This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
The following formulae give the ultimate strength of acid and basic open-hearth steel in terms of their principal chemical constituents, where C = 100 X per centage of carbon, P = 100 X per centage of phosphorus, Mn = manganese, x Mn = a coefficient for manganese in acid steel, of which the values are given in Table No. 2, y Mn = a coefficient manganese in basic steel, of which the values are given in Table No. 3, and E = a variable based on heat treatment.
1 The term " pure iron " is arbitrary, and intended to express simply the datum plane from which to start in order to find the strength of steel by a simple formula. "Absolutely pure iron never has been, and in all probability never will be, made."
Formula for Acid Open-hearth Steel. (Carbon estimated by combustion.)
17.85 + 0.44 C+ 0'44 P+ x Mn + R = ultimate tensile strength in tons per square inch.
Formula for Acid Open-hearth Steel. (Carbon estimated by colour.)
17.76 + 0.508 C + 0.44 P+ x Mn+ R = ultimate tensile strength in tons per square inch.
Formula for Basic Open-hearth Steel. (Carbon estimated by combustion.)
18.52 + 0.34C + 0.44P+ y Mn + B = ultimate tensile strength in tons per square inch.
Formula for Basic Open-hearth Steel. (Carbon estimated by colour.)
18.75 + 0.366 C + 0.44P+ y Mn + R=ultimate tensile strength in tons per square inch.
In the above formula, R, the variable for heat treatment, is zero, in angles and plates about 3/8 inch to ½ inch thick finished at a fairly high temperature.
The influence of manganese upon the tensile strength of basic steel is given in the following table: -
Percentage of carbon.
Increase in tensile strength in tons per square inch corresponding to the percentages of manganese and carbon.
Percentage of manganese.
As an example of the application of the above formulae, let us assume a specimen of open-hearth acid steel of which the chemical analysis gives a percentage of carbon (estimated by combustion) of 0166, phosphorus 0053, manganese 0.58; then by the formula we have -
Ultimate tensile strength in tons per square inch
1.00 (see Table No. 2)
. In steels containing less than 0.25 per cent, of carbon, the effect of small proportions of silicon upon the ultimate strength is inappreciable.
Sulphur in ordinary proportions exerts no appreciable influence upon the tensile strength.
It will be observed, from a comparison of the above formulae, that phosphorus causes an addition to the tensile strength for each 0 01 per cent, equal to that caused by carbon for each 0.01 per cent., and this consideration gives force to Mr. Campbell's remark that "it is well not to assume the truth of all tradition, but if there is one fact which seems demonstrated, it is that phosphorus will hide its true character in the testing machine, but will certainly make itself known at some future time."
The great bulk of the material known as mild steel is, in Europe and America, produced by the following processes, viz: -
The Acid Bessemer process; The Basic Bessemer process; The Acid Open-hearth process; The Basic Open-hearth process.
The process by which high carbon steels are produced, known as the "Crucible," need not here be further alluded to, as the quality of steel produced by this method is not that used in those forms of construction with which this work principally deals, being, in fact, chiefly used in the manufacture of machine tools and implements, cutting instruments of keen temper and fine edge, and for other similar purposes.
With regard to the above-mentioned processes, it will be observed that they consist of two principal divisions, viz. the Bessemer and open-hearth (otherwise the Siemens or Siemens-Martin process), each division being further subdivided into the processes known as acid and basic.
The authority previously quoted1 has defined each of these methods of manufacture in general terms, as follows: -
"The acid Bessemer process consists in blowing air into liquid pig-iron for the purpose of burning most of the silicon, manganese, and carbon of the metal, the operation being conducted in an acid-lined vessel, and in such a manner that the product is entirely fluid."
"The basic Bessemer process consists in blowing air into liquid pig-iron for the purpose of burning most of the silicon, manganese, carbon, phosphorus, and sulphur of the metal, the operation being conducted in a basic-lined vessel, and in such a manner that the product is entirely fluid."
"The open-hearth process consists in melting pig-iron, mixed with more or less wrought-iron, steel, or similar iron products, by exposure to the direct action of the flame in a regenerative gas furnace and converting the resultant bath into steel, the operation being so conducted that the final product is entirely fluid."
We have seen that the open-hearth process may be either acid or basic. Of the latter the same author says -
"The basic (open-hearth) process, as herein discussed, consists in melting a charge of pig-iron, or a mixture of pig-iron and low carbon metal upon a hearth of dolomite, lime, magnetite, or other basic or passive material, and converting it into steel in the presence of a stable basic slag by the action of the flame, with or without the use of ore, and by the addition of the proper recarbonizers, the operation being so conducted that the product is cast in a fluid state."
Amplifying the above general description, the following essential points of difference may be noted. In the Bessemer process the high temperature required for combustion and to effect the necessary chemical changes is maintained by blowing air through the molten pig-iron.
In the open hearth no such blowing through process takes place, the bath of steel being exposed to the direct influence of the flame and intense temperature produced by the use of the Siemens regenerative furnace, which forms an essential feature in this method of manufacture.
When we next consider the essential differences which underlie the use of the terms "acid" and "basic," we find, however, points
1 H. H. Campbell, "The Manufacture and Properties of Structural Steel." of detail which are of importance as regards the quality of the resultant material.
The influences, mainly hostile, exerted by the elements of phosphorus and sulphur, but more especially the former, upon the physical qualities of the finished product have already been enlarged upon in the foregoing remarks.
The extent, therefore, to which the elimination of these hostile influences can be carried by the various processes of steel manufacture, having regard to the original quality of the ore used, must consequently claim our attention, if we are to make any selection as to the method by which the finished product desired is to be manufactured.
It is beyond the scope of these notes to enter fully into the complete history of the changes which take place in the contents of the acid-lined Bessemer converter from the commencement to the end of the "blow." Suffice it to say that while the original carbon content has been burnt out until practically none is left, the ultimate desired percentage of carbon being obtained by recarbonization by means of the addition of spiegel or ferro manganese, the element of phosphorus remains at nearly the same percentage as that in the original stock of molten pig-iron or scrap at the commencement of the blow.