|Heat Resisting Austenitics - Corrosion Resistance|
The heat resisting austenitics are not usually used in aqueous conditions, but are usually used in high temperature gaseous environments. The following aspects of this dry corrosion are pertinent.
In many processes, isothermal (constant temperature) conditions are not maintained and process temperatures vary. Expansion differences between the base metal and the scale during heating and cooling can cause cracking and spalling of the protective scale. This allows the oxidising media to attack the exposed metal surface. The high chromium and nickel content of the heat resisting austenitics provides good resistance to high temperature oxidation. The spalling resistance is greatly improved with the higher nickel contents of the U-310S types because nickel reduces the expansion differential between the scale and the base metal.
EFFECT OF ATMOSPHERE
The presence of water vapour increases the corrosion rate. However, the increased nickel and chromium contents of the heat resisting austenitics provide good resistance to moist air at temperatures in excess of 980°C. They also have good scaling resistance to carbon dioxide and can be used at temperatures similar to those quoted for service in air.
Sulphur vapour readily attacks the austenitic grades. Typical corrosion rates for various stainless steels after 1 300 hours exposure to flowing sulphur vapour at 570°C are shown alongside.
The rate of corrosion in hydrogen sulphide depends on concentration, temperature, pressure and permeability of the scale. The presence of chromium in the steel helps to stabilise the scale and slow the diffusion process. At high pressure and temperature when hydrogen is present, the attack is more severe and alloys such as the U-309S types are more suited to these conditions.
It is extremely difficult to generalise corrosion rates in flue and process gases since gas composition and temperature may vary considerably within the same process unit.
Combustion gases normally contain sulphur compounds, as sulphur dioxide is present as an oxidising gas, along with carbon dioxide, nitrogen, carbon monoxide and excess oxygen. Protective oxides are generally formed and depending on exact conditions, the corrosion rate may be similar or slightly greater than for service in air.
Reducing flue gases contain varying amounts of hydrogen sulphide, hydrogen, carbon monoxide, carbon dioxide and nitrogen. The corrosion rates encountered in these environments are sensitive to hydrogen sulphide content and temperature, and satisfactory material selection often necessitates service testing. The high nickel content of the U-310S types may be deleterious in some instances due to sulphidation, in which case, the U-309S types may be the preferred material.
High chromium and nickel contents result in a slower diffusion rate of carbon into the steel. The heat resisting austenitics therefore have good resistance to carburising atmospheres.
The high nickel content of the U-310S types ensures a good resistance to ammonia atmospheres at high temperatures. Typical corrosion rates for U-310S types in an ammonia converter containing 5-6% NH3 after 30 000 hours at 500°C, are in the region of 0,003mm/yr.
Annealing of the heat resisting austenitics is achieved by heating to between 1 030°C and 1 150°C for 60 minutes per 25mm thickness (2.5min/mm) followed by water quenching. Annealing will ensure that any carbide precipitates are taken back into solution.
The heat resisting austenitics can be forged, hot headed and upset satisfactorily. Uniform heating of the steel in the range of 1 150°C to 1 250°C is required. The finishing temperature should not be below 950°C. Forgings should be cooled rapidly in air or water.
The heat resisting austenitics can be deep drawn, stamped, headed and upset without difficulty. Since austenitic stainless steels work harden, severe cold forming operations should be followed by annealing.
Like all the austenitic stainless steels, this alloy group machines with a rough and stringy swarf. Rigidly supported tools with as heavy a cut as possible should be used to prevent glazing.
The heat resisting austenitics can be satisfactorily welded and brazed by all methods, giving a tough weld. Welding procedures for the U-310S types have to be selected with care in order to avoid hot cracking due to the fully austenitic weld microstructure obtained from using matching filler metals.