The duplex stainless steels have a microstructure, when heat treated properly, of nearly equal proportions of austenite and ferrite. This microstructure ensures that the duplexes are much more resistant to stress corrosion cracking (SCC) than austenitic stainless steels. The specification 0.2% Proof Stress of the duplexes is more than double that of austenitic stainless steels such as the U-304 types and U-316L types. This often allows down gauging in the design, depending on Young’s Modulus and buckling limitations.

APPLICATIONS

The duplexes have a ductile to brittle transition temperature of about -40°C or lower. These steels can also become embrittled when exposed to temperatures between 300°C and 550°C (475°C embrittlement) and 550°C and 1 000°C (sigma (σ) and chi (χ) phase formation). Thus, application temperatures are generally limited from -50°C to 300°C.

The higher chromium, molybdenum and nitrogen contents give U-2205 significantly improved pitting and crevice corrosion resistance in the presence of chlorides. U-2205 also has better general corrosion resistance than the U-316L types in most environments.

U-2205 is a highly suitable material for service in environments containing chlorides and hydrogen sulphide such as marine environments and the oil and gas extraction and processing industries.

Typical applications also include the chemical industry (processing, transport and storage, e.g. pressure vessels, tanks and piping), the pulp and paper industry (digesters and liquor tanks) and the mining industry.

U-2205 has thus found widespread use in production tubing and flowlines for the extraction of oil and gas from sour wells, in refineries and in process solutions contaminated with chlorides. U-2205 is also particularly suitable for heat exchangers where chloride bearing water or brackish water is used as the cooling medium.

CHEMICAL COMPOSITIONS

In accordance with ASTM A240 and EN 10088-2

• Compositions are ranges or maximum values.

MECHANICAL PROPERTIES

In accordance with ASTM A240 and EN 10088-2.

• Minimum values, unless max or range is indicated.
• ( ) indicates applicable gauge range.
• HR is hot rolled, CR is cold rolled.
• The table assumes certification to both ASTM A240, EN 10088-2 and EN 10095, where applicable.

The values given below are for 20°C, unless otherwise stated.

PHYSICAL PROPERTIES

• May become slightly magnetic in the cold worked condition.

Thermal processing and fabrication

ANNEALING

Annealing of the duplexes is achieved by heating to between 1 020°C and 1 100°C for 90 minutes per 25mm thickness (3.5min/mm) followed by quenching in an agitated water bath down to room temperature. Controlled atmospheres are recommended in order to avoid excessive oxidation of the surface.

STRESS RELIEVING

The duplexes can be stress relieved at 525°C to 550°C for 60 minutes per 25mm thickness (2.5min/mm) in special cases but it is preferable to fully anneal. Stress relieving contributes significantly to improving the resistance to stress corrosion cracking by lowering the residual tensile stresses.

HOT WORKING

The duplexes can be readily forged, upset and hot headed. Uniform heating of the steel in the range of 1 150°C to 1 250°C is required. Initial hot working should be affected without large reductions or change of shape (especially if upsetting or staving up). Once the material starts to flow, progressively more deformation can be accomplished. The finishing temperature should not be below 950°C. If the temperature after forging is still above 1 000°C, rapid cooling (water quenching) can be carried out directly from the working temperature. Otherwise, all hot working operations should be followed by annealing and pickling and passivating to restore the mechanical properties and corrosion resistance.

COLD WORKING

The duplexes have good formability, but due to the higher proof strength, more power is required for most cold forming operations than austenitic stainless steels. Roll forming can be readily applied to the duplexes, but loadings will be about 60% higher than for mild steel and slower speeds should be used. Severe deep draws may require an intermediate anneal. Cold bending reduces the maximum gauge capacity of the machine by about half, compared with austenitics. The minimum inner bend radius for the duplexes is three times the plate thickness and four times is recommended. Severe bends should be carried out transverse to the rolling direction. The duplexes exhibits greater spring back than mild steel and this should be compensated for by slight over bending.

MACHINING

The high strength that makes the duplexes useful in many applications also reduces their machinability. U-2001 has the best machinability of the duplexes and is similar to the U-316L types. In general, for the duplexes, cutting speeds are approximately 20% slower than those for U-304. Machine tools should be ground to close tolerances to avoid the risk of excessive work hardening in the outer layer of the stock. Larger tools should be used to give stability and efficient heat dissipation. Tools with large rake angles, sharp edges and smooth surfaces reduce the work hardening and the risk of built up edges. Relatively large feed rates and cutting depths minimise the work hardening of the surface layer. A suitable cutting fluid should be used to minimise the risk of built up edges. The work should be flooded to ensure maximum heat removal.

WELDING

The duplexes have good weldability in most applications, provided that the recommended procedures are adopted. They can be welded with most standard welding methods (MMA/SMAW, MIG/GMAW, TIG/GTAW, FCAW, SAW and PAW). If the duplexes are autogenously welded, the fabrication should be solution annealed to restore the desirable duplex microstructure and hence the toughness. Only welding consumables specifically specified for the duplexes should be used to ensure that the deposited metal has the correctly balanced duplex microstructure. 2209 filler welding electrodes are recommended for optimum properties. Nitrogen, added to the shielding gas, will also assist in ensuring adequate austenite in the microstructure.

The heat input should be controlled to between 1 and 2kJ/mm in order to keep the Heat Affected Zone (HAZ) narrow and to ensure there is at least 20% austenite in the HAZ. The interpass temperatures should not exceed 150°C. The lower coefficient of thermal expansion of the duplexes, compared to austenitic stainless steels, reduces distortion and the associated stresses.

Preheating, although not essential, is beneficial on thicker gauge sections. Typical preheat temperatures are between 100°C and 250°C. Post-weld heat treatment is not normally required, but solution annealing will restore the toughness and confer the optimum stress corrosion cracking resistance to the fabrication.

Corrosion Resistance

GENERAL CORROSION

The duplex stainless steels have general corrosion resistance ranging from similar to 304L to being superior to the 316L types, and this is dependant on the corrosion media. For example, 2001 has significantly better corrosion resistance than 316L-1.4404 in sulphuric acid (H2SO4) solutions.

PITTING CORROSION

Pitting resistance is important, mainly in applications involving contact with chloride solutions, particularly in the presence of oxidising media. These conditions may be conducive to localised penetration of the passive surface film on the steel and a single deep pit may well be more damaging than a much greater number of relatively shallow pits.

Where pitting corrosion is anticipated, steels with high pitting resistance equivalents (PRE), such as the duplexes, should be considered. The below diagram shows the critical temperature for initiation of pitting (CPT) at different chloride contents for 304, U-316L and 2205 types (potentiostatic determination at + 300mV SCE, pH = 6.0).

ATMOSPHERIC CORROSION

The atmospheric corrosion resistance of duplex stainless steels is unequalled by virtually all other uncoated engineering materials. 2205 should be used in areas where the atmosphere is highly polluted with chlorides, sulphur compounds and solids, either singly or in combination.

OXIDATION RESISTANCE

The duplexes have good oxidation resistance, both in intermittent and continuous service, up to 980°C for 2205. However, continuous use of the duplexes between 300°C and 950°C may embrittle the steel and lower the corrosion resistance. At the lower temperature range, the embrittlement is due to the precipitation of α’ (475°C embrittlement) and nitrides or carbides. In the high temperature range, χ and σ phases precipitate. However, during normal production and fabrication procedures, the times at these critical temperatures are such that the risk of embrittlement and/or a decrease in corrosion resistance are small.

In addition, this effect does not necessarily affect the behaviour of the material at the operating temperature and is less pronounced in thinner gauges. For example, heat exchanger tubes are used at high temperatures without any problems. A full anneal and rapid cooling treatment will restore the toughness and corrosion resistance of the duplexes.

INTERGRANULAR CORROSION

Sensitisation may occur when the Heat Affected Zones of welds in some austenitic stainless steels are cooled through the sensitising temperature range of between 450°C and 850°C. At this temperature, a compositional change may occur at the grain boundaries.

If a sensitised material is then subjected to a corrosive environment, intergranular attack may be experienced. This corrosion takes place preferentially in the heat affected zone away from and parallel to the weld. Susceptibility to this form of attack, often termed ‘weld decay’, may be assessed by the following standard tests:

a) boiling copper sulphate/sulphuric acid test as specified in ASTM A262, Practice E.

b) for non titanium stabilised grades only, boiling nitric acid test as specified in ASTM A262, Practice C.

The Columbus Stainless Cr-Ni-Mo austenitics have low carbon contents and are resistant to sensitisation and can be specified for welded structures unless the higher carbon types are required for their increased strength at elevated temperatures. In this case,  316Ti should be specified.

STRESS CORROSION CRACKING

Stress corrosion cracking (SCC) can occur in austenitic stainless steels when they are stressed in tension in chloride environments at temperatures in excess of about 60°C. The stress may be applied, as in a pressure system or it may be residual arising from cold working operations or welding. Additionally, the chloride ion concentration need not be very high initially, if locations exist in which concentrations of salt can accumulate.

Assessment of these parameters and accurate prediction of the probability of SCC occurring in service is therefore difficult. Where there is a likelihood of SCC occurring, a beneficial increase in life can be easily obtained by a reduction in operating stress and temperature. Alternatively, specially designed alloys, such as duplex stainless steels, will have to be used where SCC cracking is likely to occur.

EROSION CORROSION

Austenitic stainless steels are attacked by erosion corrosion if exposed to flowing media containing highly abrasive solid psections, e.g. sand, or to media with very high flow velocities. Owing to its combination of high initial hardness, work hardenability and corrosion resistance, the duplexes displays very good resistance under such erosion corrosion conditions.

CORROSION FATIGUE

The duplexes possesses higher strength and better corrosion resistance than ordinary austenitic stainless steels. The duplexes, therefore, also possess better fatigue strength under corrosive conditions than such steels. For example, in rotary bending fatigue tests in a 3% NaCl solution (6 000rpm, 40°C, pH 7), 2205required 430MPa stress in the unnotched condition to bring about rupture after 2x107 cycles, while 316N failed at only 260MPa. The corresponding notched figures were 230MPa and 140MPa for 2205 and 316N respectively.