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

Literature Overview

The research 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 TIG welding of YG30 cemented carbide to 45# steel using Ni-Fe and Ni-Fe-C alloy filler metals. Conducted jointly by Dalian Railway Institute and Dalian Heavy Industry Group, this work addresses the practical challenge of joining dissimilar materials in cutting tools and wear-resistant components, where cemented carbide inserts are frequently brazed or welded to steel shanks.

Core Technical Findings

The study employed scanning electron microscopy (SEM), X-ray diffraction (XRD), and flexural strength testing to characterize the welded joints. The key finding is that the formation of eta (η) phase, identified as M₆C-type Fe₃W₃C, at the YG30/weld interface is the primary cause of low flexural strength. The selection of Ni-Fe-C alloy filler metal with appropriate carbon content significantly improves the flexural strength of the joint.

Filler Metal	η Phase Formation	Flexural Strength	Interface Quality
Ni-Fe Alloy	Extensive Fe₃W₃C formation	Low	Poor bonding, brittle interface
Ni-Fe-C Alloy (optimized C)	Reduced η phase	Improved	Better bonding, more ductile interface
Ni-Fe-C Alloy (excess C)	Carbon segregation possible	Variable	Potential cracking

Microstructural Analysis

The YG30 cemented carbide consists of tungsten carbide (WC) grains in a cobalt (Co) binder matrix. During TIG welding with Ni-Fe filler metals, several detrimental reactions occur at the interface:

Tungsten from the WC grains dissolves into the molten weld pool
Iron from the filler metal and 45# steel reacts with dissolved tungsten and carbon to form Fe₃W₃C (η phase)
The η phase forms a continuous or semi-continuous layer at the YG30/weld interface
This brittle intermetallic compound acts as a crack initiation site and severely reduces flexural strength

The XRD analysis confirmed the M₆C crystal structure of the η phase, with the Fe₃W₃C composition indicating that iron, tungsten, and carbon from the various sources in the joint participated in the reaction. The SEM micrographs revealed the η phase as a distinct layer at the interface, with varying thickness depending on the filler metal composition.

Mechanism of Strength Improvement with Ni-Fe-C Filler

The addition of carbon to the Ni-Fe filler metal improves joint strength through several mechanisms:

Carbon in the filler metal reacts preferentially with tungsten to form WC, consuming dissolved tungsten before it can react with iron to form η phase
The resulting WC particles are more compatible with the YG30 cemented carbide substrate than the brittle Fe₃W₃C η phase
The controlled carbon content prevents excessive carbon accumulation that could cause cracking
The Ni-Fe-C matrix provides better wetting and bonding with the cobalt binder in YG30

Carbon Content in Filler	Effect on η Phase	Effect on Flexural Strength	Optimal Range
0% (Ni-Fe only)	Maximum η phase	Lowest strength	Not recommended
Low carbon	Reduced η phase	Improved strength	Moderate
Optimal carbon	Minimum η phase	Maximum strength	Target range
Excess carbon	Carbon-rich phases	Reduced strength	Avoid

Engineering Practice Considerations

In the manufacturing of cutting tools, mining tools, and wear-resistant components, the joining of cemented carbide inserts to steel substrates is a common requirement. Traditional brazing methods are widely used but have limitations in terms of joint strength and thermal stability. TIG welding offers the potential for stronger joints but requires careful control of the interfacial reactions.

The findings of this study provide clear guidance for filler metal selection:

Ni-Fe-C alloy filler metals should be preferred over pure Ni-Fe alloys for cemented carbide to steel TIG welding
The carbon content of the filler metal must be carefully controlled to minimize η phase formation without causing carbon-related defects
Welding parameters should be optimized to minimize the thermal cycle and reduce tungsten dissolution from the carbide substrate
Post-weld heat treatment may be beneficial to improve the microstructure and relieve residual stresses

Welding Parameter Recommendations

Parameter	Recommended Value	Rationale
Welding Current	50-80 A	Low current minimizes tungsten dissolution
Travel Speed	150-250 mm/min	High speed reduces heat input
Arc Length	Minimum possible	Concentrated heat, reduced dilution
Shielding Gas	Pure Argon	Stable arc, good protection
Preheating	Avoid if possible	Reduces thermal cycle severity
Interpass Temperature	Below 150°C	Prevents excessive thermal input

Key Reflections and Study Insights

This research highlights the critical importance of interfacial metallurgy in dissimilar material welding. The formation of brittle intermetallic compounds at the interface is a fundamental challenge in joining cemented carbide to steel, and the identification of Fe₃W₃C as the detrimental phase provides a clear target for process optimization.

The approach of using carbon in the filler metal to suppress η phase formation is an elegant solution that leverages thermodynamic principles. By providing a preferential reaction partner for dissolved tungsten, the carbon in the filler metal diverts the reaction away from the formation of brittle iron-tungsten carbides and toward the formation of more compatible tungsten carbide particles.

For engineering practice, this study provides a practical framework for optimizing TIG welding of cemented carbide to steel joints. The systematic evaluation of filler metal composition effects on interfacial microstructure and mechanical properties is a model approach that can be applied to other dissimilar material joining problems.

The findings have direct implications for the manufacturing of mining tools, cutting tools, and wear-resistant components where cemented carbide is used. Proper filler metal selection and welding parameter control can significantly improve joint reliability and service life.