CLADDING TECHNOLOGY SHANXI CO., LTDSintered Flux Cladding Repair of 650mm Rolling Mill Rolls
Literature Overview
This 1997 publication from Dalian Second Steel Plant, authored by Zhu Yan, documents the application of sintered flux submerged arc welding (SAW) for the overlay repair of φ650 rolling mill rolls. The work was published in the journal "Mechanical Design and Manufacturing" and represents early Chinese industrial practice in roll refurbishment using powder metallurgy consumables. Rolling mill rolls are subjected to extreme thermo-mechanical loads during hot rolling operations, experiencing temperatures exceeding 1000 °C on the working surface, combined with compressive stresses, abrasive wear from iron scale, and thermal fatigue cracking. When rolls become worn or damaged beyond dimensional tolerance, complete replacement is prohibitively expensive, making overlay welding repair an economically essential practice.
Core Technical Approach
The selection of sintered flux for submerged arc overlay welding in this application is driven by several metallurgical and practical advantages. Sintered fluxes offer superior slag fluidity and deoxidation capability compared to fused fluxes, which is critical when depositing hardfacing alloys onto low-carbon steel roll bodies. The sintering process allows precise control of flux composition, enabling the introduction of alloying elements such as chromium, molybdenum, cobalt, tungsten, and carbon to tailor the hardfacing layer properties.
The typical process parameters for this application include:
| Parameter | Typical Range |
|---|---|
| Base material | 45 steel or 50Cr (roll body) |
| Overlay material | Cr-Mo-Co-W based hardfacing |
| Flux type | Sintered, low-hydrogen |
| Wire diameter | 3.2–4.0 mm |
| Current | 350–550 A |
| Voltage | 28–36 V |
| Travel speed | 100–200 mm/min |
| Preheat temperature | 150–250 °C |
| Layer thickness per pass | 4–8 mm |
| Number of overlay layers | 2–4 |
| Post-weld treatment | Stress relief at 550–650 °C |
Technical Points and Interpretation
Metallurgical Considerations
The interface between the roll body steel and the hardfacing overlay layer is the most critical region from a metallurgical standpoint. Sintered flux SAW produces a relatively wide weld bead with deep penetration, which can lead to significant dilution of the overlay composition by the base metal. For a φ650 roll, the curvature presents challenges for maintaining consistent arc characteristics and bead geometry. The effective dilution rate in the first overlay pass typically ranges from 20% to 40%, depending on the wire diameter, travel speed, and flux type.
To achieve the desired surface hardness (typically 50–60 HRC for hot strip mill rolls), a multi-pass approach is employed. The first pass serves as a transition layer, while subsequent passes progressively enrich the overlay composition. The sintered flux provides a protective atmosphere and acts as an additional alloying source, contributing chromium and carbon to the solidifying weld metal.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at interface | Excessive cooling rate, high carbon in base | Increase preheat to 250 °C, use lower travel speed |
| Porosity | Flux moisture absorption | Dry flux at 300 °C for 2 hours before use |
| Incomplete fusion | Excessive travel speed | Reduce travel speed by 20–30% |
| Hardness variation | Dilution inconsistency | Maintain constant wire feed speed, monitor voltage |
| Excessive undercut | Arc instability on curved surface | Adjust torch angle to 5–10° leading |
Process Optimization Insights
The φ650 diameter presents a specific geometric challenge. The circumference is approximately 2042 mm, and the roll is typically mounted horizontally during overlay welding. Gravity effects cause slag to pool on the lower side of the roll, leading to asymmetrical bead profiles. Practical solutions include rotating the roll at a controlled speed synchronized with the welding torch, or welding in segments with intermediate cooling intervals.
The sintered flux selection is particularly important for this application. Unlike fused fluxes which have fixed compositions, sintered fluxes can be formulated to include alloy additions that promote the formation of hard carbides (Cr7C3, WC, Co3W) in the overlay layer. The optimal flux composition typically contains 18–25% SiO2, 8–12% Al2O3, 5–8% CaF2, and 2–5% alloying powders.
Integration with Engineering Practice
In the context of modern rolling mill operations, the φ650 roll size is typical for medium plate mills and bar mills. The overlay repair cycle is generally triggered when the roll diameter decreases by 2–3 mm from the nominal working diameter, or when surface cracks exceed 2 mm in depth. The total overlay thickness applied during a single repair typically ranges from 12 to 25 mm, restoring the roll to slightly above nominal diameter for subsequent grinding.
The economic analysis strongly favors overlay repair over replacement. A new φ650 roll costs approximately 15–25 times the material and labor cost of overlay repair, with significantly longer lead time. However, the service life of the overlay layer must be carefully evaluated. Typical service life ranges from 30 to 80 tonnes of steel rolled per mm of overlay thickness, depending on the product being rolled and the operating conditions.
The 1997 publication reflects the state of practice at that time, when sintered flux SAW was one of the more accessible and cost-effective overlay methods for industrial applications. Modern alternatives such as plasma transferred arc (PTA) welding and laser cladding offer superior dilution control (typically 5–15% for PTA versus 20–40% for SAW) and more uniform layer properties, but at significantly higher equipment and consumable costs. For many industrial applications, the sintered flux SAW approach remains economically justified.
Key Questions and Reflections
The durability of sintered flux SAW overlay layers on rolling mill rolls remains a subject of ongoing investigation. The primary failure mode is typically spalling of the hardfacing layer due to thermal fatigue cracking at the overlay-to-overlay interfaces. The thermal conductivity mismatch between the hardfacing layer and the roll body creates cyclic thermal stresses during each rolling cycle. Understanding and mitigating this mechanism requires careful attention to the thermal cycling characteristics of the specific mill operation.
The use of sintered flux also raises concerns about hydrogen-induced cracking, particularly when the flux moisture content is not tightly controlled. In the humid coastal environment of Dalian, flux storage and handling procedures become critical quality control measures. The recommended maximum moisture content for sintered flux is 0.5% for low-hydrogen applications, requiring careful drying and storage protocols.
Study Insights and Implications
This literature provides valuable insight into the practical application of sintered flux SAW for heavy industrial roll repair. The work demonstrates that with proper process control, sintered flux overlay welding can produce reliable hardfacing layers with adequate hardness and wear resistance for demanding rolling mill applications. The key success factors identified include: controlled preheating, careful flux moisture management, multi-pass overlay strategy with progressive composition enrichment, and appropriate post-weld stress relief treatment.
The publication also highlights the importance of process parameter optimization specific to the geometry of the component being repaired. The cylindrical geometry of rolling mill rolls demands careful consideration of gravity effects, arc stability, and heat input distribution. These geometric considerations remain relevant in modern overlay welding practice, regardless of the specific welding process employed.
The enduring relevance of this 1997 publication lies in its demonstration of fundamental metallurgical principles that continue to govern overlay welding practice. While equipment and consumables have evolved, the core challenges of dilution control, interface integrity, and thermal stress management remain central to achieving reliable overlay repair of heavy industrial components.