Application of Surface Weld Overlay Technology in Equipment Repair

Literature Overview

This 2006 technical report by Gao Shirong from Xingfeng Group Hongyue Coal Chemical Co., Ltd. provides a comprehensive overview of surface weld overlay technology applications in equipment maintenance and repair within the coal chemical industry. The publication serves as a practical guide for maintenance engineers dealing with wear and corrosion damage to critical process equipment. The breadth of applications covered makes this document particularly valuable as a reference for maintenance planning and repair strategy development.

Scope of Application

The study covers a wide range of equipment repair applications in the coal chemical processing environment:

Technical Methodology

Classification of Overlay Applications

The author categorizes weld overlay applications into three primary functional types:

1. Wear-resistant overlay (耐磨堆焊)

• Purpose: Restore or enhance surface hardness and wear resistance
• Typical materials: Cr-C hardfacing, Ni-Cr hardfacing, Co-Cr (Stellite)
• Application examples: Pump impellers, fan blades, mill liners
• Target hardness: HRC 45–65 depending on service conditions

2. Corrosion-resistant overlay (耐蚀堆焊)

• Purpose: Provide a corrosion-resistant surface layer on carbon steel components
• Typical materials: Austenitic stainless (309L, 316L), Ni-based (Inconel 625, Hastelloy C-276)
• Application examples: Heat exchanger tube sheets, valve bodies, pipeline elbows
• Target properties: Resistance to specific corrosive media (acid, alkaline, chloride)

3. Combined wear-corrosion resistant overlay (耐蚀耐磨堆焊)

• Purpose: Provide simultaneous resistance to both wear and corrosion mechanisms
• Typical materials: Ni-Cr-C systems, Co-Cr alloys with Ni additions
• Application examples: Pump casings in slurry service, valve seats in corrosive media
• Target properties: HRC 40–55 with excellent corrosion resistance

Process Selection Matrix

Engineering Practice in Coal Chemical Industry

Case Application 1: Pump Impeller Repair

A typical coal chemical slurry pump impeller made of cast iron was subjected to severe erosion-corrosion damage. The repair procedure was:

1. Assessment: Metallographic examination confirmed 15 mm material loss from the original 25 mm wall thickness.
2. Preparation: Grind surface to remove all damaged material; establish proper weld groove geometry.
3. Preheating: Apply 300°C preheat to the cast iron base to prevent cracking.
4. Transition layer: Apply EZCA-1 cast iron electrode to create a ductile transition zone.
5. Build-up layer: Apply 309L stainless steel weld metal to restore geometry.
6. Hardfacing layer: Apply Ni-Cr hardfacing (E5156-A1 equivalent) for final wear resistance.
7. Post-weld treatment: Controlled cooling in insulating blanket to prevent cracking.
8. Machining: CNC machining to restore impeller balance and hydraulic profile.

Case Application 2: Heat Exchanger Tube Sheet Overlay

Carbon steel heat exchanger tube sheets exposed to acidic process fluids required corrosion-resistant overlay:

1. Surface preparation: Chemical cleaning followed by grinding to bare metal.
2. Transition layer: Single pass of 309L (high Cr-Ni austenitic) to reduce dilution effects.
3. Face layer: Multiple passes of 316L to achieve final corrosion resistance.
4. Post-weld treatment: Solution heat treatment at 1050°C followed by water quench (where feasible) or controlled furnace cool.
5. Intergranular corrosion testing: ASTM A262 Practice A confirmed satisfactory resistance after proper PWHT.

Quality Control Framework

The study emphasizes a systematic quality control approach based on the following framework:

Study Insights and Reflections

This publication, while appearing as a practical maintenance guide, contains several important technical insights that merit deeper consideration:

First, the author's systematic classification of overlay applications by functional requirement (wear, corrosion, or combined) provides a clear decision framework for maintenance engineers. This classification approach simplifies the material selection process and ensures that the overlay solution is matched to the actual failure mechanism rather than being selected based on habit or availability.

Second, the emphasis on transition layers for dissimilar material combinations reflects a fundamental principle of weld overlay technology: the successful integration of overlay and base materials requires careful management of the metallurgical transition zone. The use of austenitic 309L as a transition layer between carbon steel and more alloyed overlay materials is a well-established practice that effectively accommodates the thermal expansion mismatch and reduces residual stress at the interface.

Third, the practical emphasis on preheating and controlled cooling for cast iron base materials highlights a critical consideration often overlooked in field repairs. Cast iron has inherently poor ductility and high carbon equivalent, making it extremely susceptible to cracking during and after welding. The specified preheat temperatures and cooling rates are not arbitrary but are derived from the fundamental metallurgical behavior of cast iron during thermal cycling.

Fourth, the inclusion of intergranular corrosion testing as a quality control step for corrosion-resistant overlays demonstrates an understanding of a subtle but important failure mechanism. Inadequate heat treatment of austenitic stainless steel overlays can lead to chromium carbide precipitation at grain boundaries, creating paths for intergranular corrosion attack. This testing requirement ensures that the overlay provides genuine long-term corrosion protection rather than merely a superficial improvement.

The practical nature of this publication makes it particularly valuable for maintenance teams in coal chemical and petrochemical industries where rapid, reliable repair solutions are essential for maintaining production continuity. The systematic approach to overlay selection, process planning, and quality verification provides a replicable framework that can be adapted to specific equipment and service conditions.

Reference Value and Outlook

The methodology and classification system presented in this publication can serve as a foundation for developing more comprehensive equipment repair standards within coal chemical and related industries. Future developments should include:

• Standardization of overlay repair procedures into formal company or industry standards with clear qualification requirements for welders and inspectors
• Development of advanced hardfacing consumables specifically formulated for coal chemical service conditions (high temperature, multi-phase corrosion, abrasive slurry)
• Integration of condition monitoring data with overlay repair planning to enable predictive rather than reactive maintenance
• Application of advanced joining technologies such as laser cladding and cold spray for improved overlay quality and reduced thermal distortion
• Development of digital twin models of critical equipment to predict remaining useful life and optimize overlay repair timing

The continued evolution of weld overlay technology, combined with advances in consumable metallurgy and welding automation, promises ever-improved repair capabilities that extend equipment life, reduce maintenance costs, and enhance operational safety in demanding industrial environments.