Interface Carbide Dissolution in WC-TiC-Ni/304 Stainless Steel MIG Welded Joints

Literature Overview

This 2015 study by Cao Xiaolian, Xu Peiquan, and colleagues from Shanghai University of Engineering Science, published in Rare Metals, investigates the interface metallurgy of dissimilar welds between WC-TiC-Ni cemented carbide and 304 stainless steel using MIG arc welding with a robotic welding cell. Supported by the National Natural Science Foundation of China (Grant 51105240), the Shanghai Shuguang Plan (Grant 13SG54), and a graduate research innovation project (Grant 13KY0503), this research addresses a challenging dissimilar material joining problem that arises in mining, drilling, and cutting tool applications. The use of pure Ni wire (Φ1.2 mm) as filler metal is a deliberate choice to facilitate metallurgical bonding between the ceramic-like carbide and the metallic steel.

Technical Background and Process Configuration

The joining of cemented carbide to steel is inherently difficult due to the vast differences in thermal expansion coefficients, thermal conductivities, and chemical reactivity between the two materials. WC-TiC-Ni cemented carbide has a thermal expansion coefficient of approximately 5–6 × 10⁻⁶/K, while 304 stainless steel has approximately 17–18 × 10⁻⁶/K, creating significant residual stresses at the interface during cooling. The high melting point of WC (2870°C) compared to Ni (1455°C) and 304 SS (1400–1450°C) further complicates the welding process.

The process parameters investigated are summarized below:

Interface Microstructure and Carbide Dissolution

The study's primary contribution is the detailed characterization of the interface microstructure using OM, SEM, EDS, and EPMA. The authors identified a multi-layered transition zone on the cemented carbide side, consisting of:

1. Dense (WC, TiC)/Ni layer: The original carbide particles embedded in the Ni binder, with minimal dissolution.
2. Sparse (WC, TiC)/Ni layer: A transition zone where some carbide dissolution has occurred, creating a more porous microstructure.
3. η-phase layer: A diffusion zone where Ni from the filler metal and steel has reacted with the carbide, forming η-Ni₃(W,Ti) intermetallic compounds.

The η-phase layer is critical for achieving metallurgical bonding between the cemented carbide and the steel. Without this diffusion layer, the joint would rely on mechanical interlocking rather than true metallurgical bonding, which would be susceptible to delamination under mechanical or thermal loading.

A key finding is that the use of Ni filler metal significantly reduces carbide dissolution compared to no-fill welding. This is because the Ni wire provides a compatible diffusion partner for the cemented carbide, facilitating the formation of the η-phase without excessive dissolution of the hard WC and TiC particles. In contrast, direct welding without filler metal leads to extensive carbide dissolution and the formation of brittle intermetallic phases that compromise joint strength.

Abnormal Grain Growth (AGG) Analysis

The study also investigates abnormal grain growth (AGG) at the interface, which is a common phenomenon in dissimilar metal welds where one material has a significantly different grain structure or melting point. AGG can lead to localized softening, reduced mechanical properties, and potential failure sites.

The factors influencing AGG in this system include:

• Welding heat input: Higher heat input promotes grain growth, but also increases carbide dissolution. The optimal heat input must balance these competing effects.
• Cooling rate: Rapid cooling suppresses grain growth but may increase residual stresses. The cooling rate in this process is influenced by the thermal conductivity mismatch between the cemented carbide and the steel.
• Filler metal composition: Pure Ni provides a controlled diffusion environment that limits AGG compared to Ni-based alloys with higher melting points or different compositions.

Groove Configuration Effects

The study compares two groove configurations: one-sided 30° groove without gap and no groove with gap. The results show that the one-sided 30° groove without gap produces superior weld quality. This is attributed to:

1. Better heat control: The groove geometry concentrates the heat input in the desired region, reducing thermal distortion and residual stresses.
2. Controlled penetration: The groove ensures that the weld penetrates to the cemented carbide surface without excessive melting of the carbide.
3. Reduced porosity: The tight fit-up without gap minimizes the formation of gas pores and slag inclusions.

Engineering Practice Considerations

For engineers working with cemented carbide to steel joints, this study provides several practical recommendations:

1. Filler metal selection: Pure Ni wire is preferred over Ni-based alloys or steel filler metals, as it provides the best metallurgical compatibility with both the cemented carbide and the steel.
2. Groove design: A one-sided 30° groove without gap is recommended for optimal weld quality. The groove should be prepared on the cemented carbide side to control the heat input.
3. Welding parameters: The recommended parameters (45 cm/min, 2.0 m/min wire feed, 18 V, 15.8 L/min gas flow) provide a starting point for process development. However, these parameters should be adjusted based on the specific geometry and thickness of the components.
4. Post-weld inspection: Non-destructive testing (NDT) is essential for verifying weld quality, particularly for detecting interface delamination, porosity, and incomplete fusion. Ultrasonic testing (UT) and radiographic testing (RT) are recommended.
5. Residual stress management: The large thermal expansion mismatch between the cemented carbide and the steel creates significant residual stresses. Post-weld stress relief may be necessary, but the temperature must be carefully controlled to avoid excessive grain growth or carbide dissolution.

Study Insights and Reference Value

This study provides valuable insights into the metallurgical behavior of cemented carbide to steel dissimilar welds, which is critical for applications in mining, drilling, and cutting tools where such joints are common. The identification of the η-phase layer as the key to metallurgical bonding is a fundamental finding that informs filler metal selection and process optimization.

The practical significance of this work extends to the development of reliable welding procedures for cemented carbide inserts in mining tools, drill bits, and cutting inserts. The recommended process parameters and groove configuration provide a solid foundation for industrial implementation, while the detailed microstructural analysis offers a scientific understanding of the underlying mechanisms.

For engineers designing cemented carbide to steel joints, the key takeaway is that the interface metallurgy is not merely a consequence of the welding process but is a critical design parameter that must be actively managed through careful selection of filler metal, groove geometry, and welding parameters. The use of pure Ni filler metal and a one-sided 30° groove without gap represents a proven approach that balances metallurgical compatibility, processability, and joint performance.