CO2 Gas Shielded Automatic Overlay Welding on SS1 Electric Locomotive Driving Wheel Gear Seats

Literature Overview

The paper by Sun Zhixiang, published in Locomotive and Car Manufacturing Technology (1991, No. 5, pp. 48-49), documents the implementation of CO2 gas shielded automatic overlay welding for repairing the driving wheel gear seats on SS1-type electric locomotives at Taiyuan Locomotive and Car Factory. The technical challenge addressed is the restoration of dimensional accuracy on the outer cylindrical surface of the gear seat after disassembly of the gear core for gear ring replacement. This is a classic application of overlay welding for dimensional restoration in heavy mechanical components.

Technical Background and Problem Statement

The SS1 electric locomotive driving wheel gear seat is a large forged steel component that interfaces with the traction gear core through a shrink-fit or press-fit connection. During overhaul repair, the gear core must be removed and a new gear ring installed, after which the gear core is press-fitted back onto the gear seat. The repeated assembly/disassembly cycle causes wear on the cylindrical bore of the gear seat, reducing the interference fit and compromising assembly pressure. Since the internal bore of the gear core cannot be overlaid (due to stress concentration concerns at the tooth root), the solution is to overlay weld the outer cylindrical surface of the gear seat to restore its diameter.

Component Specifications and Requirements

Process Development: From Manual to Automatic Welding

The initial approach employed manual arc welding (SMAW), which suffered from:

• Low deposition efficiency: Manual SMAW deposition rates of approximately 0.5–1.0 kg/h.
• High labor intensity: Continuous welding in confined positions with poor accessibility.
• Extended repair cycle: Multiple operators required for shift work, extending turnaround time.
• Inconsistent quality: Operator-dependent bead geometry and dilution control.

The transition to CO2 gas shielded automatic welding (GMAW with automatic wire feed and mechanized torch travel) addressed these limitations:

• Deposition rate: Increased to 3.0–5.0 kg/h with automatic wire feed.
• Consistent bead geometry: Machine-controlled parameters ensure uniform bead width and height.
• Reduced repair cycle: Single shift coverage possible with mechanized operation.
• Improved quality reproducibility: Automated parameters reduce operator variability.

CO2 Gas Shielded Automatic Overlay Welding Parameters

Quality Control and Defect Prevention

The automatic overlay welding process introduces specific quality concerns:

• Spatter: CO2 shielding produces significant spatter due to the dual-stage arc characteristic. Countermeasures include the use of a flux-containing wire or a thin layer of anti-spatter agent on the substrate.
• Porosity: Inadequate gas flow or wind contamination can cause gas porosity. A minimum gas flow of 15 L/min with wind shielding is recommended.
• Surface waviness: Automatic travel on a cylindrical surface requires precise mechanical guidance to maintain constant stand-off distance.
• Residual stress: Multi-pass overlay on a large forging can induce significant residual stress. Post-weld machining removes the outer layer, effectively relieving surface stresses.

The quality verification protocol includes:

1. Visual inspection of all overlay welds for surface continuity.
2. Magnetic particle testing (MT) for surface cracks.
3. Hardness testing at multiple locations to verify no excessive softening or hardening.
4. Dimensional check after machining to confirm final geometry meets H7 tolerance.

Engineering Practice and Efficiency Gains

The implementation of CO2 automatic overlay welding at Taiyuan Locomotive and Car Factory resulted in:

• Repair cycle reduction: From 3–5 days (manual) to 0.5–1 day (automatic).
• Material utilization improvement: Precise control of overlay thickness reduced excess material by 30–40%.
• Operator productivity: One operator could supervise multiple welding stations simultaneously.
• Quality consistency: Defect rate reduced from approximately 5% to below 1%.

This case study exemplifies the broader principle that automation of overlay welding is most beneficial when applied to large, geometrically regular surfaces where the overlay thickness is relatively uniform and the component is accessible to mechanized torch travel.

Study Insights and Reflections

The SS1 locomotive gear seat overlay welding case represents an early and practical application of mechanized GMAW for dimensional restoration. The paper's value lies not in novel metallurgy but in the systematic engineering approach to process selection and implementation. The decision to overlay the outer surface rather than the inner bore demonstrates sound engineering judgment — it avoids stress concentration at the tooth root while maintaining the structural integrity of the gear core. The transition from manual to automatic welding also illustrates the economic case for mechanization in batch repair operations. For contemporary engineers, this paper serves as a reminder that process selection must be driven by the specific geometry, production volume, and quality requirements of the application, not by a preference for the most advanced welding technology available.