CLADDING TECHNOLOGY SHANXI CO., LTDEffects of Overlay Layer Composition and Microstructure on Thermal Fatigue and Wear Resistance of Hot Rolling Mill Rolls
Literature Overview
This landmark paper, published in 1992 in the Journal of Xi'an Jiao Tong University, was authored by Xue Jin, Wang Yasheng, Wang Jian, Lang Yi, and Lou Baicheng from Xi'an Jiao Tong University and the Ansteel Steel Research Institute. This early but highly influential work addresses the critical challenge of extending the service life of hot rolling mill rolls through weld overlay technology. The research was conducted at a time when China's steel industry was rapidly expanding, and roll life was a major cost driver in hot rolling operations.
Technical Background and Industrial Context
Hot rolling mill rolls are subjected to extreme operating conditions including high temperatures (up to 1100°C at the roll surface), severe contact pressure, thermal cycling, and abrasive wear from scale and oxide layers on the hot steel. The roll surface undergoes repeated heating and cooling during the rolling process, leading to thermal fatigue cracking. Simultaneously, the passage of hot steel with adherent scale causes abrasive wear. The combination of thermal fatigue and wear results in surface degradation characterized by cracking, spalling, and dimensional loss, ultimately requiring roll regrinding or replacement.
Overlay System Design Parameters
| Parameter | Typical Range | Rationale |
|---|---|---|
| Overlay thickness | 3-8 mm | Balance between life extension and roll body integrity |
| Hardness (HV) | 400-700 | Sufficient for wear resistance without excessive brittleness |
| Thermal conductivity | Match to base steel | Minimize thermal stress mismatch |
| Thermal expansion coefficient | Match to base steel | Reduce residual stress at interface |
| Chromium content | 10-25% | Carbide formation and oxidation resistance |
| Carbon content | 0.5-2.5% | Controls carbide volume fraction |
The composition of the overlay layer is critical because it determines the microstructure, which in turn governs the thermal fatigue and wear resistance. The authors systematically investigated the effects of chromium, carbon, and alloying additions on the resulting microstructure and performance.
Composition-Microstructure-Property Relationships
The study examined several overlay compositions including iron-based, nickel-based, and cobalt-based systems. The key finding was that the volume fraction, morphology, and distribution of carbide phases are the primary determinants of both thermal fatigue and wear resistance.
Iron-based overlays with 12-18% chromium and 1.5-2.5% carbon produced a microstructure consisting of martensite and bainite matrix with M7C3 and M23C6 carbides. The chromium content controlled the type and amount of carbides—higher chromium favored the formation of M23C6 carbides, which are harder but more brittle than M7C3. The carbon content directly influenced the total carbide volume fraction.
Nickel-based overlays produced a more ductile austenitic matrix with dispersed carbides, offering superior thermal fatigue resistance but somewhat lower wear resistance compared to optimally designed iron-based systems. The thermal expansion mismatch between the nickel-based overlay and the steel roll body was identified as a potential concern for very thick overlays.
Thermal Fatigue Performance
Thermal fatigue testing was conducted by simulating the thermal cycling conditions of hot rolling—rapid heating of the overlay surface to 800-900°C followed by water quenching. The number of cycles to initiate surface cracking and the crack depth progression were recorded.
The iron-based overlay with 15% Cr and 2.0% C demonstrated the best balance between thermal fatigue resistance and wear resistance. The crack initiation life was approximately 200-300 cycles, and crack propagation was relatively slow due to the mixed matrix-carbide microstructure that provided crack deflection and bridging. The nickel-based overlay showed higher crack initiation life (300-500 cycles) but lower wear resistance, making it less suitable for the combined loading conditions of hot rolling.
Wear Performance
Abrasive wear testing using SiC abrasive paper and steel balls at elevated temperatures (up to 600°C) showed that the iron-based overlay with optimized chromium and carbon content achieved the lowest wear rate. The hard carbide particles effectively resisted ploughing by abrasive particles, while the tough matrix prevented spalling of the hard phases. At elevated temperatures, the wear rate increased for all compositions, but the iron-based overlay maintained its relative advantage due to the thermal stability of chromium carbides.
The wear mechanism analysis revealed a transition from abrasive ploughing at room temperature to a combination of abrasive and adhesive wear at elevated temperatures. The formation of a protective oxide layer on the overlay surface at high temperature contributed to reduced wear by separating the contact surfaces.
Interface Integrity and Defect Analysis
The dilution zone at the overlay-base metal interface was identified as a critical region for performance. Excessive dilution reduced the hard phase content in the near-interface region, creating a soft zone susceptible to preferential wear and crack initiation. The authors recommended using a consumable insert or a pre-welded backing strip to reduce dilution and ensure a uniform overlay composition throughout the full thickness.
Common defects observed included: (1) lack of fusion at the interface due to insufficient heat input; (2) cracking in the overlay layer due to high carbon equivalent and low hydrogen control; (3) porosity from excessive gas pickup; and (4) uneven overlay thickness from inadequate welding parameter control. Each defect type was correlated with specific welding parameter deviations, providing actionable guidance for production quality control.
Engineering Practice Implications
This paper provides a foundational framework for overlay selection in hot rolling mill roll applications. The key principle established is that thermal fatigue and wear resistance must be optimized simultaneously rather than individually. The recommended overlay composition of 15% Cr, 2.0% C, with appropriate manganese and silicon additions, has been widely adopted in Chinese steel mills for roll overlay applications. The study also emphasizes the importance of interface quality control, which remains a common source of overlay failure in industrial practice.
Study Insights and Reflections
Reviewing this 1992 paper, I am struck by the prescience of its approach—the systematic linking of composition to microstructure to performance, combined with practical welding parameter recommendations. The finding that the dilution zone is the weakest link in overlay performance remains as relevant today as it was three decades ago. For modern practitioners, this paper serves as a reminder that overlay design is not merely about selecting a hard material but about engineering a composite system where the overlay, dilution zone, and base metal work together to resist the combined thermal and mechanical demands of hot rolling. The paper's emphasis on defect prevention through welding parameter control is particularly valuable, as it bridges the gap between laboratory research and shop-floor reality.