CLADDING TECHNOLOGY SHANXI CO., LTDCrater Cracks and Shrinkage Cavities in TIG Overlay Welding Acceptance
Literature Overview and Context
Crater cracks and shrinkage cavities at the termination of a weld overlay bead represent a critical quality concern in TIG cladding operations. These defects form during the final stage of solidification when the molten pool contracts and the last solidifying region becomes vulnerable to cracking due to high restraint, compositional segregation, and thermal gradients. In overlay welding, where the weld metal is often a dissimilar alloy deposited onto a base metal substrate, the susceptibility to crater defects is amplified by the additional thermal and metallurgical complexity of the joint.
Defect Mechanism and Classification
Crater cracks are classified as hot cracks that form during the final solidification of the weld pool. The mechanism involves the following sequence:
- The welding arc is terminated, and the arc crater begins to solidify
- The center of the crater solidifies last, creating a region of high thermal gradient
- Compositional segregation concentrates low-melting-point phases (sulfides, phosphides) at the solidification front
- Thermal contraction stresses exceed the solidification strength of the segregated region
- A crack initiates and propagates along the grain boundary or interdendritic region
Shrinkage cavities, while morphologically distinct, share a common origin in the final solidification stage. When the feeding of molten metal from the surrounding pool is insufficient to compensate for solidification shrinkage, a void or cavity forms at the crater center.
Defect Characteristics Comparison
| Feature | Crater Crack | Shrinkage Cavity |
|---|---|---|
| Location | Crater surface, often visible | Crater center, may be subsurface |
| Morphology | Linear, branching | Rounded, irregular |
| Detection Method | Visual, PT, MT | UT, RT, PT |
| Root Cause | Thermal stress + segregation | Insufficient feeding |
| Severity | Always reject | Depends on size and depth |
| Repair Difficulty | Moderate (grind + re-weld) | Difficult (requires full penetration) |
Acceptance Criteria and Standards Requirements
The acceptance criteria for crater cracks are unequivocally strict: any visible crater crack is classified as a rejectable defect. This is explicitly stated in ASME Section IX, which requires that welds be free of surface-breaking cracks at all locations, including the crater. Similarly, NB/T 47013.1 and GB/T 3323 mandate that crater cracks be rejected regardless of length or depth.
| Standard | Crater Crack | Shrinkage Cavity | Acceptance Basis |
|---|---|---|---|
| ASME IX | Any visible crack = reject | > 1.5 mm = reject | QW-452 |
| NB/T 47013.1 | Any visible crack = reject | > 2.0 mm = reject | Level II |
| GB/T 3323 | Any visible crack = reject | > 2.0 mm = reject | Level B |
| API 934 | Any visible crack = reject | > 1.5 mm = reject | Overlay spec |
For shrinkage cavities, the acceptance is governed by size and location. A cavity exceeding 1.5 mm in diameter at the crater surface is generally rejected, while smaller cavities may be acceptable if they do not penetrate beyond a specified depth into the overlay layer.
Process Control and Defect Prevention
The prevention of crater cracks and shrinkage cavities in TIG overlay welding requires careful attention to the termination sequence. The following procedural measures are essential:
- Crater fill technique: Before terminating the arc, reduce the current by 30–50% and maintain the arc for an additional 2–3 seconds to allow the crater to fill completely before solidification
- Post-heating: Apply a localized heat source (induction or resistance) immediately after arc termination to slow the cooling rate and reduce thermal stress
- Backing ring or filler: For critical applications, use a backing ring or add a small filler rod to the crater before arc termination to ensure complete filling
- Travel speed optimization: Avoid abrupt changes in travel speed near the end of the bead, as sudden deceleration can enlarge the pool and increase crater volume
A practical case from a hydrogenation reactor tube cladding project demonstrated the effectiveness of a systematic termination protocol. The original process produced crater cracks in approximately 12% of beads. After implementing a reduced-current termination sequence with a 2-second hold and immediate post-heat to 150°C, the crater crack rate was reduced to less than 1%, with the remaining defects being minor shrinkage cavities within acceptance limits.
Key Questions and Reflections
The study of crater defects raises an important question about the role of material chemistry in overlay welding. In nickel-based alloy overlays such as Inconel 625, the high solidification range and susceptibility to microsegregation make crater cracking particularly likely. The engineer must therefore consider not only the welding parameters but also the filler metal composition and its solidification behavior. Furthermore, the repair of crater cracks — grinding and re-welding — introduces additional heat input and potential dilution, which may compromise the corrosion resistance of the overlay layer. This creates a dilemma where defect prevention is always preferable to defect repair, and the investment in process optimization yields a higher return than post-weld inspection and repair.