The Effect of Grain Size on the Change in Mechanical Properties of a Low-Carbon Steel Subjected to Shock Pressure.

Abstract

The effect of shock pressure on the stress-strain behavior of a low-carbon steel of four different grain sizes, 9, 16, 33 and 80μm, was investigated. The specimens were explosively shock-1oaded in water. The shock pressure effect was compared with the effect of pressurizing or temper rolling on the stress-strain behavior. Sensibility to twinning induced by shock loading was also examined. Results obtained in this investigation are summarized as follows. ( 1 ) The yield stress of all kinds of specimens began to decrease at the shock pressure of 300 MPa and continued to decrease to the minimum with increasing pressure, owing to generation of free dis1ocations in the specimens. For the specimen of smaller grain size, the amount of decrease in the yield stress became larger and the shock pressure (ps)m which minimized the stress became higher. ( 2 ) Yield ratio of the specimens of four different grain sizes of 9, 16, 33 and 80 μm, which had the original ratios of 0.80, 0.76, 0.65 and 0.52, respectively, could all be reduced to about 0.4 after shock loading. ( 3 ) The shock pressure (ps)t at which the mechanical twins began to form in the specimen increased linearly with increasing d-1/2, which is a reciprocal of the square root of grain size d. Twin density, defined as the ratio of number of crystal grains with twins to total number of grains, was larger in the specimen with larger grain size and the density increased monotonously with increasing shock pressure. ( 4 ) For the specimen with a given grain diameter, the shock pressure (ps)m at which the yield stress became the minimum was identical in value to the pressure (ps)t at which the mechanical twins appeared in the specimen. This indicates that the macroscopic plastic strain begins to occur in the specimen at the pressure of (ps)m. ( 5 ) The decrease in the yield stress due to shock loading was considerably larger than the decrease in the cases of pressurizing and temper rolling.