IE 21: INDUSTRIAL MATERIALS AND PROCESSES Lecture 4b: Alloys; Heat Treatment MMT 1 st Sem/AY ’07-’08
IE 21: INDUSTRIAL MATERIALS AND PROCESSESLecture 4b: Alloys; Heat Treatment
MMT
1st Sem/AY ’07-’08
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.
ANNEALING
2) Annealing - used to reduce hardness, alter toughness, ductility, or other mechanical or electrical properties
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.
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.
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.
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.
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
-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
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
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:
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
HEAT TREATMENT FURNACES
batch furnace insulated chamber heating system access door
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
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
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