Estimation and Causes of Internal Defect Properties of Spring Flaw Detector - Master's thesis - Dissertation

Probe 420*4820 spring exposed needle head diameter is 5.0 high current needle

Estimation of the nature of defects in the flaw detector and its causes and preventive measures:

Stomata:

The single pore echo height is low, and the waveform is single slit, which is relatively stable. Detecting from all directions, the reflected waves are generally the same, but the probe disappears after a little movement, and a cluster of reflected waves appears in the dense air holes. The wave height varies with the size of the air holes. When the probe rotates at a fixed point, the phenomenon will occur. The main reason for such defects is that the welding consumables are not dried at the specified temperature, the welding strips are deteriorated, the core is rusted, the welding wire is not cleaned, the current is too large during manual welding, the arc is too long, and the voltage is too high during submerged arc welding. Or the network voltage fluctuation is too large; the purity of the shielding gas during gas shielded welding is low. If there are pores in the weld, the compactness of the weld metal is destroyed, the effective cross-sectional area of ​​the weld is reduced, and the mechanical properties are reduced. Especially when the chain-shaped pores are stored, the bending and impact toughness are significantly reduced. . Measures to prevent such defects are: do not use the electrode cracking, peeling, deterioration and corrosion of the core, the rusted wire must be rusted before use. The welding materials used shall be dried at the specified temperature, the groove and its sides shall be cleaned, and appropriate welding current, arc voltage and welding speed shall be selected.

Slag:

The point slag echo signal is similar to the point stomata. The strip slag echo signal is mostly sawtooth amplitude is not high, the waveform is mostly dendritic, there is a small peak on the main peak, and the probe translation amplitude changes, detecting from all directions. The reflected amplitude is not the same. The causes of such defects are: the welding current is too small, the speed is too fast, the slag is too late to float, the welded edges and the welds of the layers are not cleaned, the chemical composition of the metal and the welding material is improper, and the sulfur and phosphorus are compared. Wait a minute. The preventive measures are: correct selection of welding current, the groove angle of the welded parts should not be too small, the groove must be cleaned before welding, and the welding slag must be removed layer by layer during the multi-layer welding; and the welding speed of the moving angle is reasonably selected.

3, not welded through:

The reflectivity is high and the amplitude is also high. When the probe is translated, the waveform is relatively stable, and the same reflected wave amplitude can be obtained when the flaw is detected on both sides of the weld. Such defects not only reduce the mechanical properties of the welded joint, but also form a stress concentration point at the notch and the end of the incomplete penetration, which often causes cracks after the load, which is a dangerous defect. The reason for this is generally: the pure edge gap of the groove is too small, the welding current is too small or the speed of the moving bar is too fast, the groove angle is small, the angle of the moving bar is wrong, and the arc is blown. Preventive measures include: rational selection of groove type, assembly clearance and proper welding process.

Not fused:

When the probe is translated, the waveform is relatively stable. When detecting on both sides, the reflection amplitude is different, and sometimes it can only be detected from one side. The reason for this is that the groove is not clean, the welding speed is too fast, the current is too small or too large, the angle of the welding rod is wrong, and the arc is blown. Preventive measures: Correctly select the groove and current, clean the groove, and operate correctly to prevent welding deviation.

crack:

The height of the echo is large, the amplitude of the wave is wide, and multiple peaks appear. When the probe is translated, the amplitude of the reflected wave changes continuously. When the probe turns, the peak has a phenomenon of up and down. Crack is one of the most dangerous defects. In addition to reducing the strength of the welded joint, it also has a pin-shaped notch at the end of the crack. After the weldment is loaded, stress concentration occurs and becomes the origin of structural fracture. Cracks are classified into three types: hot crack, cold crack and reheat crack. The reason for the hot crack is that the cooling rate of the molten pool is very fast during welding, causing segregation; the weld is unevenly heated to generate tensile stress. Preventive measures: limit the content of easily segregated elements and harmful impurities in the base metal and welding materials, mainly limit the sulfur content, increase the manganese content; increase the alkalinity of the electrode or flux to reduce the impurity content, improve the degree of segregation; improve the welded structure form, Use a reasonable welding sequence to increase the degree of freedom when the weld shrinks.