CLADDING TECHNOLOGY SHANXI CO., LTDSurface Crack Analysis and Treatment in Hydrogenation Heat Exchanger Cladding Test Plates
Problem Statement and Background
The literature under review documents a quality issue encountered during the cladding of hydrogenation heat exchanger tube sheets, where surface cracks were observed in the overlay layer of test plates prior to full-scale production cladding. This is a critical finding because the test plate serves as the qualification coupon for the production weld procedure, and any defects in the test plate indicate systematic issues that would propagate to the production component if not resolved.
Hydrogenation heat exchangers operate under conditions of high pressure (20–30 MPa), high temperature (350–420 °C), and hydrogen-containing process fluids. The tube sheets are typically clad with Inconel 625 or Hastelloy C-276 to provide resistance against high-temperature hydrogen attack (HTHA) and corrosion. The overlay layer must be free of cracks, porosity, and other defects that could serve as initiation sites for hydrogen-induced cracking (HIC) or stress corrosion cracking (SCC) during service.
Crack Characterization and Investigation
The surface cracks were identified through visual inspection and confirmed by dye penetrant testing (PT) in accordance with JB/T 4730. The following table summarizes the crack characteristics:
| Parameter | Observation |
|---|---|
| Crack location | Surface of Inconel 625 overlay, primarily in the last 1–2 passes |
| Crack length | 5–25 mm (individual cracks) |
| Crack spacing | 20–80 mm between individual cracks |
| Crack orientation | Transverse to welding direction (perpendicular to weld travel) |
| Crack depth | 0.2–1.5 mm (surface-breaking, not through-thickness) |
| Crack morphology | Fine, branching, with slight oxidation (indicating high-temperature formation) |
| Affected area | Approximately 15–20% of test plate surface |
The investigation employed multiple analytical techniques to determine the root cause:
- Metallographic examination: Cross-sections through the cracks revealed that they initiated at the surface and propagated inward to a depth of 0.2–1.5 mm. The crack tips terminated within the overlay layer without reaching the base-overlay interface. The microstructure around the cracks showed a columnar grain structure typical of weld overlay deposits.
- Chemical analysis: The overlay composition was verified by optical emission spectroscopy (OES). The results showed that the nickel and chromium contents were within specification, but the carbon content was slightly elevated at 0.08% (specification: ≤ 0.06% for Inconel 625).
- Residual stress measurement: Hole-drilling method revealed residual stresses of 280–350 MPa in the overlay layer, significantly exceeding the typical post-stress-relief values of 100–150 MPa.
- Hardness mapping: Hardness values of 280–320 HV were measured in the overlay layer, slightly above the typical 230–270 HV for Inconel 625, indicating possible over-tempering or dilution effects.
Root Cause Analysis
The investigation identified three contributing factors to the surface cracking:
| Factor | Mechanism | Contribution Level |
|---|---|---|
| Excessive residual stress | High cooling rate + constraint from thick base plate | Primary |
| Elevated carbon content | Dilution from base metal + wire composition | Secondary |
| Inadequate stress relief | Insufficient temperature or duration of PWHT | Contributing |
The primary mechanism is believed to be stress-assisted cracking resulting from the combination of high residual stresses and a microstructure susceptible to cracking. The thick base plate (80–120 mm) acts as a thermal sink, causing rapid cooling of the overlay layer and generating high tensile residual stresses. The slightly elevated carbon content in the overlay promotes the formation of carbides (Ni₃(C,N) and Cr₇C₃) at grain boundaries, which reduces crack resistance. The inadequate post-weld heat treatment failed to sufficiently relieve these stresses.
Corrective Actions and Verification
The following corrective measures were implemented:
Process Modifications
| Parameter | Original | Modified | Rationale |
|---|---|---|---|
| Preheat temperature | 250 °C | 300–350 °C | Reduce cooling rate |
| Interpass temperature | 200 °C | 250–300 °C | Maintain thermal mass |
| Welding speed | 350 mm/min | 280–300 mm/min | Reduce heat input per unit length |
| Current | 380 A | 420–450 A | Increase penetration, reduce dilution |
| Post-weld heat treatment | 580 °C × 2 h | 620 °C × 4 h | More effective stress relief |
| Cooling rate (PWHT) | Furnace cooled | Controlled at 50 °C/h | Avoid thermal shock |
Material Modifications
- Wire composition: Switched to a low-carbon Inconel 625 wire with carbon content ≤ 0.04% (from ≤ 0.06%).
- Flux selection: Changed to a low-hydrogen, low-carbon ESW flux to minimize carbon pickup.
- Shielding gas: Verified argon purity at 99.999% to prevent oxidation.
Verification Results
After implementing the corrective measures, new test plates were fabricated and inspected:
| Inspection Method | Result |
|---|---|
| Visual inspection (VT) | No surface cracks observed |
| Dye penetrant testing (PT) | No indications (Level I acceptance) |
| Ultrasonic testing (UT) | No internal defects detected |
| Bond strength test (ASTM A263) | 380 MPa (required: ≥ 250 MPa) |
| Hardness | 240–265 HV (within specification) |
| Residual stress (after PWHT) | 85–120 MPa (within limits) |
| Intergranular corrosion (ASTM A923 Practice E) | No intergranular attack |
| HIC/SSC test (NACE TM0177) | No cracking after 720 h exposure |
Technical Reflections and Lessons Learned
This case study provides several important lessons for engineers involved in overlay cladding of pressure vessel components:
First, the test plate is not merely a procedural qualification coupon—it is a critical diagnostic tool for identifying systematic process issues before they manifest in production components. The investment in thorough investigation of test plate defects is always justified by the cost of rework or, worse, in-service failure.
Second, the interaction between residual stress and microstructure in determining crack susceptibility is a fundamental principle that cannot be overlooked. Even when individual factors (residual stress, carbon content, cooling rate) appear to be within acceptable limits, their combined effect can push the system beyond the cracking threshold. Engineers must think in terms of system interactions rather than isolated parameter limits.
Third, the post-weld heat treatment parameters require careful consideration for thick-section cladding. The conventional PWHT of 580 °C for 2 hours may be insufficient for thick base plates where the thermal mass creates high residual stresses. Increasing both temperature and duration—as done in this case—provides a more effective stress relief.
Fourth, the dilution control strategy must be comprehensive. Simply specifying a low-carbon wire is insufficient if the flux or shielding gas contributes additional carbon. A systematic approach to carbon control—covering wire, flux, gas, and surface cleanliness—is essential for maintaining overlay composition within specification.
Finally, the verification protocol after corrective action should be comprehensive, covering not only the specific defect that triggered the investigation (surface cracks) but also related properties such as bond strength, hardness, intergranular corrosion resistance, and hydrogen damage resistance. This ensures that the corrective measures do not introduce new issues while resolving the original problem.
Summary
The surface cracking encountered in hydrogenation heat exchanger cladding test plates serves as a valuable case study in the interaction between residual stress, microstructure, and crack initiation in weld overlay deposits. The systematic investigation—combining metallographic examination, chemical analysis, residual stress measurement, and hardness mapping—provided a clear understanding of the root cause and enabled the development of effective corrective measures. The key takeaways for engineering practice are: (1) always treat test plate defects as serious indicators of process issues; (2) consider the combined effect of multiple factors rather than isolated parameters; (3) ensure adequate post-weld heat treatment for thick-section cladding; and (4) implement comprehensive verification protocols after corrective action. These principles are directly applicable to other cladding applications involving nickel-based alloys on thick carbon steel or low-alloy steel base plates.