Background

      Alloy 2205 is a 22% Chromium, 3% Molybdenum, 5-6% Nickel nitrogen alloyed duplex stainless steel with high general, localized and stress corrosion resistance properties in addition to high strength and excellent impact toughness.
      Alloy 2205 provides pitting and crevice corrosion resistance superior to 316L or 317L austenitic stainless steels in almost all corrosive media. It also has high corrosion and erosion fatigue properties as well as lower thermal expansion and higher thermal conductivity than austenitic.
      The yield strength is about twice that of austenitic stainless steels. This allows a designer to save weight and makes the alloy more cost competitive when compared to 316L or 317L.
2205 is the most widely used duplex (ferritic/austenitic) stainless steel grade. It finds applications due to both excellent corrosion resistance and high strength.
      Alloy 2205 is particularly suitable for applications covering the -50°F/+600°F temperature range. temperatures outside this range may be considered but need some restrictions, particularly for welded structures.
      2205 is not generally suitable for use at temperatures above 300°C as it suffers from precipitation of brittle micro-constituents, nor below -50°C because of its ductile-to-brittle-transition.

Key Properties
      These properties are specified for pipe product in ASTM A790/A790M.
Composition
      Typical compositional ranges for grade 2205 stainless steels are given in table 1.
Table 1. Composition ranges for 2205 grade stainless steels.

Mechanical Properties
      Typical mechanical properties for grade 2205 stainless steels are given in table 2.
Table 2. Mechanical properties of 2205 grade stainless steels

Physical Properties
      Typical physical properties for grade 2205 stainless steels are given in table 3.
Table 3. Typical annealed condition physical properties for 2205 grade stainless steels

Grade Specification Comparison
      Approximate grade comparisons for 2205 stainless steels are given in table 4.
Table 4. Grade specification comparisons for 2205 grade stainless steels

Possible Alternative Grades
      Possible alternative grades to grade 2205 stainless steels are given in table 5.
Table 5. Grade specification comparisons for 2205 grade stainless steels

Corrosion Resistance

      Because of its high chromium (22%), molybdenum (3%), and nitrogen (0.18%) contents, the corrosion resistance properties of 2205 are superior to that of 316L or 317L in most environments.
Excellent general corrosion resistance; superior to Grade 316 in most environments. Excellent resistance to localised corrosion including intergranular, pitting and crevice corrosion; the CPT of 2205 is generally at least 35°C. The grade is also resistant to chloride stress corrosion cracking (SCC) at temperatures of up to about 150°C
      Grade 2205 will often perform well in environments which cause premature failure of austenitic grades. It has better resistance to sea water than grade 316.
Localized Corrosion Resistance
      The chromium, molybdenum, and nitrogen in 2205 also provide excellent resistance to pitting and crevice corrosion even in very oxidizing and acidic solutions.
Stress Corrosion Resistance
      The duplex microstructure is known to improve the stress corrosion cracking resistance of stainless steels.
      Chloride stress corrosion cracking of austenitic stainless steels can occur when the necessary conditions of temperature, tensile stress, oxygen, and chlorides are present. Since these conditions are not easily controlled, stress corrosion cracking has often been a barrier to utilizing 304L, 316L, or 317L.
Corrosion Fatigue Resistence
      Alloy 2205 combines high strength and high corrosion resistance to produce high corrosion fatigue strength. Applications in which processing equipment is subject to both an aggresively corrosive enviroment and to cycle loading can benefit from the properties of 2205.
General Corrosion in Wet Process Phosphoric Acids

Stress Corrosion Cracking Resistance

(P=Pass, F=Fail)
Structure
      The chemical analysis of 2205 is optimized to obtain a typical 50 a/ 50 g microstructure after solution annealing treatment at 1900°/1922°F (1040°/1080°C).
      Heat treatments performed above 2000°F may result in an increase of ferrite content.
      Like all duplex stainless steels, 2205 is susceptible to precipitation of intermetallic phases, usually referred to as sigma phase. Intermetallic phases precipitate in the range of 1300°F to 1800°F, with the most rapid precipitation occurring at about 1600°F. Thus, it is prudent to have 2205 pass a test for the absence of intermetallic phases, such as those in ASTM A 923.
Heat Resistance
      Although 2205 has good high temperature oxidation resistance this grade, like other duplex stainless steels, suffers from embrittlement if held for even short times at temperatures above 300°C. If embrittled this can only be rectified by a full solution annealing treatment. Duplex stainless steels are almost never used above 300°C.

Processing
Cold Forming
      Alloy 2205 is readily sheared and cold formed on equipment suited to working stainless steels. However, because of the high strength and rapid work hardening of 2205, forces substantially higher than those for austenitic steels are required to cold form 2205. Also because of the high strength, a somewhat larger allowance must be made for springback.
Heat Treatment
      Alloy 2205 should be annealed at 1020~1100℃ minimum, followed by rapid cooling, ideally by water quenching. This treatment applies to both solution annealing and stress relieving. Stress relief treatments at any lower temperature carry the risk of precipitation of detrimental intermetallic or nonmetallic phases
      This grade cannot be hardened by thermal treatment, but does work harden.
Welding
      Alloy 2205 possesses good weldability. The goal of welding 2205 is that the weld metal and heat-affected zone (HAZ) retain the corrosion resistance, strength, and toughness of the base metal. The welding of 2205 is not difficult, but it is necessary to design welding procedures that lead to a favorable phase balance after welding and will avoid precipitation of detrimental intermetallic or nonmetallic phases.
      Nitrogen added to the shielding gas will also assist in ensuring adequate austenite in the structure. Heat input must be kept low and no pre- or post-heat should be used. The lower co-efficient of thermal expansion of all duplex stainless steels compared with austenitic grades reduces distortion and associated stresses.
??? 2205 can be welded by: GTAW (TIG); GMAW (MIG); SMAW ("stick" electrode); SAW; FCW; and PAW.
Machining
      The high strength that makes 2205 useful in many applications also reduces its machinability. Cutting speeds are approximately 20% slower than for grade 304. There is as yet no Improved Machinability version of 2205.

Fabrication
      The high strength of 2205 also makes bending and forming more difficult; these operations will require larger capacity equipment than would be required for austenitic stainless steels. The ductility of 2205 is less than that of an austenitic grade, so severe forming operations, such as cold heading, are not generally possible. If severe cold working is required it is recommended that intermediate annealing be carried out.

 Applications

Typical Applications include

• Pressure vessels, tanks, piping, and heat exchangers in the chemical processing industry
• Piping, tubing, and heat exchangers for the handling of gas and oil
• Effluent scrubbing systems
• Pulp and paper industry digesters, bleaching equipment, and stock-handling systems
• Rotors, fans, shafts, and press rolls requiring combined strength and corrosion resistance
• Cargo tanks for ships and trucks
• Food processing equipment