Overlay Welding Repair Process for Four-Roll Crusher Roller Shell

Literature Overview

This technical paper by Gan Zhixi, published in Welding (1997, No. 4, p. 22), describes the overlay welding repair and strengthening of roller shells for a four-roll crusher used in a metallurgical enterprise for coke crushing. The author is from the Huaguang Equipment Repair Center of Xiangtan Iron and Steel Company. The paper provides practical details on the repair process, including pre-weld preparation, welding parameters, and economic benefits. The classification number TG455 indicates its focus on welding processes and technology.

Equipment Background and Failure Analysis

The four-roll crusher is a critical piece of equipment in metallurgical enterprises for coke crushing. It consists of two sets of rollers, and the gap between the rollers is adjusted to control the coke particle size to meet production requirements. The roller shells are mounted on the rollers and are classified as wear parts with a normal service life of 3 to 4 months under typical operating conditions.

The primary failure modes of the roller shells are:

The roller shell material is 30CrMo cast steel, which is a medium carbon alloy steel with good mechanical properties but limited wear and corrosion resistance for the severe conditions in a coke crusher. The annual consumption of approximately 50 roller shells per plant indicates a significant cost and maintenance burden.

Repair Process Details

Pre-Weld Preparation

The paper describes a thorough pre-weld preparation process that is essential for successful overlay welding:

1. Machining: The cracked and fatigued surface layer of the roller shell must be completely removed by machining. This is critical because welding over existing cracks will result in crack propagation and weld failure.
2. Surface Repair: Deep depressions are filled with J506 electrodes (a low-hydrogen, iron-powder, cellulosic electrode) and ground smooth. This ensures a uniform base for the overlay welding.
3. Preheating: The roller shell is placed in a heating furnace and heated at a rate of 50°C/h to approximately 250°C, followed by a 2-hour hold. This controlled preheating is essential for preventing hydrogen-induced cracking in the 30CrMo base metal.

Welding Process

The overlay welding process involves multiple passes to build up the wear-resistant layer. The specific welding parameters are not detailed in the brief paper, but based on the material and application, the following parameters would be appropriate:

• Welding Process: Shielded metal arc welding (SMAW) or submerged arc welding (SAW)
• Electrode Type: Low-hydrogen, high-alloy electrodes suitable for 30CrMo base metal
• Deposition Rate: Multiple passes to build up the required overlay thickness
• Interpass Temperature: Maintained above 200°C to prevent excessive HAZ hardness
• Post-Weld Heat Treatment: Stress relief at 550-650°C to reduce residual stresses

Metallurgical Considerations

The 30CrMo base metal is susceptible to hardening in the heat-affected zone due to the formation of martensite during rapid cooling. The preheating to 250°C is specifically designed to slow the cooling rate and promote the formation of tempered martensite or bainite rather than untempered martensite. The controlled heating rate of 50°C/h prevents thermal shock that could initiate cracks in the existing material.

The overlay weld material selection is critical for achieving the desired wear and corrosion resistance. For coke crusher roller shells, the overlay material should have the following properties:

• High hardness (above 500 HV) for wear resistance
• Good toughness to resist impact loading
• Corrosion resistance to coke gases and moisture
• Compatibility with 30CrMo base metal to prevent cracking at the fusion line

Common overlay materials for this application include high-chromium cast irons (such as Cr20 or Cr26), nickel-based alloys, or martensitic stainless steels. The choice depends on the specific operating conditions and cost considerations.

Economic Analysis and Benefits

The paper emphasizes the economic benefits of overlay welding repair over replacement. With an annual consumption of 50 roller shells, the cost savings from repair are substantial. The overlay welding approach extends the service life of each roller shell by 2 to 3 times, effectively reducing the annual replacement requirement to 15-25 shells.

Engineering Practice Integration

From a maintenance engineering perspective, this paper exemplifies the systematic approach to equipment repair that should be adopted in industrial settings. The key elements of a successful repair program include:

1. Failure Analysis: Understanding the root causes of failure to select appropriate repair methods
2. Process Development: Developing and validating repair procedures before implementation
3. Quality Control: Implementing rigorous inspection protocols to ensure repair quality
4. Documentation: Maintaining detailed records of repair procedures and outcomes
5. Continuous Improvement: Refining repair procedures based on field performance data

The preheating protocol described in the paper is particularly noteworthy. The controlled heating rate of 50°C/h and the extended hold time of 2 hours reflect a deep understanding of the metallurgical requirements for 30CrMo steel. This level of process control is essential for preventing hydrogen-induced cracking and ensuring long-term repair reliability.

Key Questions and Reflections

One important question is the long-term durability of the overlay weld in the severe coke crusher environment. The combination of mechanical wear, thermal cycling, and chemical corrosion creates a challenging service environment. The overlay material must be selected not only for initial hardness but also for resistance to progressive degradation under these combined loading conditions.

Another consideration is the effect of repeated repair cycles on the base metal. Each repair cycle subjects the base metal to additional thermal cycles, which can affect its mechanical properties. The number of repair cycles before replacement is required should be determined through systematic monitoring of the base metal condition.

Study Insights and Practical Recommendations

This paper provides a practical example of how overlay welding can be used to extend equipment life and reduce maintenance costs in industrial settings. The systematic approach to pre-weld preparation, particularly the controlled preheating protocol, is a model that should be adopted for all overlay welding repairs on alloy steel components.

For engineers undertaking similar repair operations, I recommend the following: first, conduct a thorough failure analysis to understand the operating conditions and select appropriate overlay materials; second, develop a detailed repair procedure including preheat, welding, and post-weld heat treatment parameters; third, implement a quality control plan including visual inspection, magnetic particle testing, and hardness measurement; and fourth, monitor the repaired component during service to validate the repair effectiveness.

The economic benefits of overlay welding repair are clear and compelling. For high-value wear components in continuous production processes, the investment in developing and implementing overlay welding repair procedures pays for itself quickly through reduced replacement costs and minimized downtime. This paper serves as a practical guide for engineers seeking to implement such programs in their own operations.