Modification of Sensitization Resistance of AISI 304L Stainless Steel through Changes in Grain Size and Grain Boundary C
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TRODUCTION
INTERGRANULAR corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) are well-known failure modes experienced by austenitic stainless steels in several environments. These typical failures primarily arise due to the onset of sensitization in the temperature range of 550 C to 800 C during processing or welding of stainless steels. The corrosion attack in sensitized stainless steels preferentially propagates in an intergranular fashion, which is further accelerated by the resulting stresses developed during operation. Components may encounter sensitization temperatures during welding or solution annealing, which leads to the formation of carbides of varied sizes. Interestingly, once nuclei are formed, they can grow even at a temperature (such as at ~300 C) much lower than the sensitization temperature and may significantly bring down the chromium level in the matrix. To minimize sensitization and related failures, several efforts such as reduction of carbon below 0.03 wt pct and addition of nitrogen as well as strong carbide formers (such as titanium/niobium) to the existing stainless steels have been made. As a result, the RAGHUVIR SINGH and INDRANIL CHATTORAJ, Scientists, Applied Chemistry & Corrosion Division, and SANDIP GHOSH CHOWDHURY, Scientist, Materials Science & Technology Division, are with the National Metallurgical Laboratory, Jamshedpur - 831 007, India. Contact e-mail: [email protected] Manuscript submitted January 9, 2008. Article published online July 22, 2008 2504—VOLUME 39A, OCTOBER 2008
evolution of ‘‘L’’ and ‘‘LN’’ varieties (such as 304L and 304LN) have been developed, which are more sensitization-resistant austenitic stainless steels. These alterations have resulted in various degrees of success; however, they have led to the enhancement in the cost of materials without producing a sensitization-free material.[1–3] Grain boundary engineering (GBE) based on the coincident site lattice (CSL) model, introduced by Watanabe, has emerged as a better alternative to improve various properties including corrosion resistance of low stacking fault energy materials.[4] Sensitization in stainless steel is said to be minimized by enhancing the lattice sites common to two or more grains and thus reducing the ‘‘grain boundary energy.’’ Such boundaries are known to have special properties and are designated by R. The success in enhancing the low R boundaries in materials with low stacking fault energy has been achieved mainly by thermomechanical processing (TMP). The two primary thermomechanical-processing paths used to improve grain boundary character distribution (GBCD) that have been commonly adopted are strain annealing and strain recrystallization. Strain annealing involves low levels of strain in the range of 3 to 7 pct followed by long annealing hours,[5] whereas strain recrystallization used low to moderate deformations in the range of 5 to 30 pct followed by annealing for short duration at relatively high temperatures.[6] Iterative, rather than single step, deformation and an
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