| Thermal cycling effect on the mechanical properties of the high temperature austenitized AISI 4340 steel was studied. Specimens austenitized at 1200℃ were thermal cycled at 840℃ and tempered in the range of 200-400℃, after which the tensile properties and impact strengths of the specimens were examined. Thermal cycled specimens tempered in the range of 300-400℃ showed higher tensile and yield strength than that of untreated specimens. Impact strength was also increased by the thermal cycling treatment in all the tempering range of this study. However, after the initial increase in impact strength with increasing tempering temperature, there was an anomalous loss in impact strength when tempering temperature was 300℃. This loss in impact strength was attributed to the tempered martensite embrittlement. High T.S., Y.S. and impact strength were obtained by three cycling treatment and subsequent tempering at 200℃, and their values were 195 ㎏/㎟, 165 ㎏/㎟ and 4.3 ㎏·m respectively. The results indicated that the mechanical properties of AISI 4340 steel was affected by the austenite grain size substantially and the increase of mechanical properties was attributed to the grain refinement by thermal cycling. |
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