Weld cracks are critical defects that demand immediate repair or rework to ensure structural integrity and safety.
When a weld develops a crack, the joint’s load-bearing capacity is significantly compromised, often leading to failure under stress. Recognizing and addressing weld cracking is essential for maintaining the durability of welded components. While weld cracks can appear alarming, a comprehensive understanding of their formation mechanisms and preventive measures can greatly reduce their occurrence.
Why Do Welds Crack?
Weld cracks occur when internal stresses within the joint surpass the strength limits of either the base metal or the filler material. These cracks are generally categorized into two main types: hot cracks and cold cracks. Example of an under-bead crack
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Various factors contribute to weld cracking, including hydrogen embrittlement, improper welding procedures, unfavorable weld geometry, stress concentration points, low ductility of the materials, and inadequate post-weld heat treatment. Understanding these factors is vital for implementing effective prevention techniques. Later sections will delve into the specifics of hot and cold cracking, their causative factors, and mitigation strategies. It’s important to note that weld cracks are classified as weld discontinuities and are considered defect types within weld quality standards. For a detailed classification, refer to our comprehensive guide on weld defects.
What Causes Internal Stresses in Welded Joints?
Internal stresses in welds primarily stem from thermal cycles during welding. Two dominant sources are:
- External Loads: Mechanical forces applied during service, such as tension, compression, or cyclic stresses.
- Residual Welding Stresses: Internal stresses that develop due to uneven heating and cooling during welding, leading to expansion and contraction of the metal.
While external loads are straightforward, residual stresses are more insidious, forming within the heat-affected zone (HAZ) and the weld metal itself as a result of thermal contraction upon cooling. These residual stresses generate tensile forces that, if exceeding the material’s strength, cause cracks. Residual stress development due to thermal contraction
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Cracks are essentially the weld’s way of relieving these internal tensile stresses. However, the presence of additional applied loads can act as triggers, leading to crack initiation and propagation. These combined stress factors necessitate careful control of welding parameters and post-weld procedures to ensure joint integrity.
Hot Cracks – Causes, Types, and Characteristics
Hot cracking, also known as solidification cracking, occurs during or immediately after welding at temperatures exceeding 1000°F (538°C). The key causes include stress concentration, low melting point constituents like sulfur, rapid cooling, and the inherently low ductility of the material. These factors weaken the grain boundaries, making them susceptible to rupture during solidification.
Common manifestations of hot cracks include longitudinal cracks, lamellar tears, and crater cracks. © weldguru.com – Image rights apply
Hot cracks typically develop along partially fused grain boundaries as the weld metal solidifies and contracts. The shrinkage creates tensile stresses, which fracture these weak grain boundaries. The resulting cracks can be longitudinal, transverse, or occur in other configurations depending on the welding conditions.
Types of Hot Cracks
- Longitudinal Cracks: These run parallel to the weld seam, often appearing during or immediately after welding. They are common in welds with high concavity or in steels with high sulfur and low manganese contents.
- Lamellar Tears: These occur in the parent metal underneath the weld, especially in rolled steel plates with high sulfur content or low ductility. Proper steel selection with a low sulfur level and adequate STRA values can mitigate this risk.
- Crater Cracks: Formed at the end of a weld bead due to insufficient filler metal deposition, leading to residual stresses that cause cracking during solidification. Proper filling of the crater can prevent these defects.
Cold Cracks – Causes, Types, and Characteristics
Cold cracking develops post-welding, often manifesting hours to weeks after the process. It is predominantly caused by hydrogen-induced embrittlement, where hydrogen atoms diffuse into the metal, creating internal stresses that lead to cracks. Hydrogen sources include oils, greases, dirt, rust, coatings, cleaning agents, and even the filler metal itself. © weldguru.com – Image rights apply
Unlike hot cracks, cold cracks are more challenging to detect early because they can remain hidden within the weld or heat-affected zone (HAZ). They typically appear as root, toe, transverse, or fusion-line cracks and are associated with high-strength steels more susceptible to hydrogen embrittlement.
Essentially, hydrogen dissolves in the weld metal during welding, then diffuses into the HAZ. Over time, hydrogen atoms recombine into molecules, causing microstructural stresses that result in cracking if internal tensile stresses are present. The critical prerequisites include high residual stresses, microstructures sensitive to hydrogen, and ample hydrogen presence.
Types of Cold Cracks
- Toe Cracks: Cracks that initiate at the weld toe, where hydrogen tends to accumulate, especially under high stress concentration zones.
- Root and Underbead Cracks: These occur along the fusion line due to hydrogen embrittlement, often linked to joint design and welding conditions.
- Transverse Cracks: Crosswise cracks, particularly in high-strength steels, visible even under low magnification, indicating severe internal stresses.
- Fusion-Line Cracks: Cracks that run parallel to the fusion zone, usually forming in microstructurally vulnerable regions.
11 Proven Strategies for Crack Prevention
Preventing weld cracks requires a comprehensive approach that addresses both hot and cold cracking mechanisms. The following strategies are proven effective in minimizing or eliminating crack formation:
1. Select the Appropriate Filler Material
Choosing the right filler metal is critical. For high-strength steels or to mitigate hydrogen-induced cracking, opt for low-hydrogen electrodes like E7018. Ensure all filler materials, rods, and wires are thoroughly dried and free from moisture to prevent hydrogen entrapment. Matching filler composition with base metals ensures compatibility and reduces residual stresses.
2. Preheat the Metal Before Welding
Preheating steel reduces thermal gradients, thereby decreasing residual stresses and enabling hydrogen to escape more effectively. Different steel grades require specific preheat temperatures, so always consult manufacturer specifications. Proper preheating improves ductility and minimizes crack susceptibility.
3. Control the Cooling Rate
Slowing the cooling process allows hydrogen to diffuse out of the weld and HAZ. Use heat-retaining blankets, oven heating, or induction heating to maintain controlled cooling, especially when welding high-strength steels or thick sections.
4. Avoid Welding with High Sulfur Content Steel
High sulfur levels reduce weld ductility and increase hot cracking risk. Use steel with low sulfur content and specify steel grades with low sulfur and phosphorus levels. This minimizes the formation of liquid films at grain boundaries during solidification, reducing the likelihood of hot cracking.
5. Optimize Weld Bead Geometry
Refrain from creating excessively concave or convex weld beads. Concave beads lack sufficient filler material to withstand stresses, increasing cracking risk. Conversely, overly convex beads create high reinforcement, leading to internal stresses during solidification. Aim for a balanced weld profile with smooth transitions at the toes to reduce stress concentrations.
6. Use High-Quality Materials
Employ top-grade filler and base materials. Avoid using materials with inconsistent grain structures, improper chemical compositions, or those stored improperly, which can introduce contaminants or moisture. Proper storage and handling prevent issues like rust, dirt, or moisture absorption that contribute to cracking.
7. Maintain Appropriate Welding Speed
Welding at excessively high travel speeds results in insufficient filler metal deposition, leading to weak, concave welds. Moderate speeds ensure complete fusion, proper weld reinforcement, and reduce internal stresses that cause cracking.
8. Use Suitable Shielding Gases
When welding ferrous metals such as carbon steels, avoid hydrogen-rich shielding gases like pure hydrogen. Instead, opt for inert gases like argon or helium mixtures to prevent hydrogen absorption. Proper shielding reduces hydrogen entrapment, lowering cold cracking risks.
9. Maintain Proper Weld Width to Depth Ratios
A ratio between 0.5 and 0.8 is recommended to minimize stress concentration and hot cracking. Deviations from this range can cause uneven solidification and promote the pooling of low melting point elements like sulfur in the weld centerline, increasing crack susceptibility.
10. Finish the Weld Properly
Ensure complete crater filling at the end of each weld bead. Insufficient filler metal at the weld terminus can result in crater cracks. Proper finishing prevents residual stress buildup and crack initiation at these critical points.
11. Accommodate Expansion and Contraction
Allow room for the metal to expand during heating and contract during cooling. Avoid restraining the welded parts excessively, as that can amplify internal stresses. Use appropriate fixturing and post-weld cooling procedures to promote natural movement and reduce cracking risks.
In Conclusion
When weld cracks occur, prompt repair is necessary to restore the joint’s integrity. However, preventive measures are far more effective and economical. Proper material selection, meticulous cleaning, controlled preheating, and appropriate welding techniques significantly reduce crack formation. Whether you’re a professional welder or a hobbyist, understanding these principles ensures safer, stronger welds and minimizes costly rework.