INTERGRANULAR CORROSION IN AUSTENITIC STAINLESS STEEL AND TTS TEST ANALYSIS

摘要

Stainless steel is generally chosen for service condition where corrosion and oxidation resistance is important. Generally, steels with more than 12 percent Cr is self-passivating and is known as stainless steel. This is mainly due to formation of protective passive film of Cr_2O_3 on the steel surface at this percentage of Cr. Among the stainless steels, Austenitic stainless steel is widely used in chemical industries due to its greater corrosion resistance. Austenitic S.S. contains 18 percent Cr, 8 percent Ni and 0.03 to 0.08 percent C. During welding, weld metal and surrounding heat affected zone undergoes certain metallurgical and chemical changes due to heating and cooling cycle especially at the temperature range of about 427-899 deg C, the carbon in SS migrates to the grain boundaries and combines with chromium to form chromium carbides. This phenomenon is called as sensitization, which gives depletion of chromium and the lowering of corrosion resistance in areas adjacent to the grain boundaries. This not only affects its corrosion resistance but also mechanical properties because of Inter - granular corrosion (IGC), which is one of the major problems of Austenitic S.S. Sensitization may also result from slow cooling from annealing temperatures, stress relieving that occurs at sensitizing range, or welding. Due to the longer time at temperature of annealing or stress relieving, it is possible that the entire piece of material will be sensitized, whereas the shorter time at temperature characteristics of welding can result in sensitization of a band, adjacent to but slightly distance from the weld, known as Heat Affected Zone (HAZ). There are five tests for detecting IGC in stainless steel as per ASTM-A262. In the paper an attempt has been made to give detailed analytical report/procedure of ASTM-A262, Practice 'B' for understanding IGC of AISI 304 and titanium stabilized stainless steel, AISI 321.