It is to be noted that while malleable cast iron far surpasses “semi-steel” in this property, though their tensile strengths are ordinarily somewhere near the same, cast steel, in turn, offers more than six times the resistance of the malleable iron to shock and has nearly double its tensile strength. It is this great strength and resistance to shock, heat and pressure, with freedom from “permanent growth” under alternate heatings and coolings that make cast steel such a valuable material for the many purposes for which castings are to-day employed. Millions of steel castings annually find varied application.
| Alloy | Tensile Strength | Number of Blows | Total Foot Pounds |
|---|---|---|---|
| Cast Iron | 23,400 | 7 | 102 |
| Semi-steel | 35,050 | 11 | 206 |
| Malleable Cast Iron | 37,140 | 22 | 1,580 |
| Cast Steel | 72,120 | 92 | 10,112 |
In modern power houses and other commercial steam and hydraulic installations particularly, steel castings have come to be the materials usually specified and approximately the only ones which satisfactorily serve under the severe conditions of to-day.
Pouring from a Lip-pour Ladle
Undoubtedly the first steel castings were poured from crucible steel, though we must remember that the crucible is a melting and not a refining furnace. This was only natural. In the crucible the metal can be made very hot and fluid, and if of proper composition and properly “killed” crucible steel makes very fine castings. Crucible steel castings, however, are not in as fortunate a position as are other products of this high grade material. Tool steels ordinarily bring high enough price that there remains a profit to the manufacturer though his manufacturing cost is necessarily high. In the steel casting line, however, there is much keener competition and crucible steel has had considerable difficulty in maintaining its place. It seems to be a matter of price alone.
Open-hearth steel is very largely used for steel castings, more than two-thirds of all made in this country being of this material. About one half of these are poured from basic open-hearth metal, and the other half from acid metal. It is generally considered that the product of the acid-lined furnace is a little freer from over-oxidation.
Tapping Side of Two-ton Oil-burning Open-hearth Furnace for Steel Castings
Open-hearth steel cannot generally be as hot and fluid as are the steels made in other types of furnaces. For this reason as well as because of the larger size of the usual open-hearth furnace, small castings are not generally poured from this steel. It is for steel castings of considerable size and where there are sufficient orders to warrant a steady and large output that the open-hearth has its place. True, smaller open-hearths are now built, some of them of only two or three tons capacity, but, in general, the standard open-hearth for steel castings is of fifteen tons or more capacity and of the style of the open-hearth furnaces which were described in Chapter IX.