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UNIT 3: Metal Alloys Unit 3 Copyright © 2012 MDIS. All rights reserved. 1 Manufacturing Engineering
OBJECTIVES By the end of the lecture, you will be able to: -Understand the Phase transformation diagrams Interpret TTT curves for steel Co relate the heat treatment process to steel properties 2Unit 3 Copyright © 2012 MDIS. All rights reserved.
Metals and Alloys Metals- a vast group of elements that have the ability to form a crystalline structure ;conductivity by virtue of the availability of a vast amount of free electrons. Metals often when freshly exposed are shining, conductive and in general hard. However, some metallic materials are soft and even liquid at room temperature (mercury) or soft and plough able like play dough (gallium) Alloy - Metallic properties can be greatly changed by manipulating the crystalline structure of the metals. This is done by either introducing an interstitial atom unlike the surrounding metals atoms, or by creating new chemical bonding with base atoms 3Unit 3 Copyright © 2012 MDIS. All rights reserved.
Structure of Alloys Iron is the base -alloys is based on the carbon and alloying elements used Steels, stainless steels, too and die steels, cast irons, and cast steels are some examples. A typical passenger vehicle has about 816kg of steel, at 55% of its weight Carbon and alloy steels Low carbon steel(0-0.3%) Medium carbon steel (0.3-0.6%) High carbon steels(more than 0.6%) 4Unit 3 Copyright © 2012 MDIS. All rights reserved.
Low carbon steels are used in hardware nuts bolts etc. Medium carbon steels are used in machine elements that require higher strength High carbon steels are used in hardness applications, like springs, wires, cutlery etc. Dual Phase steels grades of steel have a mixed ferrite and martensite structure. They have high work hardening, ductility and formability. Micro alloyed steels superior properties and eliminate the need for heat treatment- typically contain 0.5% C, 0.8% Mn and 0.1% V. 5Unit 3 Copyright © 2012 MDIS. All rights reserved.
Stainless Steels primarily corrosion resistant, high strength, and ductile with high Cr content Chromium oxide coat that protects the rest of the material in presence of Oxygen (passivation) As carbon content rises, their corrosion resistance drops- carbon bonds with the chromium preventing the passivation Austenitic steels (200 and 300 series)- Cr, Ni and Mn in Iron, they are non magnetic Ferritic Steels ( 400 series)- Cr content upto 27%. These are magnetic Martensitic Steels (400 and 500 series)- do not contain Ni but have upto 18% Cr. These are also magnetic, good ductility, igh strength at elevated temperature, but lower corrosion resistanc 6Unit 3 Copyright © 2012 MDIS. All rights reserved.
Precipitation Hardening steels- contain Cr, Ni and copper, aluminium, titanium or molybdenum. They have good strength at elevated temperature, corrosion resistance and ductility Duplex structure steels- a mixture of austenite and ferrite, higher strength and corrosion resistance, and resistance to stress-corrosion cracking 7Unit 3 Copyright © 2012 MDIS. All rights reserved.
Iron Carbon (Fe-C) diagram A phase diagram shows the user the temperature and related phase equilibrium. Steels and cast irons are represented by the Iron carbon binary systems concerned with the idea of cause and effect. Iron-iron carbide phase diagram can be extended to 100%C (pure graphite) range of interest to engineering is in the 0- 6.67% C Pure iron melts at 1537C 8Unit 3 Copyright © 2012 MDIS. All rights reserved.
Ferrite: (alpha ferrite) is a solid solution of BCC iron with 0.022% C solubility at 727C. Delta ferrite is stably only at very high temperatures and has no practical significance. Ferrite is relatively soft and ductile, it is magnetic right up to 768C Significant amounts of Cr, Mn, Ni, Mb, Si etc can be contained in the iron in solid solution Austenite: polymorphic transformation from BCC to FCC makes ferrite into gamma iron with solid solubility of up to 2.11% C at 1148C It is denser than ferite, ductile at elevated temperatures and has good formability. 9Unit 3 Copyright © 2012 MDIS. All rights reserved.
Cementite The far right boundary of the Fe-C diagram is the cementite region at 6.67% C we have the highest concentration that the iron crystals can hold. Iron carbide is formed at these higher concentrations. It is hard and brittle inter-metallic compounds. As with austenite, it can also contain other alloying elements in steel. 10Unit 3 Copyright © 2012 MDIS. All rights reserved.
Main microstructure Pearlite-Ferrite and Cementite in laminae are thin and closely packed fine pearlite, They both differ in the rate of cooling from the eutectoid (transform from ferrite to pearlite) range. High cooling gives fine and slow cooling gives coarse structure. Spheroidite pearlite is heated just below the eutectoid and then held at that temperature for a period of time the cementite convert to spheroidite. 11Unit 3 Copyright © 2012 MDIS. All rights reserved.
are less prone to stress concentration and lend higher toughness, lower hardness than pearlite. This form in alloy steels makes cold forming possible without cracking. Bainite: It is a very fine ferrite and cementite microsturcture visible in a microscope. If steel is cooled with alloying elements at higher cooling rates we obtain bainite. I t is stronger and more ductile than pearlitic steels at the same hardness. Martensite: A BCC tetragonal structure obtained at high cooling by quenching Due to lower slip systems it lacks toughness is extremely hard and brittle. 12Unit 3 Copyright © 2012 MDIS. All rights reserved.
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