CLADDING TECHNOLOGY SHANXI CO., LTDCrack Mechanisms in Roll Overlay Welding
Literature Overview
This study note examines the crack formation mechanisms in overlay welding of guide rolls and pinch rolls, published in 2010 in the journal Transactions of the China Welding Institution. The authors, Wang Qingbao from Beijing University of Technology and China Metal Welding Science and Technology Company, along with Li Zhuoxin and Shi Yaowu from Beijing University of Technology, conducted a systematic investigation into the cracking behavior observed during the overlay welding of industrial rolls used in steel strip processing lines.
The research is significant because roll overlay welding is a critical maintenance and manufacturing process in the steel industry, where rolls are frequently refurbished through weld overlay to restore surface geometry and enhance wear resistance. Cracking in overlay layers leads to roll failure, production downtime, and significant economic losses.
Core Technical Analysis
The cracking in roll overlay welding is fundamentally related to the high carbon equivalent of the base metal, the high cooling rate associated with the relatively thin overlay layers, and the severe plastic deformation imposed by the rolling process. The base metal of industrial rolls is typically a medium-carbon steel or low-alloy steel with a carbon equivalent (CE) in the range of 0.45-0.65 percent, which places it in the high susceptibility range for cold cracking according to the hydrogen embrittlement mechanism.
The study identifies three distinct crack types and their respective mechanisms:
- Type 1: Solidification cracks in the overlay layer. These occur when the overlay material has a wide solidification range and is susceptible to hot cracking. The mechanism involves the formation of low-melting-point liquid films at grain boundaries during the final stages of solidification.
- Type 2: Hydrogen-induced cold cracks in the HAZ. These are the most common and most damaging crack type in roll overlay welding. The mechanism involves three synergistic factors: susceptible martensitic microstructure in the HAZ, diffusable hydrogen concentration exceeding a critical threshold, and tensile residual stress above the yield strength of the microstructure.
- Type 3: Thermal fatigue cracks at the overlay-to-base metal interface. These develop during the rolling service life due to cyclic thermal stresses imposed by the alternating contact with hot and cold steel strips.
Key Process Parameters and Countermeasures
The study provides detailed recommendations for process optimization, summarized in the following table:
| Process Parameter | Critical Range | Effect on Cracking |
|---|---|---|
| Preheat temperature | 200-300°C | Reduces cooling rate below 10°C/s and limits martensite formation |
| Interpass temperature | ≥ 200°C | Maintains HAZ above martensite start temperature |
| Heat input | 15-25 kJ/mm | Sufficient to reduce cooling rate without excessive grain growth |
| Wire diameter | 1.6-2.0 mm | Balances deposition rate and heat input |
| Shielding gas | Ar + 2% CO₂ | Provides adequate shielding and arc stability |
| Post-weld heat treatment | 600-650°C for 2 hours | Diffuses hydrogen and relieves residual stress |
| Wire composition | Low carbon, low hydrogen | Minimizes hydrogen pickup and martensite formation |
The authors emphasize that the hydrogen content in the weld metal is the most critical factor for cold crack prevention. The hydrogen content should be maintained below 10 ml/100g, and ideally below 5 ml/100g for high-carbon-equivalent base metals. This requires strict control of welding wire moisture, gas purity, and joint surface cleanliness.
Engineering Practice Integration
In industrial roll manufacturing and maintenance, the overlay welding process is typically performed using submerged arc welding (SAW) for high deposition rates and gas metal arc welding (GMAW) for better process control on smaller diameter rolls. The study notes that SAW is preferred for thick overlay layers (greater than 5 mm) due to its high deposition rate and low spatter, while GMAW is more suitable for thin overlay layers (less than 3 mm) and for repair welding of localized defects.
The NDT protocol recommended in the study includes:
- Visual inspection (VT): 100 percent coverage for surface defects, undercut, and geometric irregularities.
- Magnetic particle testing (MT): 100 percent coverage for surface and near-surface cracks, particularly at the overlay-to-base metal transition zone.
- Ultrasonic testing (UT): Spot check at 20 percent coverage for subsurface cracks and lack of fusion, using a contact probe with frequency of 2.5 MHz.
- Hardness testing: Hardness profile across the overlay-to-base metal interface to verify the absence of brittle martensitic phases.
The study also discusses the importance of overlay layer design. For guide rolls in hot strip mills, the overlay layer should be designed to provide adequate wear resistance (hardness of 35-45 HRC) while maintaining sufficient toughness to resist thermal fatigue cracking. A multi-layer overlay design with a transition layer (low carbon austenitic) and a wear-resistant surface layer (martensitic or duplex) is recommended.
Study Insights and Reflections
This literature provides valuable mechanistic understanding of cracking in roll overlay welding that can be applied to other overlay welding applications involving high-carbon-equivalent base metals. The three-factor model of cold cracking (susceptible microstructure, hydrogen, and stress) is well-established, but the specific quantification of critical values for roll overlay welding is a significant contribution.
One particularly insightful aspect of this work is the recognition that the cracking problem in roll overlay welding is not solely a welding process issue but is also related to the rolling service conditions. The thermal fatigue cracking that develops during service is influenced by the rolling speed, strip temperature, and the thermal conductivity of the overlay layer. This means that the overlay layer design must consider both the welding process requirements and the service environment requirements.
The practical implication for engineers is that the welding procedure specification (WPS) for roll overlay welding must be qualified not only for mechanical properties and crack resistance but also for thermal fatigue resistance. This requires accelerated thermal cycling tests or finite element analysis to predict the thermal stress distribution during rolling service.
The long-term value of this study lies in its systematic approach to crack prevention, which integrates material selection, process optimization, and inspection protocols into a comprehensive quality control framework. This framework can be adapted to other overlay welding applications in the steel industry, including work rolls, backup rolls, and finishing rolls.