Interface Microstructure Analysis of YG30 Cemented Carbide and 45 Steel TIG Welding

Literature Overview

This 2005 study published in Cemented Carbide by Song Yiguo, Zhao Xiujuan, Yang Dexin, Zhao Xiaohui, and Wang Qingzhang from Dalian Jiaotong University and Dalian Heavy Industry Group's Welding Technology Research Institute investigates the interface microstructure of TIG welded joints between YG30 cemented carbide and 45 steel. The research was supported by the Dalian Municipal Science and Technology Plan Project (Grant No. 2001145) and addresses a practical engineering challenge in the repair and refurbishment of carbide tooling and wear-resistant components.

Technical Background and Challenge

YG30 cemented carbide (WC-Co with 30% cobalt binder) is widely used for wear-resistant applications due to its exceptional hardness and abrasion resistance. However, when carbide components require joining to steel structures, the significant differences in thermal expansion coefficients, melting points, and metallurgical compatibility create substantial challenges. The thermal expansion coefficient of YG30 is approximately 6.5 × 10⁻⁶ /°C, while 45 steel is approximately 12 × 10⁻⁶ /°C, creating significant residual stresses during cooling. Additionally, the melting point of WC is approximately 2870°C compared to 1510°C for cobalt and approximately 1520°C for 45 steel.

Experimental Design and Filler Metal Selection

The researchers selected three filler metals for comparison: pure Fe, Ni-Fe alloy, and Co-Fe alloy. This selection was based on the principle that the filler metal composition should minimize the formation of brittle intermetallic compounds at the carbide-steel interface while maintaining adequate wetting and bonding.

Filler Metal Comparison

The η-phase (Fe₃W₃C and Co₃W₃C) is a hard, brittle intermetallic compound that forms at the interface between the cemented carbide and the weld metal. The presence and morphology of these phases directly influence the joint's mechanical properties and service life.

Microstructural Analysis

Interface Zone Characteristics

The scanning electron microscopy and X-ray diffraction analysis revealed distinct interface microstructures depending on the filler metal used:

• Pure Fe filler: Large blocks of η-phase (Fe₃W₃C) formed at the interface, creating a discontinuous but extensive brittle layer that would significantly reduce joint toughness.
• Co-Fe alloy filler: A mixture of Fe₃W₃C and Co₃W₃C phases formed at the interface, with intermediate amounts of brittle phases. The Co₃W₃C phase may have slightly different mechanical properties compared to Fe₃W₃C.
• Ni-Fe alloy filler: The minimum amount of η-phase (Fe₃W₃C) was observed, indicating that nickel addition to the filler metal effectively suppresses brittle intermetallic formation.

Engineering Implications and Practical Recommendations

The selection of Ni-Fe alloy as the filler metal with the minimum η-phase formation has direct practical implications for repair welding of carbide components. In industrial settings where carbide-lined equipment requires refurbishment, or where carbide inserts need to be joined to steel tool bodies, the choice of filler metal is critical for joint durability.

Welding Process Considerations

1. Heat input control: Lower heat input reduces the extent of interfacial reaction and limits η-phase growth. Pulsed TIG welding is preferred over continuous TIG for this reason.
2. Travel speed: Higher travel speeds reduce the time available for interfacial diffusion and reaction, limiting brittle phase formation.
3. Preheating: Moderate preheating (150–250°C) can reduce thermal gradients and residual stresses but should not be excessive to avoid promoting interfacial reactions.
4. Post-weld cooling: Controlled cooling rates help manage residual stress levels while avoiding excessive η-phase growth.

Metallurgical Mechanism Discussion

The formation of η-phases at the interface is driven by the thermodynamic tendency for iron and cobalt from the weld metal to react with tungsten carbide from the cemented carbide. The reaction can be represented as:

3Fe (from weld) + 3WC (from carbide) → Fe₃W₃C (η-phase)

The addition of nickel to the filler metal suppresses this reaction through several mechanisms:

• Nickel has a lower thermodynamic driving force for reaction with WC compared to iron.
• Nickel forms a more continuous, less reactive diffusion barrier at the interface.
• The presence of nickel modifies the solidification microstructure of the weld metal, reducing the availability of iron-rich phases at the interface.

Quality Control and Inspection

For production welding of carbide-to-steel joints, the following quality control measures are recommended:

Study Insights and Broader Relevance

This research, while focused on a specific material combination, establishes general principles that are applicable to dissimilar material welding more broadly. The finding that Ni-Fe alloy filler metal minimizes brittle intermetallic formation at the carbide-steel interface is consistent with the general principle that nickel-based fillers provide better metallurgical compatibility in dissimilar material joints.

For the pipe and fitting industry, while the specific application of carbide-to-steel welding may be limited, the principles investigated here are relevant to other dissimilar material joints encountered in practice, such as stainless steel to carbon steel welds, or nickel alloy cladding on carbon steel pipe. The systematic approach of investigating filler metal effects on interfacial microstructure and correlating with mechanical performance provides a methodology that can be applied to any dissimilar material welding challenge.

The study also highlights the importance of interface engineering in dissimilar material welding. The mechanical properties of the joint are determined not only by the bulk properties of the weld metal and base metals but critically by the microstructure and composition of the interface zone. Engineers designing dissimilar material joints should always consider the potential for intermetallic compound formation and select filler metals and welding processes that minimize these detrimental phases.