Controlling Grain Boundary Energy to Make Austenitic Stainless Steels Resistant to Intergranular Stress Corrosion Cracking D.N. Wasnik, V. Kain, I. Samajdar, B. Verlinden, and P.K. De (Submitted 17 December 2002; in revised form 7 February 2003) Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study [1] demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries. Keywords austenitic stainless steel, corrosion, deformation pro- cessing, grain boundaries, intergranular corrosion, in- tergranular stress corrosion cracking, sensitization stress corrosion cracking 1. Introduction Austenitic stainless steels (ASS) have excellent resistance to general corrosion. They are, however, prone to localized cor- rosion like crevice, pitting, intergranular corrosion (IGC), and stress corrosion cracking (SCC). While ASS are inherently prone to transgranular stress corrosion cracking (TGSCC) even in a solution-annealed condition, certain microstructural fea- tures can make them prone to intergranular stress corrosion cracking (IGSCC). [2-4] The two forms of localized corrosion, IGC and IGSCC, are directly caused by sensitization. [5] Sen- sitization is typically found when a stainless steel is welded or heat treated in the temperature range of 500-800 °C. This leads to precipitation of chromium rich carbides at the grain bound- aries. Growth of such carbides can lead to the formation of chromium-depleted zones in the immediate surrounding. When the level of chromium in the depletion regions falls below 12-13 wt.%, the passive film over the depleted regions weakens and breaks easily in contact with aggressive solutions. This makes the sensitized ASS prone to IGC and IGSCC. The common methods [5] used to control sensitization, hence IGC and IGSCC, are (1) solution annealing (to dissolve chro- mium rich carbides and erase the chromium depletion regions), (2) lowering the carbon levels (to prevent precipitation of chro- mium rich carbides), and (3) stabilizing carbon by precipitating it with titanium or niobium. Other than the control of chemis- try, sensitization control can also be implemented through op- timizing grain boundary nature and grain size. [1,6-11] The former is often distinguished from the coincident site lattice (CSL) concept. [12-15] It is the only practical way, at least pres- ently, to relate experimentally obtained local orientation mea- surements with grain boundary nature or energy 1 , but such classification can be far from exact. For example, a so-called 3 tilt or twist boundary is identical in its misorientation ma- trix, but has a large difference in energy. Despite such restric- tions, the CSL concept is the best available approach to study grain boundary and its properties. It is to be noted that the CSL theory [12-15] defines  as the inverse of the coincident sites at the grain boundary, i.e., 3 denotes that one out of three atomic sites are coincident. 3-29 are taken as special boundaries and  >29 are taken as random boundaries. These are usually mea- sured in a scanning electron microscope with an orientation imaging microscopy (OIM) attachment. Until recently, the gen- eral understanding was that the presence of random, non-CSL high angle boundaries are detrimental to local corrosion resis- tance. This generalized concept has been repeatedly high- lighted in the published literature, patented products, and pro- cesses. [8-11] A recent study introduced an alternative: [1] the presence of a very large fraction of random boundaries was observed to be beneficial in making an ASS resistant to sen- sitization and IGC. Similar trends in experimental data con- trolling the nature of grain boundaries to control the resistance to sensitization and IGC were also reported in unrelated studies D.N. Wasnik and I. Samajdar, Department of Metallurgical Engi- neering and Materials Science, Indian Institute of Technology, Mum- bai, India 400 076; V. Kain and P.K. De, Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India 400 085; and B. Verlinden, Dep. MTM, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44–B-3001–Belgium. Contact e-mail: vivkain@ apsara.barc.ernet.in. 1 The grain boundary energy may vary widely depending on its exact nature. For example, between special and random high angle bound- aries of AISI 304L ASS, an energy difference of 20 to 835 mJ/m 2 can exist. [16] JMEPEG (2003) 12:402-407 ©ASM International 402—Volume 12(4) August 2003 Journal of Materials Engineering and Performance