| Summary: | 碩士 === 長庚大學 === 機械工程研究所 === 91 === Due to their outstanding corrosion resistance, stainless steels have been widely used in various industries, especially in petrochemical industry. In order to lower material cost, stainless steels are commonly welded with carbon and alloy steels to produce dissimilar welds. Stainless steels provide the much needed corrosion resistance while inexpensive carbon steels are used as structural materials. However, hard, brittle martensite may form in the weld metal due to excessively lower contents of alloying elements, including Ni and Cr, along the fusion boundary regions where high base metal dilution is expected. Consequently, highly alloyed filler metals such as ER309 are normally utilized to compensate the reduction of Ni and Cr in the weld metal to avoid the formation of detrimental martensite and possible subsequent hydrogen cracking.
However, the existence of a stagnant layer in the weld pool immediately adjacent to the fusion boundary may result in the composition in this region significantly different from that of the bulk weld metal, and form the reported “partially mixed zone”. As a result, the region adjacent to the fusion boundary of the carbon steel side may form a thin layer of hard martensite due to its low Ni and Cr and higher C contents. Therefore, hydrogen cracking of the weldments may arise from this region.
In this study, dissimilar welds between a plain carbon steel (A36) and a dual phase stainless steel (2205) were prepared by buttering a layer of ER309 onto the low carbon steel and ER2209 were utilized as the filler metal for making the single-V groove welds. Various magnetic coil/current combinations were designed and used to induce externally-applied electromagnetic fields which enhanced weld pool convection and stirring. Consequently, the formation of the partially mixed zone along the fusion boundary may be significantly reduced even completely eliminated.
Effects of the externally-applied electromagnetic fields on mechanical properties of the dissimilar weldments were studied by conducting various mechanical testing and metallurgical analysis. The mechnical tests included microhardness testing, tensile testing and Implant testing. The microstructural characterization of the weldments, especially along the fusion boundary was conducted using optical microscopy and transmission electron microscopy.
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