The 300 to 693 K tensile characteristics of ADI with upper bainitic structure were studied by varying the austenitization temperature (1173 and 1223 K) and austempering temperature (643 and 693 K) of a spheroidal graphite cast iron which contains 3.6 mass% C and 2.4 mass% Si. The experimental results show that the flow stress decreases with increasing austenitization and austempering temperatures. The tensile characteristics can be divided into two temperature regimes. In the lower temperature regime below about 550 K, the presence of acicular ferrite should be the main factor which leads to the common features of dynamic strain aging including strengthening, elongation drop and flow instability. The flow instability, which occurs in the temperature range of about 373 to 493 K, is more distinct with coarser microstructure. The absence of flow instability for the specimens with the finest microstructure is probably caused by its nature of high dislocation density. In the higher temperature regime above 550 K where the flow stress decreases with increasing tensile temperature, the elongation starts to drop with temperature from about 600 K. The elongation drop, which should be due to the formation of quasi-continuous carbide particles from austenite during tensile deformation, can sometimes give rise to the appearance of quasi-cleavage fracture.
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