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INTRtiUCTItN T#PHYSICAL METALLURGY

Secnd E ititn

SliNEY H. AVNERProfessor

iview York City Community College".

.

V;I

City University of New York

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McGRAW-HILLNew York

BQQK COMPANYKuala LumpurLondon Panama

St. Louis

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San Francisco Dtisseldorf

Mexico

Rip de Janeiro Singapore

MontrealNew Delhi

Sydney

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Johannesburg

Toronto:

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in Publication Dat

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CONTENTSPreface Introduction1.

Tools of the Metallurgist Metal Structure and CrystallizationPlastic Deformation

,

65 .....

3

.

107

W? Annealing and Hot Workingi i

yi?. Constitution of Alloysr

; reserved.

publicar transnicalir,

The Iron-lronCarbide Eauilibrium Diagramv- iflvfi fi. The Heat Treatment of Steel IP IiSR7.

V f

.

-Alloy

Steels

|349*|,387 '

10. 11.

Tool Steels Cast Iron

42i3

:.

mm

12.I

Nonferrous Metals and Alloys

461 * gg|547567 583' .

13.14.rna

Metals at.High and Low TemperaturesWear of Metals

- g of temperature is required,

range of -100 to 1000F, are very rugged, and require virtually no main-

J into two general classifications::

tenance. Their main disadvantage is that, owing to the necessity for enclosing the element in a protecting tube, the speed of response may be lower

|plysystems which deal of a metal, " with the expansion with resisal pyrometers.

I1-4

than that of other instruments.

Liquid-expansion Thermometers

als expand when heated and,

on the temperature and the co-

The remainder of the mechanical system temperature-measuring instruments, whether liquid-expansion or gas- or vapor-pressure, consist of a bulb exposed to the temperature to be measured and an expansible device, usually a Bourdon tube, operating an-

corporated in the bimetallic stripThe bimetallic strip is made by side with a low-expansion metal

ture changes, the strip will curveal circuit which will control the

erature indicator, thelined (Fig. 1-1).

bimetallic

r

f which is fixed so that on expan-

indicating pointer or a recording pen. The bulb and Bourdon tube are' connected by capillary tubing and filled with a suitable medium (Fig. 1-2). The liquid-expansion thermometer has the entire system filled with a suitable organic liquid or mercury. Changes In bulb temperature cause the liquid to expand or contract, which in turn causes the Bourdon tube to expand or contract. Temperature changes along the capillary and at the case also cause some expansion and contraction of the liquid, and some form of compensation is therefore required. Figure 1 -3 shows a fully compensated liquid-expansion thermometer using a duplicate system, less bulb, arranged so that motions are subtracted. Some of the liquids used and the temperatures covered by them are;,

11

MercuryAlcohol Pentane

-

35 to+950F

-

110 to+160F

+ 330 to+ 850F

Creosote

+20 to+400oF

1 -5 Gas- or Vapor-pressure Thermometers In the vapor-pressure thermometer a volatile liquid partially fills the bulb. Different temperatures of the bulb cause corresponding pressure variations in the saturated vapor above the,

liquid surface in the bulb These pressure variations are transmitted to the.

m

; 4 INTRODUCTION TO PHYSICAL METALLURGY

Bourdon\ tube.

1 -7 Thermoelectric Pyrometerlurgical temperature mesAuxiliary compensating

up to about 3000F

.

/ capillary

The simple thermoelec following units: 12 3

The thermocouple comp,

The junction block just c The extension leads,.

Compensating system3/

4 The indicating instrumen

The operation of this p;Peltier Effect If two diss contact an emf will exist,

Capillary

Bulb )

the emf developed will be wires and the temperaturtThomson Effect If there

Fig. 1 -3 A fully compensated liquid-expansion thermometer.

(From "Teniperature Measurement," American Society forMetals, 1956.)

single homogeneous wire The magnitude of the emtion, the chemical uniforn

m

Bourdon tube, the pressure indications acting as a measure of the temperature in the bulb. By suitable choice of volatile liquid, almost any temperature from -60 to +500oF can be measured. Some liquids used are methyl.

The total emf in a thSeebeck effect is thereto,

emf s at the hot and cole

'

chloride, ether, ethyl alcohol, and toluene.

of the wires.

The gas-pressure thermometer is similar to the vapor-pressure thermometer except that the system is filled with a gas, usually nitrogen. The range of temperature measured by the gas-pressure thermometer is from-

1

The cold junction or ref perature. This is usually ( usually done by means of a,

200 to+800oF.

its resistance with fluctuatinstrument at 320F. If the,

Filled-system thermometers are used primarily for low-temperaturecooling water and oil temperatures, and subzero temperatures in the cold treatment of metals. These instruments are relatively inexpensive but are not

applications such as plating and cleaning baths, degreasers,

constant temperature theibe a definite function of ti

used where quick repair or exceptionally high accuracy is required. 1 6 Resistance Thermometer The principle of the resistance thermometer

depends upon the increase oi electrical resistance with increasing tem

-

Extension leadsL

perature of a conductor. If the temperature-resistance variations of a metal are calibrated, it is possible to determine the temperature by measuringits electrical resistance. The resistance coil is mounted in the closed endmeasuring instrument, usually a Wheatstone bridge..

"

of a protecting tube and the leads are extended to a suitable resistance;i

v

.

Resistance coils are usually made of copper, nickel, or platinum. Nickel and copper are most satisfactory for temperatures between 150 and SOO-F, whereas platinum may be used between-350 and+1100oF The resis.

K

IIndicating-. Instrument

tance thermometer is very accurate and is of great importance in the >

mCold junction

laboratory.-However, its industrial use is limited because it is fragile andrequires many precautions in use.Fig. 1-4

A simple thermoelectric pyrometer

illJi

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TOOLS OF THE METALLURGIST

S

ifm

1-7

iluxiliory"

Thermoelectric Pyrometer This is the most widely used method for metallurgical temperature measurement and control; it will perform satisfactorilyup to about 3000oF. The simple thermoelectric pyrometer, shown in Fig: 1-4 consists of the,

bmpensQting

iopillary

t

Dbtinq syslem

following units:1 23

The thermocouple, composed of two different metals or alloys The junction block, just outside the furnaceThe extension leads

4

The indicating instrument or recorder

m

The operation of this pyrometer is based upon two principles: Peltier Effect If two dissimilar metallic wires are brought into electrical contact an emf will exist across the point of contact. The magnitude of the emf developed will be determined by the chemical composition of the,

;

wires and the temperature of the junction point. Thomson Effect If there is a temperature difference between the ends of a single homogeneous wire an emf will exist between the ends of the wire.,

m/:

iting as a measure of the temperaolatile liquid, almost any temper3d. Some liquids used are methyla.

The magnitude of the emf developed will bo determined by the composition, the chemical uniformity of the wire, and the temperature difference. The total emf in a thermoelectric pyrometer sometimes called the,

Seebeck effect, is therefore the algebraic sum of four emf's, two Peltier emf s at the hot and cold junctions and two Thomson emf s along each'

'

of the wires.-

lilar to the vapor pressure therwith a gas, usually nitrogen. The

jas-pressure thermometer is from d primarily for low-temperaturei:;:,

44T

ning baths, degreasers, cooling-

o temperatures in the cold treat

The cold junction or reference junction, must be kept at a constant temperature. This is usually 0C, or 32F. At the indicating instrument this is usually done by means of a cold-junction compensating coil which changes its resistance with fluctuations in ambient temperature always keeping the instrument at 32F. If the cold junction or reference junction, is kept at constant temperature, then the measured emf in the pyrometer circuit will be a definite function of the temperature of the hot junction. By suitable, , ,

relatively inexpensive but are not f high accuracy is required.of the resistance thermometerProteclmg tubeExtension leads

il resistance with increasing temjre-resistance variations of a metal ne the temperature by measuring

ThenriocoupleuJunctionHot

coil is mounted in the closed end,

extended'

to a suitable resistance-

block

junction

tone bridge.Indicating-

Stopper, nickel, or platinum. Nickel y peratures between 150 and SOCF,' -

en -350 and -MIOOT. The resisnd is of great importance in the

=

-* Instrument

is limited because it is fragile and

f

Cold junction

Fig. 1 -4 A simple thermoelectric pyrometer

.

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.

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1

1

6 INTRODUCTION TO PHYSICAL METALLURGY

TABLE 1 2 Temperature vs. Electromotive ForceEmf In millivolt*; cold |uncllon 32 F

*

The purpose of the e to a point where the teIRON VS.

PT+10%RHTEMP, "F VS. PLATINUM00.

CHROMELVS.

COPPERVS.

usually not long enouglinstrument. The extern

ALUMEL00.

CONSTANTAN00.

CONSTANTAN00.

> :

32 100 200

as the thermocouple wii vidual covering color-cmay be used in some cablock instead of the ins1-8

0 221.

1 52.

1 94.

1 517.

0 595.

3 82.

4 91.

3 967.

300i

1 017.

6 09.

7 94.

6 647.

400 500

1 474.

8 31.

11.03

9 525.

1 956.

10.5712.86

14.12 17.18

12.575 15.773

600 700

2 458.

constant temperatures. Thermocouple Materials will develop an emf whe junction points. Industri

2 977.

15.1817.53

20.2523.32

19.100

80(3

1

3 506.

4 046.

19.8922.2624.63

26.4029.5232.72

used for thermocouples electric potential reasc,

temperature-emf curve

,

i

looo1100

4 596.

to be measured. The firs

5 156.

to the positive terminal.Chromel-Alumel Chron

1200 13001400;

5 726.

26.9829.3231.65

36.0139.43

6 307.

6 897.

42,96 46.53 50.05

15001600'

7 498.

33.93

Alumel (94 percent nick( percent silicon) is one c It has a fairly linear calibis most useful in the ran(

8 110.

36.1938.43

fi

17001900

8 732.

Iron-Constantan

Const:

% 1800f;c

9 365.

40.6242.78 44.91 47.00 49.05 51.05 53.01

10.00910.662 11. 323

f 2000;

21002200

;

11.98912.657

percent copper and 46 p the range from 300 to 1 tively low cost, high the atmospheres. Copper-Constantan Thtfrom that used with Iron

\

23002400

13.325 13.991 14.656 15.319 15.979 16.637 17.292"

I2500 26002700 54.92;

and iron. This comblnatic

down to -420 . The up[The above combination

I

28002900 3000

Platinum, 10 percent Rh< mocouple. It is used forbase-metal thermocouple

.By permission from P. H. Dike.

Thermoelectric Thermometry,

"

p. 82, Leeds and Northrup Company, 1954.

are not satisfactory. It is ito 3000oF but deteriorates

m

Thermocouples are me

.

m

m

calibration, it is possible to determine an exact relationship between the developed emf and the true temperature of the hot junction (Table 1-2).

the two wires; the ends ar

Another1 useful thermoelectric law states that, if a third metal is introduced intb the circuit, the total emf of the circuit will not be affected if the temperature of this third metal is uniform over its entire length.1

or sometimes butted togei head (Fig. 1-5a). The thermocouple wire junction, since contact at

Si

TOOLS pF THE METALLURGIST

V

DN

COPPERVS.

bNSTANTAN.

CONSTANTAN00.

0 9491

1 517.

The purpose of the extension leads is to move the reference junction to a point where the temperature will not vary. Therrnocouple wires are usually not long enough nor well enough insulated to run directly to the instrument. The extension leads are usually made of the same material as the thermocouple wires and are placed in a duplex cable with the individual covering color-coded for identification. Copper extension leads may be used in some cases, but then the cold junctions are at the junctionblock instead of the instrument and may be more difficult to maintain atconstant temperatures.1-8

.

3 967.

.

.

94 03 1218

6 647.

9 525.

.

12.575

.

15.773

.

) 25.

19.100

$ 32.

3 40.

Thermocouple Materials Theoretically any two dissimilar metallic wires will develop an emf when there is a temperature difference between.their junction points. Industrially, however, only a few combinations are actually used for thermocouples. These were chosen primarily for their thermoelectric potential, reasonable cost, grain-size stability, linearity of the 1:emperature-emf curve, and melting points higher than the temperature,

9 52.

to be measured. The first material in the combination is always connectedto the positive terminal. Chromel-Alumel Chromel (90 percent nickel, 10 percent chromium) vs. Alumel (94 percent nickel, 3 percent manganese, 2 percent aluminum, 1 percent silicon) Is ohe of the most widely used industrial combinations.

12.726 01.

9 43.

2 96.

6 53.

0 05.

It has a fairly linear calibration curve and good resistance to oxidation. It is most useful in the range from 1200 to 2200oF. Ifon-Constantan Constantan is an alloy containing approximately 54 percent copper and 46 percent nickel. This combination may be used in-

the range from 300 to 1400F. The primary advantages are its comparatively low cost, high thermoelectric power, and adaptability to differentatmospheres.

Copper-Constantan

The constantan alloy used with copper differs slightly

from that used with iron and may contain small amounts of manganese and iron. This combination is most useful for measuring low temperatures,

down to -420F. The upper limit is approximately 60bF.Platinum

The above combinations are known as base-metal thermocouples. 10 percent Rhodium-Platinum This is a "noble-metal" ther,

i;

p Leeds and Northrup Company.,

1954.

mocouple. It is used for measuring temperatures which are too high for base-metal thermocouples and where radiation or optical pyrometers are not satisfactory. It is suitable for continuous use in the range from 32 to 3000F but deteriorates rapidly in a reducing atmosphere.

Thermocouples are manufactured by cutting off suitable lengths of.

e an exact relationship between the jre of the hot junction (Table 1-2).

states that, if a third metal is mtrof the circuit will not be affected if the orm over its entire length.

!

the two wires; the ends are carefully twisted together for about two turns,

,

or sometimes butted together and welded to form a smooth well-rounded head (Fig. 1-Sa).

The thermocouple wirefe should be in electrical contact only at the hotjunction, since contact at any other point will usuallyi result in too low a

8

INTRODUCTION TO PHYSICAL METALLURGY

1 -9 Measurement of Emf

The tc

measuring the emf generamost accurate Instrument

4.

tially the emf developed bemf and is measured in ter3:'

;

calibrated in millivolts or ccalibrated. This informat

mi

strument should be used o

\

-

instrument.

[a)I

A simple direct-indicati Current from the dry cell i slide-wire and an adjustab resistance wire which may of divisions. With the poh potential along the slide-w

upon the current flowing tdivision. In order to stand

is of uniform resistance, t

to the fixed markings on tl fixed voltage Is connectei standard cell (S.C.) posliicis such that the current flI

-

JWWV

Xv a telescope and a control box. The'

Galv.

WWWW.'

0I

1---

/-

u

Filament too bright

--

==

"

glass filter mounted in front of thed filament upon which the objectivele electric circuit of the lamp, and an nge of the pyrometer.

OJ

\CJ

\iy Stdcell

Knob moves potentiometijr contact Z3, only-is pressedto close switch 5"

.

v

.

V . lose temperature is being measured.

Switch on

telescope

Filament correct

id] Circuit diagram

(c) Filament appearance

ii

parts of the measuring circuit shown

Fig. 1 -8 The disappearing-filament type of optical pyrometer. (a) Telescope; (6) circuit diagram; (c) filament appearance. (Leeds & Northrup Company ).

mI-

r -

14

INTRODUCTION TO PHYSICAL METALLURGY

iLi .,I

Some; advantages of the optical and radiation pyrometers are:1 2

Measurement of high temperature.Measiirement of inaccessible bodies.

a specimen is comparative! developed only after constatduce a flat, scratch-free, min

i. .

34

Measurement of moving or small bodies

a metallographic specimen p

No part of the instrument is exposed to the destructive effects of heating The principal-disadvantages are:

f;M4 Sampling

The choice of a sa

portant. If a failure is to beas close as possible to the artaken from the normal sectic If the material is soft, suchtreated steels, the section r the material is hard, the secti

1

Errors! introduced because the photometric match is a matter of individual

judgments

2

Errors introduced by smoke or gases between the observer and the source.

.

3 Uncertainty as to the amount of departure from blackbody conditionsMETALLOGRAPHY1-13 Introduction

off wheel. This wheel is a th

highspeed. The specimen sh>

Metallography or microscopy consists of the microscopic.

1-15

Rough Grinding

structural characteristics of a metal or an alloy The microscope is by far the most important tool of the metallurgist from both the1 scientific and and the size' '

study of the

'

Whenever p( is convenient to handle. Asi

technical standpoints. It is possible to determine grain size shape, and distribution of various phases and inclusions which have a great effect on the mechanical properties of the metal The,.

V'

"-

-

microstrlicture will reveal the mechanical and thermal treatment of the

-

i

.

-

metal, arjd it may be possible to predict its expected behavior under a givenset of conditions.I

it up and back across the sl specimen may be rough-groi cool by frequent dropping ii grinding and polishing opei pendicular to the existing sc stage when the deeper seracharacteristic of the finer ab the surface is flat and free of hacksaw or cutoff wheel are

Experience has indicated that success in microscopic study dependslargely upon the care taken in the preparation of the specimen.

The most1-16

expensive microscope will not reveal the structure of a specimen that hasj;

grinding is shown in Fig. 1-1Mounting Specimens that amounted to facilitate intermi

been pobrly prepared. The procedure to be followed in the preparation of

i

sheet metal specimens, thinin a suitable material or rigic

Synthetic plastic materials10s

mounts of a uniform conve,,

mI

1a

mm((7) W,

I..

diameter) for handling in sut when properly made, are ve ordinarily used. The most Bakellte, Fig. 1-9a. Bakellte colors, which simplifies th( specimen and the correct art are placed in the cylinder of ally raised to ISO , and a i simultaneously. Since Bakt-

Fig. 1 -9 1

(a) Specimen mounted in Bakellte enlarged 2X.

reached, the specimen moiit is still hot.Lucite is the most commc

(Q Specimen mounted in Lucite, enlarged 2X. (c) Specimenheld in metal clamp, enlarged 2X.

Ii

m

f i

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iTOOLS OF THE METALLURGIST tS. . ..

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1

iadiation pyrometers are:-

. 1

a specimen is comparatively simple and involves a technique which is developed only after constant practice. The ultimate objective is to produce a flat

le destructive effects of heating.i

I;

scratch-free, mirrorlike surface. The steps required to prepare a metallographic specimen properly are covered in Sees. 1 -14 to 1 -19.,

1-14

etric match is a matter of individual

Sampling The choice of a sample for microscopic study may be very important. If a failure is to be investigated, the sample should be chosen as close as possible to the area of failure and should be compared with onetaken from the normal section.

|/veen the observer and the source.5 from blackbody conditions.

such as nonferrous metals or allocs and non-heattreated steels, the section may be obtained by manualjihacksawing, If,

If the material is soft

.

.. .

opy consists of the microscopic )f a metal or an alloy. The micro1 of the metallurgist from both the is possible to determine grain size of various phases and inclusionscal and thermal treatment of the

the material is hard, the section may be obtained by use of an abrasive cutoff wheel. This wheel is a thin disk of suitable cutting abrasive, rotating at high speed. Thespecimen should be kept cool during the cutting operation. 115 Rough Grinding Whenever possible, the specimen should'be of a size that

is convenient to handle, A soft sample may be made flat by slowly movingit up and back across the surface of a flat smooth file. The soft or hard

specimen may be rough-ground on a belt sandor, with thife specimen kept cool by frequent dropping in water during the grinding operation. In all

;

inical properties of the metal. Thets expected behavior under a given

grinding and polishing operations the specimen should! be moved perpendicular to the existing scratches. This will facilitate recognition of thestage when the deeper scratches have been replaced by shallower ones characteristic of the finer abrasive. The rough grinding is continued untilthe surface is flat and free of nicks, burrs, etc., and all scratches due to the

sss In microscopic study depends aration of the specimen. The most e structure of a specimen that has o be followed in the preparation of-

1-16

hacksaw or cutoff wheel are no longer visible. (The surface after rough grinding is shown in Fig. 1-10a.) Mounting Specimens that are small or awkwardly shaped should bemounted to facilitate intermediate and final polishing. Wires, small rods,

sheet metal specimens, thin sections, etc., must be appropriately mounted in a suitable material or rigidly clamped in a mechanical mount. Synthetic plastic materials applied in a special mounting press will yieldmounts of a uniform convenient size (usually 1 in. 1.25 in., or 1.5 in. in,

i

-

ic)

diameter) for handling in subsequent polishing operations. These mounts, when properly made, are very resistant to attack by the etching reagents ordinarily used. The most common thermosetting resin for mounting is Bakelite, Fig. 1-Qa. Bakelite molding powders are available in a variety ol colors, which simplifies the identification of mounted specimens. The specimen and the correct amount of Bakelite powder or a Bakelite preform, are placed in the cylinder of the mounting press. The temperature is gradually raised to 150oC, and a molding pressure of about 4 000 psi is applied simultaneously. Since Bakelite is set and cured when thi's temperature is reached, the specimen mount may be ejected from the molding die while,

,

it is still hot.I

Ij

--

Lucite is the most common thermoplastic resin for mounting. Lucite is

i

n

INTRODUCTION TO PHYSICAL METALLURGY

completely transparent when properly molded, as shown In Fig. 1 -db. This transparency is useful when it is necessary to observe the exact section that is being polished or when it is desirable for any other reason to see the entire spepimen in the mount. Unlike the thermosetting plastics, the thermoplasticresins do not undergo curing at the molding temperature; rather

they set on cooling. The specimen and a proper amount of Lucite powder are placed in the mounting press and are subjected to the same temperature and pressure as for Bakelite (150oC and 4,000 psi). After this tem-

|,

perature has been reached, the heating coil is removed, arid cooling fins are placed around the cylinder to cool the mount to below 750C in about'

J

:

1

-17

7 min while the molding pressure is maintained. Then the mount may be ejected from the mold. Ejecting the mount while still hot or allowing it to 1 cool slowly in the molding cylinder to ordinary temperature before ejection will cause the mount to be opaque. Small specimens may be conveniently mounted for metallographic preparation in a laboratory-made clamping device as shown in Fig. 1-9c. Thin sheet specimens, when mounted in such a clamping device, are usually alternated with metal filler" sheets which have approximately the same hardness as the specimens. The use of filler sheets will preserve surface irregularities of the specimen and will prevent, to some extent, the edges of the specimen from becoming rounded during polishing. Intermediate Polishing After mounting the specimen is polished on a series of fernery papers containing successively finer abrasives. The first paper is usually No. 1, then 1/0, 2/0, 3/0, and finally 4/0."

'

,

iiFig. 1-10100X. (t>) Surface after intermediate polishing

The surface after intermediate polishing on 4/0 paper is shown in Fig. The intermediate polishing operations using emery paper are usually done dry; however, in certain cases such as the preparation of soft materials, silicon carbide abrasive may be used. As compared to emery1-10/j.

paper, silicon carbide has a greater removal rate and, as it is resin-bonded,

(a) Surface after rough grinding, ms

can be used with a lubricant. Using a lubricant prevents overheating thesample, minimizes smearing of soft metals, and also provides rinsing action to flush away surface removal products so the paper will not become clogged. ;1-18

magnification 100X. (c) Scratch-free surface al polishing, magnification SOX. Black spots areimpurities.

Fine Polishing The time consumed and the success of fine polishing depend iargjely- upon the care that was exercised during the previous polishing steps. The final approximation to a flgt scratch-free surface is obtained by use of a wet rotating wheel covered vyith 3 special cloth that is charged with carefully sized abrasive particles. A wide range of abrasives is available for final polishing. While many will do a satisfactory job, there appearsto be a preference for the gamma form Of aluminum oxide for ferrous and copper-based materials, and cerium oxide for aluminum, magnesium, and their alloys. Other final polishing abrasives often used are diamond paste, chromiunj oxide, and magnesium oxide.

The choice of a proper pol

terial being polished and thecloths are available of varyinjas silk, to those of intermec

and canvas duck and finally,

'

ishing cloths are also availaltwo, under the trade names

used. A properly polished sar and will be scratchf ree (Fig. 1

I1

"

--

:v.

i

.

TOOLS OF JHt METALLURGIST.

17

tiolded, as shown in Fig. 1-9). Thisj'

ssary to observe the exact section "'i able for any other reason to see the-

'

Ii'.

-

tie thermosetting plastics, the therat the molding temperature; rather I a proper amount of Lucite powder ire subjected to the same temperaPC and 4 000 psi). After this tem,

PI

g coil is removed, and cooling fins.

?ilri

.

the mount to below 75C in about'

iiSaintained. Then the mount may beaunt while still hot or allowing it to

I

&

11'"' Jmm \

f 'J

rdinary temperature before ejection&

irtfm

f'A

y mounted for metallographic prep-

|-f

device as shown in Fig. 1-9c. Thin M&ch a clamping device, are usually. .

'

.

:-

'

'-

"

:

|hich have approximately the samepf filler sheets will preserve surface the edges

'

'

C

-

?-]prevent, to some extent, ted during polishing.

i . p&

l the specimen is polished on a ;| jcessively finer abrasives. The first f"

'0

,

and finally 4/0.

.

.

.

-

.

hing on 4/0 paper is shown in Fig.rations using emery paper are usu-

*

*

es such as the preparation of soft

i

y be used. As compared to emeryicval rate and, as it is resin-bonded,

1

lubricant prevents overheating thenetals, and also provides rinsing oducts so the paper will not becomed the success of fine polishing de-

{

Fig. 1-10 (a) Surface after rough grinding, magnification 100X. (b) Surface after intermediate polishing on 4/0 paper,

magnification lOOX. (c) Scratch-free surface after

'

final

polishing, magnification SOX. Black spots fire oxide impurities.

| Objective

ww

//Front focal point of objective Metallographic specimen

Object area of

specimen (Erect andunreversed)Final virtual image (Inverted and reversed)

\\ll

IiK

of the coils, and the image strength of the coils while the microscope the image is bro

Fig. 1 -12 Illustrating the principle of the metallurgical compound microscope and the trace of rays through the

optical system fromlthe object field to the final virtualImage. (By permission from G. L. Kehl, "Principles of Metallographic Laboratory Practice," 3d ed., McGraw-Hill BookCompany, New Yorfc, 1949.)J

(I

Since metallographic spec is necessary to prepare, by sp to be studied. The specim( metallographic practice. It is of suitable plastic on the etc plastic begins to flow and pr between the plastic and the fully peeled off. To improve sten is evaporated onto the t

the shadowed replica is fragil

i

i

i

I.

i

:? >.

.

TOOLS OF THE METALLURGIST

21

'"

;

)

Retinal Image

(Erect and unreversed)Cornea

Entrance pupil of eye

> Huygenion eyepiecePrimary real image

I

of object formed by

\\

\\\w

objective and fieldlens at principal focal

point of eye lens, orwithin focus distance

[o) ( ).

as illustrated (Image

inverted and Reversed)

Fig. 1-13 (a) Metallurgical microscope, (b) Bench-type metallograph. (Bausch & Lomb, Inc.)

!

\\

I\

\

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microscope. Under certain circumstances, high-velocity electrons behave like light of very short wavelength. The electron beam has associated with it a wavelength nearly 100,000 times smaller than the wavelength of visible light, thus increasing the resolving power tremendously. An electron microscope is shown in Fig. 1 -143. Although In principle the electron microscope is similar to the light microscope (Fig. 1-14to), its appearance is very much different. It is much larger because of the highly regulated power supplies that are needed to produce and control the electron beam. The entire system must be kept pumped to a high vacuum since air would interfere with the motion of theelectrons.

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The lenses of the electron microscope are the powerful magnetic fields of the coils, and the image is brought into focus by Changing the field strength of the coils while the coils remain in a fixed position. In the optical microscope the image is brought into focus by changing the lens spacing. Since metallographic specimens are opaque to an electron beam, it is necessary to prepare, by special techniques, a thin replica of the surface . to be studied. The specimen is polished and etched following normal metallographic practice. It is then placed on a hot plate with a small pellet of suitable plastic on the etched surface. As the temperature rises, the plastic begins to flow and pressure is applied to ensure intimate contact between the plastic and the surface. After cooling, the replica is carefully peeled off. To improve contrast, a thin coating of carbon or tungsten is evaporated onto the replica at an angle and from one side. Since the shadowed replica is fragile, it is supported on a disk of very fine copper'

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TABLE 1-3

Etching Reagents for Microscopic Examination*COMPOSITIONWhite nitric acid1 -5 ml USES

ETCHING REAGENTNitric acid

REMARKS

H31

In carbon steels: (1) to darken

Etching rate is increase'd selectivity,

OO c

(nital)

Ethyl or methyl alcohol (95% or absolute)

100 ml..

pearlite and give contrast between pearlite colonies, (2) to reveal ferrite boundaries, (3) to differentiate ferrite from martensite

(also amyl alcohol)Picric acidPicric acid

decreased, with increasing percentages of HNO3. Reagent 2 (picric acid) usually superiorEtching time a few seconds to 1 min

o H

oz H

4g100 ml

For all grades of carbon steels;annealed, normalized, quenched,

More dilute solutions occasionallyuseful. Does not reveal ferrite grainboundaries as readily as nital Etching time a few seconds to 1 minor more

oI

(picral)

Ethyl or methyl alcohol (95% or absolute)

and tempered, spheroidized, austempered. For all low-alloy steels attacked by this reagentStructure of austenitic nickeland stainless steels

v

Ferric chloride and

Ferric chloride

5 g

Sm

hydrochloric acid Ammonium hydroxide and hydrogen peroxide

Hydrochloric acidWater

50 ml100 ml

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c

Ammonium hydroxideWater

5 parts5 parts

Generally used for copper and many of its alloys

Peroxide content varies directly withcopper content of alloy to be etched Immersion or swabbing for about 1 min. Fresh peroxide for good results

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