Chemical Composition Differences Between 600 and 625 Inconel - Knowledge

1. Standard Chemical Composition (ASTM B166 for Inconel 600; ASTM B443 for Inconel 625)

Element	Inconel 600 (wt%)	Inconel 625 (wt%)
Nickel (Ni, Bal.)	≥72.0 (balance)	≥58.0 (balance)
Chromium (Cr)	14.0–17.0	20.0–23.0
Iron (Fe)	6.0–10.0	≤5.0
Molybdenum (Mo)	≤1.0	8.0–10.0
Niobium (Nb) + Tantalum (Ta)	≤0.5	3.15–4.15 (Nb+Ta)
Carbon (C)	≤0.15	≤0.10
Manganese (Mn)	≤1.0	≤0.50
Silicon (Si)	≤0.5	≤0.50
Phosphorus (P)	≤0.015	≤0.015
Sulfur (S)	≤0.015	≤0.015
Copper (Cu)	≤0.5	≤0.50

2. Core Chemical Composition Differences

(1) Nickel Content

Inconel 600 has a higher nickel content (minimum 72 wt%), which is the key factor for its excellent resistance to chloride ion stress corrosion cracking (SCC) and good ductility at room temperature.

Inconel 625 has a relatively lower nickel content (minimum 58 wt%), and its performance advantages are not mainly dependent on nickel content, but on the synergistic effect of other alloying elements.

(2) Chromium and Molybdenum Content

Chromium: Inconel 625 contains higher chromium (20.0–23.0 wt%) than Inconel 600 (14.0–17.0 wt%), which significantly enhances its high-temperature oxidation resistance and pitting corrosion resistance in oxidizing media.

Molybdenum: This is the most obvious difference between the two alloys. Inconel 600 contains only trace molybdenum (≤1.0 wt%), while Inconel 625 has a high molybdenum content (8.0–10.0 wt%). Molybdenum is the core element to improve the alloy's resistance to reducing media corrosion (such as hydrochloric acid, sulfuric acid) and local corrosion resistance.

(3) Niobium-Tantalum Alloying System

Inconel 625 is characterized by the addition of 3.15–4.15 wt% of niobium-tantalum mixture, while Inconel 600 has almost no niobium-tantalum content (≤0.5 wt%). Niobium in Inconel 625 acts as a strengthening element: it forms a stable intermetallic phase (Ni₃Nb) during aging treatment, realizing precipitation strengthening and significantly improving the alloy's high-temperature strength and creep resistance. In contrast, Inconel 600 is a solid solution strengthened alloy without precipitation strengthening phases, and its high-temperature strength is relatively lower.

(4) Iron Content

Inconel 600 allows a higher iron content (6.0–10.0 wt%), which helps reduce the alloy's production cost while maintaining basic performance. Inconel 625 strictly limits iron content to ≤5.0 wt% to ensure the purity of the alloy and avoid the adverse effects of iron on corrosion resistance and mechanical properties.

3. Impact of Composition Differences on Performance Orientation

The chemical composition differences determine the different application scenarios of the two alloys:

Inconel 600 is suitable for low-to-medium temperature corrosion resistance and heat exchange scenarios, such as nuclear reactor heat exchangers, steam generator tubes, and chemical industry heating elements, relying on its high nickel content and good SCC resistance.

Inconel 625 is oriented towards high-temperature, high-strength and strong corrosion resistance working conditions, such as aerospace engine components, offshore oil drilling equipment, and flue gas desulfurization systems, benefiting from the synergistic effect of high Cr, Mo and Nb-Ta elements.