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Defect distribution and evaluation method of welded steel pipe
Date:2020-11-10      Views:229Time      Label:Defect distribution and evaluation method of welded steel pipe

1. Distribution mechanism and hazards of welded steel pipe defects

The production process of welded steel pipes is to roll steel plates, steel strips, etc. straightly or in a spiral direction into the required cross-sectional shape by various forming methods, and then use different welding methods to weld the welds with the help of heating and pressure. Together, the steel pipe is obtained. Therefore, the defects of welded steel pipes are divided into two parts: the defects of the base metal of the steel plate and the defects of the weld.

 

Steel base material defects

The defects in the plate are mostly flat and parallel to the surface after rolling and other processes; the main defects are delamination, inclusions, cracks, folds, etc., among which delamination is the most common internal defect. Delamination will produce various cracks. When the plate is subjected to tensile stress perpendicular to the surface, the delamination will seriously affect the strength of the steel pipe. It is an impermissible defect.

 

Weld defect

Weld defects refer to the defects produced in the weld during or after welding and are divided into weld defects such as cracks, pores, slag inclusion, incomplete penetration, infusion, undercut, and other weld defects. Dense pores and slag inclusions in the weld are dense three-dimensional defects, and cracks and infusion are planar defects, which are harmful. Striped slag inclusions, incomplete penetration, etc. are stripped defects, which are harmful. Pores, small slag inclusions, etc. are point defects. Defects in the weld are more likely to cause problems such as the strength and plasticity of the steel pipe, which seriously affect the quality of the steel pipe. The quality of the welded steel pipe directly affects the safe operation and service life of the oil and gas transmission pipeline. Therefore, the weld inspection is mainly for welding Dangerous defects such as cracks, pores, slag inclusion, incomplete penetration, and infusion in the seam shall be inspected for flaw detection.

 

2. Defect-related evaluation methods

On-site ultrasonic automatic flaw detection is one-way and one-way, and reciprocating detection is generally not allowed, so one-pass detection accuracy is required. However, under dynamic production conditions, the on-line flaw detection is fleeting, and once the false positives are missed, the false positives cannot be recovered and verified, and the function of the automatic flaw detection system is mainly realized by software. Taking the flaw detection of welded steel pipe welds as an example, the defect-related judgments are introduced. The design of the method illustrates that the defect-related evaluation method is to use knowledge, analyze and judge and solve the problem of the defect wave signal. If the time of ultrasonic reflection in the weld is evenly divided into 8 parts, and each part is represented by δt, the defect judgment alarm condition of the defect-related judgment method is:

  |Ftn-Ftn-1|≤δt (1)

  3.5δt(Bt-Ftn)7δt (2)

   Number of continuous effective defects n4 (3)

Conditions (1) and (2) establish the allowable conditions for the actual possible damage points. In addition to conditions (1) and (2), the automatic alarm module must meet conditions (3) by comparing the echo height When the amplitude of the echo height exceeds the amplitude height of the set defect detection sensitivity, it is judged as a defect, and the automatic alarm function will automatically send out an alarm message.

 

Defect time evaluation method

Condition (1) indicates that the defect echo time Ft has continuous correlation characteristics. In a continuous short time, the difference between two consecutive defect echo times Ft is less than or equal to 1 δt. Condition (1), as a time-dependent identification condition, further improves the anti-interference ability. For example, if δt is 1/8 of the width of the wave gate, the anti-interference ability can be increased by 8 times. If δt/2 is set, the anti-interference ability can be increased by 16 times, that is, the software program represented by condition (1) makes the system The anti-interference ability has been improved by an order of magnitude. Under the condition that the maximum repetition frequency of the inspection system and the detection speed are satisfied, the user can reset the δt according to the defect type or the detection sensitivity, which can greatly improve the anti-interference ability.

Condition (2) means that the detection range of the flaw echo is surrounded by the initial wave, the bottom wave, and the front and the last two interface waves. The difference between each flaw time Ft and the average Bt of the bottom wave time should be 3.5 δt to 7δt to ensure that the central area of the welded pipe weld is repeatedly scanned and prevent false alarms caused by the bottom wave entering the gate. The setting of 3.5δt is to prevent false alarms of the bottom wave. The bottom wave that continuously enters the defect wave gate will not be able to alarm. It must be a defect wave that is 3.5δt ahead of the average value of the bottom wave time Bt. . The 7δt setting is based on the fact that most inspection standards stipulate that the distance between the sample pipe weld sample hole and the edge is 1/4 of the weld width, that is, 2ΔL, so the distance from the other edge wave (the edge where the Bt wave appears) is 6ΔL. In the actual flaw detection process, the reflected wave generated by the edge of the welding seam on the probe side is low and will not generate an alarm. Therefore, if the flaw detection requirements are high, you can set 7δt to 8δt or 8.5δt. Because the judgment conditions are realized by software programs, users can easily realize their intentions. During the detection process, they can be adjusted appropriately according to the alarm situation to make the formula Bt-Ft take an appropriate value to ensure that false alarms are minimized at the same time Broaden the detection range. Condition (2), as a position-related identification condition, can greatly improve the system's ability to avoid false alarms. Even when the bottom wave enters the fault gate, if condition (3) is not met, the system will not alarm.

 

Condition (3) means that the non-defect echo has a small number of consecutive occurrences or discontinuities, while the defect echo is continuous, the number of consecutive occurrences of the defect wave, that is, the number of consecutive defect points are confirmed to be more than 4 Defect echo, condition (3) is used as a behavior-related identification condition to improve the anti-interference ability of the system. You can change the judgment conditions according to the actual detection situation, according to the defect type, detection sensitivity, and different detection standards. For example, when small defects are also confirmed as defects, the number of continuous effective defects can be reduced, and vice versa, When the small defect is not judged, the value can be increased.

 

The location of defects, the continuity of their appearance, the time range of their appearance, and the orientation of their appearance are varied, and the reflected wave height amplitude of ultrasonic waves is also different, which is related to the production process and testing equipment. The adopted A-type pulse reflection ultrasonic inspection can only provide information on the time and amplitude of the defect echo. According to the defect characteristics such as the shape, size, and density of the defect echo and the bottom wave, the flat defect of the defect can be performed. , Point-shaped defects, dense-shaped defects, strip-shaped defects related assessment, and evaluation.

 

(1) Planar defects

Detected in two different directions, longitudinal and transverse, on both sides of the weld, the height of the defect echo is significantly different and changes irregularly, and the height of the bottom wave has no obvious change. When the defect echo is strong and the bottom wave disappears, it can be considered as a large area defect. The detection in the direction perpendicular to the defect all shows a single zigzag echo, the defect echo is high, and the waveform is sharp and steep. The echo amplitude randomly fluctuates when the probe is moved (wave amplitude difference>±6dB); the detection is parallel to the defect direction, The defect echo is low, or even no defect echo; the detection shows a bell-shaped pulse envelope in the oblique direction of the defect, and the bell-shaped pulse envelope contains a series of continuous signals, which usually show changes in position (but little change) Strong and multi-spike-shaped, and there are many small peaks. When the probe moves, each small peak moves in the pulse envelope, and the amplitude gradually rises from zero to the maximum and then drops to zero. The signal amplitude fluctuates randomly (>±6dB). According to the different heights of the defect echoes in the vertical and horizontal directions, and irregular changes, it can be evaluated as a flat defect.

Common planar defects include cracks, planar infusion, and planar incomplete penetration. Such defects have length and obvious height. The surface can be smooth or rough.

 

(2) Point defects

The inspection is carried out in two different directions, longitudinal and transverse on both sides of the weld. The defect echo equivalent is small and not necessarily very high. The defect indication length ΔLt (t is the wall thickness), the height has no obvious change, and the bottom wave There is no obvious change in height. When the defect wave coexists with the bottom wave, it can be considered as a point defect or other defect with a smaller area. The defect echo shows a light circular wave (sharp echo), which is smaller than the sound field diameter. The waveform characteristics of the point-shaped defect and the fluctuation of the defect echo with the movement of the tube body change greatly and rapidly. Keeping the sound path distance constant, there is no obvious change in the height of the defect echo in the vertical and horizontal directions, and the characteristic of a light circular wave can be evaluated as a point defect.

 

Common point defects include pores, small slag inclusions, and other small defects. Most of these defects are spherical volumetric defects, but also irregular shapes, which are small volume defects and can appear in different positions in the weld.

 

(3) Dense defects

The inspection is carried out in two different directions, longitudinal and transverse on both sides of the weld. The defect echoes appear in different positions and the display order is irregular. Each single echo signal shows a single sharp echo, but the bottom wave disappears or height reduces. When the probe is moved in different positions for detection, the echo signal shows a group of dense defect echoes, the defect waves are densely connected, and the height is different. The reflected signal rises and falls, and if it can be distinguished, each echo signal is displayed. Characteristics of point defects. According to the position and display sequence of the defect echoes in the vertical and horizontal directions, the irregularity, the continuous occurrence of the disappearance of the bottom wave, or the reduction of the bottom wave amplitude by 50% can be evaluated as a dense defect.

Common dense defects include dense pores, reheat cracks, and other defects. This type of defect is a collection of defects. Each small defect is very close to each other, and it is impossible to locate and quantify each small defect separately.

 

(4) Strip defects

Detecting in two different directions, longitudinal and transverse on both sides of the weld, the defect echo amplitude is usually very high, the shape is regular and single, the height is roughly the same and there is no obvious change, and the defect echo occurs continuously in a larger range. And at the same position, there is no obvious change in the height of the bottom wave. As long as the signal is not significantly disconnected by a large distance, the defect is continuous, and the measurable defect indication length ΔL. The peak of the defect echo rises steadily from zero to the peak and maintains a straight section, and then steadily drops from the peak to zero, which can be detected from both sides of the weld. According to the characteristics that the height of the defect echoes in the vertical and horizontal directions are approximately the same, there is no obvious change, and the echo peaks rise and fall steadily, which can be evaluated as strip defects.

Common strip defects include strip slag inclusion, incomplete penetration, infusion, and other defects. This type of defect can be measured to indicate the length, but it is not easy to measure the cross-sectional dimensions (height and width); however, it may also be discontinuous in the length direction. , Such as chain slag inclusion, intermittent incomplete penetration, intermittent infusion, etc. 

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