Inconel 625 and Inconel 718 Powders: Characteristics and Applications

The Inconel alloy family is known for its high temperature resistance, corrosion resistance and high strength, and is widely used in energy and chemical industries. This article focuses on the differences in the characteristics of Inconel 625 and 718 powders and their applications in traditional industry and additive manufacturing. Through chemical composition, strengthening mechanism, performance comparison and typical scenario analysis, the selection logic of the two is analyzed.

Alloy Powders Products List

• High-Temperature Alloy Powders
• Stainless Steel Powders
• Precision Alloy Powders
• High-Entropy Alloy Powders
• Other Alloy Powders

Basic Characteristics and Chemical Composition

The main components of Inconel 625 include nickel (≥58%), chromium (20-23%), molybdenum (8-10%), niobium (3-4%), supplemented by a small amount of iron and aluminum. Solid solution hardening is the main method, and auxiliary precipitation hardening is provided by γ'' phase (Ni3Nb) formed by niobium. The main components of Inconel 718 include nickel (50-55%), chromium (17-21%), niobium (4.75-5.5%), molybdenum (2.8-3.3%), and aluminum (0.5%) and titanium (0.9%) are added to form γ' phase (Ni₃Al/Ti). Ultra-high strength is achieved through double precipitation hardening of γ' phase (face-centered cubic) and γ'' phase (body-centered tetragonal, Ni₃Nb).

625 focuses on corrosion resistance and long-term high temperature stability, while 718 is known for its high strength, with a room temperature yield strength of 1040-1050 MPa, far exceeding 625's 480-450 MPa.

Comparison of Mechanical and Thermal Properties

625 is 208 GPa, 718 is 211 GPa, and the stiffness of the two is similar. 718 (1040 MPa) is significantly higher than 625 (480 MPa), which is attributed to the double precipitation hardening effect. Both maintain good elongation, but 718 loses strength faster at high temperatures, while 625 can still maintain high stability at 650°C.

The melting point of 625 (1290°C) is slightly higher than that of 718 (1260°C), which is suitable for more demanding high-temperature environments.

The thermal conductivity of 718 (11.4 W/m·K) is better than that of 625 (9.8 W/m·K), and the heat dissipation capacity is stronger.

The thermal expansion coefficient of 625 (13.58×10⁻⁶ K⁻¹) is slightly higher than that of 718 (12.8×10⁻⁶ K⁻¹), so attention should be paid to thermal matching design.

Corrosion Resistance and Processing Characteristics

625 performs better in acidic and marine environments due to its higher chromium (22% vs. 19%) and molybdenum (9.4% vs. 3.1%) contents. The corrosion current density of both after welding is lower than that of the parent material, but 625 has better corrosion resistance in Cl⁻-containing media. 718 has higher cutting forces due to its high strength, and low-temperature cooling or ultrasonic-assisted processing is required to improve surface quality. The residual stress of 625 after SLM forming is significant, and solid solution + aging treatment is required to optimize performance. The crack growth rate of 718 is faster than that at low temperature, and the damage tolerance needs to be designed specifically.

Industrial Application Fields

Inconel 625:

Chemical processing equipment and desalination pipelines: The chemical industry and desalination systems commonly use Inconel 625 because of its outstanding resistance to both corrosion (especially from chloride environments) and oxidation. The alloy maintains exceptional resistance to corrosion from molten fluoride salts and nitrates which makes it ideal for chemical reactor applications.

Flue gas desulfurization system: The 625 alloy passivation layer composed predominantly of Cr₂O₃ provides effective protection against pitting and intergranular corrosion within acidic settings like flue gas containing SO₄²⁻.

Inconel 718:

Oil drilling tools and gas turbine blades: 718 alloy still maintains excellent fatigue resistance at high temperatures (≤700°C), especially under cyclic loads, the precipitation of δ phase (Ni₃Nb) can inhibit crack propagation and extend the service life of gas turbine blades.

Process Selection and Optimization

Powder bed fusion (PBF) technology: Powder bed fusion (PBF) technology is mainly used in additive manufacturing of 718 and 625 alloys. Laser parameters (such as linear energy density) need to be optimized to reduce porosity. For example, increasing laser power can improve molten pool stability and make the density reach more than 99.7%.

Powder particle size control: Fine powder (such as 15-45 μm) can improve surface finish and reduce surface roughness caused by "spheroidization effect" (can be reduced to 1.25-7.4 µm).

Heat treatment: Solution annealing (such as 980°C/1h) can eliminate residual stress and promote the precipitation of γ'' phase (Ni₃Nb), significantly improving hardness (from 273 HV to 397 HV) and high-temperature oxidation resistance.

Surface treatment: Sandblasting or polishing can further reduce the roughness. At the same time, the 718 alloy formed by laser selective melting (SLM) has a dense Cr₂O₃ passivation film, and its corrosion resistance is better than that of traditional rolled materials.