Precipitation and growth of carbide particles in a cyclically strained low-carbon steel

Abstract

Quenched samples of a low-carbon steel were fatigue-tested, at 300 c/min and with a maximum strain amplitude of ±0.003, at selected temperatures in the range 20°–90°C. During strain-cycling at 60°–90°C, the active slip-bands formed narrow channels within a matrix of relatively high dislocation density which was strengthened by dynamic strain-ageing. In the matrix, carbon was removed from solid solution rapidly, by segregation to dislocations and by fine-scale precipitation of an intermediate carbide. Inside the slip bands, high-amplitude dislocation motion prevented dislocation locking and precipitate nucleation, but the slip-band/matrix interfaces and slip-band intersections were sites of abnormal carbide particle growth. At these sites, irregular platelets of cementite grew up to about 1 μm in diameter during fatigue at 90°C for a few hours. Of the processes involved in precipitation at the boundaries of active slip-bands, the distinctive features which favour particle growth at relatively low temperatures are considered to be the following: most of the dissolved carbon within active slip-bands is available for precipitation on locked dislocations at their boundaries: dislocation ‘pipe’-diffusion is expected to permit rapid transfer of carbon through the boundary network: plastic deformation of the matrix will tend to relax certain restraints on cementite particle coarsening. It is suggested that the combined effects of these three conditions of precipitation may provide a sufficient explanation of the locations and the extents of cementite particle coarsening observed during fatigue at temperatures above about 60°C.