Metallurgical Studies of Welded Joints of Local and Imported Low Carbon Steels

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The effects of heat-treatment and variations in alloy content on the microstructure and mechanical properties welded joints of local and imported low carbon steels have been studied. The joints were fabricated using the shielded metal-arc welding technique. The chemical analyses of the samples revealed that in imported steel the contents of the main alloying elements responsible for strength (i.e. carbon and manganese) were 0.19% and 0.54% by weight respectively, whereas in the local steel they constituted 0.06% and 0.29% by weight respectively. The relatively higher carbon and manganese contents in the imported steels as compared to those in locally-made steel accounted for the high Ultimate Tensile Stress (UTS) of 689.00 ± 7.72 MPa, Yield Strength (541.60 ± 6.15 MPa), Moduli of Elasticity (244.00 ± 11.44 GPa) and Impact Strength (214.50 ± 5.57 J), and lower ductility (23.66 ± 0.67%) of the imported steel. This also confirmed the large proportions of perlites in the imported steel as compared to those in the local steel from their optical micrographs. Local steel had lower UTS (528.25 ± 13.66 MPa), Yield strength (381.75 ± 5.88 MPa), Moduli of Elasticity (240.25 ± 3.66 GPa) and Impact strength (160.67 ± 7.23 J) and higher ductility (28.03 ± 1.33%). When the steels were welded there were significant drops in the mechanical properties. The heat-treatment improved the Ductility and Impact strength at the expense of UTS, Yield Strength and Young‘s Modulus. The ductility and impact strengths of welded local steels through heat-treatment increased by 42.00% and 100.50% respectively over the welded samples. With imported steel ductility and impact strengths increased by 47.30% and 125.10% respectively over the welded samples. It was concluded heat-treatment lowered the level of internal stresses at the joints thereby improving the ductility and impact toughness.
A Thesis submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfillment of the requirements for the degree of Master of Science,November-2012