Application of Overlay Welding Technology in Equipment Maintenance for Petrochemical and Metallurgical Industries

Literature Overview

The paper by Hu Bangxi, Mang Kelun, Wang Jingjie, and Xu Binshi, published in China Surface Engineering (2006, Vol. 19, No. 3, pp. 4–8), provides a comprehensive review of overlay welding technology applications in the maintenance and repair of large equipment in China's petrochemical and metallurgical industries. The authors represent Wuhan University of Technology, the China Equipment Engineering Magazine editorial office, and the National Key Laboratory of Remanufacturing Technology. This paper serves as both a technical review and a practical guide for equipment engineers responsible for maintenance decisions.

Technical Scope and Industry Context

Equipment Categories Addressed

Industry	Equipment Type	Typical Service Conditions	Overlay Welding Application
Petrochemical	Gas turbine blades/vanes	High temperature, oxidation, erosion	Thermal barrier coatings, hot corrosion resistance
Petrochemical	Pump impellers	Erosion-corrosion, cavitation	Hardfacing for wear resistance
Petrochemical	Valve seats	High pressure, erosion	Stellite overlay for sealing
Metallurgical	Mill rolls	Abrasive wear, thermal fatigue	Hardfacing for surface hardness
Metallurgical	Hot work dies	High temperature, thermal cycling	Hot work resistant overlays
Metallurgical	Rolling mill guides	Abrasive wear	Hardfacing for dimensional restoration

Overlay Welding Processes Used

The paper discusses multiple overlay welding processes applied in industrial practice:

Process	Typical Application	Advantages	Limitations
SMAW (stick welding)	Field repair, small areas	Portable, versatile	Low deposition rate, operator dependent
Submerged Arc Welding (SAW)	Large area overlay, thick deposits	High deposition rate, clean weld	Requires flux, limited positional flexibility
Plasma Arc Transfer (PAT)	Precision overlay, thin deposits	Good dilution control, uniform deposit	Equipment cost, limited to shop conditions
Electroslag Welding (ESW)	Very thick deposits, large surfaces	Highest deposition rate	Limited to flat/horizontal positions
Laser Cladding	Thin, precise coatings	Low dilution, high precision	High equipment cost, limited part size
Flame Spraying + Welding	Pre-weld preparation	Surface cleaning, roughening	Additional process step

Detailed Case Studies

Case 1: Gas Turbine Blade Overlay Welding

Gas turbines in petrochemical power generation plants operate under severe conditions where hot gas corrosion and oxidation degrade blade surfaces. The overlay welding approach involves:

Base preparation: Grinding of damaged areas, removal of oxidation scale, and surface roughening
Welding procedure: Multi-pass overlay using nickel-based or cobalt-based hardfacing alloys (such as Stellite 6 or Inconel 625)
Post-weld treatment: Stress relief annealing to reduce residual stresses and prevent cracking
Performance target: 500–600 HV surface hardness, resistance to hot corrosion at 800–950°C

The key technical challenge is controlling dilution with the base material (typically a nickel-based superalloy or high-strength steel) to maintain the alloying elements that provide corrosion resistance.

Case 2: Mill Roll Overlay Welding

Mill rolls in hot rolling mills experience:

Abrasive wear from scale and oxide removal
Thermal fatigue from repeated heating and cooling cycles
Indentation from hot metal contact

The overlay welding repair procedure typically involves:

Inspection and assessment: Measurement of roll diameter reduction, identification of damaged areas
Surface preparation: Grinding to remove damaged material, ensuring adequate overlap with sound material
Welding: Application of hot work resistant hardfacing alloys (such as Fe-Cr-Ni-B-Si systems) using submerged arc or plasma arc welding
Grinding and finishing: Return to specified diameter and surface finish
Verification: Hardness testing, dimensional verification, and visual inspection

Performance Data Summary

Equipment	Original Life	Overlay Welded Life	Improvement Factor	Cost Saving
Gas turbine blade	2000 hours	5000+ hours	2.5×	60%
Mill roll (hot)	800 tons rolled	2500+ tons	3×	70%
Pump impeller	6 months	18+ months	3×	50%
Valve seat	12 months	36+ months	3×	65%

Engineering Practice Guidelines

Decision Framework for Overlay Welding vs. Replacement

Engineers should consider the following factors when deciding between overlay welding repair and component replacement:

Economic factors: Overlay welding typically costs 30–50% of the replacement cost for large components.
Downtime considerations: Overlay welding can be performed in-situ or in a repair shop, often with shorter turnaround times than manufacturing new components.
Structural integrity: The base material must have sufficient remaining strength to support the overlay deposit without catastrophic failure.
Service conditions: The overlay alloy must be compatible with the actual service environment, including temperature, chemical exposure, and mechanical loading.
Regulatory compliance: For pressure-containing equipment, overlay welding repairs must comply with applicable codes (ASME, GB, TSG) and require qualified welding procedures and inspection.

Quality Assurance Requirements

Quality Aspect	Requirement	Verification Method
Welding procedure qualification	WPS/PQR required	Code-compliant qualification records
Welder qualification	Certified welders	Qualification records within currency
Pre-weld inspection	Surface preparation verification	Visual, MPI for surface defects
In-process control	Interpass temperature, weld appearance	Thermocouple monitoring, visual
Post-weld inspection	NDT for volumetric and surface defects	UT, MT, PT as required
Performance verification	Hardness, dilution, composition	Hardness test, PMI, metallography

Key Questions and Reflections

This paper raises several important questions for modern equipment maintenance engineering:

Integration with remanufacturing technology: The concept of "remanufacturing" (装备再制造) has evolved significantly since 2006. Modern approaches combine overlay welding with advanced surface treatments (such as laser alloying, cold spray, or plasma nitriding) to achieve superior performance.
Digital documentation: The paper predates the widespread adoption of digital welding procedure documentation and real-time welding monitoring. Today, engineers can track welding parameters in real-time and create digital quality records for traceability.
Predictive maintenance integration: Modern condition monitoring systems can predict when overlay welding repair is needed, enabling planned maintenance rather than reactive repair.
Environmental considerations: Modern overlay welding practices must address fume extraction, slag disposal, and energy efficiency, which were not primary concerns in 2006.

Study Insights and Conclusions

This paper provides a valuable historical perspective on the industrial application of overlay welding technology in China's petrochemical and metallurgical sectors. The documented case studies demonstrate that overlay welding can extend equipment life by 2–3 times while reducing costs by 50–70%, making it an economically attractive maintenance strategy. For contemporary engineers, the fundamental principles remain valid: proper consumable selection, qualified welding procedures, thorough inspection, and appropriate post-weld treatment are the cornerstones of successful overlay welding repair. The evolution from traditional arc welding to advanced processes such as laser cladding and cold spray represents a natural progression of the same fundamental approach to surface engineering and equipment restoration.