1987 Volume 73 Issue 6 Pages 699-706
The influence of grain size on the yield strength at moderate temperature in ferrite-pearlite steels was studied with paying attention to the ky value in the Petch equation.The lower yield stress of a ferrite-pearlite steel has been considered to depend on the grain size, and is estimated by the Petch equation.The ky value in the Petch equation is considered as the value which ralates with the dislocation mobility and the Pierce-force.The ky value is reported to be constant as 2.2kgf/mm2/mm-1/2 below the room temperature.On the other hand, the investigation on ky above the room temperature has not been reported.The authors have verified that the ky value decreased with the increase in temperature above the room temperature.The temperature dependence of ky is clasified to four ranges between 25°C and 550°C.The dependence can be explained by the decrease in pinning force of dislocations by carbon and nitrogen, the active multification of dislocations, and I.S.(Interstitial-Substitutional) effect.
The ky value becomes negative, when the grain boundary sliding occurs because of the creep phenomenon under the lower strain rate at relatively high temperatures.The ky value can not become negative without the creep phenomenon, but the ky value approaches to zero.
