ELEMENTS OF DISINTEGRATION.
It has been stated time and again that these impurities are elements of disintegration, and that it would be wise in every case to restrict the quantities allowable within reasonable limits, giving the steel-maker sufficient leeway to enable him to work efficiently and economically, and at the same time to keep the quantities of these impurities as low as possible.
On the other hand, able, successful, and conservative engineers have claimed that if the steel-maker meets their physical requirements as shown by prescribed tests they, the engineers, should be satisfied; that they should not interfere with chemical composition, as they had no fear of subsequent disintegrations.
This argument was answered by the statement that skilled steel-workers could manipulate poor steel so as to bring it up to the requirements; that the well-trained workers in the bridge-shops would not abuse the steel; that the inherent deficiencies would not be developed; the work would go out apparently satisfactory; and that it might remain so for a long time, in the absence of unusual shocks or strains, but that in an emergency such material might fail because of deterioration where a purer material would have held on. In the absence of proofs such statements have been met with a smile of incredulity.
Fortunately some proofs are now at hand, and as the method of getting them has been obtained, more will follow from time to time.
In Engineering, Jan. 17, 1896, Mr. Thomas Andrews, F.R.S., M.Inst.C.E., gives the following cases:
A fracture of a rail into many pieces, causing a serious accident.
A broken propeller-shaft which nearly caused a disastrous accident.
Analysis of the rail:
| Carbon | 0.440 |
| Silicon | 0.040 |
| Manganese | 0.800 |
| Sulphur | 0.100 |
| Phosphorus | 0.064 |
It is clear that the sulphur is excessive, and that it was neutralized so as to make the steel workable by an excess of manganese.
Of the propeller-shaft Mr. Andrews says chemical analysis of outside and central portions of the shaft showed serious segregation.
“The percentage of combined carbon was nearly 50 per cent greater in the inside of the shaft than on the outside; the manganese was also in excess in the inside of the shaft; the phosphorus and sulphur had also segregated in the interior of the shaft to nearly three times the percentage of these elements found near the outside of the shaft.”
Unfortunately Mr. Andrews does not give the analysis of the shaft.
A number of micro-sections of the rail and of the shaft were made and examined.
“Numerous micro-sulphur flaws were found, varying in size from 0.015 inch downward, interspersed or segregated in the intercrystalline junctions of the ultimate crystals of the steel, and being located in such a manner as to prevent metallic cohesion between the facets of the crystals, thus inducing lines of internal weakness liable to be acted upon by the stress and strain of actual wear.”
The dimensions of these flaws in the rail varied from .0150 × .0012 to .0010 × .0004 parts of an inch.
In the shaft from .0160 × .0030 to .0020 × .0016 parts of an inch.
In the rail he found as many as 14 flaws in an area of only 0.00018 square inch, equal to nearly 60,000 flaws per square inch.
In the shaft he found as many as 34 flaws in an area of only 0.00018 square inch, equal to nearly 190,000 per square inch.
In speaking of the shaft he says: “In addition to blow-holes, air-cavities, etc., the interior of the shaft was literally honeycombed with micro-sulphide of iron flaws, which were meshed about and around the primary crystals of the metal in every direction.” “The deleterious effects of an excess of manganese in interfering with the normal crystallization of the normal carbide of iron areas were also perceptible.”
As the number of micro-sulphur flaws in the shaft were about three times as many as in the rail, we may assume that the shaft contained at least as large a percentage of sulphur as the rail, and, owing to the general honeycombed structure, it would not be a far guess to assume that the steel was teemed wild.
“The deleterious effect of these treacherous sulphur areas and other microscopic flaws, with their prolonged ramifications spreading along the intercrystalline spaces of the ultimate crystals of the metal and destroying metallic cohesion, will be easily understood.”
“Constant vibration gradually loosens the metallic adherence of the crystals, especially in areas where these micro-flaws exist. Cankering by internal corrosion and disintegration is induced whenever the terminations of any of the sulphide areas or other flaws in any way become exposed at the surface of the metal, either to the action of sea-water, or atmospheric or other oxidizing influences. In many other ways, also, it will be seen how deleterious is their presence.”
“Internal micro-flaws of various character are nevertheless almost invariably present in masses of steel, and constitute sources of initial weakness which not unfrequently produce those mysterious and sudden fractures of steel axles, rails, tires, and shafts productive of such calamitous results. A fracture once commencing at one of these micro-flaws (started probably by some sudden shock or vibration, or owing to the deterioration caused by fatigue in the metal) runs straight through a steel forging on the line of least resistance, in a similar manner to the fracture of glass or ice.”
It is understood that similar investigations are being carried out on an extensive scale by Prof. Arnold; in the meantime the above cases should satisfy any one that these impurities are elements of disintegration, and that the less there are of them in any steel the better for the steel.
It seems clear that if 10 sulphur will cause 60,000 flaws per square inch, 01 sulphur ought not to cause more than one tenth of that number; or, if an equal number, then they could only be one tenth of the size.
The segregation found in the shaft is so excessive that it would seem probable that there was a good deal of sin there also; but, even if it were unavoidable segregation, the harm would have been just so much the less if there had been less of total impurities present to segregate.