final reportwithout bracket

7/31/2019 Final Reportwithout Bracket

1/26

Chapter 7

HEAT TREATMENT

Heat Treatment the operation of heating and cooling a metal in itssolid state to change its physical properties.

IRON-IRON CARBIDE DIAGRAM

Critical Points structural changes occurAc (c- chauffage- to heat)Ar (r refroidissement- to cool)

Allotropic Change reversible changes in the atomic structure with acorresponding change in properties

Austenite solid solution of carbon in gamma iron

Ferrite solid solution of carbon in alpha iron

0.20% carbon-ferrite is rejected from austenite until no free ferrite isrejected (0.80% carbon)

Eutectoid steel (100% pearlite)

Eutectoid Point lowest temperature at which changes occur in a solidsolution

Acm Line (carbon content > eutectoid) Fe3C is rejected from austeniteinstead of ferrite

Fe3C (iron carbide, cementite) extremely hard and brittle

Hypoeutectoid:

8.0

%%

CPe =

Hypereutectoid:

]87.5

)%67.6(1[100%

CCe

=

GRAIN SIZE

Coarse-grained steels

less tough

greater tendency for distortion

better machinability

greater depth-hardening power


2/26

Fine-grained steels

tough

more ductile

less tendency for distortion or crack

Aluminum (as deoxidizer) raises the temperature at which rapid graingrowth occurs

Low-carbon steel (heated)

no change in grain size up to Ac1 pt.

Ac3 , minimum grain size

Further heating increases austenitic grain size.

Quenching from Ac3 fine grainSlow cooling/quenching from high temperature coarse grain

Final grain size depends on prior austenitic grain size.

Coarsening Temperature

rapid grain size increase occurs

not immediately above Ac

*medium-carbon steels*alloy steels*steels deoxidized with Aluminum

ISOTHERMAL TRANSFORMATION DIAGRAMS

Cooling rate, time grain structure, structure obtainable when thequench is interrupted at certain elevated temperature

Shape depends on carbon content, alloys present, austenitic grain size

1. HARDENING - The process of heating a piece of steel to atemperature within or above its critical range and then cooling itrapidly

Determination of proper heating temperature:

- If the carbon content of steel is known: reference to the phasediagram

- If not: heat-quenching of small specimens of the steel atvarious temperatures; results observed by hardness testing orby microscopic examination


3/26

Slow rate of heating is necessary to obtain a uniform temperaturethroughout the structure, especially for irregularly shaped andthick steels.

Hardness depends on:

1. Quenching rate

Rapid-quenching is necessary to obtain a hard structure(martensite)

Different quenching mediums:

Water bath - for low/medium plain-carbonsteels

Oil - for high carbon and alloy steel (not as

severe as water)

brine/water spray - for extreme cooling

air-cooling - for certain alloys

2. Carbon content

Hardness obtainable increases as carbon contentincreases up to 0.60% (above this, only slightly)

Low-carbon steel will not respond appreciably tohardening treatments

Pearlite responds best to heat treatment.

3. Work size

As size increases, surface hardness decreases.

If the heat inside a large piece cannot escape fasterthat a certain critical rate, there is a definite limit tothe inside hardness.

Hardenability of steel

- refers to the response of a metal to a quenching operation

Jominy end-quench test comparing depth-hardenabilityof different steels.

1. A normalized specimen of the steel is machined to adiameter of 1 inch (12.7mm) and a length of 4 inches


4/26

(100mm) and then heated to its austenizingtemperature.

2. It is quickly placed in the quenching fixture held0.5inch (12.7mm) above a inch diameter orifice.

3. Water is directed against the bottom surface until theentire specimen is cool.

Alloys increase the hardenability of steel.

Constituents of Hardened steel

Extreme quenching of steel from a high temperaturepreserves some of the austenite (half as hard as martensite)at ordinary temperature.

If hypoeutectoid steel is cooled slowly, austenite istransformed into ferrite and pearlite (soft and ductile).

- Faster cooling (harder and less ductile)

- Rapid cooling martensite (hardest)

Martensite

- Hardness depends on carbon content

- Rockwell C45 to C67

-

Cannot be machined, quite brittle, strongly magnetic

Fine pearlite

- Steel is quenched at slightly less than the critical rate

- Softer than austenite

- Quite tough and capable of resisting considerableimpact.

Maximum Hardness of Steel

Depends on carbon content

Carbon must be completely in solution in the austenite whenquenched


5/26

Critical quenching rate must be used Austenite must not beretained in high percent

2. TEMPERING

- Reheating quench-hardened steel below critical range followed byany rate of cooling

- Reduction of hardness and brittleness to the desired point forservice conditions (hardened steels are brittle and not suitable formost uses).

- reduce tensile strength

- increase ductility and toughness

Special processes:

1. Austempering - interrupted quenching process- converts austenite to bainite

2. Martempering - minimize distortion, cracking and

internal stresses that result from quenchingin oil or water

3. ANNEALING

- To soften steel so that it may be machined or cold-worked

- Also relieves internal stresses previously set up in the structure

Full annealing wipes out all traces of previous structure, refines thecrystalline structure, softens steel.

Heating rate

- size is considered

- Uniform throughout


6/26

- 45 min for each inch (25mm)of thickness

Isothermal Annealing - short annealing cycle; gives pearlite amore uniform structure

Process Annealing - results in the usual pearlitic structure

- used to relieve stress in a cold-worked carbon steel

4. NORMALIZING

- Heating about 50 to 100F (10 - 40C) above the uppercritical range and cooling in still air to room temperature

- Rate of cooling is faster than that of annealing but slower thanthat of quenching

- More uniform grain structure

- For high residual stresses (induced by forging, casting,machining, forming or welding)

- For a better response to heat treatment

- For dimensional control

- For parts subjected to impact

5. SPHEROIDIZING


7/26

- Spheroidite forms when carbon steel is heated to approximately700 C for over 30 hours.

- The result is a structure of rods or spheres of cementite withinprimary structure.

-

to soften higher carbon steels and allow more formability

Spheroidite - the softest and most ductile form of steel

6. STRESS RELIEVING- Uniform heating below the lower transformation temperature and then

cooling uniformly- Applied to cold-worked, formed, machined, flame-cut, weld-fabricated

parts to reduce residual stress

Softer and more ductile (precipitation of iron carbide)

ADVANTAGES AND DISADVANTAGES OF HOT AND COLDWORKING OF METAL

-Advantages-HOT WORKING OF METAL COLD WORKING OF METAL

Porosity is eliminated (mostingots contain many smallblow holes; pressed andeliminated).

Cold-finished products aremore commerciallyacceptable.

Impurities are broken upand distributed throughout.

Accurate dimensionalcontrol

Coarse and columnargrains are refined.

No oxidation for a smoothsurface

Ductility increased Hardness increased (formetals that do not respondto heat treatment)

Resistance to impactincreased

Close dimensionaltolerance can bemaintained

Strength increased Rapid production

Greater homogeneity -

Less deformation forces -


8/26

CHAPTER 12HOT WORKING OF METAL

PLASTIC DEFORMATION An exploit of the metals ability to flow plastically in the solid state

without deterioration of properties.

-Disadvantages- Poor surface finish because

of rapid oxidation (scaling) Higher pressure and

heavier equipment areneeded.

Close dimensionaltolerance cant bemaintained

Brittleness results if themetal is overworked.

Equipment andmaintenance costs arehigh.

Stresses are set up.

- Distortion anddefragmentation

- Loss in ductility


9/26

Above crystallization temperature

Ductility improved (moderate shaping forces)

ROLLING

1. Ingots are rolled into intermediate shapes: blooms, billets and slabs.

2. Blooms, billets and slabs are further rolled into plates, sheets, barstocks, structural shapes or foils.

Definitions:

Ingots a mass of metal

Bloom has a square cross section with a minimum size of 6x 6 in (150 x 150 mm).

Billet square section from 1 in (38.1 mm) - 6in (150mm).

Slab - may be rolled from either an ingot or a bloom

- Rectangular cross sectional area with a minimum widthof 10 in (250mm) and minimum thickness of 1 in(38.1mm).

Process: Ingots remain in molds until solidification is about complete

and the molds are removed.

While still hot, ingots are placed in gas-fired furnaces (soakingpits), until uniform working temperature is attained.

Ingots are taken to the rolling mill to be rolled intointermediate shapes.

Q1 = Q2 A1V1 = A2V2

1

2

2

1

V

V

A

A=


10/26

As the cross-sectional area is decreased the velocity increases, asdo lengths of the material.

Two-high reversing mill

The piece passes through the rolls

Stopped and reversed in direction

Repeated (frequent 90 - turns for uniform section and torefine the metal throughout.)

Advantage:

It has a range of adjustment as to size of piecesand rate of reduction.

Disadvantage:

It is limited by the length that can be rolled and bythe inertia forces that must be overcome for eachreversal made.

Three-high mill

Advantage:

No limitations as in the reversing mill

Less expensive to manufacture

Has a higher output

Disadvantage: Elevating mechanism is required.

FORGING

1. Hammer or Smith Forging

- hammering the heated metal either with hand tools orbetween flat dies in a steam hammer

Hand Forging - oldest form of forging

- inaccurate

- cant make complicated shapes


11/26

Steam Hammer - the force of the blow is closelycontrolled by the operator

- considerable skill is required

2. Drop Forging

-(similar to hammer forging) uses closed impressionrather than flat dies.

Two Principal Types of drop-forging hammers:

1. Steam hammer

Ram and hammer are lifted by steam; the force ofthe blow is controlled by throttling the steam.

300 blows/min.

2. Gravity drop hammer

Impact pressure is developed by the forces of thefalling ram upon the lower fixed die.

Uses air or steam to lift the ram

Permits the preselection of short and long-strokeblows

Operator is relieved of the responsibility ofregulating stroke heights.

Greater uniformity

o Impact forging hammer - has two opposing cylinders in ahorizontal plane which actuate the dies toward each other

Advantages of forging operation:

Fine crystalline structure

Closing of any voids

Reduced machining time

Improved physical properties


12/26

Disadvantages:

Scale inclusions

High cost of dies (prohibits short-run jobs)

Die alignment is difficult to maintain

Die design (should prevent cracks)

3. Press forging - employs a slow squeezing action in deforming theplastic metal (as contrasted to the rapid impact blows of ahammer).

-can exert 500 1000 tons (4 90 MN) force.

Press Capacity:

F = press capacity, tons

P = pressure required, psi (usually about 15000 psi formild steel)

A = area of the forging at the parting line, in2.

Closed-impression dies are used for small press forgings.

Only one stroke of the ram is normally required.

Impact is absorbed by the machine and foundation.

Faster reduction

Cost of operation is lower.

Closer tolerance

4. Upset forging entails gripping a bar of uniform section in diesand applying pressure on the heated end, causing to beupset or formed to shape.


13/26


14/26

- The metal is extruded through the die opening untilonly a small amount remains.

- The metal is sawed off next to the die and the buttend is removed.

Indirect Extrusion

PIPE AND TUBE MANUFACTURING

Butt Welding

- Heated skelp with slightly beveled edges are used.

- One end of the skelp is trimmed to a V shape topermit the entry into the welding bell.

- When the skelp is brought up to welding heat, theend is gripped by tongs that engage a draw chain.

- As the tube is pulled through the welding bell, skelpis formed to a cylindrical shape and the edges arewelded together.

-

The pipe is passed between sizing and finishing rollsfor correct sizing and scale removal.

Continuous butt welding

- Skelp is supplied in coils.

- As the skelp enters the furnace, flamesimpinge on the edges of the strip to bringthem to welding temperatures.

-

Leaving the furnace, the skelp enters aseries of horizontal and vertical rollers thatform it into pipe.

Electric Butt Welding

- Cold forming is necessary prior to the welding operation.


15/26

- The plate is passed through a continuous set of rolls thatprogressively change its shape (roll-forming).

- The welding unit is placed at the end of the roll formingmachine (consisting of three centering and pressure rollsto hold the formed shape in position and two electrode

rolls that supply current to generate heat).

- Sizing and finishing

Lap Welding

- Skelp edges are beveled as it emerges from furnace.

- The skelp is drawn through a forming die or between rolls

to give it cylindrical shape with the edges overlapping.

- After being reheated, the bent skelp is passed betweentwo grooved rolls. (between the rolls is a fixed mandrel tofit the inside diameter of the pipe).

- The edges are lap-welded by pressure between rolls andthe mandrel.

(lap-welded pipes are made in sizes 2 16 in (50 400mm) in diameter)

Piercing (seamless tubing)

- Cylindrical billets are passed between two conical shaped rolls (between the rolls is a fixed point or mandrelthat controls the size of the hole)

- Thick walled tube passes between grooved rolls over aplug held by a mandrel (converted to a longer tube withspecified wall thickness)

- The tube is straightened and sized by passing throughthe reeling machine.

- Final sizing and finishing


16/26

Continuous Piercing:

- A round bar is pierced and conveyed to the mandrel mill, where amandrel is inserted (reduce the tube diameter and wall thickness).

- Mandrel is then removed.

- Tube is reheated.

- Tube enters a stretch reducing mill (reduces wall thickness anddiameter)

(maximum delivery is 1300 ft/min (6.6 m/s) for apipe around 2 in (50mm) in diameter)

Tube Extrusion

- Direct extrusion but uses a mandrel to shape the inside

- The die containing the mandrel is pushed through the ingot

- The press stem advances and extrudes the metal (speedsup to 10 ft/s (3m/s).

DRAWING

a bloom is heated to forging temperature

with a piercing punch operated in a vertical press, the bloom is formedinto a closed-end hollow forging


17/26

forging is reheated and placed in the hot drawbench (consisting ofseveral dies of successively decreasing diameter mounted in oneframe)

the hydraulic punch forces the heated cylinder through the full lengthof the draw bench

SPECIAL METHODS

HOT SPINNING- Dish or form of any circular plates over a rotating form and to

neck down or close the ends of tubes.- A lathe is used to rotate the piece rapidly- Shaping is done with a blunt-pressure tool or roller.

WARM FORGING- Uses a temperature between that normally used for cold and for

hot working.

ADDITIONAL METHODS

For thinner sections of forgings:

dies are heated

lubricant is used to reduce surface oxidation, to obtain closertolerances, to obtain longer periods of time in which the work remainspliable, to increase production rate

die life is decreased (greater cost)

High-energy rate forming:

uses explosive charges or capacitors discharges

parts are completed with one blow

die life is relatively short

useful in forging high-temperature, difficult to form alloys

Environmental hot forming:

for metals that are difficult to forge (titanium)

metals are cast in a press surrounded by inert gas

eliminates most oxidation and scaling

prolong die life

Aluminum sheets: molten aluminum is poured into a revolving perforated cylinder

transported by air to a preheating chamber

hot rolled into sheet and coiled


18/26

CHAPTER 13COLD WORKING OF METAL

PRINCIPLES OF COLD WORKING

Cold working brings about charged in the grain structure of metals:- grain fragmentation- movement of atoms- lattice distortion

As grain deformation proceeds, greater resistance to this action resultsin increased strength and hardness of metal.

The higher the ductility, the more a metal is able to be cold-worked.

TUBE FINISHING

steel is hot-rolled and treated by pickling any washing to remove all

scales lubricant is applied to prevent galling, reduce friction and increase

surface smoothness

one end of the tube is reduced in diameter (by swaging operation topermit it to enter the die)

the reduced end if gripped by tongs fastened to the chain of thedrawbench (tube is drawn through a die smaller than the outsidediameter of the tube)(inside surface and diameter are controlled by a fixed mandrel)

Tube Reducer-has semicircular dies with tapered grooves through which the

previously hot-rolled tubing is alternately advanced and rotated-the dies rock back and forth as the tubing moves through them-a tapered inside mandnel regulates the size to which the tube

will be reduced-can make the same reduction in one pass that might take fouror five passes in a draw bench-much longer lengths of tubing

WIRE DRAWING

- Wire is made by cold drawing hot-rolled wire rod through one ormore dies, to decrease its size and increase the physicalproperties

Process: wire rod is rolled from a single billet and cleaned in an acid bath

a coating is applied

a coil is placed on a reel or frame and the end of the printed so that itwill enter the die

the end is grasped by tongs on a drawbench and pulled through tosuch length as may be wound around a drawing block or reel


19/26

the rotation of the draw block pulls the wire through the die and formsit into a coil

repeated with smaller dies and blocks

%Reduction in Area =0

0 )100(

A

AA f

%Elongation = 1000

0

L

LLf

Q0 = Qf= A0V0 = AfVf

FOIL MANUFACTURE

Foils are made from a broad variety of pure metals and alloys bycold rolling to thicknesses as thin as 0.00008 in.

In most instances the foil is continuously cast, cooled and rolled asit comes from the furnace.

Continuous operations roll aluminum to about 0.006 in. (0.15 mm.)in thickness at velocities approaching 2000 ft/min (10 m/s).

METAL SPINNING

Shaping thin metal by pressing it against a form while it is rotating(limited to symmetrical articles).

Parts are formed with the aid of blunt hand tools that press themetal against the form.

Parts may be formed either from flat disks of metal or from blanks

that have been previously drawn in a press (finishing operation).Lubricants such as soap, beeswax, white lead and linseed oil reducethe tool friction.

One or more annealing operations may be necessary.

For short-run production jobs

Labor costs are high.

Production rate is less.

SHEAR SPINNING- For spinning thick metal plates

Process:

The plate is initially held securely against the mandrel by a holder.

The roll formers force the plate to conform to the mandrel[maintaining a uniform wall thickness from the starting point untilcompletion]

)2

(sin

sf tt =


20/26

= included angle of cone

Advantages:

increased length

material savings

reduction in cost

good surface finish

STRETCH FORMING-Forming large sheets of thin metals involving symmetrical shapesor double-curve bends.

Process:

A single die mounted on a ram is placed between two slides thatgrip the metal sheet.

The die moves in a vertical direction and the slides movehorizontally.

Large forces of 50 to 150 tons (0.5-1.3 MN) are provided for the dieand slides.

Advantages:

adopted to both production and short-in jobs

inexpensive

large double-curvature parts (difficult by other methods are easilymade with this process)

can be used with many hard-to-form alloys

the problem of unequal metal thin-out is minimized

Disadvantages:

scrap loss is fairly high

limitation to the shapes that can be formed

P = 1.25 Ys A

P = stretch-forming pressure, lbYs= yield strength of metal, psi

A = cross-sectional area, in.2

1.25 = empirical constant

SWAGGING and COLD FORMING- done with a compressive force or impact that causes the metal toflow in some predetermined shape according to the design of the dies

Sizing - the simplest form of cold-forging


21/26

- is a process of slightly compressing a forging, casting orsteel assembly to obtain close tolerance and a flat surface or a flashremoval operation.

Rotary Swaging - a means of reducing the ends of bars and tubesby rotating dies that open and close rapidly on the work, so that the

end of the rod is tapered or reduced in size by a combination ofpressure and impact

Cold Forming (Cold Heading/Upsetting) - another form of swaging-the rod is fed by straightening rolls up

to a stop and then cut off and moved into theheader die

Nail-making

the head is formed before shearing the wire

the wire is fed forward, clamped, headed and pinched orsheared off to form the point and is expelled

the nails are tumbled together in sawdust to remove thelubricants and whiskers before packaging

Bolt-making

cutting off an oversize blank

extending the shank

heading, trimming, pointing and roll threading

Intraforming - metal is squeezed at a pressure of about 300 tons(4000 MPa) of less, onto a die or mandrel to produce an internalconfiguration

HOBBING-forcing a hardened steel form a hob into soft steel to make mold

cavities

The hob is heat treated to obtain necessary hardness and strengthto withstand the tremendous pressures involved.

Several alternative pressings and annealings are necessary.

The flow of metal in the blank is retrained from appreciable lateralmovement by a heavy retainer.

Advantages: multiple identical cavities can be produced economically

the surfaces of the cavities have a highly polished finish

machine work is unnecessary[other than to remove surplus metal from the top and sides of theblank]

COINING and EMBOSSING


22/26

Coining- performed in dies that confine the metal and restrict its flow

in a lateral direction- shallow configurations are produces- limited to relatively soft alloys

Embossing- drawing or stretching operation [does not require the high

pressures in coining]- punch should not touch the mating die- embossed design is raised from the parent metal- mating die conforms to the same configuration as the punch

[so there is very little metal squeezing in the operation andpractically no change in the thickness of the metal]

- rotary embossing using cylindrically-shaped dies isextensively used on thin sheet metal and foils

RIVETING, STAKING and STAPLING- used to fasten parts together

Riveting:-a solid rivet is placed through holes made in the parts to be

fastened together, and the end is pressed to shape by a punch

Staking:-the metal of one part is upset to cause it to fit tightly against

the other part

Stapling:-used to join the two or more sheets of metal and to join sheet

to wood

ROLL FORMING- forms strips of metal into different shapes

A machine produces tubular sections with fine pairs of rolls.

The tubular section enters a resistance welder after being formedand is continuously welded.]

Window Screen Section

the vertical center or pass line is established [so that the number ofbends on either side is about the same]

forming starts at the center and progresses out to the two (2) edgesas the sheet moves through the successive roll passes

Tolerance of Roll Forming:


23/26

affected by the size of the section, material, product, and the gageand gage tolerances of the material

length of the piece is influenced by the speed of operation, lengthof the piece to be cut, and the accuracy of the cutting device

Guidelines to follow when designing a product to be cold rollformed:

A slight angle is more favorable than long vertical side walls.

Blind corners and radii should be avoided [to prevent inaccuracyresulting on rolls without control features].

Smaller-bend radii are easier and less expensive to make thanthose that are larger.

PLATE BENDING

- another method of bending metal plates and strips into cylindricalshapes

SEAMING- used in the manufacture of the metal drums, pails, cans and productsmade of light-gage metal

Types:

lock seam - adapted for joints that do not have to be absolutelytight

compound seam - much stronger and tighter than the lock seam

double seaming - seaming for containers with a flat bottom islimited to one end of a container as the container must be open tomake the joint.

- seaming with recessed bottoms can be done onboth ends

- edge flanging, curling, flattening

HIGH-ENERGY RATE FORMING- part are formed at a rapid rate by extremely high pressures- processes materials that cant be formed by conventional

forming- die costs are low

-good tolerance

- production cost minimized

Explosive Forming- an excellent method of utilizing energy at a high rate because the

gas pressure and rate of detonation can be controlled

Low Explosives:


24/26

- expanding gas is confined and pressures may buildup to 100000 psi (700 Mpa)

High Explosives:- need not be confined and may attain pressures of up

to 20 time that of low explosives

Expanding-gas methods

- Presses against the workpiece and forces it to conformthe die

- Gases act against a piston that forces the confined rubberpunch over the blank and die. (similar to drop hammerbut much more rapid)

- Slow explosive forming forms thin wall tubing by using apowder that deflagrates rather than detonates; theexpanding gases are trapped inside a boot within thetubing and the expanding boot forces the tubing into theconfiguration of the die.

Electrohydraulic Forming (Electrospark Forming)- A process whereby electrical energy is directly converted into

work

Process:

A bank of capacitors is first charged to a high voltage andthen discharged across a gap between two electrodes in a

suitable non-conducting liquid medium. This generates ashock wave that travels radially from the arc at a highvelocity.]

Magnetic Forming- Another process of direct conversion of electrical energy into

useful work

Process:

the charging voltage E is supplied by a high-voltagesource into a bank of capacitors connected in parallel

when charging operation is complete, a high-voltageswitch triggers the stored electrical energy through thecoils establishing a high-intensity magnetic field

the magnetic field induces a current into the conductiveworkpiece placed in or near the coil

when the force exceeds the elastic limit of the material, itcauses permanent deformation

Different Forming Possibilities:


25/26

1. the coil surrounds a tube that when energized forces thematerial tightly around the fitting

2. if the coil is placed inside an assembly, the force will expand thetube into a collar

3. flat plates may be embossed or blanked (used to assemblefragile parts)

Advantages:

pressure on the work is uniform

production rates are rapid

reproducibility is excellent

lubricants are unnecessary

no moving mechanical parts

relatively unskilled labor is required Limitations:

complex shapes are impossible to form

pressures cannot be varied over the workpiece

OTHER METHODS

Impact Extrusion- used in the manufacture of collapsible tubes

For Toothpaste tubes:

a small hole is punched in the center of the blank and thedie cavity is shaped to form the neck of the tube

on the upstroke, the tube is blown from the ram withcompressed air

tubes are then threaded, inspected, trimmed, enameledand printed

Hookes process (small tubes):

small slugs or blanks are used in the impact extrusionprocess (but in this case the metal is extended downwardthrough the die opening

(0.004 to 0.010 in. (0.10-0.25 mm.) thickness, 12 in. (30mm.) length)

High-speed extrusion (axle shafts):

upset billets are hand-loaded onto a three-station

cascade-type loading magazine shaft is elongated by forcing a ring die over the shaft

Shot Peening- to improve the fatigue resistance of the metal by setting up

compressive stresses in its surface- the process uses an air blast or some mechanical means as

centrifugal force for hurling steel shot onto the work at high velocity


26/26

Process:

This is done by blasting or hurtling a rain of small shots at highvelocity against the surface to be peened.

As the shot strikes, small indentations are produced causing a slightplastic flow of the surface metal to a depth of a few thousandths of

an inch. This stretching of the outer fibers is resisted by those underneath,

which tend to return them to their original length, thus producingan over layer having a compressive stress while those below are intension.

The surface is slightly hardened and strengthened.

final reportwithout bracket

Documents