Early during the homogenization of hot-rolled AISI 4340 low alloy steel at 1583 K manganese sulfide inclusions became cylindrical. At a later stage each cylinder broke into spheres ("ovulated"). It was found that: 1) the kinetics of the "cylinderizing" process can be described by the theory proposed by Nichols and Mullins for the decay of circumferential perturbations on an infinite rod; and 2) the rate of cylinderizing for inclusions with final diameter of 5)< 10-6 m or less is controlled by surface diffusion. In addition, it was predicted that "ovulation" of manganese sulfide rod-like inclusions in steel, which according to both theory and experiment is much slower than cylinderizing, should also be surface diffusioncontrolled.DURING hot-rolling of steel, manganese sulfide inclusions are flattened out and elongated in the rolling direction. A preliminary investigation showed that during a subsequent homogenization treatment these elongated plates coarsen by first becoming cylindrical. At a later stage the cylindrical inclusions broke into segments which spheroidize and decrease in number with time. In this work the transition from flattened to cylindrical sulfides during high temperature homogenization of AISI 4340 low alloy steel was investigated. The transition from cylindrical to spherical morphology, which occurs at a later stage, was the object of a separate, parallel investigation. 1,2Each one of these geometric transformations has been described theoretically to some extent, the main work on shape changes being by Nichols and Mullins 3 while that on particle size changes being by Lifshitz and Slyozov, 4 and Wagner. 5 Modifications of these theories and other treatments have also been given s -10 Experiments on ovulation, defined as the breaking of a cylinder into spheres, and particle size coarsening have been conducted on a variety of materials 11-21 including manganese sulfide. 22-24The theoretical treatment that applies to the present investigation was developed by Nichols and Mullins, 3 who considered the decay of circumferential perturbations of an infinite cylinder. The perturbation of interest is the one that describes a rod with a flattened cross-section:where r and B are cylindrical coordinates of the rod surface, R o is the radius of the rod after the perturbation of amplitude S has decayed. The rate of decay is given by:where k and n are constants which depend on whether Y. V. MURTY and R.the shape change is controlled by surface or by volume diffusion. Corresponding to Herring's scaling laws, 25 n = 3 for volume and n = 4 for surface diffusion control.Although Eq.[2] was derived for small perturbations, in which case the surface Laplacian can be linearized, it will be applied, for the sake of simplicity, to all rods considered in this investigation. As mentioned above, the kinetics of shape changes in the solid state can be controlled by either volume or surface diffusion, depending on particle size or temperature. This dependence can be shown by considering the ratio of perturbation decay ra...
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