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

Literature Overview

This 2006 review article published in China Surface Engineering, authored by researchers from Wuhan University of Technology, China Equipment Engineering Magazine, and the National Key Laboratory of Equipment Remanufacturing Technology, provides a comprehensive overview of the industrial application of overlay welding (hardfacing) technology for equipment repair and maintenance in the petrochemical and metallurgical sectors. The article draws upon extensive field experience to document typical failure modes, overlay welding solutions, and quality assurance practices for critical equipment components.

Core Technical Points

Equipment in petrochemical and metallurgical operations is subjected to severe wear, corrosion, erosion, and thermal cycling conditions that lead to progressive material degradation and eventual failure. Overlay welding offers a cost-effective and efficient means of restoring worn surfaces, extending equipment service life by 2–5 times the original design life, and reducing unplanned shutdown costs.

The article categorizes the primary applications of overlay welding in these industries:

Typical Failure Analysis and Repair Strategies

The article documents several representative case studies that illustrate the systematic approach to equipment repair:

Case 1: Ball Mill Liner Repair

• Failure mode: Progressive wear of high-manganese steel liners due to impact and abrasion by grinding media (steel balls) and ore particles.
• Wear rate: 2–3 mm/month in wet grinding conditions.
• Repair approach: Removal of worn liner, surface preparation by grinding, application of 25–35 mm thick high-chromium white iron overlay by submerged arc welding with flux-cored wire (FCAW).
• Key process parameters: Current 350–450 A, voltage 30–35 V, travel speed 300–400 mm/min, preheat 100 °C.
• Result: Restored liner thickness with hardness of 58–62 HRC, extending service life by 3–4 times compared to the original manganese steel liner.

Case 2: Centrifugal Pump Impeller Restoration

• Failure mode: Erosion-corrosion damage to 304 stainless steel impeller due to cavitation and chlorinated process fluid.
• Damage pattern: Localized pitting and material loss of 3–8 mm on the pressure surface.
• Repair approach: Weld buildup of damaged areas with 316L stainless steel filler, followed by PTA cladding with Ni-Cr-Mo alloy (equivalent to Stellite 6) to a thickness of 2–3 mm.
• Key process parameters: GTAW buildup at 120–150 A, 10–12 V; PTA cladding at 200–250 A, powder feed rate 150–200 g/min.
• Result: Restored impeller geometry with surface hardness of 38–42 HRC, resistant to further erosion-corrosion.

Case 3: Rotary Kiln Shell Repair

• Failure mode: Erosion and oxidation damage to the kiln shell at the charging zone, with material loss of 5–15 mm.
• Repair approach: Application of 10–20 mm thick overlay by strip ESW using a high-aluminum (12% Al) steel strip electrode, providing resistance to both abrasion and high-temperature oxidation.
• Key process parameters: Current 600–800 A, voltage 38–42 V, travel speed 250–350 mm/min.
• Result: Restored shell thickness with oxidation resistance up to 1100 °C, extending repair interval from 6 months to 18–24 months.

Quality Assurance and Inspection Requirements

The article emphasizes the critical importance of quality assurance in overlay welding repair operations:

1. Surface preparation: Thorough removal of rust, scale, oil, and existing weld defects by grinding to bare metal. The substrate surface must be free of cracks and porosity that could propagate into the overlay.
2. Procedure qualification: Each overlay welding procedure must be qualified per NB/T 47014 or ASME IX, including mechanical property testing, hardness profiling, and macrographic examination of the weld cross-section.
3. Non-destructive testing (NDT):

• Magnetic particle testing (MT) of the substrate surface before overlay to detect existing cracks.
• Dye penetrant testing (PT) of the overlay surface after welding to detect surface cracks and porosity.
• Ultrasonic testing (UT) for detection of lack of fusion at the overlay-substrate interface.
• Radiographic testing (RT) for critical applications requiring volumetric defect detection.

1. Mechanical property verification: Hardness testing across the overlay thickness to confirm uniform composition and absence of dilution effects. The hardness profile should show a gradual transition from the substrate hardness to the overlay hardness, with no localized soft zones.
2. Dimensional verification: Confirmation of overlay thickness, flatness, and geometric accuracy according to the repair specification.

Industry Trends and Technology Development

The review identifies several trends in the evolution of overlay welding technology for equipment maintenance:

• Shift from manual to mechanized processes: Automated SAW and FCAW systems with multi-wire configurations achieve deposition rates of 5–10 kg/h, significantly improving repair productivity.
• Introduction of hot-wire TIG (HWO): This process combines TIG arc stability with high deposition rates (3–5 kg/h), enabling precise control of overlay geometry on complex shapes.
• Laser cladding adoption: Laser cladding with alloy powder offers high dilution control (<10%), fine microstructure, and minimal thermal distortion, though the equipment cost and processing speed remain limiting factors for large-scale applications.
• Advanced consumable development: New overlay alloys with improved wear-corrosion synergy, such as high-aluminum austenitic steels and functionally graded Ni-based alloys, are extending the service life of critical components.
• Condition-based maintenance integration: The integration of wear monitoring systems with overlay repair planning enables proactive maintenance scheduling, reducing unplanned downtime.

Key Questions and Reflections

A significant challenge highlighted in the review is the limited availability of qualified overlay welding operators in the field. The quality of the overlay deposit is highly dependent on operator skill, particularly for GTAW and PTA processes. Investment in operator training and certification programs is essential for maintaining consistent repair quality.

Another concern is the long-term reliability of overlay repairs compared to new components. The residual stresses from overlay welding, combined with the cyclic loading in service, can initiate fatigue cracks at the overlay-substrate interface. Post-weld stress relief treatment should be considered for critical components subjected to fatigue loading.

The environmental impact of overlay welding consumables, particularly those containing cobalt and chromium, also warrants attention. The disposal of spent welding flux and the emission of metal fumes during welding operations require compliance with occupational health and safety regulations.

Study Insights and Conclusions

This comprehensive review underscores the vital role of overlay welding technology in maintaining the reliability and availability of critical equipment in the petrochemical and metallurgical industries. The systematic approach to equipment repair—encompassing failure analysis, material selection, process optimization, quality assurance, and performance verification—provides a robust framework for engineering practice. The key message is that overlay welding is not merely a repair technique but a strategic tool for asset management, enabling significant cost savings and production continuity through planned and preventive maintenance. As equipment ages and operating conditions intensify, the demand for advanced overlay technologies and qualified personnel will continue to grow, making investment in this technology area increasingly important for industrial competitiveness.