Effect of Multipasses on Microstructure and Electrochemical Behavior of Weldments
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INTRODUCTION
MEDIUM carbon steels have wide applications in machine parts, gears, and objects, where parts may be hardened by heat treating. AISI 1045 is used for shock absorber struts and shafting such as motor shafts and pump shafts.[1] The tendency of martensite formation in the heat-affected zone (HAZ) has made carbon steels difficult to weld as compared to low-carbon steels and, hence, deterioration of properties.[2] During welding, interpass temperature, among other variables, plays a significant role with respect to microstructure and mechanical properties.[3] In many engineering fields of applications, the design problems are resolved using optimization tools, e.g., CAD or CAE,[4] but such techniques are limited in welding technology due to a lack of comprehensive knowledge of transformations within the material as a result of transient thermomechanicalchemical boundary conditions. This emphasizes a deep understanding of in-situ/ex-situ microstructural changes along with other parameters.[5] The high heating and cooling rates of the thermal cycle during welding change the microstructures of the weld zone (WZ) and HAZ drastically compared to the usual normalizing cycle of the same steel. This is due to the high peak temperature of HAZ during welding, which is normally not maintained in ordinary normalizing treatments, i.e., 1773 K (1500 C). The intensity of this effect has a direct relation with the number of passes.[2,6,7] Therefore, the difference in chemical compositions and the presence of inclusions causes metalMUHAMMAD ATIF MAKHDOOM and MUHAMMAD KAMRAN, Assistant Professors, and SEHRISH MUKHTAR, Lecturer, are with the Department of Metallurgy and Materials Engineering, University of the Punjab, Lahore 54890, Pakistan. Contact e-mail: [email protected] GUL HAMEED AWAN, Professor, Department of Metallurgical and Materials Engineering, University of Engineering and Technology Lahore, Pakistan. Manuscript submitted November 27, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
lurgical transformations to take place across the HAZ and weld metal. These transformed structures can alter considerably the intrinsic corrosion rate of steel.[8–11] Microstructural variations in the base metal (BM), HAZ, and WZ result in a condition favoring galvanic corrosion.[12] The chloride ions can cause corrosion due to the latter’s intrinsic acidic nature at normal or elevated temperature.[13,14] The present article is laconically concerned with the corrosion behavior of medium carbon steel weldment regions of three, five, and seven passes in plain water and 3.5 pct (w/v) NaCl solution. The results are compared with the corresponding microstructures produced during welding thermal cycles. The corrosion kinetics is also simulated using the potentiodynamic Tafel scan technique.
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EXPERIMENTATION
Rolled plates of medium carbon steel (AISI 1045) having 8-mm thickness were used as the BM. Shielded metal arc welding processes were employed for multiple pass butt joints using filler metal (8018-B2) per AWS codes and standards. Thre
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