How To Detect Defects in Mild Steel Tubes?

In industrial production and construction, mild steel tube is favored for their good performance and wide application. However, various defects may occur in carbon steel pipe during production and use, which not only affect the quality and performance of steel pipes, but may also cause safety hazards. Therefore, it is crucial to accurately detect the defects of mild steel tubes. The following are some detection methods for defects in mild steel tubes:

Appearance inspection

1. Visual observation

This is the most direct and simple detection method. The inspector observes the surface of the mild steel tube with the naked eye to see if there are obvious defects. For example, cracks, folds, pits, scratches, etc. may appear on the surface of the steel pipe. For some minor defects, a magnifying glass can be used to assist in observation. At the welding site, carefully check whether the weld is flat, whether there are pores, slag inclusions, etc.

2. Gauge measurement

Use tools such as calipers and micrometers to measure the outer diameter, inner diameter, wall thickness and other dimensions of the steel pipe. Through measurement, it can be determined whether the size of the steel pipe meets the standard requirements, and it can also be found whether the steel pipe has local deformation. For example, if the outer diameter tolerance of a steel pipe is required to be ±0.5mm, a caliper is used to perform multi-point measurements. If it exceeds this range, there may be quality problems.

Nondestructive testing

1. Ultrasonic testing (UT)

Ultrasonic testing uses the characteristics of reflection and refraction when ultrasonic waves propagate inside the steel pipe and encounter defects. During testing, an ultrasonic probe is placed on the surface of the steel pipe to emit ultrasonic waves into the steel pipe. When the ultrasonic wave encounters defects such as cracks and inclusions, the reflected waves are received and analyzed by the instrument to determine the location and size of the defect. This method is suitable for detecting defects at deeper locations inside the steel pipe, and has better detection effects for mild steel tubes with thicker walls.

2. Magnetic particle testing (MT)

Magnetic particle testing is mainly used to detect surface and near-surface defects of mild steel tubes. When the steel pipe is magnetized, surface or near-surface defects will cause distortion of magnetic lines of force. Spraying magnetic powder on the surface of the steel pipe will gather at the defect, forming obvious magnetic marks, thereby showing the location and shape of the defect. For mild steel tubes with tiny cracks on the surface, magnetic particle testing can detect these defects very sensitively.

3. Radiographic testing (RT)

Radiographic testing includes X-ray and γ-ray testing. After the rays penetrate the steel pipe, an image is formed on the film or detector. If there are defects inside the steel pipe, such as pores, slag inclusions, etc., different degrees of grayscale changes will be shown on the image, and the type, size and location of the defects can be determined. However, radiographic testing has certain radiation hazards to operators, and protective measures need to be taken, and the equipment cost is relatively high.

4. Eddy current testing (ET)

When the alternating magnetic field is close to the mild steel tube, eddy currents will be generated on the surface of the steel pipe. If there are defects on the surface or near the surface of the steel pipe, the distribution and size of the eddy current will change, and this change will be measured by the detection coil to detect the defects. Eddy current testing is fast and can realize automatic detection. It is mainly used to detect defects on the surface and near the surface of the steel pipe, such as cracks, holes, etc.

Physical and chemical properties testing

1. Tensile test

Through the tensile test, the mechanical properties of mild steel tubes such as yield strength, tensile strength and elongation can be tested. The steel pipe is processed into a standard tensile specimen, stretched on a universal testing machine, and the stress-strain curve is recorded to determine whether the steel pipe meets the strength and toughness requirements. If the yield strength of the steel pipe is lower than the standard value, plastic deformation may easily occur during use.

2. Hardness test

Hardness test can reflect the wear resistance and strength of mild steel tubes. Commonly used hardness test methods include Brinell hardness (HB), Rockwell hardness (HR) and Vickers hardness (HV). Hardness test is performed on the end or side of the steel pipe. If the hardness does not meet the standard range, it may indicate that the heat treatment process of the steel pipe is improper or the material is uneven.

3. Chemical composition analysis

The chemical composition of mild steel tubes is analyzed using equipment such as spectrometers to check whether the content of elements such as carbon, silicon, manganese, phosphorus, and sulfur meets the standard requirements. If the chemical composition is unqualified, it may affect the performance of the steel pipe. For example, too high a carbon content will increase the hardness of the steel pipe, but reduce its toughness.