Solidification Mechanism of Steel l ngots

by H. F. Bishop, F. A. Brandt, and W. 8. Pellini

The solidification mechanism of experimental steel ingots ( 7 x 7 ~ 2 0 in.) was studied by thermal analysis. I t was deter- mined that solidification proceeds in wave-like fashion a t rates which are determined by the carbon level, superheat, and mold thickness. The thermal cycles of the mold walls were related

to the course of solidification.

ESPITE marked advances in the field of solid D state transformation, metallurgical research has contributed comparatively little exact quantitative data on the mechanism of solidification of metals. There is, therefore, a great need for such data in the various metallurgical industries.

The mechanics of solidification of ingots have been investigated in the past primarily by studies of the rate of skin formation as indicated by bleeding or "pour out" tests. The "pour out" method, however, is a tool which gives only approximate information. In the case of alloys with wide solidification ranges, such as irons and certain nonferrous alloys, the method will not work at all; in the case of alloys of intermediate solidification ranges, such as comm~rc ia l steels, the information may be misleading. Thus, the general adoption of this method has resulted in diver- gent conclusions regarding the solidification process.

Chipman and Fondersmith' by means of bleeding tests have shown that the linear growth of a solidi- fying ingot wall follows a parabola of the general form, D = K ~ Y - C, with the start of solidification delayed until superheat is exhausted, as indicated by the constant C. These tests were carried only to a wall thickness of about 5 in. using an ingot of approximately 17x39 in. in cross-section; hence

1 the latter stages of solidification were not studied.

i Matuschka2-Vndicated that linear solidification of ingots is rapid a t first, then slow, but toward the end of solidification the rate becomes extremely

1 rapid again. Spretnak's" bleeding studies indicated that wall growth is expressed more rigorously by two parabolas, and that their point of intersection corresponds to a change of solidification mode from columnar to equiaxed. Spretnak also showed that the K values of the first parabola are always the same regardless of superheat. NelsonQled ingots of square cross-section and found that linear wall growth is initially rapid but decreases continually until the end of solidification. He also concluded that rate of solidification in ingots of square cross-section increases 2.15 pct for every 10 pct increase in cross- sectional area of the mold. The mold ratios con- sidered (ratio of cross-sectional area of the mold to cross-sectional area of the ingot) were all less than 2 to 1.

The subject of solidification has also been treated mathematically in many cases, but because of the

H. F. BISHOP and F. A. BRANDT are Metallurgists and W. 5. PELLINI, Associate Member AIME, is Head, Me ta l Processing Branch, Metallurgy Div., Naval Research Laboratory, Washington, D. C.

Discussion on this paper, TP 3208E. may be sent, 2 copies, to A I M E by April 1, 1952. Manuscript, Aug. 13, 1951. N e w York Meeting, February 1952.

Fig. 1-Test assembly.

lack of accurate thermal constants and the simplify- ing assumptions required, as their authors generally acknowledge, they represent only approaches to the actual conditions of ingot solidification.

A third method of studying solidification is the electrical analogue method promulgated by Pasch- kiss-' and by Jackson and coworkers .This method

treats solidification as a heat transfer problem with the solidification cycle synthesized on an electrical circuit. Paschkis in his' treatment of solidification considered the fact, which was generally ignored, that solidification of steel is not simply the growth of a plane solid wall but a more complex process occurring over a temperature range as indicated by the constitution diagram. Undoubtedly, the anom- alous results obtained by bleeding tests arise from the inability to measure quantitatively this mushy condition. The shape of Paschkis' solidification curves are more nearly in accord with those of Matuschka, in that they indicate rapid linear solidification at the beginning and end of solidification with intermediate solidification occurring at a slower rate. Paschkis indicates a definite lengthening of solidification time with increasing superheat.

Thermal analysis is a direct method providing exact information for all types of metals regardless of solidification range and was thus adopted in the present program to follow the entire course of solidi- fication from the surface to the centerline of the ingots. The method has the added advantage of be- ing adaptable to following the thermal cycle of the ingot mold during the course of solidification.

Test Methods The ingots studied were of square cross-section,

20 in. long, tapered from 7Y4 in, at the top to 6% in. a t the bottom, and fed with a hot top 7 in. in diam and 12 in. high. The molds were uniform in wall

44--JOURNAL OF METALS, JANUARY 1952 TRANSACTIONS A l M E

'tHERMOCOUPLE.

Fig. 2-Thermocouple locations in ingot and mold.

MOLDINGOT

a 0 0 00 a 0

MOLD

:t~~~:j;;THERMOCOUPLETHER:~~~PLE JUNCTIONS

tCR_- AL)~§_§~~~

were obtained from each thermocouple on 15-seccycles. The response characteristics of the thermo­couple bead in the quartz tube were studied by sud­den immersion into molten steel. Near metal tem­peratures were reached in approximately 5 sec, andfull equalization was obtained in the ensuing 10 sec.It was concluded accordingly that the response totemperature changes which occurred during solid­ification was of the order of 10 sec.

The effect of the following three variables onsolidification were studied: I-mold ratio (ratio ofthe cross-sectional area of the mold to the cross­sectional area of the steel ingot); 2-pouring tem­perature, and 3--earbon content. Specifically, theingot molds employed had wall thicknesses of 1%,2V2, and 4% in., thus making the mold ratios ap­proximately 1 to 1, 1 to 2, and 1 to 4. The ingots werepoured at three different temperatures (measured inthe ladle with a Pt-Pt-Rh immersion couple) whichwere equivalent to superheats of between 25 ° and200°F. The effects of these mold ratio and superheatvariations were studied with steels of two nominalcarbon contents: 0.08 and 0.60 pet C. The steels weremelted in a 1000 lb induction furnace and 0.10 petAl was added to the ladle as a final deoxidizer.Liquidus and solidus temperatures were determinedfrom inverse rate cooling curves of the couple locatedat the center of the ingot. These curves were deter­mined by plotting the time required for the tem­perature to drop consecutive steps of 5°F. It is esti­mated that the accuracy of the liquidus and solidusdeterminations was ±3 OF. The solidus in these ex­periments represents a "technical solidus," indicat­ing essential solidity except for possible thin filmsof intE:rdendritic liquid metal, the solidification ofwhich cannot be determined thermally.

Temperature Distribution in Ingot and MoldFigs. 3a and 4a show the temperature cycles

at various locations in the 0.60 pct C steel ingots and

. // WIRES {PT-RH'

F~~~{~A -~-~::; ~\..~I~f..in, I /TUBES I I Ii ill

~I- I- I- 11f--- "-If---

I 11..-0 THERMOCOUPLE~¥ I:iJUNCTIONS

II

I /J-IIHi'r-IIHIHIIf--- I-,f---

~1/-----iIHHIH~HHI'Hf-------'I ""----''-- THERMOCOUPLE WIRES \PTI

thickness from top to bottom and were made of grayiron containing approximately 3.50 pct C, 2.00 petSi, and 0.80 pet Mn. To prevent damage to thethermocouples extending into the mold cavity it wasnecessary to gate the ingots into the bottom as shownin Fig. 1. The ingates were molded in sand and thebottoms of the ingots were also sand instead of theconventional chill.

Temperatures were measured across both the in­got and the mold in a plane at the midheight of theingot. At this position bottom and riser gradientswere essentially eliminated and solidification couldbe considered unidirectional; i.e. as growth of a solidskin in a direction perpendicular to the vertical moldsurface. Steel temperatures were measured by meansof Pt-Pt-Rh (13 pet) thermocouples enclosed in 1/8in. OD, 1/16 in. ID quartz tubes which extendedcompletely through the ingot and mold (Fig. 2)parallel to and at varying distances from a refer­ence mold wall. All hot junctions were immersedequally in 3% in. of steel and thus temperature in­accuracies due to conductivity losses along the ther­mocouple wires were minimized. The V8 in. ODquartz tubes were the smallest which could protectthe thermocouples from the erosive action of themetal, and even these softened to some extent andwere bowed upward by the buoyant force of theliquid steel. X-ray examination of sections contain­ing the tubes' showed, however, that the deforma­tion was never more than 1 in. and was entirely inthe vertical plane, so that the beads maintained thecorrect alignment with respect to the interface.

The temperatures in the ingot molds were measuredby means of chromel-alum~l couples flash weldedto the bottoms of 3/32 in. diam holes which weredrilled in the mold wall to the desired distance fromthe interface. These couples were spaced lf2 in. apartalong the vertical centerline of one of the mold sur­faces near its midlength.

Interface temperatures were measured on boththe mold and hot metal side of the interface. Themetal interface temperatures were obtained bymeans of a couple enclosed in a quartz tube whichlay against the inner mold surface and extendedthrough the mold in the same manner as the otherthermocouple tubes. In this case, however, a pro­tection tube of only 1/16 in. OD was adequate sincethe metal solidified rapidly at this position. The inter­face temperature on the ingot mold side was ob­tained by first drilling a hole through the mold walluntil the point of the drill pierced the inside sur­face. The bead of the chromel-alumel couple wasflash welded into this opening so that it was flushwith the inside mold surface. The partially exposedbead was then covered with a thin coating of silicawash. Except at this point no mold wash was usedin any of the tests; the oxide coating which hadformed on the molds when they were cast was notremoved and served to prevent welding of the ingotsto the mold walls.

The platinum and platinum-rhodium wires were26 gage, the chromel and alumel wires 28 gage. Theingot temperatures were recorded on a 16 pointautomatic potentiometer recorder, 1500° to 3000°Fscale, with an accuracy of 0.25 pct of full scale anda sensitivity of 0.1 pet. The mold temperatures wererecorded on a similar instrument 0° to 2500°F scale.These instruments permitted sequential temperaturereadings from 16 thermocouple stations within 30sec. Since in the majority of tests there were notover 8 couples attached to each instrument, readings

TRANSACTIONS AIME JANUARY 1952, JOURNAL OF METALS----45

...§ 1600

!<i 1400'r--~~-T

...I-

10 14

TIME - MINUTES

8 to 12 14

TIME - MINUTES

Fig. 3a-Temperature cycles In ingot and mold.

'4 16 18 20

800

14001-----

1200r-----

3000 I

II i Ii2800 LIQUOOS

26001--... l"V~ SOLIDUS i

..... "\k\:~ I I I I

2400 r-... POURING TEMP. .2725 0 F.

'r-.. 1'):['\ MOlD THICKN£SS • 2 'II M

220STEEL ANALYSIS:

0

~C 64 -

'"- '~"M. 52

200° 14· 51 .45 - t--

r:::,~~1800 18.,

R16001----- --f---...

u ...

H-+--+- ~ ;H--t---l

I--+-+--+-~ ~H-+----l

-N:8M'NlOOOf-----+-+---+-+---' /'6--·

r--.: 1~l-'2_f----J,\[":::~ :

6001----+--+---+-+--+ffi,"',..-"-.,--f""t 41----+---1

4001------+----1- !-t---+.v\'ct',~","'"2 r-------i~.2oo~-+--\---+--i-----+->p...--'l"'-l1

I i I'-;;;r-.'"

L,nUI"'''-

" 2' SOLIDUS

t'"t\\4~\ 'I.v,

r--,~~~I I J

POURING TEMP. -2790-F.

t---,4......MOLD ma<l';£SS • 2 'iI'STEEL ANALYSIS'

'6 C .56 -1-18b::::-~ Mn .5720MI SI .34 ~ I-----........

~t-- ...

l<..J ...

I----- ffi u

!Z ....I----- ~ "

::::..

l'::::20 MIN. I-----

~1612

\~ ." 8

..... 4

I \'\1\' 2

\~I v.- r-----

"-.:: b,.

I i". '" LIQUIDUS

I"":: ~

~~"SOLIDUS

r'-.6 I I8 POURING TEMP, -2880-F.

j-....l"~~ MOLD THICKNESS- 2 IIiSTEEL ANALYSIS'

'1.01\0-t'\ c .62-

I M• .64

~ 1 Si .48-

I

f- f-\- 1---- -!I! ... I

l--i u ...i:!--~...

j!Z I ..e--~ -iil ,

~~;;-)4M'N._...--'210

~~8-_6

\\' ""4 __

\ t\."2__

~,_-

~1/44 I 0 I 2

DiSTANCE FROM INTERFACE -INCHES

55432 10 234554 0

DiSTANCE FROM INTERFACE - INCHES DISTANCE FROM INTERFACE - INCHES

l<'ig. 3b-Thermal gradients In Ingot and mold.

Fig. 3-Superheot series for high carbon steel.

mold walls during the solidification process. Thesedata are replotted in Figs. 3b and 4b to showthe thermal gradients from the casting centerline tothe outer edge of the chill wall at various timesafter pouring. Fig. 3 illustrates the thermal effectsresulting from increasing superheat and Fig. 4 ofincreasing mold wall thickness. Figs. 5 and 6 showsimilar data for 0.08 pct C steel ingots.

Thermal Course of Mold Walls: It is apparent fromFigs. 3 and 5 that the superheat condition has rela­tively little effect on the thermal course of the moldwalls. Temperature differences can be observedmainly at the mold interfaces for the first 2 minafter pouring. This is due to the initial temperaturedifferences in the metal in contact with the mold.The higher pouring temperature is reflected in ahigher rate of heating and a higher maximum tem­perature at the interface. The differences are greatlyreduced in the later stages of the solidification process.

Variations in the volumetric thermal capacities ofthe mold walls, however, develop pronounced effects

46-JOURNAL OF METALS, JANUARY 1952

on the entire thermal course of the walls, Figs. 4 and6. The effects are not marked during the period ofinitial surface heating of the walls, for during thisperiod the walls are heated appreciably only nearthe interface. At this stage all of the walls, 1% to4% in. thick, behave essentially as if of the samethickness. However, as solidification proceeds andgeneral heating occurs throughout the walls, thevarious volumetric heat capacities of the mold wallsbegin to be reflected markedly in the thermal courseof the mold walls.

The first indication of the heat capacity effect isshown by the timing and nature of a thermal dis­turbance at the interface due to the formation of anair gap. In the systems with the thinnest walls thisdisturbance is developed at approximately 1 minafter pouring, as indicated by inflections in thethermal course of the ingot and mold interfaces. Thesystems with the intermediate and large size moldsundergo a gradual temperature reversal in the chillside at about 1% min. When the air gap forms in

TRANSACTIONS AIME

L1QUIOUS

81012141 I 20

TIME - MINUTES

...... "- SOLIDUS

B 10 12 14 16 18 20 0 2 4

TIME - M'NUTES

4

0

0 LIQUIDUS

0\ r-... r--;;:~2'.f' SOLIDUS

\.'-~ '::::""i... ;~ .--3""''' ICENTERLINE)

00,," 1':""':::----;

~~ 'NGOT

..:....~ r-.......:00 .......

~INTERFAeE- r--:

00

~ I MOJo00 e---

0INTERFAeE~ --=:b=

/b::~~~~00 'Ir""'V~/Y~/,. ",," ISURFACE)

'//1/' II I·

.e2800of.POURING TEIIP.00

IMCt.O THICKNESS" lilt"STEEL ANALYS'5'

0 - - l- e .56-+=

I "'" .4751 .33

I I

40

o

200

2000

22

24

260

1000

280

300

~ 18I

~ 16!;;ffi 14

~.. 12

Fig. -la-Temperature cycles in ingot and mold.

,I II,

,,OU, US

"'"I OLIO S

'",-'\~~ POURING TEMP. -2190'F

\, MOLD THICKNESS =4 'I,"STEEL ANALYSIS' -12 ",l'\ C .•3

r-,. ....48

16 M';;;::: r--;51 .50 -

I"'-'f---

,...- ~-0:

f--- ii:r--

\.\\\'~~~ f--\\ :'4-.....:--\, 2"

\~~~~r-(4....., ....... -...

LIQUIDUS

"- '''2 so 10 5 -

I"\.\.\ '4 I1-'"

..........,:~~ ~RI'NG TEMP. '21J.'F.MOLD T1-IICKHESS ~2 lit"

-."'~ STEEL ANALYSIS:-

-,~~~C .58Mn .~7

- 20MlN 51 .34 - f---.......~I-..--~ ..

I~-'U

~-15 II: :.!Z "' 0:.. '"-~ !:

~20 MIN. f---

~~~I.128

..... 4

\'\ .\"'-... 2

\~'I/t-n'-..:I'-F:.;'.

55432 1 01245543210234

DISTANCE FROM INTERFACE - INCIES DISTANCE FROM INTERFACE - INCHES

Fig. 4~Thermalgradients In Ingot and mold.

Fig. 4-Mold thickness series for high corbon steel.

32.' 0

DISTANCE FROM ,,.TERFACE - INCMES

000

800)/4 V,

'OUIDUS

600~~2~ ioLinuS

.... "~ POURING ~EMP. '2800"400 12~~~""""4

MOLD THiCkNESS 0: 11J"II

200 MIN: ~ STEEL ANALYSIS: -"-~

e .56Ill" ,44

~Si .33

!rI ~ "' I I1800~~ ~

- :"' 0:0-

'60Cf-- ~ I---i '--- i I I

1400'4 MIN. _ f--

~12

10120

.'""..... :

1000

\\4

800

\\ 2

600

l\ r"-,'lOO

\' i' ....200

i' '4

~I

these systems, the cooling rates of the ingot side ofthe interfaces are markedly reduced. That thesetemperature disturbances are the result of air gapformation was verified experimentally by pouring asimilar ingot against a 2% in. thick mold with a 45 °tilt such that the bottom face remained in continuouscontact with the mold, while the opposite side couldshrink away to form an air gap. The mold interfacetemperature on the top side fell when the air gapformed, but on the bottom side it climbed steadilythroughout solidification.

The air gap times noted in these experiments arein close agreement with the air gap time found byMatuschka" in a 0.65 pct C, 10 in. diam ingot castinto a mold having walls 33Js in. thick. Matuschkafound that an electrical circuit through the mold andthe ingot was broken by the air gap 1 min and 25 secafter pouring.

The fact that an air gap forms earlier with thinmolds than with thick molds is ascribed to the morerapid heating of the thin mold, which then expandsand pulls away from the ingot.

Following the stage of air gap formation with itsconsequent drop in rate of heat transfer across theinterface, further indications of the relative heatcapacities of the mold walls are provided by thetemperature course of the various mold walls. Thethinnest mold wall shows insufficient heat capacityto prevent continued rapid heating throughout theentire wall. The intermediate mold wall has sufficientheat capacity to maintain an approximately constantinterface temperature, while the heaviest is over­sufficient in that the interface temperature falls.

An indication of the relative heat capacities of thethree mold wall sizes can be made by comparingouter surface temperatures at a given time. Forexample 2 min after pouring, the surface tempera­ture of the 1l/2, 2Y2 and 4lAl in. molds, Fig. 6, are770°,425°, and 140°F, respectively.

Solidification Course of IngotsFor a basic understanding of the process of solid­

ification from mold walls, it is essential to considerthe nature of heat transfer from the ingot to the

TRANSACTIONS AIME JANUARY 1952, JOURNAL OF METALs-47

'0 12 14 16 18 20

T1ME- MINUTES

i'e,ou,L~

.\~ .........~~;\ SOLIDUS

i \ 1'--- '......... "2' 2~t:--3'" (CENTERUNEIi""..... ~ _

r--.....'"""~~~I --. 't"--:"-- INGOT /"

INTERFACE ........

I--r----

MOeD

./"-..INTERFACE

I,."f :-- TIl." ::.:( ~~

...~::.-

~ ;::;::::;-

(/a~ 2 1/2" (SURFACE)

fiJ 1/POURING TEMP.· 2975·FMOLD THICKNESS- 2~·STEEL ANALYSIS,

~!IC ,0'Mn .6'"SI ,S8

II

° 2 •10 J2 14 16 18 20

TIME - MINUTES

r/; ~~r--2"" ISURFACE)

II,/, // j POURING TEM•• 2900"FfI '/ MOLD THICKNESS- 2'12:"

STE£L ANALVSIS.

/I C ,09i Mn .57

Iff-t--t--t--t---i Si .50

20 °

400

BOO f-'f--~..4-, ",""?-''f'--+_,L,,--'._- _. '-,--'--1

600 'rfj"-V'oM-'

1000

1200

1600

11300

1400

Fig. 5a-Temperature cycles in ingot and mold.

2 4001---1--1-+

2200

2000

1600

1400

1200

1000

DISTANCE FROM INTERFACE-INCHES

LloLous

1'.\I\.\~~,' SOLIDUS

"~1{~ POURING TEMP.• Z.97!>"F

........... ""~MOLD THICKNESS- 21lt"

1"2 STEEL ANALYSIS.

C ,0'

~M. .~4

51 .38

I--- r----t:

f---<[ w::0 "'" it

I-- 'I'Lt- 12 MI. <I>1O,

6~"::;~~~I r- \\f\"

- 1-- I--.\\"-

f\>I3 2 0 I 2 3

015T ANCE FROM INTERFACE - INCI-lES

t'lg. ~b-Thermalgradients in ingot and mold.

Fig. 5-Superheat series fDr law carbon steel.

mold. This may be deduced from the shape of thethermal gradient curves shown in Figs. 3 to 6. It isrecognized that heat from three sources is meteredfrom the ingot to the mold during solidification: 1­specific heat of liquid metal, 2-heat of solidification,and 3-specific heat of the solid metal which hasalready formed. The nature and progression of solid­ification at any given time is governed by the ratesat which these various heat components are meteredto the mold.

It will be noted from Figs. 3 to 6 that the usualcontinuous thermal gradients generally associatedwith heat flow are disturbed by the formation of a"knee" in the temperature range of transformationfrom liquid to solid. The abrupt change in gradientsat the knee is due to the necessity of removing heatof solidification from this zone before the thermalgradients can move inward. The knee zone thus rep­resents a band of active solidification which formsa "thermal block" to the central liquid portion ofthe ingot. The development of this thermal block

occurs early in solidification during the formation ofthe initial skin on the mold wall. Since the solid­ification knee zone becomes established at and belowthe liquidus temperature, it will not act to block thespecific heat of the metal above that temperature.Thus, during the formation of the initial skin at thewall surface, the superheat of the liquid metal issimultaneously metered out at a very rapid rate, asindicated by the short time required for the gradientcurves in the central portion of the casting to reachthe liquidus temperature. It will be noted for exam­ple that the 180°F superheat in the ingot of Fig. 3poured at 2880 OF is lost in little over a minute andin this time a completely solid skin 1jz in. in thick­ness has also been formed.

Since significant cooling of the central portion ofthe ingot below the liquidus temperature cannotoccur because of the thermal block, the central por­tion of the ingot gravitates to an essentially iso­thermal condition at the liquidus temperature andremains as such until reached by the solidification

48-JOURNAL OF METALS, JANUARY 1952 TRANSACTIONS AIME

2001--+--+-+--+-+-, -,-,.....--,.---j

1800 1--+-+---+-+--1----1 -+----+-+----1

'6001---+-+--+-+--+-+--+-+--1----1

6 8 10 12 14 /6 18 20TIME-MINUTES

lIdUiDUl

"-~"- 2''2-SOLIDUS,

"~ "'" ~_3\1v.·(CENTEAL.INE)

2'1'\'

I ~lt2 ", I~~~-I---- i'- "".:.:~r--....

I INGOT 7"-~"INTERFACE~

_. POURING TEMP. 2900·F\ ,:~lD TtflGKESS" 4',,'STEEL ANALYSIS:

C .09Mn .53Si .38

C.:::---"- 1~~FAC~,":"'0,-= =

I r ," ::--_I"'O~

/'"/---~-,/~;/

'41/e"(SURFACE}

I I '~

I IUQUIDUS

\ r--.... ~~~~SOLIDUS I I I

i'-~ "-~~~ 3~1:~ CENTERLINE)

,. 112

'-I---.1/2- :-...:

INGOT l)'---,-INTERFACE

IOJLD

r ____ INTERFACE

I -k-1J'C·--: ====~ ;,,14':';;'-

/ ~~~.P~'f;~~

r--Zlll- (SURFACE)

'/,~POURING TEMP· 2900·'MOLD TH1CMNESS· 2"z-STEEL ANALYSIS,

VI C .08Mn .51Si .50

'II

~'N"H'bR~~sl~STEEL ANALYSIS'

C .09Mn .39Si .20

4 6 8 10 /2 14 16 18 20 0 8 10 12 14 16 18 20 0

TIME - MINUTES TIME - MINUTES

Fig. Ga-Temperature cycles In Ingot and mold.

400

~rt600

o

I ILIQUIDUS

,,"-'~~'"SOLIDUS

" .'" ~~POURING TEMP.,. 2900·F

" ..~ MOLD THICKNESS_ IIII~STEEL ANALYSIS

"-'2"M~C .09Mn 39

i'"'Si 20

c- w- <.>

"''" ~ 12MIN :

~ ~~~-

I ."~I '2.

i \t'-'-.

"\i'..

DISTANCE FROM INTERFACE-INCHES

~V1 ,,;",n,,;t"-."r'\. 2 SOLIDUS

4-

"'~~POURING TEMP .. 2900·FMCtD THICKNESS. 4 III-

........,2"'~'STEEL ANAL"l'SI5

C 09

'"['\toIn .53Si 38

II--- w

~w<.>

l'!r---- ffi ~ ;; -... '"I~ l" '"~ "'" -

~

l\\~~~~:\\\~ ~ ~08-::\\l'\f"'-2~--

.:;--.., 's---

VO,,'",-'I\l~ SOLIDUS I

'"\~t\~ POURING TEMP ... 2900-':"

~'~" .\MalO THICKNESS~2 'It"

IN ~ STEEL ANALYSISC 08

"-~Wn 51SI .50

1--"'z "'::i ~ "'I--~ u. <.>... ~ ~z ... !!;

I--~ z

~2MIN\\ ~t\\

4

1'2 I

\ ,'-

1'\.\-.....

4o

DIST ANCE FROM INTERFACE-INCHES

432: I 0 I 234554

DISTANCE FROM INTERFACE-INCHES

5 54o

200

600

800

400

100

3000

'000

2800

2600

2400

2000

2200

1400

1800

1200

1600

Fig. 6b-Thermal gradients In Ingot and mold.

Fig. 6-Mold thickness series for low carbon steel.

knee. The passage of the solidification knee is de­noted by a temperature drop which is consequentto the removal of heat of solidification; for example,the center of the ingot poured in the small size mold.Fig. 4, remains near the liquidus temperature until10 min after pouring; however, in the interval be­tween 10 and 11 min, during which time solidifica­tion is completed, a drop of approximately 125 0

occurs. With the passing of the transformation kneethe remaining, now solid, metal on the mold wallside of the knee cools further, thus contributingspecific heat of solid to the total flow of heat to themold. These basic features of solidification are ob­served to hold for all the test ingots.

The specific contributions of superheat and moldwall thickness to solidification may be observed morereadily by replotting the thermal data in terms ofthe progression of solidification waves or "fronts"through the ingot. Fig. 7 shows the course of solid­ification of the high and low carbon steel ingots thussummarized from the thermal data of Figs. 3 to 6.The progression of the forefronts and ends of the

knee zones, which represent the position of the twofreezing waves, are shown as "start of freeze" and"end of freeze" curves respectively. These data arealso plotted on square root of time basis in Fig. 8.

The effects of superheat and mold thickness on thecourse of solidification are shown in both the "start"and "end" curves; the specific effects being similarfor both curves but somewhat more pronounced forthe "end" curves. Thus, in order to simplify discus­sion, and because of the greater practical signifi­cance of the "end of freeze" curves, further discus­sion will be restricted to the wave of completesolidification.

It was observed earlier from the gradient curvesthat superheat is lost rapidly during the stage ofinitial skin formation. Since the mold has a limitedcapacity for accepting heat, the presence of super­heat necessitates a reduction in the flow of solidifi­cation heat. The bottom graphs of Fig. 7 show thatthis is reflected in decreased initial rates of solidifi­cation. Following the complete release of superheatthe rates of solidification of all ingots of a given

TRANSACTIONS AIME JANUARY 1952, JOURNAL OF METALS--49

Fig. 7-Effect ofmold ratio (upper

graphs) and super­heat (lower graphs)on linear progression

of solidification.

4~·62~· 4y,," I~·

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carbon level become essentially equal regardless ofinitial superheat; i.e. the solidification curves arethereafter displaced by a time interval which isproportional to the amount of initial superheat. Thistime displacement is retained throughout the re­mainder of solidification. For the high carbon steelan increase in superheat from 25° to 180°F increasessolidification time approximately 13 pct (9.5 to 10.7min); for the low carbon steels an increase in super­heat from 50° to 200°F increases solidification timeapproximately 6 pet (9.8 to 10.4 min).

The volumetric heat capacities of the mold wallsalso affect solidification as shown in top graphs ofFig. 7. During the first lh min the solidification ratesof the three ingots in each series are identical. Thisshould be expected inasmuch as during this period

only the mold wall surfaces are heated, hence allact as of infinite thickness. After the first lf2 minthe solidification of the ingots in the lowest heatcapacity mold wall (1% in.) becomes slower. Thesolidification rates of the high carbon steel ingots inthe two larger molds (2 Y2 and 4% in. walls) areidentical for the first 4 min after which the rate forthe intermediate 2% in. wall becomes somewhatslower. This divergence is not as pronounced in thelow carbon ingots, the solidification rates in the twolarger molds remaining essentially the samethroughout the entire process. It is noted for thehigh carbon steels that the time of total solidifica­tion is decreased approximately 13 pct (11.5 to 10min) by increasing the chill thickness from 1% to4 % in. Increasing the chill thickness from 1% to

0.5 1.0 1.5INCHES

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Fig. 9-Effect of mold ratio (upper graphs) and superheat (lowergraphs) on width of solidification bond.

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Fig. 8-Linear progression of solidification showing extent to which

the parabolic relationship (C = kyltJ is valid. Superheat, uppergraphs, and mold ratio, lower graphs.

50-JOURNAL OF METALS, JANUARY 1952 TRANSACTIONS AIME

2% in. decreased the solidification time by approxi- I

mately 9 pct (11.5 to 10.5 min) while an increase from 2Ih to 4% in. further decreased this time only 9" 5 pet (10% to 10 min) , thus indicating rapidly $,, diminishing returns in this mold ratio range. A . similar trend is noted in the low carbon steels.

While variations of chill thickness or of super- ,, heat may have approximately the same effect on the time for final solidification, the mechanisms by 0

which this time difference is developed are markedly TIME-MINUTES TIME - MINUTES

diff'erent. The retarding effect of low heat capacity mold walls does not begin until after considerable I

solidification has occurred and then becomes in- creasingly greater as solidification progresses, while 2 the superheat effect retards the solidification process "'

only during the initial stage of solidification. From Fig. 8 it can be seen that growth becomes

approximately parabolic soon after pouring. How- 20

ever, when the wall has solidified for approximately half of its total thickness, growth rates deviate

TIME -MINUTES rapidly from the parabolic condition. These solidifi- TIME-MINUTES

cation curves have shapes very similar to those in- Fig. 10-Effect of mold ra t io (upper graphs) and superheat (lower

dicated by the work of Matuschka and Paschkis. graphs) on volumetric progression of solidif ication.

The constants for the period of parabolic relationship ceeds as a band of solidification by the simultaneous

( d = kv,q are indicated on the curves of Fig. 8. If travel of "start" and "end of freeze" waves. The

the straight line portions are extrapolated back (as space separation of these two waves is relatively

a straight line) to zero thickness, the superheat de- narrow, hence the solidification process consists

lay factor of the classic solidification formula basically of the movement of the solidification band from the mold wall into the near-isothermal liquid

= lid7 - may be deduced- The effect, which is retained at the liquidus tern- is not a simple initial postponement in the start of perature, However as the s;lidification moves freezing as predicted this but rather is inward, the perimeter of the liquid center decreases one of decreasing solidification rates due to the and the width of the solidification zones increases as presence as shown the various shown in Fig, 9, i t can be noted for the high carbon curvcs of Fig. 7 Thus, the delay factor has physical steel that the solidification zone widths in the ingots significance only after superheat has been eliminated of the pouring temperature series are approximately completcly and a fixed lag in the progression of the same a t comparable locations, the solidification has been established. The lag is re- difference being about ,,, in, However in the mold

solidification. much as 36 in. in zone widths of the ingots cast in It was observed that solidification of steel from the smallest and largest molds.

metal, the term "wall thickness" or "skin thickness" Because of the presence of the solidification zone, ' S ~ r n ~ l a r s tud~es for sand mold walls (ref. 10) indicate a ge~ le ra l

yathrr t h a n a "b;ind" so l~rh f ica t~on mechanism tor the rolld~ficotion which a mixture of liquid and o i the same steels and ~ n a o t sizes. - - - - - - metal, the term "wall thickness" or "skin thickness"

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Pouring temperature 2880°F. Pouring temperature 279S°F. Fig. 1 I-Effect of pouring temperature on macrostructure of 0.60 pc t C steel ingots.

TRANSACTIONS A l M E JANUARY 1957 IOlIRNAl nF M F T A I CCI

r-----------------,

~n .~~ ASi 48POURING TEMP. ' 2880'FMOLD RATIO '2-1

~n ~~ B5, 34POURING TEMP ,2795°FMOLD RATIO '2-1

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TIME - MINUTES TIME - MINUTES TIME -MINUTES

Fig. 12-Cumulative heat evolution by solidifying ingots.

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in describing solidification phenomena of steel ingotsis ambiguous. Furthermore, the constantly decreas­ing solidification perimeter as solidification pro­gresses toward the center of the ingot results inpossible misleading conceptions of solidificationwhen linear rates alone are considered. Accordingly,the volume rate of solidification, including the solidwhich is present in the mushy region, was deter­mined for the various ingots and plotted in Fig 10.It will be noted from these curves that the volumerates of solidification constantly diminish during thesolidification cycle, whereas linear rates of solidifica­tion become very rapid at the end of freezing cycle.The increased rates of linear solidification resultfrom the increased proportion of solid developed inthe central regions (general solidification) as the"end of freeze" wave moves toward the center andalso from the decreasing perimeter of the solidifica­tion front which gravitates to a point as the centeris approached. The increase in linear rates is, how­ever, insufficient to prevent a decrease in volumerates resulting from a decreasing perimeter. It is ofinterest to note that half of the liquid metal becomessolid in approximately Ilf2 min, while an additional8 to 10 min is required for the last half of the liquidto solidify.

Variations in thickness of chill wall were found tohave no significant effect on the macrostructural char­acteristics of the ingot. Increasing superheat, how-

ever, produced a marked coarsening effect through­out the ingot. Fig. 11 shows typical macrostructuresof the high carbon ingots resulting from variationsin superheat. It is hypothesized that the fine struc­tures present when superheat is low result from theformation of many small crystallites during pouringwhich are uniformly distributed throughout theliquid and serve as nuclei or centers of growth asthe "start of freeze" wave moves through the cast­ing. With high superheat fewer nuclei are developed,hence the start of the freeze wave moves throughcomparatively nuclei-free liquid providing conditionsfor the growth of fewer but larger crystals.

Heat Transfer in Solidifying IngotsFig. 12 shows the cumulative amounts of the vari­

ous heats (the determination of which is given in theappendix )-liquid and solid specific heats and heatof fusion-which are liberated from a 1 in. thicksegment at the midheight of each of the high carbonsteel ingots. The heat of fusion curves are, of course,the same as the volume solidification curves plottedas Btu's rather than percent. While the rate at whichheat of fusion is liberated diminishes with time, therate at which heat is liberated from the metal thathas already solidified increases and nearly balancesthe diminishing rate of heat evolved by solidifica­tion. Thus the rate of total heat evolved from aningot after the air gap is formed is nearly constant.

52-JOURNAL OF METALS, JANUARY 1952 TRANSACTIONS AIME

50 100 150 200SUPERHEAT of

I-I 2-1 3-1 4-(MOLD RATIO

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II

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12

fication proceeds by the development and move­ment across the ingot of three distinct zones: l--acompletely solid zone adjacent to the mold wall, 2­a completely liquid zone at the center of the ingot,and 3-a solidification zone located between zones 1and 3 which is a mixture of liquid and solid metal.It should be noted that variations in the tempera­ture range of solidification arising from differencesin carbon contents affect only the relative widthsof the three zones leaving the basic mechanism ofsolidification essentially unchanged.

The major effects of variations in the temperaturerange of solidification are illustrated in the linearand volumetric solidification curves of the high andlow carbon steel ingots poured at intermediate tem­peratures into molds of 2 to 1 mold ratio, Fig. 13.It may be noted from the linear curves that at com­parable times in the solidification process the "startof freeze" front in the high carbon steel is moreadvanced and its "end of freeze" front is less ad­vanced than the comparable fronts of the low carbonsteel. While the "start of freeze" front of the highcarbon steel is active, the rate of travel of the "endof freeze" front of the high carbon steel is slowerthan that of the low carbon steel. During this period,as can be seen at the bottom of Fig. 13, the separa­tion of the "end of freeze" waves of the two steelsis constantly increasing. However, with the comple­tion of the "start of freeze" wave the central portionof the high carbon steel ingot is in a partially solidcondition with the consequent result that the "endof freeze" wave thereafter moves forward at a morerapid linear rate than that of the low carbon steel.After completion of the "start" wave the relativerates of travel of the "end" waves for the two steelsare reversed as should be expected.

Despite the differences in distribution of solidwithin the two ingots freezing over wide and narrowsolidification ranges, Fig. 13 (lower graph), the vol­umetric solidification rates are very nearly alike asshown in the top graph of Fig. 13. The high carbonsteel requires a slightly longer time (approximately5 pct) to complete solidification than does the lowcarbon steel poured under comparable conditions ofsuperheat and mold ratio, as shown in Fig. 14. This isascribed in part to the lower temperature differentialbetween the mold and metal for the high carbonsteel which results in a lower rate of heat transfer,and also to the fact that the high carbon steel, in

14

14

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12

4 6 8 10TIME-MINUTES

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Fig. I3-Effect of carbon content on volumetric (upper graph)and linear (lower graph) progression of solidification.

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Most of the liquid specific heat is lost very early assuperheat and the small amount remaining is lostas the liquid cools into the mushy region.

The heat evolution curves of the various ingotsare plotted in one group, Fig. 12F, in order to affordan easier quantitative comparison. It is noted thatthere is a marked spread between ingots only in theamounts of total heat and of specific heats of liquidand solid metal evolved. The spread between theheat of fusion curves is relatively nil. The heat com­ponents of ingots poured at the same temperatureinto molds of small and large mold ratios indicateinteresting relationships. More total heat is absorbedby the heavy mold than by the thin mold at anygiven time, but this extra heat is specific heat fromthe solid metal which has already solidified. Similarlywhen the mold ratios are the same and superheatis varied, more total heat is absorbed by the moldwhen superheat is high. This extra heat is primarilyspecific heat of liquid. Thus while thin molds andhigh superheat prolong solidification times, thesedifferences are not as great as would be expectedfrom a consideration of only the heat capacities ofthe molds or the initial heat within the liquid metal.

Effect of Carbon Content on the SolidificationThe wide range of carbon contents between the

two steels studied permits evaluation of the com­bined effect of differences in phase transformation,temperature range of solidification, and temperaturelevel of solidification On the solidification mechanismof steel ingots. The 0.08 pct C steel solidifies to 8iron over the temperature range of 2775° to 2740°F(35°F) while the 0.60 pct C steel solidifies to 'Y ironover the range of 2700° to 2570°F (l30°F).

A complete evaluation of the carbon effect neces­sitates consideration of the entire course of solidi­fication. As discussed in the foregoing sections solidi-

TRANSACTIONS AIME JANUARY 1952, JOURNAL 5)F METALS:-53

solidifying over a temperature range of approxi­mately 130°F must liberate concurrently with heatof fusion more specific heats of both the liquid andsolid components of the metal within this tempera­ture range than the low carbon steel which solidifiesover a range of only 35 of.

Conclusions

I-The lateral solidification of stccl in ingot moldsis characterized by the simultaneous travel of "start"and "end of freeze" fronts by encroachment intoliquid which remains essentially isothermal at theliquidus temperature. The space separation of thesefronts, which indicates a zone of intermixed liquidand solid, is proportional to the width of the liquidusto solidus temperature range and, within limits, in­versely proportional to mold ratio.

2-Linear solidification rates based on "end offreeze" are parabolic only during approximately thefirst half of the solidification cycle, after which thereis a rapid deviation from the parabolic condition.The classic relationship thickness = K y'time ap­plies only to this first stage. The K values of theparabolic relationship increase as the carbon contentdecreases.

3-Ingots of 7x7 in. cross-section solidified com­pletely in 10 to 12 min within the limits of carboncontent, superheat, and mold thickness investigated.These ingots developed an air gap at 1 to 1% minafter pouring with consequent marked temperaturefluctuations at the ingot-mold interface.

4-Volumetric solidification rates of 0.08 and 0.60pct C steels poured under like conditions are closelysimilar. Linear solidification rates of the low carbonsteels are greater during the early period of para­bolic progression and slower in the later stages ofsolidification. This is the result of the difference inwidths of the zones of intermixed liquid and solidin the two steels.

5-Superheat is completely liberated at earlystages of solidification. During the period of super­heat elimination the progression of solidification isretarded. Following this period linear solidificationrates become essentially equal irrespective of initialsuperheat. Increasing superheat produces a generalcoarsening of the macrostructure.

6-Variations in mold thickness under like condi­tions of superheat and steel composition have noeffect on solidification rates in the very early stagesof solidification. At early stages molds of lower heatcapacity show reduced rates of solidification. Moldratio variations in the range of 1 to I to 4 to 1 donot affect macrostructure.

7-Heat balance determinations show that follow­ing the formation of the air gap the total heat trans­fer of any particular ingot in a given mold remainsessentially constant. A constant rate of heat transferis maintained by the simultaneous metering of thevarious heat components (specific heat of liquid,specific heat of solid, and heat of fusion) to the moldas solidification progresses.

AppendixThe various components of the total heat liberated

after any time in the solidification process were calcu­lated from the gradient curves shown in Figs. 3 to 6.The following thermal constants were employed:

Specific Heat-Liquid Steel 0.2 Btu per Ib, ofSpecific Heat-Solid Steel 0.165 Btu per lb, OFHeat of Fusion 117 Btu per lbDensity (Liquid and Solid Steel) 0.26 lb per cu in.

54-JOURNAL OF METALS, JANUARY 1952

To determine heat of fusion which had been liberatedup to any given time it was necessary to determine theamount of solid present not only in the completelysolid region which had cooled below the solidus tem­perature, but also in the mushy region above the solidustemperature. The percentage of solid in the mushyzone of each ingot was determined by averaging thepercentage of solid at three equispaced points withinthe mushy zone. The fractional part of the tempera­ture range between the liquidus and solidus throughwhich each point had cooled was determined from thegradient curves of the particular ingot under consid­eration. This point was transferred to the comparablelocation on the Fe-C equilibrium diagram and the per­centage of solid was determined by the lever arm rela­tionship.

In calculating specific heats of liquid and solid metalliberated between the liquidus and solidus tempera­ture, where liquid and solid coexist, transformationfrom liquid to solid was considered as occurring iso­thermally at a temperature where, according to theequilibrium diagram, the metal was 50 pct liquid and50 pet solid. This condition exists in 0.55 to 0.65 pet Csteels when the metal has cooled to a point below theliquidus equal to one third of the temperature rangeof solidification.

The heat liberated by the liquid metal as specificheat up to any given time included: I-superheat, 2­specific heat of the liquid within the mushy region,and 3-specific heat of the solid metal below the solidustemperature which necessarily had to give up specificheat as a liquid within the mushy zone before it solidi­fied.

The cumulative amounts of specific heat liberated bythe cooling of solid metal include the heat liberatedby the solid at temperatures both within and below themushy zone. All of the solid present in the ingot at anyparticular time is considered to have cooled from the50 pct liquid-50 pct solid point. The heat evolved incooling below the solidus was obtained by figurativelydividing the ingot element into 1h in. wide concentricrings and from the gradient curves obtaining the meannumber of degrees below the solidus that each ringhad cooled. This figure for each ring when multipliedby the density and specific heat of the steel, and bythe volume of the ring is the specific heat of solidevolved by the ring. The total of the heats evolved bythe ingot is equal to the sum of the heats evolved byeach ring in the ingot.

References'J. Chipman and C. R. Fondersmith: Rate of Solidifi­

cation of Rimming Ingots. Trans. AIME (1937) 125,pp. 370-377.

2 B. Matuschka: The Solidification and Crystalliza­tion of Steel Ingots: The Influence of Casting Tem­perature and Undercooling Capacity of the Steel.Journal Iron and Steel Inst. (1931) 124, pp. 361-386.

8 B. Matuschka: Solidification in Open Topped andClosed Topped Ingot Molds. Journal Iron and SteelInst. (1938) 137, No. I, pp. 109-126.

• J. W. Spretnak: Kinetics of Solidification in KilledSteel Ingots. Trans. A.S.M. (1947) 38, pp. 569-676.

5 L. H. Nelson: Solidification of Steel in Ingot Molds.Trans. A.S.M. (1934) 22, pp. 193-226.

6 V. Paschkis: Studies on Solidification of Castings.American Foundryman (December 1945) 8, pp. 26-37.

7 V. Paschkis: Theoretical Thermal Studies of SteelIngot Solidiikation. Trans. A.S.M. (1947) 38, pp. 117-147.

8 R. Jackson, R. Sarjant, J. Wagstaff, N. Eares, D.Hartree, and J. Ingham: Variable Heat Flow in Steel.Journal Iron and Steel Inst. (1944) 150, No.2, pp. 211­268.

• B. Matuschka: Heat Equilibrium Between Ingotand Ingot Mold Wall. Archiv. f. Das Eisenhutten­wesen (1929) 2, pp. 405-411.

,. H. F. Bishop, F. A. Brandt, and W. S. Pellini:Solidification of Steel from Sand and Chill Walls.Trans. Amer. Foundrymen Soc. AFS Preprint No. 51­21 (1951).

TRANSACTIONS AIME