Cast Iron for Large Castings*

By E. LO N G D E N , A.M .I.Mech.E. (Member)

The qualities required in m etal for different types o f large castings vary greatly; analyses are given in T able I:—

Ta b l e I .— A n a ly sis o f M etal fo r Large Castings.

T.C. Si. Mn. P.

S

17-ton caus­

tic p o t . . 3 .4 0 1 .0 0 0 .7 5 0 .3 0 0 .0 8 1 .0 21-ton sp in ­

dle cast­

ing 3 .1 0 0 .8 5 0 .4 0 0 .6 0 0 .1 3 20-ton h y ­

dr a u 1 i c

cylinder . . 3 .1 5 0 .9 0 1 .0 0 0 .3 5 0 .1 0 32-ton 94-

in. lathe

headstock 3 .2 0 1 .4 0 0 .6 5 0 .6 5 0 .0 9 50-ton ham ­

m er anvil

block 3 .2 5 1 .2 0 0 .6 5 0 .5 5 0 .0 9

G enerally, the use o f alloying elements to im ­ prove the qualities o f cast iron is growing. The im provem ents sought m ay be all or any o f the following:— (1) Increase in strength; (2) sound­

ness with uniform ity o f structure; (3) resistance to various therm al conditions prom oting growth and disintegration o f the metal; and (4) resist­

ance to w ear or corrosive conditions.

There are sound reasons for em ploying special metals to im prove the flowing power o f the m etal and the service life o f special classes o f castings which enter the category o f small and small medium weights. But for large and medium- weight castings, the incorporation of expensive metals in the cast iron is, to pu t it mildly, both unnecessary and extravagant.

A part from the special-purpose metals con­

taining large quantities o f the expensive alloying or m odifying elements, the alloy additions vary usually between 0.25 and 2.0 per cent. In such small percentages the added metals do no t confer directly, to any serious extent, special properties on the cast iron. T he substantial effects o f alloy­

ing elements are indirect through their influence, mainly, on the conditions o f the carbon and ca r­

• See fo o tn o te to P a p e r N o. 713.

bides, either as graphitisers or carbon stabilisers, and on the freezing rate of the metal. All these conditions can be also influenced considerably by the freezing rate of the m etal, due to the section and mass of m etal and the condition of the m ould m aterial.

Structural Control

T he object o f the au th o r’s attention has been to secure a defined structure in a particular class o f casting. It m ay no t always be possible to secure the desired structure and properties in sm all and m edium lightweight castings, due to the rate of freezing preventing the reduction of the carbon and graphitisers to the effective percentages. In such cases the em ploym ent o f m odifying element^ brings about the desired re ­ sults. But with general heavy, m edium and large castings, the therm al conditions are equally favourable to the production o f the desired struc­

ture simply through the control o f the elements norm ally present in the cast iron. The slower casting rate in heavy castings tends to level up heat gradients. The hot-m ould process for the production o f pearlitic cast iron m ay be cited as a case o f control of the structure o f cast iron through the norm al elements present in cast iron and therm al conditions o f the mould.

The m etal specification for a large caustic pot w hich the author considered includes 1 per cent, o f nickel introduced as a graphitiser in p refer­

ence to about 0.3 per cent, o f silicon. A lthough this specification was met, the author is uncon­

vinced of the need to em ploy nickel in a casting carrying a uniform section o f approxim ately 3 in. T he structure desired in caustic pots is one which will no t be too dense to withstand therm al conditions o f expansion and contraction due to the firing on the outside o f the casting, and yet be uniform in structure so as to resist the corrosive influence o f the caustic fluids.

It is the au th o r’s experience to expect little trouble from fluid shrinkage in m etal o f the hem atite class, o r w ith m etal containing over 3.30 per cent, total carbon and phosphorus below 0.3 per cent. T he analysis for the caustic pot altered by raising the silicon by 0.25 to 1.25 per cent, an d om itting the nickel would be equally effective in producing the desired structure.

M any years ago it was the practice to p our such pots in N o. 2 hem atite iron, because o f the need to w ithstand therm al shocks. A t the sam e time, the m axim um life was no t obtained so far as resistance to the caustic solution is concerned, because of the open grain structure com m on to hem atite irons containing silicon above 1.50 per cent. T h e rem edy lay in reducing the silicon

content to ab o u t 1.20 per cent, an d the total carbon to ab o u t 3.20 per cent, along w ith m ore exact control in the operations o f heating up the pot in service.

O f far m ore im portance than one o r two points in silicon or carb o n is the question of pouring m etal into a m ould in such a way as to avoid the collection of sullage at any particular

> 4 S Q

J _

4' g ‘

z ^ r G A pproxim ate C asting T em ­ peratures:— A , 1,380 deg. C.;

B, 1,330 deg. C.; J, 1,340 deg. C.; R em ainder, 1,375 deg. C.

F ig . 1.— In f l u e n c e o f Co m p o s i t i o n a n d Pr o c e s s e s o n t h e Ph y s i c a l Pr o p e r t i e s o f Va r i o u s Ty p e s o f Ca s t Ir o n.

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p o in t as previously stated. In m any pots exam ined it was quite obvious th a t sullage porosity an d n o t fluid shrinkage porosity was the direct cause o f early failure due to penetration by the corrosive solution.

Influence o f Elements on Structure Fig. 1 is interesting in connection with the effects o f adding special metals and m etalloids to cast iron. This investigation was carried out by the author som e three years ago. It is ex­

trem ely difficult to com pare the effects o f various alloying elements. U sually investigation p ro ­ ceeds in stages w hereby the effects o f perhaps elements on the structure and soundness. Tensile and transverse test-bars were poured a t the

nickel, copper, copper-nickel and m anganese, and sufficient alum inium (1 oz. per 10 lbs.) to refractory-lined and heated hand-ladles were pre­

pared and m arked on the lining to indicate the pletely grey throughout all sections.

D carrying the nickel addition created graphi­

a m ild carbon stabiliser and no t as graphitisers w hen em ployed separately.

O, w hich includes alum inium , shows th a t a core o f graphite penetrates nearly the full length o f the 1-in. section.

H , w ith m anganese at 1.04 per cent., indi­

cates th a t graphite precipitation is reduced.

W hite iron had penetrated the 1-in. section by ab o u t 10 per cent.

I test clearly shows the hardening effect of the increased sulphur content. W hite iron had penetrated the 1-in. section.

J is soda-ash treated, producing a lowered sulphur content, and shows th a t white iron had slightly penetrated the 1-in. section, indicating th a t softening effect o f the lowered sulphur had been m ore than offset by the lowering o f the

silicon to 1.08 per cent., 0.06 per cent, being elim inated during desulphurisation.

K test w ith the calcium silicide addition p ro ­ duced a silicon content of 2.13 per cent., result­

ing in graphitisation th ro u g h o u t the step section. F urtherm ore, a re-in. section gas-relief riser gate leading from the 1-in. section end was also grey.

T he table of Fig. 1, show ing the physical and m echanical test results, confirm s the state of the structure o f the various examples, w ith the exception o f the soda-ash treated m etal J. It is difficult to understand why the hardness value and tensile test results are so low in J. The Brinell hardness test was taken on a 1-in. depth m achined surface located on the centre side of the 2-in. section.

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