Judgment of Bonding Interface Cracks in Explosive Welded Stainless Steel Clad Plate

Introduction and Significance

The bonding interface in explosive welded stainless steel clad plates is the most critical region for service performance, as it determines the integrity of the corrosion barrier and the mechanical continuity between the facing and backing layers. Cracks at the bonding interface are among the most severe defects, as they can lead to corrosion penetration into the base metal, catastrophic loss of cladding integrity, and ultimately pressure vessel failure. This study note addresses the methods, criteria, and technical considerations for the judgment of bonding interface cracks in explosive welded stainless steel clad plates, drawing from standards interpretation, metallurgical analysis, and engineering inspection experience.

Types and Morphology of Bonding Interface Cracks

Classification of Interface Cracks

Visual and NDT Characterization

The detection and characterization of bonding interface cracks require a combination of non-destructive testing methods:

1. Ultrasonic testing (UT): The primary method for interface crack detection. Frequency of 2.5–5 MHz is recommended, with dual-probe or pitch-catch arrangements to detect discontinuities parallel to the plate surface. According to JB/T 4730.3, the acceptance criteria for clad plates typically require no disbonding exceeding 10 mm in length or 5 mm in width.
2. Magnetic particle testing (MT): Effective for surface and near-surface cracks in the ferromagnetic base metal. The stainless steel clad layer is non-magnetic, so MT is performed on the base metal side after clad layer removal or on exposed edges.
3. Eddy current testing (ET): Useful for surface crack detection in the stainless steel clad layer, particularly for thin cladding layers (< 3 mm).
4. Radiographic testing (RT): Limited effectiveness for interface cracks due to the parallel orientation of the interface relative to the X-ray beam, but useful for volumetric defects.
5. Acoustic emission (AE): Can detect active crack propagation during loading but is not suitable for static inspection.

Judgment Criteria and Acceptance Standards

Standards-Based Acceptance Criteria

The judgment of bonding interface cracks must be performed in accordance with applicable standards:

• GB/T 150-2011: Requires that clad plates be free from cracks, delamination, and other defects that would impair the intended function.
• ASTM A263: Specifies that the clad plate shall be free from cracks, seams, or other defects in the clad layer, and the bonding shall be continuous.
• ASME VIII Div.1 UHA-51: Requires qualification testing including peel tests, bend tests, and intergranular corrosion tests to demonstrate adequate bonding.
• NB/T 47002.3: Provides additional requirements for clad plates used in pressure vessels.

Quantitative Judgment Parameters

Engineering Judgment Considerations

In practice, the judgment of bonding interface cracks involves not only quantitative measurements but also qualitative engineering assessment:

1. Crack orientation relative to service stresses: A crack oriented parallel to the principal tensile stress direction is more critical than one oriented perpendicular to it.
2. Crack location relative to the clad layer: A crack that penetrates through the entire clad layer is unacceptable, as it compromises the corrosion barrier function.
3. Crack propagation tendency: Cracks in regions of high residual stress or in susceptible microstructures (e.g., strain-induced martensite) have a higher tendency to propagate under service loading.
4. Repair feasibility: Even if a crack is detected, the decision to repair or reject depends on the feasibility of repair without compromising the clad layer integrity.

Root Cause Analysis of Interface Cracks

Metallurgical Causes

1. Oxide inclusions at the interface: During explosive welding, oxide films from both surfaces are compressed and fragmented at the bonding interface. These oxide inclusions (primarily Al2O3, Cr2O3, and Fe2O3) can act as crack initiation sites under stress.
2. Strain-induced martensite in the clad layer: In 304 stainless steel, severe plastic deformation during explosive welding can induce martensitic transformation in the wave zone. Martensite is harder and more brittle than austenite, making it susceptible to cracking under stress.
3. Microstructural heterogeneity: The wave zone exhibits a gradient of microstructural features from the heavily deformed interface to the undeformed bulk clad layer. This heterogeneity creates stress concentrations and preferential crack paths.
4. Intermetallic compound formation: In some cases, intermetallic compounds (e.g., FeCr, FeNi) may form at the interface during the explosive welding process or subsequent heat treatment, creating brittle phases that are prone to cracking.

Process-Related Causes

1. Insufficient welding velocity: If the collision velocity is too low, the jetting mechanism is incomplete, leading to poor bonding and the retention of oxide films at the interface.
2. Excessive collision angle: An improper collision angle can lead to unstable wave formation and localized regions of poor bonding.
3. Surface contamination: Oil, grease, or other contaminants on the plate surfaces before welding can be incorporated into the interface, weakening the bond and creating crack initiation sites.
4. Thermal effects during welding: Although explosive welding is a cold process, localized heating at the impact points can cause thermal stresses that contribute to cracking.

Environmental and Service-Related Causes

1. Hydrogen-induced cracking (HIC): In hydrogen-containing environments, hydrogen atoms can diffuse into the microstructure and accumulate at the interface, causing delayed cracking.
2. Sulfide stress cracking (SSC): In sour service (H2S-containing environments), the combination of hydrogen and sulfide ions can cause stress cracking in susceptible microstructures.
3. Corrosion fatigue: Cyclic loading combined with corrosion can initiate and propagate cracks at the interface, particularly in regions of high residual stress.

Inspection Methodology and Technical Procedures

Recommended Inspection Protocol

Based on engineering practice, the following inspection protocol is recommended for bonding interface crack detection in explosive welded clad plates:

1. Visual inspection (VT): Inspect the clad surface and edges for visible cracks, delamination, or wave distortion. Use magnification (5–10×) for detailed examination.
2. Magnetic particle testing (MT): Apply MT to the base metal side after clad layer removal or on exposed edges. Use wet or dry methods with appropriate magnetic field strength.
3. Ultrasonic testing (UT): Perform dual-probe or pitch-catch UT on the clad layer side to detect subsurface cracks and disbonding. Use a phased array technique for improved resolution and coverage.
4. Eddy current testing (ET): Apply ET to the clad surface for surface crack detection, particularly for thin clad layers.
5. Destructive testing (DT): For qualification purposes, perform peel tests, bend tests, and intergranular corrosion tests on test coupons from the same heat treatment lot.

Defect Evaluation and Disposition

The following decision tree is recommended for the evaluation of detected defects:

1. If the defect is a surface crack in the clad layer:

• Measure the crack length, depth, and orientation using MT or ET.
• If the crack does not penetrate the clad layer and is within the acceptance limits of the applicable standard, it may be accepted with documentation.
• If the crack penetrates the clad layer, the plate must be rejected or repaired by removing the affected area and re-cladding.

1. If the defect is a subsurface crack or disbonding:

• Characterize the defect size, location, and orientation using UT.
• If the defect is within the acceptance limits (e.g., ≤ 10 mm × 5 mm per ASTM A263), it may be accepted with appropriate documentation and monitoring.
• If the defect exceeds the acceptance limits, the plate must be rejected or repaired.

1. If the defect is an interfacial crack:

• Interfacial cracks are generally considered unacceptable, as they compromise the corrosion barrier function.
• The plate must be rejected, or if repair is feasible, the affected area must be removed and re-cladded.

Engineering Case Studies

Case 1: Edge Cracking in 2+20 mm 304/Q345R Clad Plate

A batch of 2+20 mm 304/Q345R explosive welded clad plates was rejected due to edge cracking detected during MT inspection. The cracks were located within 50 mm of the plate edge, extending from the clad surface into the base metal. Root cause analysis revealed that the edge region experienced higher residual stresses due to the free boundary condition, and the combination of residual stress and strain-induced martensite in the wave zone led to cracking. The solution was to implement a stress relief heat treatment at 620 °C for 2 hours before edge machining, which eliminated the cracking.

Case 2: Delayed Cracking in Hydrogen Service

A hydrogenation reactor clad with 3+25 mm 304/Q345R explosive welded plate experienced delayed cracking at the bonding interface after 18 months of service in a hydrogen-containing environment. Post-failure analysis revealed that hydrogen had diffused into the microstructure and accumulated at oxide inclusions at the interface, causing stress-assisted cracking. The root cause was inadequate hydrogen bake-out before welding and insufficient stress relief after welding. The lesson learned was to implement a rigorous hydrogen control program including pre-heat bake-out, post-weld stress relief, and periodic in-service inspection.

Key Questions and Reflections

Critical Questions for Further Investigation

1. How can the sensitivity of UT for interface crack detection be improved? Current UT methods have limited resolution for thin cracks at the interface. Development of advanced UT techniques (e.g., phased array, TOFD) with optimized探头 configurations may improve detection capability.
2. What is the role of data analysis in defect evaluation? Can data analysis algorithms be trained to analyze UT signals and automatically classify defects as acceptable or unacceptable? This could improve inspection efficiency and consistency.
3. How does the wave morphology affect crack initiation and propagation? The wave amplitude, frequency, and regularity of the bonding interface may influence crack initiation sites and propagation paths. Further metallurgical study is needed to establish quantitative relationships.
4. What are the long-term implications of accepting marginal defects? If defects within acceptance limits are allowed, what is the risk of crack propagation over the service life of the pressure vessel? Risk-based inspection approaches may be needed.

Independent Technical Insights

From my experience, I have observed that the most challenging aspect of bonding interface crack judgment is the distinction between true cracks and false indications in UT signals. The wave morphology at the interface can generate strong UT reflections that may be mistaken for cracks. To address this, I recommend the following practices:

• Use multiple UT techniques (e.g., conventional UT combined with phased array UT) to cross-verify indications.
• Perform UT on both sides of the plate (clad side and base metal side) to confirm the location of indications.
• Use known reference standards with calibrated interface cracks for comparison.
• Conduct destructive verification on a representative sample to correlate UT signals with actual defect morphology.

Additionally, I have found that the combination of MT and UT provides the most reliable assessment of bonding interface cracks. MT is highly sensitive to surface and near-surface cracks, while UT is effective for subsurface defects. Using both methods together reduces the risk of missed defects and false positives.

Summary and Conclusions

The judgment of bonding interface cracks in explosive welded stainless steel clad plates is a critical quality control activity that requires a comprehensive understanding of metallurgy, non-destructive testing, and standards interpretation. The study emphasizes that bonding interface cracks are severe defects that can compromise the corrosion barrier function and mechanical integrity of pressure vessels. A systematic inspection protocol combining visual inspection, magnetic particle testing, ultrasonic testing, and eddy current testing is recommended for comprehensive defect detection. The judgment of detected defects must be performed in accordance with applicable standards (ASTM A263, ASME VIII Div.1, GB/T 150) and engineering judgment, considering the defect type, location, size, orientation, and service conditions. Root cause analysis of interface cracks reveals that oxide inclusions, strain-induced martensite, and residual stresses are the primary metallurgical causes, while insufficient welding velocity, surface contamination, and inadequate stress relief are the primary process-related causes. Engineers must adopt a proactive approach to defect prevention through process control, material selection, and heat treatment, rather than relying solely on inspection and rejection. The study also highlights the need for continued research into advanced NDT techniques and long-term service behavior of bonding interfaces to further improve the reliability and safety of explosive welded clad plates in pressure vessel applications.