Spraying, Spray Welding, and Cladding Materials: A Comprehensive Review

Literature Overview

This 1995 publication by Gui Yewei from Sino-US Shanghai Stellite Co., Ltd., published in Powder Metallurgy Industry journal, provides a comprehensive overview of thermal spray, spray welding, and weld overlay materials. Published during a period of rapid growth in thermal spray technology in China, this reference offers valuable insights into the material systems available for surface protection and repair, as well as the fundamental principles governing material selection for different applications.

Core Technical Content

Classification of Surface Engineering Materials

The authors present a systematic classification of materials used in thermal spray, spray welding, and weld overlay applications:

Thermal Spray Process Comparison

The authors compare several thermal spray processes, highlighting their characteristics and applicable materials:

Spray Welding vs. Weld Overlay

The authors draw important distinctions between spray welding and traditional weld overlay:

Spray Welding:

• Involves preheating the base metal to a high temperature (typically 400-800 °C)
• Molten spray particles fuse with the hot base metal surface
• Creates a metallurgical bond between the coating and the substrate
• Suitable for materials that can be melted without excessive oxidation
• Commonly used with nickel-based and cobalt-based alloys

Weld Overlay:

• Uses conventional welding processes (SAW, GMAW, FCAW, etc.)
• Creates a true metallurgical bond through fusion welding
• Can deposit thicker layers (10-50 mm or more)
• Requires careful control of heat input and cooling rate
• More suitable for thick coatings and structural applications

Material Selection Guidelines

The authors provide practical guidelines for material selection based on service conditions:

For Abrasive Wear:

• Mild abrasion: Medium-carbon steel or manganese steel
• Severe abrasion: High-chromium iron or tungsten carbide composite
• Slurry abrasion: Tungsten carbide-cobalt or tungsten carbide-nickel

For Corrosive Wear:

• Mild corrosion: Stainless steel (304, 316)
• Severe corrosion: Nickel-based alloy (Inconel 625, Hastelloy C276)
• Combined corrosion and abrasion: Stellite 6 or Stellite 21

For High-Temperature Service:

• Below 600 °C: Nickel-based alloy (Inconel 625)
• 600-1000 °C: Cobalt-based alloy (Stellite 6, Stellite 21)
• Above 1000 °C: Ceramics (alumina, chromia)

Quality Control and Testing

The authors emphasize the importance of quality control in surface engineering applications:

1. Coating thickness measurement: Using ultrasonic or eddy current methods
2. Hardness testing: Vickers or Knoop hardness, with proper indentation spacing
3. Bond strength testing: ASTM C633 or ASTM B671 pull-off tests
4. Porosity evaluation: Metallographic examination, ASTM C205
5. Corrosion testing: Salt spray, immersion, or potentiodynamic polarization
6. Wear testing: Pin-on-disk, dry sand-rubber, or slurry abrasion

Engineering Practice Integration

The comprehensive material review in this publication serves as an excellent reference for engineers selecting surface engineering solutions. Several practical considerations deserve emphasis:

• Cost-benefit analysis: High-performance materials such as tungsten carbide or cobalt alloys provide superior performance but at significantly higher cost. The selection should be based on the expected service life and the cost of downtime.
• Process-material compatibility: Not all materials are suitable for all processes. For example, tungsten carbide coatings are best applied by HVOF to minimize carbide decomposition, while nickel-based alloys are well-suited for plasma spray or arc spray.
• Substrate preparation: The quality of the coating is heavily dependent on the substrate preparation. Surface roughness, cleanliness, and preheat temperature must be carefully controlled.
• Post-processing: Some coatings require post-processing to achieve optimal properties. For example, tungsten carbide coatings may require low-temperature stress relief, while nickel-based coatings may benefit from solution heat treatment.

Case Study: Stellite 6 Application

The authors include a case study of Stellite 6 (a cobalt-chromium-tungsten alloy) applied by plasma spray to a chemical pump impeller:

The Stellite 6 coating provided excellent resistance to cavitation erosion and corrosion in a sulfuric acid service environment, demonstrating the effectiveness of cobalt-based alloys in severe chemical environments.

Key Questions and Reflections

Several questions arise from this comprehensive review that remain relevant today:

• Advanced materials: How have newer materials, such as high-entropy alloys and functionally graded coatings, expanded the range of available surface engineering solutions?
• Process development: How have advances in HVOF, cold spray, and laser cladding changed the landscape of surface engineering since 1995?
• Environmental regulations: How have environmental regulations affected the use of certain materials, such as those containing chromium or cobalt?
• Digital integration: How can digital simulation and optimization tools improve the design and application of surface engineering solutions?

Study Insights and Implications

This 1995 publication remains a valuable reference for surface engineering material selection, despite the significant advances in technology that have occurred since its publication. The systematic classification of materials, the comparison of thermal spray processes, and the practical guidelines for material selection provide a solid foundation for engineers working in this field. The emphasis on quality control and testing is particularly important, as the performance of surface engineering solutions is heavily dependent on the quality of the coating application. I would recommend that engineers supplement this reference with more recent publications on advanced materials and processes, but the fundamental principles and material selection guidelines presented here remain highly relevant to modern practice.