On the Transition Temperature of Strain-Aged Mild Steels

Studies of load-time relations by Charpy impact tests-II

Abstract

Under the impact bending load, an investigation was made on the change of the transition temperatures and fracture modes of the strained and strain-aged specimens, which were of a low-carbon killed steel containing 0·17%G. In this experiment, as described in the previous paper (Tetsu-to-Hagané, Vol. 46 (1960) p. 140), 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 piezo-electricity of quartz crystals and a cathode-ray oscillograph.
The specimens were stretched by 3% and 10%, in static tension after the annealing at 920°C for 1 hour in vacuum. They were strain-aged at room temperature, 100°C and 200°C for various durations.
A new transition temperature, which was the highest testing temperature at which the marked propagation of a crack appeared, was defined in the previous paper. This transition temperature was very sensitive to the internal structures of the specimens. This definition was used also in this report.
The results in this investigation were summarized as follows:
(1) The strain-aging for 3 to 100 days at room temperature after the stretching of 3% and 10% did not change the transition temperature showing 25°C. The transition temperature for the 3%-stretched specimen aged at 100°C for 120 minuites and that for the 10%-stretched one aged at 100°C for 60 minuites remained to be 25°C. On the other hand, both the 3%-stretched specimen aged at 200°C for 60 minuites and the 10%-stretched one aged at 100°C for 120 minuites had the transition temperature of 40°C, which was 15°C higher than that for the other specimens.
(2) The absorbed energy vs. testing temperature curves for the specimens stretched 10%, and for the strain-aged ones were not decreased gradually with the testing temperature but decreased abruptly at two temperatures and there appeared a horizontal part, where the load-time curve of type III was recorded. This phenomenon, however, was not observed so much clearly with the 3%-stretched specimens and with the strain-aged ones.
(3) The more cold-worked the specimen was, the more steeply decreased the load acting on the specimen after the maximum load was reached.
The brittleness introduced by the cold working seemed to be different from that introduced by the quench-aging and strain-aging. The aging raised the transition temperature but the cold working did not raise it.