494 ENGINEERING MATERIALS. CHAP. XV. 



Crucible Steel is commonly made in pots or crucibles holding about 80 pounds of metal. 

 The raw material may be steel scrap; blister steel bars; wrought iron with charcoal; cast iron with 

 wrought iron or with iron ore; or any mixture that will produce a metal having the desired chemical 

 constitution. Manganese in some form is usually added to prevent oxidation of the iron. Some 

 silicon is usually absorbed from the crucible, and carbon also if the crucible is made of graphite 

 and clay. The crucible being covered, the steel is not affected by the oxygen or sulphur in the 

 flame. The quality of crucible steel depends on the freedom from objectionable elements, such as 

 phosphorus, in the mixture, on the complete removal of oxide, slag and blowholes by " dead- 

 melting " or " killing " before pouring, and on tHe kind and quantity of different elements which 

 are added in the mixture, or after melting, to give particular qualities to the steel, such as carbon, 

 manganese, chromium, tungsten and vanadium. 



Bessemer Steel is made by blowing air through a bath of melted pig iron. The oxygen of 

 the air first burns away the silicon, then the carbon, and before the carbon is entirely burned away, 

 begins to burn the iron. Spiegeleisen or ferro-manganese is then added to deoxidize the metal 

 and to give it the amount of carbon desired in the finished steel. In the ordinary or " acid " 

 Bessemer process the lining of the converter is a silicious material, which has no effect on phos- 

 phorus, and all the phosphorus in the pig iron remains in the steel. In the " basic " or Thomas 

 and Gilchrist process the lining is of magnesian limestone, and limestone additions are made to the 

 bath, so as to keep the slag basic; and the phosphorus enters the slag. By this process ores that 

 were formerly unsuited to the manufacture of steel have been made available. 



Open-hearth Steel. Any mixture that may be used for making steel in a crucible may also 

 be melted on the open hearth of a Siemens regenerative furnace, and may be desiliconized and 

 decarbonized by the action of the flame and by additions of iron ore, deoxidized by the addition 

 of spiegeleisen or ferro-manganese, and recarbonized by the same additions or by pig iron. In the 

 most common form of the process pig iron and scrap steel are melted together on the hearth, and 

 after the manganese has been added to the bath it is tapped into the ladle. In the Talbot process 

 a large bath of melted material is kept in the furnace, melted pig iron, taken from a blast furnace, 

 is added to it, and iron ore is added which contributes its iron to the melted metal while its oxygen 

 decarbonizes the pig iron. When the decarbonization has proceeded far enough, ferro-manganese 

 is added to destroy iron oxide, and a portion of the metal is tapped out, leaving the remainder to 

 receive another charge of pig iron, and thus the process is continued indefinitely. In the Duplex 

 process melted cast iron is desiliconized in a Bessemer converter, and then run into an open 

 hearth, where the steel-making operation is finished. 



The open-hearth process, like the Bessemer, may be either acid or basic, according to the 

 character of the lining. The basic process is a dephosphorizing one, and is the one most generally 

 available, as it can use pig irons that are either low or high in phosphorus. 



Strength of Steel. The properties most desired in steel are strength and ductility. Pure 

 iron has a tensile strength of about 40,000 Ib. per sq. in. and is very ductile. This strength is 

 usually increased by the impurities found in steel. 



Carbon is the important impurity as it gives strength with the least decrease in ductility. 

 Campbell states that each o.oi per cent of carbon will increase the strength of acid open-hearth 

 steel by 1000 Ib. per sq. in., and of basic open-hearth steel by 770 Ib. per sq. in. The maximum 

 tensile strength of steel is reached with 0.9 to i.o per cent of carbon. 



Silicon has little effect on the strength of rolled steel, but in castings 0.3 to 0.4 per cent of 

 silicon increases the tensile strength of steel castings and produces soundness. 



Sulphur has little effect on the strength of open-hearth steel, but it produces " red-shortness," 

 and produces checks and cracks during the rolling or during the cooling of castings. 



Phosphorus increases the static strength of steel about 1000 Ib. for each o.oi per cent of 

 phosphorus. The increase in strength is obtained at a great loss in ductility and produces a steel 

 that is brittle and unreliable. 



Manganese when above 0.3 to 0.4 per cent increases the tensile strength of steel. The 

 increase in strength above 0.4 per cent is about 300 Ib. per sq. in. for acid open-hearth and 130 Ib. 

 per sq. in. for basic open-hearth steel for each additional o.oi per cent of manganese. 



From the above discussion it will be seen that if certain physical characteristics are required 

 in a steel the manufacturer must be left free to vary part of the impurities. For example if a 

 high grade structural steel with an ultimate tensile strength of 60,000 Ib. per sq. in. is desired, the 

 phosphorus and sulphur may be limited in addition to the prescribed physical limits if the carbon 

 is left open. 



