Welding defect analysis

　　The danger of welding defects to product components, especially the hidden dangers brought about by boiler pressure vessels and pressure pipelines during use, cannot be underestimated.

　　1. Welding defects in the weld reduce the bearing cross-sectional area of the weld, weakening its static tensile strength.

　　2. If a defect forms a notch, stress concentration and embrittlement will occur at the notch tip, easily leading to crack initiation and propagation.

　　3. Defects may penetrate the weld, causing leakage and affecting the tightness of equipment or pipelines.

　　Regardless of the type, welding defects pose a certain degree of harm to structural components. Minor defects can reduce the mechanical properties of products and shorten their service life; severe defects can lead to brittle fracture, and in serious cases, catastrophic accidents, causing huge losses to the national economy.

　　They can be divided into two types: surface defects and internal defects.

　　Common surface defects: porosity, slag inclusions, incomplete penetration, lack of fusion, cracks, etc.

　　Porosity: During welding, bubbles in the weld pool that fail to escape in time during solidification remain, forming voids known as pores or porosity. Porosity is one of the most serious process defects in welding. Pores weaken the effective working area of the welded joint and also cause stress concentration, reducing the joint's strength, plasticity, and fatigue strength. Depending on where they occur, pores can be classified into surface pores and internal pores. Depending on their distribution, pores can be classified into single pores, dispersed pores, clustered pores, and continuous pores. Based on their shape, pores can be classified into linear pores, needle-like pores, circular pores, elliptical pores, etc. Image description of porosity defects on X-ray film: On the film, the image appears as black dots, or as black elongated shapes or other irregular shapes. The outline of pores is generally smooth, with greater darkness at the center of the pore, slightly decreasing towards the edges.

　　Causes of Porosity:

　　Specifically divided into two types:

　　1. Influence of metallurgical factors on porosity.

　　During welding, pores can be generated when metallurgical reactions occur in the weld metal. Chemical reactions of electrode coatings and fluxes during arc welding also produce pores when they act as protection. Oil stains, rust spots, and the decomposition of contaminants on the surface of the base metal, as well as the chemical composition of the flux, can all lead to porosity.

　　2. Influence of process factors on porosity.

　　Process factors include: welding procedures, current type, arc length, operating technique, etc. The tendency for porosity is minimized with DC reverse polarity, as the workpiece is the negative pole.

　　The following will specifically explain the influence of these two factors, metallurgical factors and process factors, on porosity:

　　1. Electrodes or flux not dried according to regulations or drying temperature not meeting requirements before use. Using such welding materials and flux during welding is highly prone to producing porosity. The drying temperature for basic electrodes is 350-450℃, while for acidic electrodes it should not exceed 200℃, typically 70-150℃. Electrodes taken from storage must be kept in an electrode warming oven and used within 4 hours. Electrodes not used within 4 hours must be returned to storage and redried. In submerged arc welding, if the flux is damp, continuous porosity will occur in the weld bead.

　　2. Electrode coating failure, spalling, or excessively high drying temperature, causing some components in the coating to deteriorate.

　　3. Failure to remove metal rust and oil stains from the base metal before welding. Or the flux contains mixed gas-producing substances.

　　4. Molten metal cooling too fast, low arc heat, or excessively fast welding speed. This prevents gases in the weld pool from rising and escaping in time.

　　5. Using excessive current, which causes the electrode to glow red and leads to coating failure, or an excessively long arc when welding with basic low-hydrogen electrodes.

　　6. Low argon purity and poor shielding during manual TIG welding. Use argon with 99.99% purity for welding; pre-purge gas before welding and post-purge after welding; correctly connect gas and water hoses without mixing them up; perform thorough cleaning before welding; select appropriate shielding gas flow, nozzle size, electrode stick-out length, etc.

　　7. Arc too long or arc blow, resulting in poor weld pool protection and air intrusion into the weld pool. When welding with low-hydrogen electrodes, short arc welding should be used, combined with weaving to facilitate gas escape.

　　8. Improper electrode manipulation, too fast arc termination, prone to shrinkage pores; incorrect arc striking at joints, prone to dense porosity.

　　9. Welding current too low or welding speed too fast, causing the weld pool existence time to be too short, and gases unable to escape from the molten metal in time.

　　10. High carbon content in the base metal and electrode core, poor deoxidation capacity of the electrode coating.

　　Measures to prevent porosity:

　　1. Before welding, thoroughly clean oil, rust, and moisture within 20-30mm on both sides of the groove.

　　2. Strictly dry electrodes according to the temperature and time specified in the electrode manufacturer's instructions.

　　3. Select correct welding process parameters, operate correctly, and preheat.

　　4. Use short arc welding as much as possible, and provide wind protection for outdoor welding operations.

　　5. Do not use expired electrodes, such as those with rusted cores, cracked or spalling coatings, or excessive eccentricity.

　　Causes of Inclusions:

　　Types of inclusions in welds:

　　1. Oxides. When welding steel materials, oxide inclusions are common. In manual arc welding and submerged arc automatic welding of low carbon steel, the main component of oxide inclusions is SiO2; these oxide inclusions are mainly produced during the weld pool reaction process.

　　2. Nitrides.

　　3. Metallic Inclusions

　　4. Slag inclusions. The inclusions mentioned above are precipitates or reaction products, all of which are microscopic inclusions. There is also another type, usually called slag inclusion, which is directly mixed in due to improper process, such as when using manual arc welding, improper stick manipulation can cause slag inclusions at the groove edge, often occurring in horizontal welding.

　　5. Tungsten inclusions, formed due to the burn-off of the tungsten electrode in tungsten inert gas welding, or the tungsten electrode touching the weld pool or wire shedding and melting into the weld.

　　Measures to prevent slag inclusions in welds:

　　The most important measure to prevent slag inclusions in welds is the correct selection of electrode and flux types. Secondly, attention should be paid to process operations.

　　1. Select appropriate welding parameters to ensure the weld pool does not solidify too quickly and remains molten for a sufficient time.

　　2. When multi-layer welding, pay attention to removing slag from the previous weld layer.

　　3. The electrode should be appropriately oscillated, and the electrode angle and manipulation method adjusted at any time to separate molten iron from slag, facilitating slag floatation.

　　4. During operation, pay attention to protecting the weld pool to prevent air intrusion into the weld pool.

　　5. The welding speed should not be too fast, ensuring sufficient time for slag in the weld pool to float up.

　　6. The welding current should not be too low.

　　Image description of slag inclusion defects on film:

　　Slag inclusions appear as irregular black spots, black lines, or black blocks on the film, with irregular darkness variation and unsmooth contours, some with sharp edges. Tungsten inclusions appear as white spots on the film.

　　Concept and hazards of lack of fusion

　　Lack of fusion refers to a defect where the weld metal and base metal, or weld metals within the weld, are not melted and joined together. Lack of fusion can be divided into lack of side wall fusion, lack of root fusion, and interpass lack of fusion. Lack of fusion is an area-type defect; lack of side wall fusion and lack of root fusion significantly reduce the bearing area, leading to severe stress concentration and great harm.

　　Causes of lack of fusion:

　　1. Welding current too low, welding speed too fast.

　　2. Incorrect electrode angle.

　　3. Arc blow occurs.

　　4. Welding in a downhill position, where the base metal is covered by molten iron before it melts.

　　5. Dirt or oxides on the base metal surface affect the fusion bonding between the deposited metal and the base metal.

　　Measures to prevent lack of fusion:

　　1. Increase welding heat input.

　　2. Ensure full penetration of the groove edges.

　　3. Correct welding parameters, proper operation, uniform welding speed, and adequate electrode oscillation.

　　4. Clean the dirt on the groove surface or the bottom edge of the groove.

　　Image description of lack of fusion on film:

　　Typical images of lack of side wall fusion are continuous or intermittent black lines, with varying widths and uneven darkness. One side of the contour is relatively neat and darker, while the other side is irregular and lighter. Its position on the film is generally at 1/2 of the distance from the weld center to the edge, extending longitudinally along the weld.

　　Typical images of interpass lack of fusion are blocky shadows with moderate darkness and irregular shapes. If accompanied by slag inclusions, the areas with slag inclusions will be darker.

　　Concept and hazards of incomplete penetration:

　　Incomplete penetration refers to a defect where the base metal does not melt, and the weld metal does not reach the root of the joint. Incomplete penetration is divided into double-sided incomplete penetration and single-sided incomplete penetration. Incomplete penetration is a relatively dangerous defect, and its harmfulness depends on the shape, depth, and length of the defect.

　　Causes of incomplete penetration:

　　1. Welding current too low, welding speed too fast.

　　2. Groove angle too small.

　　3. Root face too thick.

　　4. Groove gap too small.

　　5. Improper electrode angle.

　　6. Arc too long.

　　Image description of incomplete penetration on film:

　　The typical image of incomplete penetration is a thin, straight black line, with both contours neat and representing the trace of the root face, and the width exactly matching the root gap width. Sometimes, if part of the root face melts, the image contour becomes less neat, and the width and darkness locally change, but as long as it can be determined to be a linear defect at the weld root, it should be judged as incomplete penetration. Incomplete penetration is located at the projection position of the weld root on the film, generally in the middle of the weld, but may also be shifted to one side due to deviation in radiography or welding. Incomplete penetration appears as discontinuous or continuous distribution, sometimes spanning the entire film.

　　Measures to prevent incomplete penetration:

　　1. Reasonably select the groove type. 2. Ensure appropriate assembly gap. 3. Adopt correct welding process.