Casting Deformation And Prevention

- Jun 26, 2017 -

The selection of the surface roughness of the castings is listed in Table 1. Such as magnesium, zinc alloy pressure casting surface roughness up to RaO. 8 ~ 25.0μm, and cast steel sand casting surface roughness can only reach Ra50μm ~ Rz1600μm.

Casting deformation and its prevention

If the casting has internal stress, Casting Parts the casting is in an unstable state, the thick part of the casting by the tensile stress, thin part of the compressive stress, if the stress exceeds the yield limit, the casting itself is always trying to reduce the internal stress by deformation , Therefore, thin and long or large and thin castings prone to deformation.

In order to prevent deformation of the casting, the design of the parts of the casting should be as uniform as possible or symmetrical shape, in the casting process, to take the same solidification principle, which can make the casting stress is small, easy to produce deformation and cracks. But in the casting center area is often loose, the organization is not dense. This principle is mainly used for solidification shrinkage of small alloys (such as gray cast iron) and the wall thickness uniformity, the crystallization temperature range and the casting of the compact requirements of the casting is not high. In addition, the anti-deformation method can be used in the molding process (the shape is made in the opposite direction to the deformation direction of the casting, and the deformation of the casting should be calculated more accurately).

Practice has proved that although the casting part of the deformation when cooling, but the internal stress has not been completely eliminated. Casting Parts After the machining of internal stress re-distribution, casting will still be deformed, affecting the accuracy of parts. Therefore, for some important precision castings, such as lathe bed, etc., must be taken to stress annealing or natural aging and other methods, the residual stress to eliminate; if necessary, after roughing for stress annealing or artificial aging, and then Perform finishing to ensure the accuracy of the part.

Casting cracks and prevention

If the casting stress exceeds the strength limit of the alloy, the casting will crack. Cracks are divided into hot cracking and cold cracking. The thermal cracking is formed at a high temperature in the later stage of solidification, mainly due to the mechanical resistance caused by shrinkage. It has a short crack, the shape of twists and turns, wide gap, section has a serious oxidation, no metallic luster, cracks along the grain boundary generation and development characteristics, in cast steel and aluminum alloy castings common. Casting Parts Cold cracking is formed at lower temperatures, often in the casting by the tensile site, especially where the stress concentration. The cracks are small, continuous straight line, sewn clean, and sometimes slightly oxidized color. Wall thickness difference, the shape of complex or large and thin castings tend to produce cold cracking. Therefore, those who can reduce the casting stress or reduce the alloy brittle factors, can prevent the formation of cold cracking. At the same time in the cast steel and cast iron to strictly control the phosphorus content of the alloy.

The definition and significance of casting salvage

Casting is the repair of defective castings to make it a qualified product or to restore its performance of a technology. In the casting production, due to the complexity of the process, Casting Parts the production of castings will inevitably occur in a certain number of bad products. Which has a large part of the defective products can be saved by some means, the premise is that does not affect the performance of the case and must be the product design department of the permit, if necessary, need to be tested and identified before.

Casting quality inspection is an indispensable link in casting production. Casting quality includes appearance quality, intrinsic quality and performance. Appearance quality includes: casting dimensional tolerances, casting surface roughness, casting weight tolerance, the amount of riser residue, casting welding quality and casting surface defects, etc., for the exposed surface and the skin under the defects, the production commonly used in the naked eye or chisel , Sharp mouth hammer and other tools to test. Internal quality and performance include: chemical composition, microstructure, internal defects, mechanical properties and the relevant standards and casting and delivery requirements of the technical conditions required for a variety of special physical properties and chemical properties, inspection methods are generally non-destructive testing X-ray, Y-ray inspection, ultrasonic, magnetic powder, electromagnetic induction eddy current testing, fluorescence inspection and color detection, etc.) inspection, Casting Parts chemical composition testing, metallographic examination, mechanical properties test, fracture macroscopic and microscopic tests and hydraulic test.