FORGING

FORGING

Forging is the controlled plastic deformation of piece of metal into a useful shape, usually at an elevated temperature. Pressure or repeated press strokes may be used. Forging can carry out at room temperature (cold forging) or at elevated temperatures (warm or hot forging) depending on the homologous temperature.

Forged parts have good strength and toughness, and are reliable for highly stressed and critical applications.

CLASSIFICATION OF FORGINGOPEN DIE FORGING

CLOSED DIE FORGING

IMPRESSION DIE FORGING

PRECISION FORGINGOpen-die ForgingOpen-die forging is where a solid work piece is placed between two flat dies and reduced in height by compressing itAlso called upsetting or flat die forgingWork piece is deformed uniformly under frictionless conditions Copyright 2010 Pearson Education South Asia Pte Ltd

Open-die ForgingBarreling is caused by frictional forces that oppose the outward flow of the workpiece at the die interfacesMinimized by using an effective lubricantCogging is an open-die forging operation where thickness of a bar is reduced by successive forging steps at specific intervals Copyright 2010 Pearson Education South Asia Pte Ltd

Stages in Open-Die Forging (a) forge hot billet to max diameter(b) fuller: tool to mark step-locations(c) forge right side(d) reverse part, forge left side(e) finish (dimension control)[source:www.scotforge.com]Closed-die ForgingClosed-die ForgingIn true closed-die forging, flash does not form and the workpiece completely fills the die cavityUndersized blanks prevent the complete filling of the die cavityIt is applied to impression die forging with flash generation Copyright 2010 Pearson Education South Asia Pte Ltd

Stages in Impression-die (Closed-Die) Forging [source:Kalpakjian & Schmid]IMPRESSION DIE FORGINGIn impression-die forging, the workpiece takes the shape of the die cavity while being forged between two shaped dies Copyright 2010 Pearson Education South Asia Pte Ltd

PRECISION FORGINGPrecision ForgingIn true closed-die forging, flash does not form and the work piece completely fills the die cavityUndersized blanks prevent the complete filling of the die cavityPrecision forging requires: Special and more complex diesPrecise control of the blanks volume and shapeAccurate positioning of the blank in the die cavity Copyright 2010 Pearson Education South Asia Pte LtdVarious Forging OperationsCoiningA closed-die forging process used in the minting of coins, medallions and jewelleryMarking parts with letters and numbers can be done rapidly through coining HeadingAlso called upset forgingAn upsetting operation performed on the end of a round rod or wire in order to increase the cross sectionProducts are nails, bolt heads, screws, rivets, and fasteners

Various Forging OperationsPiercingA process of indenting the surface of a workpiece with a punch in order to produce a cavity or an impressionPiercing force depends on: Cross-sectional area and the tip geometry of the punchStrength of the materialMagnitude of friction at the sliding interfacesVarious Forging OperationsHubbingProcess consists of pressing a hardened punch with a tip geometry into the surface of a block of metalHubbing force can be estimated from 3(UTS)(A)UTS is obtained from Table 2.2 and A is the projected area of the impressionOrbital ForgingUpper die moves along an orbital path and forms the part incrementallyOperation is quiet, and parts is formed within 10 to 20 cycles of the orbiting dieVarious Forging OperationsIncremental ForgingIn this process, a tool forges a blank into a shape in several small stepsSimilar to cogging where the die penetrates the blank to different depths along the surfaceIsothermal ForgingKnown as hot-die forging process where it heats the dies to the same temperature as workpieceComplex parts with good dimensional accuracy can be produceVarious Forging OperationsRotary SwagingA solid rod or tube is subjected to radial impact forces by a set of reciprocating dies of the machine

Tube SwagingThe internal diameter and/or the thickness of the tube is reduced with or without the use of internal mandrels

Forgeability of MetalsForgeability is defined as the capability of a material to undergo deformation without cracking2 simple tests: Upsetting test- greater the deformation prior to cracking, the greater the forgeability of the metalHot-twist test- maximum number of turns occurs then becomes the forging temperature for maximum forgeability Forging DefectsForging DefectsWhen there is an insufficient volume of material, the web will buckle and develop lapsIf the web is too thick, excess material flows will develops internal cracksForging DefectsForging DefectsInternal defects may develop from Non uniform deformation of the material in the die cavityTemperature gradients throughout the work piece during forgingMicro structural changes caused by phase transformationsForging defects can cause fatigue failuresTYPICAL CHARACTERISTICSGrain flow-Controlled grain structure is the primary benefit of the forging process. With proper design, it is possible to align grain flow with directions of the principal stresses that will occur when the part is loaded in service.

TYPICAL CHARACTERISTICSForging helps ensure structural integrity from piece to piece. Internal pockets, voids, inclusions, laps, and similar flaws are easier to avoid by good forging quality control than they are in castings.Strength, ductility, and impact resistance along the grain are significantly higher than they would be in the randomly oriented crystals of cast metal or weld metal.

APPLICATIONSBecause of high-strength and light-weight requirements , makers of aircraft engines and structures, along with other aerospace manufacturers, are the most significant users of forgings on a value basis.Moving parts are forged to reduce inertia forces, and parts that must be supported by other structures are forged to reduce overall weight and complexity.Decorative parts, even when stressed very lightly, may be produced from forgings to reduce scrap losses and ensure a plate able surface, since forged or machined surfaces of forgings can be polished and plated without revealing blemishes or other internal flaws.

APPLICATIONSParts whose failure would cause injury or expensive damage are forged for safety.Some typical forging applications are the following: landing gear parts for aircraft, automotive connecting rods, universal joints, crankshafts, off-highway and farm equipment parts, plumbing valves, tees, elbows, ordinance components, railroad wheels, axles, gears, oil-field machinery components, turbine disks and blades, and bearing assemblies.

Forging NomenclatureRib -Any wall filled by flow parallel to die motion is a rib.Boss-A projection is called a boss when it is filled parallel to die motion.Recess-A recess is a small web area surrounded by thicker metal.

Forging NomenclatureWeb - The wall filled by generally horizontal flow, perpendicular to die motion and parallel to the parting plane, is the web.

Flash- To be sure that the die cavities will fill completely, excess metal is usually provided. As the die halves come together, the excess is extruded into a gutter at the parting line, producing a part with a fringe of flash metal around it. This flash is trimmed off in a separate operation.

Die materials

Required properties

Thermal shock resistance Thermal fatigue resistance High temperature strength High wear resistance HIgh toughness and ductility High hardenability High dimensional stability during hardening High machinability

Die materials: alloyed steels (with Cr, Mo, W, V), tool steels, cast steels or castiron. (Heat treatments such are nitriding or chromium plating are required toimprove die life)Die materials

Carbon steels with 0.7-0.85% C are appropriate for small tools and flat impressions.

2) Medium-alloyed tool steels for hammer dies.

3) Highly alloyed steels for high temperature resistant dies used in presses and horizontal forging machines.Die DesignDesign of forging dies includes shape and complexity of the workpiece, ductility, strength and sensitivity to deformation rate and temperature, and frictional characteristicsWorkpiece intermediate shapes should be planned so that they properly fill the die cavitiesSoftware is available to help predict material flow in forging-die cavitiesDie DesignPreshapingIn a properly pre-shaped workpiece:Material should not flow easily into the flashGrain flow pattern should be favorable for the products strength and reliabilitySliding at the workpiecedie interfaces should be minimized in order to reduce die wearDie DesignDie Design FeaturesThe parting line should locate at the largest cross section of the partFor simple symmetrical shapes, the parting line is a straight line at the center of the forgingFor complex shapes, the line may not lie in a single planeDraft angles are needed to facilitate removal of the part from the dieSelection of the proper radii for corners and fillets is to ensure smooth flow of the metal into the die cavity and improving die lifeLubricationLubricationGreatly influences friction and wearAffects the forces required, die life, and material flows into the die cavities

Die FailuresDie FailuresFailure of dies results inImproper die designDefective die materialImproper finishing operationsOverheating and heat checkingExcessive wearOverloadingImproper alignment MisuseImproper handling of the die Copyright 2010 Pearson Education South Asia Pte Ltd Die FailuresDie FailuresThe proper design of dies and selection of die materials is importantLarge cross sections and clearances of a die is needed to withstand the forcesOverloading of tools and dies can cause premature failure Copyright 2010 Pearson Education South Asia Pte LtdFormiing machiines

Forging MachinesMechanical PressesThey are either the crank or the eccentric typeMechanical presses are stroke limited since speed varies from a maximum to zeroDue to linkage design, very high forces can be applied in this type of pressMechanical presses are preferred for forging parts with high precisionForging MachinesHydraulic PressesOperate at constant speeds and are load limitedHydraulic presses are slower and involve higher initial costs but require less maintenanceTypical Speed Rangers of Forging EquipmentEquipmentm/sHydraulic press0.06-0.3Mechanical press0.06-1.5Screw press0.6-1.2Gravity drop hammer3.6-4.8Power drop hammer3.0-9.0Counterblow hammer4.5-9.0Economics of ForgingDepending on the complexity of the forging, tool and die, costs range from moderate to highCosts are spread out over the number of parts forged with that particular die setThe more expensive the material, the higher the cost of the material relative to the total costSize of forgings also has some effect on costDESIGN RECOMMENDATIONSForging DrawingsParting LineDraftRibs, Bosses, Webs, and RecessesMachining AllowanceRadiiTOLERANCESLength and Width TolerancesDie-Wear TolerancesDie-Closure TolerancesMatch TolerancesStraightness TolerancesFlash-Extension TolerancesDraft AnglesTotal TolerancesRadii

DESIGN RECOMMENDATIONSForging Drawings

Shapes and dimensions of a part as it will be forged, before any machining is done, are shown on this drawing. It is often advisable to use metal-flow simulation software to study blocker and finisher shapes for forgings.The simulation software shows how a metal bar changes shape under the action of the forging press or hammer, predicts total forging loads and tooling stresses, indicates where laps and other defects may form, and describes grain flow patternsforging design should be developed in partnership between the forging user and the forging producer.

DESIGN RECOMMENDATIONSParting LineAs the die halves come together and confine metal in their cavities, their mating surfaces define a parting line around the edges of the forgingThe parting line is indicated on the forging drawing, and determining its location is a critical step in forging design.Ideally, the parting line will lie in one plane perpendicular to the axis of die motionDESIGN RECOMMENDATIONParting line

DESIGN RECOMMENDATIONParting line Depending on the way in which the part will be loaded, it may be desirable to change parting-line location to control grain flow.

DESIGN RECOMMENDATIONDraftDraft is specified as an angle with respect to the die-motion axisDie impressions are tapered so that forgings can be removed from their dies, and forged surfaces that lie generally parallel to die motion are correspondingly tapered. This taper, called draft, also promotes flow into relatively deep die cavities. a standard draft angle will be specified for all affected surfaces on a forgingLow-draft and no-draft forgings can be produced in some metals, such as alluminum and brass . Typical draft angle for some commonly used material

DESIGN RECOMMENDATIONRibs, Bosses, Webs, and RecessesMetal flow is relatively easy to manage when ribs and bosses are not too high and narrow , and it is easiest when the web is relatively thick and uniform in thickness.Correspondingly, forging becomes more difficult when large amounts of metal must be moved out of relatively thin webs into such projections as deep ribs and high bosses.

DESIGN RECOMMENDATIONRibs, Bosses, Webs, and Recesses

DESIGN RECOMMENDATIONRadiiForgings are designed with radii on all their external corners except at the parting line.It would require a sharp internal angle in the die to form a sharp corner on the forging.Fillet radii on a forging correspond to corners in die impressions that metal must round to fill ribs and bosses. DESIGN RECOMMENDATIONMachining Allowance

Design features that promote easy forging add to the metal that must be machined away.Machining allowances or finishing allowances are added to external dimensions and subtracted from internal dimensions. DESIGN RECOMMENDATIONTOLERANCESLength and Width TolerancesDie-Wear TolerancesDie-Closure TolerancesMatch TolerancesStraightness TolerancesFlash-Extension TolerancesDraft AnglesRadii

TOLERANCESLength and Width TolerancesDimensions generally parallel to the parting plane and perpendicular to die motion are subject to length and width tolerances.Length and width tolerances are commonly specified at 0.3 percent of each dimension, rounded off to the next higher 12 mm or 164 in.TOLERANCESDie-Wear TolerancesThese tolerances apply only to dimensions generally parallel to the parting plane and perpendicular to die motion. The corresponding variations parallel to die motion are included in die-closure tolerances.Die-wear tolerances are plus variations of external dimensions and minus variations of internal dimensions. They do not affect center-to-center dimensions. Thus they allow for erosion of die metal and corresponding enlargement of the forged parts.TOLERANCESDie-Closure TolerancesDimensions parallel to die motion between opposite sides of a forging are affected by failure of the two die halves to close precisely. The plus tolerances on such dimensionsThere is no minus tolerance in this category. Effects of die wear on these vertical dimensions are included in the die-closure tolerances. An added tolerance of 0.3 percent

TOLERANCESMatch TolerancesA lateral shift of one die half with respect to the other moves all features on opposite sides of the forging correspondingly.

Straightness TolerancesFor relatively long, thin parts, a typical straightness tolerance is 0.3 percent of length. When this aspect of forging accuracy is critical, forged parts are often straightened in secondary cold operations.TOLERANCESFlash-Extension TolerancesAlthough there are many other possibilities, the most common flash-removal method is by a punching operation in contoured dies. This may produce clean, trimmed edges , but a small bead of flash is allowed. Draft AnglesCommon tolerances on draft angles are 2and 1.TOLERANCESRadiiThe conventional tolerance on all corner and fillet radii is plus or minus one-half the radius. On any corner where metal will be removed later, the plus radius tolerance governs how much metal will be left for producing a sharp corner on the finished part. The minus radius tolerance, which would only limit sharpness of the forged corner, is not enforced