CLADDING TECHNOLOGY SHANXI CO., LTDMeasures to Prevent Hot Cracking in Continuous Casting Roll Cladding
Literature Overview
Published in 1995 by Miao Hailiang from the Tianjin No. 2 Metallurgical Machinery Repair Center, this technical paper addresses one of the most persistent and challenging problems in the cladding of continuous casting (CC) rolls: hot cracking in the overlay weld. Continuous casting rolls are subjected to extreme thermal and mechanical loads during steel casting operations, necessitating hardfacing overlays to extend service life. However, the welding of hardfacing alloys to the roll substrate frequently results in hot cracking, leading to premature failure and costly roll replacement.
Core Technical Content
Hot cracking in CC roll cladding occurs during solidification of the overlay weld metal and is fundamentally driven by the combination of: (1) high sulfur and phosphorus content in the weld metal forming low-melting-point eutectics at grain boundaries, (2) high tensile residual stresses from thermal contraction, and (3) restricted deformation of the weld zone due to the rigid substrate constraint.
Root Cause Analysis
The metallurgical conditions that promote hot cracking in CC roll overlays include:
| Factor | Description | Impact |
|---|---|---|
| Sulfur content | S > 0.03% in weld metal | Forms FeS eutectics (1193°C) at grain boundaries |
| Phosphorus content | P > 0.05% | Lowers solidification range, promotes segregation |
| Solidification range | Wide freezing range alloys | Greater time in mushy zone, more crack susceptibility |
| Thermal stress | High constraint from rigid roll body | Tensile stress exceeds solidification strength |
| Weld geometry | Long, continuous weld beads | Accumulated thermal stress along weld length |
Preventive Measures Proposed
The paper proposes a multi-pronged approach to prevent hot cracking:
- Consumable modification: Using welding consumables with low sulfur (<0.02%) and low phosphorus (<0.03%) content. Adding microalloying elements such as titanium, zirconium, or rare earth elements to modify the morphology of sulfide inclusions from elongated FeS to dispersed TiS or ZrS particles.
- Process parameter optimization:
- Reducing heat input to minimize the size of the mushy zone
- Using short bead lengths (100-150 mm) with frequent interruptions to relieve thermal stress
- Applying controlled preheat (150-250°C) to reduce thermal gradients
- Maintaining interpass temperature below 200°C to promote stress relief between passes
- Weld sequence design: Implementing a break-welding strategy where the continuous overlay is divided into segments with overlap, allowing thermal stress to dissipate between segments.
- Post-weld stress relief: Applying controlled stress relief annealing at 600-650°C for austenitic overlays or 550-600°C for martensitic overlays to reduce residual stresses below the cracking threshold.
Metallurgical Mechanism of Hot Cracking
Hot cracking follows the strain rate cracking theory, where the crack initiates when the strain rate in the mushy zone exceeds the accommodation strain rate of the remaining liquid film. The critical condition is expressed as:
- When ε̇ (applied strain rate) > ε̇_acc (accommodation strain rate), cracking occurs
- The accommodation strain rate depends on the liquid fraction, surface tension, and viscosity of the remaining liquid
In CC roll cladding, the rigid substrate prevents free thermal contraction of the weld, creating high strain rates during solidification that frequently exceed the accommodation capability of the remaining liquid phase.
Engineering Practice Implications
The practical implementation of these preventive measures requires careful coordination between the welding engineer and the repair shop operator. The break-welding strategy, while effective, increases production time and requires precise planning of weld segments. The use of low-S, low-P consumables may involve additional cost but significantly extends the service life of the cladded roll. In modern CC roll repair operations, the combination of these measures with proper substrate preparation (removal of decarburized or contaminated surface layers) provides a robust defense against hot cracking.
Study Insights and Reflections
This 1995 publication, though predating many modern advances in consumable metallurgy, established fundamental principles that remain applicable today. The emphasis on controlling sulfur and phosphorus in the weld metal, combined with process parameter optimization and stress relief, represents a holistic approach to hot cracking prevention. The paper's practical orientation—written by a repair center engineer rather than an academic researcher—makes it particularly valuable for shop-floor implementation. The break-welding technique described here has since been refined into more sophisticated stress-relief welding sequences used in modern hardfacing applications. Understanding the metallurgical basis of hot cracking, as presented in this work, remains essential for any engineer working on CC roll repair or similar constrained hardfacing operations.