CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructural Changes in the Heat-Affected Zone of 718 Alloy TIG Welding
Literature Overview
This study by Ye Xin, Hua Xueming, and Wu Yixiong from the Welding Engineering Research Institute of Shanghai Jiao Tong University and the Shanghai Key Laboratory of Laser Manufacturing and Material Modification was published in the Welding Journal in 2015. The paper investigates the microstructural evolution in the heat-affected zone (HAZ) of IN718 (Haynes 188) superalloy during TIG welding.
IN718 is one of the most widely used nickel-based superalloys in aerospace and power generation applications due to its excellent combination of strength, creep resistance, and corrosion resistance at elevated temperatures. Understanding HAZ microstructural changes is critical for ensuring the performance and service life of welded components.
Core Technical Content
HAZ Microstructural Zones
The TIG welding of IN718 produces a distinct HAZ with multiple sub-zones characterized by different thermal histories and resulting microstructural changes:
| HAZ Sub-zone | Peak Temperature | Microstructural Feature | Mechanical Implication |
|---|---|---|---|
| TC (Thermal Cycle) | >1300°C | Coarse dendritic grain | Reduced yield strength |
| TC/TC1 | 1100–1300°C | Partial grain boundary melting | Cracking susceptibility |
| TC1 | 950–1100°C | δ-ferrite precipitation | Reduced ductility |
| TC2 | 800–950°C | γ' coarsening | Reduced creep resistance |
| TC3 | 600–800°C | Precipitate dissolution | Softening |
Key Microstructural Phenomena
The authors identified several critical microstructural changes:
- δ-ferrite (Ni3Nb) precipitation: The most detrimental phase in the HAZ, forming preferentially at grain boundaries during slow cooling. δ-ferrite is brittle and serves as crack initiation sites under tensile or creep loading.
- γ' precipitate coarsening: The primary strengthening phase (Ni3(Al,Ti,Nb)) undergoes coarsening in the TC2 zone, reducing the overall strengthening effect and lowering the creep strength.
- Grain boundary carbide precipitation: TiC and NbC particles may form at grain boundaries, affecting the fracture behavior of the HAZ.
- Grain growth: In the TC zone, significant grain coarsening occurs due to recrystallization and grain growth during the thermal cycle.
Thermal Cycle Effects
The TIG process produces a relatively slow thermal cycle compared to laser welding, which results in:
- Longer time above 1000°C, promoting δ-ferrite formation
- Slower cooling rates, allowing coarsening of γ' precipitates
- Greater thermal distortion and residual stress accumulation
- Larger HAZ width (typically 3–5 mm for TIG vs. 1–2 mm for laser)
Engineering Practice Integration
IN718 is extensively used in hydrogenation reactors, heat exchanger tubes, and high-temperature pressure vessel components where resistance to creep and corrosion at elevated temperatures is required. The HAZ microstructural changes identified in this study directly impact the design and qualification of welded joints in these applications.
For pressure vessel design under ASME VIII Div.2 or NB/T 47002, the reduced mechanical properties in the HAZ must be accounted for in stress analysis. The δ-ferrite content in the HAZ can be controlled through post-weld heat treatment (PWHT) at 1150°C for 1 hour followed by rapid cooling, but this treatment must be compatible with the overall vessel fabrication schedule.
In the context of nickel-based alloy cladding on steel substrates, understanding the HAZ behavior of IN718 is essential when applying overlay layers to IN718 base components, such as turbine casings or reactor internals.
Key Reflections
The study provides a comprehensive understanding of the complex microstructural evolution in the IN718 HAZ during TIG welding. The identification of δ-ferrite as the primary detrimental phase reinforces the importance of thermal cycle control and post-weld heat treatment in ensuring long-term service performance.
From a practical standpoint, the findings have direct implications for welding procedure development. Minimizing the time spent in the critical temperature range (950–1100°C) through process optimization—such as using pulsed TIG or reducing heat input—can significantly reduce δ-ferrite formation and improve HAZ properties.
The research also highlights the need for careful inspection of IN718 welds, particularly using techniques sensitive to grain boundary phenomena such as PT and MT for surface defects and UT or TOFD for subsurface indications.