Effect of Transition Layer on Hot Cracking in Clamp Roller Overlay Welding

Literature Overview

This paper by Wang Lihua from Xi'an Aeronautical Vocational and Technical College, published in Hot Working Technology in 2006, investigates the influence of a transition layer on hot cracking susceptibility in the overlay welding of clamp rollers. Clamp rollers are critical components in continuous casting machines and rolling mills, where they must withstand extreme temperatures, mechanical loading, and abrasive wear. The overlay welding of hardfacing materials on steel substrates with high sulfur and phosphorus content is prone to hot cracking, and the paper demonstrates that a carefully designed transition layer can significantly mitigate this defect.

Metallurgical Background of Hot Cracking

Hot cracking in overlay welds occurs during solidification when the following conditions coincide:

1. A high fraction of solid with limited liquid film feeding capability
2. Tensile stress in the solidifying region
3. A liquid film enriched with low-melting-point impurities at grain boundaries

In the clamp roller application, the substrate typically contains sulfur and phosphorus at levels of 0.03-0.06% S and 0.02-0.05% P, which are relatively high for welding applications. The overlay material, being a high-carbon, high-hardness martensitic steel (typically 2-4% C, 10-20% Cr), has a wide solidification range and is inherently susceptible to hot cracking.

The mechanism involves the formation of a liquid film at grain boundaries containing:

• Iron sulfide (FeS) with melting point of 1193°C
• Iron phosphide (Fe₃P) with melting point of 1060°C
• Iron carbide (Fe₃C) with melting point of 1147°C
• Chromium carbides with melting points ranging from 1400-1500°C

Transition Layer Design

The transition layer serves as an intermediary between the high-sulfur, high-phosphorus substrate and the high-carbon, high-chromium overlay material. Its design principles include:

The transition layer material is typically a medium-carbon, medium-chromium alloy with low sulfur and phosphorus content. Common choices include:

• 0Cr13Ni4Mo (modified 410 stainless steel)
• Ni-base alloys such as Ni-27Cr-2Mo-2Fe
• Modified austenitic stainless steel with added Nb and Ti

Experimental Results and Analysis

The paper presents experimental evidence demonstrating the effectiveness of the transition layer:

Without transition layer:

• Hot crack frequency: 60-80% of welds
• Average crack length: 15-30 mm
• Crack location: primarily in the first overlay pass at the fusion boundary

With transition layer:

• Hot crack frequency: 5-15% of welds
• Average crack length: 2-5 mm (when present)
• Crack location: primarily in the overlay layer, not at the interface

The reduction in hot cracking is attributed to several mechanisms:

1. Sulfur and phosphorus dilution: The transition layer dilutes the sulfur and phosphorus from the substrate, reducing their concentration at the grain boundaries of the subsequent overlay passes
2. Grain boundary modification: The chromium and nickel in the transition layer promote the formation of higher-melting-point intermetallic compounds at grain boundaries
3. Stress relief: The different thermal expansion coefficient of the transition layer partially accommodates the thermal stresses that would otherwise concentrate at the substrate-overlay interface
4. Wetting improvement: The transition layer provides a more compatible surface for the overlay material to wet, reducing the likelihood of incomplete fusion

Process Parameters

The recommended welding sequence is:

1. Substrate preparation: mechanical grinding to bare metal, degreasing
2. Transition layer: 2-3 passes of medium-carbon medium-chromium alloy, total thickness 2-3 mm
3. Overlay layer: 4-6 passes of high-carbon high-chromium martensitic alloy, total thickness 8-12 mm
4. Post-weld heat treatment: 550-600°C for 1-2 hours to relieve residual stress and transform retained austenite

The welding process typically uses submerged arc welding (SAW) or flux-cored arc welding (FCAW) for the transition layer and plasma arc welding (PAW) or gas tungsten arc welding (GTAW) for the overlay layer, depending on the required precision and deposit thickness.

Engineering Practice Considerations

For continuous casting clamp rollers, the overlay must withstand:

• Slab temperatures of 1200-1400°C at the casting point
• Thermal cycling of 50-100°C per pass
• Mechanical loading of 5-15 kN per roller
• Abrasive wear from oxide scale and slag

The transition layer approach has been successfully applied in industrial practice, with reported service life extensions of 2-3 times compared to direct overlay without transition layer. The additional cost of the transition layer (approximately 15-20% increase in material and welding cost) is justified by the dramatic improvement in overlay integrity and service life.

Study Insights

The transition layer concept is fundamentally about managing the metallurgical incompatibility between dissimilar materials through intermediate composition design. This principle extends beyond welding to include diffusion bonding, brazing, and even coating technologies. The key insight is that the transition layer must be designed not merely as a mechanical interlayer but as a metallurgical buffer that addresses the specific failure mechanisms at play.

The paper's approach to hot cracking mitigation through transition layer design is particularly instructive because it addresses the root cause (sulfur and phosphorus enrichment at grain boundaries) rather than merely treating symptoms through process parameter optimization. This represents a materials engineering approach to a welding problem, which is the correct methodology for complex dissimilar material joining challenges.