SandvikSanicro 31HT
Seamless tube and pipe

Air
Sandvik Sanicro 31HT is highly resistant to oxidation, both at constant and at cyclically varying temperatures. See Fig. 2. The service temperature in air should not exceed about 1100°C (2010°F).

Isothermal oxidation at 1100°C (2010°F) for 100h results in a corrosion rate of about 0.8 mm/year (32 mpy) and exposure at the same temperature for 1000 h causes about 0.2 mm/year (8 mpy). This corresponds to an oxide layer of about 0.1 mm (0.004 inch) thickness after the first year, in accordance with the law of parabolic growth.

Cyclic oxidation at 1100°C (2010°F) for 5 x 24 h, with cooling to room temperature every 24 hours gives a corrosion rate of less than 1.9 mm/year (75 mpy). The corresponding corrosion rate at 1150°C (2100°F) is less than 2.3 mm/year (92 mpy). The oxidation process is accelerated by temperature fluctuations and also by plastic deformation, e.g. creep.



      Figure 2. Oxidation in air resulting from cyclic exposure for 5x24 h, with cooling to room temperature every 24 hours and isothermal exposure for 100 and 1000 h respectively. The shaded area indicates the deviation in the values obtained.


Carburizing atmosphere
Carburization can occur when a material comes into contact with hot gases with high carbon activity, e.g. hydrocarbons. The degree of carburization depends on the composition of the material and on the carbon and oxygen contents. Consequently, a protective oxide is easily formed on the surface of the material, and this grade has, therefore, good resistance to carburization in carbon monoxide and carbon dioxide. See Fig. 3.

The material carburizes in gases with low oxygen content, but the depth of penetration is limited by the high content of chromium and nickel. Fig. 4 shows carburization in a mixture of 10% methane and 90 % argon. The figure also indicates to the influence of surface finish on carburization resistance.

In alternately oxidizing and carburizing atmospheres, alloys of high nickel content are sometimes attacked by what is known as "green rot". Sandvik Sanicro 31HT has good resistance to this type of attack.


      Figure 3. Carburization of a cylindrical test piece after exposure in charcoal, with barium carbonate added, for 500 h 1000      °C (1830°F).


Other gaseous atmospheres
In addition to its very good resistance to oxidation and carburization, Sandvik Sanicro 31HT also has good resistance to combustion gases and other aggressive gaseous mixtures. Its maximum service temperature in a sulphurous atmosphere, however, is lower than in air, and is mainly determined by the oxygen and sulphur contents of the gas.

This grade can also be used in synthesis gas (ammonia synthesis) at high process temperatures since the material has a resistance to nitrogen absorption, which is far superior to that of the ordinary 18/8 grades, and in gases containing chlorides (e.g. EDC) up to 650 °C (1200 °F) if the Ti+Al content is restricted to 0.7 % (see "Structural stability").

Salt melts
Sandvik Sanicro 31HT has good resistance to cyanide melts and neutral salt melts at high temperatures.



      Figure 4. Carburization of samples with various surface finishes in 10% CH      4 + 90% Ar for 1000h at 900°C (1650°F).
      I Electrolytically polished surface
      II Machined surface
      III Surface pre-oxidized in air for 5 hours at 1000      °C (1830°F).


Wet corrosion
Sandvik Sanicro 31 HT has roughly the same resistance as Alloy 800 to general corrosion and pitting. Thanks to the high nickel content its resistance to stress corrosion is good.

Owing to its relatively high carbon content and the heat treatment applied, Sandvik Sanicro 31HT can be prone to intergranular corrosion. This problem can be disregarded where high temperature operation is concerned, but should be observed in cases when the alloy is exposed to corrosive condensates or pickling solutions.


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A document from the Sandvik Materials Technology web-site.