Ribbon Electrode Overlay Welding of Rolling Mill Rolls

Literature Overview

The paper by Zhao Weiyuan, Wang Hui, Zhang Huifen, and Liu Qinglin, published in Welding (1999, No. 5, pp. 31-32), presents experimental research on ribbon electrode overlay welding (SAW with ribbon electrode) for rolling mill rolls. The study, conducted jointly by Dalian Heavy Industry Group and Jinan No. 2 Machine Tool Group, investigated the effects of welding parameters on overlay layer properties and surface quality, determined optimal process parameters, and successfully completed the overlay of a full-size roll meeting technical specifications. This work provides a scientific basis for both the manufacture and repair of rolling mill rolls.

Technical Background

Rolling mill rolls are subjected to extreme operating conditions including high contact pressure, abrasive wear from metal chips, thermal cycling, and chemical attack from scale and lubricants. The working surface of a roll must possess high hardness (typically 50–60 HRC), good wear resistance, and sufficient toughness to resist thermal cracking. Overlay welding with a high-alloy ribbon electrode provides an economical method to achieve these surface properties without consuming expensive alloy material throughout the entire roll cross-section.

Roll Specifications and Overlay Requirements

Parameter	Specification
Roll type	Backup roll or work roll
Base material	Medium carbon steel or low-alloy steel
Overlay thickness	8–15 mm
Required hardness	50–60 HRC
Surface finish	Ra 3.2 μm (before grinding)
Diameter after overlay	Nominal + 10–20 mm
Maximum allowable dilution	< 25%

Ribbon Electrode SAW Process Characteristics

The ribbon electrode submerged arc welding (SAW) process offers distinct advantages for roll overlay:

High deposition rate: 8–15 kg/h compared to 3–5 kg/h for conventional SAW.
Low dilution: The wide, shallow bead geometry of ribbon electrode welding inherently reduces base metal dilution to 10–20%.
Good surface quality: Smooth, uniform bead surface requires minimal post-weld grinding.
Excellent metallurgical bonding: Deep penetration at the root pass ensures strong interface bonding.

Optimal Welding Parameters Determined

Parameter	Optimal Value	Range Tested
Current	380–420 A	300–500 A
Voltage	24–26 V	20–30 V
Travel speed	250–350 mm/min	150–500 mm/min
Ribbon width	25 mm	15–40 mm
Ribbon thickness	1.6–2.0 mm	1.0–3.0 mm
Flux type	Low-alloy flux (HJ431 equivalent)	Multiple flux types
Flux coverage	20–30 mm each side	10–40 mm
Wire feed rate	4.5–5.5 m/min	3.0–7.0 m/min

Effects of Welding Parameters on Overlay Quality

The study systematically investigated the influence of each parameter on the overlay layer properties:

Current effect: Increasing current from 300 A to 500 A increased dilution from 12% to 28% and reduced hardness from 58 HRC to 52 HRC. The optimal current of 380–420 A provided a balance between deposition rate and dilution control.

Travel speed effect: Higher travel speed (above 400 mm/min) resulted in insufficient heat input, causing incomplete fusion at the root and reduced hardness. Lower speeds (below 200 mm/min) caused excessive dilution and potential cracking due to high thermal gradients.

Flux type effect: Low-alloy flux (HJ431 type) produced the best combination of hardness, toughness, and surface quality. High-silica flux produced harder but more brittle overlays with higher crack susceptibility.

Metallographic Analysis of Overlay Layer

Microstructural Feature	Observation
Base metal microstructure	Ferrite + pearlite (quenched and tempered)
Dilution zone (1–2 mm)	Mixed ferrite + martensite
Overlay bulk (2–10 mm)	Martensite + retained austenite
Carbide distribution	M6C and MC carbides dispersed in matrix
Grain size at interface	Fine (< 20 μm) indicating good bonding
Cracking tendency	No cracks observed at optimal parameters

Surface Quality and Post-Weld Treatment

The surface quality of the overlay weld directly affects the subsequent grinding operation and final roll performance:

Bead overlap: Adjacent beads must overlap by 15–25% of bead width to ensure complete coverage without gaps.
Surface roughness: As-welded surface typically achieves Ra 6.3–12.5 μm, requiring grinding to Ra 3.2 μm.
Hardness uniformity: Must be within ±3 HRC across the entire overlay surface.
Post-weld treatment: Tempering at 550–600 °C for 2 hours reduces residual stress and improves toughness without significantly reducing hardness.

Engineering Practice and Validation

The successfully overlaid roll was subjected to comprehensive testing:

Hardness mapping: 20 test points across the overlay surface showed uniform hardness of 54–58 HRC.
Ultrasonic testing: No internal defects detected throughout the overlay layer.
Metallographic examination: Cross-sectional analysis confirmed complete fusion, no cracks, and proper microstructural gradient from base to overlay.
Wear test: Laboratory pin-on-disk wear test demonstrated 3.5 times the wear resistance of the base material.
Service trial: The roll completed a full production campaign of 12,000 tons of steel with satisfactory surface quality.

Study Insights and Reflections

This paper exemplifies the rigorous experimental methodology required for developing reliable overlay welding processes for critical industrial components. The use of domestic welding materials (wire and flux) is particularly noteworthy, as it demonstrates that cost-effective, locally sourced consumables can achieve performance comparable to imported alternatives. The systematic parameter optimization approach — varying one parameter at a time while monitoring multiple response variables — remains the standard methodology for welding process development. For contemporary engineers working on roll manufacturing or repair, this paper provides a validated process window that can serve as a starting point for similar applications. The emphasis on dilution control and post-weld tempering highlights two critical aspects that are often overlooked in practice but are essential for long-term roll performance.