Annealing the foundry gray cast iron heat treatment to explain in detail: 1.
The stress relieving in order to eliminate the residual stress of casting
s, stabilize its geometrical size, reduce or eliminate machining to produce distortion, need to go to the casting stress annealing.
To the determination of stress annealing temperature, the chemical composition of cast iron must be considered.
Ordinary grey cast iron when the temperature about 550 ℃, which may be part of the graphitization of cementite and the granulation, the lower the strength and hardness.
When containing alloy elements, the cementite starts to break down the temperature can be increased to around 650 ℃.
Usually, the ordinary gray cast iron to stress annealing temperature to 550 ℃ advisable, low alloy cast iron is 600 ℃, high alloy cast iron is increased to 650 ℃, the heating speed is generally choose 60 ~ 120 ℃ / h.
Heat preservation time depends on the size of the heating temperature, casting and the requirement of structure complicated degree and the degree of eliminating stress.
Casting stress annealing cooling speed must be slow, lest produce residual internal stress, secondary cooling speed general control in 20 ~ 40 ℃ / h, cooled to below 200 ~ 150 ℃, can empty out cold.
Some gray cast iron pieces in the stress relieving specification is shown in table 1.
Graphitizing annealing graphitizing annealing is gray cast iron parts are made in order to reduce the hardness, improve the processing performance, plasticity and toughness of cast iron.
If the casting does not exist in the eutectic cementite or its number soon, for low temperature graphitization annealing;
When the casting of the communist party of China crystal cementite quantity is large, must be high temperature graphitizing annealing.
Graphitizing annealing at low temperature, low temperature annealing eutectoid cementite occurs graphite cast iron and granulating, and thus reduce the casting hardness, plasticity increased.
Gray cast iron casting is low temperature graphitization annealing process be heated to a temperature below cut-off Ac1, heat preservation for a period of time the eutectoid cementite decomposition, and then with furnace cooling.
High temperature graphitizing annealing, the high temperature graphitizing annealing process is the casting temperature above the heated to above Ac1, made of cast iron free cementite decomposition of austenite and graphite, heat preservation after period of time according to the requirements of the matrix organization according to the different ways for cooling.
Normalized gray cast iron is the purpose of normalizing improve casting strength, hardness and wear resistance, or as preliminary heat treatment, surface quenching to improve matrix organization.
Casting general normalizing is be heated to Ac cap + 30 ~ 50 ℃, which changes the original organization for austenitic, heat preservation after period of time from air cooling.
Complex shape or more important castings after normalizing treatment to eliminate internal stress annealing.
Such as cast iron excessive amounts of free cementite original organization, you must first heated to Ac1 cap + 50 ~ 100 ℃ temperature, high temperature graphitization first conducted to eliminate free cementite in normalizing temperature range, the higher the temperature, the higher the hardness also.
Requirements after normalizing, therefore, cast iron has high hardness and wear resistance, the choice of heating temperature limit.
After normalizing cooling speed affect ferrite amount of precipitation, so as to affect the hardness.
The greater the cooling speed, the number of ferrite precipitation less, the higher the hardness.
So can adopt the method of controlled cooling speed)
Air cooling, air cooling, cold fog)
In order to achieve the purpose of adjust the hardness of cast iron.
Quenching and tempering.
Quenching casting iron quenching process is be heated to Ac1 cap + 30 ~ 50 ℃ temperature, generally take 850 ~ 900 ℃, which changes the organization to austenite, and under the temperature of heat preservation, in order to increase the solubility of carbon in austenite, and then quenching, usually adopt oil quenching.
For complex shape or large castings should slow heating, in 500 ~ 650 ℃ preheat, when necessary to avoid uneven heating and cause cracking.
With the austenitizing temperature, the higher the hardness after quenching, but too high the austenitizing temperature, not only increase the risk of cast iron deformation and cracking, and generate more retained austenite, the hardness decreased.
Hardenability of gray cast iron and graphite size, shape, distribution, chemical composition and austenite grain size.
Graphite to lower thermal conductivity of cast iron, making it the hardenability of decline, graphite, the more thick, more, this influence is bigger.
Tempering in order to avoid the graphitization, tempering temperature should be lower than 550 ℃, tempering holding time at t = [
The thickness of the casting (
+ 1 (
Isothermal quenching in order to reduce the quenching distortion and improve comprehensive mechanical properties of castings, CAM, gear, such as cylinder liner parts often using isothermal quenching.
The isothermal quenching heating temperature and holding time is the same as the conventional quenching process.
The classification of cast iron (before the lesson
Cast iron, cast iron 6 engineering the graphitization of 1.
An overview of the cast iron is the mass fraction of carbon WC>
11% of the iron carbon alloy.
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