How does heat treatment affect the mechanical properties of printed Inconel 718 vs. Inconel 625?
How does heat treatment affect the mechanical properties of printed Inconel 718 vs. Inconel 625?
Inconel 718 and Inconel 625 are two of the most commonly printed nickel-based superalloys, but their metallurgical responses to heat treatment are radically different. This is because Inconel 718 is precipitation-hardenable (strengthened by nanoscale intermetallic phases), while Inconel 625 is solid-solution-strengthened (strengthened primarily by molybdenum and niobium in solid solution, with minimal precipitation response). Below is a detailed comparison based on superalloy 3D printing practices.
For a direct alloy comparison, see the dedicated blog: Inconel 625 vs 718 for 3D Printing: Choose the Right Alloy for Custom Metal Parts.
1. As-Printed Baseline Mechanical Properties (DMLS/SLM)
Before any heat treatment, both alloys exhibit high strength but with residual stresses and some anisotropy. Typical as-printed room temperature tensile properties (build direction perpendicular to layers):
Alloy | Ultimate Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
Inconel 718 (as-printed) | 1100–1200 | 800–950 | 10–15 |
Inconel 625 (as-printed) | 900–1050 | 550–700 | 25–35 |
As-printed Inconel 718 already shows higher strength but lower ductility than Inconel 625 due to its inherent precipitation potential (some fine precipitates form during rapid cooling). However, both contain residual stresses that require post-processing.
2. Heat Treatment of Inconel 718: Mandatory Precipitation Hardening
Inconel 718 derives its exceptional high-temperature strength from the precipitation of metastable gamma double-prime (γ'', Ni₃Nb) and gamma prime (γ', Ni₃(Al,Ti)) phases. The standard heat treatment for 3D printed Inconel 718 follows aerospace specifications (AMS 5662/5663) and consists of:
Solution treatment: 980°C ± 10°C for 1 hour, followed by rapid cooling (argon or oil quench). This dissolves any unwanted phases (e.g., Laves phase) and prepares the matrix for uniform precipitation.
Two-step aging: 720°C for 8 hours, furnace cool to 620°C at 50°C/hour, then hold at 620°C for 8 hours, air cool.
As documented in How Heat Treatment Improved Mechanical Properties of 3D Printed Parts, this process dramatically increases strength:
Inconel 718 Condition | UTS (MPa) | YS (MPa) | Elongation (%) |
|---|---|---|---|
As-printed | 1150 | 900 | 12 |
Solution + aged | 1350–1450 | 1100–1250 | 12–18 |
Additionally, heat treatment boosts resistance to wear and fatigue and keeps better material stability. However, Inconel 718 is limited to service temperatures below ~650°C for long-term creep applications because the γ'' phase coarsens above this temperature (see Inconel 718 Maximum Service Temperature).
For critical rotating parts, Hot Isostatic Pressing (HIP) is often performed before heat treatment to close micro-porosity and further enhance fatigue life. HIP also maximizes durability and performance.
3. Heat Treatment of Inconel 625: Minimal Strength Change
Inconel 625 is primarily strengthened by solid-solution elements (Mo, Nb, Cr) and the precipitation of carbides (MC, M₆C) and the intermetallic delta phase (Ni₃Nb) – but the latter is not used for significant hardening in the standard heat treat condition. Typical post-processing for 3D printed Inconel 625 includes:
Stress relief: 650–750°C for 1–2 hours, air cool. This reduces residual stresses without altering microstructure.
Solution annealing: 980–1040°C for 1 hour, followed by rapid quench. This homogenizes the composition, dissolves any secondary phases formed during printing, and maximizes ductility and corrosion resistance.
Unlike Inconel 718, Inconel 625 does not develop a strong age-hardening response because its niobium content is lower and the gamma double-prime phase is not stable enough to provide significant strengthening. As a result, heat treatment has a minimal effect on tensile strength:
Inconel 625 Condition | UTS (MPa) | YS (MPa) | Elongation (%) |
|---|---|---|---|
As-printed | 980 | 620 | 30 |
Stress relieved (700°C) | 1000 | 650 | 32 |
Solution annealed (980°C) | 950–1020 | 550–650 | 30–40 |
The main benefits of heat treating Inconel 625 are:
Reduction of residual stress and prevention of distortion (see how heat treatment releases stress and prevents deformation).
Improved ductility and toughness.
Enhanced corrosion resistance by dissolving chromium carbides that may have precipitated during printing.
Better thermal stability for high-temperature service (Inconel 625 can be used up to 980°C).
However, unlike Inconel 718, you cannot "age" Inconel 625 to higher strength. For applications requiring high strength at intermediate temperatures (e.g., 650°C), Inconel 718 is superior after heat treatment. For applications requiring excellent corrosion resistance, weldability, and ductility over a wide temperature range, Inconel 625 is preferred.
4. Combined Effect of HIP and Heat Treatment
For both alloys, HIP is often performed before the final heat treatment. HIP (typically 1120–1180°C at 100–200 MPa) closes internal porosity, improving density to near 100% and significantly enhancing fatigue life and ductility. The effect on tensile strength is moderate, but the impact on reliability is substantial. After HIP, the standard heat treatment sequence is applied as described above.
For Inconel 718, HIP + full heat treatment yields the highest combination of strength, ductility, and fatigue resistance. For Inconel 625, HIP + solution annealing produces a fully dense, homogenized, and highly ductile material with consistent properties.
5. Practical Recommendations Based on Application
Requirement | Recommended Alloy & Heat Treatment |
|---|---|
Highest room temperature / moderate temperature strength (up to 650°C) | Inconel 718 – mandatory solution + aging heat treatment |
High temperature service (up to 980°C) with moderate strength requirements | Inconel 625 – stress relief or solution annealing |
Excellent corrosion resistance, weldability, and formability | Inconel 625 – solution annealed |
Fatigue-critical rotating parts (turbine discs, shafts) | Inconel 718 – HIP + solution + aging |
Cost-sensitive, large parts with minimal post-processing | Inconel 625 – stress relief only (or as-printed) |
6. Validation of Mechanical Properties After Heat Treatment
To ensure that the heat treatment has achieved the desired properties, all critical parts undergo rigorous testing. Tensile testing (UTS/YS/elongation certification) is standard. For Inconel 718, fatigue testing is often required. Additionally, metallographic microscopy verifies the absence of unwanted phases (e.g., Laves phase in Inconel 718) and the presence of fine precipitates.
All heat treatment processes are managed under a PDCA quality management system, with traceable records for each batch.
7. Conclusion
Heat treatment is essential for unlocking the full potential of 3D printed Inconel 718, transforming it from a moderately strong as-printed material into a high-strength, precipitation-hardened superalloy suitable for turbine discs, shafts, and other critical rotating parts. In contrast, Inconel 625 shows minimal strength change after heat treatment but benefits from stress relief and improved ductility/corrosion resistance. Therefore, when designing a post-processing route, engineers must recognize that Inconel 718 requires a full solution and aging cycle to achieve its advertised properties, while Inconel 625 is often used in the as-printed or simply stress-relieved condition for many applications. For more detailed case studies, refer to superalloy 3D printing case studies and the heat treatment services overview.
