Chemical composition (nominal) %

Forms of supply

Seamless tube – Finishes and dimensions

Seamless tube and pipe is supplied in dimensions up to 260 mm outside diameter in the solution annealed and white-pickled condition, or solution annealed in a bright annealing process. Seamless tubes are available from stock in ANSI pipe and heat exchanger tube sizes. Details of our manufacturing and stock programme are given in catalogue S-110-ENG.

Using modern bending equipment, Sandvik can bend tubes to customers' particular requirements and, if required, anneal after bending.

Materials for oil and gas production

Cold worked seamless tube and pipe

For production tubing and casing in oil and gas production. Sanicro 28 is supplied cold worked with high strength properties (Sanicro 28-110 and Sanicro 28-125). Specific Oil and Gas information is available on request.

Other forms of supply

• Welded tube and pipe
• Welding wire and wire electrodes
• Covered electrodes
• Strip, annealed or cold rolled to different degrees of hardness
• Wire, drawn or ground
• Bar steel
• Plate and sheet
• Forged products
• Cast products
• Fittings

Further details concerning sizes and finishes are available on request.

Mechanical properties

The following values apply to solution annealed material, unless otherwise stated.

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 S₀ the original cross-sectional area.

Tube, pipe, plate and sheet with material thicknesses >20 mm (0.787") and bar with diameters >100 mm (3.937")

Seamless cold worked tube and pipe

Intended for oil and gas production

Impact strength

Due to its austenitic microstructure, Sanicro 28 has very good impact strength, both at room temperature and at cryogenic temperatures.

Tests have demonstrated that the steel fulfils the requirements according to the European standards EN 13445-2 (UFPV-2) ( min. 60 J (44 ft-lb) at -270 ^oC (-455 ^oF)) and EN 10216-5 (min. 60 J (44 ft-lb) at -196 ^oC (-320^oF)).

At high temperatures

Due to embrittlement caused by precipitation of intermetallic phases, Sanicro 28 should not be exposed to temperatures above 600 °C (1110 °F) for prolonged periods.

Tube, pipe, plate and sheet with material thicknesses <20 mm (0.787") and bar with diameters <100 mm (3.937")

Tube, pipe, plate and sheet with material thicknesses >20 mm (0.787") and bar with diameters >100 mm (3.937")

Creep strength

Physical properties

Density: 8.0 g/cm³, 0.29 lb/in³

Corrosion resistance

General corrosion

Sanicro 28 was originally developed for use in the manufacture of phosphoric acid, especially for heat exchangers in the concentration unit, where corrosive conditions are at their worst.

Phosphoric acid, manufactured by the "wet" method, contains varying concentrations of impurities derived from the raw material, the phosphate rock. The most dangerous of these impurities are chlorides, Cl ^– , and fluorides in free form, F ^– . Laboratory tests carried out in wet process phosphoric acid have shown that Sanicro 28 is far more resistant to impurities of this kind than other high alloy materials. Figure 1 shows the corrosion rate in contaminated phosphoric acid at different chloride concentrations.

Temperature is another factor that has a great influence on corrosion. See figure 2.

Laboratory tests at 200°C (390°F) in contaminated 95% super phosphoric acid gave the following corrosion rates after 20 days: Sanicro 28, 0.03 mm/year (1.2 mpy); Alloy 904L, 0.10 mm/year (4.0 mpy); UNS N08020, 0.23 mm/year (9.2 mpy); Alloy G, 0.03 mm/year (1.2 mpy).
Table 1 Chemical compositions of materials tested

Figure 1. Corrosion rate in contaminated phosphoric acid at different chloride concentrations 100°C (210°F). Comparision of Sanicro 28 and other alloys (chemical compositions given in table 1).

Figure 2. Corrosion rate in contaminated phosphoric acid at different temperatures for Sanicro 28 and some other alloys (chemical compositions given in table 1).

Figure 3 is an isocorrosion diagram for Sanicro 28, Alloy 904L and AISI 316L in deaerated sulphuric acid. As can be seen from the figure, Sanicro 28 is more resistant than the other alloys. Naturally aerated sulphuric acid is more corrosive than deaerated acid in the intermediate concentration range. Sanicro 28 exhibits very good corrosion resistance in concentrated acid.

Figure 3. Isocorrosion diagram for Sanicro 28, Alloy 904L and AISI 316L, in deaerated sulphuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy).

Sulphuric acid is sometimes contaminated with chlorides which increases the corrosivity of the solution. However, Sanicro 28 has good resistance, better than 904L, also in chloride contaminated sulphuric acid, especially at high concentrations. Above about 20% sulphuric acid Sanicro 28 is even more resistant than the super-duplex stainless steel SAF 2507, see iso-corrosion diagram in figure 4.

Sanicro 28 is more resistant to hydrochloric acid than stainless steels with lower chromium and molybdenum contents and can, therefore, be used to advantage in cases where chemical process solutions are contaminated with hydrochloric acid, see iso-corrosion diagram in figure 5.

Figure 4. Isocorrosion diagram for Sanicro 28 in sulphuric acid containing 2000 ppm chloride ions at a corosion rate of 0.1 mm/year (4 mpy).

Sanicro 28 resists hydrofluoric and hydrofluosilicic acid very well and can be used where these acids occur as impurities (see corrosion diagram for hydrofluoric acid, figure 6). Both Sanicro 28 and AISI 316L are completely resistant to pure acetic acid at all temperatures and concentrations at atmospheric pressure. However, at elevated temperatures and pressures, AISI 316L will corrode while Sanicro 28 will remain resistant. Acetic acid is often contaminated with formic acid, which renders it more corrosive. Laboratory tests show that Sanicro 28 is more resistant than AISI 316 and AISI 317L in such solutions.

Sanicro 28 is far more resistant to formic acid than conventional stainless steels of the AISI 316L type and more resistant than 904L, see isocorrosion diagram in figure 7. In nitric acid Sanicro 28 performs also very well. In test according to ASTM A262 Practice C (Huey test, 5x48 h in boiling 65% HNO₃) corrosion rates lower than 0.15 mm/year (6 mpy) are obtained.

The high alloying contents of chromium and nickel give Sanicro 28 considerably better resistance to sodium hydroxide than standard stainless steels of the type AISI 304 and AISI 316. At moderate temperatures and concentrations, Sanicro 28 is a suitable alternative to pure nickel, which may be attacked by erosion corrosion.

At high temperatures the general corrosion rate increases. The risk of stresss corrosion cracking (SCC) also increases when chlorides are present. Table 2 and 3 demonstrate the good resistance of Sanicro 28 against general corrosion and SCC in sodium hydroxide contaminated with chlorides.

Table 2. SCC in boiling 43% NaOH + 6.7% NaCl, 142°C (288°F), 500h.

Table 3. General corrosion in NaOH and in NaOH+NaCl, mm/year.

As can be seen, Sanicro 28 is superior to both Alloy 800 and Alloy 904L.

Figure 5. Iso-corrosion in hydrochloric acid. The curves repesent a corrosion rate of 0.1 mm/year (4 mpy).

Figure 6. Corrosion rates in hydrofluoric acid at 20°C (68°F) for Sanicro 28, Alloy 904L and AISI 316.<

Figure 7. Isocorrosion diagram for Sanicro 28 and other alloys in formic acid at a corrosion rate of 0.1 mm/year (4 mpy).

Pitting corrosion

Sanicro 28 can withstand very high temperatures in aggressive environments without being attacked by pitting. Figure 8 shows the critical pitting temperature (CPT) for some alloys in chloride-bearing water with a salinity comparable to that of sea water. The figure shows that Sanicro 28 has a higher critical pitting temperature (CPT) than Alloy 904L and Alloy 825 even in acidic chloride solutions. The curves are displaced at higher temperatures in solutions with lower salinities.

Crevice corrosion

Laboratory tests show that Sanicro 28 has good resistance to crevice corrosion. In tests according to ASTM G-48 method B (6% iron(III)chloride), the material exhibited better resistance than Alloy 825.

Stress corrosion cracking

Ordinary austenitic steels of the AISI 304 and AISI 316 types are susceptible to stress corrosion cracking (SCC) in chloride bearing solutions at temperatures above about 60°C (140°F). This susceptibility declines with increasing nickel content. Chromium contents above 20% can also be beneficial. Sanicro 28, which is alloyed with 27% Cr and 31% Ni, exhibits very good resistance to SCC, both in laboratory tests and in practice. This is demonstrated in figure 9, which shows results of SCC tests in a 40% calcium chloride solution.

Tensile specimens which were spring-loaded to stresses close to the proof strength and tested for SCC in aerated water at temperatures of up to 200–250°C (390–480°F), were not attacked, see figure 10. These tests were performed in autoclaves with an oxygen content in the water of 4.6 to 10 ppm and a pH-value at room temperature of 4.5-7.1. The testing time was 1000 hours. The curve for AISI 316/316L and AISI 304/304L is based on experimental data and practical experience.

Sanicro 28 also displays very good resistance to SCC in environments where hydrogen sulphide is present together with chlorides. This is true for both solution annealed and cold worked material, as well as for welded joints. For further information, see Sandvik R&D lecture S-58-7-ENG.

Figure 8. Critical pitting temperature (CPT) at +400 mV SCE for different alloys in synthetic seawater (3% NaCl), at different pH values (chemical compositions give in table 1).

Figure 9. Results of stress corrosion cracking tests on different steel grades in 40% CaCl₂, at 100°C (210°F), pH = 6.5.

Intergranular corrosion

The TTC diagram, figure 11, shows results of intergranular corrosion testing according to ASTM G-28 (120 hours in boiling iron(III)sulphate and sulphuric acid solution). As the figure illustrates, Sanicro 28 can be kept in the critical interval of 600-700°C (1100-1300°F) for at least 30 minutes without intergranular corrosion occurring in this highly corrosive medium. As can be seen in figure 11, Alloy 904L is more susceptible to intergranular corrosion than Sanicro 28. In normal welding operations, heat input to the parent metal takes place for a much shorter time than 30 minutes. This means that the risk of intergranular attack after welding of Sanicro 28 is minimal, which is also verified by tests on welded specimens.

Figure 11. TTC diagram for Sanicro 28 with two different carbon contents and for Alloy 904L (Sandvik 2RK65). The curves repesent normal limit values for the carbon content.

Heat treatment

Tubes are delivered in the heat treated condition. If another heat treatment is needed after further processing, the following is recommended.

Solution annealing
1100-1160°C (2010-2120°F), 10-30 minutes, quenching in water. Thin walled tubes can also be cooled rapidly in air.

Welding

Sanicro 28 possesses good weldability. Suitable welding methods are manual metal-arc with covered electrodes and gas shielded arc welding, such as TIG and MIG. The heat input should be 1 kJ/mm maximum and the interpass temperature 150°C (300°F) maximum. A stringer bead welding technique should be used. Welding should be undertaken without preheating and, if correctly performed, there will be no need for any subsequent heat treatment. To maintain full corrosion resistance of the weld joint, welding must be followed by thorough cleaning to ensure removal of all oxides and slag particles.

In common with all austenitic stainless steels, Sanicro 28 has low thermal conductivity and high thermal expansion. Welding should, therefore, be carefully planned in advance so that distortion of the welded joint can be minimized.

The welding of fully austenitic steels usually involves the risk of hot cracking in the weld metal, particularly if the weldment is under constraint. However, since Sanicro 28 is an alloy with a very low impurity content, it is not particularly prone to this type of cracking.

As a filler metal for gas shielded arc welding, Sandvik 27.31.4.LCu wire is recommended. For manual metal-arc welding, covered electrodes of the Sandvik 27.31.4.LCuR type are recommended.

When Sanicro 28 is welded to carbon steel, it is also possible to use nickel base filler metals, such as Sanicro 60.

Fabrication

Bending

The excellent formability of Sanicro 28 permits cold bending to very small bending radii. Annealing is not normally necessary after cold bending.

For operating temperatures over 400°C (800°F), heat treatment should be carried out after bending to ensure good ductility after prolonged service times.

Expanding

Sanicro 28 is expanded into tube sheets in the same way as standard austenitic stainless steels.

Machining

The machining of Sanicro 28, as with other stainless steels, requires an adjustment of tooling data and machining method, in order to achieve satisfactory results. Compared to Sanmac 316/316L, the cutting speed must be reduced by approximately 50-55% when turning Sanicro 28 with coated, cemented carbide tools. Much the same applies to other operations. Feeds should only be reduced slightly and with care.

Detailed recommendations for the choice of tools and cutting data are provided in the brochure S-02909-ENG. Data should be selected as for steel grade Sanmac 316/316L, while taking into account the provisions above.

Applications

Due to its outstanding corrosion properties, Sanicro 28 can be used in the most diverse environments. Listed below are a few examples of applications for which this alloy is particularly suitable.

Phosphoric acid

Today, Sanicro 28 is the most widely used metallic material for evaporator tubes in the manufacture of phosphoric acid by the "wet " method. Several units have now been in service for more than 10 years. The graphite heat exchangers, replaced by Sanicro 28, often had repeated problems with broken tubes and loss of production.

Sulphuric acid

Sanicro 28 is a suitable material for piping and heat exchangers, particularly at concentrations of between 40 and 70% of deaerated acid and over 85%. Sanicro 28 has approximately the same resistance as Alloy C in concentrated acid (98% H₂SO₄).

Oil and gas

Sanicro28 is used for production tubing, casing and liners in deep, sour gas wells. The material is also recommended for oil wells with a corrosive environment. For these purposes, tubes are supplied cold rolled with high strength. In the solution annealed condition, Sanicro 28 is also used as piping for transporting of corrosive oil and gas and for heat exchangers in treatment facilities. Sanicro 28 wirelines are used for lowering tools and controlling instruments in deep oil and gas wells.

Fluoride-bearing media

Fluoride-bearing off-gases can form during the manufacture of phosphoric acid and mixed fertilizers. These off-gases must be disposed of for environmental reasons. Sanicro 28 is ideal for this purpose. Tests have shown it to be preferable to higher alloyed CrNiMo grades for the recovery of fluoride-bearing gypsum.

Nuclear power plants

Due to its high resistance to SCC, pitting and crevice corrosion, Sanicro 28 has been selected for heat exchangers in nuclear power plants.

Seawater and chloride-bearing cooling water

Its high resistance to pitting and crevice corrosion makes Sanicro 28 a very suitable material for seawater-carrying piping and seawater-cooled heat exchangers. This is confirmed by practical experience.

Sanicro 28 has replaced nickel alloys, CuNi, bimetallic tubes and coated carbon steel tubes, which failed due to corrosion. The performance of Sanicro 28 has been excellent.

In seawater-cooled heat exchangers and heat exchangers that work with chloride-bearing cooling water, Sanicro 28 offers high corrosion resistance to both the water and the cooled medium.

When a seawater-cooled plant is shut down, there is no need to drain the piping system or flush with fresh water, provided that the shutdown period is shorter than one month and the water temperature is lower than 30°C (85°F).

The duplex stainless steel Sandvik SAF 2507 is more resistant than Sanicro 28 in sea water.