CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Technology of φ850 Steel Rolling Mill Rolls Using Sintered Alloy Flux
Literature Overview
This technical study focuses on the overlay welding repair of φ850 steel rolling mill rolls using sintered alloy flux, addressing a significant industrial need in the steel rolling industry. Rolling mill rolls are critical consumables that undergo severe plastic deformation, friction, and thermal cycling during hot rolling operations. The working surface of rolls typically requires high hardness (HRC 50-60), excellent wear resistance, and sufficient toughness to resist spalling. The use of sintered alloy flux in submerged arc welding (SAW) provides a cost-effective and efficient method for applying wear-resistant overlay layers on large-diameter rolls.
Core Technical Analysis
Sintered Alloy Flux Selection
Sintered alloy flux differs fundamentally from fused flux in its composition, melting behavior, and metallurgical effects. The sintered flux is typically composed of:
- Mineral components: SiO2, CaO, Al2O3, TiO2 (providing slag chemistry for weld protection and arc stability)
- Alloying powder: Cr, Mo, V, W, Co, C (providing hardening elements to the weld metal)
- Binder: Sodium silicate or calcium carbonate (holding the mixture together before sintering)
The sintering process (typically at 900-1100°C) creates a porous, low-density flux with a melting range of 1050-1350°C, which is significantly lower than fused flux (1500-1700°C). This lower melting point results in a thinner, more fluid slag layer that promotes better arc penetration and allows for more precise control of alloy transfer to the weld metal.
The study evaluates several sintered flux formulations:
| Flux Type | Alloy Composition (wt%) | Resulting Weld Hardness (HRC) | Wear Resistance Index |
|---|---|---|---|
| Cr-Mo type | 8-10% Cr, 3-4% Mo, 0.8-1.2% C | 48-52 | 1.0 (baseline) |
| Cr-V type | 6-8% Cr, 1.5-2.0% V, 0.8-1.2% C | 52-56 | 1.4 |
| Cr-Mo-V type | 8-10% Cr, 2-3% Mo, 1.0-1.5% V, 0.8-1.2% C | 55-58 | 1.8 |
| Cr-Mo-W type | 8-10% Cr, 2-3% Mo, 3-5% W, 0.8-1.2% C | 56-60 | 2.2 |
| Cr-Mo-Co type | 8-10% Cr, 2-3% Mo, 5-8% Co, 0.8-1.2% C | 58-62 | 2.5 |
Process Parameters for φ850 Roll Overlay
The overlay welding of a φ850 roll presents unique challenges due to its large diameter, cylindrical geometry, and the need for uniform hardness distribution around the circumference. The study employs the following process parameters:
| Parameter | Range | Typical Value |
|---|---|---|
| Wire diameter | 2.0-3.2 mm | 2.6 mm |
| Wire composition | High-C, high-Cr/Mo | C 1.0-1.3%, Cr 8-10%, Mo 2-3% |
| Current (DC+) | 300-500 A | 400 A |
| Voltage | 26-34 V | 30 V |
| Travel speed | 120-200 mm/min | 160 mm/min |
| Weld bead width | 10-15 mm | 12 mm |
| Overlap ratio | 30-50% | 40% |
| Number of passes | 2-3 | 3 |
| Interpass temperature | ≤ 250°C | 200°C |
| Preheat temperature | 100-150°C | 120°C |
The DC+ polarity is preferred for SAW overlay welding as it provides deeper penetration and better arc stability. The multi-pass approach ensures adequate overlay thickness (typically 3-5 mm for roll repair) while maintaining a controlled dilution ratio of 5-15%.
Microstructure and Hardness Distribution
The overlay weld metal exhibits a complex microstructure characteristic of high-carbon, high-alloy martensitic steels:
- Primary phase: Retained austenite (γ) matrix with high carbon content (0.8-1.2%)
- Secondary phases: Cementite (Fe3C), chromium carbides (Cr7C3, Cr23C6), molybdenum carbides (Mo2C), and vanadium carbides (VC)
- Microstructure morphology: Lath martensite with fine carbide precipitation, retained austenite in interlath regions
The hardness distribution across the roll circumference is critical for uniform rolling performance. The study demonstrates that with proper process control, the hardness variation around the circumference can be maintained within ±3 HRC, which is acceptable for most hot rolling applications.
The hardness profile through the overlay thickness shows:
- Surface layer (0-0.5 mm): 55-60 HRC (highest hardness due to rapid cooling)
- Mid-layer (0.5-2.0 mm): 50-55 HRC
- Transition zone (2.0-3.0 mm): 40-48 HRC
- Base metal: 25-30 HRC
Quality Control and Inspection
Non-Destructive Testing
The overlay welds on rolling mill rolls must be inspected for defects that could lead to roll failure during service. The recommended NDT approach includes:
| NDT Method | Purpose | Acceptance Criteria |
|---|---|---|
| Magnetic Particle Testing (MT) | Surface cracks, undercut | No linear indications > 0.5 mm |
| Ultrasonic Testing (UT) | Internal porosity, lack of fusion | Per GB/T 11345, level B |
| Hardness Testing | Uniformity verification | ±5 HRC variation across surface |
| Penetrant Testing (PT) | Surface-breaking defects | No indications > 0.3 mm |
Common Defects and Their Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Undercut | Excessive travel speed, low current | Reduce travel speed by 10-15%, increase current |
| Porosity | Flux moisture, base metal contamination | Dry flux at 200°C for 2 hours, clean base metal thoroughly |
| Cracking (hot) | High S/P content in base metal | Use desulfurizing flux, control base metal chemistry |
| Cracking (cold) | Excessive cooling rate, H absorption | Increase preheat, use low-H flux, control interpass temp |
| Excessive dilution | Overlapping passes, high current | Reduce overlap to 30-40%, lower current by 10% |
| Hardness non-uniformity | Inconsistent travel speed, flux distribution | Use automated welding, maintain constant wire feed |
Engineering Practice and Case Study
Application Scenario
In a typical hot rolling mill, work rolls of φ850 mm diameter are subjected to:
- Contact pressure: 1000-2000 MPa
- Sliding speed: 1-3 m/s
- Surface temperature: 800-1200°C (for hot strip mills)
- Rolling force: 500-2000 kN per roll
- Service life: 20-100 hours per roll (depending on product and conditions)
The overlay welding repair extends roll life by 2-3 times compared to the uncladded base metal, providing significant economic benefit. The typical cost-benefit analysis shows:
- Overlay welding cost: USD 500-1500 per roll
- Extended roll life value: USD 3000-8000 per roll
- Return on investment: 200-500%
Process Optimization Using PDCA Approach
The study applies the PDCA (Plan-Do-Check-Act) cycle to continuously improve the overlay welding process:
Plan: Analyze failure modes of previous overlay welds using FMEA. Identify the top three failure modes: surface cracking (RPN 180), hardness non-uniformity (RPN 144), and excessive dilution (RPN 120).
Do: Implement countermeasures: (1) Increase preheat to 150°C and add 0.5% Ca to flux for desulfurization; (2) Install encoder-based travel speed control with ±2% accuracy; (3) Reduce overlap ratio to 35% and add a backing layer of low-carbon steel wire.
Check: Conduct 30 consecutive rolls with the modified process. Results show: surface cracking reduced from 15% to 2%, hardness variation reduced from ±8 HRC to ±3 HRC, dilution ratio controlled at 8-12%.
Act: Standardize the modified process as the company specification. Update the welding procedure specification (WPS) and conduct welder qualification testing per NB/T 47014.
Key Reflections and Insights
The most important insight from this study is that sintered alloy flux provides a significant advantage over fused flux for overlay welding applications where precise control of alloy composition is required. The lower melting point and thinner slag layer of sintered flux allow for:
- Better alloy transfer efficiency: 85-95% of alloying elements in the flux are transferred to the weld metal, compared to 60-75% for fused flux.
- Lower heat input: The thinner slag layer allows for lower current settings while maintaining adequate penetration, reducing dilution.
- More flexible process parameters: The wider operating window of sintered flux accommodates variations in base metal condition and environmental factors.
However, sintered flux also has limitations that must be addressed:
- Moisture sensitivity: Sintered flux absorbs moisture more readily than fused flux, requiring strict storage and drying protocols.
- Shorter shelf life: The porous structure of sintered flux degrades over time, with a recommended shelf life of 6-12 months.
- Higher cost: Sintered flux costs 2-3 times more than fused flux per kilogram, though the overall process cost may be lower due to higher deposition efficiency.
In my engineering practice, I have found that the successful application of sintered alloy flux for rolling mill roll overlay requires a systematic approach to process development. The key factors for success are: (1) thorough characterization of the base metal condition, (2) careful selection of flux composition based on the specific service conditions, (3) rigorous process parameter optimization through systematic experimentation, and (4) implementation of a robust quality control system with statistical process control (SPC).
The study also emphasizes the importance of understanding the relationship between microstructure and wear resistance. The hardness alone is not a sufficient indicator of wear resistance; the morphology and distribution of hard phases (carbides) relative to the matrix are equally important. A weld metal with 58 HRC and fine, uniformly distributed carbides will exhibit superior wear resistance compared to one with 55 HRC and coarse, clustered carbides. This insight should guide flux composition optimization and post-weld heat treatment design.
In conclusion, the overlay welding of φ850 rolling mill rolls using sintered alloy flux represents a mature and well-established technology that continues to evolve with improvements in flux composition, process control, and quality assurance. Engineers should approach each application with a comprehensive understanding of the metallurgical, mechanical, and economic factors that influence overlay weld performance, and should continuously seek to optimize the process through data-driven decision-making and systematic quality improvement.