Chemical composition (nominal) %

Forms of supply

Seamless tube and pipe- finishes and dimensions

Seamless tube and pipe in Sandvik SAF 31803 is supplied in dimensions up to 260mm outside diameter. The delivery condition is solution annealed and either white pickled or bright annealed.

Mechanical properties

The following values apply to material in the solution annealed condition. Tube and pipe with wall thicknesses above 20 mm (0.787 in.) may have slightly lower values.

More detailed information can be supplied on request.

At 20 °C (68°F)

Tube and pipe with a wall thickness max. 20 mm (0.79 in.)

1 MPa = 1 N/mm²
a) R_p0.2 corresponds to 0.2% offset yield strength.

Impact strength

Sandvik SAF 31803 possesses good impact strength both at room temperature and at low temperatures, as is evident from Figure 1. The values apply for standard Charpy-V specimens (10 x 10 mm, 0.39 x 0.39 in.).

The impact strength of welded Sandvik S31803 is also good, despite the impact strength values in the as-welded condition being slightly lower than for weld-free material. Tests demonstrate that the impact strength of material, welded by means of gas-shielded arc welding, is good in both the weld metal and the heat-affected zone down to -50°C (-58°F). At this temperature, the impact strength is a minimum of 27 J (20 ft lb). If very high impact strength demands are made on the weld metal at low temperatures, solution annealing is recommended. This restores the impact strength of the weld metal to the same level as that of the parent metal.

Figure 1. Curve showing typical impact strength values (Charpy-V) for Sandvik SAF 31803. Specimen size 10x10 mm (0.39 x 0.39 in.).

At high temperatures

If Sandvik SAF 31803 is exposed to temperatures exceeding 280°C (540°F), for prolonged periods, the microstructure changes, which results in a reduction in impact strength. This does not necessarily affect the behavior of the material at the operating temperature. For example, heat exchanger tubes can be used at higher temperatures without any problems. Please contact Sandvik for more information.For pressure vessel applications, 280°C (540°F) is required as a maximum according to VdTÜV-Wb 418 and NGS 1606.

Tube and pipe with wall thickness max. 20 mm (0.79 in.)

According to ASME B31.3 the following design values are recommended for UNS S31803 (Sandvik S31803)

Physical properties

Density: 7.8 g/cm³, 0.28 lb/in³

Thermal conductivity

Thermal expansion, mean values in temperature ranges (X10^-6)

Sandvik SAF 31803 has a far lower coefficient of thermal expansion than austenitic stainless steels and can therefore offer certain design advantages.

Figure 2. Thermal expansion per °C (30-100°C).

1) ( x10³ )

Corrosion resistance

Pitting corrosion

The pitting resistance of a steel is determined primarily by its chromium and molybdenum contents, but also by its nitrogen content and its slag composition and content. The manufacturing and fabrication practices, e.g. welding, are also of vital importance for actual performance in service.

A parameter for comparing the resistance of different steels to pitting, is the PRE number (Pitting Resistance Equivalent). The PRE is defined as, in weight-%: PRE = % Cr + 3.3 x % Mo + 16 x % N

The PRE number for Sandvik SAF 31803 is compared with other materials in the following table:

a) Sandvik SAF 2205 has a chemical composition within UNS S32205, which is optimized within the UNS S31803 range in order to provide a high PRE value.

The ranking given by the PRE number has been confirmed in laboratory tests. This ranking can generally be used to predict the performance of an alloy in chloride containing environments. Because of the high Mo and N contents, the PRE number for Sandvik SAF 2205 is significantly higher than what would be the case with lower Mo and N contents which are still within the limits of UNS S31803.

Nevertheless, based on practical experience, Sandvik SAF 31803 can be used at considerably higher temperatures and chloride contents than AISI 304 and AISI 316 without pitting. Sandvik SAF 31803 is, therefore, far more serviceable in chloride-bearing environments than standard austenitic steels.

Stress corrosion cracking

The standard austenitic steels AISI 304L and AISI 316L are prone to stress corrosion cracking (SCC) in chloride-bearing solutions at temperatures above 60 °C (140 °F).

Duplex stainless steels are far less prone to this type of corrosion. Laboratory tests reveal good resistance to stress corrosion cracking of Sandvik SAF 31803. Results from the tests are presented in Figure 3. The diagram indicates the temperature-chloride range within which Sandvik SAF 31803 and the standard steels AISI 304L and AISI 316L have low susceptibility to stress corrosion cracking.

Figure 3. Resistance to stress corrosion cracking (SCC) in neutral chloride solutions with an oxygen content of about 8 ppm. Laboratory results for Sandvik SAF 31803 of constant load specimens loaded to the proof strength at the test temperature.

Heat treatment

Tubes are normally delivered in the heat-treated condition. If additional heat treatment is needed due to further processing, the following is recommended.

Solution annealing

1020 - 1100 °C (1870-2010 °F), rapid cooling in air or water.

Welding

The weldability of Sandvik SAF 31803 is good. Suitable welding methods are manual metal-arc welding with covered electrodes or gas shielded arc welding. Welding should be undertaken within the heat input range 0.5-2.5 kJ/mm. Max. interpass temperature is 250°C (482^oC). Preheating or post-weld heat treatment is normally not necessary.

Matching filler metals are recommended in order to obtain a weld metal with optimum corrosion resistance and mechanical properties. For gas-shielded arc welding we recommend Sandvik 22.8.3.L or Sandvik 22.8.3.LSi, developed to give improved welding properties by gas metal arc welding (MIG/MAG).

For manual metal-arc welding, the covered electrode Sandvik 22.9.3.LR is recommended. These filler metals can also be used for welding Sandvik SAF 31803 to carbon steels and stainless steels. The covered electrode Sandvik 23.12.2.LR and the welding wire Sandvik 22.15.3.L, both of type AWS 309 Mo with low carbon content, can also be used.

Fabrication

Bending

The starting force needed for bending is slightly higher for Sandvik SAF 31803 than for standard austenitic grades (ASTM 304L and 316L). Sandvik SAF 31803 can be cold-bent to 25% deformation without requiring subsequent heat treatment. For pressure vessel applications in Germany and the Nordic countries, heat treatment may be required after cold deformation in accordance with VdTÜV-Wb 418 and NGS 1606.

Under service conditions where the risk of stress corrosion crackingstarts to increase, heat treatment is recommended even after moderate cold bending, for example, where the material temperature is nearly 150°C (300°F) in an oxygen-bearing, environment with around 100 ppm Cl^-.

Heat treatment is carried out in the form of solution annealing (see under Heat treatment) or resistance annealing. Hot bending is carried out at 1100-950°C (2010-1740°F) and should be followed by solution annealing.

Expanding

Compared with austenitic stainless steels, Sandvik SAF 31803 has higher proof and a tensile strengths. This must be borne in mind when expanding tubes into tube-sheets. Normal expanding methods can be used, but the expansion requires higher initial force and should be undertaken in one operation.

Machining

Being a two-phase (austenitic-ferritic) material, Sandvik SAF 31803 will present a different tool wear profile from that of single phase steels of types ASTM 304/304L and 316/316L. The cutting speed must, therefore, be lower than that recommended for ASTM 304/304L and 316/316L. Built-up edges and chipping are to be expected. It is recommended that a tougher insert grade is used than when machining austenitic stainless steel, e.g. ASTM 304L.

A version with improved machinability, Sanmac 2205, is available form of bar and hollow bar.

Applications

Due to its excellent corrosion properties, Sandvik SAF 31803 is a highly suitable material for service in environments containing chlorides and hydrogen sulphide. Sandvik SAF 31803 is mainly available as heat exchanger tubing and is particularly suitable for heat exchangers where chloride-bearing water or brackish water is used as a cooling medium.

The high strength of Sandvik SAF 31803 makes the material an attractive alternative to the austenitic steels in structures subjected to heavy loads.

The good mechanical and corrosion properties make Sandvik SAF 31803 an economical choice in many applications by reducing the life cycle cost of equipment.