Complete Guide on Inconel 625

Inconel 625 Description

Inconel 625 is a nickel-chromium-molybdenum alloy that is used for its high strength, high toughness and excellent corrosion resistance. The strength of alloy 625 is derived from the stiffening effect of molybdenum and niobium on its nickel-chromium matrix. Although the alloy was developed for high temperature strength, its highly alloyed composition also provides a significant level of general corrosion resistance. Inconel 625 with excellent resistance to pitting, crevice and corrosion cracking. Inconel 625 is highly resistant in a wide range of organic and mineral acids. Good high temperature strength.

Alloy 625 is a nonmagnetic , corrosion - and oxidation-resistant, nickel-based alloy. Its outstanding strength and toughness in the temperature range cryogenic to 2000°F (1093°C) are derived primarily from the solid solution effects of the refractory metals, columbium and molybdenum, in a nickel-chromium matrix. The alloy has excellent fatigue strength and stress-corrosion cracking resistance to chloride ions. It has the best all-round combination of strength, high temperatures and corrosion resistance of the Inconel family of alloys. It is pre-qualified for use in pressure vessels up to 815°C. Inconel 625 LCF is a special quality of the alloy for use in bellows.

Inconel 625 with excellent oxidation resistance, corrosion resistance, and strength at high temperatures. Age-hardening treatments are not required to obtain the high strength properties of Alloy 625 due to the solid solution effects of molybdenum and columbium on the nickel-chromium matrix. I625 is resistant to chloride-ion stress-corrosion cracking, pitting, and stress-corrosion cracking making it suitable for seawater applications. Inconel 625 can operate from cryogenic temperatures up to 1800°F (980°C). Some typical applications for alloy 625 have included heat shields, furnace hardware, gas turbine engine ducting, combustion liners and spray bars, chemical plant hardware, and special seawater applications.

Inconel 625 alloy is based on nickel, with 21-23% chromium, 8-10% molybdenum and 3.15-4.15% niobium added. It is a typical solid solution strengthened alloy with excellent mechanical properties, corrosion resistance and welding performance, and its operating temperature range is from low temperature to 982°C. From a composition, Inconel 625 alloy is more stringent in controlling impurity elements, such as lower sulfur and manganese contents, which makes Inconel 625 alloy have better instant performance and durability. Alloy 625 has a high nickel content and a higher molybdenum content giving it a better foundation to resist corrosion.

Mechanical properties of Nickel Alloy 625

Corrosion Resistance

Alloy 625 has withstood many corrosive environments. In alkaline, salt water, fresh water, neutral salts, and in the air, almost no attack occurs. The nickel and chromium provide resistance to oxidizing environments. Nickel and molybdenum provide for resistance to nonoxidizing atmospheres. Pitting and crevice corrosion are prevented by molybdenum. Niobium stabilizes the alloy against sensitization during welding. Chloride stress-corrosion cracking resistance is excellent. Alloy 625 has good resistance to oxidation and scaling at high temperatures. At 1800°F, scaling resistance becomes a significant factor in service. It is superior to many other high temperature alloys under cyclic heating and cooling conditions. The combination of the alloyed elements in alloy 625 enables it to withstand a wide variety of severe corrosive environments. There is almost no attack in mild environments, such as fresh and seawater, neutral pH environments, and alkaline media. The chromium content of this alloy results in superior resistance to oxidizing environments.

Pickling

Sodium hydride baths are necessary to descale alloy 625. After the sodium hydride treatment, the material should be immersed in a sulfuric acid bath 165°F (74°C) for approximately 3 minutes. A 25-minute immersion in a nitric-hydrofluoric bath 145°F (63°C) is then necessary. Rinse. Sulfuric solution: 16% by weight, H2SO4. Nitric solution: 8% HNO3 by weight and 3% HF by weight. Acid etching for macro-inspection-expose material electrolytically to a 3-to-1 HCl to HNO3 solution, saturated with CuCl2 at a current density of 0.645 amp/in² (25.4 A/m).

Fabrication

Alloy 625 can be formed using various cold and hot working processes. Alloy 625 resists deformation at hot working temperatures, therefore higher loads are required to form the material. Hot forming should be performed within a temperature range of 1700° to 2150°F. During cold working, the material work hardens more rapidly than traditional austenitic stainless steels.

Hot Working

Hot working may done at 2100°F (1149°C) maximum furnace temperature. Care should be exercised to avoid frictional heat build-up which can result in overheating, exceeding 2100°F (1149°C). Alloy 625 becomes very stiff at temperatures below 1850°F (1010°C). Work pieces that fall below this temperature should be reheated. Uniform reductions are recommended to avoid the formation of a duplex grain structure. Approximately 15/20% reduction is recommended for finishing.

Heat Treatment

Alloy 625 has three heat treatments: solution annealing at 2000/2200°F and air quench or quicker, annealing 1600/1900°F and air quenching or quicker and stress relieving at 1100/1500°F and air quenching. Solution annealed (grade 2) material is commonly used for applications above 1500°F where resistance to creep is important. Soft-annealed material (grade 1) is commonly used for lower temperatures and has the optimum combination of tensile and rupture properties.

Weldability

Inconel 625 is readily welded by GMAW (MIG), GTAW (TIG), SMAW (manual) and SAW processes. Preheat, post heat and post weld heat treatment are not required. The area around the weld must be clean to prevent contamination of the weld pool. Argon shielding gas is used.

Inconel 625 Uses & Applications

Aircraft ducting systems

Jet engine exhaust systems

Engine thrust-reverser systems

Bellows and expansion joints

Turbine shroud rings

Flare stacks

Seawater components

Chemical process equipment handling mixed acids both oxidizing and reducing.