Microstructure and Properties of Tungsten Carbide Cladding Layer Prepared by TIG Arc

Literature Overview

This study, conducted by Yang Lijun and colleagues at the School of Materials Science and Engineering, Tianjin University, and supported by the National Natural Science Foundation of China (Grant No. 51875403), investigates the microstructure evolution and mechanical performance of tungsten carbide (WC) hardfacing deposits produced using gas tungsten arc welding (GTAW/TIG). Published in the Journal of Tianjin University (Natural Science and Engineering) in 2019, the paper addresses a long-standing challenge in wear-resistant cladding technology: the decomposition of WC during the high-temperature thermal cycle of arc welding. The research was carried out under the auspices of the Tianjin Key Laboratory of Modern Joining Technology, reflecting the strong institutional commitment to advancing hardfacing technologies in China.

Core Technical Content

The fundamental problem in WC cladding by TIG welding lies in the thermodynamic instability of the WC phase under arc temperatures. When WC particles are introduced into the molten weld pool, the carbon in WC reacts with the liquid iron matrix to form iron carbides (Fe₃C, Fe₇C₃) and free cementite, significantly reducing the hardness and wear resistance of the deposit. The authors systematically examined the microstructural features of the cladding layer, including the morphology and distribution of retained WC particles, the type and quantity of secondary carbides formed during solidification and cooling, and the resulting hardness profile through the deposit thickness.

Key findings include the following:

The cladding layer microstructure consists of a dendritic matrix with secondary carbides distributed along grain boundaries and interdendritic regions.
Retained WC particles appear as irregularly shaped dark phases, while decomposed WC manifests as Fe₃C and Fe₇C₃.
The hardness of the cladding layer is directly correlated with the proportion of retained WC and the morphology of secondary carbides.
Excessive heat input leads to greater WC decomposition, reducing hardness from the ideal range of 1500-2000 HV down to values as low as 600-800 HV.

Process Parameters and Their Influence

The following table summarizes the typical process windows and their effects on cladding quality:

Parameter	Typical Range	Effect on Microstructure	Effect on Hardness
Arc current	80-150 A	Higher current increases dilution and WC decomposition	Decreases with increasing current
Travel speed	50-200 mm/min	Lower speed increases heat input per unit length	Decreases with decreasing speed
Shielding gas flow	8-15 L/min	Insufficient flow causes oxidation of WC and deposit	No direct effect if adequate
Wire feeding rate	2-6 m/h	Higher rate increases dilution ratio	Decreases with higher dilution
Preheating temperature	100-250 °C	Excessive preheat promotes WC decomposition	Slight decrease with higher preheat

The dilution rate is a critical parameter that governs the final performance of the cladding layer. In this study, dilution rates ranging from approximately 20% to 60% were observed depending on the combination of arc current, travel speed, and wire feed rate. Lower dilution rates were associated with better retention of WC particles and higher overall hardness, but at the expense of bond strength and weldability.

Microstructural Analysis

The authors employed optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to characterize the cladding layer. Metallographic examination revealed a complex microstructure comprising:

Retained WC particles - these appear as dark, angular particles with high contrast under SEM, and EDS analysis confirms their composition as predominantly W and C.
Iron carbides (Fe₃C and Fe₇C₃) - these form as a result of WC decomposition and appear as lighter phases in the microstructure.
Dendritic matrix - the solidification structure of the weld metal, which in high-carbon steels appears as a network of cementite and martensite.
Inclusion-free zones adjacent to retained WC particles - these zones exhibit lower hardness due to carbon depletion caused by the dissolution of WC.

The interaction between WC particles and the molten matrix was found to be strongly dependent on the local cooling rate. Higher cooling rates, achieved through lower heat input, promote the retention of WC particles and suppress the formation of coarse cementite networks. This finding has direct implications for process optimization in industrial hardfacing operations.

Engineering Practice Integration

From a practical standpoint, this research provides valuable guidance for engineers designing TIG-based hardfacing processes for applications such as valve seats, pump impellers, and mining equipment components. The following recommendations emerge from the study:

Use the minimum arc current necessary to achieve adequate penetration and wetting, typically in the range of 80-120 A for single-pass cladding.
Maintain travel speeds above 100 mm/min to limit heat input and reduce WC decomposition.
Consider using flux-cored wire with WC pre-dispersed to minimize particle size and improve dispersion uniformity.
Implement multi-pass cladding with lower heat input per pass to achieve better hardness distribution through the deposit thickness.
Perform hardness mapping across the full cross-section of the cladding layer to verify that the surface hardness meets the specification (typically >1200 HV for severe wear applications).

Key Questions and Reflections

The study raises an important question about the fundamental trade-off between dilution rate and cladding performance. While lower dilution rates preserve more WC and yield higher hardness, they may compromise the metallurgical bond between the cladding layer and the substrate. In pressure vessel applications where the clad layer must withstand cyclic loading and thermal fatigue, this trade-off becomes particularly critical. Engineers must carefully balance hardness requirements against fatigue and fracture toughness requirements when selecting process parameters for TIG hardfacing of WC-based alloys.

Another reflection concerns the scalability of laboratory results to production environments. The study was conducted under controlled laboratory conditions, and the transition to industrial-scale hardfacing operations may introduce additional variables such as surface preparation quality, wire consistency, and operator technique that can significantly affect the final cladding performance.

Study Insights and Implications

This research contributes meaningfully to the understanding of WC behavior during TIG arc cladding and provides actionable process recommendations for improving the wear resistance of hardfaced components. The systematic correlation between process parameters, microstructure, and hardness offers a solid foundation for developing process windows in production environments. Future work should explore the use of advanced simulation tools to predict WC decomposition kinetics under various thermal cycles and investigate the potential of hybrid processes such as laser-TIG to achieve even better WC retention with reduced dilution.