Overlay Welding Repair of Converter Support Rings in Steelmaking

Literature Overview

The paper by Qiu Fuxiang and Xu Kejian, published in Welding Technology (Vol. 29, No. 4, 2000, pp. 44), describes the overlay welding repair of converter support rings at Xiangtan Iron and Steel Group. Converter support rings are critical structural components in steelmaking converters that support the converter shell and withstand severe thermal and mechanical loading during the steelmaking process. Wear and damage to the support rings can lead to production stoppages and safety hazards, making their timely repair essential for maintaining plant productivity and safety.

Core Technical Content

Service Conditions and Failure Modes

Converter support rings operate under extremely harsh conditions in the steelmaking environment:

• Thermal cycling: The support rings are exposed to repeated heating and cooling cycles as the converter is charged with hot scrap and molten steel, and then cooled during tapping and relining.
• Mechanical loading: The support rings bear the weight of the converter shell and its contents, as well as dynamic loads from tilting and rotating operations.
• Abrasive wear: The support rings are exposed to abrasive materials including molten slag, refractory particles, and scale, which cause progressive wear of the support surface.
• Thermal shock: Rapid temperature changes can cause thermal cracking and spalling of the support ring surface.

The primary failure mode is progressive wear of the support surface, which reduces the effective contact area and increases stress concentrations, eventually leading to structural failure if not repaired.

Overlay Welding Repair Process

The overlay welding repair process for converter support rings involves the following key steps:

1. Inspection and assessment: Evaluate the extent of wear and damage, measure the remaining thickness, and determine the required repair thickness.
2. Surface preparation: Clean the worn surface by grinding or machining to remove oxide scale, refractory deposits, and damaged material.
3. Preheating: Apply controlled preheating to reduce residual stresses and prevent cracking, especially for thick-section support rings.
4. Overlay welding: Apply multiple passes of wear-resistant overlay weld metal to restore the support surface to its original dimensions.
5. Post-weld treatment: Perform stress-relief annealing if required by the material specification.
6. Inspection: Inspect the overlay weld for defects using magnetic particle testing or penetrant testing.

Selection of Overlay Welding Materials

The selection of overlay welding materials is critical for the success of the repair. The overlay weld metal must provide:

• Adequate hardness: To resist abrasive wear from slag and refractory particles.
• Good toughness: To withstand thermal shock and mechanical loading without cracking.
• Good weldability: To produce sound welds without excessive cracking or porosity.
• Compatibility with base metal: To avoid adverse metallurgical interactions at the weld interface.

Common overlay welding materials for converter support rings include:

Process Parameters and Quality Control

The paper describes the following process parameters for the overlay welding repair:

• Welding process: Shielded metal arc welding (SMAW) or submerged arc welding (SAW), depending on the thickness and accessibility of the repair area.
• Welding electrode: Selected based on the required hardness and toughness, with consideration of the base metal composition.
• Preheat temperature: Typically 200-350°C for carbon steel support rings, higher for alloy steels.
• Interpass temperature: Controlled to prevent excessive grain growth and softening, typically maintained below 300°C.
• Welding sequence: Carefully planned to minimize distortion and residual stresses, using a balanced welding pattern.

Quality control measures include:

• Visual inspection of all welds for surface defects.
• Magnetic particle testing of the overlay weld surface to detect cracks.
• Hardness testing of the overlay weld metal and heat-affected zone.
• Dimensional inspection to verify restoration of original dimensions.

Engineering Practice and Implementation

Challenges in Converter Support Ring Repair

The repair of converter support rings presents several unique challenges:

• Limited access: The support rings are often located in confined spaces within the converter structure, making welding access difficult.
• Thick sections: Support rings can have thicknesses of 50-100 mm or more, requiring extensive preheating and careful welding to prevent cracking.
• Residual stresses: The existing residual stresses from the original fabrication and service history can be exacerbated by the repair welding, increasing the risk of cracking.
• Production pressure: Steelmaking plants operate continuously, and any downtime for repair is costly, creating pressure to complete repairs quickly.

Integration with Plant Maintenance Programs

The overlay welding repair of converter support rings should be integrated into the plant's overall maintenance program:

• Scheduled inspection: Regular inspection of support rings during converter relining to identify wear before it becomes critical.
• Predictive maintenance: Use of wear monitoring techniques to predict remaining service life and plan repairs proactively.
• Pre-planned repair: Schedule overlay welding repairs during planned maintenance outages to minimize production disruption.
• Documentation: Maintain detailed records of all repairs, including welding procedures, materials used, and inspection results, for traceability and future reference.

Study Insights and Reflections

The paper by Qiu Fuxiang and Xu Kejian provides a concise but practical description of the overlay welding repair of converter support rings in a steelmaking environment. The work highlights the importance of overlay welding as a cost-effective and reliable repair method for critical components in harsh industrial environments. The emphasis on process parameter control and quality assurance is particularly relevant, as the success of the repair depends on the ability to produce sound welds with adequate mechanical properties under challenging conditions.

One observation from my perspective is that the paper could benefit from more detailed discussion of the long-term performance of the repaired support rings. While the immediate success of the repair is described, the durability of the overlay weld under continued service conditions is not addressed. Follow-up studies on the service life of overlay-welded support rings would be valuable for establishing confidence in the repair method and for optimizing the process parameters for maximum service life.

Additionally, the paper does not discuss the potential for alternative repair methods, such as surfacing with laser cladding or thermal spraying, which may offer advantages in terms of dilution control, microstructural refinement, and surface finish. However, the practical considerations of equipment availability, cost, and operator skill may favor traditional arc welding methods in many steelmaking environments.

Summary

This paper presents a practical approach to the overlay welding repair of converter support rings in steelmaking applications, addressing the challenges of severe thermal and mechanical loading, abrasive wear, and limited access. The described process, which involves careful selection of overlay welding materials, controlled preheating and welding parameters, and rigorous quality control, provides a reliable method for restoring worn support rings to serviceable condition. Engineers involved in steelmaking equipment maintenance should study this work to understand the overlay welding repair process and to consider its application to other worn components in their facilities, contributing to improved equipment reliability and reduced downtime.