Morphology and crystal structure of carbides precipitated from solid solution in alpha iron

MORPHOLOGY PRECIPITATES

AND CRYSTAL STRUCTURE OF CARBIDES FROM SOLID SOLUTION IN ALPHA IRON*

W, C. LESLIE,P

R

M. FISHER?

and N. SENf

The carbides precipitated from solid solution in high-purity iron-carbon alloys, in two low-carbon steels and in an iron-carbon-3.25% silicon allay were studied by electron microscopy and diffraction. In the iron-carbon alloy and in the two low-carbon steels the carbide precipitates in a dendritic form during aging following quenching. In specimens quenched directly to the aging temperature oblong plates appeared, along with dendrites. Both of these forms are cementite (FE&) and no diffraction evidence of precipitation of epsilon carbide was found. The time required for formation of identi~able particles at 260°C corresponds to the time reported by others for 76 per cent aompbtion of the precipitation reaction, as memured by internal friction. At 150°C the time required for form&ion of such particles was much longer t&m the time for completion of precipitation, as measured by internal friction. The dendritic carbides in Fe-C alloys &ppea,rto p~cipitat~ on (110) planes of ferrite, with brnnches along (11 I> directions. In the iron-3.25% silicon alloy the carbides take t,he form of lentieular disks precipitated on
ET STRUCTURE CRISTALLINE DES CARBURES PARTIR D’UNE SOLUTION SOLIDE DE FER cc

PRECIPITES

A

-4 l’aide de la microscopic et de la diffraction Blectroniques, les auteurs ant etudie les carbures preeipitbs & pertir de la solution solids dans des a&ages fer-carbone de t&s haute pm&+, dans deuaz zmicrsk bas carbone et dans un &age fer-carbone-3,25% silicium. Dsns l’&sge Fe-C et dens les deux seiors It bas earbone, Its carbums pritcipitent sous une form% dendritique pendttnt le vieiflissement ~pr&s trempe. Dans tes Bchantillons tramp& directement B la temperature de vieilfissement, epparaissent des plaquettee oblongues en m&me temps que des dendrites. Ces deux formes sont const&ubes par la c&nentite (Fe&) et b diffraction ne r&v&? aucune trace de carbure E. Le temps necessaire 8, la, formation de psxticules identi~abl0a k 25O’C correspond au temps mesu& pm frnttement interne et requis pour que la precipitation soit r&&s& ii 76 per cent. Au contraire 2r 156”C, 1%temps r&&if k la formation de ces particules est beaucoup plus long que eelui indique par le frottement interne pour le d~roulem~nt eomplet de la pr&ipitation. Len carbures dendritiques dans les allixges Fe-C precipitent sur fes plans {l 101 de la ferrite, &vec des ramifications le long des directions {ill). Dans l’alliage Fe-C-3,255(/, Si, les carbures prennent la forme de disques lenticulniresprecipitant sur les plans (100) de la matrice ferritique. La structure cristalline bien que non d&ermintie, ne peut etre celle du citrbura E ni de la cementite. Au come d’un vieillissement prolong& ces disques sont deplaces par des films de cementite le long des joints de grains. ~~ORPHOL~GIE

UND KRISTALLS~~UKT~~ VON KA~,~ID~~USS~H~IDUNGEN A~P~A~~ISEN-MIS~HK~ISTAL~~N

-4US

Die K~rbid~usseheidungen sue ~is~hkrist&llen von hu~ll~einen risen-Kohlensto~egierun~en, zwei kohl~~toffarInen Stithlen und einer Eisen-Kohlenstoff-3,25°/~ Silizium-Le~~~g wurden mit dem ~lektronenmikroskop und mit ~lektronen~u~ng untsrsucht. Aus der Eisen-KohlenstoffLegierung und mxsden zwei kohlanstoffa,rmen Stiihlen scheidet sich das Karbid w&hrend des Auslagerns nach dem Absehreeken in dendritisaher Form aus. In Proben, die direkt auf die Auslagerungstemperatur abgesehreckt worden waren, erschienen l&ngliehe Pliittehen ~~~~rnrnenmit Dendriten. Bcide Formen sind Zementit (Fe&) und die Bougungs~ufnahmeu g&en keinen Hinweis auf die Ausseheidung von Epsilon-Karbid. Die Zeit, die bei 250°C notwendig ist, damit sich identifizierbare Teilcben bildan, entspricht der von anderen mitgeteilten Zeit, in der die Ausscheidungsresktion nach Ausweis von Dtlmpfungsmessuugen zu 70% abgelaufen ist. Bei 156°C WRPdie zur Bildung solcher Teilchen notwondigo Zeit vie1 liinger als die &it, in der die Ausscheidung nach Ausweis van DBmpfungsmessungen vollstiindig abgelaufen ist. Die dendritischen Karbide der Fe-C-Legierungen scheinen sich auf den jli6)-Ebenen des Ferrits auszusaheiden mit Asten ltings { 111 )-Richtungen. In der Eisen-3,250& Sifizium-tegieruug s&&den sich die Karbide in der Form van Iinsenf&migen Seheiben auf den {106)-Ebenen der Ferritmatrix aus. Die Kristallstruktur dieser Ksrbide wurde nicht bestimmt, sie ist aber weder Epsilon no& Zementit. Nach Itigerem Ausfagern werden diese Sehciben durch ~o~~en~enzementit ersetzt. * Received November 13, 1958. t Edgar C. Brain Laboratory for Fundamental Research, United St&es Steel Corporation Research Center, Monroeville, Pennsylvania. ACTA METALLURGICA,

VOL. 7, SEPTEMBER

I359

632

LESLIE,

FISHER

SEN:

AND

MORPHOLOGY

AND

INTRODUCTION

CRYSTAL

STRUCTURE

micrograph*

of Fig.

1.

OF

Electron

The precipitation of carbon from super-saturated ferrite during quench-aging has been a subject of

several years ago of an extraction

considerable

as

interest

for

a number

of

years.

A

Fro. I. Carbides precipitated in 0.02yJ C rimmed steel held 10 min at 705”C, quenched to 425”C, aged 10 min. brinequenched. Picral etch. x 1000.

aged low-carbon plates

actually

a number of discrepancies

In addition,

be resolved. calculated

For example,

to be spherical from

dependence

calculations must

be

appear

of the time during

micrographs purporting to been publish~.(3) Similar

plate-like(4~6~ and electron micrographs to support this view have been pre-

sented.(59g)

reports

been

have been made to show that the particles

which

solution

analysis

have

of the change of internal friction

aging(l12) and electron show this shape have

carbide

t#he carbides

It has been claimed

are precipitated

from

that two forms

super-saturated

in alpha

iron. f7*8~9$3) According cementite (Fe,C) forms during

of

633

mi~ro~aphs

made

replica of a quenchthat

in optical

what

appeared

nlicro~aphs

were

Fm. 2. Grain boundary and dispersed carbides in 0.047& C rimmed steel held 20 min at 72O”C, quenched to S15”C, aged 20 min, brine-quenched. Extraction replica. x 20,000.

review of the numerous papers on this topic shows that have arisen whioh need to

steel showed

or needles

CARBIDES

more complex

shapes,

the electron

particles extracted not compatible

as shown in Fig, 2.

diffraction

patterns

from

from a number of specimens were

with the crystal

strrrcture of epsilon

carbide. These observations, and the conflicting claims in the literature, provided the impetus for a more thorough

investigation

carbon from super-saturated

of the precipitation

of

alpha iron.

As will be described later, improvements in techniques of electron microscopy and diffraction now make it possible to determine

solid

structure

to these aging at

certainty.

of

the

precipitated

the shape and crystal carbides

with

more

about 200°C or above whereas at lower temperatures

In a paper published after this work was completed, Pitsch and Sehrader(20) have reported results similar

epsilon

to part of the work reported here.

(Fe,,,C)

measurements

occurs. indicate

However,

internal

that

precipitation

the

frietion of

carbon from solid solution in ferrite is a single-stage process.(lJ0J” Also, a recent analysis by Harn(l@) has shown precipitation shape

that in general reaction

providing

geometrically

the

the time rate law of a

does not depend upon particle precipitated

particle

similar, in contradiction

remains

to the earlier

suggestion by Zener(lg) which was used in the calculations previously mentioned.

2. MATERIALS

AND

PROCEDURES

The ~ompo~it,ions of the materials used are listed in Table 1. The high-purity iron was obtained in the form of a 25 lb ingot from National poration.

The major

impurity

Research

was 0.033%

Cor-

silicon.

Steel I was an enameling iron in the form of hot-rolled sheet, 0.140 in. thick. Steel P was hot-rolled lowcarbon sheet 0.127 in. thick. Steel A was a hot-rolled silicon steel in the form of sheet 0.110 in. thick.

Many observations, made at this laboratory, of the carbides precipitated in low-carbon steel during quench-aging indicated that these carbides were

A + in. thick slice was cut, perpendicular to the long axis of the high-purity iron ingot, then cold

certainly not spherical (12) but rather, seemed to taka the form of plates or needles, as shown in the optical

* All micrographs reduced approximately ductions.

50% in repro-

ACTA

METALLUXGICA,

TABLE 1. % Composition of materials used

VOL.

Electron

7,

1959

diffraction

patterns

were

obtained

from

particles on these replicas. 3. RESULTS

A.

Precipitate

AND

DISCUSSIONS

morphology

The extraction

replica technique

of electron micro-

scopy is very well suited to the determination _.--

---

--

shape and crystal

* Al. 0.001; N, 0.0013; 0, 0.0057; MO, 0.003.

precipitated

structure

from

solid

solution.

particles are not obscured

of the

of very small particles Details

by shadowing

of

the

and electron

rolled to a thickness of 0.035 in. Strips 1 in. wide were annealed in dry, purified hydrogen at 740°C for

diffraction patterns can be obt,ained without interference from the matrix or from thin surface films.

6 h, then cooled in hydrogen

in the cold zone of the

There are a few inherent

furnace.

carburized

should

They

were

then

in

Hs-CH,

be

limitations,

recognized.

During

the

however,

that

second

etch,

PIG. 3. Dendritic carbides precipitated during aging of Fe-0.014C alloy,brine quenched from 74O"C, aged as noted. Extraction replicas. ~40.000.

mixtures

at 740°C to carbon

0.009 to 0.014%, furnace.

The nitrogen

was 0.0003°/0.

specimens solution

contents

in the range

and cooled in the cold zone of the content

after this treatment

After cutting into smaller sections, the

were sealed treated

at

into

740°C

evacuated for l/2

silica tubes,

hr, then

brine

through the replica, the replica.

More particles

brine quenching.

to use extraction

tion-treated

steels were solu-

for 20 min at 72O”C, then either

quenched to room temperature aging temperature, or quenched

brine

and reheated to the directly to the aging

temperature. The specimens were brine quenched at the end of the aging period. Observations of the microstructure

were made as soon as possible

after

and etched surface,

so

that attempts to determine the density of precipitate from extraction replicas are not justified. During the separation,

low-carbon

sur-

appear than can be seen

on an equal area of polished

quenched. The subsequent aging treatments were done in lead-bismuth baths and were terminated by The two commercial

particles below the original

face may be exposed sufficiently to be pulled off with

the

angular

relationships

between

the

particles may be changed slightly so it is not advisable tions.

replicas

in habit plane determina-

The angles within a given particle,

remain unchanged

however,

by the extraction.

The dendritic shape of the carbide precipitate, as suggested in Fig. 2 can be seen clearly in the micrographs

of

Fig.

3 at higher

magnification.

These

micrographs show the particles in extraction replicas taken of specimens aged at 150” and at 260°C following

aging. Metallographic specimens were polished in the usual manner. Most of the electron observations were made on extraction The etchant used with these replicas was

and etched microscope

directions in the dendrite subtending an angle in the range 68”-73”. A third direction is faintly visible in

repIicas.(r3) picral, with

an addition

per 100 ml.

Fig. 3A, 3B and 3C, subt~n~ng angles of about 50” and 60” with the two principal directions. The branches of the dendrite appear to be made up of

of 2 ml Zephiran

chloride

a brine-quench

from 740°C.

There are two principal

AND

CRYSTAL

following the

STRUCTURE

a quench.

detail

of the

OF

CARBIDES

The shadowing dendrites.

angle between the specimen

635

process obscures

Depending

upon

the

surface and the plane of

the particle, shadowed replicas can give the appearance of either plate-like or spherical particles. The dendritic high-purity employed.

carbides

were observed

smaller for corresponding tures and exhibited comparing

bundles

of narrow

rods.

carbide

will be discussed

dealing

with

electron

particles. The distribution

tion

of

further

are shown

Fig. 7B with Fig 3C.

difference

branching

than

This can be seen by There was no dis-

between the carbides in the two i.e. the difference in manganese

low-carbon steels; content,, 0.05% and 0.520/o, did not seem to influence the size or shape of the carbide particles.

analysis

of

particles

the

is fre-

Typical denuded grain boundary in Figs.

1 and 2.

carbides

These probably

carbides

of this

alloys.

in a later section

diffraction

stringers of the dendritic shown in Fig. 4.

morphology

of the precipitate

quently nonuniform. regions

The

cernible

aging times and tempera-

less extensive

those in the iron-carbon FIG. 4. String of dendritic carbides in Fe-0.014C alloy, quenched from 74O”C, aged 3 min at 250°C. Extraction replica. X 40.000.

both in the

iron-carbon alloys and in the two steels The particles formed in the steels were

along

&ringers were observed

Occasionally

were observed,

as

result from nuclea-

dislocation

lines.

Similar

at earlier stages of precipita-

tion, as shown in Fig. 5, taken from a specimen of the 0.014°/0 carbon iron aged 30 min at 300°C following quench.

The contrast

of the material

deposited

a on

the replica is very low. It is probably carbon from condensed along the dislocation line. “atmospheres” This

material

did

not

give

a crystalline

diffraction pattern. The difficulty of resolving the complex dendritic

particles

by

conventional

electron

shape of the

surface

replica

methods 6A and

is illustrated by the comparison of Figs. 6B. Both micrographs were taken of a

specimen

of O.OOO”/OC iron aged for 1 min at 315°C

Fro. 6. Cornrarison between appearzu~ce of’ rxrbides on a shadowed replica end on an extraction replica. Fe- O.OO!JC alloy, brine-quenched from 74O’C, aged I nlin at 315°C. x 50,000.

In

all the

persisted

over

alloys

examined

the

dendritic

as can be seen in the micrographs

of Fig. 7, covering

aging temperatures from 150” to 524’C. higher aging temperatures the dendrites shorter

and thicker.

persists

even

when

The

characteristic

coalescence

(Fig. 7E). Some of the specimens of aggregates

form

a wide range of aging temperatures,

is well

contained

At the become 70’

angle

advanced,

a small number

such as are shown in Figs. 3B and 8.

These take the form of sets of L-shaped lines. The lines are approximately 30 A wide and 100 A apart. No electron diffraction patterns could be obtained

Fro. 5. An early &age in t’he precipitation of dendritic carbides showing darkened unresolvable areas. Fe-0.014C alloy, brine-quenched from 74O”C, aged 30 min at 200°C. Extraction replica. x 50.000.

from this precipitate. Pitsch(21) has found particles with the same morphology and with the epsilon carbide structure in quench-aged iron--carbon-nitrogen alloys. The presence of such particles in our specimens containing only 0.0003°/0 nitrogen, however,

636

ACTA

METALLURGICA,

VOL.

7,

1959

FIG. 7. Dendritic carbides precipitated in steel I (except A) in the aging temperature range 150”-425°C. Extraction raplicas.

FIG. 8. Carbide aggregate in Fe-O.OSGC alloy, aged 1 hr at 2GO’C. Extraction replica. x 200,000.

FIG. 9. Dendrite and oblong plates of cementite in 0.014% C ferrite, solution treated 74O*C, quenched to 372”C, aged 30 min. Extraction replica. x 55,000.

iA FIG. 10. Electron diffraction pattern of dendritic eementite particle in Fig. 9.

i..

FIG. 11. Indexing of spot pattern cementite structure.

l

on basis of

LESLIE,

FISHER

AND

SEN:

MORPHOLOGY

AND

CRYSTAL

STRUCTURE

OF

CARBIDES

637

FIG. 12. Large dandrkic carbide in Fe-0.096C alloy, held 1 hr at 260°C. Extraction replica. x 200,000.

makes it doubtful

that nitrogen

aging temperature of carbide

is required for their

80 kV.

As a check on the calibration

some diffrac-

tion patterns were taken of particles which had been

formation. When specimens

were quenched

directly

to the

after solution treatment, two forms

were observed.

These take the form

of

shadowed

with gold so that both patterns were super-

imposed.

Using this method the accuracy

is about

l/2 per cent.

However,

dendrites and oblong plates, as shown in Fig. 9. The

in comparing

frequency

and very thin crystals with X-ray diffraction data from bulk samples. The relative intensities of diffrac-

of occurrence

of the two forms

upon the aging temperature. plates were formed

Relatively

depends

few of the

at 315”C, but this form predomi-

nates when the aging temperature is 455°C. Dendrites grown in this manner become quite large. The largest dendrites observed were those separated from the ferritic areas of a high-purity iron specimen carburized to 0.096%

carbon, cooled in the carburizing gas in the

cold zone of the furnace, peratures. reached

The

then aged at various tem-

dendrites

a maximum

length

grown

in

of about

this

manner

1O-3 cm.

One

a great many diffraction

lines will be missing entirely.

This effect is also observed for X-ray diffraction

and is

particularly troublesome in identification of cementite because of the complex structure. The large change in the X-ray

diffraction

progress of tempering

pattern of cementite

during the

has been discussed by Jaokos).

Table 2 lists transmission

electron diffraction

carbides

data

of a sample of the

0.014% carbon iron aged for 100 hr at 100°C. A sufficient number of particles had formed to give a

B. Identi$cution of crystal structure of carbides precipitated from ferrite

powder pattern.

The carbide particles which precipitate during aging from ferrite are too small and too few in number in the samples to be identified by X-ray diffraction However, it was possible to obtain techniques. electron diffraction powder patterns from groups of particles and cross-grating patterns from single particles on extraction replicas. Most of the patterns I operating

data from such small

tion lines may be quite different and if the particle has only a few unit cells along one or even two dimensions

for a group of dendritic

such is shown in Fig. 12.

were taken with a Siemens Elmiskop

electron diffraction

of the data

caution is required

at

Similar patterns were obtained from

a number of other specimens. The data are compared with the standard pattern for cementite and epsilon carbide. These measurements, as well as visual comparison of the actual plates with similar patterns from known cementite and epsilon carbide, leave no doubt that the dendrites are cementite rather than epsilon. Identification

of the powder patterns as cementite

ACTA

638

METALLURGICA,

2. Interplanar spacings (A) for dendr itic: carbides IFeO.O14C, aned 100 hr at 200°C:)

TABLE

Observed spacing

Epsilon carbide?

1Intensity*

>ementite. (Fe&)

(Fe,.&)

2.15

W

k W

1

1.60

:W W

t!]

3.358 2.536 2.371 2.248 2.207 2.098 2.018 1.862 1.679 1.403 1.326 1.189

3.33 2.54 2.40 2.21 2.11 2.02 1.85 1.66 1.40 1.33 1.18

Miller indices of crystal planes

1.36 1.23

002 020 021 200 120 121 022 113 023 024 312 025

VOL.

apparent

7,

1959

in Figs. 3, 6 and 12.

These rods are 35 to

50 A wide, which is only 5 to 7 unit cell distances, so the diffraction

spots are stretched out along the direc-

tions of the branches of the dendrites. Even the large dendrites formed by quenching directly to the aging temperature

or by

gas cooling

temperature

exhibit

this diffraction

W-weak,

solution The

separate crystal although they are all oriented in the same way. The

oblong

following

plates

a quench

temperature

which

directly

during

aging

to an elevated

occur

aging

give an electron diffraction

which can also be interpreted M-medium,

the

streaking.

micrograph of Fig. 12 shows the make-up of such a dendrite. Each rod in the branches diffracts as a

~.._~ ~. * Intensities: S-strong, very weak. t Data from Jack’=‘.

from

VW-

instance the diffraction

pattern

obtained

of cementite

from lamellae

spot pattern

as cementite.

In this

is the same as that formed

during

the growth of pearlite. (13) The direction through the is confirmed by analysis of electron diffraction patterns of single particles.

thin plate is along the c axis of the orthorhombic

Fig. 10 shows the electron diffrac-

The small size and complex

tion pattern of the carbide particle in Fig. 9. Similar electron diffraction patterns were obtained from

habit plane of this precipitate.

dendritic

at least five directions

particles

over

the

full

range

of

aging

temperatures

and times.

This pattern is actually the

superposition

of two patterns at an angle of 70” and

carbide

increase

the

difficulty

of determining The observation

of the precipitate

can be seen

or {ill>

habit planes, unless more than one carbide is

present or more than one habit plane is adopted.

cipal

constant

and

so diffracts

as two

crystals.

During the early stages of growth of the dendrites third direction viously,

is faintly

visible,

as mentioned

but it does not appear to contribute

diffraction

a

preto the

pattern and vanishes as aging progresses.

When the particle

has relatively

few side branches,

the that

in one grain (Fig 1) eliminates the possibility of (100)

is a result of the fact that the dendrite has two prindirections

cell.

shape of the dendritic

70” angle observed

The

between the branches of

the dendrites corresponds to the 70’ 32’ angle between (111) directions

on (110)

paper on this subject,

plane and growth direction. able.

planes.

IIn his

most recent

Pitsch(2a) suggests this habit The choice seems reason-

Fig. 13 is a sketch showing the structure

and

one of the line patterns is very much weaker than the other. With the selected area electron diffraction technique it is possible with the

to relate the crystallographic

dimensions

diffraction

pattern

of the

obtained

section through the reciprocal crystal.

The coordinates

crystal.

direction

The

is equivalent

lattice of the diffracting

of the diffracting

spots may

be equal to the Miller indices of the crystal related to them in a simple manner. shown in Fig. 10 cannot be obtained carbide,

electron

to a plane

or else

The pattern from epsilon

but as shown in the sketch of Fig. 11, it can

be obtained

[ill1 a

from cementite.

The crystal grows with the b axis of the orthorhombic cementite unit cell aligned along the length of branches of the dendrite, the c axis perpendicular to the branches and in the plane of the particle and the a axis aligned through the particle. The streaking of the diffraction

spots along the C* direction is a result

of the fact that the branches of the dendrites are actually a parallel array of very narrow rods, as is

FIG. 13. Orientation

of growth of dendritio cementite ferrite.

in

LESLIE,

FISHER

AND

MORPHOLOGY

SEN:

AND

CRYSTAL

STRUCTURE

OF

CARBIDES

639

PREClP#TATE OBSERVED

FIG 14. Time required for precipitation of dendritic carbide from supersaturated alpha, iron (0.014% C, brine-quenched from 74O”C, reheated to temper&es indicated).

1

0.1

10

to

10 TIME

IN

l0

IO

MINUTES

15. Time required for precipitation of dendritic carbide from two low-carbon steels (brine-quenched from 72O”C, reheated to temperatures indicated).

FIG.

growth orientation of the dendrites. The carbides grow along the b axis of the orthorhombic cell which is the close-packed direction of iron atoms in the cementite structure. This same growth direction is observed for the carbides precipitated during growth of pearlite and bainite.(22) The third direction of growth, faintly visible in several of the figures, and making angles of about 50” and 60’ with the two principal directions, cannot be tt (111) direction.

The time required for precipitation of carbides from solution-treated and brine-quenched alpha iron, contaming 0.014°h carbon, in the temperature range 1.50” to 3WC, is indicated in Fig. 14. The times

required for precipitation of carbides in the two commercial low-carbon steels, at 200’ and 26O”C, are shown in Fig. 15;. The curves are based on the 6rst observation of a clearly defined precipitate on an extraction replica in the electron microscope, These observations were supplemented by the usual techniques of optical microscopy. Results obtained by both procedures agreed well. When clearly defined precipitates were obtained on the replicas, preoipitation could be observed on polished and etched surfaces by light microscopy, although the precipitate could not be resolved. When the time required for formation of the first recognizable precipitate is plotted vs. the reciprocal of the absolute temperature, the slope of the line

640

ACTA

METALLURGICAL

VOL.

7,

1959

0.2-

FIG. 16. Kinetics saturated solution

corresponds

to an aetivation

commonly

energy of 20,100 cal, the

accept.ed value for the activation

for diffusion of carbon in alpha iron. for

observation

however,

of the

is much

relationship. crepancy

of precipitation of carbon from superin alpha iron, as measured by internal friction.

A

The time required

first precipitate

longer

than

possible

energy

at

predicted

explanation

of

15O”C, by

this

this

dis-

is discussed later.

This may on the

replica, even though they are above the limit of resolmion of the microscope, are not recognized as carbides. Another

possibility

is that rate of precipitation

in

the specimens used may differ from those used in the

Very little difference was noted between the rates of precipitation

in the two commercial

considerable

difference

cipitation

of carbon as measured by internal friction.

be due to the fact that very small particles

steels despite the

in manganese

content.

Pre-

internal

friction

perform

both

work. internal

electron microscopy

It

would

friction

he desirable

measurements

to and

on the same specimen.

was slightly more rapid in the high-purity

iron than in either of the two steels, but the difference in solution

tcmperat.ures

makes the comparison

certain. At 2OO*C, a precipitate was observed 15 min in the high purity iron as compared l-2 hr for the steels. It is interesting to compare

unafter with

the cnrves of Figs. 14

and 15 with the data for the rate of precipitation

of

carbon,

in

as measured

by internal

Fig. 16. At 26O”C, a precipitate high-purity internal

iron-carbon

friction

precipitation

alloy

data(n)

is about

friction,

shown

was observed after

2 min.

at 250°C indicate

in the Wert’s

that the

70 per cent completed

after

2 min. At 17O”C, the internal friction measurements of Pitsch and Liickeo*) indicat.e that the precipitation of carbon is completed found

in about 200 min, which was

in this in~estigat,ion

to be about

the time

required to form the first recognizable precipitate. At 15O”C, complete precipitation occurred in about 400 min, according cipitate after

to Pitsch and Liickeo4’,

was observed

10,000 min.

decreases,

the

on extraction

Thus,

formation

as the aging of the

first

precipitate lags behind the completion

but no prereplicas

until

temperature recognizable

of precipitation

FIG. 17. Approximate length of time required for start a,nd for 50 per cent completion of isothermal precipitation of cementite in 0.02% C rimmed steel. Prior solution treatment 10 min at 705”C, quenched directly to the aging temperature.

LESLIE,

FISHER

SEN:

AND

MORPHOLOGY

AND

CRYSTAL

STRUCTURE

OF

CARBIDES

641

FIG. 18. Rate of growth of dendritic carbides at 200°C after quenching from 740°C and reheating (C!=O.O14%).

In discussing carbon

from

rate of isothermal

supersaturated

iron, a distinction

precipitation

solid solution

of

in alpha

must be made between (1) the rate

measured after quenching

from the solution tempera-

ture to room temperature

and reheating

temperature,

and (2) the rate measured after quench-

could be obtained by reflection from the surface of the

This is shown by a comparison directly

is observed

after from

about

740°C

5 set; to

room

A sufficient number of high-purity accurate

this

if the specimen temperature,

is

then

precipitate.

of observations determination

of the rate of

The length of the each aging period

Because of the shape of the particles,

is a measurement

of one-dimensional

sample.

Table 3 lists the interplanar

grain boundary with those

film.

Comparison

of cementite

table, identifies

Fe,C,(i6)

the carbide

also listed in the

film as cement&e.

result agrees with that recently and Leako5). The

occurrence

in silicon

which has a needle-like

spacings of the

of these spacings

reported

steel

appearance

This

by Leak

of a precipitate in a polished and

etched cross section has been observed for many years.

were made of

iron aged at 200°C to allow

growth of the carbide particles. largest particle observed after was measured.

After

bath at dOO”C, about 30 min

is required to form a recognizable

a reasonably

of Fig. 14 with

and Kristufek.02)

from 705” to 2OO”C, a precipitate

placed in a lead-bismuth

samples

from the solu-

The latter is the more rapid of the

Fig. 17 taken from Rickett

quenched

carbides

on an extraction replica, but deep etching left the carbide in relief so that electron diffraction patterns

tion temperature.

quenching

Fig. 19 shows typical grain boundary

to the aging

ing directly to the aging temperature two.

effects.

in a 3.20% Si, 0.01 %C alloy, cooled slowly from 870°C. These grain boundary carbides could not be removed

This precipitate

can be formed by various treatments,

including air cooling from high temperatures, heating to high temperatures followed by an isothermal treatment

in

the

temperature

range

250” to 6OO”C, and brine quenching temperature

followed

by

from

about

from an elevated

an isothermal

treatment.

growth.

The rate can be represented as a logarithmic function of time, as shown in Fig. 18. An extrapolation of the curve indicates

that the length of the particle would

be zero at 10 min which is a good check on the direct observat8ion (Fig. 14). D. Carhides precipitated

from

silicon ferrite

The presence of up to 0.5% manganese appears to have little effect on the kinetics of precipitation or the form of presence

the carbide in iron-carbon alloys. The of silicon, however, produces pronounced

FIG. 19. Grain boundary cementite in 3.20% Hi, 0.01% Picral-nital etch. >: 2000. C alloy, slowly cooled from 870°C.

642

ACTA

TABLE 3. Interplanar

METALLURGICA,

spacings (A) for grain boundary in silicon steel

-.

I

Miller indices of crystal planes

5.072 2.536 1.691 1.268 1.015

010 020 030 040 050

3.75 3.018 2.098 1.539 1.202

101 111 121 131 141

1.87 1.75 1.51 1.25 1.05

1.862 1.756 1.506 1.255 1.052

202 212 222 232 242

1.245 1.23 1.12 1.00

1.250 1.212 1.123 1.005

303 313 323 333

2.54 1.70 1.268 1.016

I

3.74 3.02 2.10 1.54 1.21

I

I

7,

1959

carbide

FeJYG

Observed spacing

VOL.

FIG. 22. Lenticular carbide from steel 1315T to 315”C, held 2 hr. Extraction

A, quenched from replica. x 20,000.

particles are believed to be carbides.

They are found

only

in iron-silicon

alloys

containing

they are displaced by grain boundary after prolonged heating. of round,

lenticular

micrograph

of Fig.

carbon,

and

cementite films

The particles have the shape

disks, as shown in the electron 22.

The

diameter-to-thickness

ratio is about 25 to 1. Fig. 20 is a micrograph of this precipitate

steel was completely The

quenching

Although

their

(at which

temperature

the

ferritic) then air cooled to room relatively

in Fig. 21 were obtained 1315”C,

occurrence

in Steel A, heated in an evacuated

silica capsule to 1315°C temperature.

showing a typical

large

particles

shown

by heating the same steel to

to 315”C, then holding composition

is

for 2 hr.

unknown,

these

Since not more than three directions cipitate

are observed

inferred

that the lenticular

one grain, carbides

of the preit can be

precipitate

clearly defined precipitate directions. The poles of the { lOO}planes of the grain fell on the normals to the traces of the precipitate.

This same observation

was

recently reported by Suits and Low(17). A typical

electron

is shown

diffract,ion

in Fig. 23.

pattern

of a single

The diffraction

spots

20.

FIG. 21.

FIG. 20. Lenticular carbides in steel A, air cooled from 1315°C. Picral-nital etch. x 100. FIG. 21. Lenticular carbides in steel A, quenched from 1315°C to 315”C, held 2 hr. Picral-nital etch. x 2000.

on

(100) planes of the ferrite. This was checked by determining the orientation of a large grain showing

particle

Fm.

within

FIG. 23. Electron diffraction spot pattern obtained from lenticular carbide shadowed with palladium.

LESLIE,

FISHER

\

SEN:

AND

MORPHOLOGY

AND

\a,

TRYSTAL TABLE

STRUCTURE

Observed

4.

0

MEDIUM

Intensity

W M W

1.43

S

form

a square

although

many

missing and others are very weak.

points

Except

are

1.34 1.94

WM WM

for inten-

sity, all points in such a pattern are equivalent

and so

1.47

W

1.56 1.04 1.21

WM W

the innermost spots must be indexed as (030), etc, in order to avoid fractional indices. This is shown in Fig. 24.

The interplanar

accurately palladium the

by shadowing

were determined

an extraction

replica with

so that the electron diffraction

palladium

and

superimposed. be concluded carbon

distances

a

precipitated

that the lenticular

precipitate

cementite

carbide but another carbide of unknown

S-strong,

were

were unsuccessful,

100 200 300 400 500 600 110 220 330 440 120 240 130 260 140 150 230 350 340

4. SUMMARY

spots is

coupled with the observation indicates that the iron

atoms have the same arrangement in the particles as in the ferrite matrix (in directions parallel to the That is, the iron atoms are

coherent on the surface of the particle;

account

for the lenticular

this

and electron of existing

The relatively

of the diffraction

spots

confirmed

from carbon atoms is too weak to be observed.

The

other possibility is that the crystal structure of the precipitate is actually complex with a large unit cell

concerning

the

supersaturated

solid

Some of the previous

inter-

techniques

by the observations

supersaturated

dendrites,

oblong

precipitate

This might

from

The carbides precipitated

may

reflections.

carbon

has led to the

used.

Calcula-

tion of particle shape from rate of precipitation has recently been shown to be fallacious,d*) and this is

parallel

be due to superlattice

diffraction

in alpha iron.

other than those already mentioned suggests that they indicate that the precipitate contains silicon atoms in an ordered arrangement, inasmuch as the scattering

of

CONCLUSIONS

discrepancies

tions of the experimental

of low intensity

of silicon in the

pretations(3T5yg) were erroneous because of the limita-

shape of even the

largest particles.

AND

clarification solution

could

I

of improved techniques of electron

microscopy

iron.

probably

I

W.M-media-weak

but the presence

The development

composition

precipitation

surface of the particle).

,I

in low-

the same as the (110) and the (200) spacings in alpha This observation,

M-medium,

I

carbide was indicated.

nor epsilon

and structure. The spacing of the most intense diffraction

that the habit plane is {loo},

for

-

pattern of

particle

The data are listed in Table 4. It can

silicon steel is neither

Intensities: W-weak.

1

6.06 3.03 2.02 1.515 1.212 1.01 4.28 2.14 1.427 1.07 2.67 1.335 1.938 0.969 1.468 1.19 1.563 1.038 1.212

of spot pattern of Fig. 23.

network

spa&es

Co-ordinates of diffraction spots

S

2.02 1.515 1.212 1.01

643

Calculated spacing

<-;MISSING FIG. 24. Indexing

CARBIDES

and calculated interplanar lenticular carbides _

06

Observed spacing

OF

lines

have

alpha

iron

take

plates

and

L-shaped

been

observed.

in the temperature

they are the predominant tures.

reported herein.

during the quench aging several The

forms;

arrays

of

dendrites

range 150”-48O”C,

and

form at the lower tempera-

The oblong plates predominate

at temperatures

above about 400°C. Both forms appear to be cementite, Fe.&. The branches of the dendrites seem to be

so that it contains a number of planes with a very low density of atoms. Although the square array of

made up of bundles of thin cementite rods. These branches have two principal directions of growth, with an angle of 70” between them, and a third minor

diffraction spots suggests a cubic or tetragonal structure, these are unlikely because of the large number of omissions in the pattern.

direction. No indication was found of the presence of epsilon carbide. However, no diffraction patterns could be

Attempts to determine the silicon content of the particles by use of the electron probe microanalyzer

is unknown.

obtained from the L-shaped arrays, and their structure

ACTA

644

At low aging temperatures,

METALLURGICA,

the rate of precipita-

VOL.

7,

1959

Smith and J. C. Swartz for their help in other phases

tion, measured by electron and optical metallography,

of the work, and to R. L. Rickett for suggestions which

is much slower than the rate of precipitation

led to the investigation.

by internal friction. to

an inability

This disagreement

to

recognize

particles

on extraction

specimen

composition

very

replicas

measured

may be due

fine precipitate

or to differences

and history,

in

solution tempera-

ture, and aging procedure. The habit

planes

of Fe&

precipitated

from

un-

alloyed alpha iron are probably { 1 lo},. The principal directions of growth in this plane are (ill),. A third minor direction

of growth remains unknown.

The presence of up to 0.5% effect

upon

the morphology,

or habit, of the carbides but the presence of 3.25% effect.

manganese

has little

rate of precipitation,

precipitated

from

ferrite,

silicon has a pronounced

The carbides precipitated

during aging of the

iron-carbon-silicon alloy take the form of lenticular disks on (100) planes of the ferrite. The crystal structure

of these carbides is unknown,

neither cementite

but they are

(Fe.&) nor epsilon carbide

(Fe,,,C).

ACKNOWLEDGMENT

The

thanks

of

the

authors

Szirmae for the preparation micrographs,

are

due

to

Albert

of many excellent electron

to C. P. Stroble,

K. G. Carroll, R. P.

REFERENCES 1. C. WERT, J. Appl. Phys. 20, 943 (1949). 2. C. WERT and C. ZENER, Ibid. 21, 5 (1950). 3. J. RADAVICH and C. WERT. Ibid. 22. 367 (1951). a. W. PITSCH, Acta Met. 3, 542 (1955).’ ’ 5. W. PITSCH, Ibid. 5, 175 (1957). 6. R. H. DOREMUS, Ibid. 5, 393 (1957). 7. A, L. Tsou, J. NUTTING and J. W. MENTER, J. Iron St.Ist.172,163 (1952). 8. 0. KRISEMENT, A&iv Pys. 7, 353 (1955). 9. G. LAGERBERG and B. S. LEMENT, Trans. Amer. Sot. Metals 50,141 (1958). 10. L. J. DIJKSTRA, Tmns. Amer. Inst. Min. (Metall.) Engre. 185, 252 (1949). 11. C. WERT, Thermodynamics in Physical Metallurgy, p. 178. American Society for Metals, Cleveland, Ohio (1950). 12. R. L. RICKETT and F. C. KRISTUI~EK, Trans. Amer.Soc. Metals 41, 1113 (1949). 13. R. M. FISHER, Symposium. 0% Techniques for ElectTon Microscopy. ASTM (1953). 14. W. PITSCH and K. LOCKE, Arch. Eisenhiittenw. 27, 45 (1956). 15. D. A. LEAK and G. M. LEAK, J. Iron St. Inst. 189,256 (1958). 16. K. H. JACK, J. Ivon St. Inst. 169,26 (1951). 17. J. C. SUITS and J. R. Low. JR., Acta Met. 5. 285 (1957). 18. F. S. HAM. J. Phvs. Chem. Solids 6. 335 (1958). 19. C. ZENER,‘J. Ap& Phys. 20, 950 ii949).‘ 20. W. PITSCH and A. SCHRADER, Arch. Eisenhiittenw. 29, 485 (1958). 21. W. PI~sca. Private Communication. 22. R. M. FISHER, 4th International Congress on Electron Microscopy, Berlin (September 1958). 23. F. W. C. BOSWELL, Actu Cryst., Camb. 11, 51 (1958).