S.I. Specification for Cast Iron The iron foundry trade has m ade great efforts

during the last few years to bring about im­

provements in the properties of grey cast iron, and m ore especially to produce a series of products of consistent quality. Every possible means of im provem ent has been studied, but special attention has been paid to obtaining higher strengths, coupled with greater uni­

form ity of structure in castings of com plicated design and where widely divergent sectional thicknesses are exhibited in one and the same casting.

The B.S.S. for general iron castings N o. 321 draw n up in 1928 enum erated two grades, the highest minim um tensile strength being that called for in G rade C, nam ely, 12 tons per sq. in. on 0.785 in. dia. test bars as cast, the corresponding figures for 1.2 in. and 2.2 in. dia.

test bars being 11 and 10 tons respectively.

D uring 1938 a new British Standard Speci­

fication 786 was issued to cover high-duty cast irons. This specification detailed three grades of high-duty cast iron, the highest grade, num ber III, calling fo r a minim um tensile strength of 22 tons per sq. in. on 0.785 in. dia. test bars, with corresponding values o f 18 and 15 tons from G rades II and I.

A lthough for several years previously alloyed and special process cast irons had been used to meet specifications calling fo r over 22 tons per sq. in. tensile strength, the issue of this new B.S.S. specification did m uch to bring high- strength cast iron to the notice of engineers.

It is interesting to note th at 786, G rade III, calls for nearly double the highest strength called for only 10 years previously. Of course, this does not m ean th at cast irons of higher tensile strength were not being m ade in 1928, but until 1938 there was no British Standard Specification which could be quoted. A still higher strength grade has recently been added, nam ely G rade IV, with a m inim um tensile strength o f 25 tons per sq. in. on the 0.875 in. dia. bar.

The question im m ediately arises in any gather­

ing of foundrym en, w hat is the best, easiest, and most economical m ethod of meeting these speci­

fications for any particular application, and w hat are likely to be the founding difficulties.

D uring m ore norm al times when choice of raw m aterials is alm ost unlim ited there are, com ­ paratively speaking, very few real difficulties, but at present choice o f raw m aterials tends to be som ew hat limited.

3 g 2

Broadly speaking, the easiest and m ost re­ controlled com position has been widely accepted. In the sam e way it has been found produced in any foundry possessing a reasonably m odern cupola furnace if full use is taken of consider the broad tendencies exhibited by various com m only recognised elem ents either

used singly or in a variety o f com binations.

sider very briefly the structure and constitution of cast iron. C ast iron is essentially an allay that the graphite particles possess practically no tensile or shear strength. T herefore the u lti­

Alloy additions produce two distinct effects in grey cast iron. In the first place they will

Both the above tendencies produce changes in the physical properties, and m uch research

boron. These elements, by reason of being very pow erful carbide form ers, increase chill, stabilise carbides and refine and harden the m atrix in pearlitic cast irons.

Chill-Restraining Additions

The chill-restraining elements m ost com m only used, ap art from carbon and silicon, are nickel and copper, the latter only within the limits of its solubility. These two elements also refine and strengthen the pearlitic m atrix. O ther graphitising or chill restraining elements are alum inium , titanium and zirconium . O f these, alum inium acts like carbon and silicon, in that it produces ferrite and softens the m atrix and also coarsens the graphite structure.

On the other hand, titanium , although p ro ­ titanium additions, while certainly r'efining graphite structures, m ay under certain circum ­ stances lead to the form ation o f so-called extremely im portant alloying element, namely, m olybdenum , which has been described as and distribution of the graphite.

The m ost striking im provem ents in the pro­ in castings of widely varying sectional thick­

ness.

(2) Better physical properties, including 85

higher tensile and transverse and compression strength and resistance to shock, etc.

(3) Im proved w ear resistance.

(4) Im proved heat resistance.

(5) Im proved corrosion resistance.

(6) Im proved m achinability when correlated with any o r all of the above improvements. but special-purpose alloy cast irons prepared particularly fo r their wear, heat and corrosion resisting properties, will be excluded.

Current Practice for Meeting B.S.S. 786 mostly w ithout alloy additions have mechanical properties sufficiently high to meet this grade, low nickel cast irons in this category exhibit extremely good machining qualities, take a high quality finish and are generally foolproof.

F urtherm ore, they can be produced in any foundry w ithout special equipm ent and with very little metallurgical control. Their wear resist­

ance is also good, but this can be further im­

In order to m eet the requirem ents of G rade III, 22 tons per sq. in. m inim um tensile strength, the sam e type o f cast iron, i.e., w ith 1 to 2 per cent, nickel, w ith the addition of 0.3 to 0.5 per cent, m olybdenum , is useful, p a r­

ticularly w here com paratively small tonnages are required in foundries lacking the necessary m etallurgical control fo r the production of the ferro-silicon, calcium silicide and other graphitis­

ing agents.

Meeting Grade IV with Pseudo Austenitic Irons A m ongst G rade III m aterials N i-Tensyl iron described as the general utility high-strength cast irons. T h e m atrix in the as-cast condition Tensyl iron is particularly suitable for pistons, cylinder heads and cylinder blocks in high-speed supercharged types o f diesel engines. O ften strength an d w ear resistance have to be coupled w ith freedom from any risk o f picking up or galling under very severe conditions and with extrem ely lim ited clearance tolerances under high-speed lubricated ru bbing conditions. O ther im portant fields o f application are for dies, com plicated designs w ithout after-treatm ent, thus ensuring speedy deliveries.

T hey do no t exhibit the sam e degree o f liquid shrinkage porosity difficulties in unequal sec­

tions as is experienced in the p roduction of steel castings. H ence their im m ense value to the engineering industry under present condi­

tions. driving gears, sliding gears, tool holders, spindle couplings, form ing and profiling cam s as well as horizontal tailstock an d collet cylinders in hydraulically operated lathes. These com ­ ponents w ould norm ally be m ade from steel

fication, it m ust be rem em bered that, when

dries, particularly even the best jobbing foun­

dries. It is no t readily applicable to any but new specification, nickel-m olybdenum cast irons of som ew hat higher alloy contents should prove

W hen discussing the individual influences of alloying elements on cast iron, it was observed transverse strength, and deflection with inter­

m ediate hardness values (up to 350 B.H.N.) and outstanding m achinability in relation to strength.

Their resistance to shock is m uch in advance of anything previously achieved for grey cast irons with norm al carbon contents. This type of cast castings with tensile strengths considerably less than this, and electric furnace irons held the increased as the sectional thickness increases, bu t care must be taken not to over-alloy the m aterial with the form ation of true unm achin- able martensite.

Practical Applications

So far m ost of the practical applications of acicular-structured nickel-molybdenum cast iron has been confined to rather lumpy thick section castings, particularly in connection with a variety of types of hot heading and forging

tough-ness has been found extremely useful for hot pressing and forging dies, highly stressed sliding gears, crankshafts, cam shafts, levers and num erous heavily stressed com ponents. Even w hen the carbon content is increased to 3.3 per cent., as m ay happen under certain cupola operating conditions, tensile strengths over 28 tons per sq. in, will be m aintained, provided the alloy content is adjusted to the m ass of the casting. Its w ear resistance is extrem ely good, a n d these properties appear to be retained up to quite heavy sections.

Limits of the Phosphorus Content I t has already been strongly em phasised th a t increase o f phosphorus from , say, 0.3 to 0.7 per cent., or even 1 per cent., does no t greatly re­

duce tensile strength in pearlitic cast irons (other considerations being equal), but this increased phosphorus does increase or widen the freezing o r centrifugal casting m ethods.

A lthough the changes which have taken place m ethod naturally fails where pressure-tight cast­

ings o f com plicated design are being produced.

H igh phosphorus, of course, is not detrim ental to attainm ent of solidity if adequate feeding facilities can be provided, such as by m eans of self-feeding heads, progressive solidification, etc.

U nfortunately, the degree of liquid shrinkage tendency produced by phosphorus increm ents so fa r cannot be estim ated accurately. M ost of

High Phosphorus and M oulding Technique M oulding and core m aking practice fo r the

Consequently in producing castings in high- strength cast irons attention m ust be given to progressive solidification. T he com m onest m ethods em ployed a re : example, flat circular castings should always be run with ring type runners with ingates equally distributed ro und the castings, o r in other cases be run from a central point.

L ong cylindrical castings should invariably be 88

run from the top, with the addition, when absolutely necessary, of auxiliary or rather initial bottom running to provide a cushion or solidification from the bottom upwards.

(2) Progressive solidification m ay also be

fortunately been grossly over-stated by engineers and machine shop executives. A fter liquid shrinkage porosity imm ediately beneath risers. On no account should rod or puddle

pseudo-m artensitic structures, which, besides being one of the latest phases in the production of cast irons, was of extrem e im portance. M r. M cRae Smith knew that the com pany with which he (the speaker) was associated, was probably one of the first in the country to undertake the m anu­

facture o f such high-duty cast irons. Their extraordinary toughness was undoubtedly a particularly outstanding feature.

nection with the production of high-strength irons in the cupola had reference to the operating conditions necessary to be developed in the cupola. If a cupola were run entirely