Measures to Prevent Hot Cracking in Hardfacing of Continuous Casting Rolls

Overview of the Technical Challenge

Continuous casting rolls are critical components in steelmaking, subjected to extreme thermal cycling, mechanical abrasion, and corrosive molten steel contact. Hardfacing weld overlay is the primary method for extending roll service life, yet hot cracking during the overlay process remains one of the most persistent and costly failure modes. The study of preventive measures for hot cracking in this application reveals deep interactions between metallurgy, thermal management, and process parameter control. The core challenge stems from the high carbon and alloy content of typical hardfacing alloys (such as Ni-Cr-Mo, Co-Cr, or high-carbon martensitic compositions), which produce wide freezing ranges and elevated susceptibility to solidification cracking, particularly at the weld pool boundary and in the last-to-freeze interdendritic regions.

Metallurgical Mechanisms of Hot Cracking

Hot cracking in hardfacing deposits is fundamentally a solidification phenomenon driven by the combination of three factors: a wide liquid-solid temperature range, restrained shrinkage strain, and the presence of low-melting-point segregations at grain boundaries. In continuous casting roll hardfacing, the substrate material—typically a high-chromium cast iron or austenitic manganese steel—imposes additional constraints. The thermal conductivity mismatch between the hardfacing alloy and the roll body creates steep thermal gradients, intensifying the tensile stress state in the solidifying deposit.

The following table summarizes the key metallurgical factors contributing to hot cracking susceptibility:

The Role of Sulfur and Phosphorus

Sulfur content in hardfacing consumables must be tightly controlled, typically below 0.015%, because manganese sulfide (MnS) inclusions form at grain boundaries during solidification. These sulfide films melt at temperatures approximately 50-100°C below the solidus temperature of the base deposit alloy, creating a network of liquid films that propagate under tensile stress. Phosphorus, while less detrimental than sulfur in isolation, synergistically enhances cracking when combined with sulfur. The study emphasizes that consumable chemistry must be specified with both S and P controlled to trace levels, ideally below 0.010% each.

Dilution and Its Effect on Cracking Behavior

Dilution from the roll substrate into the first weld pass is a critical variable. When a high-carbon martensitic hardfacing alloy is deposited on a low-carbon or austenitic substrate, the resulting dilution can shift the microstructure of the first pass toward a more crack-susceptible composition. For example, dilution of 20-30% from a low-carbon substrate into a Cr12MoV-type hardfacing alloy can reduce the carbon content below the threshold needed for stable martensite formation, leading to a mixed microstructure with retained austenite and ferrite that exhibits poor hot cracking resistance. The recommended countermeasure is to either use a sacrificial first pass of pure iron or low-alloy filler to buffer the dilution, or to increase the number of passes so that subsequent passes dilute the first pass further.

Preventive Measures and Process Optimization

Preheating and Interpass Temperature Control

Preheating is the single most effective process parameter for reducing hot cracking in roll hardfacing. The recommended preheat temperature depends on the specific hardfacing alloy and substrate combination:

The purpose of preheating is twofold: it reduces the thermal gradient between the substrate and the weld pool, thereby decreasing the restraint stress imposed on the solidifying deposit, and it slows the cooling rate, which promotes more equiaxed grain growth and reduces the severity of interdendritic segregation. However, excessive preheating must be avoided, as it can lead to coarse grain structures and reduced hardness in the hardfacing deposit.

Welding Process Selection and Parameter Control

The choice of welding process significantly influences the thermal input and thus the cracking susceptibility. Submerged arc welding (SAW) provides high thermal efficiency and deep penetration, but the high heat input can exacerbate dilution and widen the affected zone. Gas metal arc welding (GMAW) with a shielded metal arc welding (SMAW) electrode combination offers better control over heat input and is often preferred for the first few passes. The following process parameters are critical:

• Travel speed: Higher travel speeds reduce heat input and cooling rate but may cause incomplete fusion if too high. A balanced approach targeting a heat input of 15-25 kJ/cm for hardfacing passes is recommended.
• Arc voltage: Higher voltage increases penetration and dilution; moderate voltage settings (22-28 V for GMAW) minimize dilution while maintaining good fusion.
• Wire diameter: Larger wire diameters (1.6-2.0 mm) provide higher deposition rates but require higher currents, increasing heat input. A 1.2-1.4 mm wire is often optimal for balancing deposition rate and thermal control.
• Weld pass layout: A multi-pass strategy with the first pass deposited perpendicular to the rolling direction (circumferential direction of the roll) helps distribute thermal stress and reduces the probability of longitudinal cracking.

Consumable Selection and Chemistry

The chemical composition of the hardfacing consumable is the foundation of crack resistance. The study highlights several key compositional strategies:

• Ni-Cr-Mo alloys (e.g., Stellite-type): These austenitic deposits have excellent hot cracking resistance due to their narrow freezing range and solid solution strengthening. They are suitable for high-temperature applications but offer moderate abrasion resistance.
• Co-Cr alloys (e.g., CoCr15Mo): Cobalt-based alloys exhibit superior hot cracking resistance because of their low thermal expansion coefficient and high solid solution strength. They are ideal for severe thermal cycling environments.
• High-carbon martensitic alloys (e.g., Cr12MoV): These offer excellent abrasion resistance but are inherently susceptible to hot cracking. They require strict process control, including high preheat, low heat input, and careful consumable chemistry management.

Post-Weld Heat Treatment

A properly designed post-weld heat treatment (PWHT) can relieve residual stresses that contribute to delayed cracking. For high-carbon martensitic hardfacing deposits, a tempering treatment at 500-550°C for 1-2 hours per 25 mm of deposit thickness is recommended. This treatment transforms the as-welded martensite into tempered martensite, reducing hardness from approximately 60-65 HRC to 45-50 HRC while significantly improving toughness and reducing residual stress. For austenitic and cobalt-based deposits, PWHT is generally not required, but a stress relief anneal at 850-950°C for 2 hours may be beneficial to reduce residual stresses without altering the microstructure.

Engineering Practice and Case Study

In a practical application at a steel plant, a continuous casting tundish roll with a Cr12MoV hardfacing overlay exhibited hot cracking in approximately 35% of weld passes during initial trials. The root cause analysis identified three contributing factors: insufficient preheating (only 150°C instead of the recommended 250-300°C), excessive sulfur content in the consumable (0.025% instead of the specified maximum of 0.010%), and an unbalanced weld pass layout that concentrated thermal stress in the circumferential direction.

The corrective actions implemented were:

1. Increase preheat temperature to 300°C using induction heating with continuous temperature monitoring via thermocouples embedded in the roll surface.
2. Switch to a low-sulfur consumable grade with S < 0.008% and P < 0.008%.
3. Reorient the weld pass layout so that the first pass is deposited in the circumferential direction, with subsequent passes crossing the first pass at 60-90° angles.
4. Introduce a sacrificial first pass of pure low-carbon steel wire to buffer substrate dilution.

After implementing these measures, the hot cracking rate dropped to below 3%, and the roll achieved a service life exceeding 18 months compared to the previous 6-8 months. This case demonstrates that hot cracking prevention is not a single-variable problem but requires a systematic, multi-factorial approach integrating metallurgy, process parameters, and quality control.

Study Insights and Implications

The study of hot cracking prevention in continuous casting roll hardfacing reinforces several broader engineering principles. First, the interaction between consumable chemistry and process parameters is synergistic—optimizing one variable without considering the others yields diminishing returns. Second, the dilution effect from the substrate is often underestimated in practice, leading to unexpected microstructural changes and cracking in the first weld pass. Third, the economic cost of hot cracking extends beyond rework; it includes production downtime, reduced roll life, and potential quality issues in the cast product.

From a quality assurance perspective, the implementation of a robust FMEA (Failure Mode and Effects Analysis) approach is recommended. The FMEA should systematically evaluate each potential cracking mode—longitudinal, transverse, and center-line cracking—and assign risk priority numbers based on severity, occurrence, and detection. This structured approach ensures that preventive measures are prioritized according to their impact on the overall process reliability.

In conclusion, preventing hot cracking in continuous casting roll hardfacing demands a comprehensive strategy that integrates careful consumable selection with strict process parameter control, adequate thermal management, and thorough quality verification. The engineer who masters this balance will achieve significant improvements in roll life, production efficiency, and cost reduction, making the investment in process optimization well worth the effort.