CLADDING TECHNOLOGY SHANXI CO., LTDCrack Failure Analysis of Weld Overlay Layer on Remanufactured Large Hot Rolling Support Rolls
Literature Overview
The 2022 study by Nie Binying from the School of Chemistry and Biological Engineering at Yichun University, published in Materials Science and Technology, presents a detailed failure analysis of cracking in the weld overlay layer on a large-diameter hot rolling support roll that had been remanufactured through weld overlay. This case study addresses a critical and increasingly common problem in the steel industry: the premature failure of remanufactured rolls due to overlay cracking. As the cost of new support rolls continues to rise and environmental pressures favor remanufacturing over replacement, understanding the root causes of overlay failure becomes essential for extending roll service life and ensuring production reliability.
Failure Mode and Fractography Analysis
The investigation began with a systematic examination of the cracked overlay surface using both macroscopic and microscopic techniques. The failure mode was identified as a combination of surface cracking and subsurface spalling, with the following characteristics:
| Defect Type | Location | Orientation | Maximum Length |
|---|---|---|---|
| Surface cracks | Overlay surface | Transverse to roll axis | 15–30 mm |
| Interpass cracks | Between overlay passes | Along pass boundaries | 20–50 mm |
| Subsurface spalling | 2–5 mm below surface | Irregular | 50–100 mm |
| Fusion zone cracks | Overlay-base interface | Along interface | 10–25 mm |
Scanning electron microscopy (SEM) fractography revealed that the surface cracks exhibited intergranular fracture characteristics, with cleavage facets visible along prior austenite grain boundaries. This fracture mode is indicative of either hot cracking during solidification or cold cracking during cooling, both of which are common in high-carbon, high-alloy overlay deposits. The interpass cracks showed a mixture of intergranular and transgranular fracture, suggesting a combination of solidification cracking and hydrogen-induced cracking.
Root Cause Analysis Using Fishbone Diagram Methodology
A systematic root cause analysis was conducted using the fishbone (Ishikawa) diagram approach, identifying the following contributing factors:
Material factors:
- Excessive carbon content in the overlay wire (0.8–1.2% vs. recommended 0.4–0.6%)
- High sulfur and phosphorus content in the base metal (S: 0.035%, P: 0.045% vs. recommended <0.025% each)
- Inadequate base metal surface preparation, leaving residual oxide and scale
Process factors:
- Insufficient preheat temperature (measured at 80–100°C vs. recommended 200–250°C)
- Excessive interpass temperature in some areas (measured up to 400°C vs. recommended <300°C)
- Inconsistent travel speed leading to variable heat input
- Inadequate post-weld stress relief treatment
Environmental factors:
- High ambient humidity during welding (70–80% vs. recommended <60%)
- Wind exposure during welding without adequate shielding
Design factors:
- Excessive overlay thickness (8–10 mm vs. recommended 3–5 mm)
- Inadequate consideration of thermal mismatch between overlay and base metal
- No buffer layer deposited to accommodate thermal expansion differences
Metallurgical Investigation
Detailed metallographic examination of the cross-section revealed several critical findings:
Overlay Layer Microstructure
The overlay layer contained a predominantly martensitic structure with retained austenite and dispersed carbides. The hardness distribution across the overlay thickness showed significant variation:
| Depth from Surface (mm) | Hardness (HRC) | Retained Austenite (%) |
|---|---|---|
| 0.0–0.5 | 62–65 | 12–18 |
| 0.5–1.5 | 58–62 | 15–22 |
| 1.5–3.0 | 55–58 | 18–25 |
| 3.0–5.0 | 50–55 | 20–28 |
| 5.0–8.0 | 45–50 | 25–35 |
The increasing retained austenite content with depth is attributed to the decreasing cooling rate in deeper passes, which reduces the martensite start temperature and leaves more austenite untransformed. While retained austenite provides some toughness, excessive amounts (>25%) can lead to dimensional instability and reduced wear resistance.
Fusion Zone and Heat-Affected Zone
The fusion zone between the overlay and the base metal showed a width of 0.8–1.5 mm with a hardness of 50–55 HRC, indicating significant dilution and hardening. The heat-affected zone (HAZ) of the base metal extended 2–4 mm into the roll body, with a hardness increase from the base metal value of 25 HRC to 35–40 HRC at the fusion zone boundary. This hardening in the HAZ is attributed to the formation of martensite during the rapid cooling that follows the welding thermal cycle.
The most critical finding was the presence of microcracks at the fusion zone interface, which were not visible during surface inspection but were revealed by cross-sectional examination. These microcracks, measuring 0.1–0.5 mm in length, were oriented parallel to the interface and were attributed to the high residual stresses generated during welding combined with the brittle nature of the hardened fusion zone.
Residual Stress Analysis
Residual stress measurements were conducted using X-ray diffraction on both the as-welded and post-failure specimens. The results revealed:
| Location | As-Welded Stress (MPa) | Post-Failure Stress (MPa) |
|---|---|---|
| Overlay surface | 450–550 (tensile) | 200–300 (tensile) |
| Mid-overlay depth | 350–450 (tensile) | 150–250 (tensile) |
| Fusion zone | 500–600 (tensile) | 250–350 (tensile) |
| Base metal HAZ | 200–300 (tensile) | 100–200 (tensile) |
The high tensile residual stresses in the as-welded condition, particularly at the fusion zone (500–600 MPa), were identified as a primary contributing factor to the cracking. These stresses exceed the yield strength of the hardened fusion zone material, creating conditions favorable for crack initiation and propagation. The stress relief observed in the post-failure specimen indicates that the cracking process itself relieved a significant portion of the residual stress, confirming the stress-driven nature of the failure.
Corrective Actions and Prevention Strategies
Based on the failure analysis, the following corrective actions were recommended:
Process Parameter Optimization
| Parameter | Failed Condition | Recommended Value | Rationale |
|---|---|---|---|
| Preheat temperature | 80–100°C | 200–250°C | Reduce cooling rate, minimize HAZ hardness |
| Interpass temperature | 200–400°C | 150–250°C | Control thermal cycling, prevent excessive grain growth |
| Heat input | 25–40 kJ/mm | 15–25 kJ/mm | Reduce dilution, minimize HAZ width |
| Overlay thickness | 8–10 mm | 3–5 mm | Reduce total thermal input, minimize residual stress |
| Wire composition | C: 0.8–1.2% | C: 0.4–0.6% | Reduce hot cracking susceptibility |
Additional Recommendations
- Implement a buffer layer strategy: Deposit 1–2 mm of austenitic stainless steel (309L) before the final overlay layer to accommodate thermal expansion differences and reduce fusion zone hardness.
- Mandate post-weld stress relief: Apply a stress relief treatment at 550–600°C for 2–4 hours per 25 mm of roll diameter, with controlled cooling rates not exceeding 100°C/h.
- Implement in-process monitoring: Use thermal imaging to monitor interpass temperatures in real-time, with automated shutdown if temperatures exceed 300°C.
- Enhance base metal preparation: Perform thorough mechanical and chemical cleaning of the roll surface to remove all scale, oxide, and contaminants before overlay application.
- Establish acceptance criteria: Define quantitative acceptance criteria for overlay quality, including maximum allowable crack length, minimum bond strength, and maximum residual stress levels.
Key Reflections and Industry Implications
This failure analysis highlights a critical gap in the remanufacturing industry: the lack of systematic failure analysis and process optimization for weld overlay applications on large-diameter rolls. The root causes identified—insufficient preheat, excessive overlay thickness, inadequate stress relief, and poor base metal preparation—are all well-known issues in welding engineering, yet their combination in this application led to catastrophic failure. This suggests that the remanufacturing industry needs more rigorous engineering controls and quality assurance procedures.
The finding that microcracks at the fusion zone interface were the initiation sites for the larger cracks observed on the surface is particularly significant. These microcracks are invisible to surface inspection methods (MT, PT) and can only be detected by cross-sectional examination or advanced NDT techniques such as phased array ultrasonic testing (PAUT). This implies that conventional inspection methods may be insufficient for ensuring overlay quality, and that more advanced inspection techniques should be considered for critical applications.
The residual stress analysis also underscores the importance of post-weld stress relief in overlay applications. The high tensile stresses measured in the as-welded condition (500–600 MPa at the fusion zone) are well above the threshold for stress corrosion cracking and hydrogen-induced cracking in hardened materials. Without adequate stress relief, even a metallurgically sound overlay can fail prematurely due to residual stress-driven cracking.
Summary
This failure analysis provides a comprehensive understanding of the cracking mechanisms in weld overlay layers on remanufactured hot rolling support rolls. The systematic investigation, combining fractography, metallography, residual stress analysis, and process review, identified multiple contributing factors that acted synergistically to produce the observed failure. The key engineering lessons are clear: adequate preheating, controlled interpass temperatures, appropriate overlay thickness, proper base metal preparation, and mandatory post-weld stress relief are all essential for preventing overlay cracking. The remanufacturing industry would benefit significantly from adopting more rigorous engineering controls, implementing in-process monitoring, and establishing quantitative acceptance criteria for overlay quality. This case study should serve as a cautionary example for engineers and practitioners in the roll remanufacturing sector, emphasizing the importance of systematic approach to overlay design, fabrication, and inspection.