Reasons and solutions of casting deformation
Casting deformation is one of the most common defects in casting. Unfortunately, after more than ten processes, you find that the requirements of customers cannot be met due to the deformation of castings. Today, we will roughly discuss the causes of casting deformation and its solutions. I hope this article is helpful to you.
Castings often experience varying degrees of deformation, such as bending, warping, distortion, dents, irregular circles and ellipses, and uneven wall thickness, which are called deformation. The degree of deformation of castings is usually relative to the overall deformation. In addition, the situation that does not conform to the drawing size due to the change of external dimension is called deformation.
Casting deformation law
It is not difficult to find that deformed products are usually thick and concave inward. Thin wall products are mostly convex.
Hazards of casting deformation
The deformation of the casting will cause the product size not to meet the drawing requirements and affect the processing of subsequent products. Part of the position deformation may also affect the clamping, resulting in deviation after machining. The severe deformation of castings may lead to insufficient machining allowance, or even scrap of castings, resulting in roughness and machining loss.
This shows that controlling casting deformation is very important. Casting stress is the main cause of casting deformation. Therefore, reducing and preventing casting stress is an effective measure to prevent casting deformation. However, it should be noted that the casting stress can only be reduced rather than completely eliminated.
Causes of casting deformation
1. Wax mold deformation.
Because wax molds are produced in the production process, they need to be taken and placed. In this process, if the mold is not installed correctly, the wax mold is easy to deform due to external force when it is not completely cooled. The placement after taking the mold is also very important.
2. Deformation caused by product structure.
For some products with uneven wall thickness, wax mold and pouring process will have shrinkage process. The shrinkage at the wall thickness position will be greater than that at the thin wall position, resulting in problems such as depression and deformation.
3. Deformation caused by placing the mold shell after pouring.
The mold shell has just been poured for a long time, and the product still maintains high temperature. If we can’t standardize it and stack it together, it will exert an external force on our castings, and the castings are cooling and solidification. The performance of the casting is not completely stable, resulting in casting deformation.
4. Deformation caused by improper operation of workers when cleaning sand and shaking shell.
Under the high frequency vibration of the shell vibrator, the casting is unstable, resulting in the deformation of the casting.
5. Deformation caused by heat treatment.
The main reason is that the strength of castings decreases after thermal expansion and softening during heat treatment. The deformation of casting is caused by its own gravity, and may also be caused by the release of stress accumulated in the structure during structural deformation. This deformation has a great influence on the size and shape of castings.
6. Deformation caused by welding of wax mold and group tree.
This deformation mainly occurs in products with multiple dumping risers and complex structures. Due to the external force during welding, the two gates that should be in the same line seem to be bent.
7. Deformation caused by improper operation of the coating shell when processing the shell.
In the process of shell making, there are problems in the mixing and drying of slurry, which will expand the product, which is also a kind of deformation.
8. Deformation caused by subsequent processes.
In the post-treatment process, excessive shot peening and a large number of product stacking will lead to casting deformation.
How to prevent casting deformation
1. Reasonably design the product structure to make the product wall thickness more uniform and not too large. In the sample development stage, the casting inspection of the product structure must be carried out to reduce the risk of product deformation caused by the structure.
2. Design the deformation resistance of the product. Our Besser casting achieves this goal by measuring the dimensions of billets and product wax molds. Of course, in order to obtain the deformation resistance and correct the die, it needs a lot of data to support it.
3. For open castings, our Bessel casting company often controls the deformation of products by adding process ribs at the opening.
4. If the structure of some products cannot be changed and it is difficult to meet the drawing requirements by forming and other methods, sufficient allowance must be added to the products and controlled by machining.
5. In the wax mold manufacturing stage, there will be a shrinkage process when the wax mold is cooled. This process can easily cause wax mold deformation. Deformation is usually controlled by reasonable placement and water cooling. In some cases, the shrinkage variables caused by different wall thickness are not easy to control. As a result, the standard block is usually used to control the size of the wax mold, and the standard block is taken out after the wax mold is cooled.
6. Reasonable placement and process control of castings in the subsequent process.
How to correct the deformation of castings
1. Cold correction
The cold correction method is based on the size of the deformed casting and the thickness of the profile. Castings can be hammered, impacted or extruded at room temperature by hand hammer, straightener, water (oil) press or some simple pressurization methods. Practice has proved that it is difficult to achieve the expected effect when cold straightening is used to correct the deformation of ordinary castings. However, for correcting the deformation of high manganese steel castings, cold correction can only be used (determined by material properties).
2. Thermal correction
The thermal correction method is to heat the deformed casting above the elastic-plastic temperature zone of the material, and restore the deformation of the casting through external forces such as extrusion and support, so as to achieve the purpose of correction.
The heating temperature of thermal correction method can be roughly divided into two types. One is high temperature (i.e. austenitizing temperature); The other is high temperature. The other is low temperature, that is, the heating temperature of low-temperature annealing or high-temperature tempering. The two heating temperatures have their own advantages and disadvantages. “High temperature” has high requirements for the operator, which is difficult to operate, but the correction time is short. “Low temperature” has less requirements for operators, simple operation and high correction success rate, but the correction time is relatively long.
In the production process of castings, it is necessary to fully consider the factors such as product structure and materials, determine the key characteristics, and take corresponding casting technology and measures according to the specific situation. All these steps mentioned above help to reduce the occurrence of casting deformation.
