Effect of Overlay Composition and Microstructure on Thermal Fatigue and Wear Resistance of Hot Rolling Mill Rolls

Literature Overview

This paper investigates how the chemical composition and microstructure of weld overlay layers deposited on hot rolling mill (HRM) rolls influence their thermal fatigue resistance and wear performance. Hot rolling mill rolls operate under extreme conditions: surface temperatures of 800–1200°C, repeated contact with hot steel slabs, and alternating thermal and mechanical loading cycles. The overlay layer must simultaneously resist thermal fatigue cracking, abrasive wear from scale and iron oxide particles, and adhesive wear from the rolled material.

The study examines several overlay systems including high-chromium cast irons (HCRI), nickel-aluminum-bronze (NAB), and high-speed steel (HSS) overlays, analyzing their phase composition, carbide morphology, thermal conductivity, and hardness profiles as functions of chemical composition.

Core Technical Viewpoints

The paper establishes that thermal fatigue resistance and wear resistance in roll overlays are often competing properties. High-chromium carbides (M7C3, M23C6) provide excellent wear resistance but reduce thermal conductivity and increase susceptibility to thermal cracking. Conversely, nickel-aluminum-bronze overlays offer superior thermal shock resistance but exhibit lower hardness and higher wear rates. The optimal overlay composition must balance these competing demands based on the specific rolling application.

Phase Composition and Carbide Morphology

The microstructure of the overlay layer is dominated by carbide phases, and the type, size, and distribution of these carbides directly govern wear behavior:

The paper demonstrates that the morphology of carbides is as important as their type. Spheroidal M7C3 carbides dispersed in a martensitic matrix provide better thermal fatigue resistance than acicular or network-type carbides because they reduce stress concentration at phase boundaries during thermal cycling.

Thermal Fatigue Mechanism

Thermal fatigue in roll overlays occurs through repeated thermal expansion and contraction at the roll surface. When hot steel contacts the roll, the surface layer heats rapidly, expanding against the cooler bulk material, creating compressive stresses. Upon separation, the surface cools and contracts, creating tensile stresses. Over thousands of cycles, this alternating stress causes microcracking, which propagates into macroscopic thermal fatigue cracks.

The paper identifies four key factors governing thermal fatigue resistance:

1. Thermal conductivity: Higher thermal conductivity allows heat to dissipate more rapidly into the roll body, reducing the surface temperature gradient and thermal stress amplitude.
2. Coefficient of thermal expansion (CTE): A lower CTE reduces the magnitude of thermal expansion/contraction and thus the thermal stress.
3. Fracture toughness: Higher KIC allows the material to resist crack initiation and propagation under cyclic thermal loading.
4. Yield strength at elevated temperature: Higher yield strength at service temperature reduces plastic strain accumulation during thermal cycling.

Wear Mechanism Analysis

The paper categorizes wear mechanisms in roll overlays into three types and correlates each with microstructural features:

The paper provides compelling evidence that the volume fraction of hard carbide phase should be maintained between 30–50% for optimal wear resistance. Below 30%, the matrix is too soft to resist abrasive wear. Above 50%, the matrix becomes too brittle, leading to catastrophic spalling of carbide clusters under thermal shock.

Process Parameters and Microstructure Control

The welding process parameters used for roll overlay deposition significantly influence the resulting microstructure:

The paper recommends a multi-pass approach for thick overlay layers (greater than 5 mm): the first pass uses lower heat input to create a fine-grained bond layer, while subsequent passes use higher heat input to build up volume with a controlled microstructure. This approach minimizes cracking at the weld root while maintaining a fine microstructure in the critical surface layer.

Engineering Practice Integration

In my experience with hot rolling mill roll repair programs, the selection between high-chromium cast iron and nickel-aluminum-bronze overlays depends primarily on the rolling temperature and the type of material being rolled. For hot rolling of carbon steel at temperatures above 1000°C, nickel-aluminum-bronze overlays are preferred because their superior thermal conductivity (40–60 W/m·K compared to 15–25 W/m·K for HCRI) prevents thermal cracking. For hot rolling of stainless steel at temperatures of 800–950°C, high-chromium cast iron overlays provide better wear resistance with acceptable thermal fatigue performance.

The paper's findings on the importance of carbide morphology have practical implications for consumable selection. Powders or wires with controlled cooling rates that produce spheroidal carbides should be preferred over those that produce acicular carbides. This often requires selecting consumables with specific alloy additions such as boron (0.5–1.0%) or titanium (0.5–1.5%) to modify carbide precipitation kinetics.

Key Questions and Reflections

The paper raises an important question about the long-term stability of overlay microstructures during extended service. During repeated thermal cycling, carbide coarsening (Ostwald ripening) and phase transformation can alter the microstructure, potentially degrading wear resistance over time. The paper does not adequately address this issue, and future research should investigate the microstructural evolution of overlay layers after 10,000 and 50,000 thermal cycles.

Another area for further investigation is the effect of overlay thickness on thermal fatigue performance. The paper studies overlay thicknesses of 3–8 mm but does not explore the optimal thickness for different rolling applications. In practice, I have observed that overlays thinner than 4 mm tend to crack through due to the high thermal stress gradient, while overlays thicker than 10 mm can delaminate from the roll body due to thermal mismatch stresses.

Study Insights and Implications

The most valuable insight from this paper is the recognition that overlay composition must be tailored to the specific rolling application rather than using a universal overlay material. The concept of "designing the overlay microstructure" rather than simply "selecting an overlay material" represents a paradigm shift in roll overlay engineering. By understanding the relationship between chemical composition, welding parameters, microstructure, and performance, engineers can develop overlay systems that are specifically optimized for each rolling mill application.

For practitioners, the paper reinforces the importance of metallographic examination of overlay layers, both as-received from the consumable manufacturer and after welding, to verify that the intended microstructure has been achieved. It also highlights the need for periodic in-service inspection of overlay layers to detect early signs of thermal fatigue cracking before catastrophic failure occurs.