Thermal Fatigue Performance of Roller Cladding Metals and Crack Initiation Mechanism

Literature Overview

Feng Lingzhi et al. from Tianjin University, in collaboration with Xingtai Special Roller Works (Welding Journal, 2001, Vol. 22, No. 2, pp. 19–22), conducted a systematic study on the thermal fatigue resistance of three self-designed flux-cored wires and one commercial solid wire for roller cladding. This research addresses a critical failure mode in hot rolling mills, where roller surface cladding undergoes repeated thermal cycling due to contact with hot steel strips, leading to thermal fatigue cracking and premature failure.

Core Technical Findings

Thermal Fatigue Test Methodology

The authors designed a specialized thermal fatigue test method that simulates the thermal cycling experienced by roller cladding surfaces during hot rolling operations. The test involves cyclic heating and cooling of the cladding surface, replicating the temperature gradients encountered in actual service conditions.

Key Findings on Crack Initiation and Propagation

The study establishes that thermal fatigue cracking in cladding metals results from the combined action of:

1. Cyclic thermal stress: Generated by differential thermal expansion across the cladding thickness and between the cladding and substrate.
2. Oxidation attack: High-temperature oxidation at crack tips and surface defects accelerates crack initiation and propagation.

Influence of Microstructural Factors

Crack Formation and Propagation Mechanism Model

The authors proposed a mechanistic model for thermal fatigue cracking:

1. Stage 1 – Crack Initiation: Surface oxidation creates preferential sites for crack nucleation. The combination of cyclic tensile stress (from thermal expansion mismatch) and local oxidation weakening initiates microcracks at surface defects, inclusions, or grain boundaries.
2. Stage 2 – Subsurface Crack Growth: Once initiated, cracks propagate along the thermal stress gradient, typically perpendicular to the roller surface. Oxidation at the crack tip accelerates propagation.
3. Stage 3 – Surface Linking: Multiple subsurface cracks eventually link with surface cracks, forming through-thickness cracks that lead to spalling and material loss.

Comparison of Test Materials

The three self-designed flux-cored wires and one commercial solid wire were evaluated. The study found that:

• Materials with superior oxidation resistance (higher Cr, Al content) demonstrated stronger resistance to crack initiation.
• Materials with more homogeneous microstructure (uniform carbide distribution, absence of segregation) exhibited longer thermal fatigue life.
• The presence of large or stringer-type inclusions (particularly MnS, SiO2, or oxide clusters) served as crack initiation sites and significantly reduced thermal fatigue life.

Engineering Practice Integration

Roller Cladding Design Guidelines

Based on this research, the following guidelines can be established for roller cladding design:

1. Filler material selection: Prioritize alloys with high oxidation resistance (Cr ≥ 25 wt%, Al ≥ 5 wt%) to maximize crack initiation resistance.
2. Inclusion control: Implement strict slag removal and shielding gas quality control to minimize oxide and sulfide inclusions in the cladding layer.
3. Microstructure homogeneity: Multi-pass cladding with consistent parameters ensures uniform microstructure throughout the cladding thickness.
4. Cladding thickness optimization: Sufficient cladding thickness (typically 3–5 mm) ensures that thermal fatigue cracks remain within the sacrificial cladding layer and do not reach the substrate.

Application to Pipeline Engineering

While this study focuses on roller cladding, the thermal fatigue principles are directly applicable to:

• Pipeline components in thermal cycling service: Heat exchanger tubes, boiler tubes, and refinery heater tubes experience similar thermal fatigue mechanisms.
• Cladding layers on heat-resistant components: Any overlay applied to components subject to thermal cycling must be evaluated for thermal fatigue resistance, not just wear or corrosion resistance.
• Quality control criteria: Inclusion content and microstructural homogeneity should be specified in cladding quality standards for thermal cycling applications.

Key Reflections

This study provides a fundamental understanding of thermal fatigue cracking that transcends the specific application of roller cladding. The insight that oxidation resistance governs crack initiation is particularly valuable because it suggests that conventional thermal fatigue tests conducted in inert atmospheres may significantly overestimate component life in oxidizing service. Engineers designing cladding systems for thermal cycling applications must consider both mechanical (stress) and chemical (oxidation) degradation mechanisms simultaneously.

The proposed crack formation and propagation model offers a framework for evaluating alternative cladding materials and processes. For instance, processes that minimize inclusion content (such as CMT with its clean arc and low spatter) would inherently produce cladding layers with superior thermal fatigue performance. Similarly, alloys that form protective oxide scales (such as Cr-Al-Mo superalloys) would provide dual protection against both oxidation and thermal fatigue cracking.

The collaboration between academia (Tianjin University) and industry (Xingtai Special Roller Works) exemplifies the value of industry-academia partnerships in addressing practical engineering challenges. The self-designed flux-cored wires demonstrate that proprietary filler development, guided by fundamental metallurgical understanding, can yield significant performance improvements over commercial alternatives.

Summary

These five studies collectively illustrate the breadth and depth of cladding technology research, spanning from fundamental microstructure-property relationships to process optimization and engineering application. The Fe3Al study emphasizes thermal management for brittle intermetallic cladding. The Co-Al-W study provides quantitative wear performance data for next-generation cobalt alloys. The X80/2205 composite pipe study demonstrates the superiority of CMT for corrosion-critical pipeline applications. The magnetic field study reveals electromagnetic stirring as a microstructure control tool. And the thermal fatigue study establishes the combined role of cyclic stress and oxidation in crack initiation. Together, these works provide engineers with a comprehensive knowledge base for selecting materials, optimizing processes, and predicting service life of cladding systems across diverse industrial applications.