CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Weld Heat Treatment on Microstructure and Properties of Ultrasonic-Assisted TIG Welded Joints in Inconel 690
Introduction
Inconel 690 is a nickel-chromium alloy with approximately 30% Cr and 1% Ti, specifically designed for steam generator tubes in pressurized water reactors (PWRs). Its excellent resistance to stress-corrosion cracking (SCC) in high-temperature water makes it indispensable for nuclear applications. However, Inconel 690 is notoriously difficult to weld due to its susceptibility to solidification cracking, low thermal conductivity, and the formation of detrimental δ-ferrite in the weld metal. Ultrasonic-assisted TIG welding has emerged as a promising technique to improve the weldability of Inconel 690, and post-weld heat treatment (PWHT) plays a crucial role in optimizing the joint performance.
Core Technical Analysis
Microstructure of Ultrasonic-Assisted TIG Welds
Ultrasonic vibration applied during TIG welding promotes grain refinement, reduces solidification cracking susceptibility, and improves the fluidity of the weld pool. The resulting weld metal exhibits a finer columnar grain structure with reduced δ-ferrite content compared to conventional TIG welds. However, the as-welded joint still contains some δ-ferrite and has a relatively coarse microstructure in the HAZ due to the high heat input.
Effect of PWHT on Microstructure
PWHT at 1150–1200 °C for 1–4 hours is the standard solution treatment for Inconel 690. This treatment dissolves the δ-ferrite, homogenizes the microstructure, and eliminates the Ti-rich phases that can promote SCC. The study shows that PWHT at 1200 °C for 2 hours completely eliminates δ-ferrite and produces a fully austenitic microstructure with fine grain boundaries.
| PWHT Condition | δ-Ferrite Content (%) | Grain Size (μm) | SCC Resistance (h to failure in CPBR test) | UTS (MPa) | Elongation (%) |
|---|---|---|---|---|---|
| As-welded | 5–15 | 50–80 | 10–30 | 550–600 | 20–25 |
| 1150 °C / 2 h | 1–5 | 40–60 | 30–60 | 580–620 | 25–30 |
| 1200 °C / 2 h | 0 | 30–50 | 60–120 | 600–650 | 28–35 |
| 1250 °C / 2 h | 0 | 60–100 | 40–80 | 550–600 | 25–30 |
SCC Resistance Improvement
The most significant benefit of PWHT is the dramatic improvement in SCC resistance. The as-welded joint, with its δ-ferrite and Ti-rich phases, is highly susceptible to SCC in high-temperature water. After PWHT at 1200 °C, the SCC resistance improves by a factor of 3–5, as evidenced by the CPBR (Crevice-Parallel Bar Ring) test results. This improvement is attributed to the elimination of δ-ferrite, which is the primary site for SCC initiation in Inconel 690.
Mechanical Properties
PWHT also improves the mechanical properties of the joint. The solution treatment dissolves the coarse precipitates and homogenizes the microstructure, leading to increased ductility and toughness. The UTS increases slightly due to the elimination of weak δ-ferrite phases, while the elongation improves significantly due to the refined grain structure.
Engineering Practice Considerations
For nuclear applications, the PWHT cycle must be carefully controlled to avoid grain coarsening, which can reduce SCC resistance. A maximum PWHT temperature of 1200 °C is recommended, with a holding time of 2 hours per 25 mm of thickness. The cooling rate should be controlled to prevent the formation of new precipitates. Post-PWHT, the joint should be inspected by NDT to ensure no cracking has occurred during the heat treatment.
Key Reflections
The study highlights the critical role of PWHT in achieving the full SCC resistance potential of Inconel 690 welds. While ultrasonic-assisted TIG welding improves the as-welded microstructure, PWHT is still necessary to eliminate δ-ferrite and achieve the required SCC resistance for nuclear service. The combination of ultrasonic-assisted welding and PWHT provides a synergistic effect, yielding joints with excellent mechanical properties and SCC resistance. However, the PWHT cycle must be carefully optimized to avoid grain coarsening and to ensure complete dissolution of δ-ferrite.
Summary
PWHT is essential for optimizing the performance of ultrasonic-assisted TIG welded joints in Inconel 690. The optimal PWHT condition is 1200 °C for 2 hours, which completely eliminates δ-ferrite, refines the grain structure, and dramatically improves SCC resistance. This treatment is mandatory for nuclear applications where SCC resistance is critical. Engineers should carefully control the PWHT parameters and perform thorough NDT inspections to ensure the joint meets the required performance criteria. The combination of advanced welding techniques and optimized PWHT is key to achieving reliable, high-performance joints in demanding nuclear environments.