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Defect distribution and evaluation method of welded steel pipe

Views:5     Author:Site Editor     Publish Time: 2019-06-03      Origin:Site

Takeaway: in this article, welded steel pipe automatic ultrasonic flaw detection system defects in the relevant evaluation method took an important step in the direction of intelligent testing instrument, based on the theory of related technologies, compiled by the time, location, behavior, characteristics, properties of five kinds of method for software programs, overcame many man-made factors, the influence of the test reliability and anti-interference ability and the judgment accuracy has been greatly improved, can meet the requirements of continuous automatic detection, although not absolutely eliminate system flaw detection of false positives, but has to eliminate the omission of the system, and the various technical indexes reach the expected goal.

1. Distribution mechanism and hazard of welded steel pipe defects

The production process of welded steel pipe is to roll steel plate and strip into the required cross section shape by various molding methods or by bending in the spiral direction. Therefore, the defects of welded steel pipe can be divided into two parts: steel plate base material defects and weld defects.

Steel plate base defects

Defects in the plate after rolling and other processes, most of the plane shape, parallel to the surface; The main defects include layering, inclusions, cracks and folds, among which layering is the most common internal defect. Delamination will produce various cracks, when the plate is perpendicular to the surface of the tensile stress, delamination will seriously affect the strength of the steel tube, it is not allowed to exist defects.

Weld defect

Weld defects refer to the defects in the welding seam during or after fusion welding, which are classified into cracks, pores, slag inclusion, incomplete welding, incomplete fusion, undercut and other weld defects. The dense porosity and slag inclusion in the welding seam belong to the dense vertical defects, while the cracks and undissolved defects belong to the plane defects, which are harmful. Sliver slag inclusion and lack of penetration are sliver defects with great harmfulness. Stomata, small slag and other defects are point - like. Of welding defects are more likely to cause the problem such as the strength of the steel, plastic, seriously affect the quality of steel tube, and welded steel pipe quality directly affect the safe operation of oil and gas pipeline and service life, and therefore in weld inspection is mainly for weld crack, air hole, slag inclusion, incomplete fusion, not amalgamation dangerous defects such as detection of flaw detection.

2. Defect related evaluation methods

Field ultrasonic automatic flaw detection is one-way, one-way, generally not allowed to reciprocate testing, so there needs to be a once through the detection accuracy. But the scene in the dynamic production conditions, the online triage a transient, once leaving out false positives can't recover and validation, and the function of automatic detection system is mainly composed of software implementation, defects in welded steel pipe weld inspection, for example to introduce the design of the relevant evaluation method, the relevant evaluation method is defects on defect injury wave signal analysis using the knowledge, judgment, and the computing method program to solve the problem. If the reflection time of ultrasonic wave in the welding seam is evenly divided into 8 parts, each of which is denoted by delta t, then the defect detection alarm condition of defect correlation evaluation method is:

| | Ftn - Ftn - 1 or less delta t (1)

3.5 delta t≤ (Bt-Ftn) ≤7 delta t (2)

Number of continuous effective defects n≥4 (3)

Conditions (1), (2) established the actual may break points allowed condition, at the same time meet the condition (1), (2) two conditions must also satisfy the condition (3) the automatic alarm module by comparing the back wave of high amplitude, high when the wave amplitude than setting height when the flaw detection sensitivity of amplitude judged defect, automatic alarm function will automatically send alarm information.

Defect time evaluation method

Condition (1) means that the flaw echo time Ft has continuous correlation characteristics. Within a small continuous time range, the difference of echo time Ft between two consecutive adjacent defects is less than or equal to 1 delta t. And condition (1), as the time correlation discrimination condition, further improves the anti-interference ability. For example, if delta t is 1/8 of the gate width, the anti-interference ability can be increased by 8 times; if it is set to delta t/2, the anti-interference ability can be increased by 16 times, that is, the software program represented by condition (1) improves the anti-interference ability of the system by an order of magnitude. The user can reset delta t according to the defect type or flaw detection sensitivity, which will greatly improve the anti-interference ability if the maximum repetition frequency and flaw detection speed of the detection system are satisfied.

Defect location evaluation method

Condition (2) means that the detection range of defect echo is between the initial wave, the bottom wave, the front wave and the back wave, and the difference between each time Ft and the average Bt of the bottom wave time should be between 3.5 delta t and 7 delta t to ensure that the central area of the welding seam is repeatedly scanned and prevent false alarm caused by the bottom wave entering the gate. The setting of 3.5 delta t is to prevent the false alarm of the bottom wave, the bottom wave that continuously enters the flaw gate will not alarm, only the flaw wave that is 3.5 delta t before the average Bt of the bottom wave time will alarm. And 7 delta t set is based on the majority of inspection specified in the standard sample be weld on the pitch on the edge of the distance to a quarter of the width of the weld, i.e. Δ 2 L, so with another edge wave (Bt wave appeared on the verge of) the distance is 6 Δ L. 7 delta t will be set to 8 delta t or 8.5 delta t if the detection requirement is high. Because the condition of fault determination is realized by software program, users can easily realize their intention. In the process of fault detection, appropriate adjustment can be made according to the alarm situation, so that the formula bt-ft takes an appropriate value to ensure that the range of detection can be broadened while minimizing false alarm. Condition (2), as a condition of location correlation identification, can greatly improve the system's ability to avoid false alarm. Even when the bottom wave enters into the gate of the damaged wave, the system will not alarm if the condition (3) is not met.

Defect behavior evaluation method

Conditions (3) according to the defect echo successive occurrences or less appears discontinuous, and the defect echo is continuous, defect wave for the number of occurrences of, namely the number of points in four consecutive defects above is confirmed the defect echo, condition (3) as a condition of behavior related to identify used to improve the system anti-interference ability. This value can be changed according to the actual detection needs and according to the defect type, flaw detection sensitivity and different detection standards to modify the defect conditions. For example, when small defects are also recognized as defects, the number of continuous effective defects can be reduced; on the contrary, when small defects are not judged, the value can be increased.

Defect feature evaluation method

The location, continuity, time range and orientation of defects are various, and the high amplitude of ultrasonic wave reflection is not the same, which is related to the production process and testing equipment. The type A pulse reflection ultrasonic detection can only provide the time and amplitude information of the defect echo. According to the shape, size, density and other defect characteristics of the defect echo and the condition of the bottom wave, the plane defect, point defect, dense defect and strip defect can be evaluated and evaluated.

(1) planar defect

When detected in two different directions, longitudinal and horizontal, on both sides of the weld, the height of the defect echo was significantly different and showed irregular changes, while the height of the bottom wave was not significantly changed. When the defect echo is very strong and the bottom wave disappears, it can be considered as a large area defect. In the detection perpendicular to the defect direction, the single zigzag echo was shown, the defect echo was relatively high, and the waveform was obviously sharp and steep. When the probe moved, the echo amplitude had large random fluctuation (amplitude difference >±6dB). In the detection parallel to the defect direction, the defect echo is low or even no. In tilt in the direction of defect detection shows that bell pulse envelope, the bell with a series of continuous signal in pulse envelope, usually characterized by changing position (but not change) more strong peak shape, and appeared a lot of small wave, the probe moves, every little wave in the pulse envelope mobile, amplitude by zero gradually rises to the maximum, and then dropped to zero, the random fluctuation amplitude of the signal (> + 6 db). According to the height difference of defect echoes in the longitudinal and horizontal directions and their irregular change characteristics, it can be estimated that they are planar defects.

The common plane defects include cracks, surface undissolved, surface incomplete welding and other defects, which have length and obvious height of their own, and the surface is both smooth and rough.

(2) punctate defect

In weld on the longitudinal and lateral direction of two different sides, the defect echo equivalent smaller, does not necessarily high, Δ indicated defect length L t for wall thickness (t), or less height has no obvious change, Deborah height also has no obvious change, when the defect wave and Deborah coexistence, can be considered a dot defects or other defects, smaller size of the defect echo showed a light wave (sharp echo), which is less than the sound field of the diameter of the waveform characteristics of point defects, and with the movement of the pipe body, defect echo has changed a lot of ups and downs and fast. With the sound distance unchanged, the defect echo in the longitudinal and transverse directions has no obvious change in height and shows a circular wave of light, which can be estimated as a point-like defect.

The common spot defects include porosity, slag inclusion and other small defects. Most of these defects are of spherical volume type, and some are irregular in shape, which are small volume defects and can appear at different positions in the welding seam.

(3) dense defect

When detected in the longitudinal and horizontal directions on both sides of the welding seam, the defect echoes appear in different positions and show irregular display order. Each single echo signal shows a single sharp echo, while the bottom wave disappears or the height decreases. When the probe moves in different positions, the echo signal shows a group of dense defect echoes, and the defect waves are closely connected with each other, with different heights. The reflected signals rise and fall one after another, with different heights and lows. If it is distinguishable, each single echo signal shows the characteristics of point-like defects. According to the irregular position and display order of defect echoes in the longitudinal and horizontal directions, continuous disappearance of bottom wave or reduction of bottom wave amplitude by 50%, it can be estimated and judged as a dense defect.

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

(4) strip defect

When detected in the longitudinal and horizontal directions of both sides of the weld, the defect echo amplitude is usually very high, the shape is simple, the height is roughly the same, and there is no obvious change. In a large range, the defect echo appears continuously. And in the same position, the bottom wave height also has no obvious change, as long as the signal is not obvious disconnect large distance, basic continuous defects, measurable Δ indicated defect length L. The peak of defect echo rises smoothly from zero to the peak, maintains a straight section, and then drops smoothly from the peak to zero, which can be detected from both sides of the weld. According to the features that the height of defect echo in the longitudinal and horizontal directions is roughly the same and there is no obvious change, and the echo peak rises and falls smoothly, it can be estimated and judged as a stripe defect.

The common strip defects include strip slag inclusion, lack of penetration, and lack of fusion, etc. This defect can be measured indicating length, but it is not easy to measure the section size (height and width). However, it may be discontinuous in the length direction, such as chain slag inclusion, intermittent lack of penetration, intermittent lack of fusion, etc.

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