This most valuable material has of late years come into prominent use in a variety of forms, although in ordinary building construction the expense attending its adoption has been, and still is, the principal obstacle to its extended use in that direction. A material possessing some four times the direct tensile resistance of ordinary cast iron, and a range of elasticity to which cast iron offers but little parallel, appears at first sight an ideal metal to be cast into constructional forms. But, as above stated, the high price of steel castings at present forms an effective commercial obstacle to any rivalry with the cheaper metal in ordinary construction. There are also other facts to be taken into account which somewhat discount the advantages of increased strength and elasticity. These are, in the present stage of the art of steel-founding, first, the want of finish on the exterior of the casting, the roughness of the surface contrasting somewhat unfavourably with the smooth finish obtainable on the skin of a good casting in iron - a result commonly supposed to be due to the high temperature of cast steel, and its injurious effect upon the surface of the mould exposed to the intense heat and attrition of the molten metal. The second defect, of a more vital nature, is one which probably causes the inspectors and users of this material considerably more anxiety than the want of external finish. It consists in the .almost invariable presence of gas-holes, or, as would be called in iron castings, air- or blow-holes, more or less numerous, mainly of small dimensions, but frequently not showing themselves on the surface of the casting until machining has revealed their existence, and possibly not even then.
The judgment of the inspector will frequently be exercised as to the extent to which such defects may be considered to affect the soundness of the casting, and an indiscriminate rejection of work on this ground only is generally considered an impracticable course to adopt in the present condition of steel founding.
The problem will again and again present itself to the mind of the inspecting official as to how far a gas-hole manifesting itself on the surface, let us say of a machined casting, may extend into the body of the work, what may be its cubic capacity or its ramifications, and to what extent is it likely to interfere with the practical soundness of the casting, and with its fitness for the purpose for which it has been designed?
When the depth of such a defect cannot be well ascertained by pricking with a wire, owing possibly to its crooked shape, a measure of its cubic capacity may be obtained by pouring water into the hole, and observing the volume of water absorbed.
But when every allowance has been made for the defects above mentioned, it must still be conceded that the high tensile resistance, elasticity, and toughness of this material render it extremely valuable under certain conditions.
But little is at present known as to its compressive resistance in the form of long columns, or in the compression flange of simple girders, and the proportions of girder flanges •determined by the early and well-known experiments of Hodgkinson would probably require considerable modification in the use of cast steel for simple H-shaped girders.
In machinery and engine work the use of cast steel is extensive and various. Spur wheels of all sizes up to large diameters are in frequent use, though the high rate of shrinkage of cast steel as compared with cast iron gives some little trouble in this direction occasionally.
Mitre wheels, small gearing and clutches, and generally details exposed to severe shock are all instances of its use.
In ship construction we have perhaps the finest examples of the use of this material in large and complicated castings often of several tons in weight, as in rambow castings, screw frames (known in the shipbuilding yard as "spectacles"), rudder frames, and the like.
In the Table of tests which follows, a considerable variety of purposes to which this material has been applied will be observed. Thus in Table No. 20, tests Nos. 1 to 40, we find a series of tests of steel castings used for pawl racks in slipways for hauling craft up for repairs having a displacement of some 200 tons or more. The racks having to resist the shock of the pawls in the event of the hauling apparatus giving out, were required to be of a tough and very strong material, and cast steel was used for the purpose.
In Table No. 20, tests Nos. 41 to 97, we have a series of tests on cast-steel bollards for mooring purposes on a wharf or breakwater wall.
Tests Nos. 270 to 272 are the results from castings for wheels to bogie waggons carrying concrete blocks of 50 tons weight. These are small wheels of crucible cast steel.
Tests Nos. 98 to 193 are from steel castings for the roller paths of large sliding caissons for dock entrances, these paths being laid under water. Tests Nos. 194 to 226 are from the roller castings themselves.
Specimens from cast-steel cylinders for hydraulic rams, and their covers, together with various other items of machine construction, also find their place in the tables.
In several cases the details of these castings are found in the illustrations referred to, in order that the student may fully realize the class of work to which the tests have reference.
It is perhaps unnecessary to remark that when cast steel is specified, it is desirable to be sure that cast steel is supplied. All is not cast steel that is called by that name.
An examination of tests Nos. 1 to 8 in Table No. 20 will show considerable fallings off in the metal offered from the specified standard, but the improved results successively indicated in tests Nos. 9 and onward show the beneficial result of a steady adherence to the requirements of the specification.
The tests applied to steel castings usually consist of tensile and bending tests on specimens cast on the castings and cut off for that purpose after annealing. Large castings are frequently hammered, or let fall from a height, to test their soundness.