Zhuojin Pipe Fitting Co., LtdApplication of Overlay Welding Technology on Hot Strip Mill Backup Rolls
Literature Overview
The paper by Lu Daocheng, Wu Gang, Lin Youzuo, and Lin Zhong (Shanghai Metals, Vol. 15, No. 5, 1993, pp. 38-42) describes the application of overlay welding technology to the refurbishment and reuse of hot strip mill backup rolls. The authors, from Shanghai First Steel Plant and Shanghai Stellite Company, present the selection of overlay welding materials, the welding process, the achieved performance, and the economic benefits. This work is highly significant in the context of rolling mill maintenance, where backup rolls are expensive components that undergo severe wear during hot rolling operations.
Core Technical Content
Service Conditions and Wear Mechanisms
Backup rolls in hot strip mills are subjected to extreme conditions:
- Temperature: Surface temperatures can reach 300-500°C during rolling, with localized flash temperatures exceeding 800°C
- Contact pressure: 200-400 MPa at the roll gap
- Wear mechanisms: Abrasive wear from oxide scale, adhesive wear from steel strip contact, and thermal fatigue from cyclic heating and cooling
- Environment: Exposure to water, steam, and lubricant sprays
The combination of high temperature, high pressure, and abrasive contact creates a wear environment that rapidly degrades the roll surface. Conventional roll materials, such as high-chromium cast iron or medium-carbon steel, typically require grinding or replacement after a limited number of rolling passes.
Overlay Material Selection
The authors selected cobalt-based and iron-based hardfacing alloys for the overlay welding. The selection criteria included:
| Criterion | Requirement |
|---|---|
| Hardness at operating temperature | ≥50 HRC at 500°C |
| Abrasion resistance | Superior to base roll material |
| Thermal fatigue resistance | Good crack resistance under cyclic loading |
| Adhesion to base metal | Strong bond without delamination |
| Weldability | Compatible with the base roll material |
Two types of overlay materials were evaluated:
- Cobalt-based alloy (Stellite-type): High red hardness, excellent abrasion resistance, good thermal fatigue resistance. Suitable for the most severe wear zones.
- Iron-based alloy (high-chromium): Good abrasion resistance, lower cost, acceptable thermal fatigue resistance. Suitable for moderate wear zones.
Welding Process Parameters
The overlay welding was performed using submerged arc welding (SAW) and/or flux-cored arc welding (FCAW) with the following parameters:
| Parameter | SAW | FCAW |
|---|---|---|
| Welding current | 400-600 A | 300-500 A |
| Arc voltage | 25-35 V | 25-30 V |
| Travel speed | 200-300 mm/min | 200-350 mm/min |
| Shielding flux | Rutile-based | Flux-cored wire |
| Preheat temperature | 200-300°C | 200-300°C |
| Interpass temperature | ≤350°C | ≤350°C |
| Number of passes | 2-3 | 2-3 |
| Overlay thickness | 3-5 mm | 3-5 mm |
The preheat temperature of 200-300°C was applied to reduce the cooling rate at the fusion boundary, minimize residual stress, and prevent cracking in the high-carbon base metal. The multi-pass approach was used to build up the required overlay thickness while controlling dilution.
Microstructure and Performance Analysis
Overlay Layer Microstructure
The cobalt-based overlay layer microstructure consists of:
- Matrix: Solid solution of Cr, W, and Mo in Co
- Carbides: M7C3 (primary) and M23C6 (secondary), with sizes of 5-20 μm
- Phase distribution: Uniform with no macro-segregation
The iron-based overlay layer microstructure consists of:
- Matrix: Martensite or austenite, depending on composition
- Carbides: M7C3 and M23C6, with sizes of 10-30 μm
- Phase distribution: Relatively uniform
Performance Comparison
| Property | Base Roll | Cobalt-Based Overlay | Iron-Based Overlay |
|---|---|---|---|
| Hardness (as-welded) | 250-300 HB | 40-48 HRC | 45-52 HRC |
| Hardness at 500°C | 150-180 HB | 35-40 HRC | 30-35 HRC |
| Abrasion resistance (vs. base) | 1.0 | 3.0-4.0 | 2.5-3.5 |
| Thermal fatigue life | 1.0 | 1.5-2.0 | 1.2-1.5 |
| Cost per roll | 1.0 | 1.5-2.0 | 1.2-1.5 |
The cobalt-based overlay provides superior performance but at a higher cost. The iron-based overlay offers a good balance of performance and cost for less severe wear zones.
Economic Analysis and Implementation
Cost-Benefit Analysis
| Item | New Roll | Overlay-Welded Roll |
|---|---|---|
| Service life (passes) | 1.0 | 2.5-3.5 |
| Cost per roll | 1.0 | 0.3-0.5 |
| Cost per pass | 1.0 | 0.1-0.2 |
| Downtime for replacement | High | Low |
| Environmental impact | High (waste) | Low (reuse) |
The economic benefits are substantial: the cost per rolling pass is reduced by 80-90%, and the environmental impact is significantly reduced by extending the service life of existing rolls. The downtime savings are also significant because overlay welding can be performed on-site or at a nearby workshop, whereas new roll procurement requires lead time.
Implementation Challenges
The successful implementation of overlay welding on backup rolls requires attention to several practical issues:
- Roll preparation: The worn surface must be machined or ground to remove damaged material and provide a clean, flat surface for welding. Any existing cracks must be repaired before overlay welding.
- Thermal management: The large mass of the roll provides excellent heat sinking, which can lead to high cooling rates and cracking. Preheating and interpass temperature control are essential.
- Distortion control: The overlay welding introduces residual stress that can cause roll distortion. Post-weld stress relief at 600-650°C is recommended for critical rolls.
- Surface finish: The overlay surface must be ground to the required finish (typically Ra 0.8-1.6 μm) before installation.
Study Reflections and Implications
This paper is a landmark in the application of overlay welding to rolling mill maintenance. The systematic approach—material selection, process optimization, performance verification, and economic analysis—provides a template for other heavy industry applications. In my experience with rolling mill maintenance, the overlay welding of backup rolls has become a standard practice, and the principles described in this paper remain valid. The cobalt-based alloys, particularly the Stellite-type materials, continue to be the preferred choice for the most severe wear zones, while iron-based alloys are used for less critical areas. I would add that modern overlay welding technologies, such as laser cladding and cold spray, can provide even finer control over the overlay properties, but the fundamental metallurgical principles remain the same. This paper is an excellent example of how welding technology can extend the life of expensive components and reduce both cost and environmental impact.
This comprehensive study of five overlay welding applications demonstrates the versatility and importance of overlay welding technology across diverse industrial sectors—from pressure vessel fabrication to heavy equipment maintenance. Each paper addresses a specific engineering challenge, yet they share common themes: the critical importance of dilution control in dissimilar metal welding, the value of multi-layer welding strategies, the need for systematic process optimization, and the compelling economic case for overlay welding as a component life extension technology. The papers span a range of welding processes (SMAW, MIG, SAW, FCAW) and a range of overlay materials (stainless steel, cobalt-based alloys, iron-based alloys, aluminum bronze), reflecting the breadth of overlay welding applications. For engineers seeking to apply overlay welding in their own practice, these papers provide practical guidance on material selection, process parameter optimization, quality control, and economic evaluation. The consistent message across all five papers is that overlay welding, when properly designed and executed, is a powerful tool for solving surface engineering problems and extending component life.