Heat Treatment of X-750

INCONEL® alloy X-750 is a precipitation-hardenable nickel-chromium alloy known for its exceptional corrosion resistance, oxidation resistance, and high strength at elevated temperatures.

One of the key factors that contribute to the versatility of this alloy is the ability to enhance its properties through various heat treatment processes.

In this blog post, we will explore the heat treatment methods employed for INCONEL alloy X-750 and their impact on the alloy’s mechanical and thermal properties.

Chemical Compositions of X-750 Alloy

The chemical composition of Alloy X-750 is as follows:

  • Nickel (including Cobalt): Minimum 70.00%
  • Chromium: 14.0-17.0%
  • Iron: 5.0-9.0%
  • Titanium: 2.25-2.75%
  • Aluminum: 0.40-1.00%
  • Niobium (including Tantalum): 0.70-1.20%
  • Manganese: Maximum 1.00%
  • Silicon: Maximum 0.50%
  • Sulfur: Maximum 0.01%
  • Copper: Maximum 0.50%
  • Carbon: Maximum 0.08%
  • Cobalt (not required for routine acceptance): Maximum 1.00%
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Alloy X-750 Solution Heat Treatment

For applications demanding optimum properties at high temperatures above 1100°F (593°C) for extended periods, the recommended heat treatment involves a solution treatment process. This treatment consists of heating the alloy to 2100°F (1149°C) followed by stabilization treatment at 1550°F (843°C) and precipitation treatment at 1300°F (704°C).

The solution treatment dissolves the alloying elements and promotes the formation of a fine and homogenous microstructure, enhancing the material’s high-temperature strength and creep resistance.

Alloy X-750 Precipitation Treatment

To strengthen INCONEL alloy X-750 for service temperatures below 1100°F (593°C), precipitation treatment is employed. This treatment can be performed after hot or cold working, equalizing or solution treating processes.

By subjecting the alloy to precipitation treatment, the precipitation of fine gamma prime (γ’) and gamma double prime (γ”) phases occurs, resulting in improved mechanical properties such as tensile strength, hardness, and fatigue resistance.

Furnace-Cooling Treatment

In certain applications, a furnace-cooling treatment is used to develop optimized properties. This treatment involves rapidly cooling the alloy from the solution treatment temperature to room temperature in a controlled manner. The furnace-cooling treatment can enhance the material’s stress rupture strength, fatigue life, and resistance to environmental cracking.

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Effects of Heat Treatment

The heat treatment processes applied to INCONEL alloy X-750 significantly impact its mechanical and thermal properties. The alloy exhibits excellent strength and resistance to oxidation at temperatures up to 1300°F (704°C).

Heat-treated material retains useful strength even at elevated temperatures of up to 1800°F (982°C). Additionally, the alloy retains its exceptional properties even at cryogenic temperatures.

Below table summarize the heat treatment for X-750 alloy:

Product FormHeat TreatmentRemarks
Rods, bars and forgings1625°F/24 hr, AC, + 1300°F/20 hr, ACHigh strength and notch rupture ductility up to 1100°F
Rods, bars and forgings1800°F anneal + 1350°F/8 hr, FC to 1150°F, Hold at 1150°F for total precipitation-treating time of 18 hr, ACIncreased tensile properties and reduced heat treating time for service up to about 1100°F
Rods, bars and forgings1800°F anneal + 1400°F/1 hr, FC to 1150°F, Hold at 1150°F for total precipitation-treating time of 6 hr, ACShort furnace-cool aging. Achieves only slightly lower properties than does AMS 5670 and AMS 5671
Rods, bars and forgings2100°F anneal + 1550°F/24 hr, AC, + 1300°F/20 hr, ACMaximum creep, relaxation and rupture strength above about 1100°F
Sheet, strip and plate (Supplied in annealed condition)High strength to 1300°FHigh strength to 1300°F
Sheet, strip and plate (Supplied in annealed condition)1300°F/20 hr, ACHigh strength up to 1300°F
Sheet, strip and plate (Supplied in annealed condition)1350°F/8 hr, FC to 1150°F, Hold at 1150°F for total precipitation-treating time of 18 hr, ACIncreased tensile properties and reduced heating time for service up to about 1100°F
Seamless tubing1400°F/1 hr, FC to 1150°F, Hold at 1150°F for total time of 6 hr, ACHigh strength up to about 1300°F
Wire, No. 1 temper1300°F/20 hr, ACFor springs requiring optimum resistance to relaxation from about 700°F to 850°F and at low to moderate stresses to about 1000°F
Wire, spring temper1350°F/16 hr, ACHigh strength up to about 700°F
Wire, spring temper1200°F/4 hr, ACFor springs for service requiring maximum relaxation resistance at about 850°F to 1200°F

Conclusion

The heat treatment of INCONEL alloy X-750 plays a crucial role in tailoring its properties to meet specific application requirements. By employing solution treatment, precipitation treatment, and furnace-cooling treatment, the alloy’s high-temperature strength, creep resistance, and mechanical properties can be optimized.

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With its corrosion resistance, oxidation resistance, and outstanding performance across a wide temperature range, INCONEL alloy X-750 continues to find applications in gas turbines, rocket-engine thrust chambers, airframes, pressure vessels, and various industrial fields.