A Comparison of INCONEL Alloys 625 and 725

Both INCONEL® alloy 625 and 725 are nickel-based corrosion-resistant alloys, widely used in high-temperature, high-pressure and highly corrosive environments. However, there are significant differences between them in terms of composition design, strengthening mechanisms, properties and application scenarios. The following is a detailed comparison from multiple dimensions: 

I. Chemical Composition Comparison

Both alloys are nickel-based, with core alloying elements being chromium (Cr), molybdenum (Mo), and niobium (Nb). However, the differences in the content of key elements (such as titanium, aluminum, and iron) determine their performance variations. The specific compositions (mass fractions, %) are as follows:

II. Comparison of Strengthening Mechanisms

The strengthening mechanism is the core reason for the performance difference between the two:

INCONEL® 625: Predominantly solid solution strengthening. By dissolving elements such as chromium, molybdenum, and niobium into the nickel matrix, a stable solid solution is formed, which hinders dislocation movement and thereby enhances strength. This mechanism makes the alloy less prone to softening at high temperatures (no problem of precipitate phase decomposition), and it has excellent high-temperature stability.

INCONEL® 725: Predominantly precipitation hardening (age hardening). After solution treatment (dissolving alloy elements at high temperatures), followed by aging treatment (holding at approximately 700-800°C), fine and uniform γ' phases (Ni₃(Ti,Al)) precipitate, significantly enhancing strength through dispersion strengthening. This mechanism enables it to achieve a much higher yield strength than 625 at room temperature and medium temperatures. 

III. Comparison of Mechanical Properties

IV. Corrosion Resistance Comparison

Both alloys exhibit excellent corrosion resistance, but due to differences in composition, their application environments have slight emphases:

Commonalities: Both are resistant to seawater, salt spray, neutral and weakly acidic media, and have strong resistance to pitting and crevice corrosion caused by chloride ions (containing Cr, Mo, and Nb elements).

Differences:

625: With a higher molybdenum content (8-10%), it offers superior corrosion resistance in strongly reducing media (such as hydrochloric acid and hydrofluoric acid) and high-temperature oxidative environments (such as flue gas desulfurization).

725: With higher titanium and aluminum content but slightly lower chromium and molybdenum, it shows slightly inferior corrosion resistance in strongly oxidizing media compared to 625, but performs exceptionally well in the H₂S/CO₂ environment of oil and gas extraction (resistant to sulfide stress corrosion). 

V. Comparison of Processing and Heat Treatment

VI. Comparison of Application Fields

VII. Summary and Selection Recommendations

From the above comparison, it can be seen that 625 is more suitable for scenarios with high temperatures, strong corrosion, and easy processing, while 725 focuses on the high-strength requirements in medium-temperature and high-pressure environments. The two are irreplaceable in their respective fields.