Microstructure and Bending Strength of TIG Welded Joints Between YG30 Cemented Carbide and 45# Steel

Literature Overview

The paper by Wang Hao, Yang Dexin, Zhao Xiujuan, Chen Chunhuan, and Wang Qingzhang, published in Welding Technology (2005, Vol. 34, No. 1, pp. 9–11), investigates the microstructure and bending strength of TIG welded joints between YG30 cemented carbide and 45# steel (a medium carbon steel equivalent to AISI 1045). The research was conducted at the Department of Materials Science and Engineering, Dalian Railway Institute, in collaboration with the Welding Technology Research Institute of Dalian Heavy Industry Group Co., Ltd. The study employed scanning electron microscopy (SEM), X-ray diffraction (XRD), and bending mechanical property testing to characterize the welded joints fabricated using Ni-Fe and Ni-Fe-C alloy filler metals.

Core Technical Findings

The study reveals critical insights into the metallurgical challenges of joining cemented carbide to steel:

• The η phase (M₆C type Fe₃W₃C) formed at the YG30/weld interface is the primary cause of low bending strength in the welded joints.
• The η phase is hard, brittle, and promotes crack initiation and propagation under bending loads.
• The use of Ni-Fe-C alloy filler metal with appropriate carbon content can improve the bending strength of the welded joints.
• The addition of carbon to the filler metal suppresses the formation of brittle η phase by promoting the formation of more ductile intermetallic compounds.

Technical Analysis of Interfacial Metallurgy

The joining of YG30 cemented carbide (which contains approximately 30% cobalt binder with tungsten carbide particles) to 45# steel presents unique metallurgical challenges due to the vast differences in thermal properties, chemical reactivity, and microstructural characteristics between the two materials:

The formation of η phase (Fe₃W₃C, M₆C type carbide) at the interface between the cemented carbide and the weld metal is a critical issue in dissimilar metal welding. This phase forms through the reaction between tungsten from the YG30 carbide and iron from the weld metal, with carbon from both materials. The η phase is extremely hard (approximately 1900 HV) but very brittle, with a fracture toughness significantly lower than the base materials. Under bending loads, cracks initiate at the η phase particles and propagate through the interface, leading to premature failure of the welded joint.

The mechanism by which carbon addition to the filler metal suppresses η phase formation is related to the thermodynamic stability of competing carbide phases. In the Fe-W-C system, several carbide phases can form, including:

• M₆C (Fe₃W₃C, η phase) - very hard, brittle
• M₇C₃ (Fe₄W₃C) - moderately hard, less brittle
• M₂C (FeW₂C) - hard, but less common
• Cementite (Fe₃C) - moderately hard, relatively ductile

By increasing the carbon activity in the weld metal, the thermodynamic driving force shifts from M₆C formation toward M₇C₃ or cementite formation, which are less brittle and provide better mechanical properties. The optimal carbon content in the Ni-Fe-C filler metal represents a balance between sufficient carbon to suppress η phase and avoiding excessive carbon that could lead to other detrimental phases or porosity.

Engineering Practice Implications

The findings of this study have significant practical implications for the fabrication of tooling and components where cemented carbide is joined to steel:

1. Filler metal selection: For TIG welding of YG30 cemented carbide to steel, Ni-Fe-C alloy filler metal with appropriate carbon content should be selected over Ni-Fe alloy without carbon. The carbon content should be optimized through experimental qualification for the specific application.
2. Application areas: This technology is relevant to the manufacture of cutting tools, mining tools, wear-resistant components, and tool holders where cemented carbide inserts are joined to steel bodies. The bending strength improvement achieved through proper filler metal selection directly translates to improved tool life and reliability.
3. Welding parameter optimization: TIG welding parameters should be optimized to minimize the thermal input to the cemented carbide side, as excessive heating can cause decomposition of the cemented carbide structure and formation of additional brittle phases.
4. Quality control: X-ray diffraction analysis should be incorporated into the quality control procedure for cemented carbide-steel welded joints to verify the absence of η phase at the interface. Bending strength testing should be performed on qualification specimens to confirm that the welding procedure meets the required mechanical properties.
5. Alternative joining methods: For applications where TIG welding cannot provide adequate mechanical properties, alternative joining methods such as brazing, diffusion bonding, or mechanical fastening should be considered.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

• What is the optimal carbon content in the Ni-Fe-C filler metal for different cemented carbide grades (YG6, YG8, YG15, YG20, YG30)?
• How does the welding heat input affect the extent of η phase formation and the resulting bending strength?
• Can post-weld heat treatment modify the interfacial microstructure and improve the mechanical properties of cemented carbide-steel welded joints?
• How do other filler metal compositions (such as Ni-Cr, Ni-Co, or Cu-based alloys) compare to Ni-Fe-C in terms of interfacial metallurgy and mechanical properties?
• What is the effect of filler metal microstructure (powder metallurgy vs. cast) on the weld joint properties?

Study Insights and Implications

This paper provides valuable insights into the metallurgical challenges of joining cemented carbide to steel and demonstrates that the selection of appropriate filler metal composition is critical for achieving acceptable mechanical properties. The identification of η phase as the primary cause of low bending strength is a key finding that has direct implications for welding procedure development and quality control.

For practicing engineers in the tool and die industry, this study reinforces the importance of understanding interfacial metallurgy in dissimilar metal welding. The simple but effective solution of adding carbon to the Ni-Fe filler metal to suppress η phase formation demonstrates the power of metallurgical insight in solving practical welding problems. The technology has direct applications in the manufacture of cutting tools, mining tools, and wear-resistant components where cemented carbide is joined to steel bodies.

The practical recommendation is to use Ni-Fe-C alloy filler metal with optimized carbon content for TIG welding of YG30 cemented carbide to 45# steel, with the carbon content determined through experimental qualification for the specific application. Quality control should include X-ray diffraction analysis to verify the absence of η phase and bending strength testing to confirm mechanical property requirements. For applications where TIG welding cannot provide adequate properties, alternative joining methods should be considered.