Annealing Processes –All the structural changes obtained by hardening and tempering may be...
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Annealing Processes
– All the structural changes obtained by hardening and tempering may be eliminated by annealing.
• to relieve stresses
• to increase softness, ductility, and toughness
• to produce a specific microstructure
– Process consists of
• heating to the desired temperature
• holding
• cooling to room temperature
– annealing time must be long enough to allow for any necessary transformation reactions
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– Normalizing - used to refine the grains
• cooling in air, less expensive, some sections of a part may cool too fast
– Full anneal: Utilized in low and medium carbon steels that will be machined or plastically deformed
• cooling in furnace to room temperature
• final product is coarse perlite (soft and ductile)
– Spheroidizing
• for medium and high carbon steels
• Fe3C will turn into the spheroids
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– Some parts should be hard on the surface but soft and ductile inside
• shafts, gears, guideways of machine tools
• carious surface hardening processes
• heating on the surface only and
• quenching it
• flame hardening (by torch)
• induction hardening
• carburizing
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Precipitation Hardening
• Hardening of non-allotropic alloys– The properties of nonferrous substances that do not readily form
allotropes cannot be changed by controlled cooling.
– Such substances are known as non-allotropic alloys, and include aluminum, copper, and magnesium alloys as well as stainless steels containing Ni.
– The method of hardening is called precipitation hardening and age hardening.
– Precipitation involves the formation of a new crystalline structure through the application of controlled quenching and tempering. Precipitation disperses hard particles throughout the existing more ductile material.
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– These particles disrupt long dislocation planes of the material, restricting the movement of dislocations and increasing the strength and stiffness of the alloy.
– The final step is to hold the material at a specific temperature for a given amount of time.
– When aluminum is quenched in water to room temperature, the solubility of copper is drastically decreases and a compound of copper aluminide forms that slowly disperses along grain boundaries and slip planes to harden the alloy.
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• Summary– primarily interest to increase YS
– it means increase the load-carrying capacity of elements
– the methods to increase YS are based on the various mechanisms of interfering with dislocation movements.
– Methods:
• Obtaining fine grain material
– combination of hot working and recrystallization
– annealing after cold work
• Cold work (strain hardening)
– the total amount of strain is limited by a total loss of ductility
• Solid solution treatment
– for alloys in which a solute-rich solid-solution phase exists at room temperature
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• Precipitation hardening
– for alloys in which substantial solid solubility exists at an elevated temperature
– followed by quenching it yields a supersaturated solution that releases fine, coherent precipitate
• Allotropic hardening
– only for steels
– heating up to austenite and quenching leads to a martensitic structure (very hard and brittle)
– tempering is followed in order to get some ductility
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Engineering Metals
– This section describes the most common metallic alloys, their properties, and their usage. The alloys considered are steels, cast irons, alloys of aluminum, copper, titanium, and Ni- and Co-based superalloys.
– The production volumes in tons per year in the United States of the individual classes of metals are given below:
• Steels and cast irons: 100 million• Aluminum alloys: 36 million• Copper alloys: 1 million• Ni-based alloys: <100,000
– The significance of the individual classes of metals is not entirely expresses by these tonnages. For example, the Ni-based alloys are extremely important for gas turbines and jet engines; without them modern aircraft would not exist, and there would not be such a great need for aluminum alloys.
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• Steels:– Carbon Steels
• no alloying elements other than C
• impurities: Si, S, and P
– adverse effect on ductility and toughness
• Mn improves hardenability
• low carbon steels (nonhardenable) with less than 0.2% C
– thin sheet steel for car bodies, appliances, and for sidings of houses, heavy steel plates for ships and tanks, for the structures and frames of heavy machinery
• hardenable steels
– medium carbon steels, with more than 0.3% C
– high carbon steels
• hot rolled carbon steels
• cold rolled carbon steels
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• Low alloy steels
– almost always used in a heat treated (quenched and tempered) state
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• High alloy steels
– contain over 5% of alloying elements
– austenite could be present at room temperature, Ni widens the field, Cr narrows
– Two classes
» stainless steels
» tool steel
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• Tools steels:– Each grade of tool steel is designed for a specific purpose, and as
such, there are few generalizations that can be made about tool steel. Each tool steel exhibits its own blend of the three main performance criteria: toughness, wear resistance, and hot hardness.
– Some of the few generalizations possible are listed here:
• An increase in carbon content increases wear resistance and reduces toughness.
• An increase in wear resistance reduces toughness.
• Hot hardness is independent of toughness.
• Hot hardness is independent of carbon content.
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• Aluminum alloys
– pure Al is light, about three times lighter than iron
– excellent electric conductor
– excellent ductility
– good resistance to corrosion
– Tm = 660 degree C
– could be strengthened by cold work
– used from electric wire to extruded structural shapes for housing construction
– it is usually used as an alloy
– alloying elements: Si, Cu, Mn, and Mg
– above 3% of Si improves fluidity
– above 12% Si improves hardness and wear resistance
– Cu improves the age hardenability, it is a primary element in achieving high mechanical strength in aluminum alloys at elevated temperatures.
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• Magnesium
– its density is 2/3 that of Al
– Tm = 650 degree C
– is alloyed with Al, Zn, Mn, Ce, and Ag
– main use in the aerospace industry
– UTS below 350 MPa
– it is difficult to cold form
– extrusion, forging, and deep drawing possible on higher temperature