Roll Overlay Welding Process Technology

Literature Overview

This 2005 publication by Ding Jie and Shen Qianhui from Tianjin Sino-German Vocational College and Tianjin Steel Pipe Company addresses the overlay welding of rolls, a critical maintenance activity in steel rolling mills. Rolls are among the most heavily loaded components in steel production, experiencing contact pressures exceeding 3 GPa, high temperatures from hot rolled stock, and severe abrasive and adhesive wear. The ability to restore and enhance roll surfaces through overlay welding is essential for economic and operational continuity in rolling mill operations. The authors present a comprehensive process development for roll overlay welding, covering substrate preparation, electrode selection, welding parameters, and post-weld treatment.

Roll Types and Overlay Requirements

The overlay requirements vary significantly depending on the roll type and its position in the rolling mill:

The specific application in this literature focuses on the Tianjin Steel Pipe Company's production requirements, which involve overlay welding of pipe mill rolls that are exposed to hot steel pipe at temperatures of 500-900°C while experiencing severe abrasive wear from oxide scale and material deformation.

Substrate Preparation

Proper substrate preparation is the single most important factor in achieving a sound overlay weld on rolls. The authors emphasize the following preparation sequence:

1. Machining: Grind or machine the roll surface to remove damaged material, achieving a surface finish of Ra ≤ 3.2 μm. The machining depth should be sufficient to remove all decarburized material and surface cracks (typically 2-5 mm).
2. Cleaning: Remove all oil, grease, and contamination using solvent cleaning followed by mechanical scrubbing. Residual contamination is a primary cause of porosity and lack of fusion.
3. Groove preparation: For deep overlays (>3 mm), a V-groove or U-groove is machined to provide mechanical keying and reduce the thermal mass of the base material that must be melted.
4. Preheating: Rolls are preheated to 200-350°C depending on the material and overlay thickness. Preheating reduces the thermal gradient at the interface, minimizing the risk of cracking in both the heat-affected zone and the weld metal.

A critical aspect of roll preparation that the authors highlight is the importance of controlling the carbon content at the surface. Hot rolled rolls often develop a decarburized layer during service, which can lead to poor weldability and reduced hardness in the overlay. The machining step must remove this layer completely, which requires knowledge of the decarburization depth—typically 0.5-2.0 mm for hot rolled rolls after extended service.

Electrode Selection and Weld Metal Design

The selection of overlay electrode for rolls must balance hardness, toughness, thermal stability, and weldability. The authors present a systematic approach to electrode selection:

For the pipe mill roll application described, the authors recommend a two-pass approach: a nickel-based transition layer followed by a high-speed steel or Stellite-type overlay. The transition layer accommodates the thermal expansion mismatch between the high-alloy overlay and the alloy steel roll body, while the overlay layer provides the required hardness and wear resistance.

Welding Process Parameters

The welding of rolls presents unique challenges due to the cylindrical geometry, the need for uniform coverage, and the requirement for dimensional accuracy after welding. The authors describe the following process parameters:

A critical process consideration for cylindrical rolls is the management of thermal distortion. Asymmetric welding can cause the roll to warp, leading to out-of-roundness that exceeds acceptable tolerances (typically ±0.05 mm for precision rolls). The authors recommend a balanced welding sequence that alternates between opposite sides of the roll, welding in short segments (50-100 mm) and allowing cooling between passes to minimize cumulative distortion.

Post-Weld Treatment and Surface Finishing

The overlay weld metal as-deposited typically contains a coarse grain structure and residual stresses that must be addressed before the roll enters service. The recommended post-weld treatment sequence is:

1. Stress relief: Furnace annealing at 500-550°C for 2-4 hours, followed by controlled cooling. This reduces residual stresses to below 100 MPa while maintaining the hardness of the overlay.
2. Grinding: Precision grinding to achieve the required dimensional accuracy and surface finish (Ra ≤ 0.8 μm for work rolls, Ra ≤ 1.6 μm for backup rolls).
3. Heat treatment: For high-speed steel overlays, a tempering treatment at 540-580°C for 2-4 hours achieves optimal hardness-toughness balance.
4. Inspection: MT or UT examination of the overlay for cracks, porosity, and lack of fusion.

Defect Analysis and Countermeasures

Engineering Practice Integration

The practical implementation of roll overlay welding in a production environment requires careful planning and scheduling. The authors describe a typical workflow:

1. Roll removal and inspection: Measure wear, inspect for cracks, assess remaining serviceable life
2. Decision point: Overlay repair vs. replacement (based on wear depth, crack severity, and cost analysis)
3. Surface preparation: Machining, cleaning, preheating
4. Overlay welding: Multi-pass deposition following the balanced sequence
5. Post-weld treatment: Stress relief, grinding, heat treatment
6. Quality verification: Dimensional check, hardness measurement, NDT examination
7. Reinstallation: Mounting on roll housing, alignment verification

The economic justification for overlay welding over roll replacement is significant. A typical HFM work roll costs $15,000-50,000 for replacement, while overlay repair costs $2,000-5,000, yielding a cost savings of 80-90%. However, the repair must be performed correctly; a failed overlay that causes roll cracking during service can result in production losses far exceeding the cost of replacement.

Key Questions and Reflections

The 2005 publication raises several contemporary questions. The use of laser cladding and plasma transferred arc (PTA) welding for roll overlay has advanced significantly since this publication, offering lower dilution, better microstructural control, and the ability to apply thinner, more uniform overlay layers. The literature does not address these newer technologies, which now represent the state of the art for many roll applications. Additionally, the environmental and safety implications of using cobalt-based Stellite-type electrodes are not discussed, yet cobalt is classified as a carcinogen by several regulatory bodies.

The metallurgical challenge of achieving a sound bond between a high-alloy overlay and an alloy steel substrate remains the central technical issue. The nickel-based transition layer approach described in this literature is still widely used, but newer approaches using functionally graded materials or gradient composition transitions may offer improved performance.

Study Insights and Implications

This publication provides a solid foundation for understanding the practical aspects of roll overlay welding. The emphasis on substrate preparation, balanced welding sequences, and post-weld treatment reflects a mature understanding of welding metallurgy and process engineering. For contemporary engineers, the key lessons are: (1) the quality of the overlay is determined as much by preparation and post-treatment as by the welding itself; (2) the cylindrical geometry of rolls requires special attention to thermal distortion control; (3) the economic case for overlay repair is compelling but must be supported by rigorous quality assurance; and (4) the selection of overlay material must be driven by the specific failure mechanism encountered in service, not by generic hardness specifications.