Expert Guide: Welded Steel Pipe Defects and Evaluation Methods | MachineMFG

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Expert Guide: Welded Steel Pipe Defects and Evaluation Methods

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The defect evaluation method is based on relevant technologies.

By combining the analytical thinking ability and understanding judgment ability of humans with various on-site inspection experiences, five relevant methods for evaluating defects including time-related, location-related, behavior-related, feature-related, and property-related were developed and compiled into a defect judgment software program applied in the automation inspection process.

This makes the automation inspection system possess intelligent judgment function, which not only objectively ensures the accuracy and high efficiency of welded steel pipe inspection, improves the reliability and anti-interference ability of the inspection system, but also reduces the production cost of inspection and improves the integration and intelligence level of the inspection system.

1. Distribution Mechanism and Hazards of Welded Steel Pipe Defects

The production process of welded steel pipes involves using various forming methods to directly roll or spiral bend steel plates and strips into the required cross-sectional shape, and then using different welding methods to weld the weld seam to obtain the steel pipe with the help of heating and pressure.

Therefore, the defects of welded steel pipes are divided into two parts: defects in steel plate base materials and defects in weld seams.

1.1 Defects in steel plate base materials

Defects in plates are mostly in the form of flat layers parallel to the surface of the plate after processing operations like rolling. The main defects include delamination, inclusion, cracking, folding, etc., of which delamination is the most common internal defect.

Delamination can cause various cracks, and when the plate is subjected to tensile stresses perpendicular to the surface, delamination seriously affects the strength of the steel pipe, and it is not an allowable defect.

1.2 Defects in weld seams

Defects in weld seams refer to defects generated during the fusion welding process or after welding in the weld seam, including cracks, pores, slag inclusions, incomplete penetration, incomplete fusion, undercuts, etc.

Dense cavities and slag inclusions in the weld seam belong to dense three-dimensional defects, while cracks, incomplete fusion, and other defects belong to planar defects with greater hazards. Linear defects, such as strip-shaped slag inclusions and lack of penetration, are also dangerous.

Punctate defects include pores and small inclusions. Defects in the weld seam are more likely to cause problems in the strength and plasticity of the steel pipe, severely affecting the quality of the pipe.

The quality of welded steel pipes directly affects the safe operation and service life of oil and gas pipelines.

Therefore, weld seam inspection focuses mainly on the detection of potentially hazardous defects such as cracks, pores, slag inclusions, incomplete penetration, and incomplete fusion.

2. Defect Relevant Evaluation Method

On-site ultrasonic automated inspection is one-way and one-pass. Generally, it is not allowed to conduct back-and-forth inspections, so there is a need for a one-time accurate inspection.

However, in dynamic production conditions, online inspections are fleeting, and once missed or misreported, they cannot be retrieved or verified.

Moreover, the function of the automated inspection system is mainly implemented by software. Taking the inspection of welded steel pipe welds as an example, the design instruction of the defect relevant evaluation method is introduced.

The defect relevant evaluation method is a computational program for applying knowledge, analysis, judgment, and problem-solving to the defect waveform signal.

If the time it takes for ultrasound waves to reflect in the weld seam is evenly divided into eight parts, each part is represented by δt.

Then the defect judgment alarm condition of the defect relevant evaluation method is:

|Ftn – Ftn-1| ≤ δt (1)

3.5δt ≤ (Bt – Ftn) ≤ 7δt (2)

The number of consecutive valid defects n ≥ 4 (3)

Conditions (1) and (2) establish the allowable conditions for potential defect points. In addition to satisfying these two conditions, condition (3) must also be met for automatic alarm module.

By comparing the amplitude of the echo waveform, when the amplitude exceeds the set defect detection sensitivity amplitude height, it is judged as a defect, and the automatic alarm function will automatically issue an alarm message.

2.1 Defect Time Evaluation Method

Condition (1) means that the defect echo time Ft has continuous relevant features. Within a continuous small time range, the difference between two consecutive defect echo times Ft is less than or equal to 1 δt.

Condition (1) serves as a time-related discrimination condition and further improves the anti-interference ability. For example, if δt is 1/8 of the damage wave gate width, the anti-interference ability can be improved 8 times.

If set to δt/2, the anti-interference ability can be improved 16 times. The software program represented by condition (1) improves the system’s anti-interference ability by an order of magnitude.

Under the conditions of satisfying the maximum repetition frequency of the detection system and the scanning speed of the probe, users can reset δt based on the type of defect or the sensitivity of the probe, which greatly enhances the anti-interference ability.

2.2 Defect Location Evaluation Method

Condition (2) means that the detection range of the defect echo is surrounded by the start wave, bottom wave, and the front and rear interface waves.

The difference between each damage wave time Ft and the average value of the bottom wave time Bt should be between 3.5δt and 7δt to ensure that the central area of the welded pipe weld is repeatedly scanned and to prevent false alarms caused by the bottom wave entering the gate.

The setting of 3.5δt is to prevent false alarms caused by the bottom wave. Bottom waves that enter the defect wavegate continuously will not trigger an alarm.

The defect wave that exceeds the bottom wave time Bt suddenly by 3.5δt can trigger an alarm. The setting of 7δt is based on the fact that the distance between the sample hole on the welded sample tube and the edge of the weld is 1/4 of the weld width, that is, 2ΔL.

The distance from the other edge wave (where the Bt wave appears) is 6ΔL. In the actual inspection process, the reflection wave generated by the edge of the welded seam on the side of the probe is low and will not trigger an alarm.

Therefore, if the inspection requirements are high, 7δt can be set to 8δt or 8.5δt. As the judgment condition is implemented through software programs, users can easily achieve their own intentions, and adjust appropriately based on the alarm situation during the inspection process to ensure that the formula Bt-Ft takes a suitable value, while reducing false alarms to the maximum extent and relaxing the inspection range.

Condition (2) as a location-related identification condition greatly improves the system’s anti-false alarm ability. Even if the bottom wave enters the defect wavegate, the system will not trigger an alarm if condition (3) is not met.

2.3 Defect Behavior Evaluation Method

Condition (3) means that non-defective echoes appear less frequently or non-continuously, while defect echoes appear continuously. The number of consecutive defect points, that is, the number of continuous defect points, is confirmed as a defect echo only when it is more than 4.

Condition (3) serves as a behavior-related discrimination condition to improve the system’s anti-interference ability. According to the actual inspection needs, this value can be changed based on the type of defect, the sensitivity of the probe, and different inspection standards to change the judgment conditions.

For example, if small defects need to be confirmed as defects, the number of consecutive valid defects can be reduced. Conversely, if small defects are not to be considered, the value can be increased.

2.4 Defect Feature Evaluation Method

The position, continuity, time range, orientation and other characteristics of defects are diverse. The amplitude of the reflected wave of ultrasound is also different, which is related to the production process and inspection equipment.

The A-type pulse reflection ultrasonic testing can only provide information on the time and amplitude of the defect echo.

The defect features, such as the shape, size, and density of the defect echo and the bottom wave situation can be evaluated for planar defects, point defects, dense defects, and strip-like defects.

2.4.1 Planar Defects

The inspection is conducted in two different directions, longitudinal and transverse, on both sides of the weld. The height of the defect echo is significantly different and irregular, and the bottom wave height does not change significantly.

When the defect echo is strong and the bottom wave disappears, it can be considered as a large area defect.

When detecting perpendicular to the direction of the defect, a single sawtooth-shaped echo is displayed, the defect echo is higher, and the waveform is obviously sharp and steep.

When the probe is moved, the amplitude of the echo fluctuates randomly (wave amplitude difference > ±6dB).

When detecting parallel to the direction of the defect, the defect echo is lower or even absent.

When detecting obliquely to the direction of the defect, a bell-shaped pulse envelope is displayed.

In this pulse envelope, a series of continuous signals appear, usually showing a strong multi-peak shape with multiple changes in position (but not significant) and many small peaks. When the probe is moved, each small peak moves in the pulse envelope.

The amplitude gradually rises from zero to the maximum value and then decreases to zero. The signal amplitude fluctuates randomly (> ±6dB). According to the different height and irregular changes of the defect echo in the longitudinal and transverse directions, it can be evaluated as a planar defect.

Common planar defects include cracks, surface non-fusion defects, and surface non-penetration defects. These types of defects have obvious length and height, and the surface can be smooth or rough.

2.4.2 Point Defects

During the inspection in different directions, longitudinal and transverse, on both sides of the weld, the equivalent size of the defect echo is relatively small and not necessarily high.

The indicated length of the defect (ΔL) is less than or equal to the wall thickness (t), the height does not change significantly, and the bottom wave height also does not change significantly.

When the defect wave coexists with the bottom wave, it can be considered as a point defect or a small-sized area defect. The defect echo shows a sharp round wave (sharp echo), which is the waveform feature of point defects smaller than the diameter of the sound field, and the fluctuation of the defect echo changes rapidly and significantly as the probe moves along the pipe.

By keeping the path length constant, according to the characteristic that the height of the defect echo does not change significantly in the longitudinal and transverse directions and shows a sharp round wave, it can be evaluated as a point defect.

Common point defects include porosity, small inclusions, and other small defects. These types of defects are mostly spherical volume-type defects, which can also have irregular shapes. They are small volumetric defects and can appear in different positions in the weld.

2.4.3 Dense Defects

During the inspection in different directions, longitudinal and transverse, on both sides of the weld, the defect echo appears at different positions, and the display order is irregular.

Each individual echo signal displays a single sharp echo while the bottom wave disappears or its height decreases. When the probe is moved to different positions for detection, the echo signal displays a group of dense defect echoes.

The defect echoes are dense and interconnected, with different heights, and the reflection signals are fluctuating, sometimes high and sometimes low. If distinguishable, each individual echo signal displays the feature of point defects.

According to the characteristic that the position and display order of the defect echo in the longitudinal and transverse directions are irregular, and the bottom wave disappears or the bottom wave amplitude decreases by more than 50%, it can be evaluated as a dense defect.

Common dense defects include dense porosity, reheat cracks, and other defects. These types of defects are a collection of a group of defects. Each small defect is very close to each other, and it is impossible to individually locate and quantify each small defect.

2.4.4 Stripe Defects

During the inspection in different directions, longitudinal and transverse, on both sides of the weld, the defect echo amplitude is usually very high, with a single regular shape, roughly the same height, and no significant changes. Within a large range, continuous defect echoes occur.

At the same position, the bottom wave height does not change significantly as long as the signal does not significantly disconnect over a large distance, and the defect is continuous.

The defect indicator length (ΔL) can be measured. The defect echo peak steadily rises from zero to the peak value, maintains a straight section for a period, and then smoothly returns from the peak value to zero. This type of defect can be detected on both sides of the weld.

According to the characteristic that the height of the defect echo does not change significantly and the defect echo peak value rises and falls steadily in the longitudinal and transverse directions, it can be evaluated as a stripe defect.

Common stripe defects include strip-like inclusions, lack of penetration, lack of fusion, and other defects. These types of defects can measure the indicative length but are not easy to measure the cross-sectional dimension (height and width).

However, they may also be intermittent in the longitudinal direction, such as chain-shaped inclusions, intermittent lack of penetration, intermittent lack of fusion, etc.

Conclusion

In summary, the defect-related evaluation method in this welding steel pipe ultrasonic automatic testing system has taken an important step towards intelligent testing instruments. Based on the relevant technology, five related methods, including time-related, position-related, behavior-related, feature-related, and property-related, were compiled into the damage assessment software program.

It overcomes the influence of many artificial factors, and its detection reliability, anti-interference ability, and damage assessment accuracy have been greatly improved. It can meet the requirements of continuous automatic testing. Although it cannot completely eliminate false alarms in the system’s testing, it has eliminated the system’s misses, and the technical indicators have reached the expected design targets.

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