Application of Overlay Welding in Petrochemical and Metallurgical Equipment Maintenance

Literature Overview

This paper, published in China Surface Engineering (2006, Vol. 19, No. 3, pp. 4-8), provides a comprehensive review of overlay welding applications in the maintenance and refurbishment of large-scale equipment in the petrochemical and metallurgical industries. The research was conducted by researchers from Wuhan University of Technology, the editorial team of China Equipment Engineering, and the National Key Laboratory of Equipment Remanufacturing Technology. The paper surveys the practical application of overlay welding technology for strengthening and repairing critical industrial equipment, with specific case studies on gas turbines in the petrochemical industry and rolling mill rolls in the metallurgical industry.

Overlay Welding Technology Overview

Overlay welding, also known as surfacing or hardfacing, is a traditional surface engineering technology that has evolved significantly in recent decades. The technology involves depositing one or more layers of weld metal onto a base material to provide:

• Wear resistance — Hardfacing alloys with high hardness (HRC 50-70) for abrasive and erosive environments.
• Corrosion resistance — Alloy overlays resistant to chemical attack in aggressive environments.
• Dimensional restoration — Building up worn surfaces to original dimensions.
• Surface functionalization — Combining wear and corrosion resistance in a single overlay.

Common Overlay Welding Processes

Petrochemical Industry Applications

Gas Turbine Blade and Component Repair

Gas turbines are critical equipment in petrochemical plants for power generation and process driving. The hot section components (blades, shrouds, vanes) are subjected to extreme temperatures, centrifugal stresses, and corrosive hot gas environments. Overlay welding is used to:

1. Restore blade profiles — Worn blade leading and trailing edges are rebuilt with wear-resistant and heat-resistant overlay alloys.
2. Apply protective coatings — Thermal barrier coatings and oxidation-resistant overlays extend blade life.
3. Repair damaged components — Cracked or eroded components are repaired with compatible overlay welds.

Typical Overlay Materials for Gas Turbine Components

Case Study: Gas Turbine Blade Repair

A typical repair scenario involves a gas turbine blade with leading edge erosion of 2-3 mm. The repair process involves:

1. Inspection and assessment — Non-destructive testing (MT, PT) to identify cracks and erosion extent.
2. Surface preparation — Grinding to remove eroded material and expose sound base metal.
3. Overlay welding — GTAW or plasma arc welding with Ni-Cr-Al alloy to restore the blade profile.
4. Post-weld treatment — Stress relief annealing at 850-900 °C for 1-2 hours.
5. Final inspection — Dimensional check, hardness measurement, and NDT.

The overlay welding process must be carefully controlled to avoid excessive heat input that could damage the base alloy's microstructure. Preheating to 200-300 °C and limiting interpass temperature to 400 °C are typical requirements.

Metallurgical Industry Applications

Rolling Mill Roll Restoration

Rolling mill rolls are subjected to extreme conditions during hot rolling operations:

• High temperatures — Contact with hot steel slabs at 800-1200 °C.
• High pressures — Contact pressure of 100-300 MPa.
• Abrasive wear — Scale and oxide particles embedded in the roll surface.
• Thermal fatigue — Cyclic heating and cooling during rolling passes.

Overlay welding is used to restore worn rolls and to apply wear-resistant surfaces to new rolls.

Typical Overlay Materials for Rolling Mill Rolls

Case Study: Rolling Mill Roll Restoration

A typical restoration process for a worn hot rolling mill work roll involves:

1. Dismantling and inspection — Remove roll from mill; inspect for cracks (UT), surface defects (PT), and dimensional wear.
2. Surface preparation — Grind the roll surface to remove worn material and prepare for overlay welding.
3. Overlay welding — Apply 3-5 mm of wear-resistant alloy using submerged arc welding or gas metal arc welding with automatic traverse.
4. Grinding — Machine the overlay surface to precise dimensional tolerances (typically ±0.05 mm).
5. Heat treatment — Quench and temper to achieve target hardness and toughness.
6. Final inspection — Dimensional verification, hardness mapping, and NDT.

The overlay welding parameters must be carefully controlled to achieve adequate bond strength between the overlay and the roll base material. Insufficient bond strength leads to overlay spalling during rolling, which is a catastrophic failure mode.

Engineering Practice and Quality Control

Process Selection Criteria