What are the conventional non-destructive testing methods for martensitic stainless steel pipes

Martensitic stainless steel pipes are widely used in high-demand fields such as energy, chemical industry, shipbuilding, aerospace, and nuclear industry. This type of material has excellent strength and wear resistance, but due to its heat treatment characteristics and organizational structure, various defects are prone to occur during manufacturing and welding. In order to ensure the quality stability and service safety of martensitic stainless steel pipes, scientific and reliable non-destructive testing methods must be used for comprehensive inspection. Non-destructive testing technology can detect internal or surface defects without destroying the integrity of the workpiece, and is an important tool for quality control and failure prevention.

Radiographic testing (RT)
Radiographic testing is one of the conventional methods for detecting internal defects in martensitic stainless steel pipes. X-rays or gamma rays are used to penetrate the material, and film imaging or digital imaging technology is used to observe whether there are defects such as pores, inclusions, and cracks inside the material.
Radiographic testing is suitable for detecting volumetric defects in areas such as welds, base materials, and joints. Especially in key areas such as pressure pipes, heat exchanger tube bundles, and boiler tubes that require high welding quality, radiographic testing can intuitively reflect the shape and location of defects.
This method has the advantages of clear imaging and intuitive recording, but it has high requirements for the operating environment, requires shielding and protection measures, and has relatively high detection costs. It is not suitable for components with complex shapes or large sizes.

Ultrasonic testing (UT)
Ultrasonic testing is a widely used non-destructive testing technology, suitable for internal defect detection of welds, parent materials and connection areas of martensitic stainless steel pipes. When ultrasonic pulses propagate in the material, they will reflect when they encounter defects. The location, size and type of defects are determined by analyzing the reflected waves through the received signal.
Ultrasonic testing can be used to detect volumetric defects such as pores, inclusions, cracks, etc., especially for stainless steel pipes with thicker walls. Compared with X-ray testing, ultrasonic testing has high safety, strong sensitivity, fast detection speed, and is easy to operate on site.
When testing martensitic stainless steel, it is necessary to consider its coarse grains and large changes in acoustic impedance, and appropriately select low-frequency probes and high-gain equipment to improve detection resolution and accuracy.

Magnetic particle testing (MT)
Magnetic particle testing is suitable for detecting cracks, folds, slag inclusions and other defects on the surface and near the surface of martensitic stainless steel pipes. Since martensitic stainless steel is a ferromagnetic material with good magnetization conditions, magnetic particle testing technology can be effectively applied.
During the inspection process, a magnetic field is applied to the workpiece to form a magnetic leakage field at the defective part, fluorescent or colored magnetic powder is adsorbed, and magnetic traces are observed with the help of ultraviolet light or natural light to determine the existence and distribution of defects.
Magnetic particle testing has the advantages of high sensitivity, low cost, and simple operation. It is widely used for rapid inspection of welded joints, elbows, and flange connection areas. However, this method can only detect surface and near-surface defects and is not suitable for non-ferromagnetic stainless steel materials.

Penetrant testing (PT)
Penetrant testing is suitable for detecting surface opening defects of martensitic stainless steel pipes, such as cracks, pores, cold shuts, etc. This method is not limited by the magnetism of the material and is an effective means to detect surface defects in non-magnetic or weakly magnetic areas.
The operation process includes steps such as cleaning, penetration, removal of residual liquid, imaging, and observation. Fluorescent penetrants can show signs of defects under ultraviolet light, which is convenient for visual identification; colored types are suitable for use under ordinary lighting conditions.
Penetrant testing has a good effect on the detection of surface microcracks, and is especially suitable for supplementary testing of welds, heat-affected zones, processed surfaces and other parts. However, its disadvantage is that it cannot detect internal defects and has certain requirements for surface roughness.

Eddy current testing (ET)
Eddy current testing is mainly used to detect cracks, corrosion, wear and other problems on the surface and near the surface of martensitic stainless steel pipes, especially for thin-walled stainless steel pipes or online detection scenarios. By exciting the probe to generate an alternating magnetic field, eddy currents are generated on the surface of the induced material, and defects will change the eddy current path and form impedance changes.
Eddy current testing has a fast response speed and is suitable for automated and continuous testing, especially in the maintenance of heat exchangers and condenser pipelines. This method has obvious advantages in non-contact, non-destructive and high-efficiency testing.
When testing martensitic stainless steel, due to the low electrical conductivity and high magnetic permeability of the material, the frequency and probe parameters need to be accurately adjusted to avoid interference affecting accuracy.

Magnetic flux leakage detection (MFL)
Magnetic flux leakage detection is suitable for detecting corrosion, thinning and crack propagation of martensitic stainless steel pipes during use, especially in the online detection of long-distance pipelines and oil and gas transportation pipelines. This method magnetizes the pipe body. When there is corrosion or cracks, a magnetic leakage field will be generated at the defect to form a detection signal.
Magnetic flux leakage detection is suitable for large-scale, rough screening scenarios, which facilitates early detection of potential structural degradation areas and improves the safety of pipeline system operation.