CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Wear-Resistant Overlay Welding Electrodes for Hot Rolling Rolls
Literature Overview
This 1997 study by Ying Pengzhan, Ge Changlu, and Cai Yingjun from the China University of Mining and Technology, funded by the Ministry of Coal Industry, addresses the development of specialized wear-resistant overlay welding electrodes for hot rolling mill rolls. Hot rolling mills are critical equipment in steel production, and the rolls experience severe wear due to the combination of high temperature, high pressure, abrasive scale, and thermal fatigue. The replacement and refurbishment of rolls is a significant cost center in steel production, and the development of effective overlay welding consumables can substantially extend roll service life and reduce production costs.
Technical Background and Wear Mechanisms
Hot rolling mill rolls are subjected to a complex combination of wear mechanisms that vary depending on the rolling stage (roughing, intermediate, or finishing), the steel grade being rolled, and the operating conditions. The primary wear mechanisms include:
| Wear Mechanism | Description | Severity |
|---|---|---|
| Abrasive Wear | Removal of material by hard scale particles and oxide debris | High |
| Adhesive Wear | Transfer of material between roll and strip due to high pressure | Moderate |
| Oxidative Wear | Chemical attack by scale and atmosphere at elevated temperatures | Moderate |
| Thermal Fatigue | Cracking due to cyclic thermal stresses from strip contact | High |
| Corrosive Wear | Chemical interaction with scale and rolling oils | Low to Moderate |
The roll surface temperature can reach 800–1200 °C during hot rolling, and the contact pressure can exceed 1000 MPa. The scale formed on the hot steel strip is composed primarily of iron oxides (FeO, Fe3O4, Fe2O3) with a Mohs hardness of 6–8, which is significantly harder than the roll surface material. This creates a severe abrasive wear environment that demands overlay materials with exceptional hardness and wear resistance.
Electrode Development and Metallurgical Design
The authors developed a series of wear-resistant overlay welding electrodes with different compositions tailored to specific rolling applications. The electrode design philosophy was based on achieving a balance between hardness, toughness, thermal fatigue resistance, and red hardness (retention of hardness at elevated temperatures).
The electrode compositions investigated included:
| Electrode Type | Composition (wt%) | Hardness (HRC) | Application |
|---|---|---|---|
| Type A (Martensitic) | C 2.5-3.5, Cr 12-18, Mo 3-5 | 55-62 | Roughing rolls |
| Type B (Austenitic) | C 0.8-1.5, Cr 18-25, Ni 12-18 | 40-48 | Intermediate rolls |
| Type C (High-Cr High-C) | C 3.0-4.0, Cr 20-28, Mo 5-8 | 60-68 | Finishing rolls |
| Type D (Ni-Cr-Mo) | C 1.5-2.5, Cr 15-20, Ni 8-12, Mo 4-6 | 50-58 | General purpose |
The electrode coating composition was carefully designed to ensure good welding characteristics (arc stability, low spatter, good slag coverage) while delivering the desired overlay properties. The coating typically consisted of the alloying elements (Cr, Mo, Ni, W) combined with deoxidizers (Si, Mn, Ti) and fluxing agents (CaF2, CaCO3, MgO).
The microstructure of the deposited overlay was analyzed using optical microscopy, SEM, and X-ray diffraction (XRD). The martensitic electrodes (Type A and C) produced overlays with a microstructure consisting of tempered martensite with dispersed carbides (M7C3, M23C6). The austenitic electrodes (Type B) produced overlays with a duplex microstructure of austenite and martensite, with the austenite fraction controlled by the carbon and nickel content. The high-chromium electrodes (Type C) produced overlays with a high volume fraction of carbides (up to 40-50 vol%), providing exceptional abrasive wear resistance.
Performance Evaluation and Field Testing
The developed electrodes were evaluated through laboratory testing and field trials on actual hot rolling mill rolls. The laboratory testing included hardness profiling, microstructure analysis, and simulated wear testing using a pin-on-disk tribometer with iron scale abrasive media. The field trials involved refurbishing worn rolls and monitoring their service life in production rolling mills.
The results demonstrated significant improvements in roll service life:
| Roll Type | Original Service Life | With Overlay | Improvement |
|---|---|---|---|
| Roughing Roll | 80-120 hours | 200-300 hours | 2.0-2.5x |
| Intermediate Roll | 150-200 hours | 350-500 hours | 2.0-2.5x |
| Finishing Roll | 60-100 hours | 180-280 hours | 2.5-3.0x |
The overlay layers maintained their hardness and wear resistance throughout the service life, with only minor softening observed at the roll surface due to thermal cycling. The bond strength between the overlay and the roll base material was verified through shear testing and was found to be adequate for the operating conditions, with no evidence of overlay spalling or delamination during service.
Key Process Considerations
The successful application of these overlay electrodes in hot rolling roll refurbishment requires careful attention to several process parameters:
- Preheating: The roll must be preheated to 200–300 °C to prevent cracking in the base material, which is typically a high-carbon, high-chromium cast iron or forged steel.
- Welding sequence: The overlay should be applied in a spiral pattern to ensure uniform coverage and minimize residual stresses.
- Interpass temperature: Must be maintained below 300 °C to prevent excessive softening of the previously deposited overlay.
- Post-weld treatment: Light tempering at 200–300 °C may be applied to reduce residual stresses without significantly reducing overlay hardness.
- Surface preparation: The worn roll surface must be cleaned and profiled to ensure adequate bond strength; a minimum roughness of Ra 6.3 μm is recommended.
Key Questions and Reflections
The study raises several important considerations for the steel industry. First, the thermal fatigue resistance of the overlay layers under the cyclic thermal loading of hot rolling conditions requires further investigation. The thermal fatigue cracking observed at the overlay/substrate interface in some field trials suggests that the thermal expansion mismatch between the overlay and the base material may be a limiting factor. Second, the effect of the overlay on the roll's thermal conductivity and heat dissipation characteristics should be evaluated, as excessive overlay thickness may impair the roll's cooling performance. Third, the environmental impact of the electrode coating materials, particularly the fluorine-containing fluxing agents, should be considered in the context of modern environmental regulations.
Study Insights and Implications
This study demonstrates the practical value of tailored welding consumable development for addressing specific industrial wear problems. The systematic approach to electrode design—balancing hardness, toughness, thermal fatigue resistance, and weldability—provides a model for developing specialized overlay consumables for other demanding applications. The field trial results confirm that overlay welding is an economically viable solution for extending roll service life, with the cost of overlay refurbishment being significantly lower than the cost of roll replacement. For engineers in the steel industry, the key lesson is that the selection of overlay consumables must be matched to the specific wear mechanisms and operating conditions of the application, and that field validation is essential to confirm laboratory predictions.