Name.

Percentage of Carbon.

Properties.

1.

Malleable iron.

0,25

Is not sensibly hardened by sudden cooling.

2.

Steely iron.

0,35

Can be slightly hardened by quenching.

3.

Steel.

0,50

Gives sparks with a flint when hardened.

4.

Steel.

1,00 to 1,50

Limits for steel of maximum hardness and tenacity.

5.

Steel.

1,75

Superior limit of welding steel.

6.

Steel.

1,80

Very hard cast steel, forging with great difficulty.

7.

Steel.

1,90

Not malleable hot.

8.

Cast-iron.

2,00

Lower limits of cast-iron, cannot he hammered.

9.

Cast-iron.

6,00

Highest carburetted compound obtainable.

In Table XXIX the writer has attempted to classify systematically the various kinds of iron and steel used in buildings. The table has been largely based on a similar table prepared for the Stevens Indicator by Mr. William Kent.

Manganese is frequently used instead of carbon to harden steel, it hardens it considerably (though not so much so as carbon) and does not reduce its ductility as much as carbon.

Chromium and tungsten are added to and produce very hard but yet ductile, steels, but add considerably to the expense. Chrome steel being used largely for fine tools and vault-works. While cast-iron can be melted at 2732° F. and cast-steel at 3500° F., wrought-iron is practically not fusible. By adding, however, some 1/2 to 1 per cent of aluminium to wrought-iron its point of fusion is reduced some 300° to 400° F. and it becomes practicable to melt it. This is done in crucibles of fire-clay or plumbago and in petro-

Classification of Structural Iron and Steel.

Mitis.

1 Copied from " Notes on Building Construction." leum furnaces. The product is a comparatively new metal and known as mitis. Its resistance to tension is about one-fifth greater than that of wrought-iron, while its ductility is about the same.

In "Notes on Building Construction" is given such a very able short resume of the difference between cast and wrought iron and steel, that it will be given here with but slight modifications.

"The great differences in the characteristics of cast-iron, and wrought-iron and steel, are briefly recapitulated below, and these determine the uses to which they are respectively applied.

"Cast-iron has little tensile strength, but affords great resistance to compression.

"It is hard, brittle, wanting in toughness and elasticity, and gives way without warning, especially under sudden shocks or changes of temperature. It is easily melted and run into various shapes.

"The castings thus produced are liable to air-holes and other flaws, which reduce their strength. Small castings are stronger in proportion to their size than large ones." Castings should be of even thickness throughout and all sharp angles should be avoided.

"Cast-iron can be cut or turned with edge tools, but is not malleable either when cold or hot, nor is it weldable.

"It is not so easily oxidized in moist air as wrought-iron. In salt water, however, it is gradually softened and converted into plumbago.1

"Cast-iron is peculiarly adapted for columns, bedding-plates, struts, chairs, shoes, heads, and all parts of a structure which have to bear none but steady compressive strains; also for gutters, water-pipes, railings, grate-fronts, and ornamental work of nearly every description.

"It has been much employed for girders, but is an untrustworthy material for those of large size, or in important positions, it is liable to crack and give way without warning under sudden shocks, and also under extreme changes of temperature, such as occur in the case of buildings on fire, where the girders may become highly heated, and then suddenly cooled by water being poured on them." For lintels of short span it is used on account of the ease of adapting it to any condition. The main objection to the use of cast-iron is its brittle and unreliable nature, danger of flaws, and danger of snapping in case of fire and sudden cooling by a stream of water.

Resume of

Qualities.

Cast-iron.

1 Or dissolved.

"Malleable cast-iron possesses originally the fusibility of cast-iron, and eventually acquires some of the strength and toughness of wrought-iron. It may be used for heads, shoes and other joints in roofs, and for all articles in which intricacy of form has to be combined with a certain amount of toughness.

"Wrought-iron has many most valuable qualities, though these differ considerably as to degree in different varieties of the material.

"Its tensile strength is three or four times as great as that of cast-iron, but it offers not half the resistance to compression.

"It is, however, very tough and ductile, and therefore gives way gradually instead of suddenly snapping.

"Its elastic limit is equal to about half its ultimate strength, and it will bear repeated loads below that limit without injury.

"Wrought-iron is practically infusible, is malleable hot and cold, is weldable at high temperatures, and can be forged into various shapes.

"It is subject to 'hot and cold shortness' produced by impurities, and to other defects. Large sections are more likely to contain flaws than small ones. Bars are, as a rule, stronger than plates, and plates are stronger with the grain than across it.

"Malleable iron rusts quickly in moist air, but stands salt water better than cast-iron.

"The great tensile strength of wrought-iron leads to its employment for tie-rods, bolts, straps, and all members of any structure which are exposed to tensile stress; it is also much used for members which undergo compression," particularly if very long compared to their diameters, or if exposed to fire. "It should be employed for all important iron beams and girders, especially those exposed to sudden shocks. In its various forms it comes into play in a variety of ways in roofs, braced girders, and iron structures of all kinds. Corrugated sheets are much used for roof coverings.

" Steel differs even more than wrought iron in the characteristics of its several varieties.

"It has a high tensile strength, much greater than that of wrought-iron. Its resistance to compression is also much greater. Moreover, it has a harder surface, and is better able to resist wear and tear.

"Hard steels, containing a large proportion of carbon, are fusible, easily tempered, have a high tenacity and elastic limit. Their resistance to compression is enormous, especially when they are tempered, but they cannot he easily welded or forged, are brittle, and very uncertain in quality.