Lecture 4b (alloys, Heat Treatment)

IE 21: INDUSTRIAL MATERIALS AND PROCESSES Lecture 4b: Alloys; Heat Treatment MMT 1st Sem/AY ’07-’08

 Quenching

oil: mild quenching  water: cheap fairly good vaporizes easily forms 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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media

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

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 matrix Three 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 stresscorrosion 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  Decarburizationphenomenon 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  parts

furnace

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/elec tricallyheatedbox(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/durchlaufofe n.jpg  http://www.ukcar.com/features/tech/pictures/turbine_titanium.jpg  http://www.reidsteel.com/images/reid_images_large/bridges/steelgurders.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