Heat Treatment Effects on Microstructure and Properties of Powder Metallurgy Inconel 718 TIG Welds

Literature Overview

This study, published in the Chinese Journal of Materials Research (2023) by Zhao Yunmei, Zhao Hongze, Wu Jie, Tian Xiaosheng, and Xu Lei from the Shichangxu Advanced Materials Innovation Center at the Institute of Metal Research, Chinese Academy of Sciences, and the University of Science and Technology of China, investigates the effects of post-weld heat treatment on the microstructure and mechanical properties of powder metallurgy (PM) Inconel 718 alloy TIG welds. Supported by the CAS Stable Support Basic Research Young Team Program (YSBR-025), this work addresses a critical challenge in nickel-based superalloy fabrication.

Core Technical Content

Inconel 718 is a precipitation-strengthened nickel-based superalloy widely used in high-temperature pressure vessels, hydrogenation reactors, and nuclear components. The PM variant offers superior isotropy and mechanical properties compared to wrought Inconel 718, making it ideal for demanding applications. However, welding introduces severe metallurgical challenges due to the complex precipitation strengthening mechanism.

Microstructural Evolution

The PM Inconel 718 base metal contains:

• Gamma (γ) austenite matrix
• Fine L12-ordered γ' (Ni₃(Al,Ti)) precipitates (5–10 nm)
• B2-ordered δ (Ni₃Nb) phase (100–500 nm)

After TIG welding without post-weld heat treatment (PWHT), the weld zones exhibit:

Heat Treatment Conditions

The researchers evaluated multiple PWHT regimes:

Mechanical Property Recovery

After optimal PWHT (1040 °C/1h + 720 °C/8h + 620 °C/8h):

• Weld metal hardness recovered to 420–460 HV (90–95% of base metal)
• Tensile strength reached 1150–1200 MPa (85–90% of PM base metal)
• Elongation improved from 5–8% (as-welded) to 12–15%
• The δ phase was redistributed to grain boundaries, providing crack-arresting capability

Implications for Pressure Vessel Fabrication

Inconel 718 pressure vessels—particularly hydrogenation reactors operating at 300–450 °C and 15–30 MPa—require rigorous welding and heat treatment protocols. The findings have direct relevance to:

1. Welding procedure qualification: The PWHT regime must be integral to the WPS, not a separate operation
2. Acceptance criteria: Post-PWHT hardness mapping should be mandatory for all weld joints in Inconel 718 pressure vessels
3. NDE considerations: The coarsened microstructure in the as-welded HAZ may affect ultrasonic testing signal interpretation

Standards Compliance

Key Reflections

The study underscores a fundamental principle in superalloy welding: the as-welded microstructure is inherently degraded, and post-weld heat treatment is not optional but mandatory for achieving serviceable properties. For PM Inconel 718 specifically, the fine γ' precipitate distribution in the base metal is completely disrupted during welding, requiring full solution treatment to dissolve coarse δ phases before re-precipitation. The double-ageing schedule (720 °C + 620 °C) is critical—the first stage precipitates primary γ', while the second stage optimizes the size distribution for maximum strength at elevated temperatures.

In my experience with hydrogenation reactor fabrication, the most common cause of weld failure is inadequate PWHT execution. Temperature uniformity across the vessel wall thickness, proper soak time, and controlled cooling rates are all essential. The study provides valuable quantitative data for establishing engineering margins in PWHT specifications.