Microstructure Analysis of CO2 Gas Shielded Overlay Welded Roller Shaft

Literature Overview

The paper by Wen Jingxian (2002), published in Journal of Guangxi University (Natural Science Edition) (Vol. 27, No. 3, pp. 196-198), presents a microstructural and mechanical analysis of roller shafts (榨辊轴) repaired by CO2 gas shielded overlay welding. The study was conducted at the Industrial Testing and Experimental Center of Guangxi University and addresses a specific industrial application in sugar cane crushing equipment (甘蔗压榨机), where roller shafts undergo severe abrasive wear from continuous contact with fibrous cane material.

Application Context

Sugar cane crushing roller shafts are subjected to extreme operating conditions:

• Continuous abrasive contact with fibrous cane material containing silica particles
• High compressive loads from the crushing mechanism
• Wet environment with sugar juice and organic acids
• Continuous operation for extended periods during crushing campaigns
• Large diameter (typically 400-600 mm) requiring substantial overlay thickness

The repair of worn roller shafts by overlay welding offers significant economic advantages over complete replacement, particularly for large-diameter shafts where forging and machining costs are substantial.

Welding Process Parameters

CO2 gas shielded overlay welding (GMAW with 100% CO2 shielding) was selected for the following reasons:

• High deposition rate suitable for large repair areas
• Good penetration for bonding to base material
• Availability of specialized overlay consumables
• Cost-effectiveness for large-scale repair operations
• Suitability for field repair conditions

Typical Process Parameters for Roller Shaft Overlay

Microstructural Analysis

The study identified sorbite (索氏体) as the primary microstructure in the overlay weld surface. This finding is significant and warrants detailed discussion.

Microstructural Composition

Significance of Sorbite Structure

The formation of sorbite (fine lamellar pearlite with intermixed ferrite) in the overlay surface is particularly advantageous for roller shaft applications because:

1. Balanced mechanical properties: Sorbite provides a favorable combination of strength and toughness, unlike martensite which offers high hardness but poor fracture resistance, or coarse pearlite which offers low hardness.
2. Abrasive wear resistance: The fine lamellar structure of sorbite resists abrasive wear through the mechanism of work hardening—each abrasion event deforms the lamellae rather than fracturing them.
3. Impact resistance: The ductile ferrite component absorbs impact energy from cane fiber impacts, preventing crack initiation and propagation.
4. Corrosion resistance: The relatively uniform microstructure of sorbite provides more consistent corrosion behavior compared to heterogeneous structures with segregated carbides.
5. Thermal stability: Sorbite is more stable at operating temperatures (50-80°C in cane crushing) compared to martensite, which may temper unpredictably during service.

Mechanical Property Evaluation

The CO2 gas shielded overlay weld provides:

The key finding is that the overlay simultaneously maintains the original strength and toughness of the shaft while significantly improving surface wear resistance—a rare combination that makes this approach highly effective for roller shaft repair.

Comparison with Alternative Repair Methods

The CO2 gas shielded overlay welding offers the best balance of cost, wear resistance, and toughness maintenance for this specific application.

Process Optimization Recommendations

Based on the study findings and engineering experience, the following optimization recommendations are made:

1. Consumable selection: Use Cr-C or Cr-Mo-C hardfacing wire with controlled carbon content (1.5-3.0%) to promote sorbite formation rather than martensite.
2. Heat input control: Maintain moderate heat input (10-20 kJ/cm) to ensure adequate cooling rates for sorbite transformation without excessive dilution.
3. Layer design: Implement 3-4 layers with 2-3 mm thickness per layer, ensuring complete fusion between layers while maintaining the desired microstructure in the surface layer.
4. Post-weld treatment: Optional low-temperature tempering at 250-300°C for 2 hours can relieve residual stresses without significantly reducing hardness or altering the sorbite structure.
5. Surface finish: Final grinding to Ra 3.2-6.3 μm improves the tribological performance of the overlay surface.

Summary

This study demonstrates that CO2 gas shielded overlay welding is an effective and economical method for repairing worn sugar cane crushing roller shafts. The formation of sorbite microstructure in the overlay surface provides an optimal balance of abrasive wear resistance, impact toughness, and mechanical strength that matches the demanding service conditions of cane crushing operations. The approach maintains the structural integrity of the shaft while extending service life by 2-3 times at minimal cost compared to replacement. This makes CO2 gas shielded overlay welding one of the most practical and cost-effective solutions for roller shaft restoration in sugar processing industries, and the methodology can be extended to similar applications involving large-diameter rotating shafts subjected to abrasive wear.