Application of MIG Surfacing in Remanufacturing of High-Power Diesel Engine Exhaust Valves

Literature Overview

This research by Zhou Fangming, Zhang Fuqiang, Miao Baohai, Yu Dan, and Liu Wei from Jiangsu University's Key Laboratory of Advanced Welding Technology (Jiangsu Province), published in Electrician Welder in 2012 (Volume 42, Issue 5, pages 86-89), presents the development and application of a robotic flexible MIG surfacing system for the remanufacturing of marine diesel engine exhaust valve discs. The classification code U664.121 places this work within the marine engineering domain. This study addresses a significant economic and environmental challenge in the marine industry: the high cost and limited availability of replacement exhaust valves for large marine diesel engines, which are typically monopolized by developed countries.

Engineering Context and Problem Definition

Marine diesel engine exhaust valves operate under extremely severe conditions:

• High temperatures (exhaust gas temperatures typically 400-600°C)
• Corrosive atmosphere (sulfur compounds, ash deposits)
• Mechanical loading (spring force, gas pressure differential)
• Thermal cycling (repeated heating and cooling during engine operation)
• Erosion from high-velocity exhaust gas flow

These conditions lead to frequent valve failure through combined mechanisms of:

• Thermal fatigue cracking
• Corrosion of the sealing face
• Erosion of the valve disc surface
• Wear at the valve seat contact area

The manufacturing technology for these valves has historically been controlled by a few developed nations, resulting in high costs and long lead times for replacement. Remanufacturing through surfacing offers a cost-effective alternative that can restore valve functionality while reducing environmental impact through material conservation.

Technical Approach and System Development

The research team developed a robotic flexible MIG surfacing system specifically designed for exhaust valve disc remanufacturing. Key aspects of the system include:

System Architecture

Process Development

The surfacing process involves the following key steps:

1. Surface preparation: Cleaning and roughening of the worn valve disc surface to ensure adequate metallurgical bond
2. Preheating: Controlled preheating to reduce thermal stress and prevent cracking
3. Multi-pass surfacing: Sequential deposition of Ni-based alloy layers to build up the required thickness
4. Post-weld treatment: Stress relief and surface finishing to meet dimensional and quality requirements

Quality Requirements

The paper reports that the remanufactured valve discs met the following quality criteria:

• No defects exceeding 0.06 mm in size
• Complete conformance with marine diesel engine exhaust valve remanufacturing quality standards
• Adequate metallurgical bond between surfacing layer and substrate

Material Selection and Metallurgical Considerations

The selection of Ni-based alloy for surfacing SNCrW stainless steel substrate involves careful consideration of several metallurgical factors:

SNCrW Stainless Steel Substrate

SNCrW is a high-temperature stainless steel alloy designed for valve applications, characterized by:

• Excellent oxidation resistance at elevated temperatures
• Good creep strength
• Resistance to thermal fatigue
• Moderate corrosion resistance in exhaust environments

The Ni-based surfacing alloy complements the substrate by providing:

• Superior corrosion resistance in the most severe areas (sealing face)
• Enhanced erosion resistance at the gas flow path
• Improved sealing surface quality
• Extended service life under combined degradation mechanisms

Process Parameters and Optimization

The robotic MIG surfacing process requires careful optimization of several parameters to achieve the required quality:

• Wire feed speed: Controls deposition rate and heat input
• Travel speed: Affects bead width, profile, and dilution
• Shielding gas flow rate: Ensures adequate protection of the molten pool
• Arc length: Affects transfer mode and weld quality
• Preheat temperature: Typically 150-250°C for this application
• Interpass temperature: Controlled to manage thermal cycling
• Number of passes: Determined by required build-up thickness

The flexible robotic system allows for complex trajectory programming to conform to the curved geometry of the valve disc, ensuring uniform coverage and adequate build-up across the entire working surface.

Engineering Practice and Quality Assurance

The remanufacturing process requires rigorous quality assurance at each stage:

1. Incoming inspection: Assessment of worn valve condition to determine if remanufacturing is feasible
2. Surface preparation verification: Visual and dimensional inspection of prepared surface
3. Process monitoring: Real-time monitoring of welding parameters during surfacing
4. In-process inspection: Visual inspection between passes for defect detection
5. Final inspection: Comprehensive quality assessment including:

• Visual inspection for surface quality
• Dimensional verification against specifications
• Non-destructive testing (PT or MT) for surface defects
• Hardness testing to verify proper alloy composition
• Leak testing to verify sealing integrity

PDCA Cycle Application

The development of this remanufacturing process follows the PDCA (Plan-Do-Check-Act) cycle:

• Plan: Define quality requirements, select materials, develop process parameters
• Do: Implement the robotic surfacing process on actual valve discs
• Check: Inspect and test the remanufactured valves against quality criteria
• Act: Optimize process parameters based on inspection results and implement improvements

Key Reflections and Study Insights

This research demonstrates the viability of robotic MIG surfacing as a technology for remanufacturing high-value marine components, with significant implications for the marine industry:

• Economic benefits: Remanufactured valves can be produced at a fraction of the cost of new valves, reducing operational costs for shipping companies
• Environmental benefits: Remanufacturing conserves materials and reduces waste compared to manufacturing new components
• Supply chain resilience: Provides an alternative to dependence on foreign manufacturers for critical marine components
• Technology transfer: The robotic system developed can potentially be adapted for other marine component remanufacturing applications

The quality results achieved (no defects exceeding 0.06 mm) demonstrate that robotic MIG surfacing can meet the demanding quality requirements of marine applications when properly developed and controlled. This finding is particularly significant because it validates the use of MIG surfacing, which is generally considered less precise than processes such as TIG or plasma arc surfacing, for high-quality marine component repair.

The study also highlights the importance of material compatibility in surfacing applications. The successful combination of Ni-based alloy with SNCrW stainless steel substrate demonstrates that careful selection of surfacing alloys can overcome potential metallurgical incompatibilities through appropriate process control.

For the broader welding engineering community, this research contributes to the growing body of knowledge on robotic surfacing technology for component remanufacturing, providing a template for similar applications in other industries where high-value components are subject to wear and degradation.

Concluding Remarks

These five studies collectively represent significant contributions to the field of surfacing welding technology, spanning fundamental research on alloy design, process development, testing methodology, and engineering applications. From the metallurgical investigation of chromium effects in Fe-C-V-B alloys to the practical application of robotic MIG surfacing for marine valve remanufacturing, these works demonstrate the breadth and depth of the surfacing welding field. Engineers involved in steel pipe manufacturing, pipe fitting production, and welding quality control can draw valuable insights from these studies, particularly regarding alloy selection for combined wear-corrosion environments, hydrogen damage assessment methods, and robotic surfacing technology for component repair and remanufacturing. The systematic approach to understanding wear mechanisms, microstructural evolution, and process-parameter interactions presented in these papers provides a solid foundation for rational surfacing alloy design and process optimization in industrial applications.