| As the austenitic stainless cladding layer structure has excellent heat resistance, corrosion resistance, abrasion resistance and other properties, it has been widely used in nuclear power, chemical industry, etc. For the problem that it is easy to form different types of defects such as cracks in the interior of material,it is necessary to use nondestructive testing technology to evaluate it. TOFD (Time of Flight Diffraction) is based on Huygens’ principle, and it works By calculating Sonic time difference of defect diffraction wave. The methods has high precision of quantitative and it is highly sensitive to linear and area types of defects. The testing is not affected by defect orientation. For the characteristics that it has big attenuation coefficient and low signal noise ratio (SNR) while testing, the parameter optimization has been proposed to detect the defects, determine its location and size exactly.The object of study is Alloy steel with austenitic stainless steel welding layer block. The thickness of the block is 70mm. The base metal is alloy steel while the welding cladding layer is stainless steel. And the thickness of the base metal is from 55mm to 60mm while the welding cladding layer is from 10mm to 15mm. The research on the austenitic stainless cladding layer structure is through experiment. The experimental study is Surrounding the detection parameters include the probe frequency, chip size, Angle of the probe and the probe center spacing. The main research contents are as follows:(1) First, conduct the metallographic analysis of stainless steel cladding layer sample. Then measuring the sound velocity, attenuation coefficient and other parameters. The results show that austenitic stainless steel welding layer organization grain size is from dozens of micron to hundreds of micrometers range. The average attenuation coefficient is from 0.17dB/mm-0.22dB/mm, and the average velocity of sound is from 5804 m/s-5906 m/s.(2) The probe frequency mainly influence the resolution of detection. If we increase the frequency of probe, the firing pulse width will be narrowed and detecting resolution will increase, but the attenuation will increase. Probe Angle mainly affect the surface blind area. If we increase the Angle probe, the beam coverage will increase and On the surface of blind area will decrease.But if the probe Angle is too large, the diffraction wave amplitude value is low. So we should choose 70 ° wedge to reduce the surface blind area. The chip size mainly affect the beam spread Angle. If we Scan for the first time, we Should choose a smaller chip size to get larger beam coverage. The probe center spacing mainly affect the beam resolution, if we increase the probe center spacing, the beam coverage will increase and the beam resolution will be reduced. The experimental results show that cross hole defects in surfacing welding interface and surfacing layer can be detected by using the frequency of 2.25MHz to 5MHz, Angle of 45° to 60° and the chip size of 3mm to 10mm.(3) On the basis of parameter optimization, use the direct quantitative, the spectral analysis,-6dB and other methods to determine the size of defects include the surface notch defects which In the parent metal with depth of 3.0 mm and the cross hole defects which In the parent metal with depth of 10.0mm,20.0mm,30.0mm,40.0mm,50.0mm and in the surfacing layer with depth of 60.0mm,65.0mm. The results show that the absolute error is less than ±1.0mm when determine the depth of the defects, the relative error is less than 4%. The absolute error is less than ±0.2mm when determine the height of the defects. The relative error is less than 10%. And the absolute error is less than ± 1.0mm when determine the length of the defects, the relative error is less than 5%.(4) Finally, the paper analyses the causes of quantitative errors from the parameter Settings and personnel operation, etc. |
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