Selection of Process Parameters for Ultrasonic Testing of ERW Welds

The ERW tube is made by using the skin effect and proximity effect of high-frequency current to heat the bad edge of the tube to the welding temperature quickly and then extrude and weld it. Compared with seamless steel pipes, ERW welded pipes have the advantages of high dimensional accuracy, low price and high production efficiency, and their grain size and structure are better than seamless steel pipes. Compared with the submerged arc welded pipe of the same specification, the production speed of ERW welded pipe is fast, and there is no local thinning of the anti-corrosion layer at the weld. Its fields of application involve casing used in oil drilling, subsea oil and gas pipelines in the offshore oil industry, oil and gas pipelines and gas distribution pipes for trunk lines and urban pipeline networks, etc.

Ultrasonic testing of ERW welds mainly includes automatic testing and manual testing. At present, the automatic detection of ERW welds mainly adopts two forms: wheel probe detection and partial water immersion detection. Automatic detection has the advantages of high detection efficiency and fast speed, but it is not conducive to the precise positioning of defects and qualitative and quantitative analysis. In comparison, the flexibility of manual detection is higher, not only can accurately locate defects, but also conduct qualitative and quantitative analysis of defects through echo characteristics and dynamic waveforms. For defects detected by automatic ultrasonic detection, manual methods are generally used for further confirmation. The process parameters that need to be considered in the ultrasonic testing of ERW welds include refraction angle, acoustic beam width and testing frequency, etc.

1. Selection of refraction angle

Ultrasonic testing of ERW welding tanks generally adopts oblique incidence of longitudinal waves, and generates transverse waves through wave mode conversion in the workpiece and weld to achieve detection. The two basic conditions are:

1) Pure shear waves are excited in the base metal and weld.

2) The transverse wave sound beam scans the inner wall of the steel pipe,

2. Selection of detection frequency

The frequency range of ultrasonic testing is wide, generally 0.5-10MHz, and the selection of frequency should mainly consider the following factors:

(1) The sensitivity of ultrasonic testing is about half of the wavelength, and increasing the frequency is conducive to finding smaller defects. In addition, the higher the frequency, the smaller the pulse width and the higher the resolution.

(2) The frequency is high, the wavelength is short, the half-diffusion angle is small, the sound beam directivity is good, and the energy is concentrated, which is conducive to finding and locating defects. But for the same wafer size, the higher the frequency, the larger the near-field area, which is more unfavorable for detection.

(3) As the frequency increases, the scattering and absorption attenuation of ultrasonic waves increase sharply, which is unfavorable for detection.

In the detection, various factors should be considered comprehensively, and the detection frequency should be selected reasonably. For ERW welded pipes, the base metal is generally hot-rolled coils with relatively fine grains. After induction heating and extrusion molding, the width of weld fusion zone and heat affected zone is narrow, and its grain size is basically equivalent to that of the base metal. In actual work, in order to obtain higher resolution, a higher frequency should be selected as much as possible under the premise of ensuring detection sensitivity, generally between 2.5-5MHz

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