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Jun. 1938.

 

 

TH 5515

LOWS

The writer wishes to express his
sppreoistion and may thsnks to Professor
H. l. Publos under whose direction this .
investigation was carried out. for his
assistance and suggestions.

Be slso wishes to express his ep-
prsoistion to Ir. Bering, o sue-her of
Itiohipn Btsts College houlty end Ir. base,
of the «an» llotor Car Corporation. for
their assistance in asking the physical tests.

.w‘ -I"‘34 , I" ”"4
d.\/ _ ii- 3'

mm: 21-. after tine allotnents of unm- types of steel
treated under identical conditions have provokingly failed to live up to
tested require-ents. fire of a standard composition treated in the nost
approved Inner to give the required results has discouragingly failed
when put to use. fluch failure when detected are translated into huge
losses for both the consumer and the mufacturer. Industry has bent
every effort to localize the indicators of the so-called abnornl steel.
Yet the steel being discussed, exhibiting such failures is not abnornl
as the word is usually understood. Specimens that have stood 19 under
static tests have failed nieerablyundor dynamic tests. Steel which
should display certain potentialities hae failed to even fulfil linin-
specifications. Steel of identical quality and worknnnship still fails.

It has been noticed that such failures occur particularly in steel
of low carbon content: steel of vhich the highest degree of ductility is
required. Such processes as stamping and the drawing of wire have desnnded
such ductility of steel and have likewise been confronted by this failure
of steel. laturally, every effort has been expended to overcome it. but
the probl- still reuins. 'l'ho author's attention was first turned to
this probleu by the failure of some specimens of steel when eta-pod into
(pokes and tineing gear covers. two specimens, supposedly the sane, (the
difference being that the one had failed, the other had stoned perfectly"
were i-ediately subjected to a microscopical examination. the canes for
the failure was soon evident. he speoinens were not identical. no one
that had failed showed a banded peerlitic structure. laturally, further
tests were not nde on the two samples of unidentical nature.

Inperinental work done in the past offered a clue towards the

solution of the ip. Low carbon steel in some cases when examined under
an oil imersion lens revealed a thin film around the ferrite grains.

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Undoubtedly if this film consisted of cementito it would affect nterially
the ductility of the specimen when tested. !his being trua the effort to
solve the problem resolves itself for the time being into a study of this
fil- surrounding the ferrite grains, its formation and ito subsequent
effect upon steel. ‘

Ehnt this film is conentito can be readily seen by examining the
nicrop'aphs of sons steel 3. i. r. 1040, following a slow cool from 1850‘?"
(Iig's. 1-7). i'ho caentito apparently flows fro- the cenentitic areas in
the pesrlite into the grain boundaries. It would be difficult in the
lidlt of the evidence offered to euggest that all the boundaries surrounding
the ferrite grains consist of cementito. Rather it seems more probable
that none of the boundaries are composed of comentito, especially in the
case of the lower carbon stools. Attention is called to the difference in
the free cementite formed in the grain boundaries of the low carbon steel.
i'here is the large noses of the cenentite similar to that noticed in the
ridat half of rigure I. further, there are large distinct cementitic
grain boundaries formed directly between the pearlitic areas. (Iig's. 2 a. 3)
And lastly there are cenentitic grain boundaries seemingly eminating from
the peorlitic areas and surrounding the ferrite grains. High. 1, 4, 6, d 7)

In an effort to study the forntion of cenentite in the grain boundaries
in the following proceedure was carried out: Sir pieces of 20% carbon steel
(:1 Hanna»1 25mm: from thosanobar,wereplacedintwobonbs':
heated in an electric furnace to 1850‘? for one hour: all six were then
furnace cooled to 1675’}: He's. D l, 2, d 3, were removed from the furnace
and allowed to cool in the bomb: lo's. l l, 2, d 3 were cooled slowly to
" Ibo speoinen was heated in a loskins automtically controlled electric
furnace at 185001 for one hour, cooled in the furnace to 132001, more

slowly cooled at the rate of a drop of 20°! every 10 ninutoe to 1250”, and
then furnace cooled to room telperaturo.

‘2 i'he bombs consisted of one inch steel pipes six inches long sealed at
“a m.

Fig. l

Mag. 2160 A

big. 2

Mag. 2160 X

 

in

r

1.4 .,

).'L

nd

1.3
rl

J

 

 

 

 

 

Fig. 3

Mag. 2160 A

Fig. 4

5
Fig.

2160 h
Lag.

5
fig.

“160 X
Mag.

 

 

 

 

 

 

 

 

 

Fla. 7

Mag. 2160 X

“r a, u . Ir
1'. C ..‘ [ . . 1313 ...

 

11759, at the rate of a drop of 20° every 10 ninutes, fron which point
they were furnace cooled to room t-peraturo. Both A: 24.3 and D m were
heated to 12600! held for one hour and water quenched. An and us were
then heated to 12605 hold for one hour and water quenched.

In order to afford a check on the work the experiment was repeated,
giving the treatment formerly given to the I. series to the 1! series, and
giving to the A series that which was given to the D.

It is apparent from the photomiorogaphs (Iig's. 8 d 9, corresponding
to £1 and D1) that the sins of the pearlitic areas increases, while the
umber decreasee, as the time consuod in panning thru the lower critical in
lengthened. Likewise the sire of the lamellae, both in length and breadth,
increases with an increase in tine. -Ocmpering the etructures formed after
the two rates of cooling as exemplified in steels numbered 11 (lig'e. 10 6 11)
and ”1 (fig. 12), it is observed that there is in the slower cooled steel,
£1, a .rgin of cementite around the nest of the pearlitic areas and that the
filn surrounding the ferrite grains evidently contains a greater enount of
cementite. whereas in 11 the poerlite seems to fill an entire gain
(whether it is an austenito grain or not, is difficult to say) the poarlito
in DI the more rapidly cooled piece sons to start fron the grain boundaries.
It is true that there are some cases noticed in which the poerlite is-
entirely isolated frn the grain boundaries but for the most part the first
stages of the pearlitic formation seens to center around the pain boundaries
as seen in l‘iguro 7. Attention is called to the apparent fact that in the
more rapidly cooled piece, very little cenentite existed in the boundaries
surrounding the ferrite grains. mo cenontito that was present was there
as free isolated cementite in the boundaries or else as a cementito grain
boundary between two pearlitic areas.

!he sins of the pearlitic grains in i2, the slow cooled steel following

 

 

 

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the first. ”to: quench, soens to be little affected. However, there is a {
noticeable loss of cenontito in the pearlitic areas: the argins in some
cases have disappeared and the lamollae have become thinner. lsrkedly
eneuda, the ferrite has become mob disturbed, not being as clean as
formerly. no grain boundaries have not increased in size. (fig. 13)

I rem the structure in 92, the more rapidly cooled piece after the
first water quench, it is evident that the pearlite is beginning to ballup,
not nrkedly but econ“ to cause a breaking up of the pearlitic fornation.
The grain boundaries evidently have becone a little larger thru this treat-
lent. (fig. 14) ‘

In lg, the slow cooled steel after the second quench. the poerlite
in .... oasos has started to break up: the la-ellar structure where it
still exists has become nuoh thinner. loticably, the censutito particles
have started to spherodise in the grain boundaries, not necessarily near the
pearlitic areas. (fig. 15)

In D3, the more rapidly cooled piece after the second quench, the
poerlite is very Itch broken up. here was not however, the sons spherodising
of ouontito in the pain boundaries as in L3. Yet the pain boundaries.
surrounding the ferrite were larger than in D1. ” 7": ”0

no hardness of the pieces treated above is shown in table I.

Specimen Rockwell Hardness lo. ‘3' Scale
1: 11 28.8

32 , 41.6

A3 A 39.:

n1 45.3

’2 . ' 46.8

1);, , 41

the results obtained by reversing the specimens used in the onerinent

F1?. 13

F '13. 1.0

mag. 1903 X

show the carbon content to be the same. In every case the observations
seuod to check those .de previously.
the hardness. of the pieces after reversing the treatment is shown
below in fable II.
lpecimon Rockwell Hardness of lo. '3' Scale
74

73.6

52.3
72.2

.Pd’J’ JJ:
.21

71

!he hardness of the slower cooled piece in each case was much less.
However it is noticed after quenching from below the lower critical, that
it compared very favorable with the more rapidly cooled piece.

rhe- far attention has been centered .. the cementitic fornations
which follow the change from gas- to alpha iron with a lowering of tem-
perature. In the absence of gum iron, the effect of heat treatment at
a temperature Just below the lower critical produces very different results.
If the treatment is continued long enough, a spheroidal structure is obtained,
especially if the structure so treated is in a fine condition. Apparently
following a spheroidal structm'e is obtained, especially if the structure
so treated is in a fine condition. Apparently following a spherodising
treatment, the cementito particles exist as isolated spheroids. If this is
true, consentite cannot have precepitatod to the grain boundaries upon the
transforntion fron m to alpha iron, although proof of their existence
in the grain boundaries would not preclude that they were deposited there
in their entirety at the time of transforaaticn. Rather, in the face of '

the work done previously it would some probable that a shall amount

would be procepetated at the grain boundaries following the transformation
the amount increasing by miaation with time. However, ordinary examination
of the structure of sphercdined.low carbon steel does not reveal any'grain
boundary. In order to clarify the situation and if possible, throw a

little more light upon this type of cementitic formation, the following
experiment was carried out: a sample of .40! carbon steel was heated

to 18809! in.a.Hoskinfis automatically controlled electric furnace and.held
for one hour, air cooled, reheated to 1850°I and air cooled again. The.
piece was then heated to 1250°I for fifteen hours and allowed to cool slowly.
following the cooling the sample wan etched for 2 seconds, 8 seconds, 4 seconds,
and 10 seconds, with.four percent nital solution.

In figures 17 ... 18, the 2 second etch was very ught. so lidlt in
fact, that a photcsdcrograph poor in detail was obtained. Even at that one
can observe order about the arrangnent of the spheroids. rhey seen to be
in some cases, laid down in strings. Iith.a little deeper etch, 3 seconds,

a clearer detail of this line like arrangnent is seen at low ngnification’,m
while at a higher mification a faint line is seen to run from one to

another of these spheroids?” the structure obtained with a 4 second etch

is shown in.!igures 21 and 22, very stailiar to the three second etch» lith

a 10 second etch at low unification. dark lines are seen to distinctly con-
nect some of the spheroids together. (fig. 23) lhile 'at a higher spin-
cation it is evident that the spheroids are connected by channels of cementite-
-¢ain boundaries of cenontite if one so wishes to call then that. (fig. 24

d 25) Upon closer ennination it can be noticed in either figure 84 or 25.
that the ceaentite actually runs from the spheroids into the grain.boundaries.

Ihother all the spheroids are connected together in the same mner

is beyond the scope of the paper, although there seems to be some indication

Fig. 17 , Big. 18

hag. 580 x hag. 2500 A

Fig. 19 rig. 20 ,

nag. 580 X Mag. 2300 A

a

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thew ny he. he pertenent point to he pined is that in further
observation of spheroidnl structure in low carbon steel, these ce-entitic
anin haundnries last he considered, st leeet nentslly.

Iron the results ohteined in spheroidising, the force of cohesion is
one thet mt be contended with. net it exists ct the time of the cementitic
ferntien is undoubtedly true. But the force «cohesion comes into predom-
insnce when n niaction of the cenentite becones possible in the sheence of
n precipitsting constituent such es gen- irono-nore briefly, when the steel
is heeted to e point Just below the L1 point. Yet considering the tine
enwolwsd, the force must he ensll. However, if sufficient tins is sllewed.
the question arises: will the cenentite 'hsll-vup' further in pein hound-rise,
end will the min boundaries themselves. tend to “hell-up“? Before sctmlly
developing the experimnt, the feet that the mount of cementite present
my alter the circumstances enough to elter the results, lust he considered.
fhus. another wsrisnt is introduced into the experinent, which is es follows:
three types of steel (.20! csrhon, .40! carbon, end eutectoid) were considered.-
Ehe eutectoid steel ens received in g, spheroidnl condition. !he .20! csrhon
end the .40; when steel were heeted to 185001, sir-cooled, rehested te 1950”,
end sir-cooled to plece the specinens in e fine condition. 'i'he three steels:
in this condition were then pleced in the furnnce end hested to 12605.
Iron tine to time the pieces were taken cut end emined, cud phetonicrogrephs
were suds. then the .40! when steel hed been subjected to the trestnent
for 209 hours end the .20; carbon and the euctectoid steel for 223 hours, the

ssnples were rescued.
the structures considered in the enperinent ere shown in figures 26-76.

h ohserwntion ehert is shown. (hble IV)

1‘10. NO.

36-27

28-29

30-31

33-33

34-33
35.37

38
39-40

41-42

45-46

47

48-53

54-55

mun

mmmrrmrm

3.1.1. 1040 after m1.
air cool fro- 185$}.

34.3. 1040 after 15 hrs.
at 1250*.

84.1. 1040 “3.: 24.5 we
.3 125001.

34.1. 1040 after 36.5 hrs.
at 1200”.

84.3. 1040 after 46.5 hrs.
‘3 IMe

84.1. 1060 after 57 hrs.
50 min. at 1250”.
8.1.1. 1040 after 52 hrs.
50 min. at 1250.,0

2001 steel about lutectoid
composition after 72 hrs.
1250”. Structure spher-
old-.1 30 start.

BJJ. 1020 after a double
air quench fro. 1850".
followed by a heat at
1250”. £01? 72 hrs.

84.]. 1 after 72 hrs.
‘3 lzgubg’e

8.1.1. 1040 after a
spheroidising treatment
at 12500. for 15 hrs.

34.3. 1048 “t” 76 hrs.
‘3 1250 a

5.1.3. 1040 after 89 hrs.
30 min. at 125001.

0381371110! 30‘!”

Structure is very fine previous to
spheroidisin; treat-ant.

there is a tendency to spheroidise.
However, the spheroids are angu-

lar. lo novenent to the ferrite
pp. Only a few lanellae left.

Advance-cut of spheroidisin‘ process.
lots inter-cementi ts connections.

ihe spheroids are snll and have lost
their angular nature.

i'here is not much diffusion. fhe cen-
entite seems to be lengthening out.

Lengthing of the censntite particles.

rho ceaentite seems to be lengthening
into parallel rods.

fhe spheroids are larger. here are
a few indications of the spheroids

lengthening out.

fig. 35 shows the connection between
censntite particles.

11;. 36 shows cementite in the pain
boundary as a result of nee action
or surface, tension.

Parallelism 0f the censntite is notice-
able. !he grain boundaries still
exist.

A typical example of spheroidised
plain carbon steel.

Inrgins have coalesced-lanollae
appear to run from the margins.
fig. 52 d 53 show cementite in the

grain boundary and also the appear-
ance of cementite in the ferrite

grain.

Tie. 54 shows the development of the
parallel structure.

11;. 55 illustrates censntite in the
gain boundaries.

HG. '0.
5595?

58-59

63-65

66

5'?

68-69

74-75

mu 1' (can't)

turn um um mm
54. ’e IMO $fter 98 ”Is
45 I111. at 1250°r.

10 lin. ‘3 IWe

0.1.1. 1040 after 119 hrs.
so min. at 1250°r.

84.3. 1040 after 132 hrs.
35 M‘s

84.2. 1040 after 148 hrs.
It 125Me

84.3. 1 after 135 hrs.
83 1250 s

84.8. 1040 after 148 hrs.
It 1250”.

54.3. 1040 after 153 hrs.
50 .111. ‘t IWe

84.3. 1010 after 209 hrs.
at 1250”.

is recieved spheroidal
eutectoid tool steel
after 238 hrs. 30 nin.
at 1250°r.

go‘ege 1030 at” 238 ”Is
30 nin. at 1250°r.

OBSERVATI 0' IO!!!

fhe length of the cementite
particles scene to increase.

A snearing action of polishing seems
to predominate. However, the re-
lative increase in size is evident.

r15. 60 show an ennple of the parallel
causnti te rods or plates.

fig. 61 illustrates this same point and
also the lengthening of the calen-
313's

Illustrates the development of paral-
lelisn in the cementite particles.

8hows spheroids in the grain boundaries
and cementite in the ferrite.

8hows long develop-ant of cenentite
chain

Due to the cenentite and min bound-
ary has apparently increased in
size.

the devel opnent of a lanenated calen-
tite together with partial coale-
scence is noticed.

Develop-ant of the ceasntite lanellae.

fig. 71 shows colascenoe of lansllae
with spheroids in the grain bound-
me

A little develop-sat ofthe laninated
structure is noticeable but it is
exceptional.

lvidentully all the cementite has
nig'ated to the grain boundary.

figs 26 ' ‘16. 27
Mag. 950 X Mag. 1905 x

Mag. 955 x ' Mag. 1905 x

 

 

 

 

 

 

 

 

 

Fig. 50 11g. 51
Mag, 1905 1 ‘ Mag. 3075 K

Fig. 52 ._ Fig. 55
Mag. 7910 x ‘ Mag. 1910 x

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Fig. 34 . Fig. 35
Mag. 1905 X Mag. 1905 X

Fig. 35 Fig. 57
Mag. 2450 x 'aag. 950 x

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Fig. 38 Fig. 39
Mag. 1905 X Mag. 800 A

Fig. 40 _ Fig. 41
Mag. 1600 X _ Mag. 1905 X

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Mag. 1905 X

Fig. 46
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Mag. 1890 X mag. 2040 1

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Fig. 60 Fig. 61
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Fig. 6? Fig. 6?

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Fig. 64 _ Fig. 65
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It my be generalized Rom the data obtained in thie laet experiment.
that in low carbcned eteele. in particular. cementite ie precipitated to the
grain boundariee after a long heat treatment below the lower critical. Further.
that in eteele other of eutectoid nature. cementite tende to migrate to the
grain boundariee. i'he epeed' with which cementite or the carbon nigratee at a
teqerature Juet below All depende largely upon the amomte coneidered. Ehie
ie evident by the fact that after 238 houre. the epheroide reamined ae euclin
outectoid eteel. lhereae with low carbonateel. ae hee boon ehown, migration

taken place quite rapidly.
It hae boon tho firm conviction of moat anthoritiee that cementiteie

very hard and brittle. Banveur acne up tho general thought predominate from
tho time of Howe to tho recent. by eaying that. “Cementite ie an extremely
hard eubetance. being in not the hardeet of all the conetitnente occuring in
iron and eteel. harder even than hardened. hit carbon eteel.‘l Howe etatee
that it ie harder than glaee and nearly ae brittle. to it ecratchee feldepar
and not quarts, it ie generally aeeigned to rank of 6.5 (lohe ecale of herdneee).
Othere have endeavored to determine the exact hardneee. indicating euch

‘ hardneee ee hid: ae 8.20 Brinell ae the true value. Yet the writer hoe found
evidence upon different occaeicne iich would indicate that cementite, or ouch
me we know it. ie not alwaye hard. If the hardneee cf cementite doee very.
then the comentitic etructuree likewiee varying. will affect the eteel. In the
ease faehion the cementic etructuree which are being etudied ehould vary.

In order to determine the herdneee of the cementitic forntion, a micro
character herdneee nchine wee firet need. However, it no econ evident that
ouch a device could not eerve the purpoee. fhe reeulting ecretch wae much too
large. So that any relative difference that might exiet between the eamll
foramtion wae completely nullified by the large force brougt into play at the
point of the diamond need to ecratch the epocinaa. In an effort to overcue

thie difficulty. verioue methode were devieed. finally. the following wae need
to determine the relative hardneee“: the eagle to be teeted wae pcliehed in

euchannnerthat

unetched, the ecratchee ran in one direction only: the ecratchee did
not go eo deep tht they could not be rmved by the etchant: the eagle
wae ecratched at a 90° angle to the poliohing ecratchee, the ecratch being
of the em- .Qitude ae the forntion coneidered. (an. hardneae obtained
by thie aethod ie one of abraeien. the reeulte of couree can not euhetitnte
for the other forme but a difference in the abraeive hardneoe met indicate
a difference in the conetituent caueing the hardaeee. further. the author
ie trying to ehow a relative difference between the variouo forno of
cementite and not an aboolnte hardneoe of oeuentite.) rho aothod of hardneeo
ecratching ie carried out by eatunting oono broadcloth with pure aotnyl
alcdel following an impregnation with a few graine of Levipted Aldaa
powder. l'he oaaple ie then placed. etched eurface downward. on the cloth,
and moved firmly about an inch and a half acroee the eurface. Iho eharp
graino. held no they are. act ao the diamond point. cutting deeply into the
eoft nterial and ligtly into the hard. core muot be need when ecratching
low carbon eteel. for the etched eurface io deetroyed very eaeily. In the
caee of low carbon eteel it wao found that cheeee cloth. need a few tiaee
previouoly. would ecratch the eurface of the epecimen when rubbed acroee it
and to aid in dyyiag. In thie work impuritiee which might canoe miecel-
laaeoua ecratchee muot be removed or elindnated after etching. In order to
avoid any extra ecratching that nifit arioe from drying with a cloth. after
waehing with a feat otream of water. the' amplee were inneroed in xyol and
then metlvl alcohol (muet be pure or a film in left on the eurface) and
allowed to dry in the air.

the epecimene of eteel tooted in the above manner contained .4“. .795-
.oofi. .905. .9”. and a 1.201 of carbon. in both the noun one opheroidal
forme. Of theee it ie intereoting to note that of the two. the otheroidal
form wao harder to ecratch. two and ooaetineo three ecratching operationo
being aeceeeary to obtain ecratchee of the eerie ngnitude ao the fornticne.

10'

coneidering in detail the epheroidal etructuree for the five different
oteele. ehown in ligureo 77-81. it can be eeen that the ecratch ridee
ever or around the cementitie epheroide in almoot every caee. deepite the
difference in carbon content. (the photomicrographe ehown in rigure 77
wae taken from a {IO-.80! carbon eteel. l'igure 78 from a .405 carbon eteel.
Iigure 79 from eutectoid eteel. l'igure 80 from a .9” carbon eteel, and
Iigure 81 from a 1.20! carbon eteel.) ‘l'here were a few caeeo where the
ecratch cut throudi the ephoroid. but thie wao an exception. (See Figure 79)
Undoubtedly. the cementite in a hrd coaotituent: yet let uo turn our
attention to the oteelo when the cementite in in a laminated condition.

figure 82 ehowe a ecratch about the nine of the lamellae it ie cutting.
Clone examination reveale that the lamellae are bent in acme caoee. not all
being cleanly cut. Such io contrary to current enpectationo. If the
lamellae are brittle. protruding plateo or rode. they would chatter when
the fine point of the alumina pain comee in contact with it. In figure 83.
ehowing a .77-.aofi carbon eteel. the cuentitic name. did not crack or
ohattor. but were bent downward. Likewise in both the .99; carbon ‘oteel
(figure 04) and the 1.20% carbon eteel. (I'igure so) the lanellae were not
ohattored, cracked or ridden over,-they were bent. a conetituent almoot
ao brittle ae glaoo bent throng: a 45 to 90° angle! lore etartling than
that ie the fact that the ecratch while ploughing thru the cementite in
pearlitic areao. rode over the cementite in the grain boundariee of the
hifi carbon eteel (Figure 85). i'hio oame variation between the cementite
in the grain boundariee and the pearlitic areao wae noticed in the low
carbon eteel. .

lvidently the hardneee of cementite not vary.

lith a large difference enieting between the cementite in the grain

Fig. 77

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11

boundarieo and that in the poerlite. there muot be acne noticeable
effect upon the ductile propertiee of eteelo depending upon the location
of the cementite. In order to determine the exact effect theoe cementite‘
forntione have upon the ductility of low carbon eteel. epecimene of

5 inch round, no; carbon, cold rolled ....1 were prepared in the regular
manor for the Iaod iqact teete. All the piecee were heated in the
furnace to 1860” for one hour. they were then cooled to 1676”.

torieo 3 wae cooled rapidly throng the 5341-1 range. but elow enoudl
to obtain poerlite. Seriee A van cooled olowly throng: the cane range
taking at leaet two houre to cool to 1175, cooled in the furnace to room
temperature. £2 and 82 were then heated to 1260‘! and held for one hour,
after which they were water quenched. A2 and n; were heated to 1260"]I
for one hour and water quenched apin. Iqact teete were then do
following a microocopical examination. the reeulte are ehown in i‘able V.

mammncre
mnv

Izod Impact valuee Bckwell Hardneeo lo. '39 Scale
:1 26.5 . 69.6
, 31 46 . 78.6
1.2 44 77
32 47 77.6

1. 'fhe oiee of the pearlitic areao increaeee with the tine coon-ed
upon paeoing throudi the critical rangee an - 1,3 .

2. he formtion of the cementite and the cenentite in the grain
boundarieo between the pearlitic areao ie related to the epoed with
which the piece paeoeo through the critical rangee.

3. ‘l'he amount of cementitic precipitation at the gain boundariee
aurrounding the ferrite grain increaeeo with the tine consumed upon

4.

5.

6.

7.

12

paeeing through the traneforntion range.

fhe hardneoe of very elowly cooled eteel can be made to increaee

by heating to a point Juat below the lower critical and quenching.
Continued heating at a point below the lower critical decroaeee

the hardening qulitiee elightly. following a quench.

here are epheroide of cdmontite precipitated .in the grain boundary
of low carbon epheroidal eteel.

fhe readineeo with which cenentite (temperature at a point Juot below
the lower critical) nigrateo to the grain boundarico in the preeence
of alpha iron io governed by the mount of carbon preeent.

fhe hardneoe of cementite variee. Ehie variation ie moot noticeable
between the cementite in grain boundarieo and that in the pearlitic
areao.

Ihat brittleneeo of a couple cooled rapidly (olow enough to fora
poerlite) through the critical range ie leeo than one cooled very
elowly. .

018008810!
Undoubtedly an explanation ehould be forthcoming. Yet the author

heeitatee to depart radically from the generally accepted theorieo to

enpalin the facto found in the expermental work. In eo for ac pcooible,

concurrent theorieo predoninating at the preoent will be uoed‘ae a baeie.

However. the facto for the neat part are in thenoelvee radical. lot that

they are now: rather they have been over-looked or aeouned in the poet.

In order to explain than it will be neceeoary. to deviate from the general

thoudit a little.

it the outoet cognizance io ado of the idea advanced by come. that

the etructure obtained by etching ie not the true otructure. fhoee who

advance thie theory believe that the etructure obtained by etching io the

13

reault of a chemical action which midit indeed Juatify their concluaiona.
However. oince the name baoic atructuroo ehould give the cane final atructure
with the acne etch. and oince the work preeented here .ybe cowared with
the peat. the theoretical idea that the atructureo viewed are not the true
etructlu'ea will booet aeide.

rerhapa in coneidering a poeeible explanation for the forumtion of the
cementitic film. it might be well to review briefly the phenonana. no it
- ie generally underotood. that takea place no a opecimen of eteel ia olowly
cooled.

Above the upper critical. hypoeutectoid eteel coneiete of auotenite.
generally regarded no a eolid eoluticn of carbon or iron carbide in go.
iron. which in cryotalline in nature. it a eufficiently hid: temperature
(1850” no wao need in the erperinent) the auotenite graino will be hono-
geniouo. altho the compoaition uy vary from grain to grain. he the toner-
ature ie lowered to below the upper critical. alpha iron begina to form.
we newly formed ferrite hae a certain amount of cementite retained
mechanically within it. which oince it ia nore aoluhle. in an iron. migratea
upon olow cooling from the alpha iron. It io believad that above the lower
critical cementite ia aoluble in alpha iron to a certain extent. eo that
not all the cementite will paao over to the auotenite. it any definate
temperatue equilibriu io quickly eetabliohed between the cementite in m
iron and that in the alpha iron. do the tempeuture ia lowered more alpha
ferrite ia formed which will leave a onller amount of gum ferrite. [ocea-
aarily becauoe the carbide in more acluble in an iron. it will become
increaeingly rich in the carbide no the tomerature ia lowered. however. if
equilibrium ie to be eetabliohed at a lower temperature. aoeuming that the
partition conetant to re-in conetant, than more and more carbide will be
contained in the alpha ferrite. Finally. a point will be reached where the

ferrite becomea euperoaturatod and onmll carbide particlea will co-ence to

timber?!» I»- .3..er IL, '._.I..|llv .D‘1.I"l..,£l.l

1]. .4:

14

precipitate from the aolution. (fhe aoluhility of iron carbid in alpha
ferrite ie never great and not become increaaingly leeo no the taperature
ia lowered.) According to J. h. Ihitely‘. thie precipitation will occur
in the regiono of in.

Before reaching the lower critical the effect of varioua cooling
ratee upon the foreation of alpha ferrite ehould be ccneidered. In thie
diacuaaion the cooling rate ia never coneidered ae being rapid enough to
fern any other atructure than aerial poerlite. rho work of H. 0. E. carpenter
and J. I. Robertaon thrcwe acme light upon thie pcint“. Upon olow cooling.
alpha ferrite etartea to precipitate at the grain boundarieo. lot entirely
around the auotenite grain at once. for due to phyeical differencee. heat
io not radiated equally. Once the precipitation haa aterted the fora-tion
of the alpha ferrite cryotal increaaea in oiae no in the formation of
cryotala from liquid aolution upon cooling over a long period of time. he
the rate of cooling ie increaeed the forention of alpha ferrite takeo place
at more nueroua 'centera. thuo giving rice to onllor cryotalo. If the apeed
ia increaeed etill further. ferrite can be found to form in the auotenitic
areao. tending to break it up. Fhie viewpoint openo up an explanation of
the plate-like formationa of cementite. for according to Carpenter and lebert-
eon. “large neoeo of reoidual auotenite alwayo tranoforma to poerlite. but
enll filme any. under certain conditiona. give rice to cementite. Fhia i‘a
formed when the particlea of reoidual auotentite are very thin. Under theee
condition. all the carbon in the auotenite tranafcrma to the plate of
cementite and the ferrite adda iteelf to the enieting cryotalo."
*1 Ooaleacence of Peerlite. Journal Iron and Steel Inatitution lo. 2.

pp. 147. Sept. '29. '

'2 Formation of Ferrite fron Auatenite. Journal of Inatitute of Iran 5
8*..1. Ibo 1’ m0 363. .31.

15

“For the forntion of cementitic filme , however. it ie neceeeary
for all the carbon in the particlea of reoidual auotenite to form
cementite about one center. Fhia requiree time for the neceeeary dif-
fuaion to take place. and the faeter cooling the greater the tendency for
the cementite to form at a nuber of pointo. thua giving poerlite.“

Undoubtedly for ouch an explanation of cementitic filmo. the conditiono
are narrow. for the rate of cooling not be rapid enough to cauoe the aplit-
ting up of the auotenite grain and it moat be olow enoufi to provide for
diffuoion to take place. From their work it ia apparent that they did not
coneider the forention of a fine cementitic film around the ferrite pain.
Fer did they coneider the effect of paaeing thru the an upon the fornticn
of the film.

l'o continue the diacueaion. at a point Juat above the lower critical
two phaeeo eniet in equilibriu. auotenite and ferrite. Ihether there ie
a diotinct boundary oaperating the two io a hoot queation. Fheory and acme
evidence would indicate that there io. altho to the preeent time no proof
of ita exietence haa been nmde in every caee. It reamine for better
technique or better equipment to anewer thie queation. it the 1.1 the
auotenite ruining changes to pearlite. l'hie change according to J. H. mm,-
takeo place over a range of tacperaturea. fhe initial pearlitic formticn
takeo place at the grain boundary when cementite etarto to precipitate there.
(Figure '7) the came for ouch a precipitation my be due to inoculation of
the ouporoaturated aolution of you ferrite with ninute cementitic particlea
precipitated from the aux-rounding ouperoaturatod aolution of alpha ferrite
or. it my be due to nuclei of inpuritiea eituated in the grain boundarieo
canning the precipitation. Following the theory advanced by lhitely, the alpha
ferrite becomea aupereaturated with carbide with reopect to the cementite.

" Ooaleocence of Pearlite. Journal of Iron and Steel Inotitute. lo. 2
pp. 147. September '29. '

16

With the reeult that the carbide ie precipitated on the lamillae of the
pearlite at the enpenee of the gamut iron aolution. Iith continued

depoaticn of cmentite upon very olow cooling there will be little chance

for may now nuclei to form. thuo explaining the difference in atructuree
obtained (Figure'e 8 8e 9) upon continued olow cooling thru the tranoition
etage. no mrgine of the ldllae meet and coaleaco. and eventually.

over a long enough peroid of time. the cementite would. coaleaco into a

moo. which would account for the plate-like formtion in the gain boundarieo.
fhe cementite in the grain boundarieo between the pearlitic areao can be
accounted for by either the precipitation at the grain boundary of a cementitic
aeotion which later developed. the natal lamenatod extenoione at either and:

or by the growth in the grain boundary after the pearlitic .... in. developed
and the mrgino have begun to form. Since the prominent otructureo have

been found in opocimeno that have cooled rapidly. it io aaauned that the
former view io correct. Such an aeounption doee not obliterate the poeeibility
that the latter my take place in olowly cooled ateela.

Stead offero another aimilar eaplamtion for the large plateo of com-
entite which form in the grain boundarieo. In deecribing the effect he atateo.
“the eeriea of knob-like extremitiee were not there when they came out of
aolution. and only became bulboua by coaleecence and eegroption at acme
temperatm below the critical point. Had favorable conditiona for the
movement been allowed to continue for a much longer time. the luenationa
would have eegegated to ouch a thickneeo that they would have Joined to-
gether to form a eolid mveloping ring of moeivo cementite and poeeible
oapegated into a large moo."

'fo the preeent time meet men in thie field have turned their attention
to the cementitic eegregation and inter-pearlitic formtiono which have

" Journal of the Society of chemical Induatry. 1905 Volume mu

17

already been diacuaoed. I. A. Atk:ino"I wae one of the firet to notice
the formtion of cementite around the graino of ferrite. He firet
noticed thie thru hie work done on wire drawing. Althc the exact canoe
for the phenomena wao not exactly clear to him. it aeema no if the
“cementite oegregatea formed after annealing (cold drawn wire) alwayo
ehown up at the gain boundary.“ According to him he concluded that.
'the formation of cementite in mild eteel appeare to be due to the change
in grain aizo taking place below the lower critical point Acl- no the
pearlite from which the cementite io formed ie in an elongated form. it muet
therefore have travelled by a procoae of eolid diffuaion to the reformed
ferrite boundarieo. Fho cementite appeare to accmuulue at theee pointa.
and eepogate ao filma partially surrounding the graina owing to the fact
that the condition of temperature neceaeary for eutectoid formation are
not present.“ Hie concluding etatement would aeem to indicate hie further
belief that the fornation of cementite in the grain boundarieo io in acme
mnner connected with a change in grain aiae. I'Fhe olow heating and cooling
aooociated with pet annealing io particularly favorable for changeo in
grain eiaea. and. even if the temperature obtained exceedd the ‘71 point
and pearlite oeperateo. the euhoequent olow cooling ia often oufficient to
canoe alterationo in grain aize and formtion of conentite.‘I

Later. in the name year. Ihitley publiohed a paper. innocent of
Atkin'e work. indicating the formation of cementite in the grain boundariee.‘2
2o illuatrate hie point. a eagle. wao cooled olowly from elevated touperature

to room tomerature; reheated to 1260°F. held for one hour and quenched in
water: the piece wae again reheated to 1260”. held for one hour and quenched

again. Upon examination of the loot piece. according to him. opheroido of

‘ Journal of Iron a Steel Inotitute. lo. 1. p 443. 1927
‘2 Journal of Iron d» Steel Inotitute. lo. 2. 927

.I I'P."aol' 1";3-..

l.

13'

coIontito mod in tho grain bonndnrioo ahilo thoro had boon nono in
tho firot lawlo. Bo oxplainod thio by oaying that upon hoating to 1260”
tho carbon by rapid nimtion procoodod to go to tho grain boundary ohoro
it appoarod no ophoroido.

i'ho work of both thooo non haa throvn a goat deal of lidtt upon tho
procipitation of conontito in tho grain bonndarioo. Honour, it io tho
anthor'o boliof that cortnin pointo havo boon ovorlookod nhich aro nocouary
for a cloaror nndorotandin; of tho probloa; Doforo conoidoring thooo lot
no turn our attention to tho oolnbility of canontito in tho alpha torrito.
Indood tho proviono thoorotical diocnooion could load ono to conclndo that,
on olow cooling at loaot, alpha forrito my contain ooao coaontito in
oolntion bolo' tho “’1‘ '!o that toaporatnro tho carbon in oolnblo in alpha
torrito, io otill ill-dotinod. Ho'ovor, oxporinontal work dono- rocontly my
clarify tho point a littlo and at loaot indicato tho oolnbility of carbon in
forrito. saw otatod in 1925.1 that in ordinary otoolo npm torrito
containod ao Inch ao .06! carbon in aolution. During tho mo yoorpnhliohod
a papa-2 indicoting that botvoon .04 to .055 m pooont. ho yoaro lator
rm pnbliohod an’acconnt of m. work“ indicating that up. forrito con-
toinod .034 carbon in alpha at tho A1 point. “1th vorifiod thio tho
oano yoor by stating that ho had found from hio oxporinonto, that a olow
coolod opocim containod only .Ol$ conontito in aolution. Upon hoating
thoro Iao vory littlo chango until a toqoratnro of 6 30°§ no rooohod at
Ihich point a littlo noro no diooolvod. it 720° 6. tho amount of conontito
that to. man into oolntion roochod .05;
1 Journal of Iron 8: Stool Inotitnto, 1926, ”.2, r 313
2 "l'ho Phyoical Oho-iotry of Stool flaking Procooooo'. Iranoactiono of tho

‘rarady Socioty, 1925 Vol. m I? 272
3 Journal of Iron 8. Stool lnotitnto, 10.1, 1927 I 747

.qu

 

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19

lith thin in aind it in roaoonablo to onppooo tho carbon boconling
looo oolnblo upon olowly cooling in procipitatod at tho grain bonndarioo
ao ooon in Iignroo lo and 11. Iwidontly thio procipitation taboo placo
olowly for in tho opocinon coolod noro rapidly than that rotorrod to
abowo, bnt olow onongh to obtain poarlito, thoro wao no approciablo
count of coaontito to bo found in tho grain bonndarioa. (Iignro 12)
Ioroovor, aoonning tho gonoral acooptod thoory to bo trno, taht a fino
dioporoion of conontito incroaooa tho hardnooo. wo aro lod to boliowo upon
onnining tho hardnooo valnoo prooontod in hblo I. that oarbido particloo
in tho noro rapidly olow coolod piocoo aro uchanically hold in tho torrito
graino. to if to 'worify fnrthar, it wao noticod aftor rohoating to 1260”. .
holding for an hour and qnonching, that tho hardnooo of tho pioco oqnllod
that of tho aoro rapidly olow coolod pioco. lwidontly oarbido particloo
rodiooolwod into tho torrito and woro hold in tho forrito graino. Pocnliarly
onongh, thoro did not noon to bo Inch chango in tho oino of tho grain boundaries.
(rigoroo 10, 11, and 13) lot thoro was a nrbod docroaoo in nrgino ant-rounding
tho poarlitic aroao, which would indicato that tho carbido cano i'roa thoro.
lho roaoon for a proforontial diooolwing of tho concntito tron tho poarlitic
aroao io nnanoworablo for no furthor anorinontation wan carriod out along
thio lino. It in ontiroly poooiblo that a phononona of oqmligation wao taking
placo botwooa tho coaontito in tho poarlitic aroao and that in tho grain
bonndarioo. A diocnooion a littlo lattor ohonld uh plain tho roaoon for
thin boliof.

lohoating attor tho firot qnonch ohocld favor tho procipitation of tho
conontito particlea. Bowovor, tho phyoical natnro of tho forrito graino
would act an an obotruoting agont at low toqoratnroo. in tho toqoratnro
incroaood it haa boon indicatod that tho oolnbility incroaood, a nrkod

- C
offoct taking placo in tho noidxborhood of 6 30%. hit would account. thoo-
rotioally, at loaot, for a procipitation of tho particloo at tho grain

tllll'l ill. .’ ‘IJVIDIMX. I

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boundarieo if tho opecimen io hold for a long period of tine below

5 30°C: (nod’r). However, an ordinary heat to 1250‘? would not per-it

ouch a phononona to take place. The alpha ferrite would bocono fairly
well oaturatod with carbide, but no appreciable precipitation would take
place. i'ho effect of ouch a treatment upon tho cementite in both tho
pearlitic areao and tho gain boundarieo would be to canoe coaloocenco.
(Iiguro 16) In tho light of thin it would be expected that thoro would be
little or no ophorodiaing in tho fine grain boundarieo of tho rapidly
coolod piece in ouch a ohort opaco of tine. bocauoo of tho omll amount

of cementite prooont. Iith oufficient tine at a to-porature below “'1’ tho
amount of cementite in tho grain boundarieo sill incroaoo bocauoo migration
by neano of tho oaturatod alpha oolution will tabe placo botwoon tho pearlitic
arena and tho grain boundarieo. flue- io born out by oboervation. Upon
continued heating Juot below the critical thoro io a olow migration of
cementite from pearlite to the grain boundarieo. Undoubtedly, the firot
formtiono to appear are opheroido but with a longer heat treatment the
boundarieo fill out an in figure 59.

dtkino, ao otatod prowiouoly, indicated the formtion of cementite
in the grain boundarieo of cold drawn wiro after annealing for long poriodo
of tine. Although he did not know exactly what to attribute tho canoe,
believing that it wan connected in oone mnnor to a change in grain oino,
he recognized the formtion. coneidering tho formtiono in the light of
the foregoing diocuaoion it met have been due to precipitation phenomena
following a long annealing and cooling proceoo.

J. H. Ihitoly on the other hand indicated that tho cementite formed
in the gain boundarieo ao opheroido. He recognized the fact that cementite
io oolublo in alpha ferrite ao hio proviouo otatononto would indicate.
Apparently he overlooked the factathat upon olow cooling, cementite dio-
oolvod in the alpha ferrite, my be precipitated at the grain boundarieo,

altho he adaito ho hao found that the oolubility of cementite in alpha
iron at 720°C (13280?) io .05, while in a olow coolod opociaen it io .01‘.
the reeult io that he is faced with the tank, for which he hao been oererly
criticized, of contending that at one time alpha ferrite at a point Juot
bolow tho‘lowor critical will diooolvo cementite and at another precipitate
it. further, to back up thie contonotion he roachoo the concluoion that
the carbon or carbide particloo are able to diffuse at a rapid rate through
the alpha iron without paooing into aolution. It io entirely poeeible that
coaentito particleo in the pearlitic area my pace to the gain boundarieo
by going into aolution ao clearly dononotratod by tho provioua experiment
illnotrating the effect of a long heat treatment upon low carbon eteel,

but thie cannot be accomliohed in a oha't tine. Ividently he io aware

of thie and unfortunately roeorto to the ooeningly impoooiblo explanation
to fit in a few of hie facto.

Conoidoring the time noceooary to opherodioo a oanple, it in not
ourprizing to find cementite in the gain boundaries of low carbon stools
upon etching, In coneidering ooae working explanation it nuot be remabored
that the amount of the two conotituonto lavea mrkod effect upon the
formation of the gain boundarieo. it the outoot, after air quenching, it
io aoounod that the ceaentito io well diotributod throughout the. individual
gain. Quito natumlly it io to be expected aloo that, following the rapid
cool, ao in the caee of the rapid olow cool, the ferrite io oaturated with
eoaontite. [eating the opociaen at a point Juot below the lower critical
would not favor the diooolving of an noro ceaentito in the oaturatod ferrite.
However, it doeo promote dopooition and coalooconce between the nuclei
ocattorod in the gain boundarieo and tho gaino. After continued heating,
opheroido begin to appear. Iron the work carried out precipitation dooa.

not apparently occur along the gain boundarieo until after the opheroido
I.” been well found. he equiblibrim between the carbide nuclei in tho

ferrite and tho pearlite io undoubtedly ouaoptiablo to roaJuotnent upon
reheating to a temperature below ‘71 that will allow ouch a procooo to
take place. Carbon io depooitod upon nuclei in certain localitioo: fro-
the ferrite aolution. i'hio embleo the aolution to take up more carbide
at the onpenee of the omllor nuclei. ‘

Iith tine the forcoo of the two comononto, cementite and ferrite,
ohow a decided effect. In what manor thie taken place. the writer io
adnitodly at a looo to explain. Iot fre- oxporimnte it in evident that
with increaoingly larger anounto of carbon up to the eutectoid ratio, the
conotituonto tend to balance each other. an ie brought to attention,
mrkedly by etching a eanplo of eutectoid eteel eubJeoted to a opherodining
trmtnont for 238 houre, a poroid of time in which under the name conditiona,
the cementite of a .20! carbon eteel wao precipitated entirely at the gain
boundarieo. (Iiguroo 74 d 78) After a deep etch, a fine localised condition
which apparently one a gain boundary wao noticed. i'e be euro there wao a
localized otrin‘ging out of the ceamtito particloo (I‘iguroo '72 d 73) yet it
had not reached the proportiono that it nigtt be coneidored to envelope a
gain of ferrite. Iith a lowering of the carbon content the carbide in
apparently forced to the gain boundary no if the moo action of the emoooivo
ferrite wao to 'oquoooo' the ceaentito to the gain boundarieo. Bone inveot-
iptero believe that it io n mttor of ourfaco tonoion that cauoeo the forn-
ation of cementite at the gain boundary. Whatever the force io that cauoeo
carbide to precipitate at the boundary, we my aeouo that the ligation

takoo place at the ozponoe of the opheroido, oinilar to the nigration from
nuclei to nuclei.

If coacntito in a definite compound onpreoeed by the foraula read.
then an explanation of a difference in hardneoe obtained in the experimnt,

beconea an iqoooibility. On the other hand, if the baoio for aeoming

that the for-ale 1.30 lo week then perhapo another line of roaooning io

opened. Indeed, such soons to bo the case. In 1885 01639 described the
recovery of pure carbide and following mthomtical calculations baeod upon
‘the per cent carbon and ferrite present, he concluded the conetituont carbide
to have the fornula rose. However, I. D. Mbell indicated by chemical
analysis that there was acre than one carbide present in plain carbon eteel.
he even went so far as to indicate that in annealed steel the neon molecular
weiddt of the hydrocarbons evolved, hence, presumably of the carbides fro-
which the hydrocarbons are dovived decreases as the percentage of carbon
increases: that carbidee existing in hyporeuctectoid steel have a geater
nolecular weight than those in the hypeoutictoid, and that in a given stool
the lean nolocular weight of hardened steel is lower for the sane eteel
annealed. more are my notallurgists who never took kindly to Oenpbell's
findings. Bolas believe that hydrocarbons are not indicators of chemical
combinations, but of a physical relationship. Rejecting the idea that
cementite io a definite conpound logo. two nethedo of explanation are avail-
able; first, that ceaentito consists of varying nuabors of carbideo and
secondly, that ceaontite is a solid solution.

Should coaentite consist of varying amounts of carbidoe, then two
angles of approach are available. he one would hinge around the idea
advanced by Jefferies2 that in auotenite, carbon exists as a free atom
in gen. iron. is carbon steel auotenite coolo slowly from above the
critical temperature range, it transforms into alpha iron and the iron
carbide. fho carbide is supposed to fern from the atoms of carbon and the
atcas of iron after changing from gun to the alpha state. rho fermtions

l. Jourml of Iron d Steel Institute 1914
2. Transaction by io 88". larch 1928

and crystallisation undoubtedly takes place simultaneously. Yet suppose
acre than carbide is feraod. It is reasonable that upon formtion,
coincident with diffusion of the carbon atca, that it would be precipitated.
If, on the other hand, the carbides existed in the austentic condition so
noloculos, they would also be precipitated upon rofcrmtion following the
transformticn frm plan to alpha iron. fhis would account for a difference
in hardness, but upon closer inspection, especially in tho hyporoutectoid
otoola, it becones a very weak explanation. is the upper critical was
passed upon cooling in hid! carbon steels, oonontito was precipitated. out of
solution. is the teaperature was lowered still mrther, acre coaentito
was precipitated from the solution. This action necessarily become a
straigit lino function of the teuperaturo. Io date general observation of
the fcrmtion of omentite in the gain boundarieo seems to bear this out.
lot at this point the idea of the formtion of several carbides become
untomblo. If the carbides are already for-ed, their precipitation would
follow individual functions depending on each carbide. If they are gadod
in this respect, their couposit result my produce sn' effect sinilar to
that obtained if one carbide exists. Mover, this raises a great may
objections which would mks it desirable to abandon the idea. ‘ In the sane
mnner it lust be believed tlnt a graded effect exists should tho carbides-
bo formed after they are thrown out of solution. is before the reracity of
such an idea bocoaos doubtful. Until sufficient evidence would point to
the conclusion that such a gredional formtion takes place, it too, should
be held with nontal reservations.

no solid solution theory, offers a little acre sound explanation.
lo aide in this explanation a solid solution will be considered as being

an aooociation of substances, honogenous througout, the proportions of

L’Io . alga-tufsslfilh |1 ‘5. 9

whose ccnonents can be altered gadmlly without the production of
abrupt clnngos in the properties ofthe same. In such a solution, the
exact arrange-tent of the carbon atone in respect to ferrite atone is not
definitely known. i'ho general asmticn is that the arrangment is a
physical orientation of one or noro carbon stone in the space lattice of
the systm. Upon cooling below the uppu critical the coaentito, rich in
carbon, would be precipitated (Hyperoutoctoid stool). loch successive
precipitates becoming leaner in carbon until the lower critical wao reached.
Ehe laminae forming in the pearlite would necessarily be leaner than
those found in the gain boundaries, thus supporting the egerinental
results.

Concerning the precipitation of cementite in low carbon steel, seeming
cmentite to be a solid solution, it may be said that the some precoses taboo
place as in the higher carbon steel. It is to be expected that a reaJuotnont
of the carbides in both the gonna iron and the alpha ferrite takes place
continually upon cooling throng: the transfermtion ranges. cementite rich
in carbon will, after all the auotenite has transfer-ed, tend to be situated
in the alpha ferrite. lecesoarily this precipitate in the gain boundaries
will be a little harder than that in the lamellae. This is born out by
observation which indicates the gain boundary is a little harder than the
cementitic lanellao in the poerlite. lxperinental evidence also supports
tho foregoing onplonotion, that the ceaontite in tho pearlite (not reaJustod)
is of varying hardnesses.

Short Diseased-on cf the Iii-wt Paragraph, hrrite brittleneeo
'ithout a doubt a goat deal of the ferrite brittleness found in

stool can be traced to the effects of a long heat treatment Just below the
lower critical. fhis heat treatnent as has been pointed out my consist of

,Fflollv‘lp ,Inthohro‘e 1 .

a very slow cool through the range below the lower critical or a pro-
longed annealing operation below the lower critical. It has been seemed
in the past that the slower the cooling operation or the longer the
annealing operation, the loss brittle the heat treated specimen will be.
However, as has been shown, prolonged cooling or lengthy anneals below

the lower critical proacto the precipitation of cmontite in the boundaries
surrounding the ferrite gaine. Consequently, with a disrogrd for oractnese'
in heat the treatment below the lower critical, it is not to be wondered
that, while identical heat treat-onto are given to heats of steel above
the lower critical, they my very below the 11 point. Therefore, if the
heat treatment is finished at the lower critical, ferrite brittleness can
be expected in different stools having the sane couposition and heat treat-
nont. In any case, it my be concluded that acre consideration must be

given to the phenomena that occurs below the L1 while heat treating.

BIBLIOM

itihino, r. i.
one Drawing of Steel wire and Its nolot'ien to coolitioo of Steel.
Journal Iron a Stool Institute 1927 10.1 p443

”“11, '0 DO
Contribution to the fheory of Hardening and the Constitution of Stool.
Jounral Iron a Steel Institute 1914

Carpenter, H. C. H. d Robertson
rcrmtion of I'errite fron instenito
Journal Iron d Steel Institute 1931 lo.III p35

Cleo:
Ccsptos Hondus 701.85 pllOS

Hatfield
rho Elysiaal Chemistry of Steel inking Processor
Eraneactions of the Iarady Society 1925 Vol. 21 p272

Howe, H. l.
!he Hotallurgy of Steel
H. I. 1892

Howe, H. l. d Levy A. C.
the life History of he Hutsctoid Conentite
I’roc. Int. doeoc. testing Hatorials 2(13) II

Jefferies, 2’.
A contribution tothe fhoory of Hardening and the Constitution of Steel
mution b, ‘o 8e 8e!e “Ch 1’38

Hooter , I.

Influence of Heat freatnent bolow d1 Point on the Properties of

Bibo 0011' to
Eochnical Iron
Arch. lreonlrattonew 2, 503—22 (1928-29)

Wm, H. S. a Harper, J. I.
Sphoroidised Ceaontite in Hyprooctectoid Stool
Transaction of A.S.S.!. a 341-74 (lace)

Sauvour, l.
!he Austonitic-Pearlitic transfermtion
A. S. S. '1'. 17 199- I'eb. 1930
he Hotallogaphy and Heat heat-ant of Iron do Steel
Cmbridgo, llase. 1926
Dondortic Sogaticn in Iron-carbon Alloys
Journal Iron d Steel Institute 1925 Ho. II P313

3‘“, Jo to "

the Segregatory on Higatcry Habit of Solids in Alloys and in Steel
below the Critical Points. '

Journal Society of Chen. Institute 1903 Vol. 22 p30

M. I.
!he Hardness of Different Structues in Steel
Sci. Papers Inst. Pby. Chen. Research 5, 25-44 (1926)

Hotos on Pseudo-twinning in rec-rite and on the Solubility of Carbon
in Alpha-iron at the A1 point.

Journal Iron: 6 Stool Inst. 1927 lo. I p747

“1149', I.

Investiption of the lochmisn of the Solution of Conentito in Carbon
steels and the Influence of the Hetrogenity of the Hotel

Jounal Iron a Steel Inst. Sept. 1930

Solution of Ceaontite in Carbon Steel Inginooring 131, 27-30 1931

'hitley, J. H.

Eho Solution of Conentito in Alph Iron 6 its precipitation
J. Iron a Steel Institute. 1922 So.I p354

bib. ccn't 3.

Shitley, J. H.
Globular Poarlite
Journal Iron and Steel Institute 1922 10.! p339
Ccalescenco of Poarlite
Journal Iron d Steel Inst. Ho.II p147 Sept. '29
Diffusion of Ceaaltite '
Journal Iron d Steel Institute 1927 loll p293

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