1、 Overview of Non Destructive Testing Technology
Non destructive testing (NDT) refers to the technology of using physical or chemical methods to test materials, components, or structural parts without damaging the performance of the tested object. In the production of castings in Dalian, non-destructive testing technology plays a crucial role in effectively detecting defects inside and on the surface of castings, ensuring product quality, reducing scrap rates, and improving production efficiency.
Due to the special production process, castings are prone to various defects such as porosity, shrinkage, slag inclusion, cracks, and cold shuts. If these defects are not detected and dealt with in a timely manner, they will seriously affect the mechanical properties and service life of the castings. Therefore, adopting appropriate non-destructive testing techniques for comprehensive inspection of castings is an indispensable part of the casting production process.
2、 Main non-destructive testing techniques and applications
1. Radiographic Testing Technology (RT)
Radiographic testing is a method of detecting defects in castings by using X-rays or gamma rays to penetrate the castings and detect differences in the absorption of radiation in different parts of the castings. When the radiation passes through the casting, the defect areas (such as pores and shrinkage cavities) absorb less radiation, resulting in a darker image on the film or digital imaging system.
Technical Features:
Can detect internal defects, especially volumetric defects
Intuitively display the shape, size, and location of defects
The test results can be stored for a long time
High technical requirements for operators
There are radiation safety issues present
Application scope: Radiographic testing is particularly suitable for detecting defects such as pores, shrinkage cavities, and slag inclusions inside castings. It is commonly used for the detection of important castings such as engine cylinder blocks, turbine blades, high-pressure valves, etc.
2. Ultrasonic Testing Technology (UT)
Ultrasonic testing is a method of detecting defects by utilizing the principle that high-frequency sound waves will reflect, scatter, or attenuate when encountering defects during propagation in castings. By analyzing the amplitude, time, and waveform characteristics of reflected echoes, the location, size, and nature of defects can be determined.
Technical Features:
Can detect internal defects, especially planar defects
Fast detection speed and high efficiency
High technical requirements for operators
Need coupling agent
High requirements for surface roughness
Application scope: Ultrasonic testing is suitable for detecting defects such as cracks, slag inclusions, and shrinkage inside castings. It is commonly used for the detection of large castings such as wind turbine castings, machine tool beds, and large gears.
3. Magnetic particle testing technology (MT)
Magnetic particle testing is a method of displaying defects by applying magnetic particles, based on the principle that ferromagnetic materials generate a leakage magnetic field at the defect site after magnetization. When there are defects on or near the surface of the casting, magnetic powder will gather at the defect site to form visible magnetic traces.
Technical Features:
Only applicable to ferromagnetic materials
Detectable surface and near surface defects
Easy to operate, low cost
Fast detection speed
Further demagnetization treatment is required
Application scope: Magnetic particle testing is mainly used to detect defects such as cracks, folds, and pores on the surface of castings. It is commonly used for the detection of ferromagnetic castings such as automotive parts, pipeline flanges, and bearing seats.
4. Penetration testing technology (PT)
Penetration testing is a method that uses the principle of capillary action to coat the surface of a casting with penetrant, allowing the penetrant to penetrate into the surface opening defects, remove excess penetrant from the surface, and then apply a developer to extract the penetrant from the defects, thereby displaying the defects.
Technical Features:
Suitable for various non porous materials
Detectable surface opening defects
Easy to operate, low cost
Not limited by the shape of the casting
Surface that needs to be cleaned
Application scope: Penetrant testing is suitable for detecting surface cracks, pores, cold shuts, and other opening defects in castings. It is commonly used for the detection of non-magnetic castings such as aluminum alloys, magnesium alloys, and stainless steel.
5. Eddy current testing technology (ET)
Eddy current testing is a method that utilizes the principle of electromagnetic induction. When an alternating magnetic field acts on a conductive material, eddy currents are induced in the material, causing defects to change their distribution and size. By detecting these changes, defects can be discovered.
Technical Features:
Only applicable to conductive materials
Detectable surface and near surface defects
Fast detection speed
Can achieve automated detection
Sensitive to surface conditions
Application scope: Eddy current testing is suitable for detecting defects such as cracks and pores on the surface of castings, and is commonly used for detecting conductive material castings such as pipes, rods, and wires.
6. Industrial CT detection technology
Industrial CT (computed tomography) technology is an advanced detection method that reconstructs the three-dimensional structure inside castings through multi angle X-ray projection data. It can provide precise three-dimensional information of internal defects in castings.
Technical Features:
Can provide three-dimensional defect information
High detection accuracy
Expensive equipment
Slow detection speed
Complex data processing
Application scope: Industrial CT is suitable for detecting internal defects in complex castings and is commonly used for testing high value-added products such as precision castings, aerospace castings, medical device castings, etc.
3、 Selection and application strategy of non-destructive testing technology
In actual casting production, non-destructive testing methods should be reasonably selected and combined based on factors such as casting material, shape, size, defect type, and testing requirements. Here are several common application strategies:
Surface defect detection: For ferromagnetic materials, magnetic particle testing is preferred; For non ferromagnetic materials, choose penetration testing; For conductive materials that require rapid detection, eddy current testing can be considered.
Internal defect detection: For volumetric defects, radiographic testing is preferred; For planar defects, choose ultrasonic testing; For high-precision requirements, industrial CT can be chosen.
Comprehensive testing plan: For important castings, a combination of multiple testing methods is usually used, such as surface testing (MT or PT) first, followed by internal testing (RT or UT), to ensure comprehensive detection of various defects.
Automated testing: For mass-produced castings, automated non-destructive testing systems such as automatic ultrasonic testing lines, automatic X-ray testing systems, etc. can be considered to improve testing efficiency and consistency.
Non destructive testing technology plays the role of a quality "guardian" in the production of castings. By scientifically and reasonably applying various non-destructive testing methods, the quality of castings can be effectively controlled, production costs can be reduced, and product competitiveness can be improved. With the advancement of technology, non-destructive testing will develop towards a more intelligent, accurate, and efficient direction, providing stronger support for quality control in the casting industry. Foundry enterprises should establish a comprehensive non-destructive testing system based on their own product characteristics and production needs, cultivate professional testing talents, and continuously improve product quality levels.
