Traditional casting defect analysis

Firstly, we take the shrinkage porosity defect in steel castings as an example to explore how the traditional foundry solves the problem of casting defects in casting production. When the shrinkage defects of the inspection department or the customer are reported, the engineer will solve the problem as soon as possible.

Step 1 – Investigate the appearance, location and frequency of casting defects

The location of casting defects is determined by means of dye penetration test and ultrasonic flaw detection. According to time, cost and resources, choose to observe the size and shape (spherical, irregular, internal smoothness and color, etc.) of casting defects with naked eye or microscope and scanning electron microscope. If possible, measure the composition of gas in casting defects. In addition, the time of occurrence of casting defects shall be determined as far as possible.

Step 2 – Check for changes in casting process parameters

Check the charts and values of control items, including base metal grade and content, chemical composition, gas content and carbon equivalent of liquid metal, consumption rate of new sand and binder, clay or dust content, water content of mixture, permeability, etc. Check the behavior of workers or operators and try to find out the errors in operation, especially the casting defects caused by careless slag removal, discontinuous pouring, and not strictly maintaining a constant pouring temperature. Check the change and maintenance of the machine or equipment, such as component replacement frequency, die wear and deformation, accuracy control of measuring instruments, etc. Check for environmental changes such as weather, temperature and humidity.

Step 3 – Determine casting defect type and cause

The identification of casting defect types is very important because it is closely related to their causes and solutions. Generally, it is very difficult to identify the specific types of casting defects other than cracks, which needs to be combined with existing knowledge and experience, but books or manuals are not always correct or do not include the latest research results. Generally, casting shrinkage defects are divided into shrinkage porosity, pores and cracks, and gas defects are divided into pores, pinholes, bubbles and so on. For example, although holes with smooth wall and bright surface color are usually considered as casting gas defects, they may sometimes be shrinkage caused by melting or early solidification shrinkage. However, if the gas content in the molten metal is higher than the critical value and the shape is spherical or elongated, they may be casting gas defects.

At the same time, pores often occur in the parts with thick wall or serious turbulence, which makes it difficult to distinguish them from casting shrinkage defects. If several factors with obvious changes can be found at the same time, this problem will be easier to solve. For example, if the water content in the molding sand is higher than the normal value and the casting defect looks like a pore, it may be caused by the gasification of water in the molding sand, resulting in the gas entering the liquid metal. However, we often can not find obvious changes in casting defects. One reason is that the data recorded continuously is very limited, even once a day.

The traditional casting defect analysis process is not only time-consuming and laborious, but also has low accuracy. Therefore, in order to improve the casting quality, it is necessary to introduce advanced casting CAE technology.