Study Note on TIG Weld Heat-Affected Zone Microstructure of Alloy 718

Literature Overview

This 2015 publication from Shanghai Jiao Tong University's Institute of Welding Engineering Technology and the Shanghai Key Laboratory of Laser Manufacturing and Material Modification examines the microstructural evolution in the heat-affected zone (HAZ) of IN718 (Inconel 718) superalloy welded by GTAW/TIG. The research addresses a critical challenge in the fabrication of high-temperature pressure vessels, heat exchangers, and hydrogenation reactors where IN718 is specified as the cladding or overlay material due to its exceptional combination of strength, creep resistance, and corrosion resistance at temperatures up to 700°C.

Core Technical Content

IN718 is a precipitation-strengthened nickel-based superalloy whose strength derives primarily from the γ' (Ni₃(Al,Ti)) and δ (Ni₃Nb) phases. The base alloy composition typically includes 19–23% Ni, 4.75–5.50% Cr, 2.85–3.15% Mo, 0.65–1.15% Ti, 0.28–0.86% Al, and 0.65–1.10% Nb. The HAZ of IN718 is particularly vulnerable because the welding thermal cycle can dissolve the strengthening precipitates, and subsequent cooling conditions determine whether beneficial or detrimental phases reform.

Microstructural Zones in the HAZ

Zone	Temperature Range	Microstructural Changes	Mechanical Effect
Recrystallized zone	>1200°C	Complete grain recrystallization; new grain growth	Reduced strength; potential grain boundary weakness
Partial recrystallization	1100–1200°C	Mixed old/new grains	Non-uniform properties
Precipitate dissolution	900–1100°C	γ' dissolution; δ phase coarsening	Significant strength loss
Over-aging zone	700–900°C	Coarsening of existing precipitates	Moderate strength reduction
Sensitization zone	600–700°C	Carbide precipitation at grain boundaries	Intergranular corrosion susceptibility

Welding Parameters and Their Influence on HAZ

Parameter	Low Value Effect	High Value Effect
Welding current	Narrow HAZ; less property degradation	Wide HAZ; extensive precipitate dissolution
Travel speed	High heat input; large affected zone	Low heat input; steep thermal gradient; cracking risk
Interpass temperature	Rapid cooling; martensite-like δ formation	Slow cooling; excessive grain growth
Preheating	Reduced residual stress	Increased HAZ width
Post-weld heat treatment	Retains as-welded properties	Restores precipitate distribution

Critical Microstructural Concerns

δ phase formation: The δ-Ni₃Nb phase is particularly detrimental in the HAZ because it forms preferentially at grain boundaries, reducing creep resistance and promoting intergranular fracture. The equilibrium δ fraction increases significantly above 1000°C.
Laves phase precipitation: In the high-temperature HAZ region, the Mo-rich Laves phase (Ni₂MoSi) can precipitate, creating brittle intergranular networks that severely compromise toughness.
Grain boundary carbides: Ti and Al form carbides (TiC, Al₄C₃) at grain boundaries during slow cooling, leading to sensitization and reduced intergranular strength.
Recrystallized grain size: If the peak temperature exceeds the recrystallization temperature, the resulting grain size directly affects the creep life of the welded component.

Engineering Practice Integration

In pressure vessel applications, IN718 is commonly used as:

Overlay/cladding material on carbon steel or low-alloy steel backing plates for hydrogenation reactors operating at 300–450°C under high hydrogen partial pressures.
Weld overlay on heat exchanger tubes and shells for sour service (H₂S-containing environments).
Full weld material in high-pressure hydrogen storage vessels and petrochemical reactors.

Welding Procedure Development for IN718 Cladding

Step	Parameter	Specification
Preheating	Temperature	150–200°C (to reduce cracking susceptibility)
Welding	Current (DCEN)	100–150 A
Welding	Travel speed	250–400 mm/min
Interpass	Temperature	≤250°C (critical for preventing δ formation)
Post-weld	Solution treatment	1120°C ± 10°C, 1 hour
Post-weld	Aging (double)	870°C × 8h + 720°C × 8h
Shielding gas	Composition	Pure Ar or Ar/He 50/50
Wire	Specification	ERNiCrMo-3 (AWS A5.11)

NDT and Acceptance Criteria for IN718 Welds

Test	Standard	Criteria
Radiographic Testing	ASME V, T-274	No indication exceeding 1 mm for critical welds
Ultrasonic Testing	ASME V, T-276	No indication exceeding 2 mm
Hardness	ASTM E92	300–400 HV (after PWHT)
Tensile strength	ASTM E8	≥ 965 MPa (after PWHT)
Intergranular corrosion	ASTM G28	No intergranular attack in HAZ
HIC/SSC	NACE TM0177	No cracking in HAZ

Study Insights and Implications

The HAZ of IN718 welds represents a critical failure initiation site in high-temperature pressure vessel service. The fundamental challenge is that the same thermal cycle that achieves full fusion inevitably creates a zone of degraded microstructure that must be restored through post-weld heat treatment. The interpass temperature control is perhaps the single most important parameter—maintaining it below 250°C prevents excessive δ phase formation while still allowing adequate weldability. For engineers specifying weld overlay procedures for bimetallic hydrogenation reactors, understanding the precipitation kinetics of IN718 during multi-pass welding is essential for developing PWHT schedules that fully restore the alloy's design properties. The research reinforces the principle that weldability of precipitation-strengthened superalloys cannot be assessed by as-welded properties alone; the post-weld treated condition must always be the evaluation basis for design qualification.