The three-dimensional (3-D) morphology and growth kinetics of intragranular ferrite idiomorphs formed in association with MnS(ϩVN) inclusions were studied in an Fe-C-Mn alloy. Ferrite idiomorphs began to be formed at a temperature ϳ40°C lower than the grain boundary allotriomorphs, and the highest temperature of the formation of intragranular ferrite plates was ϳ60°C below the Widmanstätten start temperature of grain boundary sideplates (W s ). The 3-D shape of ferrite idiomorphs was more equiaxed than grain boundary allotriomorphs. Measured growth rates were smaller than those calculated assuming spherical growth, similar to those of grain boundary allotriomorphs previously reported. The possible reasons for the retardation of growth, e.g. deprivation of carbon supersaturation by grain boundary ferrite allotriomorphs and solute drag etc., are discussed.KEY WORDS: ferrite; inclusion; growth; serial sectioning; morphology.
Experimental Procedures
Alloy and Heat TreatmentThe alloy was melted in a vacuum induction furnace using electrolytic iron, high purity carbon and manganese. An excess amount of sulfur, vanadium and nitrogen were added to promote the formation of intragranular ferrite. The composition of the alloy is shown in Table 1. A 50 kg ingot of the alloy was hot rolled to a plate of 50 mm thickness at 1 000°C and subsequently, the slab was cold rolled by 70 %. Specimens 10ϫ10ϫ0.35 mm 3 in size, three pieces bundled and coated with anti-oxidation ceramic paint, were austenitized at 1 250°C for 10 min under high purity argon atmosphere. This treatment produced a grain size of approximately 200-300 mm. A larger austenite grain size facilitates the formation of intragranular ferrite. The use of thin specimens and a high austenitizing temperature made the austenite grain boundaries essentially perpendicular to the broad faces of the specimen.2) After austenitizing the specimens were swiftly transferred into a salt bath for isothermal reaction to occur at temperatures ranging from 610 to 750°C for varying times, and quenched into iced brine.
Serial Sectioning, 3-D Construction and Measurement of Growth RateFiducial hardness indents were put on the polished specimen surface for the purpose of image alignment and measurement of the thickness of the removed layers. The specimens were then polished by an automatic grinder polisher to remove ϳ0.5 mm per section. A digital image of each section taken by a CCD camera was stored in a personal computer. After masking the objects a stack of images were transformed into a 3-D image by means of visualization software (AVS).
11)For some ferrite particles the sizes were measured directly from 3-D constructed images and were compared with calculation. The growth rate of ferrite was measured from the variation with time of the maximum half-thickness or half-diameter of the ferrite particles in the plane of polish. The measurement of the diameter of the largest particles in the plane of polish could be representative of the thickness of one of the first formed ferrite particle...
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