By J. G. PEARCE, M.Sc., F.Inst.P., M.I.E.E.
The establishm ent in 1928 o f the first national specification for general grey-iron castings (B.S.S. 321/1928), w ith w hich the original Test- Bar Com m ittee of the Institute was actively con
cerned, was a landm ark in the history o f cast- iron testing. A m ong the revolutionary features of this specification were:—
(1) The introduction of the tensile test for use in parallel w ith the transverse test, pre
viously used alm ost exclusively.
(2) T he adoption o f cylindrical test-bars, rendering obsolete the rectangular and square bars previously used, accom panied by the option o f m achining the transverse bar.
Fi g. 1.— De f l e c t i o n a n d Di a m e t e r o f Ba r.
(3) T he acceptance o f the principle o f a d justing size of bar to section o f casting repre
sented by it.
(4) T he adoption of test-bars cast separately from the casting.
In addition, the specification both actively and passively, that is, both by w hat it said and by w hat it did not say, pow erfully reinforced the principle that for general purposes, iron castings should be specified by their mechanical proper
ties and that com position should be left to the discretion o f the founder.
Experience has confirmed the accuracy of the original basis, which has been extended, and in m inor degrees modified. T o-day there are six grades o f iron covered in two specifications, 321 and 786, ranging from 9 to 26 tons per sq. in. in tension, an d five test-bar sizes in place o f the original three. As was hoped, the basis o f test
ing has been adopted both in new and in re
visions of earlier B.S. Specifications. It has had repercussions abroad. A great deal o f ex
perience has now been recorded on the round bar, to be found in British, U.S. and Continental
sources. The w ar has given an enorm ous im
petus to the use of these specifications, both for new products and for cast-iron replacements of other m aterials. T heir existence has been a great advantage to the industry.
In some quarters, a dem and for revision has been expressed, to take care o f the m inor incon-63
sistencies arising from the grow th o f the furnish a basis for discussion, although pressure o f other w ork u nder present conditions has pre
vented this being m ade as com plete as was intended.
The Transverse Test
Skin-m achining transverse bars gives a rather better result, and, w hat is perhaps m ore im por and deflection being approxim ately constant.
Bars can therefore be readily com pared with transverse stress changes w ith size.
Deflection
T he deflection figure gives a valuable indica
tion o f toughness, b u t its value is obscured by diam eter and span variations in test-bars and by changes consequent on the change in quality between various grades. A t present, deflection is regarded principally as an indication o f re deflection and in o rdinary engineering irons ab o u t 75 per cent, o f the total deflection is
tough irons a high perm anent set, as do irons between elastic deflection and total deflection, as the perm anent set is, under these conditions, very low, and hence the error in taking total for elastic deflection is small. At fracture, the difference is considerable, and. hence elastic m oduli calculated from total deflection show large errors.
The difference between an ordinary engineer
ing iron and a high-duty iron under transverse thereby enabling greater elastic deflection to take place under load. T he total deflection in com pression than in tension, transverse failure,
Ta b l e I I .— Transverse Values for Specified Tensile tensile and transverse stresses. If the transverse rupture stress is designated R, and the ultimate
m ations, for in any equation connecting R and T, w hen one is zero, the other m ust also be zero.
T he developm ent in recent years o f high-duty irons has enabled a m uch wider range of strengths to be charted from a large num ber o f cases. F ro m the transverse and tensile strengths so obtained, the curve show n in Fig. 3 results, a n d it will be seen th a t in the m iddle range it is substantially straight and slightly curved at the upper and low er ends. This curve enables the transverse strengths to be m ore accurately estim ated w ith respect to corresponding tensile strengths. T he full line covers the range of strengths for which evidence is available.
Figures fo r the tensile strengths given in the two specifications are accom panied by the tran s
verse values deduced from this curve in T able II.
It will be seen th a t the differences are small,
series would enable the present m ixed n om en
clature of letters an d figures to be im proved.
T he straight p o rtio n o f the curve, Fig. 3, has an equation o f approxim ately R = 1.1T + 9.5, or the transverse ru p tu re stress is 1.1 times the ultim ate tensile stress plus 9.5 tons. T he curved portions w ould have equations o f such a kind th a t it is sim pler to determ ine R ap p ropriate to a p articular value of T or vice versa direct from the curve.
T he equation could be altered to give a very different appearance w ithout substantially alter
ing the results. F ro m a m uch m ore limited range o f values, the au th o r gave in 1930 the equation T = 0.59R - 4,000, the stresses here being in lbs. per sq. in.,1 and th e resulting R from a given T is n o t widely different in the two cases.
4 0 4 5
TRANSVERSE RUPTURE STRESS-TONSPERSQ/N^
Fi g. 3.— Re l a t i o n s h i p b e t w e e n t h e Te n s i l e a n d Tr a n s v e r s e St r e n g t h s o f Ca s t Ir o n
b ut inconsistencies are rem oved. A series of values w hich form a continuous range for the 1.2-in. bar o f tensiles a t intervals of 2 tons, is given in T able III. T he odd series includes four already appearing in 321 and 786, an d the even series only two, m arked w ith a n asterisk.
T he corresponding deflections can also be accurately estim ated, as indicated above. In this way, a regular range of irons could be speci
fied, covering all reasonable requirem ents, giving eight m aterials in place o f the present six. The effect is to fill the gap between G rad e A and G rade 1, retaining G rades A and C. G rades 1 to 4 are virtually retained, w ith a grade between 3 and 4 to m ake a consistent series. Such a
O ne very interesting result follows from the curve, Fig. 3. It has been supposed hitherto th a t as irons im prove, the transverse value does n o t increase as rapidly as the tensile, and con
versely th a t in the w eaker irons the tensile is low ered relatively m ore than the transverse.
Fig. 3 shows through the bending o f the high duty iron end to the horizontal axis, th a t R in
creases relatively m ore than T. A t the other end, the bend in the reverse direction shows th at the view with regard to w eaker irons was co r
rect.
T h e a u th o r is confident that, w hile fo r vari
ous reasons freak results m ay be recorded, as a general rule any com bination of tensile and
transverse figures obtained from properly priate adjustm ents to com position, the difference in strength between larger and smaller castings friends, who, incidentally, have adopted British practice of varying test-bar size with section of in strengths appropriate to lighter castings.
Secondly, it is the usual practice for a firm to separately-cast test-bars, the m etallurgical de
sirability of which, com pared with the cast-on bar, is now accepted. It is notew orthy that one im portant authority some years ago expressed its willingness, in case of dispute, to accept the Cast Iron Research Association for permission to publish this Paper.
4 P e a rc e , J . G. “ T h e C o rrelatio n o f M ech an ical T e sts fo r C ast industrial nations were giving attention to this particu lar subject. It was pleasing to record test-bars cast separately from the casting.”
A Standard Test Mould pressure tests, certain parts show ed failures which, in his opinion, were due to the absence sufficiently enlightened in regard to the sec
tions these cast irons could be used in. They such stress relieving treatm ent.
M ore Specifications Needed
H e would like to have M r. Pearce’s opinion M r . I. Y a t e s , dealing w ith the general ques- on the question of stress relief with these high- tion o f specifications, said it had been his
experi-68
ence th at co-operation between the designers and the foundrym en themselves, a feature which everybody desired to see, had been rather vague; in fact, it was often impossible to get co-operation wholly between the designer on the one hand and the m an who tried to m ake elaborate series of specifications, w hether from the cast-iron, or non-ferrous point of view, was
tainly for specification purposes, because it obviously involved fairly accurate draw ing of the curve. A lthough it might be excellent from strength would appear in the test-bar represent
ing the section o f the casting. T h at was the specifications was really a compromise. An iron which was suitable for a large bar was
obviously quite unsuited for a smaller bar, and the figures showing rising strength with decreas
ing size o f the bar bore very little relationship personal experience was that enorm ous varia
tions occurred. He did not believe that there 786 specifications, and substitute one specifica
tion approxim ately as given in Table III of
mittee of the Institute’s Technical Committee.
Author’s Reply
M r . P e a r c e , replying to the discussion, agreed with Mr. H udson th a t it was very useful to 69
m atch experience in the non-ferrous an d cast the specification values were reached, otherw ise the designers’ w ork rested on a false foundation. transverse strengths. They usually arose from faulty m aterial o r faulty testing in one way or another. I f they got sound m aterial and tested it properly there was a reasonable concordance between tensile an d transverse strengths and in rigid h ard irons a low trans p roper specifications in the future.
M r . A . L. K e y in s e c o n d i n g s a i d t h a t h e 70
thought great credit was due to the author for the case he h ad m ade in support o f his theories.
T he Ch a i r m a n observed th a t norm ally they were indebted to those who prepared Papers,
but in wartim e, w ith the pressure o f business, greater thanks were due to those who gave up
their leisure for w ork in the com m on cause.
T he vote o f thanks was carried with acclam ation.
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