Abstract
An 80kg/mm2 high tensile steel with a low carbon equivalent value and difficult to harden by quenching was tempered after water-quenching or transformed isothermally and the mechanical properties and the internal friction were measured. The internal friction at room temperature was calculated from the time during which the strain amplitude of the sample set free after being subjected to a transversal resonance vibration by the electromagnetic method decreased to 1/n of the initial value.
The experimental results are as follows:
(1) The sample quenched in water has a low strain amplitude-dependent internal friction and shows a low quenchability.
(2) The strain amplitude-independent internal friction of the specimen quenched in water from 900.. decreases remarkably by tempering in the temperature range from 20.. to 200.., shows litte change in the range from 300.. to 500.., and increases in the range from 550.. to 700... These changes correspond to the changes in the Charpy impact value. The three stages in the variation of internal friction are interpreted respectively as due to the precipitation of carbides from martensite, the decomposition of martensite and the formation of carbides.
(3) The transition temperature rise by tempering in the temperature range between 300.. and 500.., reaches a minimum after tempering at 700... The transition temperature is influenced by the form and distribution of carbides.
(4) The sample transformed isothermally has a strain amplitude-dependent internal friction.
