Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Sintering Machine Toothed Rollers and Grates
Literature Overview
This paper by Yu Guangming, Zhong Yi, and Duan Liren, published in China Surface Engineering in 2006 (Vol. 19, No. 3, pp. 51–54), documents the development and implementation of a high-chromium cast iron wear-resistant overlay welding alloy for the repair of toothed rollers and grate plates in large-scale sintering machines at Wuhan Iron and Steel Group. The study combines failure analysis of worn components with the development of a proprietary overlay welding alloy, optimization of welding parameters, and process improvement, resulting in a mature and successful manufacturing technology.
Failure Analysis and Operating Conditions
The toothed rollers and grate plates in sintering machines operate under extremely harsh conditions characterized by:
- Abrasive wear from iron ore, coke, and flue dust particles at temperatures ranging from ambient to approximately 400–600°C.
- Impact loading from the falling sinter material, which creates dynamic stress concentrations at the tooth tips and edges.
- Thermal cycling due to the hot sinter bed and the relatively cooler ambient environment, leading to thermal fatigue.
- Chemical attack from sulfur compounds and other corrosive elements present in the sintering process.
The original components, typically made from medium-carbon steel or low-alloy steel, suffer from rapid wear at the tooth tips, leading to reduced tooth profile accuracy, poor material handling efficiency, and eventual component failure requiring replacement or repair. The failure analysis revealed that the primary damage mechanism was abrasive wear, with secondary contributions from thermal fatigue cracking at the tooth roots and impact damage at the tooth tips.
Development of the High-Chromium Cast Iron Overlay Alloy
The authors developed a high-chromium cast iron alloy specifically designed for overlay welding applications. The alloy design philosophy was based on the following principles:
- High chromium content (typically 20–30% Cr) to promote the formation of chromium carbides (M7C3 and M23C6), which provide excellent abrasion resistance.
- Eutectic composition to ensure the formation of a carbide matrix in the as-cast condition, maximizing hardness.
- Weldability optimization to minimize cracking susceptibility, which is a major challenge with high-carbon, high-chromium cast irons due to their high carbon equivalent and low ductility.
| Alloy Design Parameter | Typical Value | Purpose |
|---|---|---|
| Carbon content | 2.5–3.5 wt% | Promote carbide formation |
| Chromium content | 20–30 wt% | Chromium carbide matrix |
| Manganese content | 1.0–2.0 wt% | Solid solution strengthening |
| Silicon content | 1.0–2.0 wt% | Deoxidation and carbide modification |
| Nickel content | 2–5 wt% | Improve toughness and reduce cracking |
| Molybdenum content | 1–3 wt% | Refine microstructure and improve high-temperature properties |
| Hardness target | 55–65 HRC | Balance wear resistance and toughness |
The addition of nickel and molybdenum is particularly important for weldability. Nickel reduces the carbon equivalent and improves the ductility of the weld metal, while molybdenum refines the microstructure and improves resistance to thermal fatigue. The alloy was designed to have a carbon equivalent (CE) below 4.5% to reduce cold cracking susceptibility.
Welding Process Optimization
The welding process was optimized through systematic trials, and the following parameters were determined to be optimal:
| Process Parameter | Optimized Value | Notes |
|---|---|---|
| Welding process | SMAW (shielded metal arc welding) | Most practical for field repair |
| Electrode type | Rutile-fluxed coated electrode | Good arc stability and slag protection |
| Electrode diameter | 3.2 mm or 4.0 mm | Depends on repair area size |
| Arc voltage | 22–28 V | Low voltage for controlled heat input |
| Welding current | 120–180 A | Moderate current for penetration |
| Travel speed | 150–250 mm/min | Adequate for thick deposits |
| Preheating temperature | 250–350°C | Reduce residual stress and prevent cracking |
| Interpass temperature | 250–350°C | Maintain uniform thermal conditions |
| Post-weld cooling | Controlled (air cooling or furnace cooling) | Avoid rapid cooling that causes cracking |
The preheating temperature is critical for high-chromium cast iron overlay welding. A preheat of 250–350°C reduces the cooling rate below the martensite start temperature threshold, promoting the formation of austenite-ferrite-martensite microstructures instead of 100% martensite, which would be extremely brittle and prone to cracking.
Manufacturing Technology and Process Improvement
The paper describes a comprehensive manufacturing technology that includes:
- Surface preparation: Removal of worn material by grinding or machining to create a clean, flat base for the overlay deposit. The surface should be free of scale, rust, and other contaminants.
- Multi-pass welding: The overlay is built up in multiple passes to achieve the required thickness (typically 5–10 mm). Each pass is carefully controlled to maintain the interpass temperature within the specified range.
- Post-weld heat treatment: After welding, the component is subjected to a stress-relief heat treatment at 500–600°C for 2–4 hours, followed by controlled cooling. This reduces residual stresses and improves the toughness of the overlay deposit.
- Machining and finishing: The overlay surface is machined to restore the original tooth profile geometry. The high hardness of the deposit (55–65 HRC) requires specialized cutting tools, typically polycrystalline diamond (PCD) or cubic boron nitride (CBN) inserts.
Performance Results and Engineering Significance
The optimized overlay welding technology resulted in a significant improvement in service life compared to the original components. The field trials demonstrated:
- Wear life improvement: 3–5 times the original service life.
- Reduced downtime: Fewer unplanned shutdowns for component replacement.
- Cost savings: Repair costs were significantly lower than replacement costs.
- Process reliability: The technology was mature enough for routine production use.
From an engineering practice perspective, this study exemplifies the PDCA (Plan-Do-Check-Act) cycle applied to welding technology development. The failure analysis provided the "Plan" phase, the alloy development and process optimization constituted the "Do" phase, the performance testing and field trials represented the "Check" phase, and the process standardization and documentation completed the "Act" phase. This systematic approach is a model for welding technology development in industrial settings.
Key Reflections and Practice Integration
The paper highlights several important lessons for welding engineers:
- Alloy design must consider weldability: A high-wear-resistance alloy that cannot be welded without cracking is of limited practical value. The balance between wear resistance and weldability is a fundamental design challenge.
- Process parameters are as important as alloy composition: Even a well-designed alloy can produce poor results if the welding parameters are not properly controlled.
- Field conditions must be considered: Laboratory-optimized parameters may need adjustment for field repair conditions, where preheating and post-weld cooling are more difficult to control.
- Standardization is essential: The development of a documented, repeatable process is critical for widespread adoption and quality consistency.
This study demonstrates the practical value of overlay welding as a repair technology for heavy industrial equipment. The systematic approach to alloy development, process optimization, and performance validation provides a valuable framework for similar welding repair applications in other industries.