TIG Welding Connection Between Cemented Carbide YG30 and 45 Steel

Literature Overview

This 2004 study by Zhao Xiu-Juan and Yang De-Xin from Dalian Jiaotong University investigates the TIG welding of dissimilar materials: cemented carbide YG30 and carbon steel 45. Published in New Technology and New Process (No. 3, pp. 34-35), the work addresses a significant industrial challenge in the fabrication and repair of cutting tools, mining equipment, and wear-resistant components where carbide inserts must be joined to steel substrates. The research was supported by the Dalian Municipal Planning Project (Grant No. 2001145).

Core Technical Challenge

The welding of cemented carbide to steel presents unique metallurgical challenges due to the fundamental incompatibility of these materials:

Property	YG30 Cemented Carbide	45 Carbon Steel
Thermal expansion coefficient	~5.9 × 10⁻⁶ /K	~12 × 10⁻⁶ /K
Thermal conductivity	~70 W/(m·K)	~50 W/(m·K)
Melting behavior	Decomposition rather than true melting	~1495°C
Hardness	88-92 HRA	200-250 HB
Brittleness	High	Low

The large mismatch in thermal expansion coefficients creates significant residual stresses during cooling, while the decomposition behavior of WC during heating leads to the formation of brittle intermetallic phases at the interface.

Experimental Approach and Results

Filler Metal Selection

The authors evaluated two filler metal options:

Ni-Fe alloy wire: Contains both nickel and iron, designed to bridge the compositional gap between the carbide and steel
Pure Ni wire: Nickel-based filler that forms a diffusion bond with the carbide

Welding Configuration

Parameter	Ni-Fe Wire, 4-pass welding	Pure Ni Wire, 4-pass welding	Pure Ni Wire, 4-pass + surfacing	Ni-Fe Wire, surfacing only
η-phase formation	Yes	No	No	No
Hardness gradient at interface	Sharp change	Gradual	Gradual	Gradual

η-Phase Formation Mechanism

The formation of η-phase (Fe₃W₃C) is the critical metallurgical phenomenon identified in this study. The mechanism involves:

During welding, carbon diffuses from the WC particles in YG30 into the weld metal
Simultaneously, Fe and Ni diffuse from the filler metal into the WC particles
When Ni-Fe wire is used for direct welding, the combined diffusion of C, Fe, and Ni creates conditions favorable for η-phase precipitation at the interface

The scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) analysis provided definitive evidence of the diffusion mechanism and phase formation.

Hardness Profile Analysis

The hardness distribution across the weld interface reveals the practical consequences of η-phase formation:

With η-phase (Ni-Fe direct welding): Sharp hardness gradient at the carbide/weld interface, indicating a brittle, poorly bonded transition zone
Without η-phase (pure Ni or surfacing approaches): Gradual hardness transition, indicating a more robust and damage-tolerant interface

Engineering Practice Implications

Process Selection for Dissimilar Material Welding

This study provides clear guidance for engineers working with carbide-to-steel joints:

Avoid direct Ni-Fe welding to carbide: The formation of η-phase creates a weak interface prone to cracking and delamination under service loads
Use pure Ni as a diffusion buffer: The pure Ni filler metal creates a transition zone that accommodates the thermal expansion mismatch without forming brittle intermetallics
Surfacing approach is preferred: When possible, apply a Ni-based surfacing layer to the carbide before welding to the steel substrate

Application to Pipe and Fitting Manufacturing

While cemented carbide is not commonly used in pipe construction, the principles established in this study are directly applicable to:

Wear-resistant pipe linings: Carbide-reinforced overlays on pipe interiors for slurry service
Tool and die repair: Welding carbide inserts to steel tool bodies
Mining equipment: Repair of carbide-faced components in crushers and conveyors
Cutting tool fabrication: Joining carbide tips to steel shanks

Quality Control Considerations

For production welding of dissimilar material joints:

Metallographic examination: Cross-sectional examination of the interface is essential to verify the absence of η-phase
Hardness mapping: Vickers hardness profiles across the interface should show a gradual transition (not exceeding 30 HV per 100 μm)
Fracture surface analysis: Post-failure SEM examination should reveal ductile fracture characteristics at the interface rather than intergranular or cleavage fracture

Critical Analysis

The study's primary limitation is the relatively brief publication format (2 pages), which limits the depth of discussion on several important aspects:

Long-term mechanical performance under cyclic loading was not evaluated
The effect of welding sequence on residual stress distribution was not systematically studied
The influence of preheating temperature on η-phase formation was not investigated
No comparison with alternative joining methods (brazing, diffusion bonding, mechanical fastening) was provided

Study Insights

The key insight from this research is that the choice of filler metal in dissimilar material welding is not merely a matter of matching melting points or wetting characteristics, but must account for the complex diffusion phenomena that occur at the interface during the thermal cycle. The formation of η-phase is a direct consequence of compositional interactions during solidification and post-weld cooling, and can be prevented through careful filler metal selection and process design. For engineers, the practical takeaway is that a multi-layer approach using pure Ni as an intermediate layer provides the most reliable results for carbide-to-steel TIG welding.