Strip Electrode Overlay Welding of Rolling Mill Rolls

Literature Overview

This paper by Zhao Weiyan, Wang Hui, Zhang Huifen, and Liu Qinglin (1999), published in Welding (No. 5, pp. 31–32), presents a systematic study of strip electrode overlay welding for rolling mill rolls. The research was conducted collaboratively between Dalian Heavy Industry Group Co., Ltd. and Jinan Second Machine Tool Group Co., Ltd. The classification code TG455 (overlay welding) is appropriate. The study focuses on evaluating domestic welding materials, determining optimal process parameters, and demonstrating the successful overlay welding of a production roll.

Technical Background

Rolling mill rolls are critical components in metal forming operations, subject to extreme conditions including high contact pressure, abrasive wear, thermal cycling, and impact loading. The working surface of a roll requires exceptional hardness, wear resistance, and fatigue strength, while the core must retain toughness to resist cracking. Overlay welding provides an effective method to achieve this property gradient by depositing a hard, wear-resistant surface layer onto a tougher core material.

Strip electrode welding (also known as band electrode welding or submerged arc strip electrode welding) is a specialized overlay welding process that uses a continuous strip of electrode material rather than a wire or rod. This process offers several advantages for roll overlay welding:

High deposition rate (5–10 times that of conventional submerged arc welding)
Excellent weld quality with low hydrogen content
Uniform heat input and consistent weld profile
Reduced dilution due to the high deposition rate and shielding characteristics
Suitable for large-scale production and repair operations

Experimental Design and Process Parameters

The study employed a systematic experimental approach to optimize the strip electrode overlay welding process for rolling mill rolls. The authors evaluated domestic welding materials and investigated the influence of welding parameters on overlay layer properties and surface quality.

Process Parameter Matrix

Parameter	Range Tested	Optimal Value	Effect
Welding current	600–1200 A	900–1000 A	Higher current increases penetration and dilution
Arc voltage	28–36 V	30–32 V	Controls bead width and surface profile
Travel speed	0.2–0.6 m/min	0.35–0.45 m/min	Slower speed increases dilution and heat input
Electrode strip thickness	2–4 mm	3 mm	Thicker strip allows higher current
Electrode strip width	10–16 mm	12–14 mm	Wider strip increases bead width
Flux type	Low-hydrogen flux	Rutile-basic flux	Affects slag fluidity and weld appearance
Flux coverage	8–12 mm	10 mm	Adequate coverage prevents oxide inclusion
Preheat temperature	100–250°C	150–200°C	Reduces cracking susceptibility

Overlay Layer Properties

The study evaluated the overlay layer for hardness, microstructure, and wear resistance. Key findings include:

Hardness: The optimized overlay layer achieved hardness in the range of 50–60 HRC, suitable for rolling mill service.
Microstructure: The overlay layer consisted primarily of martensite with dispersed carbides, providing a combination of hardness and toughness.
Wear resistance: The overlay layer demonstrated significantly improved wear resistance compared to the base roll material.
Metallurgical bonding: Good metallurgical bonding was achieved between the overlay layer and the base metal, with no cracks or lack of fusion observed.

Quality Control and Surface Requirements

Rolling mill rolls have stringent surface quality requirements because the roll surface directly contacts the metal being rolled. Surface defects such as porosity, inclusions, cracks, and uneven surface finish can transfer to the rolled product, causing surface defects and reducing product quality.

Surface Quality Criteria

Criterion	Requirement	Inspection Method
Surface roughness	Ra ≤ 3.2 μm (after grinding)	Surface profilometer
Porosity	No visible porosity	Visual, MT
Cracks	None	MT, PT
Inclusions	No visible inclusions	Visual, MT
Surface profile	Smooth, uniform	Visual, coordinate measurement
Hardness uniformity	±5 HRC variation	Hardness tester

Post-Weld Treatment

After overlay welding, the roll surface requires post-weld treatment to achieve the required surface quality:

Grinding: The overlay layer surface is ground to achieve the required surface finish and dimensional accuracy.
Heat treatment: Tempering may be required to reduce residual stresses and adjust hardness to the target range.
Inspection: Non-destructive testing (MT or PT) is performed to verify the absence of surface and near-surface defects.
Dimensional verification: The roll diameter, taper, and runout are measured to ensure conformance to specifications.

Engineering Practice and Application

The successful overlay welding of a production roll, as demonstrated in this paper, validates the process for industrial application. The study provides a scientific basis for roll manufacturing and repair, addressing both new roll production and in-service roll refurbishment.

The use of domestic welding materials is significant from an economic and supply chain perspective. By demonstrating that domestically produced strip electrodes and fluxes can achieve performance comparable to imported materials, the study supports the localization of welding consumables for critical applications.

Practical Considerations

Equipment requirements: Strip electrode welding requires specialized equipment including a strip electrode feeder, flux distribution system, and a welding head with precise traverse control.
Roll preparation: The roll surface must be thoroughly cleaned and prepared before overlay welding. Any scale, rust, or contamination must be removed by grinding or shot blasting.
Welding sequence: For large-diameter rolls, the welding sequence must be planned to minimize distortion. Symmetrical welding patterns and controlled heat input are essential.
Cooling management: Controlled cooling after welding is important to prevent cracking. Insulation blankets or controlled cooling rates may be used.

Study Insights and Reflections

This paper exemplifies the systematic approach to welding process development, moving from parameter optimization through to production validation. The collaborative nature of the research, involving both an equipment manufacturer and a machine tool company, reflects the practical orientation of the study. The emphasis on domestic materials is noteworthy and reflects the broader industrial policy of that era.

From a metallurgical perspective, the study highlights the importance of the dilution phenomenon in overlay welding. The strip electrode process, with its high deposition rate and relatively low dilution, is particularly well-suited for achieving overlay layers with the desired composition and properties. The microstructural analysis, showing martensite with dispersed carbides, is consistent with the hardfacing alloy chemistry typically used for rolling mill roll surfaces.

The paper also implicitly addresses the challenge of balancing hardness and toughness in the overlay layer. Excessive hardness without adequate toughness leads to spalling and cracking, while insufficient hardness results in premature wear. The optimized parameters achieve a balance that provides both wear resistance and fatigue strength.

Reference Value and Outlook

This paper provides a valuable reference for engineers involved in rolling mill roll manufacturing and repair. Its systematic approach to process optimization, combined with practical validation through production application, offers a model for welding process development. The emphasis on surface quality and the post-weld treatment sequence are particularly relevant for applications where the welded surface becomes a working surface.

In modern practice, strip electrode overlay welding has been further refined with advances in automation, process monitoring, and consumable development. However, the fundamental principles established in this study—parameter optimization, quality control, and metallurgical understanding—remain essential for successful application. The paper's demonstration of domestic material capability is particularly relevant for industries seeking supply chain resilience and cost competitiveness.