![]() Upon heating pure Iron experiences two changes in crystal structure. If grain 4 is rotated clockwise to cause its ‘A’ planes to line up with the ‘A’ planes of grain 3, then the grain boundary gets removed and the two grains become one larger grain. The ‘A’ planes in grain 4 make a much steeper angle with the horizontal than do the ‘A’ planes of grain 3. The boundary is a planar interface, generally curved, along which two grains of different orientation intersect. The nature of a grain boundary is shown in Fig 1. Iron with a bcc structure is called ferrite. In the Fig 1, the crystal lattice can be envisioned as three sets of intersecting planes of atoms, with each plane set parallel to one face of the cube. This crystal structure is called a body-centered cubic (bcc) structure, and the geometric arrangement of atoms is often called a bcc lattice. In iron at room temperature, the cubes have an atom at each of the eight corners and one atom right in the middle of the cube. As shown in Fig 1, if lines are drawn connecting the centres of the atoms, a three-dimensional array of little cubes stacked together to fill space is generated. The grains shown in Fig 1 are called crystals and they are made up of atoms, all of the atoms are uniformly arranged in layers. In pure iron the basic building blocks are the individual atoms of iron (Fe) atoms. For a comparison, the thickness of aluminum foil and the diameter of a hair are both approximately 50 microns.įig 1 Microstructure of pure iron showing grains, grain boundary and crystal structure Although a small number, this grain size is much larger than the grain size of most commercial irons. The average grain diameter for this sample has been measured to be 125 microns. In the figure at the 100× magnification, a length of 200 microns (1 micron is 0.001 mm and is also known as micrometer) is shown by the arrow so labeled. The average size of the grains is quite small. The individual regions, such as those numbered 1 to 5, are called iron grains, and the boundaries between them, such as that between grains 4 and 5 highlighted with an arrow, are called grain boundaries. If the sample is now viewed in an optical microscope at a magnification of 100 ×, it is found to have the appearance as shown in Fig 1. This process of etching causes the shiny surface to become a dull color. The shiny disk is now immersed for around 20 to 30 seconds in a mixture of 2 % to 5 % nitric acid (HNO3) with methyl alcohol, a mixture often called nital (‘nit’ for the acid and ‘al’ for the alcohol). A face of the disk is now polished on polishing wheels, starting first with a coarse grit polish and proceeding in steps with ever finer grits until the face has the appearance of a shiny mirror. A bar of pure iron (e.g., 25 mm in diameter) is sectioned to form a thin disk in the shape of a quarter. The metallurgical nature of solid pure iron can be studied from the experiment as described here. Compounds of iron with valency +2 are known as ferrous compounds while the compounds of iron with valency +3 are known as ferric compounds. Its other properties include easy corrosion in the presence of moist air and high temperatures. It conducts heat and electricity and is very easy to magnetize. Pure iron can be hammered into sheets and drawn into wires. It is easy to work and shape and it is just soft enough to cut through (with quite a bit of difficulty) using a knife. Its most important property is that it is very soft. Pure iron is silvery white colored metal and is extremely lustrous. This shows how less the impurities are in the pure iron. Wrought iron has a higher carbon content of up to 0.5 %. The carbon content in pure iron is always less than 0.008 %. The carbon content of pure iron makes it unique and different from the other metals and ferrous alloys. Pure iron is a common metal but it is mostly confused with other metals such as steel and wrought iron. It is the fourth most common element and the second most common metal in the earth crust. It is by mass the most common element on the earth, forming much of earth’s outer and inner core. It is a metal in the first transition series. It has a melting point of 1538 deg C and boiling point of 2862 deg C. Its atomic number is 26 and atomic mass is 55.85. Iron is a chemical element with symbol Fe (from Latin word Ferrum). alpha iron, austenite, delta iron, Ferrite, gamma iron, Pure iron, steel, wrought iron ,.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |