Surfacing Repair of φ650 Rolling Mill Rolls Using Sintered Flux

Literature Overview

The paper by Zhu Yan (1997), published in Machinery Design and Manufacturing (No. 3, pp. 37-38), describes the application of sintered flux submerged arc surfacing for the repair of φ650 rolling mill rolls at Dalian Second Steel Mill. The work is classified under TG333.17 (rolling mill equipment) and TG455 (surfacing welding), reflecting its dual relevance to metallurgical equipment and welding technology. The paper provides practical engineering experience from a production environment.

Core Technical Context

Rolling mill rolls are critical components in steel production, directly influencing the quality of rolled products and the operational efficiency of the mill. The roll surface is subjected to:

• Abrasive wear: Contact with hot steel during rolling.
• Thermal fatigue: Repeated heating and cooling cycles during rolling operations.
• Chemical attack: Contact with scale, oxides, and other reaction products.
• Impact loading: Dynamic forces during rolling.

The φ650 designation refers to the roll diameter, indicating a medium-sized rolling mill. The repair of worn rolls by surfacing is a cost-effective alternative to replacing entire rolls, provided the surfacing process and materials are appropriately selected.

Surfacing Repair Process

The repair process involves the following steps:

1. Removal of worn surface: The worn roll surface is machined or ground to remove the damaged layer and expose sound base material.
2. Surface preparation: The exposed surface is cleaned and prepared for surfacing, removing any scale, oil, or contaminants.
3. Surfacing deposition: Sintered flux submerged arc welding is used to deposit the surfacing layer on the roll surface.
4. Post-weld heat treatment: Annealing treatment is applied after surfacing to relieve residual stresses and optimize the microstructure.
5. Machining: The surfaced roll is re-machined to restore the original dimensions and surface finish.
6. Reuse: The repaired roll is returned to service.

Sintered Flux Characteristics

Sintered fluxes are manufactured by mixing powder ingredients (iron oxides, lime, dolomite, silica, fluorspar, and alloying elements) and sintering at high temperature to form a porous, granular material. Compared to fused fluxes, sintered fluxes offer several advantages for roll surfacing:

• Higher alloying capacity: Sintered fluxes can incorporate higher levels of alloying elements, allowing for precise control of the surfacing layer composition.
• Better wettability: The porous structure of sintered fluxes provides better wettability on the roll surface.
• Lower melting point: Sintered fluxes melt at lower temperatures than fused fluxes, reducing the heat input to the roll.
• Customizable composition: The flux composition can be tailored to specific surfacing requirements.

Surfacing Layer Composition and Properties

The surfacing layer composition is selected to provide the following properties:

• High hardness: Typically 400-600 HV for hot strip mill rolls, providing wear resistance against hot steel.
• Good thermal fatigue resistance: The ability to withstand repeated thermal cycling without cracking.
• Adequate toughness: To resist impact loading and prevent brittle fracture.
• Low thermal conductivity: To reduce heat transfer to the roll body, protecting the roll from thermal damage.

Typical Surfacing Layer Compositions for Rolling Mill Rolls

Post-Weld Heat Treatment

The annealing treatment after surfacing is critical for:

• Residual stress relief: Surfacing welding introduces significant residual stresses that can lead to cracking during service.
• Microstructure optimization: Annealing transforms the as-welded microstructure to a more stable and ductile condition.
• Hardness adjustment: Annealing can adjust the hardness to the desired level for the specific application.

The annealing temperature and time must be carefully controlled to avoid:

• Over-tempering: Excessive softening of the surfacing layer.
• Carbide coarsening: Loss of fine carbide dispersion that provides wear resistance.
• Thermal cracking: Cracking due to thermal expansion mismatch between the surfacing layer and the roll body.

Engineering Practice Implications

The practical experience described in this paper provides valuable insights for rolling mill roll repair operations:

1. Cost-effectiveness: Surfacing repair significantly reduces the cost of roll maintenance compared to roll replacement. A typical roll can be repaired multiple times before replacement is necessary.
2. Quality consistency: The submerged arc process with sintered flux provides consistent surfacing quality, which is critical for maintaining roll performance.
3. Downtime reduction: The repair process is faster than roll replacement, reducing mill downtime.
4. Material selection: The surfacing layer composition must be matched to the specific rolling application (hot strip, cold strip, plate, etc.) to achieve optimal performance.

Key Questions and Reflections

The paper, while practical in nature, raises several technical questions:

1. Long-term service life: How many repair cycles can a roll undergo before the accumulated defects (cracks, inclusions, etc.) compromise its structural integrity?
2. Interface quality: The bond between the surfacing layer and the roll body is critical. Any lack of fusion or cracking at the interface can lead to premature failure.
3. Flux consistency: The quality and consistency of sintered flux is critical for surfacing quality. Flux contamination or compositional variation can lead to surfacing defects.
4. Inspection methods: What non-destructive testing methods are used to verify the quality of the surfacing repair? Ultrasonic testing, magnetic particle testing, and hardness testing are common methods.

Study Insights and Implications

This paper represents practical engineering knowledge from the front lines of steel production. The use of sintered flux submerged arc surfacing for rolling mill roll repair is a well-established practice, but the specific details of the process parameters, flux composition, and heat treatment conditions are critical for success.

The key insight for practicing engineers is that roll repair is not simply a welding operation but a comprehensive process that involves surface preparation, surfacing, heat treatment, machining, and quality verification. Each step must be carefully controlled to ensure the repaired roll performs reliably in service.

The economic impact of roll repair is substantial. A single φ650 roll can cost tens of thousands of dollars, and a rolling mill may have dozens of rolls in service. The ability to repair rolls extends their service life and reduces the overall cost of rolling mill operations.