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Crystalline Structure


Emmrey Reyes

on 11 February 2013

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Transcript of Crystalline Structure

Gamma Iron Crystalline Structure Iron-Carbon Alloys Physical Properties Microstructure Grain The overall arrangement of grain structure, and phases present in a solid metal is called a microstructure. It is largely responsible for the metal’s properties. This change in crystal structure is reversible if cooled slowly Crystal Structures When metal is cooled, the atoms lose their mobility with the loss of heat energy An allotrope is a variant of a substance consisting of only one type of atom. It is a new molecular configuration, with new physical properties. It should not be confused with phase, which is a change in the way molecules relate to each other, not in the way that individual atoms bond together. Ferrite: This phase has a Body Centred Cubic structure which can hold very little carbon; typically 0.0001% at room temperature. It can exist as either: alpha or delta ferrite. Austenite: This phase is only possible in carbon steel at high temperature. It has a Face Centred Cubic structure which can contain up to 2% carbon in solution. Cementite: Unlike Ferrite and Austenite, Cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder Iron. Though when mixed with soft ferrite layers, its average hardness is reduced by a considerable amount. Fe3C Pearlite: A mixture of alternate strips of Ferrite and Cementite in a single grain. A fully pearlitic structure occurs at 0.8% Carbon and is relatively strong and ductile. Alpha Iron & Delta Iron As metal reverts to its solid state, the atoms attract together in definite patterns of symmetrical arrangement called the lattice system During melting process, the atoms are able to move freely due to the added heat energy When metal is heated to its liquid state, it has no distinct structure or orderly arrangement of atoms Bismuth Common Alloys % Carbon
(by mass) Alloy: A mixture or metallic solid solution composed of two or more elements. In alloys, the crystal structures will change. Depending on the portion of alloys mixed, three types of formation can occur: Substitutional solid solution: The atoms of the metal making up the minor portion of the alloy mixture will randomly replace some of the atoms of the metal making up the majority of the mixture. Interstitial solid solution: The atoms of the minor metal in the alloy mixture are much smaller than those in the major metal. They do not replace the atoms of the major metal but instead locate points between spaces in the lattice. Intermetallic compounds: The minor metal atoms in the alloy cannot completely dissolve by either interstitial or substitutional. This results in the formation of mixed kinds of atomic groupings consisting of different crystalline structure. (Ferrite & Cementite) Alpha Iron
910°C 0% Carbon, pure Iron Room Temp. Gamma Iron
1400°C Delta Iron
1540°C As molten iron cools down, it crystallizes at 1,540 °C into the allotrope Delta Iron, which has a body-centered cubic (BCC) crystal structure similar to Alpha Iron. Liquid State (molten Iron)
<1540°C Mild Steel
(0.2%) Pure Iron A group consisting its own crystalline structure is referred to as a phase; Gamma phase (Austenstine). When molten metal solidifies from its liquid state, it occurs in all directions following a pattern suited with its crystal structure. The areas between the grains are known as grain boundaries. As the metal cools, millions of tiny crystal start to form. The longer the metal takes to cool, the larger the crystal will grow. The resultant crystals formed during the process form the grains in the solid metal. Within each grain, the individual atoms form a crystalline lattice. Due to the attractive forces of the atoms, this gives the metal an elastic property To permanently change (deform) the metal, it must be cold worked by forging, stamping, or rolling. The defects or dislocations formed in the grain structure weakens the metal structure and creates movement which causes the metal to lose shape. Strong materials can be formed by slowing down and stopping the movement of the defects by means of cold work. Defect The microstructure is affected by the composition or alloy content and other factors such as cold working, straining, heat treating and etc. The properties of the weld is greatly influenced by the welding process during the formation of the microstructure Strain Hardening Cold Rolling:
The grains in the metal become elongated. This results in a permanent deformation increasing the metal's strength but also its brittleness Annealing:
When metal becomes too difficult to work, annealing can be used to induce ductility and relieve internal stress By: Emmrey and Brandon 0.8% 2% 6.7% Pearlite
White -> Ferrite
Black -> Cementite Ledeburite: A mixture of Austenite and Cementite that occurs between 2.06% and 6.67% Carbon. Metals in a solid state possess these uniform arrangements called crystals. *All metals are crystalline solids made of atoms arranged in a specific uniform manner. Iron undergoes a process known as allotropic change when it's heated, acquiring both face-centered and body-centered structures at different temperatures Allotropes of Iron Microstructure of a weld used in stainless steel Microstructure of the base metal Welding Applications When a weld is made, the following factors occur;
change of temperature
growth of dimensions
phase transformations The rate of cooling is of primary importance which is controlled by the process, procedure, type of metal and its mass
Ex. Electroslag welding has the lowest cooling rate among welding methods, while the gas metal arc has a much faster cooling rate.
Different cooling rates also occur resulting in
different microstructures.
(d) Completely liquid state
(c) Full Austenite transformation
(b) Pearlite transformed to Austenite
(a) Mixture of ferrite and pearlite grains When weld metal is deposited on the base metal, some of the base metal melts and mixes with the weld metal, producing a dilution of metal. If the two welded metals are non-identical, it will cause a variation of cooling rates and composition resulting in the variation of microstructures
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