Abstract:
Low-carbon steels have as one of their properties good weldability as a result they are often joined--into useful structures by welding, however their safe use under different operating conditions is to a large extent determined by the micro structural characteristics of the welded parts, it is against this backdrop that investigation of the micro-structures and mechanical properties of welded cold -rolled low- carbon steel plate was conducted, to achieve this low-carbon steel plates were machined and cut into various sizes.fifteen(15) of these samples were normalized while another sets of fifteen (15) were normalized using laboratory heat treatment furnace. The normalized and annealed samples were heat treated at 920°C by coolingin air and furnace respectively to produce the desired normalized and annealed structures. The as received samples were not heat treated so as to be used as control for the heat treated samples. The normalized,annealed and as-received structures were further cold worked by rolling at 5%, 10010,20010, 30% and 40% percent reduction using laboratory rolling mill. Subsequently, the rolled samples were cut into two equal halves with hacksaw and welded together using electric arc welding process. From the welded structures, specimens for hardness, tensile strength and impact (notch toughness) energy tests and
micro structural studies were produced by machining. Mechanical properties were determined at ambient temperature (250q and results analyzed graphically and micro structural characteristics obtained using optical microscope at magnification of 400xx. The micro structures revealed predominantly mixed phases of pear lite and ferrite, with the pear lite phase in ferrite matrix. Mechanical properties of the various structures were found to increase with increasing percent reduction with the non welded and welded parts of normalised structure exhibiting higher hardness and tensile at all ranges of percent reduction over that of corresponding parts of the annealed and as-received structures. Also, the welded part of normalised structureexhibited higher impact over that of the as-received structure at all ranges of percent reduction - and higher impact over welded part of the annealed structure at lower percent reduction,but lower impact at
higher percent reduction. The observed low impact (notch toughness) energy absorbed on impact by the welded part of the normalised structure at higher percent reduction is attributable to more fraction volume of pear lite phase in ferrite matrix.
