CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Microstructure and Composition on Properties of Weld Overlay Layer on Hot Rolling Mill Rolls
Literature Overview
This study examines how the microstructure and chemical composition of weld overlay layers influence the mechanical and tribological performance of hot rolling mill work rolls. Hot rolling mill rolls operate under extremely demanding conditions involving high temperatures (typically 900–1200 °C at the roll surface), intense mechanical loading, and continuous contact with hot steel slabs. The overlay layer must therefore provide exceptional wear resistance, thermal fatigue resistance, and spalling resistance while maintaining adequate toughness to prevent catastrophic failure. The literature systematically investigates the relationship between elemental composition, phase constitution, and resulting performance characteristics, offering valuable guidance for weld material selection and process parameter optimization.
Core Technical Findings
Phase Constitution and Microstructure
The overlay layer microstructure is the primary determinant of wear resistance. The key phases identified in high-performance overlay layers include:
| Phase | Composition Range | Hardness (HV) | Wear Resistance Contribution |
|---|---|---|---|
| M₇C₃ carbide | 20–35 wt% Cr | 1400–1800 | Primary wear-resistant phase |
| M₂C carbide | 20–35 wt% Cr | 1200–1500 | Secondary reinforcement |
| M₆C carbide | Lower Cr content | 1000–1300 | Moderate contribution |
| Austenite (γ) | Balanced Cr, Ni | 200–400 | Toughness and thermal fatigue resistance |
| Ferrite (α) | Low alloy | 150–300 | Ductility, lower wear resistance |
The study emphasizes that the relative proportion of these phases is governed by the chromium, carbon, and alloying element content. A chromium content in the range of 20–30 wt% with carbon content of 2.5–4.0 wt% produces a favorable balance of hard carbide phases and a ductile matrix. When chromium exceeds 30 wt%, excessive M₇C₃ carbide precipitation can lead to embrittlement and increased susceptibility to thermal fatigue cracking.
Hardness and Wear Performance
The measured hardness of the overlay layer directly correlates with wear resistance. Typical hardness values for well-designed hot rolling roll overlays range from 55–65 HRC in the as-welded condition. However, the study highlights that hardness alone is insufficient to characterize performance; the distribution and morphology of carbides within the matrix are equally critical. Coarse, irregular carbide networks serve as crack initiation sites, while fine, uniformly distributed carbides provide superior wear resistance without sacrificing toughness.
Thermal Fatigue Behavior
Thermal fatigue is a dominant failure mode in hot rolling roll overlays. The cyclic temperature gradients between the roll surface and the substrate generate alternating tensile and compressive stresses. The study demonstrates that an overlay layer with a balanced austenite-ferrite-cementite structure exhibits superior thermal fatigue resistance compared to fully martensitic or fully carbide structures. The austenite phase acts as a stress-relieving mechanism through its ability to accommodate plastic deformation under thermal cycling.
Process and Standards Analysis
Welding Process Considerations
The choice of welding process significantly affects the overlay microstructure and properties:
| Process | Typical Parameters | Overlay Characteristics | Application Suitability |
|---|---|---|---|
| Submerged Arc Welding (SAW) | Current 500–800 A, Voltage 30–40 V | Thick overlay (3–8 mm), coarse grain | Heavy-duty rolls, high production rate |
| Electroslag Welding (ESW) | Current 800–1500 A, Slag resistance controlled | Uniform microstructure, minimal dilution | Large diameter rolls |
| Gas Metal Arc Welding (GMAW) | Current 200–400 A, Wire speed 5–8 m/min | Fine grain, moderate thickness | Medium-duty rolls |
| Plasma Transferred Arc (PTA) | Current 150–300 A, Powder feed 200–500 g/min | Excellent dilution control, fine microstructure | Precision overlay, alloy roll faces |
The literature notes that SAW and ESW are preferred for industrial-scale hot rolling roll remanufacturing due to their high deposition rates, while PTA is increasingly adopted for applications requiring precise composition control and minimal dilution with the base steel.
Dilution Control
Dilution between the overlay layer and the base steel is a critical parameter. For hot rolling mill rolls, the base steel is typically a medium carbon steel or chromium-molybdenum steel. Excessive dilution (above 15–20%) reduces the effective alloy content in the overlay, degrading wear resistance. The study recommends multi-pass welding with the first pass providing a transition layer, followed by subsequent passes with the final overlay composition. This approach achieves dilution control below 10% in the final overlay layers.
Integration with Engineering Practice
In practical roll remanufacturing operations, the following considerations emerge from the literature:
- Preheating to 200–300 °C is recommended for rolls with high carbon equivalent (CE > 0.4) to prevent cold cracking.
- Interpass temperature should be maintained below 250 °C to ensure adequate hardness retention.
- Post-weld heat treatment (PWHT) at 550–620 °C for 1–2 hours per 25 mm thickness improves toughness but may reduce hardness by 3–5 HRC.
- Surface finish requirements (Ra ≤ 6.3 μm) are critical for roll performance and strip surface quality.
A common failure mode observed in service is spalling, where the overlay layer detaches from the substrate due to thermal fatigue cracking at the interface. The study attributes this to excessive hardness contrast between the overlay and substrate, inadequate interface bonding, or insufficient toughness in the overlay layer itself.
Key Questions and Reflections
The literature raises an important question regarding the optimal trade-off between hardness and thermal fatigue resistance. Maximum wear resistance is achieved with high hardness, but this often comes at the expense of toughness and thermal fatigue life. The practical solution appears to be a layered approach: a hard, carbide-rich surface layer (2–3 mm) over a tougher transition layer (2–4 mm) that provides thermal fatigue resistance and adequate bonding strength.
Another reflection concerns the role of residual stresses. The welding process inevitably introduces residual stresses that, combined with thermal cycling in service, accelerate fatigue crack initiation. The study suggests that controlled peening or shot blasting of the overlay surface can introduce beneficial compressive residual stresses that extend thermal fatigue life by 30–50%.
Study Insights and Implications
The most significant insight from this literature is that overlay performance cannot be optimized by focusing on any single parameter in isolation. The composition-microstructure-properties relationship forms a tightly coupled system where changes in one variable propagate through the entire chain. Engineers must adopt a holistic approach, considering the complete service environment—temperature, load, sliding velocity, and steel grade being rolled—when selecting overlay materials and welding processes. The transition from traditional SAW/ESW processes to advanced PTA technology represents a meaningful advancement in achieving the composition control and microstructural refinement necessary for next-generation hot rolling roll overlays. Understanding these fundamental relationships enables more rational material selection and process design, ultimately extending roll life and reducing production costs in the steel industry.