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Corrosion Resistant Alloys
Description of material
EG2 is a Nickel – Copper alloy with very high corrosion resistance and whose strength is increased by the precipitation of particles of Ni3 (Ti, Al) obtained by an exact aging heat treatment. EG2 is an age-hardened alloy with greater strength and resistance corrosion especially in sea water and offers suitable tensile strength up to around 600°even if a rapid reduction of Rm and Rp0,2 starts at around 250-350°C.
Many marine applications exposed to sea water environments such as pump shafts, marine fixtures, marine fasteners & bolting, tanks, springs, off shore devices, chains and valves for oil and gas processes. In addition, this alloy warrants good cryogenic properties and is non- magnetic even at very low temperatures. It is mostly used in off shore structures, in desalination and chemical plants and in sour gas environments. Marinox EG2 boat shafts offer a high performance in terms of corrosion resistance and high strength. (See Marinox EG2).
EG2 is not as susceptible to pitting and crevice corrosion in stagnant sea water as other grades in the martensitic and austenitic series but the effect of fouling should be always considered. In any case, these kinds of corrosion are not so rapid in the propagation of deep pits. Nevertheless, it’s important to know that this alloy, when in the age hardened condition, could be prone to stress corrosion cracking in certain aggressive environments.
EG2 offers a good cold forming thanks a low cold hardening factor but for the best results, in the case of a large amount of cold deformation, such as cold extrusion or heading, is when this alloy is soft annealed. Intermediate annealing should be evaluated.
EG2 is classified as difficult when machining but has approximately the typical machinability of austenitic structures strengthened by Nitrogen, and some difficulties could happen in turning, threading and milling processes. Operators should know that EG2 requires more rigid and powerful machines in addition to the correct choice of tools, coating carbides and cutting fluids. However, this alloy machines better when in the annealed rather than in age-hardened condition. Machining parameters must avoid operations able to cause stresses due to the risk of distortion, especially if the structure is age hardened. In this case, it’s recommended to machine with the lowest stock removal, to age harden and then finish to size. As with all precipitation hardening steels and alloys, a certain amount of dimensional changes, in terms of contraction, happens after the aging of parts: these dimensional variations should be evaluated.
EG2 surfaces to weld must be free of any kind of contaminants. Welding procedures are substantially the same of those applied for Austenitic Stainless steels but joint design and a different weld penetration behavior should be evaluated. Using shielding gas is recommended. Welding of age hardened parts must be avoided and, in any case, cannot have the same strength of the age base metal. EG2 should be welded in the annealed condition followed by a post welding aging treatment. Filler metals of matching composition should be used in order to obtain approximately the same mechanical properties of the aged base metal after PW ageing and heat tint should be eliminated to maximum corrosion resistance.
As with other Ni-alloys, the atmosphere of the furnace should be well monitored in order to avoid embrittlement and surface damage. EG2 has a good hot plasticity and is suitable for processing by drop forging, hot extrusion or by upsetting. Because its forging temperature is lower than many austenitic grades, it is recommended to choose a temperature at the lower range of forging and to avoid a long-time soaking detrimental to its structure. Overheating must be always avoided. The choice of hot working temperature and process parameters must always evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top of the range during the extrusion and forging process, could generate internal bursts, large grain size and structural permanent damage. Suitable strain in terms of section reduction (for instance: 25-30%) at a low to medium range of hot working temperatures during the last step forging operation is recommended in order to obtain a fine grain structure, which is very important for mechanical, fatigue and corrosion resistance properties and makes it easier for ultrasonic testing to detect small indications, as required by several International Norms. Forgings should be cooled rapidly in water because an air or slow cooling rate causes an aging of the structure, where the formation of precipitates generate internal bursts or splitting, in the case of subsequent reheating. In addition, a very rapid fast cooling offers a better structural situation after age hardening.
Surfaces of products must free of any contaminants and very clean before heat treatment. Structural hardening of EG2 is generated by a precipitate of submicroscopic particles of Ni3 (Ti, Al) in the matrix. This means a special care should be adopted to allow to these particles to achieve the expected mechanical properties during the age-hardening. Therefore, EG2 must be solution annealed before the aging treatment, paying attention that both heating and soaking have to be kept to a minimum followed by a fast water quenching. It’s important to underline that a long time soaking or at an excessive temperature, results in both coarse grains and the formation of detrimental Ti–Carbides, jeopardizing the required structural hardening after age treatment. Moreover, it’s important to point out that EG2 should be supplied in the hot or cold working conditions followed by a suitable choice of aging heat treatment parameters if higher mechanical properties were required.
|Commercial name||Alloy K-500|