Multi-Layer Multi-Pass Overlay Welding Repair of Sintering Car Bodies

Literature Overview

This 2005 study by Cheng Jun and Guo Changqing from Inner Mongolia University of Science and Technology, published in Hot Working Technology (Vol. 34, No. 12, p. 85), addresses the repair of sintering car bodies used in iron ore sintering operations at Baotou Steel (Baogang). The study combines failure analysis with overlay welding process development, demonstrating a systematic approach to equipment repair that is directly applicable to similar heavy-duty structural components in the iron and steel industry.

Application Background

Sintering car bodies are critical components in iron ore sintering plants, where they carry green sinter mix through the sintering bed and through the cooling and discharge sections. The operating environment is characterized by:

The combination of high-temperature service, abrasive wear, thermal cycling, and mechanical loading leads to progressive material loss and eventual structural failure of the car body. Traditional repair methods such as replacement of worn plates are often impractical due to the complex geometry and the integrated structure of the car body.

Failure Analysis and Repair Strategy

The authors conducted a systematic failure analysis of the sintering car body, which revealed:

• Primary failure mode: Progressive material loss due to abrasive wear at the bottom and side plates, combined with thermal fatigue cracking at stress concentration points
• Secondary failure mode: Cracking at existing weld joints due to thermal cycling and residual stress accumulation
• Material degradation: Localized softening of the base steel at the surface due to high-temperature exposure, reducing the remaining material's resistance to further wear

Based on this analysis, the authors developed a multi-layer multi-pass overlay welding repair strategy. The key design decisions included:

1. Welding process: Submerged arc welding (SAW) was selected for its high deposition rate, deep penetration, and ability to produce dense, defect-free weld metal suitable for multi-pass overlay applications.
2. Welding consumables: Low-alloy steel wire electrodes with appropriate composition to match the base material's strength and thermal expansion characteristics were selected.
3. Multi-layer multi-pass approach: Multiple layers of weld metal were deposited, with each layer consisting of multiple passes, to build up the required material thickness while ensuring adequate fusion and bonding between layers.

Metallurgical and Mechanical Characterization

The study reports the following characterization results for the repaired car body:

The multi-layer multi-pass approach was shown to be effective in achieving a homogeneous overlay layer with consistent mechanical properties. The absence of cracks, porosity, and lack of fusion indicates that the welding parameters were well-controlled and that the preheat and interpass temperature management were adequate.

Engineering Practice Implications

The study demonstrates several important principles for overlay welding repair of heavy-duty structural components:

1. Failure analysis is essential: Without a thorough understanding of the failure mechanism, the repair strategy may be ineffective or even counterproductive. The identification of abrasive wear as the primary failure mode guided the selection of welding consumables and process parameters.
2. Multi-layer multi-pass welding provides superior results: Compared to single-layer or single-pass welding, the multi-layer approach allows for better control of cooling rates, reduced residual stresses, and improved mechanical properties of the final overlay layer.
3. Process qualification is critical: The systematic approach to process development, including chemical analysis, mechanical testing, and metallographic examination, provides confidence in the repair quality and ensures that the repaired component meets the same performance standards as a new component.
4. Economic and environmental benefits: Repairing existing car bodies through overlay welding is significantly more economical than manufacturing new ones, and it reduces waste and energy consumption associated with component replacement.

Key Insights and Reflections

The study's demonstration that overlay welding repair can restore sintering car bodies to a service life equivalent to new components is a strong endorsement of the technique. However, several practical considerations deserve emphasis:

• Geometric constraints: The complex geometry of sintering car bodies, with their curved surfaces, internal ribs, and integrated structural features, presents challenges for welding access and heat input control. The multi-layer multi-pass approach helps mitigate these challenges by allowing gradual build-up of material and controlled thermal cycling.
• Residual stress management: Heavy section steel structures are prone to welding-induced residual stresses, which can contribute to cracking and distortion. The multi-layer approach, combined with appropriate preheat and post-weld treatment, helps manage residual stresses.
• Long-term performance: While the study reports satisfactory immediate results, the long-term performance of the repaired car body under sustained abrasive wear and thermal cycling should be monitored to validate the repair strategy over extended service periods.

The study also highlights the importance of integrating metallurgical knowledge with practical welding engineering. The selection of welding consumables that match the base steel's composition and the development of welding parameters that produce the desired microstructure and mechanical properties are both essential for successful repair.

Summary

This study provides a practical and well-documented case of overlay welding repair for sintering car bodies in iron ore sintering operations. The systematic approach — from failure analysis through process development to quality verification — demonstrates the value of a metallurgy-driven repair strategy. The multi-layer multi-pass overlay welding technique, when properly executed with appropriate process parameters and quality controls, can restore worn heavy-duty structural components to a performance level equivalent to new components. For engineers involved in equipment maintenance and repair in the iron and steel industry, the work offers a model for systematic approach to welding repair that combines technical rigor with practical effectiveness.