JMEPEG (1992) 1:341-346 9 ASM International Improving Intergranular Corrosion Resistance of Sensitized Type 316 Austenitic Stainless Steel by Laser Surface Melting U.K. Mudali and R.K. Dayal An attempt was made to modify the surface microstructure of a sensitized austenitic stainless steel, with- out affecting the bulk properties, using laser surface melting techniques. AISI type 316 stainless steel specimens sensitized at 923 K for 20 hr were laser surface melted using a pulsed ruby laser at 6 J energy. Two successive pulses were given to ensure uniform melting and homogenization. The melted layers were characterized by small angle X-ray diffraction and scanning electron microscopy. Intergranular corro- sion tests were carried out on the melted region as per ASTM A262 practice A (etch test) and electrochemi- cal potentiokinetic reactivation test. The results indicated an improvement in the intergranular corrosion resistance after laser surface melting. The results are explained on the basis of homogeneous and nonsen- sitized microstructure obtained at the surface after laser surface melting. It is concluded that laser sur- face melting can be used as an in situ method to increase the life of a sensitized component by modifying the surface microstrueture. 1. Introduction AUSTENITIC stainless steels are prone to intergranular corro- sion (IGC) and intergranular stress-corrosion cracking (IGSCC) when they are subjected to sensitizing heat treatments between 723 and 1073 K, leading to premature failure of com- ponents during service. The sensitization is attributed to the formation of a chromium-depleted zone adjacent to the chro- mium-rich M23C6 carbides along the grain boundaries during the heat treatment. Thus, the presence of this heterogeneous mi- crostructure can initiate IGC and IGSCC. To avoid this type of failure, the remedial methods include use of low-carbon stain- less steels, high-temperature solution annealing treatment to dissolve the carbides, and use of carbide stabilizing alloy addi- tions such as Ti and Nb. [1-3] However, if the components are found to be sensitized during the final stage of commissioning or during service, they cannot be used in hostile environments without solution annealing treatment. Also, it is difficult to eliminate a sensitized microstructure, such as the one formed at the heat-affected zones during welding, by a simple heat treat- ment without affecting the properties of the weldment. In such cases, an in situ method is required that can selectively elimi- nate the sensitized microstructure. In the present work, an attempt was made to homogenize the surface of the sensitized specimens by using a laser. This would eliminate the possible initiation of IGC and IGSCC at the sur- face, because most corrosion processes initiate from the sur- face. It is known that laser surface modification produces microstructures with improved properties for many applica- tions. [4-1~ Also, laser surface melting can be carried out on complicated or inaccessible components in situ if required. [51 With careful beam control, appropriate melt depths can be pro- duced, with minimum surface residual stresses and complete intermixing of the alloying components. Because melting dur- U.K. Mudali and R.K. Dayal, Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. ing laser processing occurs in a very short time, and only in the surface region, the bulk of the material remains cool, thus serv- ing as an infinite heat sink. Large temperature gradients exist across the boundary between the melted surface layer and the underlying solid substrate. This produces rapid self-quenching and resolidification, with quench rates as large as 1011 K sec -1 and accompanying resolidification velocities in the range of 20 ms -1. The rapid quenching from the liquid phase can produce extended solid solutions, metastable crystalline phases, and, in some instances, metallic glasses. In the present work, laser surface melting of sensitized type 316 stainless steel was carried out using a pulsed ruby laser sys- tem. The melted regions were characterized by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). In- tergranular corrosion resistance was evaluated by testing as per ASTM A262 practice A (etch test) and by conducting electro- chemical potentiokinetic reactivation (EPR) tests. 2. Experimental Methods Nuclear-grade type 316 stainless steel (see material in Table 1 for composition) was used for the present work. Solution an- nealed type 316 stainless steel was given a sensitization heat treatment at 923 K for 20 hr. Specimens of 10 by 10 by 2 mm were cut from this material and polished with up to 600-grade SiC emery paper. This surface preparation was preferred to minimize the reflection of the laser beam during melting. The specimens were thoroughly cleaned in methanol prior to laser surface melting. Laser surface melting was carried out using a pulsed ruby laser system (JK Industries, UK) with the follow- ing specifications: pulse width, 30 ns; ~,, 693.4 nm; energy range, 1 to 10 J/pulse; and pulse frequency rate, 6 pulses per minute. In the present work, laser irradiation was carried out at a calibrated energy of 6 J per pulse with two successive pulses at the same spot. The diameter of the melted region, corre- sponding to the diameter of the laser beam, was about 6 mm. Complete melting of the surface was ensured by immediately Journal of Materials Engineering and Perform~mce Volume 1(3) June 1992--341