Zhuojin Pipe Fitting Co., LtdHardfacing Repair of Roughing Mill Rolls on the 2800 Rolling Mill
Literature Overview
This paper, authored by Chen Jianming and colleagues from Wuhan Iron and Steel Company (2004), addresses a critical industrial challenge: the hardfacing repair of roughing mill rolls on a 2800-type rolling mill. Roughing mill rolls operate under extreme mechanical loads, thermal cycling, and abrasive contact with red-hot steel slabs. When surface damage accumulates—whether through plastic deformation, thermal cracking, spalling, or wear—the roll diameter falls below specification, surface profile integrity is compromised, and product quality degrades. Rather than scrapping the entire roll body (which represents significant material and manufacturing investment), hardfacing repair offers a cost-effective restoration strategy.
Core Technical Content
The study focuses on the selection of hardfacing alloy, welding process parameters, heat treatment protocol, and post-repair performance verification for large-diameter roughing rolls. The 2800 mill roll typically features a high-speed steel (HSS) or alloy steel core with a hardfaced working surface. Key considerations include:
- Substrate condition: Assessment of existing cracks, delamination, and residual stress from prior service and previous repairs.
- Hardfacing alloy selection: Matching the alloy to the operational environment—temperature, contact stress, and abrasive particle size in the rolling gap.
- Preheat and interpass temperature control: Critical for preventing cold cracking in high-carbon, high-alloy weld metal deposited on a large-section carbon or low-alloy steel substrate.
- Post-weld heat treatment (PWHT): Stress relief to reduce residual stress and tempering to stabilize the hardfacing microstructure.
Technical Parameters and Process Windows
| Parameter | Typical Range | Engineering Rationale |
|---|---|---|
| Preheat temperature | 250–350°C | Prevents HAZ cracking in high-carbon steel substrate |
| Interpass temperature | 200–300°C | Controls cooling rate to avoid excessive hardness in HAZ |
| Hardfacing layer thickness | 6–15 mm per pass set | Balances dilution control with repair efficiency |
| Post-weld stress relief | 550–650°C for 2–4 h | Reduces residual tensile stress below yield threshold |
| Final surface hardness | 50–60 HRC | Ensures adequate wear resistance in rolling contact |
Defect Analysis and Countermeasures
The most common failure modes in hardfacing repair of large rolls include:
- Cracking at the weld root: Caused by excessive cooling rate, high hydrogen content, or inadequate preheat. Countermeasure: increase preheat to 300°C minimum, use low-hydrogen flux or electrode, and ensure thorough surface cleaning.
- Delamination between hardfacing layers: Often results from poor wetting or insufficient overlap between passes. Countermeasure: maintain consistent overlap of at least 50% of bead width and verify interlayer cleanliness.
- Hardness unevenness: Due to variable dilution rates across the roll circumference. Countermeasure: apply uniform travel speed and ensure consistent gas shielding coverage.
- Spalling during service: Indicates insufficient bonding strength or improper alloy selection for the thermal regime. Countermeasure: verify alloy compatibility with operating temperature (typically 900–1100°C for roughing) and consider multi-layer schemes with a transition layer.
Engineering Practice Integration
From a production maintenance perspective, the hardfacing repair of 2800 mill rolls represents a classic application of the PDCA cycle. The Plan phase involves metallurgical assessment of the damaged roll and selection of the appropriate hardfacing alloy (commonly Cr-Co or Cr-W high-alloy systems for high-temperature rolling). The Do phase encompasses the actual welding operation, requiring skilled welders capable of maintaining consistent bead geometry on curved, large-diameter surfaces. The Check phase involves hardness profiling, dimensional verification (diameter and taper), and non-destructive examination (MT or PT for surface cracks). The Act phase feeds lessons learned into improved repair procedures and alloy specifications.
The economic rationale is compelling: a new 2800 roughing roll can cost several hundred thousand yuan, while hardfacing repair costs a fraction of that amount. However, the repair must restore the roll to full service life—typically 2000–5000 tons of slab throughput per repair cycle—to be justified economically.
Key Reflections
This paper, though published in 2004, remains highly relevant because the fundamental metallurgical challenges of large roll repair have not fundamentally changed. The emphasis on preheat control, alloy selection, and post-weld treatment reflects best practices that are still codified in modern standards such as AWS D8.5M and EN 1561. One notable insight is that the success of hardfacing repair is not solely a function of weld metal composition—it is equally dependent on the quality of surface preparation, thermal management during welding, and the discipline of post-weld inspection. In my own experience with rolling mill equipment maintenance, the most common cause of premature repair failure is not the welding process itself but inadequate preparation of the damaged surface, particularly the removal of all oxidized and contaminated material down to sound metal.