Quality assurance of steel castings: non-destructive testing methods and quality control strategies

01 Non-destructive testing method

According to the national standard GB\/T 37400.14-2019, non-destructive testing of steel castings mainly includes 4 methods: ultrasonic testing, radiographic testing, magnetic particle testing and penetrant testing. These methods constitute a comprehensive inspection system for castings from the inside to the surface.

Ultrasonic testing is suitable for ferritic steel castings with a thickness of no more than 600 mm, and can detect internal discontinuity defects. Radiographic testing is more suitable for castings with a thickness of more than 5 mm, and can provide intuitive images of defects.

The detection of defects on the surface of ferromagnetic materials relies on magnetic particle testing, which is highly sensitive to discontinuities such as near-surface cracks. Penetrant testing is generally applicable to surface opening defects of various steel castings.

The selection of different inspection methods requires comprehensive consideration of the material, structural characteristics and possible defect types of the casting.

02 Quality grade classification

National standards set corresponding quality levels for each non-destructive testing method. Taking magnetic particle testing as an example, the standard stipulates the upper limit length of defect display 4 allowed within the 105 mm to 148 mm assessment frame.

These grade classifications provide a quantitative basis for acceptance, ensuring that steel castings produced by different manufacturers can be evaluated according to unified standards.

Manufacturers and customers need to jointly determine the applicable quality level based on the service conditions and safety requirements of the casting. For key components that bear high loads or are in harsh working environments, higher quality level requirements are usually selected.

03 Whole process quality control strategy

Although non-destructive testing is an important part of quality control, it is only a part of the quality control system. Real quality control starts from the casting process design and runs through the entire production process.

From the inspection of raw materials entering the factory to the quality inspection of process equipment to the control of each process in the casting process, each step will affect the quality of the product. The quality of the raw materials used for casting directly affects the performance of the castings and needs to be strictly inspected according to standards.

Special casting processes, such as investment casting, metal mold casting, etc., require targeted process quality control measures. Reasonable selection of casting methods can ensure quality while taking into account economy.

04 Defect assessment and repair

Even after strict quality control, castings may still have certain defects. At this time, defects need to be evaluated according to standards to determine whether they affect performance.

Defects that do not affect performance can be repaired by welding repair, molten metal filling, metal plugging or infiltration repair, etc.

Welding repair can make the repaired part achieve mechanical properties similar to those of the casting body. Impregnation repair achieves the purpose of plugging leaks by penetrating the colloidal impregnation agent into the pores of the casting.

These repair measures should be carried out on the premise that it is technically feasible and does not affect the performance of the casting to reduce the loss of scrap.

On the production line of a steel enterprise, operators are conducting magnetic particle inspection on a batch of cast steel support rollers of large rolling mills. The inspection process is strictly in accordance with national standards to ensure that only castings that meet quality grade requirements can enter the next process.

With the application of new technologies such as digital radiography and phased array ultrasonic testing, non-destructive testing of steel castings is developing in a clearer and smarter direction. Whether it is wind power castings in the energy field or high-speed rail parts in the transportation industry, quality assurance is always based on scientific testing and strict control.