Effect of Preloading on the Relation between Lower Yield Behaviour and Tensile Speed for a Low Carbon Steel

Abstract

The effects of temperature and tensile speed under prestraining on the lower yield stress of the undeformed part of specimen under reloading were investigated. The results obtained are summarized as follows.
(1) When the specimen prestrained at 20°C was reloaded at 20°C, -78°C or-130°C, the lower yield stress of the undeformed part was higher than that of the unprestrained specimen (as annealed) at the same temperature. Moreover, the difference of lower yield stress between the prestrained and unprestrained specimens had a tendency to increase with increasing tensile speed of reloading.
(2) When the specimen prestrained at -78°C or -130°C was reloaded at the same temperature, the lower yield stress of the undeformed part was almost the same as that of the unprestrained specimen at -78°C and -130°C.
(3) When the specimen prestrained at -78°C or -130°C was reloaded at 20°C, the lower yield stress of the undeformed part was lower than that of the unprestrained specimen at 20°C. Moreovr, when the specimen loaded below the lower yield stress at -130°C was reloaded at 20°C, the lower yield stress was lower than that of the unprestrained specimen at 20°C. It is considered that some changes in the states of dislocations may occur under considerably low stress at low temperatures.