Submerged Arc Cladding Repair of Cold Rolling Backup Rolls Using Ceramic Flux

Literature Overview

This 1997 paper by Liu Chaojian, Wu Jiyuan, and Liu Jian from Taiyuan Iron and Steel Company (TISCO), published in Welding, presents a technically detailed investigation into the submerged arc cladding repair of cold rolling backup rolls. Backup rolls in cold rolling mills are subjected to extremely high contact stresses (exceeding 3 GPa at the roll gap) and must maintain precise dimensional accuracy to ensure strip quality. The study focuses on the application of ceramic-type flux in submerged arc welding for overlay repair, addressing the specific challenges of maintaining dimensional tolerance, achieving adequate hardness, and ensuring long-term service reliability.

Technical Challenges of Backup Roll Cladding

Cold rolling backup rolls present unique challenges that distinguish them from other roll cladding applications:

The combination of dimensional accuracy requirements with thick overlay deposition creates a fundamental tension: thick weld deposits inherently produce greater distortion and residual stress, while dimensional accuracy demands minimal geometric deviation. The authors addressed this through careful process design and post-weld machining strategy.

Ceramic Flux Characteristics and Selection

The selection of ceramic-type flux (as opposed to granular flux) was a critical process decision that significantly influenced overlay quality. Ceramic fluxes consist of sintered or fused flux material that melts into a thin, fluid slag layer rather than a thick granular slag pool.

Flux Comparison for Roll Cladding

The authors demonstrated that ceramic flux produced overlays with significantly lower hydrogen content (typically below 1.5 ml/100g compared to 3-5 ml/100g with granular flux), which was critical for preventing delayed hydrogen cracking in the high-carbon overlay material. Additionally, the thinner slag layer produced a more uniform heat input distribution, reducing local thermal gradients and consequently minimizing residual stress and distortion.

Process Design and Parameter Optimization

The cladding process was designed as a multi-stage operation to balance deposition rate with quality control:

Stage 1: Surface Preparation and Base Cleaning

• Shot blasting to Sa 2.5 standard
• Induction heating preheat to 250-300°C (uniform around circumference)
• Verification of base metal hardness (should be 25-35 HRC for typical 50Mn2 or equivalent)

Stage 2: Transition Layer (1-2 mm)

• Filler wire: H08Mn2SiA (low carbon, low alloy)
• Flux: Ceramic type
• Current: 500-550 A (DC, electrode negative)
• Voltage: 32-35 V
• Travel speed: 400-450 mm/min
• Wire feed: 5-6 m/min
• Purpose: Ensure metallurgical compatibility and reduce dilution in subsequent passes

Stage 3: Build-up Layer (3-5 mm)

• Filler wire: H08CrMoSiA (medium carbon, Cr-Mo alloy)
• Flux: Ceramic type
• Current: 550-650 A
• Voltage: 33-37 V
• Travel speed: 350-450 mm/min
• Wire feed: 5.5-6.5 m/min
• Purpose: Build to required thickness with controlled microstructure

Stage 4: Hard Facing Layer (2-3 mm)

• Filler wire: H10Cr30 or H08Cr28SiMn (high Cr, medium-high C)
• Flux: Ceramic type
• Current: 500-600 A
• Voltage: 30-34 V
• Travel speed: 350-400 mm/min
• Wire feed: 5-6 m/min
• Purpose: Achieve target surface hardness of 55-62 HRC

Microstructural Evolution and Property Analysis

The multi-layer approach produced a graded microstructure that was critical for achieving the required combination of properties:

The hardness gradient from 28 HRC at the base to 55-62 HRC at the surface created a favorable stress distribution under contact loading. The gradual hardness increase prevented stress concentration at the interface that would occur with a sharp hardness discontinuity, thereby improving fatigue life and reducing the risk of spalling.

Residual Stress Analysis

Residual stress measurements using the hole-drilling method revealed compressive residual stresses of 80-150 MPa in the near-surface region of the overlay, which was beneficial for fatigue resistance. The transition from compressive to tensile stress occurred at approximately 2-3 mm depth, coinciding with the transition from the hard facing layer to the build-up layer. This favorable stress profile was attributed to the ceramic flux's ability to produce a more uniform thermal cycle, reducing thermal gradients and consequently reducing residual tensile stresses.

Post-Weld Treatment and Quality Verification

The post-weld treatment sequence was critical for achieving the final dimensional and mechanical properties:

1. Stress relief: 550-600°C for 4-6 hours (depending on roll diameter) in a controlled atmosphere furnace
2. Cooling: furnace cool to below 200°C before air cooling
3. Rough grinding: remove 1.5-2.0 mm to eliminate surface defects and establish dimensional accuracy
4. Final grinding: achieve target roundness (< 0.01 mm) and surface finish (Ra < 0.4 μm)
5. Final inspection: hardness, dimensional, and surface quality verification

Quality verification criteria were stringent:

Study Insights and Reflections

This paper represents a mature application of weld overlay technology to a demanding industrial application. The authors' systematic approach—combining flux selection optimization, multi-layer design, and rigorous quality control—demonstrates the level of technical sophistication achievable in industrial repair operations. The selection of ceramic flux, while increasing material cost by 3-5 times, was justified by the superior overlay quality, reduced hydrogen content, and improved dimensional stability that it provided.

The multi-layer graded approach deserves particular emphasis as a design philosophy that transcends this specific application. The principle of creating gradual property transitions rather than sharp interfaces is fundamental to preventing stress concentration and improving component life. This approach should be considered standard practice for all thick overlay applications where the overlay hardness significantly exceeds the base metal hardness.

The dimensional control strategy—depositing generous overlay thickness followed by precision grinding—is pragmatic and effective. While this approach consumes additional material, it provides a margin for correction of any minor geometric imperfections that may develop during welding, ensuring that the final product meets the stringent dimensional requirements of cold rolling backup rolls.