lecture 4b (alloys, heat treatment)
TRANSCRIPT
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IE 21: INDUSTRIAL MATERIALS AND PROCESSESLecture 4b: Alloys; Heat Treatment
MMT
1st Sem/AY ’07-’08
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Quenching mediaoil:
mild quenchingwater:
cheapfairly goodvaporizes easilyforms air bubbles causing soft spots
brine: more severe than water may lead to rusting
hydroxides: very severe quenching
Hardenability - the ability of a steel to develop its maximum hardness when subjected to the normal hardening heating and quenching cycle. A steel is said to have good hardenability when it can be fully hardened with relatively slow cooling.
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ANNEALING
2) Annealing - used to reduce hardness, alter toughness, ductility, or other mechanical or electrical properties
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FULL ANNEALING
- Imposes uniform cooling conditions at all locations which produces identical properties
Steps:
1. Metal is heated.
Hypoeutectoid (<0.77% Carbon): 30-60oC above the A3 line
Hypereutectoid steels (>0.77% Carbon): 30-60oC above the A3 line
2. Temperature is maintained until the material transforms to austenite.
3. Cooled at a rate of 10-30oC per hour until it reaches about 30oC below A1
4. Metal is air cooled to room temperature.
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NORMALIZING
- Cooling is non-uniform, resulting to non-uniform properties
Steps:
1. Metal is heated 60oC above line A1.
2. Held at this temperature until material transforms to austenite.
3. Metal is cooled to room temperature using natural convection.
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PROCESS ANNEAL
-Used to treat low-Carbon Steels (<0.25% Carbon)
-Metal produced is soft enough to enable further cold working without fracturing
Steps:
1. Temperature is raised slightly below A1.
2. Held in this temperature to allow recrystallization of the ferrite phase.
3. Cooled in still air at any rate.
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STRESS-RELIEF ANNEAL
- Reduces residual stress in large castings, welded assemblis and cold-formed parts
Steps:
1. Metals are heated to temperatures below A1.
2. Temperature is held for an extended time
3. Material is slowly cooled.
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SPHEROIDIZATION
- Produces a structure where the cementite is in form of small spheroids dispersed throughout the ferrite matrixThree ways:
1. prolonged heating at a temperature below the A1
then slowly cooling the material
2. cycling between temperatures slightly above and below the A1
3. for high-alloy steels, heating to 750-800oC or higher and holding it for several hours
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-no significant phase transformations like that of steel
-Three purposes:
1. produce a uniform, homogenous structure
2. provide stress relief
3. bring about recrystallization
- process is usually slowly heating the material to moderate temperatures, holding it for a certain time to allow change in desired properties to take place then is slowly cooled
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Stress-relief annealing – reduces tendency for stress-corrosion cracking
Tempering – reduce brittleness, increase ductility and toughness, reduce residual stress
Austempering – provides high ductility and moderately high strength
Martempering – lessens tendency to crack, distort and develop residual stresses during heat treatment
Ausforming – ausformed parts have superior mechanical properties
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EXAMPLE
A 200-mm (8-in) round and 75-mm (3-in) long hot-extrusion die, with a hole of 75 mm, is made of H21 hot-work steel. A typical method for heat treating such a die, suitable for hot extrusion is shown in the following flow chart:
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CASE HARDENING
this process alters the surface properties of a part only, such as improving resistance to surface indentation, fatigue and wear
Decarburization- phenomenon where in alloys containing carbon lose carbon from their surfaces as a result of heat treatment
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HEAT TREATMENT FURNACES
batch furnace insulated chamber heating system access door
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HEAT TREATMENT FURNACES continuous furnace
parts are heat treated continuously through the furnace on conveyors or various designs that use trays, belts, chains and other mechanisms
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DESIGN CONSIDERATIONS Heat treating
Part design Sharp internal or external corners
Quenching method nonuniform cooling
Thickness, holes, grooves, keyways, splines, asymmetrical shapes, Cracking and warping
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Sources E.P. DeGarmo, et. al. Materials and Processes in Manufacturing.
6th Edition, Macmillan Publishing Company, 1984. S. Kalpakjian. Manufacturing Engineering and Technology. 3rd
Edition, Addison-Wesley Publishing Company, 1995. http://www.ul.ie/~walshem/fyp/iron%20carbon5.gif http://www.phys.unsw.edu.au/~jw/photos/brasslogo.jpg http://www.gvtinc.com/images/Continuous_Temper.JPG http://www.thermaldynamix.com/furnacesystems/boxfurnaces/
electricallyheatedbox(batch)furnace.jpg,3 http://www.schumag.de/wm/user_gfx/Ipsen-Kammerofen2.jpg http://www.ipmx.com/html/tour/ebner.jpg http://www.iwu.fraunhofer.de/schaumzentrum/images/
durchlaufofen.jpg http://www.ukcar.com/features/tech/pictures/turbine_titanium.jpg http://www.reidsteel.com/images/reid_images_large/bridges/
steel-gurders.jpg http://www.dastuart.com/graphics/metalworking/quenching.jpg http://www.padamelectronics.com/gifs/annealing-furnace.jpg http://www.mitchel-group.com/images/heattreat.jpg