CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Weld Overlay Technology on Hot Rolling Backup Rolls
Literature Overview
This 2004 paper by Ge Baowen from Baosteel Engineering Technology Co., Ltd., published in Baosteel Technology, documents the practical application of weld overlay technology to restore and enhance the surface properties of hot rolling backup rolls in steel production. Hot rolling backup rolls (also known as backup rolls or backup rollers) are critical components in rolling mills that support the work rolls during the hot rolling process. They are subjected to extreme thermal cycling, mechanical loading, and chemical attack from scale and coolant, leading to progressive surface degradation that necessitates periodic restoration.
Technical Challenges and Overlay Solution
The primary challenges facing backup roll restoration include the need to restore dimensional accuracy, improve surface hardness and wear resistance, and ensure thermal stability under operating temperatures that can exceed 900 °C. Traditional grinding and resurfacing methods are limited by the amount of material that can be removed without compromising the roll's structural integrity, and they do not improve the surface properties beyond the original base material specification.
Weld overlay provides a solution by depositing a hardfacing alloy layer onto the roll surface, simultaneously restoring dimensions and enhancing surface properties. The overlay layer must exhibit:
- High hardness (typically HV 500–700) for wear resistance
- Good thermal stability to resist softening at elevated operating temperatures
- Adequate toughness to resist thermal fatigue cracking
- Strong metallurgical bond with the roll base material (typically 42CrMo or similar alloy steel)
| Overlay Parameter | Specification | Rationale |
|---|---|---|
| Base material | 42CrMo or equivalent | Standard backup roll material with good toughness |
| Overlay alloy | High-Cr alloy or Ni-based hardfacing | Provides wear and thermal resistance |
| Overlay thickness | 3–8 mm | Balances dimensional restoration with bond integrity |
| Welding process | Submerged arc welding or GMAW | High deposition rate with good penetration |
| Preheating temperature | 200–300 °C | Reduces cracking risk in high-carbon overlay deposits |
| Post-weld treatment | Stress relief at 550–650 °C | Eliminates residual stresses to prevent cracking |
Process Development and Implementation
The paper describes a systematic approach to developing the overlay welding procedure, including consumable selection, welding parameter optimization, and quality verification. The authors conducted extensive trials to determine the optimal combination of welding current, voltage, travel speed, and wire feed rate that produced a defect-free overlay with the desired mechanical properties.
Key process considerations included:
- Dilution control: Maintaining dilution below 20% to preserve the overlay alloy's hardness and wear resistance
- Crack prevention: Using low-hydrogen consumables and appropriate preheating to prevent hydrogen-induced cracking in the transition zone
- Surface preparation: Thorough cleaning and grinding of the roll surface to remove scale, oxide, and contaminated material before overlay application
- Multi-pass strategy: Using a build-up pass followed by a wear-resistant overlay pass to optimize both bond strength and surface properties
Quality Verification and Performance
The overlay quality was verified through a combination of non-destructive testing (magnetic particle inspection for surface cracks, ultrasonic testing for subsurface defects) and destructive testing on coupon specimens. Hardness profiling across the overlay layer confirmed uniform hardness distribution with a gradual transition to the base material. The overlay layer demonstrated excellent wear resistance in field trials, extending backup roll service life by a factor of 2–3 compared to conventional restoration methods.
Engineering Practice Implications
This paper provides a practical roadmap for engineers involved in roll restoration and maintenance in steel mills. The documented process parameters and quality verification procedures can serve as a foundation for developing site-specific WPS for backup roll overlay. The economic analysis presented in the paper demonstrates significant cost savings from extended roll life, with the overlay restoration cost being only a fraction of the replacement cost for new rolls.
The study also highlights the importance of process control in overlay welding applications where the component geometry (large diameter cylindrical surface) and operating conditions (high temperature, high stress) impose demanding requirements on the overlay weld quality. Engineers should pay particular attention to the thermal management of the welding process, as excessive heat input can cause distortion and reduce the residual strength of the roll.
Study Insights and Reflections
This paper exemplifies the practical application of weld overlay technology in heavy industry, where the economic and operational benefits of surface restoration are substantial. The systematic approach to process development, from consumable selection through field verification, provides a model for engineers approaching similar restoration challenges. The emphasis on quality verification through both NDT and destructive testing on coupons underscores the importance of comprehensive quality assurance in overlay welding applications. The findings also reinforce the principle that successful overlay welding requires careful consideration of the entire process chain, from base material preparation through post-weld treatment, to ensure long-term service reliability.