C O NTRACTION TESTS Simple Bar Castings

C ontraction phenom ena encountered in cast­

ings holding contrasting sections disclose in the type of casting investigated by the au th o r th a t the thicker sections con tract m ore th a n the thinner sections. So far as cast iron is con­

cerned, this is quite contrary to the know n be­

h aviour o f th e m etal, th a t the thinner the sec­

tion, the finer and closer is the structure, con­

sequently the greater the contraction.

R eferring to Fig. 5, w hich is a test casting holding varying sections held together in the form o f a grid, contraction takes place as ex­

aggerated in the Sketch d. If the bars are separated fro m association as a grid o r fram e, contraction takes place as norm ally understood;

the thinner the bar, the greater the contraction.

T he graphs of Figs. 6 and 7 shows the cooling

t l

Fi g. 5 .— Te s t Gr i d s f o r As c e r t a i n i n g t h e Ef f e c t o f Co n t r a c t io n.

behaviour of three bars 1 in., 2 in., an d 4 in.

in thickness by 8 in. in depth and 32 ft. in length. T h e bars were poured a t the sam e tim e as the 32-ft. long bedplate described in the graph, Fig. 8, from m etal o f the follow ing com ­ p o s itio n :— T o ta l carbon, 3.22; silicon, 1.48;

m anganese, 0.83; sulphur, 0.11; and phosphorus, 0.79 per cent.

T est Results

T hese tests are typical o f a num ber taken by the au th o r on the contraction and distor­

tion in large castings o f the bedplate design.

T he evidence am ply confirm s the phenom ena noted in the sm all grid test casting.

[For details o f the test procedure see P aper N o. 712, p. 82 o f the present volum e.—Ed i t o r.]

Fig. 6 shows the early cooling behaviour of the simple bar castings. In 12 m inutes the 1-in. bar, A , has taken up its m axim um expansion a t 106 thousandths. Z ero is reached in ab o u t 23 m inutes. A t ab o u t 43 minutes w hen contraction equals 560 thousandths, a secondary expansion takes place w hich lasts fo r 15 m inutes. A fterw ards contraction proceeds steadily.

T he 2-in. bar, B, reaches the m axim um ex­

pansion o f 130 thousandths in 17 m inutes, re­

turning to zero in 35 m inutes. A secondary expansion is noted after 80 m inutes w hen 540 thousandths contraction has been reached. A fu rth e r and third expansion is indicated at 145 m inutes w hen 1,125 thousandths contraction had taken place. This expansion rem ained very steady fo r 70 m inutes.

The 4-in. b a r C reached a m axim um expansion o f 268 thousandths in 50 m inutes an d reached zero in 117 m inutes. A secondary expansion was noted in 180 m inutes. This expansion to o k about 65 m inutes to return to the original contraction point. T he graph o f Fig. 7 expresses in hours the com plete behaviour o f the three simple bars.

It will be noted th a t the 1-in. b ar A has taken up its total contraction o f 4,248 thousandths in ab o u t 86 hours, w hich is 248 thousandths m ore than the theoretical standard. Sim ilarly, the 2-in. bar, B, takes up its to tal co ntraction of 4,204 thousandths in ab o u t 120 hours. C o n trac­

tion is 204 thousandths above standard. T he 4-in. bar, C, takes 148 hours to reach a final co n ­ traction o f 4,050 thousandths.

It will be seen th a t between the thinnest and thickest b ar there is a difference o f 198 thousandths, confirm ing the greater contraction o f the th in n er sections as against the thicker sections when poured separately. T he percentage o f contraction over stan d ard is as fo llo w s: — 1-in.

bar, 6.25 per cent.; 2-in. bar, 5 per cent.; and 4-in. bar, 1.25 per cent. W hen it is rem em bered that the m etal is o f the m oderately close-grained 90

class, the results are also in conform ity with the greater contraction o f the close-grained iron against the softer variety. M etal of 2 per cent, silicon would correspond m ore with the standard contraction.

Bedplate

T he graph o f Fig. 8 expresses in m inutes the nine hours cooling behaviour of a bedplate, also 32 ft. in length. The curve D gives the average contraction on the to p side of the casting as

This P aper would be of too great a length if it included the whole o f the tests.

The design of the bed under consideration was uniform sideways; therefore, as revealed by the contraction figures, it was only necessary to p ro ­ duce two curves, one fo r the bottom contraction and one fo r the to p contraction. T he curve D shows th a t the top area of the bed had ex­

panded 276 thousandths in 30 m inutes, and had reached zero in 122 minutes. The bottom heavy area o f the bed had expanded 438 thousandths

Fi g. 6 .— Gr a p h o f Th e r m a l Hi s t o r y o f Si m p l e Ba r Ca s t i n g s o f In c r e a s i n g Se c t i o n, Ea r l y St a g e s.

poured, and curve E the average contraction of the bottom heavy side of the casting carrying the shears o r slide-ways. It will be rem em bered th at eight rods are em ployed to test a bed, four at each corner o f the ends— two at the top and two a t the bottom at each end.

Several beds tested which carry heavy facings along one side o f the casting show a diffeient rate o f contraction on each side, both top and bottom , as well as the difference between the top light section and the bottom heavy section.

The behaviour o f contraction in such cases is expressed by fo u r curves, and reveals some extraordinary stresses and crises during cooling.

in 35 m inutes, and reached zero in 188 minutes.

T he graph, Fig. 9, expresses the cooling curves in hours. N o clear indication of secondary ex­

pansions can be noted. Y et, if the m inutes curves be examined, one can detect arrests in the curves w hich m ay be caused by expansions.

The conflicting pulls in a large bed will undoubt­

edly m ask som ew hat expansions, since one part of the bed will be expanding when another area is beginning to contract. In tests on very heavy beds the secondary expansions are alm ost as clearly defined as on the simple bars previously outlined.

R eturning to the graph showing curves in

hours, it will be noted th a t contraction is fairly regular until at 53 hours the contraction on the to p and b ottom areas o f the casting is equal.

A t this p o int the curve E — b ottom heavy side o f the casting— crosses the curve D, from which point the contraction o f the bottom E begins to exceed th a t o f the top D. U ndoubtedly, it is at this stage th a t the severe stress and bend­

ing begins to function.

T he critical period o f 53 hours m arks the period after w hich any aids in relieving the

The bed on straightening from the cam ber given will account fo r approxim ately 30 thousandths extension o f the bed. T he am ended figures will then read 3,520 fo r the to p area o r 88 per cent, o f standard, and 3,716 fo r the bottom area or 93.06 per cent, o f standard.

A fu rth er am endm ent is needed in all the curves expressed in the graphs fo r the extension o f the testing rods, due to expansion by heat, and which is included in the total figure of ex­

pansion below the zero line. This extension of

H O U R S .

Fi g. 7 . — Th e r m a l Hi s t o r y o f Si m p l e Ba r Ca s t i n g s, Ex t e n d e d t o Em b r a c e Ov e r Ei g h t Da y s.

casting will be o f little use. In the 53 hours, the bedplate has com pleted nearly 75 per cent, of its total contraction. F rom this point the contrac­

tion of the bottom area E steadily increases its lead over th a t o f the to p area D. In 190 hours the top area had ceased to contract and the bottom in 220 hours.

T he to tal contraction o f the top is 3,550 thousandths o r 88.86 per cent, o f theoretical standard. T he to tal contraction o f the bottom is 3,746 o r 93.65 per cent, o f standard.

Finally, on the to tal contraction an allowance m ust be m ade fo r the difference in length th at a curved line will give w hen straightened out.

the rods will slightly affect the tim es stated in the curves, dim inishing and disappearing as final cooling is reached. T he final contraction is correctly stated as between the m easurem ents before pouring the casting w hen everything is cool and afte r contracting w hen all is cool.

Tests were m ade by placing rods o f the size as used in the m ajor tests between fixed points on a m achined table and setting the inside m icrom eters to a gap form ed at the end of the rod. The difference between the gap w ith the cold rod and afte r heating to a sim ilar tem pera­

ture as obtains in the various sections of cast­

ing dealt with gives the num ber o f thousandth

parts of one inch to be deducted from the first expansion figures.

The am ended figures will approxim ate t o : — F o r the 1-in. bar, A, Fig. 6, first expan­

sion, 106 less 30 thousandths equals 76 thousandths n et expansion.

F o r the 2-in. bar, B, first expansion 130 less 45 thousandths equals 85 thousandths net expansion.

F or the 4-in. bar, C, first expansion 268 less 120 thousandths equals 148 thousandths net expansion.

R eferring to the bedplate the am ended figures for the first expansion will approxim ate to : —

F o r the to p area, D , first expansion 276 less 124 equals 152 thousandths net expansion.

F o r the bottom area, E, first expansion 438 less 266 equals 172 thousandths net expansion.

Fi g. 8 ( Lo w e r Gr a p h) .— Be h a v i o u r o f t h e Up p e ra n d Lo w e r Se c t i o n s o f a 3 2 -f t. l o n g Be d p l a t e.

Fi g. 9 ( Up p e r Gr a p h).— Ex t e n d e d Ti m e Re c o r d w i t h t h e Cr o s s i n g o f t h e Li n e s a f t e r Two Da y s.

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Vital Comparisons

Be d p l a t e—

B o tto m area, E , contraction Top area, D , contraction . . E xtra b ottom contraction over top Th r e e Ba r s—

1-in. bar A , contraction 2-in. bar B , contraction 4-in. bar C, contraction

E x tra con traction on 2-in. bar over 4-in. bar

E xtra contraction on 1-in. bar over 4-in. bar

T housandths.

. 3,716 . 3,520

196, or I in.

4,248 4,204 4,050

154 198 or £ in.

The average contraction o f the three bars equals

The average contraction o f the bed ­ p late equals

Thousandths.

4,167 3,618 The average difference b etw een bars

and bed 549

In the bedplate the thick sectioned area con­

tracts m ore th a n the thinner sectioned area, which is quite opposite to the contraction on the simple bars w here the contraction of the thinner bars exceeds the contraction o f the thicker bars.

These results show clearly how the contraction o f large castings is influenced by design. In bedplates one learns th a t the conflicting expan­

sions an d contractions due to the varying sec­

tions length an d its relationship to the m ould frictional-resistance and the resistance set up by cores cause considerable stress an d strain, so m uch so th a t the castings do n o t contract as m uch as a sim ple unhindered casting. T he dif­

ference is contraction between the simple casting and the com plicated casting is quite \ in. It is easy to understand from this why castings dis­

to rt o r even fracture.

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