I. Overview of Valve Welding Defects
Valves play a crucial role in industrial production, and the welding quality of valves directly affects their performance and reliability. During the welding process, various defects may occur, such as pores, slag inclusions, cracks, incomplete fusion, and incomplete penetration. These defects not only reduce the strength and sealing performance of the weld seam but also may cause leakage or damage to the valve during use, thereby affecting the normal operation of the entire production system. Therefore, it is crucial to promptly and effectively address valve welding defects.
II. Common Valve Welding Defects and Treatment Methods
(1) Pores
1. Defect Characteristics
Pores are air holes formed when gases in the molten pool fail to escape during solidification during the welding process. Pores can be spherical, elliptical, or irregular in shape, and vary in size. Surface pores are generally easier to detect, while internal pores can only be detected through non-destructive testing methods.
2. Causes
Welding materials getting wet, such as when welding rods or wires get wet, can cause gases to decompose and form pores during welding.
Improper welding process parameters, such as excessive welding current or too fast welding speed, or too high or low arc voltage, can cause gases to not have enough time to escape and form pores.
The weld surface having oil stains, rust, etc., can produce gases during welding, increasing the possibility of pore formation.
The flow rate of the shielding gas is inappropriate, such as in shielded welding, where the flow rate of the shielding gas is too small and cannot effectively protect the molten pool, allowing air to enter the molten pool and form pores; an excessive flow rate can also cause turbulence, which is not conducive to gas expulsion.
3. Treatment Methods
(1) Surface Pore Treatment
For surface pores, the surrounding weld metal can be ground off using a grinding wheel or steel wire brush to expose fresh metal. Then, manual arc welding or gas shielded welding can be used for repair. When welding, attention should be paid to controlling the welding process parameters, with the welding current being slightly larger than normal to ensure that the gas in the pores can escape.
(2) Internal Pore Treatment
If the pores are located within the weld seam and are numerous or large in size, generally, non-destructive testing should be conducted on the weld seam to determine the location and size of the pores. Then, carbon arc air cutting or mechanical processing can be used to remove the weld metal at the pore location until the non-porous metal is exposed. Then, repair welding can be carried out. The repair welding process is similar to surface pore treatment, but the welding quality should be strictly controlled to prevent the recurrence of pores.
4. Repair Materials
Generally, welding materials of the same or similar material as the valve base material should be selected. For example, for common carbon steel valves, E4303 (J422) welding rods can be used; for stainless steel valves, corresponding stainless steel welding rods should be selected based on the specific material, such as A102 welding rods for 304 stainless steel valves and A022 welding rods for 316L stainless steel valves, etc.
(2) Slag Inclusions
1. Defect Characteristics
Slag inclusions refer to the residual slag in the weld seam after welding. Slag inclusions can be in the form of blocks, strips, or dispersed throughout the weld seam. Their presence reduces the strength and toughness of the weld seam and affects its compactness.
2. Causes
Insufficient welding current, resulting in a low temperature of the molten pool, prevents the slag and liquid metal from fully separating, causing the slag to remain in the weld seam.
Excessive welding speed, causing the slag to not have enough time to float out of the molten pool and be solidified in the weld seam.
Too small bevel angles or narrow gaps, which are not conducive to slag expulsion.
In multi-layer welding, incomplete cleaning of the layer interface, where the slag from the previous layer is not cleaned before proceeding to the next layer of welding, leads to the accumulation of slag between layers.
3. Treatment Methods
For surface slag inclusions, they can be removed using a grinding wheel or steel wire brush, and then repaired. When performing welding repair, it is important to clean the bevel thoroughly and adjust the welding process parameters to ensure a good fusion between the weld metal and the base material.
The treatment of internal slag is relatively complex. If the slag is shallow, carbon arc gouging can be used to remove the slag area, followed by welding. If the slag is deep, methods such as drilling may be needed to remove the slag, and then welding can be carried out. During the welding repair process, attention should be paid to controlling the welding heat input to avoid deterioration of the weld structure due to overheating.
4. Welding Repair Materials
The selection of welding repair materials is similar to that of gas hole treatment, and it should be determined based on the material of the valve base. For carbon steel valves, E4315 (J427) welding rods can be used for repair welding, and for stainless steel valves, appropriate stainless steel welding rods should be selected according to the material.
(III) Cracks
1. Defect Characteristics
Cracks are one of the most serious defects in valve welding and have great hazards. Cracks can be classified as hot cracks, cold cracks, and reheat cracks, etc. Hot cracks usually occur during the solidification of the weld seam and are distributed along the grain boundaries, presenting a sawtooth shape; cold cracks usually appear after welding at a lower temperature and have a delayed nature, possibly penetrating the weld seam or extending to the base material; reheat cracks occur during post-weld heat treatment or high-temperature use.
2. Causes
(1) Causes of Hot Cracks
The content of low-melting-point impurities in the weld metal, such as sulfur and phosphorus, forms low-melting-point eutectics during the solidification process, reducing the crack resistance of the weld metal.
Excessive welding stress, including thermal stress generated during the welding process and structural restraint stress. When the stress exceeds the strength limit of the weld metal, hot cracks are prone to occur.
(2) Causes of Cold Cracks
The quenching tendency of the base material is high, such as some high-strength alloy steels. When the cooling speed is too fast after welding, a quenched structure is formed, increasing the sensitivity of cold cracking.
Excessive hydrogen content in the welding joint. Hydrogen enters the weld metal during welding and accumulates in the stress concentration area during cooling. When the hydrogen concentration reaches a certain level, cold cracking is induced.
Improper welding process, such as excessive welding current, insufficient preheating temperature, and excessive post-weld cooling speed.
(3) Causes of Reheat Cracks
It is mainly related to the chemical composition of the steel and the microstructure of the welding joint. Some steels containing chromium, molybdenum, vanadium, etc., are prone to reheat cracks after welding or during high-temperature use.
3. Treatment Methods
(1) Treatment of Hot Cracks
First, determine the depth and length of the crack. For shallow hot cracks, the crack can be ground off using a grinding wheel until the crack-free metal is exposed, and then welding can be carried out. During welding repair, control the welding process parameters well, reduce welding stress, and appropriate methods such as small current, multi-layer multi-pass welding can be adopted. For deep hot cracks, the crack may need to be removed and then welded, and the removed area should be preheated before welding, and post-weld cooling treatment should be carried out promptly.
(2) Treatment of Cold Cracks
The treatment of cold cracks is more complex because it has a delayed nature. Once a cold crack is detected, it is necessary to thoroughly inspect and analyze the crack, determining its direction and depth. For shallow cold cracks, the crack can be removed and then welded, but strict preheating must be carried out before welding, the preheating temperature is generally determined based on the material and thickness of the base material, usually between 150°C - 350°C. For deep cold cracks, methods such as drilling for crack arrest, removing the crack and then welding may be used, and during the welding process, the welding heat input and interlayer temperature should be strictly controlled, and post-weld heat treatment should be carried out promptly to eliminate welding residual stress and prevent the recurrence of cracks.
(3) Treatment of Reheat Cracks
For reheat cracks, efforts should be made to avoid welding and heat treatment under conditions prone to reheat cracks. If reheat cracks have already occurred, generally, the method of removing the cracks and then re-welding is required. The welding process should be optimized according to the characteristics of the steel, such as controlling the welding heat input and selecting appropriate welding materials. During post-weld heat treatment, the heating speed, holding time and cooling rate should be reasonably controlled to reduce the risk of reheat cracks.
4. Welding repair materials
When repairing heat cracks, welding materials with good crack resistance can be selected. For example, for some low-carbon steel and low-alloy steel valves, alkaline electrodes such as E5015 (J507) can be used. For cold cracks, in addition to considering the matching of the base material, low-hydrogen welding materials should also be selected. For reheat cracks, appropriate welding materials should be selected based on the chemical composition of the steel and the reheat crack sensitivity. Special alloy elements such as niobium and titanium may be needed to improve the crack resistance of the weld seam.
(4) Lack of fusion and incomplete penetration
1. Defect characteristics
Lack of fusion refers to the phenomenon where the weld metal does not fully melt and combine with the base metal or between the layers of the weld metal. Incomplete penetration refers to the phenomenon where the root of the welding joint is not fully melted. Both lack of fusion and incomplete penetration will reduce the effective cross-sectional area of the weld, and decrease the strength and sealing performance of the weld.
2. Causes
(1) Causes of lack of fusion
If the welding current is too small or the welding speed is too fast, there will be insufficient heat, and the base metal and filler metal cannot be fully melted.
If the bevel angle is too small, the gap is too narrow or the fillet is too large, the arc cannot penetrate to the root of the bevel, resulting in the lack of fusion between the base metal and the weld metal.
If the surface of the weld piece is contaminated with oil, rust, etc., it will affect the melting and fusion of the metal.
Improper operation, such as incorrect electrode angle or improper welding technique, causes the arc to deviate from the edge of the bevel or fails to fully cover the bevel.
(2) Causes of incomplete penetration
Similar to lack of fusion, causes include insufficient welding current, too fast welding speed, inappropriate bevel size, etc.
If the arc is too long during welding, the heat is dispersed, resulting in poor melting of the root metal.
Uneven assembly gaps in the weld piece can easily lead to incomplete penetration in the areas with larger gaps.
3. Treatment methods
(1) Treatment of lack of fusion
For surface lack of fusion, the un-fused area can be ground off using a grinding wheel, and then re-welded. During re-welding, the welding process parameters should be adjusted to ensure sufficient heat input, allowing the base metal and filler metal to be fully melted. For internal lack of fusion, it is generally necessary to use non-destructive testing methods to determine the location and extent of the lack of fusion, and then use carbon arc air cutting or mechanical processing to remove the lack of fusion area, and then re-weld. During re-welding, the groove should be cleaned, and the welding angle and welding technique should be controlled properly.
(2) Treatment of incomplete penetration
If the depth of incomplete penetration is shallow, it can be removed using a grinding wheel, and then re-welded. For deeper incomplete penetration, carbon arc air cutting or mechanical processing should be used to completely remove the incomplete penetration area until good metal is exposed, and then re-weld. During re-welding, the welding current, voltage and welding speed should be strictly controlled to ensure that the root can be fully melted.
4. Welding repair materials
The selection of welding repair materials is the same as described earlier, and it should be determined based on the base material of the valve. For example, carbon steel valves can use suitable carbon steel electrodes or wires, and stainless steel valves can use corresponding stainless steel welding materials.
III. Precautions for Valve Welding Defect Treatment
1. Accuracy of defect detection and evaluation
Before treating welding defects, accurate detection and evaluation must be carried out. Appropriate non-destructive testing methods, such as radiographic testing, ultrasonic testing, magnetic particle testing, etc., should be used to comprehensively detect possible defects in the weld seam and determine information such as the type, size, location and quantity of the defects. Only by accurately understanding the defect situation can a reasonable treatment plan be formulated.
2. Strict implementation of welding procedures
Whether it is re-welding or supplementary welding, it must be carried out in accordance with the established welding procedure regulations. The welding process parameters (such as welding current, voltage, welding speed, preheating temperature, interlayer temperature, post-heating temperature, etc.) must be strictly controlled within the specified range. At the same time, attention should be paid to the standardization of welding operations to ensure the stability and reliability of the welding quality.
3. Quality control of welding materials
The selected welding materials must comply with national standards and design requirements and have quality certificates. Before use, the welding materials should be inspected, such as checking whether the coating of the welding rods is intact, whether the welding wires are rusted, etc. For damp welding materials, they should be dried before use to ensure the quality of the weld metal during the welding process.
4. Consideration of environmental factors
The welding operation environment also has an impact on welding quality. It is advisable to carry out welding in a dry and windless environment. If the environmental temperature is too low or the humidity is too high, corresponding protective measures such as preheating and moisture-proofing may be required. At the same time, attention should be paid to keeping the welding site clean to avoid the influence of dust, debris, etc. on the welding quality.
5. Subsequent inspection and acceptance
After the welding defect treatment is completed, the weld seam should be re-inspected to ensure that the defect has been completely eliminated and the weld quality meets the requirements. The inspection methods can be a combination of non-destructive testing and visual inspection. Only valves that have passed the inspection can be put into use to ensure the safety and reliability of the valves during use.
The treatment of welding defects in valves is a highly technical task that requires the selection of appropriate treatment methods and materials based on the specific defect type and situation, and strict adherence to relevant process requirements and precautions. Only in this way can the welding quality of valves be ensured and the smooth progress of industrial production be guaranteed.
