7/21/2019 Sens It Ization http://slidepdf.com/reader/full/sens-it-ization 1/5 The ize of the ensitization Zone in 304 tainless teel Welds N. S TSAI and T. W EAGAR Factors influencing the size and shape of the sensitization zone in 3 4 stainless steel welds have been studied through a statistically designed experiment. The results indicate that the width of the sensitization zone is in proportion to the magnitude of the heat input except when very little fusion occurs. The minimum sensitization size can be obtained with a broadly distributed heat source traveling at high speed and the maximum size can be obtained with low speed and a narrow heat distribution. Under certain welding conditions one can obtain welds which are free of sensitization on the surface and hence this process may lead to a new method o preventing sensitization-induced intergranular corrosion. By depositing surface sensitization-free welds on either side of a previously sensitized weld the chromium carbide precipitates dissolve and the sensitization on the plate surface is eliminated. INTRODUCTION Sensitization-induced intergranular corrosion in 304 stain- less steel weldments has become a major problem in recent years. Factors controlling the sensitization are alloy content, carbon content, and weld thermal cycles. Among these, the metallurgical factors are well known, * and a number of methods of preventing intergranular corrosion are available: Employing solution heat treatment Lowering carbon content Adding stabilizers Employing internal cooling of pipes to produce fa- vorable compressive residual stress on the surface However, these methods are either costly or difficult to Wly, particularly to weldments that have already been sen- sitized. A practical and inexpensive method of reducing sensitization of these existing welds is desirable. This paper studies the effects of the welding process parameters on the location and size of the sensitization zone and presents a N TSAI and T W. EAGAR are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 new method of preventing sensitization-induced inter- granular corrosion. EXPERIMENT L PROCEDURE A Box-Behnken experimental design was used with current, arc length, and travel speed employed as the primary vari- ables. Single-pass, bead-on-plate welds were made by gas tungsten arc welding on 30-cm/30-cm/1.3-cm 304 stainless steel plates using argon shielding gas. The chemical com- position of the stainless steel is given in Table 1 The weld cross section was etched in 10 pet oxalic acid (per ASTM-262 practice A). The location and the size of the sensitization zone were measured under an optical micro- scope at 200-times magnification, and the sensitization zone is defined as the region in which more than 30 pet of the grain boundary is attacked. RESULTS ND DISCUSSIONS Sensitization occurs in the region which experiences peak temperatures of 650Â to 900 OC for a number of seconds during the welding process. Two primary variables deter- mining the location and width of the sensitization zone are ~~~~~~~~ FOR ENERGY SYSTEMS 1984 MERIC N SOCIETY FOR METALS VOL.. 6 NO I JL NE 1984 33
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