专业英语 第六单元 Internal structure of steel专业英语 第六单元 Internal structure of steel.doc

Unit 9 Internal structure of steel Steel is our most important engineering and constructional material; it accounts for approximately 80% of all metal produced. Steel has attained this degree of prominence because it combines strength, ease of fabricability into many shapes, and a wide range of properties along with low cost. Our ability to produce steels for a wide variety of uses, ranging from relatively soft strip steel to hard tool steel, depends in many instances upon suitable heat treatments of a given steel during or after forming. Before considering the heat treatment of steel and other ferrous alloys, it will be helpful to briefly consider the internal structure of steel. The surface of a piece of steel gives no indication of its internal makeup, but if the piece of metal is broken the fracture will show a granular appearance. The grains are usually so small that a magnifying glass is needed to show their presence. A high magnification of a fracture will show little, for the rough surface can not be brought in to focus; some parts will be too near and other parts too far from the microscope. In order to examine a specimen of a piece of metal under a microscope properly, it must first be prepared by grinding a flat surface and then polishing that surface with finer an finer abrasives until a scratch-free mirror surface is obtained. The thin film of metal smeared across the surface due to the polishing action is removed by etching the surface with a suitable reagent (for example , a 5% solution of nitric acid in alcohol known as nital is often used to etch carbon steel), thereby revealing the grains. A photomicrograph is photograph taken through a microscope, in this case, a metallurgical microscope known as a metallograph. A photomicrograph of almost pure iron or ferrite shows the microstructure as revealed by properly preparing and etching the specimen. Grain boundaries generally show up as black lines. Dark areas in the photomiacrograph are caused by di