CLADDING TECHNOLOGY SHANXI CO., LTDPreventing Hot Cracks in Continuous Casting Roll Overlay Welding A Study Note
Literature Overview and Background
Continuous casting rolls are subjected to extreme thermal cycling, mechanical loading, and corrosion from molten steel. To address these challenges, hardfacing and overlay welding are commonly applied to the roll surface. However, the overlay welding of continuous casting rolls is notorious for producing hot cracks, particularly in the weld metal and at the weld-base metal interface. This study note reviews the measures proposed to prevent hot cracks during the overlay welding process and evaluates their effectiveness based on metallurgical principles and engineering experience.
Core Technical Points
Mechanism of Hot Cracking in Roll Overlay Welding
Hot cracks form in the solidification range of the weld metal when the combination of thermal contraction stresses and the presence of low-melting-point phases exceeds the material's resistance to crack formation. In continuous casting roll overlay welding, the base material is typically a ductile iron or a high-chromium cast iron, both of which have a tendency to form low-melting-point eutectic phases at the grain boundaries during solidification. The high thermal conductivity of the roll material creates rapid cooling rates that exacerbate the problem by narrowing the temperature range over which the weld metal is susceptible to cracking.
Key Measures for Hot Crack Prevention
The study proposes several measures that can be implemented individually or in combination to minimize hot cracking. These measures address both the metallurgical and thermal aspects of the welding process.
| Measure | Mechanism | Implementation Details |
|---|---|---|
| Preheating | Reduce cooling rate, increase ductility in solidification range | 300-500 °C depending on base material |
| Low thermal input | Minimize dilution and reduce residual stress | Current 150-250 A, speed 200-400 mm/min |
| Multi-pass welding | Each pass acts as a heat treatment for the previous pass | 3-5 passes with interpass temp ≤400 °C |
| Weld consumable selection | Avoid high sulfur, phosphorus, and carbon content | Use low-carbon, low-sulfur stainless steel or nickel-based alloys |
| Post-weld heat treatment | Relieve residual stresses | 550-650 °C for 2-4 hours |
| Weld geometry optimization | Reduce stress concentration | Use shallow, wide weld beads with good root penetration |
Metallurgical Approaches
The selection of welding consumable plays a decisive role in hot crack resistance. Nickel-based alloys such as Ni-Fe-Cr or Ni-Cr-Mo are particularly effective because they have a wide solidification range and good resistance to hot cracking. The addition of elements such as niobium, titanium, and vanadium can refine the microstructure and reduce the formation of low-melting-point phases. The study also highlights the importance of controlling the carbon and sulfur content of the consumable, as high carbon promotes the formation of brittle carbides and high sulfur lowers the melting point of grain boundary films.
Thermal Management Strategies
The thermal management aspect of hot crack prevention involves controlling the heat input, preheat temperature, and interpass temperature. A lower thermal input reduces the size of the heat-affected zone and the magnitude of residual stresses, but it must be balanced against the need for adequate penetration and bond strength. The preheat temperature should be selected based on the base material composition and the desired cooling rate. For high-chromium cast iron rolls, a preheat of 400 to 500 degrees Celsius is typically required to prevent cracking, while for ductile iron rolls, 300 to 400 degrees Celsius may be sufficient.
Engineering Practice Insights
In practice, the most effective approach to preventing hot cracks is a systematic combination of measures rather than reliance on any single technique. A practical workflow might begin with a preheat to the recommended temperature, followed by a transition pass using a nickel-based consumable to establish a crack-resistant interface, and then multiple overlay passes using the selected hardfacing material. The interpass temperature must be monitored and controlled throughout the welding sequence. After welding, a post-weld stress relief treatment should be applied to reduce the residual stress level below the cracking threshold.
The study also emphasizes the importance of weld sequence planning. For large rolls with multiple overlay zones, the welding sequence should be designed to minimize the accumulation of thermal stresses. A symmetrical welding pattern, where opposite sides of the roll are welded in sequence, helps to distribute the thermal input more evenly and reduce warping and cracking.
Quality control should include visual inspection for surface cracks, dye penetrant testing or magnetic particle testing for subsurface cracks, and metallographic examination of cross-sections to assess the crack-free condition of the weld metal and interface. The hardness profile should be measured to ensure that the overlay meets the specified wear resistance requirements.
Summary and Reflections
The prevention of hot cracks in continuous casting roll overlay welding is a multifaceted problem that requires a comprehensive approach addressing both metallurgical and thermal factors. The most important insight from this study is that no single measure is sufficient on its own; rather, a systematic combination of preheating, consumable selection, thermal input control, multi-pass welding, and post-weld heat treatment is necessary to achieve a crack-free overlay. Engineers should approach the development of a welding procedure for this application as a structured problem-solving exercise, using the PDCA cycle to iteratively refine the process parameters based on metallurgical evaluation and service performance feedback.