A mild steel with 0·15%C, having three kinds of grain diameter, was tested at temperatures from room temperature to liquid nitrogen one under impact and quasi-static tensile loadings.
Experimental results are summarized as follows: -
(1) The increase of lower yield stress with the lowering of temperature was much greater than that of tensile strength in both static and impact tensile test. Below the temperature at which brittle fracture was observed, the yield stress (brittle fracture strength) was almost constant.
(2) The transition from ductile to brittle fracture was observed on the unnotched test piece in both static and impact tension test. The transition temperature registered in the latter test was higher by 60°C than that in the former. The transition temperature was also affected by the grain size, i. e., the specimen N1 (grain diameter: 0·091mm), no matter what the deformation rate was changed, had the temperature higher by 40°C than one with N3 (grain diameter: 0·017mm). This 40°C was quite equal to the difference between the transition temperatures of the respective specimens (N1 and N3) in Charpy V-notch test.
(3) Although the lower yield strength and tensile strength were functions of deformation rate as σ=Alog∈+B (σ: yield strength or tensile strength, ∈: strain rate, A and B: constants) at a given temperature, the deformation rate dependence was larger for the lower yield strength. This dependence increased with the decrease of the temperature, at which ductile fracture occurred. However, it became very small when the specimen fractured in brittle manner.
(4) The lower yield stress (or brittle fracture strength) σu obeyed quite well the relation σu=σi+kd-1/2 (σi, k: constants, d: grain diameter). The value of k for the impact tensile test was larger_than that for the static one.
(5) In the range of temperatures at which ductile fracture occurred, k was found to be not a function of temperature but of deformation rate, and frictian stress σi was a function of both temperature and deformation rate. σi=α exp (-βT) (α, β: constants, T: temperature) was found at a given deformation rate. Meanwhile, when the brittle fracture occurred, the constant k became larger but independent of the temperature and of the deformation rate, and σi had not any dependence on the temperature but a slight one on the deformation rate.