The objective of this research is to investigate the effect of extension rate on tensile properties of AISI 4140 steel. The steel was supplied by local vendor in Thailand and its use for some military product. The primary material testing was needed to be done in order to obtain mechanical properties. The extension rate testing was performed on tensile testing machine ranged from 300 to 500 min / min. The results were shown that when higher extension rate was applied, the tensile stress of 4140 steel was increased. In contrast with elongation, the trend was decreased according to extension rate advancement. Moreover, extension rate is not affect to deformation mode of 4140 because ductile failure remains in the same as quasi-static tensile test.Keywords: Heat treatment, AISI 4140, extension rate, tensile properties
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