toughness of ductile iron annealing
Nodular cast iron in the process of casting the ordinary grey cast iron of chilling tendency, internal stress is bigger also, cast iron or hard to get pure ferrite pearlite matrix, in order to improve the ductility or toughness of cast iron parts, often will cast iron pieces to heated 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, graphite gathered in the original around the nodular graphite, matrix into ferrite.
If the as-cast organization by ferrite + pearlite matrix, and the composition of spheroidal graphite, to improve the toughness, simply in the pearlite ferrite and cementite decomposing into spherical graphite, so it will soon be iron to heated to 700-760 ℃ after the eutectoid temperature fluctuation by holding furnace cooling to 600 ℃ from getting cold.
3. Improve the strength of ductile cast iron is fire
Ductile cast iron is the purpose of normalizing the matrix was converted into fine pearlitic structure. 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 some of the spheroidal graphite dissolves in austenite, air cooling after heat preservation austenite transformed into 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 pieces of quenching and tempering treatment at low temperature. Process is: the 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 original 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.
Nodular cast iron as the axial parts, such as diesel engine crankshaft, connecting rod, high strength and toughness good comprehensive mechanical machinery performance, conditioning treatment was carried out on the cast iron pieces. Process is: cast iron heating to 860-900 ℃ austenitizing temperature thermal insulation for matrix, and then cooling achieve quenching in oil or molten salt, after 500-600 ℃ high temperature tempering, obtain tempered sorbite structure (usually have a small amount of essence of blocky ferrite), the original spheroidal graphite morphology is constant. After processing strength, toughness good match, adapted to the working conditions of shaft parts.
The isothermal quenching process of the ductile cast iron
Isothermal quenching process of ductile iron to make cast iron matrix organization into a strong lower bainite tissue, ultimate strength can be more than 1100 mpa, impact toughness AK 32 p j. Treatment process is: will ductile cast iron is heated to 830-870 ℃ after austenitizing temperature insulation matrix, 280-350 ℃ heat preservation in the molten salt, let austenitic part into the bainite, the original spheroidal graphite remains the same. To obtain high strength ductile iron.
Heat treatment shows that the cast iron castings heat treatment can only change the matrix organization, can't change the morphology and distribution of the graphite, the change of mechanical properties is caused by the matrix organizational change. Ordinary grey cast iron (including meehanite) graphite piece had a great influence on mechanical properties (strength and ductility), 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 steel notch sensitivity is high, so the cast iron cooling speed during heat treatment (quenching) especially to strictly control.