ASTM A240 S31254 254SMO Stainless Steel Flat Sheet High Tensile Strength

Product Specification

ASTM A240 S31254 254SMO cold rolled stainless steel flat sheet

UNS S31254 Sheet

UNS S31254 is a high-alloy austenitic stainless steel developed for use in seawater and other aggressive chloride-bearing media. The steel is characterized by the following properties:

• Excellent resistance to pitting and crevice corrosion
• High resistance to general corrosion
• High resistance to stress corrosion cracking
• Higher strength than conventional austenitic stainless steels
• Good weldability

Standards

• UNS S31254
• EN number 1.4547
• EN name X1CrNiMoCuN20-18-7
• W.Nr. 1.4529**
• SS 2378*
• AFNOR Z1 CNDU 20.18.06AZ*

* Obsolete. Replaced by EN.
** Nearest equivalent grade.

Product standards

• Seamless tube and pipe: ASTM A269, A213, A312, NFA 49-217, EN 10216-5
• Welded tube and pipe: ASTM A249, A269 , A312, A358, A409
• Fittings: ASTM A182
• Bar: ASTM A276, A479, EN 10088-3
• Forged products: ASTM A473
• Sheet and plates: ASTM A240

Approvals

• UNS S31254 (254 SMO) in the form of seamless pipe has been approved by the American Society of Mechanical Engineers (ASME) for use according to ASME Boiler and Pressure Vessel Code section VIII, div. 1. However, there is no approval for UNS S31254 in the form of seamless tube, but according to the ASME paragraph UG-15 it is allowed to use the design values for seamless pipe according to ASME section VIII, div. 1 also for seamless tube.
• NACE MR 0175 (sulphide stress cracking resistant material for oil field equipment)

Chemical composition (nominal) %

Mechanical properties

The following figures apply to solution annealed condition seamless tube and pipe.

At 20°C (68°F)

1 MPa = 1 N/mm²
a) R_p0.2 and R_p1.0 correspond to 0.2% offset and 1.0% offset yield strength, respectively.
b) Based on L₀ = 5.65 √S₀ where L₀ is the original gauge length and S₀the original cross-section area.

Impact strength

Due to its austenitic microstructure, UNS S31254 has very good impact strength both at room temperature and at cryogenic temperatures.

Tests have demonstrated that the steel fulfils the requirements (60 J (44 ft-lb) at -196 ^oC (-320 ^oF)) according to the European standards EN 13445-2 (UFPV-2) and EN 10216-5.

At high temperatures

Intermetallic phases are precipitated within the temperature range of 600–1000°C (1110–1830°F). Therefore, the steel should not be exposed to these temperatures for prolonged periods.

Figure 1. Strength values (min. values) for UNS S31254 and allowable stress according to ASME Boiler and Pressure Vessel Code section VIII, div. 1.

Physical properties

Density: 8.0 g/cm³, 0.29 lb/in³

Corrosion resistance

In solutions containing halides such as chloride and bromide ions, conventional stainless steels can be readily attacked by local corrosion in the form of pitting corrosion, crevice corrosion or stress corrosion cracking (SCC). In acid environments, the presence of halides also accelerates general corrosion.

General corrosion

In pure sulphuric acid, UNS S31254 is much more resistant than ASTM TP316, and in naturally aerated sulphuric acid containing chloride ions UNS S31254 exhibits higher resistance than '904L', see Figure 2.

Figure 2. Isocorosion diagram 0.1 mm/year (4mpy) in naturally aerated sulphuric acid containing 2000 ppm chloride ions.

Stress corrosion cracking (SCC)

Ordinary austenitic steels of the ASTM TP304 and TP316 type are prone to stress corrosion cracking (SCC) in chloride-containing solutions at temperatures exceeding about 60°C (140°F). For the austenitic steels, resistance to SCC increases with higher nickel and molybdenum contents. The tables below show the results of two accelerated tests, clearly demonstrating that UNS S31254 has a very good resistance to SCC.

* A 0.1 M NaCl solution is allowed to drop slowly onto an electrically heated
tensile test specimen at 300 ^oC (570 ^oF).

Intergranular corrosion

UNS S31254 has a very low carbon content. This means that there is very little risk of carbide precipitation during heating, for example when welding. The steel passes the Strauss test (ASTM A262, practice E) even after sensitizing for one hour at 600–1000°C (1110–1830°F).

However, due to the high alloying content of the steel, inter-metallic phases can precipitate at the grain boundaries in the temperature range 600–1000°C (1110–1830°F). These precipitations do not involve any risk of intergranular corrosion in the environments in which the steel is intended to be used. Thus, welding can be carried out without any risk of intergranular corrosion.

Pitting corrosion

Its high chromium content and particularly the molybdenum content give UNS S31254 excellent resistance to pitting and crevice corrosion. The high nitrogen content also improves pitting resistance.

The results of laboratory determination of the critical pitting temperature (CPT) in 3 % NaCl are shown in Figure 3, where it can be seen that UNS S31254 possesses very good resistance in water containing chlorides. UNS S31254 is, therefore, a suitable material for use in seawater.

Crevice corrosion

The weak point of conventional stainless steels is their limited resistance to crevice corrosion. In seawater, for example, there is a considerably greater risk of crevice corrosion under gaskets, deposits or fouling. Tests in natural seawater at 60°C (140°F) have shown that UNS S31254 can be exposed for prolonged periods without suffering crevice corrosion. Figure 4 shows the results of accelerated crevice corrosion tests.

Figure 3. Critical pitting temperature (CPT) in 3% NaCl, 600 mV/SCE.

Figure 4. Critical crevice corrosion temperature in FeCl₃ for UNS S31254, AISI 316L and 904L. According to ASTM G-48.

Heat treatment

The tubes are delivered in heat treated condition. If additional heat treatment is needed due to further processing the following is recommended.

Solution annealing

1150–1200°C (2100–2190°F), quenching in water. Thin-walled tubes min. 1130°C (2060°F), quenching in air/water.

Welding

The weldability of UNS S31254 is good. Welding should be undertaken without preheating, and if correctly performed there will be no need for any subsequent heat treatment. Suitable methods of fusion welding are manual metal-arc welding with covered electrodes and gas-shielded arc welding, mainly by means of the TIG and MIG methods.

Since the material is intended for use under severe corrosive conditions, welding must be carried out with care and followed by thorough cleaning to ensure that the weld metal and the heat-affected zone maintain the best possible corrosion properties.

The heat input during welding should not exceed 1.5 kJ/mm, and in multi-pass welding the interpass temperature should not exceed 100°C (210°F). A stringer bead welding technique should be used.

The welding of fully austenitic steels usually entails a risk of hot-cracking in the weld metal, particularly if the weldment is under constraint. However, since UNS S31254 has a very high degree of purity, the risk of this type of cracking is greatly reduced. Backing bars or similar devices of copper alloys must not be used since copper penetration into the grain boundaries in stainless steel can lead to cracking.

In common with all austenitic stainless steels, UNS S31254 has low thermal conductivity and high thermal expansion. For this reason, welding should be carefully planned in advance so that distortion of the welded joint can be minimized. If, despite these precautions, it is believed that residual stresses may impair the function of the weldment, it is recommended that the entire structure be solution annealed. See under Heat treatment.

In the as-supplied condition, the material has a homogeneous structure. Welding without filler metal leads to structural changes that reduce corrosion resistance. Such welding should be followed by solution annealing in order to ensure that the corrosion properties of the weld metal are equal to those of the parent metal.

Fabrication

Avoid abrasion against copper/copper alloys or other similar metals which, if present in metallic form, can cause cracks during subsequent welding, hot processing or heat treatment.

Bending

The excellent formability of UNS S31254 permits cold bending to very tight bending radii. Annealing is not normally necessary after cold bending.

Applications

UNS S31254 is used in the following applications:

• Equipment for handling of seawater, such as, seawater cooling, cooling water pipes, ballast water systems, firefighting systems etc.
• Hydraulic and instrumention tubing
• Equipment in pulp bleaching plants
• Components in gas cleaning systems
• Tanks and pipelines for chemicals with high halide contents

Production process