MICROSTUCTURES AND MEHCANICAL PROPERTIES OF WELDED COLD- ROLLED LOW- CARBON STEEL PLATE

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 microstructural characteristics of the welded parts, it is against this backdrop that investigation of the microstructures 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 annealed using laboratory heat treatment furnace. The normalized and annealed samples were heat treated at 920°C by cooling in 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% and40% 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 microstructural studies were produced by machining. Mechanical properties were determined at ambient; temperature (250q and results analyzed graphically and microstructural characteristics obtained using optical microscope at magnification of 400xx. The microstructures revealed predominantly mixed phases of pearlite and ferrite, with the pearlite 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 structure exhibited 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 pearlite phase in ferrite matrix.