Development Report on Hardfacing and Thermal Spraying Technologies

Literature Overview

This comprehensive review article by Zhang Ping, Zhao Junjun, Li Changjiu, Zhao Kun, and Shan Jiguo, published in Welding (2013, Issue 2, pp. 7-14), provides a systematic overview of the state of the art in hardfacing and thermal spraying technologies. The authors represent four leading Chinese research institutions: the Remanufacturing Department of the Academy of Armored Force Engineering, the School of Materials Science and Engineering at Xi'an Jiaotong University, the Harbin Welding Research Institute under the Chinese Academy of Mechanical Sciences, and the Welding Research Center of Tsinghua University. This multi-institutional authorship lends considerable authority to the review, as it synthesizes perspectives from military remanufacturing, academic research, national standards development, and fundamental welding science.

Core Technical Domains Covered

The report divides the field into two major branches—hardfacing (overlay welding) and thermal spraying—each of which serves distinct engineering purposes in surface engineering. Hardfacing involves the deposition of a wear-resistant, corrosion-resistant, or high-temperature-resistant alloy layer onto a base substrate through arc welding, flame welding, or submerged arc processes. Thermal spraying encompasses a family of processes including flame spraying, plasma spraying, detonation spraying, high-velocity oxy-fuel (HVOF) spraying, and cold spray, in which molten or semi-molten particles are accelerated onto a substrate to form a coating.

Hardfacing Classification and Process Parameters

Hardfacing alloys are typically classified by their primary hard phase:

The report highlights the critical importance of dilution control in hardfacing. Dilution—the mixing of base metal into the weld deposit—directly affects the hardness, microstructure, and service life of the overlay. Typical dilution rates range from 10% to 40%, depending on the process, number of passes, and preheating temperature. Multi-pass welding is the standard strategy to reduce dilution below 20% in critical applications.

Thermal Spraying Process Comparison

Interpretation of Key Technical Points

The report emphasizes several technological trends that remain highly relevant today. First, the shift toward low-dilution hardfacing processes—such as submerged arc surfacing with consumable electrode holders and flux-cored wire surfacing—reflects the industry's growing demand for consistent overlay composition. Second, the maturation of HVOF spraying for chromium carbide and tungsten carbide coatings has significantly expanded the range of applications in oil and gas, power generation, and aerospace sectors. Third, the development of composite coatings incorporating ceramic particles (WC, Cr₃C₂, TiC, SiC) into metallic matrices represents a hybrid approach combining the toughness of metals with the hardness of ceramics.

From a metallurgical perspective, the report discusses the formation mechanisms of hard phases during solidification. In high-chromium alloys, the competition between M₇C₃ and M₂₃C₆ formation is governed by carbon activity and cooling rate. Rapid cooling favors M₇C₃, which offers superior toughness, while slower cooling promotes M₂₃C₆, which provides higher hardness but greater brittleness. This knowledge directly informs process parameter selection for achieving target microstructures.

Integration with Engineering Practice

In the context of pipeline and piping systems, hardfacing and thermal spraying technologies find extensive application. For example, API 5L X70/X80 line pipes with internal plug valves require hardfaced valve seats to resist erosion from high-velocity flow. Wellhead equipment components—such as choke and kill manifold valves—demand Stellite-type hardfacing to withstand abrasive formation fluids. In the refining and chemical industries, pipe fittings exposed to corrosive media may receive Ni-Cr-Mo overlay welding (e.g., Hastelloy C-276 or Inconel 625) to extend service life.

A practical engineering consideration highlighted implicitly in this report is the post-weld treatment strategy. Hardfaced components often require stress relief at 650-750°C to reduce residual stresses that could cause cracking during service. However, excessive stress relief temperatures may soften the hard phase, reducing surface hardness by 100-200 HV. This trade-off must be carefully managed based on the specific alloy system and service conditions.

Key Questions and Reflections

One question that emerges from this report is how to systematically select between hardfacing and thermal spraying for a given application. The decision matrix should consider: required coating thickness, substrate geometry accessibility, allowable distortion, interfacial bond strength requirements, and production volume economics. Hardfacing is generally preferred for thick coatings (>1 mm) on complex geometries, while thermal spraying excels for thin, uniform coatings on large-area components.

Another reflection concerns the standardization gap. Unlike welding processes covered extensively by ASME Section IX or ISO 15614, hardfacing and thermal spraying processes lack equally comprehensive qualification standards. This creates challenges for regulatory compliance in safety-critical applications such as nuclear piping and pressure vessels, where NORSOK M-650 and ASTM F2329 provide some guidance but leave significant room for engineering judgment.

Study Insights and Implications

This report serves as a valuable roadmap for engineers entering or advancing in surface engineering. Its strength lies in the breadth of coverage and the integration of academic research with industrial practice. For piping engineers, the key takeaway is that surface engineering is not merely an afterthought but should be integrated into the initial design phase—selecting the appropriate base material, overlay system, and process parameters to ensure the component meets its intended service life. The report also underscores the importance of understanding the underlying metallurgy, as surface modifications without metallurgical understanding inevitably lead to premature failure.