At a recent meeting of the engineering section of the Bristol Naturalists'
Society a paper on "Chilled Iron" was read by Mr. Morgans, of which we give
an abstract. Among the descriptions of chilled castings in common use the
author instanced the following: Sheet, corn milling, and sugar rolls; tilt
hammer anvils and bits, plowshares, "brasses" and bushes, cart-wheel boxes,
serrated cones and cups for grinding mills, railway and tramway wheels and
crossings, artillery shot and bolts, stone-breaker jaws, circular cutters,
etc. Mr. Morgans then spoke of the high reputation of sheet mill rolls and
wheel axle boxes made in Bristol. Of the latter in combination with wrought
iron wheels and steeled axles, the local wagon works company are exporting
large numbers. With respect to the strength and fatigue resistance of
chilled castings, details were given of some impact tests made in July,
1864, at Pontypool, in the presence of Captain Palliser, upon some of his
chilled bolts, 12¾ in. long by 4 in. diameter, made from Pontypool
cold-blast pig iron. Those made from No. 1 pig iron--the most graphitic and
costly--broke more easily than those from No. 2, and so on until those made
from No. 4 were tested, when the maximum strength was reached. No. 4 pig
iron was in fracture a pale gray, bordering on mottled. Several points
regarding foundry operations in the production of chilled castings were
raised for discussion. They embraced the depth of chill to be imparted to
chilled rolls and railway wheels, and in the case of traction wheels, the
width of chill in the tread; preparation of the chills--by coating with
various carbonaceous matters, lime, beer grounds, or, occasionally, some
mysterious compost--and moulds, selection and mixture of pig irons, methods
and plant for melting, suitable heat for pouring, prevention of
honeycombing, ferrostatic pressure of head, etc. Melting for rolls being
mostly conducted in reverberatories, the variations in the condition of the
furnace atmosphere, altering from reducing to oxidizing, and _vice versa_,
in cases of bad stoking and different fuels, were referred to as
occasionally affecting results. Siemens' method of melting by radiant heat
was mentioned for discussion. For promoting the success of a chilled roll
in its work, lathing or turning it to perfect circularity in the necks
first, and then turning the body while the necks bear in steady brasses,
are matters of the utmost importance.
The author next referred to the great excellence for chilling purposes
possessed by some American pig irons, and to the fact that iron of a given
carbon content derived from some ores and fluxes differed much in chilling
properties from iron holding a similar proportion of carbon--free and
combined--derived from other ores and materials. Those irons are best which
develop the hardest possible chill most uniformly to the desired depth
without producing a too abrupt line of division between the hard white skin
and the softer gray body. A medium shading off both ways is wanted here, as
in all things. The impossibility of securing a uniform quality and chemical
composition in any number grade of any brand of pig iron over a lengthened
period was adverted to. Consequent from this a too resolute faith in any
particular make of pig iron is likely to be at times ill-requited.
Occasional physical tests, accompanied with chemical analysis of irons used
for chilling, were advocated; and the author was of opinion it would be
well whenever a chilled casting had enjoyed a good reputation for standing
up to its work, that when it was retired from work some portions of it
should be chemically analyzed so as to obtain clews to compositions of
excellence. Some of the physical characteristics of chilled iron, as well
as the surprising locomotive properties of carbon present in heated iron,
were noticed.
Attention was called to some German data, published by Dr. Percy in 1864,
concerning an iron which before melting weighed--approximately--448¼ lb.
per cubic foot, and contained--approximately--4 per cent. of carbon--3¼
being graphitic and ¾ combined. The chilled portion of a casting from this
had a specific gravity equivalent to 471 lb. per cubic foot, and contained
5 per cent. of carbon, all combined. The soft portion of the same casting
weighed 447¾ lb. per cubic foot, and contained 34.5 per cent. of
carbon--31.5 being graphitic and 3.5 combined. Mr. Morgans doubted whether
so great an increase in density often arises from chilling. Tool steel,
when hardened by being chilled in cold water, does not become condensed,
but slightly expanded from its bulk when annealed and soft. Here an
increase of hardness is accompanied by a decrease of density. The gradual
development of a network of cracks over the face of a chilled anvil orbit
while being used in tilt hammers was mentioned. Such minute cleavages
became more marked as the chill is worn down by work and from grinding.
Traces of the same occurrence are observable over the surface of much worn
chilled rolls used in sheet mills. In such cases the sheets get a faint
diaper pattern impressed upon them. The opening of crack spaces points to
lateral shrinkage of the portions of chilled material they surround, and to
some release from a state of involuntary tension. If this action is
accompanied by some actual densification of the fissured chill, then we
have a result that possibly conflicts with the example of condensation from
chilling cited by Dr. Percy.
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