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

Probe with a 5.0 mm diameter spring-exposed needle head, suitable for high current applications

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

Porosity:

Porosity is characterized by a low echo height and a single-slit waveform, which remains relatively stable. When detected from all angles, the reflected waves are generally consistent, but when the probe is slightly moved, the porosity disappears and a cluster of reflections appears in areas with dense air holes. The amplitude varies depending on the size of the pores. This phenomenon occurs when the probe rotates at a fixed point. Common causes include improper drying of welding consumables, deteriorated or rusted welding rods, unclean welding wire, excessive current during manual welding, long arc length, high voltage during submerged arc welding, or unstable network voltage. Low purity of shielding gas in gas-shielded welding can also contribute. Porosity reduces the compactness of the weld metal, decreases the effective cross-sectional area, and weakens mechanical properties. Chain-like porosity significantly reduces bending and impact toughness. To prevent this, avoid using cracked, peeling, or corroded electrodes. Rusty wires should be cleaned before use. Ensure welding materials are dried properly, clean the groove and surrounding areas, and select appropriate welding current, arc voltage, and speed.

Slag Inclusion:

Slag inclusion echoes resemble those of porosity. Strip slag echoes typically have a sawtooth shape with moderate amplitude, a dendritic waveform, and a small peak on the main peak. As the probe moves, the amplitude changes, and the reflection strength varies when detected from different angles. Causes include insufficient welding current, excessive speed, delayed slag floating, failure to clean the joint edges and previous layers, and improper chemical composition of the base metal or welding material, especially high sulfur and phosphorus content. Preventive measures involve selecting the correct welding current, ensuring the groove angle is not too small, cleaning the groove thoroughly before welding, removing slag layer by layer during multi-pass welding, and choosing an appropriate welding speed and angle.

Incomplete Penetration:

Incomplete penetration shows a high reflectivity and amplitude. The waveform remains stable as the probe moves, and the same amplitude is observed when detecting from both sides of the weld. This defect weakens the mechanical properties of the joint and creates stress concentration points, often leading to cracks under load. Common causes include a narrow groove gap, low welding current, fast travel speed, small groove angle, incorrect electrode angle, or arc blow. Preventive measures include selecting the right groove type, proper assembly clearance, and appropriate welding parameters.

Lack of Fusion:

Lack of fusion is identified by a stable waveform when the probe moves. Reflection amplitude differs when detecting from both sides, and sometimes it can only be detected from one side. This occurs due to an unclean groove, fast welding speed, incorrect current, improper electrode angle, or arc blow. Prevention includes selecting the correct groove and current, cleaning the groove, and operating correctly to avoid misalignment.

Cracks:

Cracks produce a high echo with wide amplitude and multiple peaks. As the probe moves, the reflected amplitude continuously changes, and when the probe is rotated, the peak fluctuates up and down. Cracks are among the most dangerous defects, reducing joint strength and creating sharp notches that cause stress concentration and lead to structural failure. Cracks are classified into hot cracks, cold cracks, and reheat cracks. Hot cracks occur due to rapid cooling of the molten pool, causing segregation, and uneven heating generating tensile stress. Prevention involves limiting the content of segregating elements and harmful impurities, especially sulfur, increasing manganese content, raising electrode or flux alkalinity, and improving the welded structure through proper welding sequence and increased shrinkage freedom.