Zhuojin Pipe Fitting Co., LtdOverlay Welding Process for Rolling Mills Rolls
Literature Overview and Context
The paper by Ding Jie (Tianjin Sino-German Technical College) and Shen Qianhui (Tianjin Steel Pipe Company), published in "Welding Technology" (2005, Vol. 34, No. 2, pp. 64–65), examines the overlay welding process for rolling mill rolls. Rolling mill rolls are critical components in steel production, subjected to extreme contact stress, high temperatures, and severe abrasive wear during the rolling process. When the working surface of a roll becomes worn or damaged, replacement of the entire roll is often economically unjustifiable, particularly for large-diameter rolls. Overlay welding provides a cost-effective and efficient means of restoring or enhancing the surface properties of rolls. The authors discuss the selection of welding consumables, the role of carbon equivalent in crack resistance, and the optimization of flux and flux-cored wire parameters for successful roll overlay welding.
Technical Analysis of Roll Overlay Welding
Base Metal Characteristics and Carbon Equivalent
Rolling mill rolls are typically manufactured from high-carbon steel or cast steel with high hardness and wear resistance. The high carbon content of the base metal significantly increases the carbon equivalent (CE), which is a key indicator of cold-cracking susceptibility. For typical roll materials such as high-carbon chrome steel or cast roll steel, the CE can range from 0.5% to 0.8%, placing them in the high-risk category for cold cracking. The authors emphasize that understanding the CE of the base metal is essential for selecting appropriate welding parameters and consumables. The CE is calculated using the standard formula CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, and for roll materials with CE above 0.6%, preheating and post-weld heat treatment are generally required to prevent cracking.
Welding Consumable Selection
The authors discuss the selection of flux-cored wires and flux for roll overlay welding. Flux-cored welding (FCAW) is preferred for roll overlay because of its high deposition rate, good weld shape, and deep penetration, which allows for efficient repair of worn surfaces. The consumable composition must be carefully matched to the base metal to ensure adequate hardness and wear resistance in the overlay layer while maintaining sufficient toughness to resist cracking. The typical overlay material for roll working surfaces contains 1.0–1.5% carbon, 4–6% chromium, and 1–2% vanadium, producing a high-carbon martensitic microstructure with dispersed carbides that provide excellent abrasion resistance.
Flux Properties and Weld Pool Behavior
The flux used in FCAW for roll overlay serves multiple functions: it stabilizes the arc, provides shielding gas, deoxidizes the weld pool, and modifies the slag composition. The authors note that the flux composition directly affects the weld metal composition, particularly the carbon content and alloy retention. A flux with appropriate basicity and alloying elements ensures that the overlay layer achieves the target hardness of 58–65 HRC, which is necessary for resisting the severe abrasive wear in rolling mill service. The flux must also be designed to minimize spatter and porosity, which are common defects in roll overlay welding due to the high carbon content of the base metal and the aggressive rolling conditions.
Process Parameters and Defect Control
The following table summarizes the typical process parameters and defect countermeasures for roll overlay welding:
| Process Parameter | Typical Range | Purpose |
|---|---|---|
| Preheat temperature | 200–300°C | Reduce cooling rate, prevent cracking |
| Interpass temperature | 250–350°C | Maintain thermal balance, reduce residual stress |
| Welding current (FCAW) | 300–450 A | Ensure adequate penetration and deposition rate |
| Welding voltage | 28–35 V | Control arc length and bead shape |
| Travel speed | 200–400 mm/min | Balance dilution rate and overlay thickness |
| Wire diameter | 1.2–1.6 mm | Match to current range and bead width |
| Overlay layer thickness | 2–4 mm | Provide adequate wear life |
Common defects in roll overlay welding include cracking, porosity, undercut, and excessive dilution. Cracking is primarily caused by high CE of the base metal, rapid cooling, and hydrogen pickup. Countermeasures include proper preheating, low-hydrogen consumables, and controlled interpass temperature. Porosity is often caused by contaminated base metal or flux, and can be mitigated by thorough surface cleaning and proper flux storage. Excessive dilution reduces the hardness of the overlay layer and can be controlled by using a shallower penetration setting and a higher alloy content in the consumable.
Engineering Practice and Quality Assurance
In practice, roll overlay welding requires careful attention to several quality factors. The base metal surface must be thoroughly cleaned to remove rust, scale, and oil, as contamination leads to porosity and poor fusion. The preheating must be uniform across the entire roll surface, not just the weld area, to prevent thermal gradients that cause distortion and cracking. After welding, the overlay layer is typically ground to the required dimensions and may be heat-treated to relieve residual stresses and optimize the microstructure.
The authors report that the overlay-welded rolls achieve a service life comparable to new rolls, with the added benefit of reduced downtime and lower cost compared to roll replacement. The overlay process also allows for the application of specialized wear-resistant materials that may not be available in the original roll material, providing an opportunity to improve roll performance.
Reflections and Practical Implications
The paper by Ding and Shen provides a practical and accessible overview of roll overlay welding, emphasizing the interplay between base metal carbon equivalent, consumable selection, and process parameters. One of the key insights is that the success of roll overlay welding depends on a holistic approach that considers not only the welding consumable but also the base metal condition, preheating, and post-weld treatment. The paper also highlights the economic advantages of overlay welding over roll replacement, which is particularly relevant for large-diameter rolls where replacement costs are substantial.
A point of interest is the discussion of flux-cored wire selection. The authors note that the flux composition must be carefully matched to the base metal to ensure adequate alloy retention and hardness. In my experience, this matching is critical, and a generic flux-cored wire may not provide the required hardness and wear resistance for roll overlay. The overlay layer must be hard enough to resist abrasion but tough enough to withstand the impact and thermal loading during rolling. Achieving this balance requires a consumable with a carefully designed alloy composition and a flux that supports the desired microstructure.
Summary
The overlay welding process for rolling mill rolls is a well-established technique that offers significant economic and operational advantages over roll replacement. The paper by Ding and Shen effectively highlights the key technical factors—carbon equivalent, consumable selection, flux properties, and process parameters—that determine the success of roll overlay welding. For engineers working in steel production, the paper provides practical guidance on consumable selection and process optimization, with clear emphasis on the importance of base metal characterization and quality control. The technique continues to be widely used in modern rolling mills, and the principles discussed in this paper remain relevant for ongoing process improvement and quality assurance.