How is the internal stress of castings formed

1. Mechanical stress

The internal stress caused by the mechanical obstruction of the mold or core during the linear shrinkage of the alloy. Mechanical stress is temporary and disappears on its own after sand falls. The combined effect of mechanical stress and thermal stress may increase the tendency for cracks in certain areas Preventive method: Improve the yielding of the mold and core.

2. Thermal stress:

Uneven casting thickness, different cooling rates, and inconsistent shrinkage occur Plastic state: During the solid cooling stage above the recrystallization temperature, the metal undergoes deformation under stress, resulting in work hardening. At the same time, the recrystallization induced hardening is cancelled out, and the internal stress disappears on its own (Simply put, in a yielding state, stress free) Elastic state: below the recrystallization temperature, the metal undergoes elastic deformation under external force, and stress continues to exist after deformation... For example: a) At the beginning of solidification, both the coarse and fine parts are in a plastic state with no internal stress. The two rods have different cooling rates, with the thin rod being faster and shrinking more, and being restricted by the thick rod, unable to contract freely and being relatively elongated, while the thick rod is relatively compressed, resulting in equal shrinkage of the two rods. b) The thin rod has a higher cooling rate, such as in the elastic stage, while the thick rod is still in the plastic stage and undergoes plastic shrinkage with the shrinkage of the thin rod, without stress. c) The contraction of the thin rod stops first, and the thick rod continues to contract, compressing the thin rod, while the thin rod prevents the contraction of the thick rod. At room temperature, the thick rod is subjected to tensile stress (+), (-) which shows the temperature difference between each part. The larger the temperature, the greater the thermal stress. The slower cooling part forms tensile stress, while the faster cooling part forms compressive stress Prevention method: 1. Uniform wall thickness 2. Solidification at the same time - a gate is set at the thin part, and cold iron is placed at the thick part. Advantages: riser saving, labor saving, and material saving. Disadvantages: shrinkage cavity or porosity is easy to occur at the center, which is applied to gray iron and tin bronze. Because gray iron has small shrinkage cavity and porosity tendency, and tin bronze solidifies in paste form, it is difficult to see microscopic shrinkage porosity through sequential solidification. During the stress cooling process of phase transition, the volume will change during solid-state phase transition As for A-P, the volume of A-P will increase, while Fe3C graphite will increase in volume If the volume change is obstructed Then internal stress is generated - the three types of stress in iron carbon alloy at different parts of the casting are shown in the following table:

The thermal stress, phase transition stress, mechanical stress, eutectoid transformation, graphitization, sand removal, and subsequent thinning or thickening of the outer layer or inner layer before or after sand removal. As mentioned earlier, stress prevention methods can be used. If stress is generated, natural aging and artificial aging methods can also be used to prevent stress Two deformations and prevention of castings from relaxing internal stress through free deformation, a spontaneous process The casting factory has experienced varying degrees of deformation For example, flat castings have slow heat dissipation at the center of the flat plate and are subjected to tension Slow cooling of the lower part of the tablet Method for preventing deformation as shown in the figure: 1. Uniform wall thickness, symmetrical shape, and simultaneous solidification 2. Anti deformation method (for long and easily deformed parts) residual stress: natural aging, artificial aging - low temperature annealing at 550-650 ℃. Cracks occur in three castings when the internal stress of the castings is prevented from exceeding the strength limit 1. Hot cracks: Cracks formed at high temperatures. Characteristics: Short cracks, wide cracks, and tortuous shapes The cracks appear oxidized with no metallic luster, and propagate along the grain boundaries, often occurring at stress concentration or solidification sites Gray iron and ductile iron have less hot cracking, while cast steel, cast aluminum, and white cast iron have more Reason: 1. At the end of solidification, the alloy appears as a complete skeleton+liquid, strong and plastic ↓ 2. It contains S - hot brittleness. 3. Poor ductility. Prevention: The design structure is reasonable, the ductility is controlled, and the content is controlled. 2. Cold cracking: Crack characteristics at low temperatures: fine cracks, continuous straight or smooth curves, dry and static crack surfaces, metallic luster, and sometimes slight oxidation color. Reason: Complex large workpieces are prone to tensile stress and stress concentration; Poor plasticity of materials; - Cold brittleness prevention: Reasonable design, reducing internal stress, controlling content, and improving retreat. § 4 Common gas defects in castings, 1/3 of waste Gas forms pores in castings