Discover the latest trends and innovations in the nickel alloy industry. We provide expert insights and in-depth analysis on the newest developments in nickel alloys.
GH738 (commercially known as Waspaloy, UNS N07001) is a precipitation-hardening nickel-chromium-cobalt-molybdenum superalloy. Its exceptional high-temperature strength, creep resistance, and fatigue performance are not inherent solely to its chemical composition but are critically dependent on a precise heat treatment regimen. The process is designed to dissolve primary strengthening phases and then re-precipitate them in a controlled, fine distribution to maximize the gamma prime [γ′] phase effectiveness. Shanghai COCESS Special Alloys Co., Ltd supplies GH738 in various conditions and provides technical guidance on heat treatment protocols to ensure optimal final component performance.
Solution Treatment: Establishing the Matrix
The first step in the heat treatment of GH738 is the solution treatment (also referred to as solutioning). This process involves heating the alloy to a high temperature to dissolve the gamma prime (γ′) precipitates and carbides into the nickel matrix, creating a homogeneous, supersaturated solid solution.
Temperature Range: Typically 1080°C ± 10°C (some specifications or optimized processes may use 1040°C to 1120°C depending on product form and desired grain size).
Soaking Time: Generally 4 hours (adjustable based on cross-sectional thickness, e.g., 1 to 4 hours).
Cooling Method: Air Cooling (AC) is standard for many forms; however, Oil Quenching or Water Quenching may be employed for specific sections to ensure a faster cooling rate, preventing premature coarse precipitation and ensuring a uniform starting structure for aging.
The goal is to achieve a uniform austenitic grain structure (usually ASTM 5-8 grade for turbine disks) without excessive grain growth, which would detrimentally affect low-cycle fatigue properties.
Aging Process: Precipitation of Gamma Prime (γ′)
Following the solution treatment, GH738 undergoes a multi-stage aging process. This is where the alloy derives its high strength. The controlled cooling and reheating allow for the precipitation of fine, coherent γ′ particles (Ni₃(Al, Ti)) which hinder dislocation movement.
First Aging (Stabilization / Higher Temp Aging): Heating to 840°C ± 10°C and holding for 24 hours, followed by Air Cooling. This step promotes the precipitation of relatively uniform, slightly coarser γ′ at grain boundaries and within grains, aiding in creep resistance and stability.
Second Aging (Lower Temp Aging): Heating to 760°C ± 10°C and holding for 16 hours, followed by Air Cooling. This lower temperature step refines the γ′ precipitate distribution, significantly increasing the yield and tensile strength as well as the stress-rupture capability.
Alternative/Optimized Cycles: Some applications may utilize a 3-step process including a stabilization at ~845°C for 4h, or adjust times/temps (e.g., 995°C solution + 845°C/4h + 760°C/16h) to prioritize specific tensile vs. creep balances.
Resulting Mechanical Properties
When the standard heat treatment (1080°C/4h/AC + 859°C/24h/AC + 760°C/16h/AC) is correctly applied, GH738 achieves the following typical properties:
Room Temperature: Tensile Strength ≥ 1200 MPa, Yield Strength ≥ 800 MPa, Elongation ≥ 15%.
High Temperature (e.g., 815°C): Tensile Strength remains ≥ 608 MPa, with excellent stress-rupture life (e.g., >23 hours at 815°C / 328 MPa).
Hardness: Typically reaches HRC 30–38 (approx. 320–360 HB).
Critical Considerations & Supplier Support
Atmosphere Control: Heat treatment should be performed in a neutral or reducing atmosphere (vacuum, hydrogen, or dissociated ammonia) to prevent oxidation, decarburization, or alpha case formation on the surface.
Dimensional Stability: Due to the long hold times at aging temperatures, fixtures may be required for complex shapes to prevent distortion.
Welding: If welding is required, it is best performed in the solution-treated (soft) condition. A full post-weld heat treatment (solutioning + aging) is highly recommended to restore the heat-affected zone (HAZ) properties to match the base metal.
Supply Condition: Shanghai COCESS Special Alloys Co., Ltd typically supplies GH738 bar, forgings, and ring rolls in the solution-treated condition or fully heat-treated condition according to customer drawings and specifications, ensuring the material is ready for final machining or immediate use.
For detailed heat treatment certificates (batch specific), or to inquire about GH738 products in specific heat-treated conditions, please contact the engineering team at Shanghai COCESS Special Alloys Co., Ltd.
Website: https://en.cocessalloys.com
Email: sales@cocessalloys.com
