Avoid welding defects with expert weld quality inspection

With the development of science and technology, welding has become more important in industrial production. From the analysis of a large number of structural accidents, it is clear that many of them are caused by poor welding quality, and the sense of responsibility and operational ability of welders directly affect the quality of welding.

To improve the quality of welders and ensure the safety and reliability of welded structures, it is necessary to provide training and evaluation to welders.

Section 1: Welding Defects

Welding defects: Defects in welding joints that do not meet the requirements of the design or process documents.

1. Classification of welding defects:

According to the location of welding defects in the weld, they can be divided into two categories: external defects and internal defects. External defects are located on the outer surface of the weld and can be observed with the naked eye or with a low-power magnifying glass.

Examples include improper weld size, undercut, weld bead, arc pit, burning, sagging, surface porosity, surface cracks, etc. Internal defects are located within the weld and require destructive testing or non-destructive testing methods to detect. Examples include incomplete penetration, lack of fusion, slag inclusion, internal porosity, internal cracks, etc.

2. Common defects of electric welding:

(1) Inadequate weld size:

Refers to irregular width and height, inadequate or excessive weld size. Too small a weld size will reduce the strength of the welded joint, while too large a size will increase stress and deformation in the structure, leading to stress concentration and increased welding workload.

Inadequate weld preparation angle or uneven mounting clearance, excessive or insufficient welding current, incorrect travel speed or angle may result in weld size nonconformity.

(2) Undercut:

Refers to the groove or depression formed along the weld tip due to improper welding parameters or incorrect operating procedures.

Undercutting reduces the effective cross-sectional area of ​​the base metal, weakens the strength of the welded joint, and can cause stress concentration and crack formation in the undercut, even leading to structural failure. The undercut that exceeds the allowable value must be repaired by additional welding.

(3) Weld bead:

Refers to the metal bead formed on the unmelted base metal outside the weld during the welding process. The weld bead not only affects the appearance of the weld, but also often hides incomplete fusion defects underneath, leading to stress concentration.

In the case of pipe joints, welding seams inside the pipe can reduce the effective area and even cause clogging.

Weld seams frequently occur in flat welding and horizontal welding. Excessive gap between welds, incorrect electrode angle and travel method, poor electrode quality, excessive welding current or welding speed that is too slow can cause weld bead formation.

(4) Burn:

Refers to the defect where molten metal flows through the back of the groove during the welding process, forming a hole. Burning usually occurs during root pass welding. Burning makes it difficult to continue the welding process and is an unacceptable welding defect.

The main causes of burnout are excessive welding current or too low welding speed, excessive grooves and gaps, or inadequate edge preparation.

To avoid burns, it is necessary to properly design the dimensions of the groove, ensure the quality of assembly and select the appropriate parameters of the welding process. For one-sided welding, methods such as using copper backing plates or flux can be employed to avoid burns. When manual arc welding of thin plates, jump welding or intermittent arc welding techniques can be used.

(5) Incomplete penetration:

It refers to the phenomenon where the joint root does not melt completely during welding. Incomplete penetration generally occurs at the root of one-sided welding and in the middle of two-sided welding.

Incomplete penetration not only reduces the mechanical properties of the welded joint, but also creates stress concentration points in incomplete penetration, leading to the formation of cracks.

The causes of incomplete penetration include insufficient welding current, excessive welding speed, improper electrode angle, arc blowing, insufficient groove angle or gap, rapid heat dissipation of the workpiece, hindered oxidation and slag, etc.

Any factor that prevents sufficient fusion between the electrode metal and the base metal can cause incomplete penetration.

Measures to prevent incomplete penetration include:

1. Proper selection of groove shape and assembly clearance and removal of dirt and slag between groove sides and weld layers.
2. Selection of appropriate welding current and speed.
3. During travel, constant attention must be paid to adjusting the electrode angle, especially when encountering arc blow or electrode eccentricity, to ensure sufficient fusion between the weld metal and the base metal.
4. For workpieces with high thermal conductivity and large heat dissipation area, preheating should be applied before welding or heating during the welding process.

(6) Lack of fusion:

Lack of fusion refers to the portion where the weld metal and the base metal or between the weld metals are not fully fused and melted during welding. Lack of fusion presents similar risks to incomplete penetration. Causes of lack of fusion include low welding heat input, arc blowing, rust and dirt on channel side walls, incomplete slag removal between weld layers, etc.

(7) Craters, sags and lack of weld metal:

Craters refer to local depressions formed on the surface or behind the weld, below the surface of the base metal. Sinking occurs when excess molten metal penetrates the back of the weld, causing the front of the weld to sink and the back to protrude. Weld metal starvation refers to the continuous or intermittent groove formed on the weld surface due to insufficient filler metal.

These defects weaken the effective cross-sectional area of ​​the weld, leading to stress concentration and a severe reduction in weld strength. Sinking often occurs in flat welding and horizontal welding, especially in pipe welding, where such defects are likely to occur due to sagging of the molten metal. In argon arc welding, attention should be paid to making the electrode remain in the weld pool for a short time during arc termination or use circular displacement to avoid craters at arc termination.

(8) Tungsten inclusion:

Causes:

1. Improper welding operation causes the tungsten electrode to come into contact with the workpiece and melt into the weld metal.
2. Using a small diameter tungsten electrode with high welding current.
3. The filler wire touches the tip of the tungsten electrode.
4. Excessive burning and overheating of the tungsten electrode.
5. Poor gas protection or severe oxidation of the tungsten electrode.

Preventive measures:

1. Use high-frequency and high-voltage arc ignition to avoid contact arc ignition.
2. Select the appropriate tungsten electrode diameter according to the required welding current. ⑶ Strengthen operational skill training and avoid contact between the filling wire and the tungsten electrode.
3. Immediately sand and replace the tungsten electrode if it becomes cracked or severely burned.
4. Ensure the appropriate protrusion length of the tungsten electrode, increase the gas flow rate and increase the post-flow time to prevent tungsten oxidation.

(9) Porosity:

• Porosity formation and hazards:

During welding, bubbles in the weld pool that cannot escape during solidification and remain behind form voids called porosity. Porosity can be classified as dense porosity and pinhole porosity. The main gas that forms porosity in the weld is hydrogen. Hydrogen in the welding area can come from several sources, including moisture in the arc column atmosphere, moisture adsorbed on the welding material, and oxide film on the surface of the base metal.

These moisture sources form bubbles in the weld pool under the high arc temperature, but are unable to rise and form porosity. Porosity has a significant impact on weld performance. Not only does it reduce the effective working cross-section of the weld and weaken its mechanical properties, but it also compromises the density of the weld, making it prone to leakage. Porosity edges can cause stress concentration, reducing the plasticity of the weld.

Therefore, strict control of porosity is essential for critical welding.

• Causes of porosity:
• Low purity of argon gas, excessive impurities or moisture in the argon pipeline, and gas leakage in the pipeline.
• Inadequate cleaning of the welding wire or base metal near the groove before welding, or recontamination with dirt and moisture after cleaning.
• Poor protection of argon gas during argon arc welding, unstable arc, excessively long arc length, excessive tungsten electrode protrusion.
• Inadequate selection of welding parameters, welding speed too fast or too slow.
• High humidity in the surrounding environment and high wind speed.

Preventive measures:

1. Ensure the purity of the shielding gas.
2. Properly clean the welding wire and base metal near the groove.
3. Choose the correct welding parameters.
4. Preheat before welding if necessary.
5. Avoid working in humid environments and implement wind protection measures.

(10) Cracks:

Cracks are gaps formed by the destruction of atomic bonding strength in localized areas of the metal in the welded joint under welding stresses and other embrittlement factors. Cracks in welded joints, especially thermal cracks in welding aluminum and aluminum alloys, are the most dangerous welding defects.

They have a severe impact on the performance, usability and safety of welded structures and are the main cause of many structural welding failures.

Causes of cracks:

1. Improper welding wire selection: When the Mg content in the weld is less than 3% or when the content of Fe and Si impurities exceeds the specified limit, the tendency of cracking increases. When the welding temperature is too high, settlement cracks occur in the heat-affected zone.
2. Inadequate selection of the welding sequence.
3. If the heat source is removed too quickly during the termination or interruption of welding, or if the crater is not filled properly, cracks in the crater are likely.
4. Concentration of welds or excessive heat in the heat-affected zone results in excessive strain stress.
5. Excessive impurities in solvents and welding wire shielding gas.
6. Irrational structural design with excessive concentration of welds, leading to excessive restraint stresses in the welded joint.

Preventive measures:

1. Proper selection of welding wire to ensure a good match between the weld composition and the base metal composition.
2. Select a reasonable welding sequence.
3. When welding is terminated or stopped, reduce the arc current, slightly extend the arc termination time, and fill the arc termination area with filler wire or install a crater filling device at the end of the weld to terminate the arc .
4. Control the temperature and deformation in the heat-affected zone and implement preheating measures if necessary.
5. Reduce the rigidity of the welding structure and avoid stress concentration in the weld as much as possible.

Section 2: Inspection of Welding Defects

The importance of welding inspection:

Welding inspection is an important measure to ensure excellent product quality and prevent scrap from leaving the factory. During the trial production process, inspection can identify quality problems, identify causes, and eliminate defects. This ensures the application of new products or processes and guarantees quality.

1. Non-Destructive Testing

Non-destructive testing refers to the method of detecting defects without damaging the tested material or the performance and integrity of the finished product. Includes visual inspection, tightness inspection and non-destructive testing.

1.1 Visual Inspection

Visual inspection of welded joints is a simple and widely used method. It is usually performed with the naked eye or with a 5 to 10x magnifying glass. The main objective is to check defects such as cracks, porosity, undercuts, weld bead, burns and craters on the weld surface.

It also examines the quality of weld formation, whether the reinforcement height meets standard requirements, and the smooth transition of the weld to the base metal.