The effect of the surface on grain boundary migration in austenitic stainless steel weld metal
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I. INTRODUCTION
IT is well known that grain boundary (GB) migration occurs after solidification in the weld metal of an austenitic stainless steel. This GB migration often affects the intergranular corrosion resistance.[1] That is, the migrated GB seceded from the solidified GB, which is rich in impurities on solidification and shows superior corrosion-resistance than the GB that has not seceded.[1] Thus, to clarify the behavior of the GB migration is important from the view of weld metal intergranular corrosion and the control of the weld structure. There have been only a few articles[2,3,4] presenting GB migration in weld metal. The authors have investigated GB migration that occurs in the austenitic stainless steel weld metal. They have also studied the direction of GB migration.[5] It is found that GBs migrate from the center of the weld to the heat-affected zone due to reduction of the interfacial energy. Namely, most of the triple junctions, which decide the migrating direction of GBs, migrated from the center of the weld to the heat-affected zone. In a detailed observation, it is also found that most of those triple junctions had shown a geometric similarity to each other in their shapes before the GB migration. Before the GB migration, these triple junctions were very far from equilibrium, which means the GBs cross at 120 deg.[6] in the vicinity of the triple junctions. These observations were because of the characteristic of the weld texture. Hence, a bias in the migrating direction was found in the weld metal. These experimental results were also confirmed by simulation, using the vertex dynamics model[7] and the grain growth behavior of the weld metal during annealing. However, the observation of GB migration has been observed in the polished surface of the weld metal approximately 100 m deep from the original surface. Also, in the
intergranular corrosion of the weld metal, the original surface contributes an important role; the intergranular corrosion begins at the original surface. In addition, the surface may affect the GB migration. Thus, it is important to investigate the GB migration in the surface. The GB migration in the interior, occurring at a depth approximately of 100 m from the original surface, was carefully observed. Therefore, the purpose of this article is to clarify the effect of the surface on the direction of the GB migration in the austenitic stainless steel weld metal. II. EXPERIMENTAL PROCEDURE A. Weld Specimen The commercial AISI310S austenitic stainless steel (25.03Cr-19.15Ni-0.042C-0.93Mn-0.81Si-0.016P-0.0014Swt pct) was selected as the weld specimen. Welding was performed on a rectangular plate of 80 ⫻ 70 mm (1 mm thickness) shown in Figure 1. The GTA welding was performed at a traveling speed 1.67 mm/s, an arc voltage 10.5 V, and a welding current 32.3 A. As a result of the welding condition, the bead width was kept to 4 mm. Two methods were applied to observe the microstructure. One method was for the original surface microstructure, such as the room temperature GB and the
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