CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair and Heat Treatment of Channel Steel Rolling Mill Rolls
Literature Overview
This technical paper by Ni Zhenhang, Tang Xin, and Xia Yang from the Second Mechanical Equipment Manufacturing Company of Maanshan Iron and Steel Co., Ltd. was published in the journal Metal Heat Treatment in 2004. The study addresses the practical challenge of repairing worn channel steel rolling mill rolls through weld overlay technology, followed by appropriate heat treatment to restore mechanical properties and service life.
Core Research Objectives and Methodology
The researchers developed a systematic approach to repair worn channel steel rolling mill rolls using weld overlay technology. The study encompasses the entire repair process from surface preparation and welding procedure selection through post-weld heat treatment and final inspection. The experimental approach involved fabricating test specimens on actual roll materials, optimizing welding parameters, and characterizing the repair deposits through metallographic examination, hardness testing, and mechanical property evaluation.
Key Technical Findings
Roll Material and Service Conditions
Channel steel rolling mill rolls are typically made from medium-carbon steel or low-alloy steel (such as 45 steel or 50Mn2) with a surface hardness of 250-300 HV after quenching and tempering. The rolls experience severe wear from friction with hot channel steel during rolling operations, leading to surface degradation and dimensional loss that necessitates periodic repair.
Weld Overlay Consumable Selection
The study evaluates several consumable options for roll repair:
| Consumable Type | Composition (wt%) | Hardness (HV) | Application Suitability |
|---|---|---|---|
| J507 (E5015) | C 0.10, Mn 1.4, Si 0.6 | 220-260 | General repair, low wear |
| D266 (cast iron) | C 3.0, Si 2.5, Mn 0.5 | 350-450 | Moderate wear resistance |
| D256 (high carbon) | C 2.5, Cr 1.5, Mo 0.5 | 450-550 | High wear resistance |
| Custom alloy | C 1.2, Cr 2.0, Mo 1.0, V 0.5 | 550-650 | Severe wear conditions |
The study recommends the custom alloy consumable for channel steel rolling mill rolls due to its superior wear resistance and adequate toughness for the service conditions.
Welding Procedure Optimization
The welding procedure was optimized through systematic experimentation:
| Parameter | Optimized Value | Rationale |
|---|---|---|
| Preheat temperature | 150-200°C | Reduce thermal gradient; minimize cracking |
| Interpass temperature | 250-300°C | Maintain weldability; control cooling rate |
| Heat input | 1.0-1.5 kJ/mm | Balance dilution and hardenability |
| Travel speed | 60-80 mm/min | Ensure adequate penetration and fusion |
| Number of passes | 2-3 | Achieve required build-up thickness |
| Backing material | Copper backing | Ensure full penetration and smooth back surface |
Heat Treatment Protocol
The post-weld heat treatment is critical for achieving the required mechanical properties and relieving residual stresses:
| Treatment Stage | Temperature (°C) | Time (h) | Cooling Method | Purpose |
|---|---|---|---|---|
| Stress relief | 550-600 | 2-3 | Furnace cool | Relieve residual stresses |
| Quenching | 850-880 | 1-2 (soak) | Oil quench | Harden the deposit |
| Tempering | 550-600 | 2-3 | Air cool | Achieve target hardness; improve toughness |
The heat treatment protocol was optimized to achieve a surface hardness of 450-500 HV while maintaining adequate core toughness. The tempering temperature was carefully selected to avoid over-tempering (which would reduce hardness) or under-tempering (which would leave excessive retained austenite).
Interpretation and Technical Analysis
Microstructural Evolution
The microstructure of the weld overlay deposit undergoes significant transformation during the welding and heat treatment processes:
| Stage | Microstructure | Hardness (HV) |
|---|---|---|
| As-welded | Martensite + bainite + carbides | 550-650 |
| After stress relief | Tempered martensite + carbides | 450-500 |
| After quenching | Fine martensite + carbides | 600-700 |
| After tempering | Tempered martensite + spheroidized carbides | 450-500 |
The final microstructure after heat treatment consists of tempered martensite with uniformly distributed spheroidized carbides, providing an optimal balance between hardness and toughness.
Dilution and Interface Effects
The dilution between the weld deposit and the base roll material affects the final properties of the repair zone. The study found that dilution of 15-25% produces the optimal balance between hardness and toughness. Higher dilution reduces the hardness of the deposit, while lower dilution increases the risk of cracking at the weld-base metal interface.
Residual Stress Analysis
The welding and heat treatment processes introduce complex residual stress patterns that must be carefully managed:
- As-welded: High tensile residual stresses (300-400 MPa) in the weld zone
- After stress relief: Reduced tensile stresses (100-150 MPa)
- After quenching: Compressive stresses in the surface (50-100 MPa) due to differential cooling
- After tempering: Stabilized stress state with compressive surface stresses
The compressive surface stresses after heat treatment are beneficial as they improve fatigue resistance and reduce the risk of surface cracking during service.
Process and Standards Analysis
Welding Process Selection
The study evaluates several welding processes for roll repair:
| Process | Advantages | Disadvantages | Suitability |
|---|---|---|---|
| SMAW | Portable; good control | Lower deposition rate | Small repairs; field work |
| SAW | High deposition rate; consistent quality | Requires flux handling | Large repairs; shop work |
| GMAW | Good deposition rate; clean welds | Higher consumable cost | Medium repairs; shop work |
| Oxy-fuel | Simple; low equipment cost | Low deposition rate; high dilution | Small repairs; emergency |
The study recommends SAW for large-scale roll repairs due to its high deposition rate and consistent quality, while SMAW is suitable for smaller repairs or field work.
Standards Compliance
For engineering applications of weld overlay repair on rolling mill rolls, the following standards are relevant:
- NB/T 47014: Welding procedure qualification requirements
- GB/T 150: Pressure vessel design and fabrication (if rolls are pressure-containing)
- JB/T 4730: Non-destructive testing requirements
- ASTM A264/A265: Clad plate specifications (for reference on overlay requirements)
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking | High sulfur/phosphorus; excessive carbon | Control base metal impurities; reduce C content |
| Cold cracking | High hardenability; hydrogen embrittlement | Preheat to 150-200°C; use low-hydrogen consumables |
| Excessive dilution | High heat input; thin deposit | Optimize heat input; use multiple passes |
| Poor fusion | Insufficient heat input; inadequate cleaning | Increase heat input; ensure proper surface preparation |
| Porosity | Flux contamination; inadequate shielding | Ensure flux dryness; improve shielding coverage |
Engineering Practice Integration
Application Scenarios
Weld overlay repair of channel steel rolling mill rolls is applicable to the following scenarios:
- Preventive maintenance: Periodic repair of worn rolls before failure
- Corrective maintenance: Emergency repair of damaged or failed rolls
- Dimensional restoration: Restoration of worn rolls to original dimensions
- Surface hardening: Application of wear-resistant overlay to new rolls for extended service life
Performance Benchmarks
The weld overlay repair process achieves the following performance levels:
- Surface hardness: 450-500 HV (compared to 250-300 HV for original roll material)
- Wear resistance: 2-3 times improvement over original material
- Service life extension: 50-100% increase in roll service life
- Cost savings: 30-50% reduction compared to roll replacement
Cost-Benefit Analysis
The weld overlay repair process provides significant economic benefits:
| Cost Component | Repair (¥) | Replacement (¥) | Savings |
|---|---|---|---|
| Material cost | 500-800 | 5000-8000 | 85-90% |
| Labor cost | 200-300 | 1000-1500 | 75-80% |
| Downtime cost | 1000-2000 | 5000-10000 | 80-90% |
| Total cost | 1700-3100 | 11000-19500 | 80-85% |
The significant cost savings make weld overlay repair an economically attractive option for roll maintenance.
Key Questions and Reflections
Several important considerations emerge from this study:
- Long-term reliability: How does the weld overlay repair perform over multiple repair cycles? Is there a risk of cumulative damage or property degradation?
- Thermal fatigue resistance: The rolls experience repeated heating and cooling cycles during service. How does the weld overlay perform under thermal fatigue conditions?
- Impact resistance: The rolls may experience impact loading during operation. How does the weld overlay perform under impact conditions?
- Alternative repair methods: How does weld overlay repair compare with other repair methods such as machining, welding build-up, or replacement?
Study Insights and Implications
This research provides practical guidance for the repair of worn channel steel rolling mill rolls through weld overlay technology. The key insight is that the combination of appropriate consumable selection, optimized welding parameters, and proper heat treatment can restore and even improve the mechanical properties of worn rolls.
From a practical engineering standpoint, the study provides actionable guidance for developing repair procedures and optimizing the repair process. The recommended custom alloy consumable with 1.2% C, 2.0% Cr, 1.0% Mo, and 0.5% V offers an excellent balance between wear resistance and toughness for the service conditions of channel steel rolling mill rolls.
The study also highlights the importance of post-weld heat treatment in achieving the required mechanical properties. The heat treatment protocol developed in this study provides a reliable method for achieving consistent hardness and toughness in the repair deposits.
Looking forward, the integration of advanced welding processes such as laser cladding and plasma transferred arc (PTA) powder cladding could further improve the quality and consistency of roll repairs. Additionally, the development of new consumable compositions tailored for specific service conditions could further extend the service life of repaired rolls.
In conclusion, this research provides a solid foundation for the development of reliable and cost-effective repair procedures for channel steel rolling mill rolls, offering significant potential for extending the service life of critical rolling mill equipment.