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TOPIC 3METAL WORK, CASTING PROCESS AND HEAT TREATMENT ON STEELPREPARED BY:AZMARINI BINTI AHMAD NAZRIPENSYARAH POLITEKNIK UNGKU OMAR

METAL WORK

Mechanical working process also known as metal forming processes, are primary shaping processes in which a mass of a metal or alloy is subjected to mechanical force.Under the shape of such forces, the shape and size of metal piece undergo a change.By mechanical processes, the given shape and size of a machine part can be achieved with great economy in material and time.Metal forming is possible in case of such metals or alloys are sufficiently malleable and ductile.Frequently, work piece of material is not sufficiently malleable and ductile ordinary at the room temperature , but may become so when heated.Thus we have both hot and cold metal forming operation.

Advantages of Mechanical Working Processes Apart from higher productivity, mechanical working processes have certain other advantages over other manufacturing processes. These are enumerated below:improves the mechanical properties of material like ultimate tensile strength, wear resistance, hardness and yield point while its lowers ductility.Grain flow lines being developed in the part being mechanically worked. The grain flow improves the strength against fracture when the part is in actual use. For example is crankshaft.

Comparison of grain flowDifference Between Cold and Hot Working

Cold working may be defined as plastic deformation of metals and alloys at a temperature below the recrystallization temperature for that metal and alloy. When this happens, then the strain hardening which occurs as a result of mechanical working does not get relieved. In fact as the metal or alloy get progressively strain hardened, more force is required to cause further plastic deformation. After sometime, if the effect of strain hardening is not removed, the forces applied to cause plastic deformation may in fact cause cracking and failure of material.

Hot working can be explained as plastic deformation of metals and alloys at such a temperature at which recovery and recrystallization take place simultaneously with the strain hardening. Such a temperature is a above recrystallization temperature. Properly done hot working will leave the metal or alloy in a fine grained recrystallization structure.Advantages and Disadvantages of Cold and Hot Working Processes

Since cold working is practically done at room temperature, no oxidation or tarnishing of surface takes place. No scale formation is there, hence there is no material loss. Besides, hot working of steel also results in partial decarburisation of the work piece surface as carbon gets oxidised as CO2.

Cold working results in better dimensional accuracy and a bright surface. Cold rolled steel bars are therefore called bright bars, while those produced by hot rolling process are called black bars (they appear greyish black due to oxidation of surface).

In cold working heavy work hardening occurs which improves the strength and hardness of bars, but it also means that high forces are required for deformation increasing energy consumption. In hot working this is not so.Due to limited ductility at room temperature, production of complex shapes is not possible by cold working processes.

Severe internal stresses are induced in the metal during cold working. If these stresses are not relieved, the component manufactured may fail prematurely in service. In hot working, there are no residual internal stresses and the mechanically worked structure is better than that produced by cold working.

The strength of materials reduces at high temperature. Its malleability and ductility improve at high temperatures. Hence low capacity equipment is required for hot working processes. The forces on the working tools also reduce in case of hot working processes.Sometimes, blow holes and internal porosities are removed by welding action at high temperatures during hot working.

Non-metallic inclusions within the work piece are broken up. Metallic and non-metallic segregations are also reduced or eliminated in hot working as diffusion is promoted at high temperatures making the composition across the entire cross-section more uniformCold WorkRolling (Cold and Hot rolling)

Rolling Process

In this process, metals and alloys are plastically deformed into semi finished and finished products by being pressed between two rolls which are rotating.The metal initially pushed into the space between two rolls, there after once the roll takes a bite into the edge of material, the material gets pulled in by the friction between the surfaces of the rolls and the material.The material is subjected to high compressive forces as it is squeeze (and pulled along) by the rolls.This is the process to deal with material in bulk in which the cross section of material is reduced and its length increased.Rolling is very convenient and economical.

Hot RollingHot rolled steel products look an unattractive greyish-black in colour. Non-ferrous materials also develop a tarnished colouring due to oxidation of outer surface. The surface finish is rough and the finished sizes of hot rolled products are far from satisfactory. However hot rolling is very economical. A great deal of hot rolled black bars and sheets/plates of steel are used in construction industry for fabrication of structuresThinner gauges, better surface finish, tighter size control and bright surfaces are obtained in cold rolling process. These products also develop greater strength and wear resistance due to strain hardening.

Cold Rolling

Rolling DeffectRolling defects divide in to classes:Surface defectsStructural defectsTypes of surface defects:Rusting and scalingSurface scratchesSurface cracksTypes of structural defects:Wavy edges

Zipper cracks

Edge cracks

Center split

Alligatoring

Wire DrawingWire drawing is a simple processIn this process, rods made of steel and non-ferrous metals and alloys are pulled through conical dies having a hole in centre.The included angle of the cone is kept between 8 to 24.As the material is pulled through the cone, it undergoes plastic deformation and it gradually undergoes a reduction in its diameter.At the same time, the length is increased proportionately.

Tube DrawingThe drawing process can also be used for tube drawing.Tube drawing does not mean manufacturing a tube from solid raw material. It means lengthening a tube reducing its diameter.Various arrangements used for tube drawing are shown in figure.

Method (a) is the common used for tube drawing. A conventional tube drawing bench is used.Method (b) employes a floating mandrel which adjust itself to the correct position.

Method (c) uses a long circular rod to control the size of tube-bore.Method (d) uses neither a mandrel nor a bar and controlling size of bar is difficult.Hot WorkingForge and Smith forgeIn forging, metals and alloys are deformed to the specified shapes by application of repeated blows from a hammer.It usually done hot; although sometimes cold forging is also done.The raw material is usually a piece of a round or square cross section slightly larger in volume than the volume of the finished component.Shows the video:

18Forward and inverted extrusionExtrusion is a process in which the metal is subjected to plastic flow by enclosing the metal in a closed chamber in which the only opening provided is through a die.The material is usually treated so that it can undergo plastic deformation at a sufficiently rapid rate and may be squeezed out of the hole in the die. In the process the metal assumes the opening provided in the die and comes out as a long strip with the same cross-section as the die-opening. Incidentally, the metal strip produced will have a longitudinal grain flow.The process of extrusion is most commonly used for the manufacture of solid and hollow sections of nonferrous metals and alloys e.g., aluminium, aluminium-magnesium alloys, magnesium and its alloys, copper, brass and bronzeType of extrusions

Advantages of extrusion:

The complexity and range of parts which can produced

The extrusion process is complete in one pass only.

Large diameter, hollow products, thin walled tubes are easily to produced.

Good surface finish and excellent dimensional and geometrical accuracy.

Forward or direct extrusion processIn this process, the material to be extruded is in the form of a block.It is heated to requisite temperature and then it is transferred inside a chamber as shown in figure.

In the front portion of the chamber, a die with an opening in the shape of the cross-section of the extruded product, is fitted.The block of material is pressed from behind by means of a ram and a follower pad.Since the chamber is closed on all sides, the heated material is forced to squeeze through the die-opening in the form of a long strip of the required cross-section.

Backward or indirect extrusionthe block of heated metal is inserted into the container/chamber.

It is confined on all sides by the container walls except in front; where a ram with the die presses upon the material.As the ram presses backwards, the material has to flow forwards through the opening in the die.The ram is made hollow so that the bar of extruded metal may pass through it unhindered.Different between forward and backward extrusion.

CASTING PROCESSManufacture of a machine part by heating a metal or alloy above its melting point and pouring the liquid metal/alloy in a cavity approximately of same shape and size as the machine part is called casting process.After the liquid metal cools and solidifies, it acquires the shape and size of the cavity and resembles the finished product required.The manufacture of a casting requires:(a) Preparation of a pattern,(b) Preparation of a mould with the help of the pattern,(c) Melting of metal or alloy in a furnace,(d) Pouring of molten metal into mould cavity,(e) Breaking the mould to retrieve the casting,(f) Cleaning the casting and cutting off risers, runners etc., (this operation is called fettling), and(g) Inspection of casting.

Cast are made in large number of metals and alloys, both ferrous and non-ferrous.Steel casting used for components subject to higher stressesBronze and brass casting used in ship and marine environmentAluminium and aluminium-magnesiumcasting used in automobileStainless steel casting used for making cutlery itemsAdvantages 1) economical to produce component 2) improve properties in certain part 3) very little metal is wastedDisadvantage less strong as compared the wrought component (forging)

Types of casting process:

Sand castingLost wax/Investment casting

sandMeltingOf metalfurnacesProduction steps in the sand casting production

Sand Casting Process

Schematic illustration of sand mold

Lost wax/investment casting processDie castingHot die casting

Cold die casting

HEAT TREATMENT

Heat treatment is a combination of heating and cooling operations, timed and applied to a metal or alloy in the solid state in a way that will produce desired propertiesThe first step in the heat treatment of steel is to heat the material to some temperature in or above the critical range in order to form austenite.Highly stressed materials produced by cold work should be heated more slowly than stress-free materials to avoid distortion.The types of heat treatmentfull annealingStress relief annealingSpheroidizingNormalizing

Full Annealingheat treating process used to soften previously cold-worked metal by allowing it to recrystallize.The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Ordinarily, annealing is carried out to: relieve stresses (often introduced when cold-working the part) (2) increase softness, ductility and toughness(3) produce a desired microstructureAny annealing process consists of three stages:1. heating to the desired temperature2. holding or soaking at that temperature3. slowly cooling, usually to room temperatureTime is the important parameter in these procedures. Process annealing is a heat treatment that is used to negate the effects of cold work that is to soften and increase the ductility of a previously strain-hardened metal.

Stress Relief Annealingis an annealing process that is utilized when internal residual stresses develop in metal pieces in response to such things as cold working.Failure to remove these internal stresses may result in distortion and warping.The internal stresses are relieved by bond relaxation as a result of heating.A stress relief is carried out by heating the piece to a recommended temperature, holding the work piece at temperature long enough to attain a uniform temperature throughout the part, and finally cooling to room temperature in air.Stress relieving can eliminate some internal stresses without significantly altering the structure of the material.

SpheroidizingAn annealed hypereutectoid steel with a microstructure of pearlite and a cementite network will generally give poor machinability.Since cementite is hard and brittle, the cutting tool cannot cut through these plates.A heat-treating process which will improve the machinability is known as spheroidize annealing.This process will produce a spheroidal or globular form of carbide in a ferritic matrix.Medium and high carbon steels having a microstructure containing coarse pearlite may still be too hard to conveniently machine or plastically deform.These steels (and in fact, any steel) may be annealed to develop the spheroidite structure.

Normalizingis an annealing heat treatment used to refine the grains and produce a more uniform and desirable size distribution.Normalizing is accomplished by heating the material to a temperature above its upper critical temperature (e.g.: just above 1333F for most fastener-related steels).After sufficient time has been allowed for the alloy to completely transform to austenite, the metal is allowed to cool in the air.