The introduction of trustworthy mild steel castings has replaced locomotive iron castings and forgings to the extent of about forty articles, and is still increasing. These are illustrated by Figs. 87-108, including stays, gusset or other-wise; horn-blocks and axle-boxes, domes, covers, and safety valve seating; spring links, brackets, and lire-box foundation rings; reversing shaft - Joy gear - motion plate and brake shaft; wheel centres, and, to a limited extent, the unhammered crank axles made by Messrs. J. Spencer & Son, Newburn Steel Works; one of which, supplied to the North British Railway for engine 460, a heavy class of goods engine working trains over the main lines, has run a mileage of 337,879 miles from February 19, 1881, to April 27, 1892.* This would of course be considered a good mileage for a mild steel forged axle, and is mentioned as an interesting fact; but under existing circumstances the author does not deliberately advocate the use of steel castings for crank axles, although the experiment deserves encomium, and their adoption may be in the future an accomplished fact.

Advantages may be gained in many points by the adoption of steel castings, because they can be guaranteed to be sound, homogeneous, and free from internal strains, at the same time possessing the specified tenacity and ductility. They contrast favourably with built-up iron forgings and those from mild steel ingots; remarkable results as to ductility having come under the personal experience of the author. The tensile strength of unforged steel castings may be taken at about three to four times that of cast iron, therefore they may be considerably lighter than those of cast iron of equal strength ; but whenever steel castings have to replace forgings or iron castings, the physical properties expected of those castings must be carefully taken into consideration, ductility being of much greater value than an unusually high tenacity, because it will give a greater amount of endurance and furnish ample warning of failures, excepting perhaps in a few cases, such as pins, where a great amount of wear and tear takes place, due to friction.

Steel castings are designed sometimes of a peculiar shape and thin in section; when this is the case, the steel founders have to contend with several difficulties in manufacture, which could be avoided if the designer would keep in view the fact that steel castings should be as simple in design and as near uniformity in thickness as the circumstances of the case will admit, and of easy curves. It is contended by some American engineers that it is preferable to bolt several small castings to one large one, than to make a complicated casting with several branches, brackets and projections; because each change in the form of the casting is a source of uncertainty. Steel wheel centres having the rim, boss, arms and balance weight formed in one casting, compare very favourably with built-up wrought iron centres, in which there are of necessity so many welds. To overcome the manufacturing difficulties, it is of the utmost importance that the rim should be in proportion to the arms, which are mostly of the oval section, the dimensions of the major and minor axes where they enter the boss being 4¬ inches by 2 inches, and at the rim 3⅜ inches by 1⅝ inches, Figs. 93 and 94. Mr. R. A. Hadfield opines that the most trustworthy job is made of wheel centres by having a small strengthening rib on the inside of the rim and another at the boss into which the arms sink, which is chipped out during fettling; also that in the case of crank boss wheels, trouble is sometimes experienced when the counterbalance weight is heavy and relatively out of proportion to the other parts of the wheel. This large mass of material near the outside cools more slowly than the rest of the metal, thereby tending to draw it out of shape. Therefore, to obtain a thoroughly sound and good steel casting, the balance weight should be distributed over a sufficient number of spaces between the spokes. The adoption of steel roof-bars for fire-boxes has proved a success see Fig. 15, p. 13. It enables the stays to be well proportioned, and convenient attachments for any form of sling stays desired can be easily added. With cast steel horn-blocks and axle-boxes there is greater strength without being cumbersome, greater security in running, and the risk of breakage is reduced to a minimum. Cast steel motion-plates give greater stability as frame stays. Being cast in one piece, there are not any rivets to work loose, as in the case of the built-up type, and, having greater strength and elasticity than cast iron, they can he made lighter, and there is not the same danger of breakage from sudden shocks. Generally, the remarks made as to the design of iron castings apply themselves doubly to steel castings, especially those relating to fillets, abrupt changes, crystallisation, and the internal strains set up by unequal contraction.

* In April 1894, Mr. Holmes, the chief mechanical engineer, informed the author that this axle was thoroughly examined and tested during the last week in March, was, quite satisfactory, and is still running.

The process and the raw materials should exercise a great influence upon the choice of steel castings, because they can be produced short and brittle, which no amount of subsequent annealing will rectify, for reasons well known to metallurgists. Up to the present, as far as the author is aware, the acid process has given the most uniformly successful castings, the basic and the Bessemer generally favour oxidation of the metal during the process, and this incorporation of the oxides of iron is particularly obnoxious, not only in steel for castings, but generally, although upon occasions splendid castings have been made by these two latter processes, and at the same time containing a low percentage content of carbon. The enormous temperature required to produce fluidity, the difficulty in obtaining a suitable facing sand sufficiently refractory to withstand this great temperature, and at the same time porous enough to allow the gases to permeate, besides the direct venting and the mould being to a certain extent fragile enough to allow the greater contraction without pulling the casting, all tend to produce a product which cannot compete, as far as surface is concerned, with iron castings.