Some Observations on Contraction in Grey Cast Iron
TESTS ON LARGE BORING BAR CASTINGS
18-in. dia., 45-ft long Solid Boring Bar The volume changes of a large solid boring bar, 18 in. dia. and 45 ft. in length and w eigh
ing 15 tons 15 cwts., is illustrated by the cool
ing curves in the graph, Fig. 2. T he casting is shown in Fig. 3.
The casting, on account of its great length, is poured in a horizontal position. A descrip
tion o f the m oulding technique would be too lengthy to include in this Paper. Briefly, the m ould was metal-faced and of special construc
tion, which produced an excellent quality
cast-Fi g. 3.— So l id Bo r in g Ba r We i g h i n g 151 t o n s.
The records o f the m ovem ent of the casting, with associated times, are plotted on graphs.
The early behaviour of the casting is expressed in minutes and the com plete movements in hours.
A line, zero, on the graphs identifies the con
dition of the m olten m etal in the mould. The penetration of the curves below the zero line indicates expansion beyond the size o f the mould. Above the zero line the curves regis
ter contraction and any further volume change, such as expansion or arrests. N ote is taken o f the expansion of the test rods due to absorp
tion of heat from the casting. T he tem pera
ture variations o f the rods are checked at in
tervals. L ater, the same class and length of rod is heated to a sim ilar tem perature as obtained during the cooling of the casting. The length of the rod, cold, as against the rod heated to the particular tem perature required, gives the am ount of extension of the rod, which must be cancelled from the early movements of the casting as recorded. A fu rth er adjustm ent is needed in castings which are cambered. On the
ing. The com position of the m etal is shown in Fig. 2.
A record of the movements of the casting was obtained in a sim ilar m anner as by the genera!
m ethod of obtaining volum e changes described earlier in these notes. Two readings were obtained from two rods located one on each end of the mould; the sum of the movements was plotted in the graph, Fig. 2. All allow ances have been m ade in the curves for the ex
pansion of the test rods due to rise in tem pera
ture. This adjustm ent also applies to the curves of the 22-in. cored bar, a description of which follows the first example.
The m inutes’ curve shows an arrest a t 10 minutes when the casting is substantially solid.
Expansion is indicated for about 65 minutes.
Zero is reached in 170 minutes. Contraction proceeds for a period until 570 m inutes is reached, when a very decided arrest and expan
sion is registered which lasts for 330 minutes.
A further arrest follows at 980 minutes, but it is of short duration and lasts about 25 minutes.
F rom this point the curve indicates only small
fluctuations and a steady rate of contraction.
T he curve in ho u rs shows the com plete be
haviour.
Finally, on cooling to room tem perature, the to tal contraction is fo und to be only 91 per cent, o f standard allow ance at 5,116 thousandths against 5,625 thousandths. T he casting had taken u p 96 per cent, o f its contraction in 10 days.
22-in. dia., 47-ft. long Cored Boring Bar A second test, applied to a cored boring bar, 22 in. dia. and 47 ft. in length, and weighing
the m etal is well advanced. Expansion con
tinues fo r ab o u t 60 m inutes. Z ero is reached in 130 m inutes. A short arrest is disclosed at 285 m inutes. A fter fu rth er contraction, a m ost m arked arrest and expansion is registered in 440 m inutes. This volum e change lasts fo r 320 m inutes, until 750 m inutes is reached. A further sm all arrest can be noted a t 850 m inutes. F rom this p o int the rate of contraction is very un i
form . T he ho u rs’ curve shows the com ple
tion o f the cooling process o f the casting. A fter cooling dow n to atm ospheric tem perature, the total contraction o f the casting was 5.400
100 200 300 400 500 600 100 800 900 1,000 1,100 COO 1,300 1.400
M IN U T E S
Fi g. 4 . — Vo l u m e Ch a n g e s o f t h e Co r e d Bo r i n g Ba r Sh o w n i n Fi g. 5.
To n s Cw t s
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2 0 AO 60 80 >00 >20 140 160 180 200 220 240 300
H O U R S —*| |»-6 —J U—20
6,000
5.000
4.000
3.000
2.000
1,000
A n a l v s \ ? C. 3-18 S l 1-34 R -69
Mn. '66
St a n d a r d Co n tra c tio n 5 . S T 5 =■= >QoI
Ac t u a l Co n t r a c t i o n 5,400 =» 9l,9?
19 tons, confirmed the tests applied to the 18-in.
solid bar. Fig. 4 shows the design and dim en
sions o f the bar, and the cooling behaviour of the casting by the curves plotted on the graph.
The casting is shown in Fig. 5.
T he casting was m ade and poured in a simi
lar m anner as practised for the solid bar, but w ith the added com plication o f a series of kidney-shaped cores, which were alm ost com pletely surrounded by m etal. A gain, a truly sound and satisfactory casting resulted from the technique em ployed. T he com position o f the m etal is set out in Fig. 4.
T he m inutes’ curve indicates an arrest and expansion at 10 m inutes w hen solidification of
thousandths against 5,875 thousandths for the standard allowance. C ontraction equalled 91.9 per cent, of standard allowance. T he casting had taken up 94 per cent, o f its contraction in 10 days.
Test on Long Bed Castings
Figs. 6 an d 7 illustrate the cooling behaviour of a 41 ft. 6 in. long bed casting fo r a roll- grinding m achine. T he design and dim ensions of the bed are outlined by the inset in Fig. 7.
A cross-sectional elevation o f the bed, across the points, A A, o f the portion o f the longitu
dinal elevation, L, is show n a t E.
T he casting is poured in the reverse position
to that which is shown in the sketches, Fig. 7, with the slideways on the bottom face of the mould. The cooling behaviour is followed and checked, as previously described. In this case, however, fo u r curves were form ed. All allowances have been m ade in the curves fo r the expansion o f the rods due to heating, and also fo r the curvature of the casting conse
quent upon the cam ber necessary to produce a straight casting. C am ber is required dow n
wards (in the centre as poured) and sideways (in the centre) in the direction indicated by the arrow , K , in Fig. 7.
T he early volum e changes o f the casting are expressed in m inutes in the graph, Fig. 6, and the com plete m ovem ents, in hours, are shown in Fig. 7. T he letters A, B, C and D identify, in
arrest, fo r 65 minutes, occurs. A t 207 m inutes a third arrest appears lasting for about 30 minutes. T he rem ainder of the curve records only small fluctuations.
Curve C .—A t 8 m inutes the record shows an arrest and expansion of 55 thousandths, lasting for ab o u t 25 minutes. A t 180 minutes, a small arrest occurs for 15 minutes. A t 240 m inutes a third small arrest can be identified. The curve following is steady until final cooling.
Curve A .— A t 10 m inutes an arrest and ex
pansion o f 75 thousandths is shown over a period o f 40 minutes. A second arrest is in
dicated at 150 m inutes, lasting for 20 minutes.
At 195 m inutes a third, sharp arrest is recorded, lasting for 15 minutes. A fourth arrest occurs a t 265 minutes, which covers a period o f 65
Fi g. 5 .— Co r e d Bo r in g Ba r We i g h i n g 19 t o n s.
the graphs, Figs. 6 and 7, the curves taken of the cooling casting, on the lines indicated by A, B, C and D in the sketch E, Fig. 7. The line of contraction, A, carries the heaviest mass of the casting— the vee slideway. T he fine, B, holds the flat slideway, being the second heaviest line of contraction. The line, C, influenced by the extra m etal at H, thereby takes the third position in order of mass influence, and D the lightest and m ost quickly cooled and contract
ing line.
R eferring to the m inutes’ curve in Fig. 6, the following movements are indicated: —
Curve D .—A t 7 m inutes an arrest and ex
pansion of 25 thousandths occurs and lasts for 28 minutes. A t 68 m inutes a second arrest is shown covering a period of about 30 minutes.
A third and fo u rth slight arrest m ay be d e
tected at 140 and 220 m inutes respectively. The curve then indicates a steady contraction with only small fluctuations.
Curve B .— A t 9 minutes an arrest and ex
pansion of 46 thousandths is noted, lasting for ab o u t 30 minutes. A t 130 minutes, a second
minutes. A fifth slight arrest is indicated at 395 minutes. A fter 430 m inutes the curve shows a steady contraction rate.
The standard contraction allowance for a 41 ft. 6 in. casting is 5,188 thousandths. The actual final contraction of the bed is as fol
lo w s:—A, 4,978 thousandths, which is 96.0 per cent, of the standard allowance; B, 4,904 thousandths, which is 94.6 per cent, o f the stan
dard allowance; C, 4,251 thousandths, which is 82.0 per cent, of the standard allowance; D, 4,182 thousandths, which is 81.0 per cent, of the standard allow ance; average contraction, 87.4 per cent, o f the standard allowance. Be
tween the lowest contraction of D and the highest contraction o f A there is a difference of 796 thousandths. A, C side has contracted an average o f 72 thousandths m ore than B, D side.
A (bottom heavy vee slideways) has contracted 74 thousandths m ore than B (bottom flat slide
way). A, bottom , has contracted 727 th o u sandths m ore than C, top. B, bottom , has con
tracted 722 thousandths m ore than D, top. The average contraction of A, B (bottom face) over C, D (top face) is 725 thousandths.
81 G
Camber Allowances
These varied contractions, recorded on the fo u r extrem es of the castings, confirm the cam ber allow ances needed to counteract distortion due to the differing heat gradients and the hin- derance, by design, to free contraction. T he cam ber allowance, dow nw ards, was 2 | in., w hich m ust be associated with the excess con
traction o f 725 thousandths o f A, B (bottom face) over C, D (top face). T h e cam ber requirem ents on the A, C (heavy) side was
| in. This allow ance m ust be associated with an excess contraction o f 72 thousandths o f the A. C side over the contraction o f the opposite side, B, D.
W ithout the heavy facing, S, on the A, C
the m ould, cooling and contraction are, subse
quently, m ore rapid than B, w hich is located in the bottom o f the m ould. A gain, m uch of the heat absorbed by the denseners is retained by the densener to slow up the rate o f cooling a t a later period.
A dverting to the arrests in the cooling curves, it is possible th a t the influences, w hen cooling has reached a stage w hen a considerable differ
ence in contraction stress is upon the casting, cause a late arrest which is n o t actually due to an expansion, bu t m ay be attributed to distortion of the casting.
Test Castings
Fig. 8 illustrates, by sketches, the design o f a test casting 10 ft. by 2 ft. by 7 in. and,
graphic-MINUTES
Fi g. 6 .— Co o l in g Be h a v io u r o f a 4 1 -f t. 6 -i n. Ro l l- Gr i n d i n g Ma c h in e Be d.
side, it is necessary to allow approxim ately } in.
m ore cam ber on the A (heavy vee slideway) line than on the B (flat slideway) line. T h e in
fluence o f the heavy side facing, S, was to re
duce slightly the dow nw ard cam ber, bu t to create, along w ith the heavy vee bottom slide
way, a side distortion if not counteracted by cam ber allowance.
It will be noted, on close exam ination of the cooling curves, th at B (bottom flat slideway) contracted slightly ahead of C (top with side fac
ing) and then later falls behind to take its place in o rd er o f mass cooling. This m ay be due to the early effect of the denseners, on the B slideway, an d the acceleration o f the cooling rate im m ediately afte r solidification o f the m etal. T he C (top side), however, is influenced by a heavy facing, carrying nearly as much m etal as B (bottom ) which is also densened.
Since the side facing is subjected to a sharper cooling, by virtue o f its outw ard position in
ally, the cooling curve and final co ntraction of the casting. T he sketches show th a t the casting consists of a series o f bars o f varying sections m aking one unit. T he outside longitudinal bars carry a 4-in. flange, 1 in. thick, form ing, with a vertical |- in . bar, a tee section. T he inner bar is heavier, having a 5-in. by 2-in. flange m ounted above a i-in . vertical bar, again form ing a tee section. T he cross bars are plain bars o f ^-in. section.
T he test is arranged to create an exaggeration o f the cooling conditions present in certain types o f castings, particularly of bed designs.
T he com position o f the m etal poured into the casting was such as to yield a grain structure as m ight obtain in a m uch larger casting, with a lower silicon content. T he analysis is set out in Fig. 8.
T he volum e changes were checked on three contraction lines, indicated by the letters X.
T he behaviour o f the tw o outside longitudinal 82
bars was so sim ilar that it becam e unnecessary to draw a curve fo r each bar. They were, therefore, m erged and are plotted together in the curve C, in the graph, Fig. 8. The curve traced for the m ovem ent o f the centre bar is identified by the letter B.
The curve C shows th a t an arrest occurs at 4 m inutes with an expansion of 28 thousandths.
A second mild arrest and slowing up o f the rate of contraction occurs at 20 minutes. W ith B curve an arrest is recorded a t 5 m inutes with a 32 thousandths expansion. A third arrest and slowing up of contraction oc
curs at 28 minutes. A steady contrac
tion follows. T he heavy centre, B, passes and exceeds the contraction of the outer lighter bars, C, at 375 minutes.
The final contraction o f the casting came out
sandths less than standard. T he gap, S, sever
ing the bar in the centre had, afte r cooling, widened by 187 thousandths.
The distortion in the second test, with the heavy centre bar split into two sections, was less pronounced than in the first test, w here the casting is not split across the centre bar. The lines C and B cam bered -h in. F urtherm ore, no fractures appeared at the points F.
Conclusions
All of these tests offer substantial support to the previous experim ents carried out by the author and presented to the Institute over a num ber o f years.
T he conclusions are that, on the classes o f iron castings surveyed, thick bars, o r sections, contract m ore than thinner bars, or sections,
A tZ S l 4.182.
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2 2 0 4 0 6 0 8 0 lOO 120 .140 160 180 2 0 0 2 20 2 4 0 2 6 0 2 8 0 3 0 0 3 2 0 3 4 0
HOURS
Fi g. 7 .— Co m p l e t e d Co o l in g Be h a v io u r o f t h e Ro l l- Gr i n d i n g Ma c h in e Be d.
a t 1,246 thousandths for the heavy centre bar, B, being only 4 thousandths below standard allowance, and 1,164 thousandths for the outer lighter bars, C, which represents only 92.5 per cent, o f standard allowance. T he casting was bent (down in the centre as poured) by f in., along the line B, and -¡% in. on the lines C.
The casting was also fractured at six points indicated by F, in Fig. 8.
The usual allowances were m ade in the final figures for the expansion o f the rods due to heat and the curvature of the distorted casting.
A fu rth er test was taken, but w ith the middle bar, B, split by a core, as described at S, Fig. 8.
The behaviour during cooling is interesting.
The lines, C, contracted 1,252 thousandths (close to standard allowance), against 1,164 thousandths fo r the same dim ension o f bar in the first test casting. This means th a t the bars in the first casting contracted 88 thousandths less than the bars in the second casting.
T he two middle bar sections, B, together con
tracted 1,238 thousandths, which is 12
thou-w here such varied sectioned members are linked together, as a single-piece casting, in a proxim ity to be affected by m utual influences. If, how ever, the sam e contrasting sections are cast as simple uniform and separate items, contraction is in line with standard allowance and know ledge— that a thin section of grey iron con
tracts m ore than a thicker section fo r the same analysis, because o f the m ore rapid freezing of the lighter section and the effect of this more rapid cooling on the grain size and graphite form ation.
A probable explanation of the two contrary degrees o f contraction referred to is that, in the case o f a one-piece casting, the thin o r com paratively light sections are subjected to an ex- tensional stress, during freezing and cooling o f the metal, created by the resistance o f the thicker sections w hich are no t ready to con
tract. T he frictional resistance o f the m ould and cores, and the expansion o f cores o n being heated up by the m olten metal, will also tend to subject the cooling m etal to extensional
o 2
stress. A gain, a study o f the very clear arrest and expansion periods noted on the cooling curves, especially those of the heavy an d large boring bars, indicates th a t a thick section of a casting m ay be undergoing an expansion a t a tim e w hen a thinner section has passed its ex
pansion phase and is in a state o f contracting.
U n d er these conditions the thin sections will suffer extensional stress.
Conversely, the thicker sections will be sub
jected to a com pressional stress by the effort o f the earlier cooled m em bers to contract.
Finally, the heavy sections, on cooling, m ust take up a shorter length by bending certain
no t expand, instead of m ild steel and by insert
ing balls between the rods an d the tube, to act as a bearing. T he a u th o r’s view of these aspects would perhaps be o f general interest.
Mr. E. Longden wrote in r e p ly :— T he sug
gestions put forw ard for im plem enting the test procedure are very w elcom e; they afford an opportunity to explain phases n ot yet disclosed.
T he possibility o f em ploying a m etal with a low coefficient o f expansion exercised m y m ind very early in the experim ents— some five years back. W ith this object in view, I later tried an alloy of nickel and iron, of sim ilar com posi
tion as com pounded for Invar metal, but I
C. 3-22
lao
Se c A A Mn.
200 250
MINUTES
20 40
HOURS
Fi g. 8 .— De s i g n a n d Th e r m a l Hi s t o r y o f Te s t Ca s t i n g.
sections o f the casting, o r by fracture in the weakened o r m ost highly stressed sections.
T he a u th o r’s thanks are tendered to M r. J. R.
G reenw ood (chairm an) and the directors of C raven Bros., Lim ited, m achine-tool engineers, fo r facilities to obtain inform ation fo r this Paper.
W R I T T E N D I S C U S S I O N
Mr. V. C. F a u lk n e r (Past-President o f the Institute) w rote: A really valuable contribution has been m ade by M r. L ongden’s researches, n o t only to the practical adjustm ent of contrac
tion allowances to be m ade when m aking p a t
terns and m oulds fo r large castings, but also to the theoretical aspects of the subject. In connection w ith the test procedure, it is con
ceivable th a t im provem ents m ight be effected by using bars o f Invar m etal, as th at m etal does
abandoned the use o f this special alloy because the tem peratures dealt w ith went beyond the range at which the alloy reactions caused a can
celling o f volum e increase. It was also found th a t the alloy test rod becam e em brittled, result
ing in partial fra ctu re at the juncture where the rod entered the m etal o f the casting, indicating intercrystalline failure. This failure would be due to subjecting the nickel alloy to the very high tem perature o f the face o f the m olten cast iron o f the casting.
T he peculiarities o f In v ar and sim ilar alloys are that, while steel expands and contracts norm ally, the special alloy shows little o r no volum e increase over a restricted and co m p ara
tively low range o f tem perature. This range o f tem perature appears to correspond w ith the m agnetic transform ation period, which is com plete a t ab o u t 220 deg. C., depending upon the
com position of the alloy. Thus the volume change is neutralised by the m agnetic trans
form ation. But at low er and higher tem pera
tures both expansion and contraction are experienced.
It was, therefore, decided to revert to the use o f mild-steel rods, which could be relied upon to give the consistency o f a regular expansion rate, according to the tem perature gradient, which would be identified and allowed for when com m itting figures to the curves on the graph.
The second suggestion, by M r. F aulkner,
“ th at balls be inserted in the space form ed by
(G) Weight, fastened to the rod (C).
(H) Baseplate.
(I) Slideway for slide (F).
(J) R ecording paper, gum m ed to the base
plate (H).
(K) The weight fo r m easuring the variations in the gap (L).
(L) G ap which discloses expansion and con
traction m ovem ents as described in the Paper.
The movements o f the casting were recorded by the pencil on the paper. W hen the pencil moves away from the m ould an expansion is
The movements o f the casting were recorded by the pencil on the paper. W hen the pencil moves away from the m ould an expansion is