Zhuojin Pipe Fitting Co., LtdStrength and Toughness Characteristics of Large Support Roller Overlay Deposits Using RZ-11 Flux-Cored Wire
Literature Overview
Published in Metal Products (2026, Vol. 52, No. 4, pp. 56–59), this study by Liu Shibin, Dong Yanchun, and colleagues from the National Key Laboratory of High-Performance Roller Materials and Composite Forming investigates the mechanical properties of overlay deposits on large forged steel support rollers using a proprietary RZ-11 high-alloy flux-cored wire with submerged arc surfacing. The research was supported by multiple Hebei Provincial funding programs focused on laser additive manufacturing and roller surface engineering. The work addresses the critical challenge of balancing strength and toughness in support roller overlay deposits used in heavy plate rolling mills.
Technical Background
Large forged steel support rollers in hot strip mills experience extreme combined loading conditions:
- Heavy radial contact loads from work rolls
- Thermal cycling from hot strip contact
- Abrasive wear from scale and oxide particles
- Cyclic fatigue from continuous rolling operations
The overlay deposit must provide wear resistance while maintaining sufficient toughness to resist cracking under impact and cyclic loading.
Microstructural Analysis
Deposit Microstructure
The overlay deposit microstructure consists of:
- Primary phase: Lath martensite (dominant)
- Secondary phase: Retained austenite distributed between martensite laths
This microstructure is characteristic of high-carbon, high-alloy martensitic steels that undergo rapid cooling during surfacing operations. The lath martensite morphology (as opposed to plate martensite) suggests a moderate carbon content in the martensite, which is generally associated with better toughness than plate martensite.
Hardness Distribution
| Location | Hardness (HRC) | Characteristics |
|---|---|---|
| Surface layer | 50–55 | High hardness for wear resistance |
| Transition zone | Gradual decrease | Stress buffering |
| Base metal interface | Lower | Dilution zone |
The surface hardness of 50–55 HRC provides excellent wear resistance against scale and oxide abrasion in the rolling mill environment.
Mechanical Properties
Tensile Properties
The maximum tensile strength achieved is 1260 MPa, which is exceptionally high for a weld overlay deposit. This strength level approaches that of quenched and tempered high-strength steels, indicating that the RZ-11 wire composition is designed to produce a high-strength martensitic structure.
Impact Properties
| Temperature | Impact Energy | Fracture Mode |
|---|---|---|
| Room temperature | 3.0–5.0 J | Quasi-cleavage with minor shallow dimples |
The extremely low room temperature impact energy (3.0–5.0 J) represents the critical limitation of this overlay system. This level of toughness is concerning for applications involving impact loading or thermal shock.
Fracture Analysis and Failure Mechanism
Fracture Morphology
The fracture surface exhibits a mixed morphology:
- Dominant mode: Quasi-cleavage fracture (indicating brittle behavior)
- Minor features: Small, shallow dimples (indicating localized ductile tearing)
This mixed morphology confirms that the material fails primarily by cleavage with limited ductile tearing capability.
Root Cause Analysis of Low Toughness
The low impact energy is attributed to two primary factors:
- Inherent brittleness of high-carbon martensite: The high carbon content in the martensite promotes cleavage fracture by:
- Reducing dislocation mobility
- Promoting {100} cleavage plane formation
- Creating high internal stresses from carbon supersaturation
- Welding residual stress: The rapid cooling during surfacing creates significant tensile residual stresses that:
- Lower the effective fracture toughness
- Promote crack initiation at microstructural defects
- Reduce the energy required for crack propagation
Strength-Toughness Trade-off
| Property | Value | Assessment |
|---|---|---|
| Tensile strength | 1260 MPa | Excellent |
| Surface hardness | 50–55 HRC | Excellent |
| Impact energy | 3.0–5.0 J | Poor |
| Fracture mode | Quasi-cleavage | Brittle |
This represents a classic strength-toughness trade-off where the composition and process parameters have been optimized for maximum strength and hardness at the expense of toughness.
Process Improvement Recommendations
Based on the study findings, the following approaches can improve the toughness-strength balance:
Composition Optimization
| Adjustment | Expected Effect | Risk |
|---|---|---|
| Reduce carbon content by 0.1–0.2% | Improved toughness | Slightly reduced hardness |
| Add Ni (2–3%) | Enhanced ductility | Potential for retained austenite instability |
| Add Mo (1–1.5%) | Grain refinement + secondary hardening | Increased cost |
| Reduce Cr slightly | Improved toughness | Slightly reduced corrosion resistance |
Process Optimization
- Post-weld tempering: Temper at 550–600°C for 2–4 hours to convert brittle martensite to tempered martensite, potentially improving impact energy to 20–30 J while maintaining acceptable hardness (45–48 HRC).
- Multi-pass with tempering between passes: Reduce residual stress accumulation through controlled thermal cycling.
- Preheat optimization: Increase preheat to 250–300°C to slow cooling rate and promote more favorable microstructure.
Engineering Application Assessment
PDCA Cycle for Implementation
| Phase | Action | Verification |
|---|---|---|
| Plan | Define target properties: ≥40 HRC, ≥20 J impact, ≥1100 MPa tensile | Property specification |
| Do | Apply RZ-11 wire with optimized parameters + tempering | Process execution |
| Check | Test hardness, impact, tensile on coupon samples | Property verification |
| Act | Adjust composition/parameters based on test results | Iterative improvement |
Application Suitability
The current RZ-11 overlay deposit is suitable for:
- Support rollers with primarily static or slowly varying loads
- Applications where wear resistance is the dominant requirement
- Environments without significant impact loading
The overlay requires modification for:
- Applications with thermal shock (rapid temperature changes)
- Impact loading conditions (e.g., slab casting rollers)
- Fatigue-critical applications requiring high fracture toughness
Study Insights and Conclusions
This research provides critical data on the strength-toughness characteristics of RZ-11 flux-cored wire overlay deposits. The findings clearly demonstrate that while the current formulation achieves excellent strength and hardness, the toughness is insufficient for demanding applications. The quasi-cleavage fracture mode with minimal ductile tearing indicates that the material has reached its inherent toughness limit for the given composition and processing conditions.
The study's most valuable contribution is the clear identification of the factors limiting toughness: high-carbon martensite brittleness and welding residual stress. This provides a clear roadmap for improvement through either composition modification (reducing carbon, adding toughness-enhancing elements) or process optimization (tempering, controlled cooling).
For engineering practice, the key takeaway is that strength and toughness in overlay deposits cannot be optimized simultaneously through process parameters alone—composition modification is essential for achieving both high strength and adequate toughness. The RZ-11 wire represents an excellent starting point for developing the next generation of support roller overlay materials that can withstand the demanding conditions of modern heavy plate rolling mills.