Inconel 625
Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum alloy strengthened mainly by niobium and molybdenum through solid-solution hardening. It does not need precipitation-hardening heat treatment to reach high strength. The alloy has strong fatigue resistance, oxidation resistance, and corrosion resistance from cryogenic temperatures up to 980°C.
1. Chemical Composition
The table below gives the elemental composition of Inconel 625. Niobium and molybdenum are the main strengthening elements that set it apart from other Ni-Cr alloys.
| GRADE | UNS Designation |
C | Mn | P | S | Si | Ni | Cr | Mo | Cu | Fe | Ti | Al | Co |
| Inconel 625 | N06625 | ≤ 0.10 | ≤ 0.50 | ≤ 0.015 | ≤ 0.015 | ≤ 0.50 | ≥ 58 | 20.0-23.0 | min 8-10 | ≤ 5.0 | ≤ 0.4 | ≤ 0.4 | Nb+Ta:3.15-4.15 |
2. Mechanical Properties
The values below are for Inconel 625 in the annealed condition for seamless pipe and tube. They show its high baseline strength without precipitation hardening.
| Grade | Condition & Size | Standard | Tensile Strength, MPa (ksi) |
Yield Strength, MPa (ksi) |
Elongation, % | Hardness |
| N06625 | Grade 1 annealed | B444 | ≥827（120） | ≥414（60） | ≥30 | — |
| N06625 | Grade 2 solution annealed | B444 | ≥690（100） | ≥276 (40) | ≥30 | — |
3. Equivalent Grade
The table below maps Inconel 625 (UNS N06625) to its equivalent designations across international standards including EN/DIN, JIS, GB/T, and ISO.
| GRADE | UNS | GB | JIS | ISO | DIN - EN | GOST | |
| ISC | NEW | ||||||
| Inconel 625 | N06625 | - | NS3306 | NCF 625 | - | 2.4856 | - |
4. Key Technical Advantages
- Solid-Solution Strengthening Without Precipitation Treatment: The 3.15–4.15% Nb+Ta interacting with the Ni-Cr-Mo matrix produces high strength through solid-solution mechanisms. This avoids the sensitization or overaging risks that come with precipitation-hardened alloys during complex fabrication sequences.
- Strong Resistance to Pitting, Crevice, and Intergranular Corrosion: The combination of 21% Cr and 9% Mo gives a Pitting Resistance Equivalent (PRE) well above that of standard stainless steels. This allows service in seawater, sour gas, and acid environments where localized corrosion is the primary failure mode.
- Fatigue Strength in Dynamic Pressure Service: Inconel 625 has superior high-cycle fatigue resistance compared to most other nickel alloys, due to the uniform distribution of niobium in the matrix. This matters for flexible risers, umbilicals, and subsea tubing under dynamic mechanical loading.
Technical Note: At temperatures above 650°C for extended service periods, delta phase (Ni3Nb) precipitation can cause embrittlement. Where long-term elevated temperature operation is required, Inconel 625LCF (low-cycle fatigue variant) or other precipitation-hardened grades should be evaluated.
5. Common Manufacturing Standards
ASTM B444: Standard Specification for Seamless Pipe and Tube — Nickel-Chromium-Molybdenum-Columbium Alloys (UNS N06625). This is the primary tubular standard for Inconel 625 in chemical processing, offshore, and power generation applications.
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes — stainless and nickel alloy grades (Material No. 2.4856)
- DIN 17751: Nickel alloy bars and tubes
JIS Standards:
- JIS H 4552: Nickel and nickel alloy seamless pipes and tubes
GB/T Standards:
- GB/T 15007: Corrosion-resistant alloy grades (NS3306)
GOST Standards:
- GOST 14965: Tubes from nickel and nickel alloys
Summary Table — Standard Equivalents:
| ASTM | EN/DIN | JIS | GB/T | GOST |
| ASTM B444 | EN 10216-5 / DIN 17751 | JIS H 4552 | GB/T 15007 | GOST 14965 |
6. Primary Applications
- Offshore Oil and Gas: Flexible pipe inner liners, subsea wellhead components, and riser tubing in aggressive sour gas service (H2S + CO2 + chlorides) where both fatigue resistance and corrosion resistance are needed.
- Aerospace Propulsion: Exhaust ducts, thrust reverser structures, and fuel and hydraulic tubing in aircraft engines. Weight, fatigue strength, and oxidation resistance all have to be met at once.
- Nuclear Waste Reprocessing: Tanks, piping, and heat exchanger tubes handling highly radioactive nitric acid solutions where sustained corrosion resistance over decades of service is required.
- Marine and Seawater Systems: Propeller blades, seawater piping, and desalination plant heat exchanger tubes in full-concentration seawater environments that need resistance to biofouling-assisted crevice corrosion.
- Chemical Processing: Reactor vessels, distillation columns, and transfer lines in the production of organic and inorganic acids, particularly in acetic acid and phosphoric acid service.