A description is made of an apparatus and method for tensile testing of cast iron in air under constant strain rate at constant temperature in the range from room temperature to liquidus. The test results on ferritic gray iron and ferrite-pearlitic gray iron are discussed. With increase of temperature, the ultimate tensile strength from room temperature to liquidus can divided into five ranges : (i) high strength range ; room temperature to about 500°C, (ii) rapid falling strength range ; about 500°C to α→γ transformation start point, (iii) strength transition range ; eutectoid transformation range, (iv) austenite transcellular fracture strength range ; α→γ transformation finish point to about 1,100°C, and (v) intercellular fracture strength range ; about 1100°C, to liquidus temperature. The elongation from range (i) to range (v) are about 1%, 3%, 2%, 4% and 4%, respectively. The strength of ferrite-pearlitic gray iron is higher than that of ferritic gray iron in ranges (i) to (iii), but both show the same strength and deformation in ranges (iv) and (v).<br>  The maximum true stress is greatly influenced by the rate of strain and temperature in range (ii) which are expressed by the relationships (1) and (2) in ferritic gray iron<br>      σ_M=C₁·exp(Q₁/RT)     ……(1)<br>      σ_M=B₁·έ^β1              ……(2)<br>where σ_M is the maximum true stress, kgf/mm₂ ; C₁ is a coefficient, 0.033kgf/mm² at strain rate of 5.2×10⁻³ sec⁻¹ ; Q₁ is an activation energy of maximum true stress, 9.3kcal/mol ; R is a universal gas constant ; T is the absolute temperature. K ; B₁ is a coefficient, 5.9 sec^0.105kgf/mm² at 730°C; ε is the strain rate, sec⁻¹ ; β₁ is a strain rate sensitivity index, 0.105 ; and the relationships (3) and (4) in ferrite-pearlitic gray iron<br>      σ_M=C₂·exp(Q₂/RT)      ……(3)<br>      σ_M=B₂·έ^β2              ……(4)<br>where C₂ is a coefficient, 5.5×10⁻³ at strain rate of 5.2×10⁻³ sec⁻¹, kgf/mm² ; Q₂ is an activation energy of maximum true stress, 13.4kcal/mol ; B₂ is a coefficient 10 sec^0.135kgf/mm² at 730°C ; β₂ is a strain rate sensitivity index, 0.135.<br>  The deformation behavior in range (iv) is dominated by a thermally activated process that can be expressed in terms of the maximum true stress, temperature and strain rate by the following relation (5)<br>      έ=K₁·σ_M^ζ·exp(—H₁/RT)    ……(5)<br>where K₁ is a coefficient, 1.9×10¹¹ (mm²/kg)^6.9 sec⁻¹ ; ζ is a stress index, 6.9 ; H₁ is an activation energy for deformation, 88.2kcal/mol.<br>  The strength in range (v) drops rapidly as the temperature rises and shows a dependence on the strain rate.