McGraw-Hill Machining and Metalworking Handbook 3rd ed - R. Walsh_ D. Cormier (McGraw-Hill_ 2006) Episode 11 pdf

Solid Freeform Fabrication Solid freeform fabrication SFF refers to a category of manufac-turing processes in which parts are built by depositing one sectional layer of material on top

706

Solid Freeform Fabrication

Solid freeform fabrication (SFF) refers to a category of

manufac-turing processes in which parts are built by depositing one sectional layer of material on top of the next It is very much like

cross-“printing” a succession of slice images one on top of the next so thatthickness is gradually built up (Fig 10.1) Owing to the fact thatmaterial is deposited in thin cross-sectional layers, there are typi-cally no concerns about tool collisions, parting lines, undercuts, etc

It is a toolless and fixtureless approach to making parts ing highly complex geometric shapes via layered manufacturingprocesses therefore is no more difficult than fabricating simple geo-metric shapes such as cubes or cylinders This is evident in Fig.10.2, which shows a very impressive geometric sculpture designed

Fabricat-by artist Bathsheba Grossman (www.bathsheba.com) The sculpture

was fabricated using ProMetal’s Direct Metal Printing process

Rapid prototyping (RP) and rapid manufacturing (RM) are terms

that typically are associated with SFF In both cases, the parts beingfabricated are built layer by layer The distinction lies simply withthe end use of the part being produced RP produces parts that will

be used as part of the iterative design process RM produces a tional end item to be delivered and used by the customer Note thatthe end item can be a steel mold or die if the customer is a molding,casting, or stamping shop

func-The terms rapid prototyping and rapid manufacturing can be

somewhat misleading owing to the fact that layered processes

generally are much slower than mass-production processes such asdie casting or injection molding However, layered manufacturingprocesses are capable of building parts with complex geometriesmuch more quickly than generally would be possible when usingconventional fabrication techniques When production quantities arenot sufficiently high to justify the expense of tooling and fixturing,then one of the SFF processes can be cost-effective.

The vast majority of early SFF processes were developed to duce nonmetallic parts Some of the early processes included stereo-lithography (3D Systems), fused deposition modeling (Stratasys),and selective laser sintering (3D Systems) The SLS process alsohas been used to make sintered parts from both metal and ceramicpowders

pro-Figure 10.2 Complex freeformstructure (Photo courtesy of Bathsheba Grossman.)

Figure 10.1 Principle of layer-additive manufacturing

More recently, literally dozens of relatively inexpensive layeredmanufacturing processes have been commercialized In order toobtain fully dense metal parts from one of these processes, it is gen-erally necessary to use a two-step process in which the prototype part

is used as the pattern for a casting or forming process For example,polymer prototype parts often are used as patterns for investmentcasting or sand casting

10.1 Direct-Metal Rapid Manufacturing Processes

Recent years have witnessed a flurry of research and developmentaimed at “direct-metal” RM processes These processes are of partic-ular interest to machinists and metalworkers Rather than using

an indirect approach such as investment casting to produce a metalpart, these processes directly produce near-net-shape functionalmetal components

Owing to the considerable expense usually associated with metal RM processes, they typically are used for one of several reasons

from very expensive alloys with special material properties Because

RM processes produce components to near-net shape, very littlematerial is wasted in finish machining In many cases these mate-rials are quite difficult to machine; hence producing them to near-netshape dramatically speeds up fabrication of the final part andreduces material costs Although titanium and its alloys (mostnotably Ti-6Al-4V) seem to have attracted the most interest, therealso has been considerable development work involving a wide vari-ety of metal-matrix composites (MMCs) involving TiB, TiC, SiC, andother reinforcements The majority of RM processes are powder-based, so particulate-reinforced MMCs are employed rather thancontinuously reinforced MMCs

weld-ing, brazweld-ing, or similar thermal methods With traditional turing methods, it is often the case that components in an assemblyare designed as separate pieces merely to accommodate the produc-tion method Because geometric complexity is not a major concernwhen RM processes are used, it is often possible to merge severalparts into one part In some cases the elimination of assembly jointsalso can have implications for improved component reliability

manufac-Design optimization. RM processes can produce complex geometricshapes that would be extremely difficult and/or costly to fabricatevia more conventional means Owing to the fact that the designerneed not be concerned with demolding a part or (in some cases) cut-ting-tool access, it is possible to optimize components for specialform or function with little regard for the manufacturing process.Some examples include

Internal wiring Electrical wiring can be routed through channels

that are fabricated in the walls of a part so that the wiring is tected from external moving components

pro-Conformal cooling channels Conventional mold/die cooling

chan-nels typically lie in a flat plane With RM, it is possible to fabricatecooling channels that conform to or flow with the actual shape ofthe core/cavity surfaces in each of the three dimensions By routingcoolant close to the mold/die surface, more uniform cooling can beachieved, thus lowering stresses brought about by uneven cool-ing Furthermore, cycle times can be reduced In addition to hav-ing cooling channels that conform to the shape of the tool surface,

it is also possible to vary the cross section of the channel itself Itneed not be round For instance, it can fan out to a larger surfacearea as it passes near the mold surface Likewise, baffles can bebuilt in to increase turbulence if so desired

Weight optimization Through finite element analysis (FEA) codes,

mechanical designers are able to determine exactly where rial can be removed from a component without adversely affectingthe function and safety of a part However, designing a weight-optimized part and fabricating it are two separate issues In manyinstances, an RM method can fabricate optimized components thatwould be very difficult to fabricate via conventional methods Forthis reason, the aerospace, automotive, and military industrieshave been among the early adopters of these direct-metal tech-nologies Figure 10.3 shows a geometrically complex componentfor a head-mounted display In order to reduce weight, the three

mate-“tentacles” coming off the device have been hollowed out

Compositional optimization Some RM processes have the ability

to independently deposit multiple materials in varying amounts.Much like a printer is able to print thousands of colors with justthree or four print heads, these systems can locally vary thematerial composition of a component to achieve material proper-ties that cannot be had any other way

10.1.1 Electron-beam melting

Electron-beam melting (EBM) is a process developed by Arcam AB

of Sweden (www.arcam.com) Alloys such as H13 steel, Ti-6Al-4V,

TiAl, and Inconel can be fabricated with this process A thin layer

of metal powder, typically 100 ␮m thick, is first spread over a plate

An electron-beam gun is selectively scanned over the powder bedwith enough power to melt the metal powder that it comes in con-tact with The platform is lowered, a new layer of powder is spread,and the process repeats itself The entire process takes place invacuum, which helps to prevent unwanted oxidation of metal pow-ders at elevated temperatures On completion of the final layer, thepart is removed from the bed of loose unmelted powder that sur-rounds it The unmelted powder surrounding the part also serves

to support any downward-facing surfaces

Parts fabricated via most powder-based RM processes, includingEBM, possess textured surfaces somewhat resembling those of sand

Figure 10.3 Ti-6Al-4V head-mounted display component

built via electron-beam melting (EBM) (CAD design

courtesy of Dr Jannick Rolland, University of Central

Florida.)

castings For example, Fig 10.3 shows an as-processed Ti-6Al-4Vhead-mounted display component built via EBM prior to finishmachining As is the case with sand castings, critical features aresubjected to any necessary finish machining operations Approxi-mately 0.5 to 1.5 mm of material typically is recommended as amachining allowance Finish machining of Ti-6Al-4V parts built withthis process has been achieved using TiAlN-coated end mills with acutting speed of 50 ft/min and a feed rate of 0.007 in/tooth Figure10.4 shows a finish-machined nozzle component built via the EBMprocess in the Ti-6Al-4V alloy.

Despite the fact that metal is melted in layers, components cated via EBM possess excellent interlayer bonding The fact thateach layer is so thin also means that melt solidification takesplace very rapidly, thus leading to a refined microstructure High-performance materials that are difficult to process, such as titaniumaluminide, can be processed via the EBM process owing primarily tothe fact that the entire part is kept at an elevated temperature dur-ing processing This significantly reduces thermal stresses thatwould be caused by depositing molten material on top of (relatively)cold previously solidified material Thermal stresses generally lead

fabri-to warping and/or cracking in the workpiece; hence proper thermalmanagement is critical for any of the direct-metal RM processes

One particularly intriguing aspect of EBM and other direct-metal

RM processes is that locating features easily can be built into eachcomponent to facilitate finish machining

Figure 10.4 Finish-machined

tita-nium component (Photo courtesy

of Arcam AB.)

10.1.2 Selective laser sintering (SLS)

3D System’s SLS process (www.3dsystems.com) is capable of

pro-cessing a wide variety of polymer, metal, and ceramic powders Ofparticular interest to the machinist or metalworker is the Laser-Form A6 steel material The LaserForm material consists of A6steel particles that have been coated with a polymeric binder Athin layer of the powder is spread over a platform, and a 100-W(typical) laser scans the cross-sectional area of the first layer to beproduced Heat from the laser causes the polymer binder to melt,thus agglomerating together the metal particles for that layer Theprocess is repeated one layer after the next until the part is com-plete The green polymer-bound part is removed from the powderbed and placed in a furnace The polymer binder is vaporized undercareful control, and the remaining steel particles are sintered Theresulting component is porous sintered A6 steel If full density isdesired, the part can be infiltrated with a secondary metal such asbronze Figure 10.5 shows a photograph of a well-known rook chesspiece that was built in steel and infiltrated with bronze using theSLS process

Figure 10.5 SLS rook chess piece

in bronze-infiltrated tool steel

10.1.3 Laser additive manufacturing (LAM)

Rapid prototyping processes based on laser forming generallyinvolve using a high-power laser to start a melt pool One or morepowder feeders then are used to direct metal powder into the meltpool at a predetermined rate The melt pool typically is shieldedwith inert gas such as argon to inhibit oxidation The laser andpowder are moved simultaneously above the surface of the sub-strate, thus producing a moving melt pool into which new powdermaterial is constantly added Slightly varying versions of laser addi-tive manufacturing processes have been commercialized by Optomec

(www.optomec.com), Aeromet (www.aerometcorp.com), and POM (www.pomgroup.com).

A particularly attractive advantage of LAM processes is thatmultiple powder feeders can be used By using multiple powderfeeders and varying the rate with which each powder is added tothe melt pool, it is possible to locally vary the material composition

of a component to match its mechanical requirements in service.For instance, the composition of a component might be tailored toprovide a tough, fracture-resistant core that is surrounded by ahard, wear-resistant outer skin Figure 10.6 shows a laser deposi-tion nozzle with multiple powder feeders

Because there typically is no powder bed surrounding the part(s)being fabricated, production of components with downward-facingsurfaces can be a challenge Systems equipped with three-axismotion controllers can deposit material only on gradually tapereddownward-facing surfaces Surfaces with steeper downward-facingslopes generally must be fabricated on systems equipped with five-axis positioning systems and specialized software to generate mate-rial deposition toolpaths that will not result in collisions between thepowder feeders and previously deposited materials

Although fabricating complex geometries with downward-facingsurfaces generally is more difficult to accomplish with LAM sys-tems that do not surround the part with powder, LAM systems arevery well suited to repairing damaged or worn surfaces on existingparts Because of their ability to control chemical composition, theyare also used to deposit special coatings on existing parts

Rapid tooling can be defined in many ways In the simplest sense,

rapid tooling refers to the practice of either directly fabricating a

tool (mold, die, etc.) from any one of the RM processes or fabricating

a tool using an RP/RM component as a pattern

10.2.1 RTV silicone molds

One of the most widely used approaches to rapid tooling involvesmaking room-temperature vulcanizing (RTV) silicone molds from apattern made on any one of the RP/RM processes The RTV moldmay be fabricated in two pieces, or it may be a one-piece cut mold.Regardless of how the RTV mold is made, polyurethane or othertwo-part resins are then cast into the mold to produce a plasticpart The urethane may include dyes and/or fillers to modify itsappearance and material properties Common fillers include glassbeads to reduce part weight and to reduce the volume (and cost) ofthe resin being used Chopped fiberglass strands sometimes areadded to increase part strength Metal powders such as bronze arealso added at times to make the part look and feel like a metal part.Provided the powder loading is sufficiently high, it is possible tobuff the parts in order to achieve a somewhat shiny metallic look.Depending on the resin being cast, the workpiece geometry, andany fillers/additives, the typical RTV mold usually is good forapproximately 30 to 50 castings

Figure 10.6 Laser-engineered net shaping with multiple powder feed nozzles

(Photo used with permission of Sandia National Laboratories.)

Fabricating One-Piece Cut RTV Molds. The process for fabricating aone-piece cut mold is relatively straightforward.

1 Any of the RP processes is used to fabricate a pattern of thepart to be molded Shrinkage allowance for the resin used must

be factored into the pattern dimensions

2 Visual inspection of the part determines the parting line for thetwo halves of the mold Note that the RTV is flexible; hencesmall to moderate undercut features generally may be moldedwithout difficulty To aid in cutting the mold apart in a laterstep, the parting line is often drawn onto the part using a per-manent marker

3 The RP part is suspended by wires and/or pins inside of a boxwith approximately 25 mm (1 in) of clearance on all sides Caremust be taken to ensure that the part is held firmly in place.When the RTV silicone is poured into the box, a loosely attachedpart can break free and either float or sink depending on itsbuoyancy in the RTV silicone At this point, it is helpful to visu-ally project the parting line from the part out to the walls of thebox A permanent marker is then used to sketch the location ofthe projected parting line around the perimeter of the box

4 The RTV silicone rubber is mixed and degassed according to themanufacturer’s recommendations and then is poured slowlyinto the box until the liquid level is approximately 25 mm (1 in)above the top of the part The box is set aside and allowed tovulcanize (cure) for approximately 24 hours Note that thereare a wide variety of RTV rubbers that vary according to theirtranslucency, hardness, tear strength, etc It is generally mucheasier to make one-piece cut molds using clear or somewhattranslucent RTV materials than opaque ones

5 Once the rubber has cured, the walls of the box are removed Anumber of knives then are used to cut the rubber down to theparting line of the part Once the parting line has been located,the knives are used to follow the parting line around the part.Note that a slight zigzag pattern should be cut as the rubberhalves are separated Doing so will provide self-interlockingfeatures so that the reassembled mold halves will align prop-erly during casting

6 Once the mold is cut apart, the RP pattern is removed, thusleaving a cavity in the rubber mold

7 The knife is used to cut a pouring basin into the rubber, alongwith any vent channels that may be needed The urethane gen-erally is poured into the highest point on the part It is helpful

to visualize the urethane filling up the mold Air vents should

be cut into the mold at any locations where air is likely to becometrapped during the filling process

two-piece RTV mold is only slightly more complicated than that used tomake one-piece cut molds

1 Any of the RP processes is used to fabricate a pattern of thepart to be molded Shrinkage allowance for the resin used must

be factored into the pattern dimensions

2 Visual inspection of the part determines the parting line for thetwo halves of the mold The part is then embedded in modelingclay or a similar material Various hand tools are used to buildthe clay up to the parting line for the part At this point, theupper half of the part protrudes above the clay along the part-ing line

3 The clay-supported pattern is then placed inside a box A suitablerelease agent is sprayed on the exposed part, clay, and innerbox walls

4 The RTV silicone rubber is mixed and degassed according to themanufacturer’s recommendations and then is poured slowlyonto the exposed part and clay surface until the liquid level isapproximately 25 mm (1 in) above the top of the part The box

is set aside and is allowed to vulcanize (cure) for approximately

24 hours

5 The following day, the box is flipped over, and the clay that thatpart was embedded in from beneath is removed At this point,the upper half of the part is now embedded in the cured RTVsilicone

6 A knife is used to cut pyramidal (or other) chunks of cured RTVsilicone away around the perimeter of the part When the sec-ond batch of RTV is poured, this will lead to the formation ofinterlocking features that keep the two mold halves aligned

7 The exposed part and RTV silicone surfaces are sprayed with asuitable release agent

8 A second batch of RTV silicone rubber is mixed and pouredover the exposed part and RTV surfaces This is allowed to curefor approximately 24 hours.

9 Once the rubber has cured, the walls of the box are removed.Provided an appropriate release agent has been used, the twocured halves of RTV silicone should just peel apart along theparting line without any need to cut them with a knife

10 The RP pattern is removed, thus leaving a cavity in the rubbermold

11 The knife is used to cut a pouring basin into the rubber alongwith any vent channels that may be needed

Casting the Polyurethane

1 Regardless of which type of mold has been produced, all moldsurfaces are sprayed with a release agent that has been matched

to the urethane being cast The mold halves then are clampedtogether between a pair of flat boards Hand clamps or evenrubber bands can be used to hold the assembled mold together.Since the RTV rubber is compressible, care must be taken toavoid distorting the cavity when the mold halves are clamped

2 The urethane and any colorants/fillers/etc are mixed togetherand poured slowly into the cavity Once the urethane has hard-ened (generally 1 to 15 minutes depending on the urethane for-mulation), the mold is opened up, and the part is removed

3 The process can be repeated until the polyurethane begins to stick

to the mold despite the use of a mold releasing agent

Caution: Be sure to follow the polyurethane manufacturer’s

recommended safety precautions regarding proper ventilation aswell as skin and eye protection Many of the uncured polyurethanecompounds are considered carcinogenic Also take note of thefact that the reaction taking place during curing is exothermic.The amount of heat released can be sufficient to burn skin insome cases

Direct-inject SLA tooling refers to the practice of using photosensitive

resins with the stereolithography process (www.3dsystems.com) to

produce an injection-mold insert The insert goes into a mold base

in an injection-molding press SLA tools are not intended for duction use, but they are a fast and relatively inexpensive way

pro-to produce mold inserts that can be used pro-to injection-mold smallbatches of functional plastic parts in the desired resin Depending

on the melting temperature of the resin being injected and the ence or absence of fillers (e.g., glass-filled nylon), direct-inject toolingmay last from just a few shots to several hundred shots

pres-When the molded part has complex geometry, it is customary toproduce a “hand mold.” Rather than assembling a complex tool withhydraulic slides and other moving components, the direct-injectinserts with removable cores are assembled by hand and placed in themold base The inserts are then removed from the mold base fol-lowing each shot, and the cores are removed from the part by hand.The tooling then is reassembled, and the next shot is made This isobviously not a suitable approach for mass production, but it is quitefeasible to produce tens or even hundreds of sample plastic parts bythis method

Figure 10.7 shows a close-up photograph of a direct-inject cavity SLA insert used to mold miniature worm gears The matinginsert is not shown in the photograph because it is nearly identical

six-to the one shown in Fig 10.7 Each insert is 35 mm (13⁄8in) squareand 9.5 mm (3⁄8in) deep It took approximately 1 hour of machinetime to fabricate this pair of inserts on a high-resolution stereolitho-graphy machine without any need to program computer numerical

Figure 10.7 Direct-inject SLA inserts for molding worm gears

control (CNC) toolpaths or to design an electrode A steel pin insert

is placed in the mold prior to injection, and the worms are slid offthe pin following injection The worms are approximately 6 mmlong, with a diameter of just under 3 mm A pair of molded worms

is shown in the lower left of the photograph The penny is includedsimply to provide the reader with an idea of the scale involved

For instances where it is helpful to have metal-like parts that donot necessarily need the same material properties as a final produc-tion metal part, it is possible to electroplate prototype components.Chapter 13 provides details on various plating processes, althoughplating nonconducting polymer components can be tricky

A simple though not terribly reliable approach to obtaining anelectrically conducting surface on the plastic prior to electroplating

is to manually paint it with conductive paints Paints containingsilver or copper powders are readily available for this purpose.This approach will work in simple cases, but the adhesion of thepaint to the plastic surface during subsequent electroplating isoften inadequate The result is blistering or flaking of the platedsurface

A more reliable approach to preparing plastics for plating is asfollows: First, the components to be plated must be completelycleaned of any oils, solvents, etc In order to improve adhesion ofthe plating to the plastic, the surface of the plastic often is etched

in a chromic acid bath Next, the surfaces of the parts are sensitizedthrough the addition of a palladium chloride solution This stepallows palladium metal to deposit on the etched surface of the plastic.Either electroless nickel or copper coatings may then be applied, withthe palladium acting as a catalyst for nickel or copper deposits Avery thin (ⱕ1 ␮m) electroless nickel coating is common At thispoint, the surface of the plastic has been made to be electrically con-ducting with reasonably good adhesion properties The surface thencan be electroplated using techniques described in Chap 13 Whenthe part is to undergo mechanical stress, a copper plating often isrecommended prior to nickel plating Nickel platings can be followed

by a chrome plate if desired for aesthetic purposes

Many electroplating compounds are highly toxic and are highlyregulated Consequently, prototype plastic parts often are sent out

to specialty shops for plating Figure 10.8 shows a batch of complex

handheld air compressor components that were fabricated in a DSMSomos Prototool material on a high-resolution stereolithographymachine by Fineline Prototyping, Inc The parts then were given aproprietary SLArmor coating that consists primarily of a copperstrike coat followed by an electroless nickel plating With minimalhand finishing, these components were assembled into a fully func-tional handheld air compressor.

Three-dimensional (3D) printing is a process that was developed atthe Massachusetts Institute of Technology (MIT) and subsequentlybrought to market by several companies, including Z Corporation,ExtrudeHone, Soligen, and Therics Z Corporation has introduced

a special powder and binder formulation for fabricating investment

casting shells The company refers to this process as ZCast At the

present time, it is used for casting nonferrous alloys such as zinc oraluminum As is the case with many RP processes, Z Corporation’sversion of 3D printing starts by spreading a thin layer of powder Inthis case, the powder being spread is a specially formulated castinginvestment An inkjet print system is then scanned across the pow-der bed The printer prints an image of the current cross-sectionalslice of the investment casting shell being produced The moisture

in the ink is the binder that causes the casting investment to

Figure 10.8 SLArmor air pump components (Photo courtesy of Fineline typing, Inc.)

Proto-harden This is akin to mixing water with plaster, except on a muchsmaller scale This process is used to fabricate the shell in twopieces, as well as any cores that are needed Figure 10.9 shows acompleted cover housing along with the ZCast tooling needed toproduce such a casting.

Figure 10.9 ZCast cover with mold (Photo courtesy of ZCorp.)

Hardening and Tempering Steels and Nonferrous Alloys

The hardening and tempering of steels and nonferrous alloys areimportant aspects of metalworking Carbon and alloy steels arerelied on to perform a great number of services in the metalworkingindustries Some of the nonferrous metals and alloys are also capa-ble of being hardened above their normal condition either throughheat treatment or cold working and find countless uses in productdesign and manufacturing

11.1 Standard Steels and Steel-Making Practices

Steel is the generic name for a large group of iron-carbon alloys.

The basic materials used in steel making are iron ore, coke, and stone A blast furnace converts these materials into a product known

lime-as pig iron, which contains considerable amounts of carbon,

man-ganese, sulfur, phosphorus, and silicon Basic oxygen furnaces arealso employed in steel making, as are other methods Steel making’sbasic constituent, pig iron, is hard and brittle and unsuitable for pro-cessing into usable wrought iron or steel products Steel making isthe process of refining pig iron and iron and steel scrap by removingunwanted elements and adding the desirable elements in predeter-mined and controlled amounts Most steel-making processes cause

a combination of carbon and oxygen to form a gas When a steel is

11

deoxidized strongly with a deoxidizing agent, no gas forms, and the

steel is called killed steel The degree of deoxidation affects some of

the properties of the steel, and the degree of gas evolution

charac-terizes steels that are known as semikilled, capped, and rimmed In

addition to oxygen, fused steel contains small amounts of hydrogenand nitrogen Special deoxidation practices, including vacuum treat-ment, may be used to control the amount of dissolved gases in thesteel

The carbon content of the common steel grades ranges from a fewhundredths of 1 percent to 0.95 percent All common steels containmanganese, sulfur, phosphorus, and silicon to some degree

Wrought steels are the most common and widely used engineeringmaterials There is no other single material that offers such a broadrange of practical applications as the various types and grades ofsteels

The unified numbering system for steels is shown in Chap 4,

“Materials and Their Uses.” This system classifies the various types

of steels and provides identification numbers

11.2 Constituents of Steel: Phases

When carbon steels are heated to various temperatures, changes

take place in the structure that are known as phases Figure 11.1 is

a diagram showing the relationship between temperature and theamount of carbon in the steel that affects the basic structure Thediagram is presented strictly for academic reasons and serves thepurpose of describing the different states that the steel assumeswhen the temperature and carbon content are varied The actionsand reactions that occur during the heating and cooling of carbonand alloy steels are complex and form the basis for the controlledheat treatments that are performed on the various types of steels.The diagram in Fig 11.1 indicates the phase transformations thatoccur in steels with up to 6.67 percent carbon content, where the

main form of the steel above 2066°F is called cementite The other

forms or states/phases are indicated by the various letters andcombinations of letters shown below the figure Some of the otherforms that occur are shown below the letter designations, the most

important of which is martensite Martensite is the phase or form

of carbon steel that is produced in the hardening process, whichwill be described in a later section

Carbon steels with less than 0.85 percent carbon are called

hypoeutectoid, and those with carbon contents greater than 0.85

percent are called hypereutectoid In binary-alloy systems, a

eutec-toid alloy is a mechanical mixture of two phases that form

simulta-neously from a solid solution when it cools through the eutectoidtemperature Alloys leaner or richer in one of the constituents ormetals undergo transformation from the solid-solution phase over

Figure 11.1 Iron, iron-carbide diagram

a range of temperatures beginning above and ending at the toid temperature The structure of such alloys will consist of pri-mary particles of one of the stable phases in addition to theeutectoid, e.g., ferrite and pearlite in low-carbon steel.

eutec-11.3 Standard Definitions of Terms Pertaining to

Heat Treatment of Metals

Many terms are associated with the heat treatment of metals, andthe American Society for Testing and Materials (ASTM) has classi-fied these terms in ASTM Standard E44-84 Figure 11.2 outlinesthe terms and definitions as described in ASTM Standard E44-84and is reproduced with permission from the ASTM

By studying the diagram shown in Fig 11.1 and the definitions

of terms in Fig 11.2, you will gain an excellent insight into thepractice of heat treatment of metals

11.4 Heat Treatment of Steels

In fully annealed carbon steels, the percentage of carbon determinesthe structural constitution of the steel Figure 11.1 shows the consti-tution or phases for varying carbon content versus temperature

carbon steels are heated above the lower critical point, which rangesbetween 1335 and 1355°F, depending on the carbon content, austen-ite is formed If the temperature continues to rise, the steel structurewill change completely to austenite The temperature at which excessferrite and cementite are completely dissolved in the austenite is

called the upper critical point This critical temperature varies with

the carbon content of the steel If the steel is cooled slowly to ambienttemperature, the steel returns to its original condition

when carbon steels are heated to a certain temperature, suddenlycooling the steel at the proper cooling rate causes the austenite totransform to martensite, which has very high hardness Thus theoperation of hardening steels consists of two steps The first step is

to heat the steel at least 100°F higher than its transformationpoint, and the second step is to cool the steel at some rate that is

faster than the critical rate (The critical rate is determined by or

depends on the carbon content and alloying elements present inthe steel.) The hardness of a martensitic steel depends on its car-bon content and ranges from 460 Brinell at 0.20 percent carbon to

710 Brinell at 0.50 percent carbon Ferrite has a hardness ofapproximately 90 Brinell, pearlite approximately 240 Brinell, andcementite approximately 550 Brinell

Figure 11.2 Standard terms relating to the heat treatment of metals (Reprinted with permission from the Annual Book of ASTM Standards, copyright 1992, American Society for Testing and Materials.)