On the Transition Temperatures of Mild Steel

Studies of load-time relations under Charpy impact test-I

Abstract

The study of the load-change of a low-carbon killed steel containing 0·17%C under-the impact bending load was carried out in the transition temperature range from ductile to brittle fracture. At the test, a miniature Charpy impact testing machine, being of 2kg-m capacity, was used and the load acting on the specimen was measured by the use of piezoelectricity of quartz crystals and a cathode-ray oscillograph. In this apparatus there was no electric disturbance between the quartz and the oscillograph, and then the precise load-time curve was recorded on the film inside the oscillograph.
The states of the specimens were: (i) cold-drawn state and the state normalized at 950°C. (ii) the states annealed at 700°, 800°, 920° and 1000°C respectively after cold drawing. (iii) the states quenched in water from 700°, 800° and 920°C respectively after normalizing at 950°C and (iv) the states aged atroom temperature (about 25°C), 50°C and 100°C for various durations after the quench from 700°C of normalized specimens.
The results in this investigation were summarized as follows:
(1) The load-time curves in the range from ductile to brittle fracture were classified into five types.
(2) The definition of transition temperature, the temperature at which the crack appeared at first, was reasonable not only theoretically but also in practice. And it was concluded that this transition temperature was very sensitive to the heat-treated states of specimens.
(3) A characteristic load-time curve was recorded in the test of cold-drawn specimen. This curve showed that the crack was propagated intermittently and then the specimen was fractured.
(4) Specimen annealed at 700°C had a low transition temperature (-70°C), while that annealed at 800°C had much higher one (10°C). The transition temperature thereafter rose slightly with the annealing temperature.
(5) Specimens quenched from higher temperatures (800° and 920°C) than the transformation point had not definite transition temperatures and showed much greater work-hardening than that of other specimens.
(6) Among three kinds of quench-age-hardened specimens, the specimen aged at room temperature showed the most remarkable change in transition temperature. This change was parallel to the marked change in Vicker's hardness number.
(7) In all tests, the rapid increase of maximum load with decrease of testing temperature was observed in transition temperature range, and then the load became smaller rapidly owing to the initiation and propagation of severe cracks.