“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.
ARSENIC.
Arsenic is known to be very harmful in tool-steel, and it is proper to assume that it can do no good in structural steel. In any case where the properties of steel do not come up to the standard to be expected from the regular analysis examination should be made for arsenic, antimony, copper, etc. These are not as universal constituents of steel as silicon, phosphorus, sulphur, and manganese, but they are present frequently, and in any appreciable amount they are bad.
XI.
THEORIES OF HARDENING.
The hardening of steel is such a marked phenomenon, and one of so great importance, that it has always attracted a great deal of attention, and many theories have been put forward in explanation.
Before chemistry was brought to bear upon the subject the proposed theories were based upon assumption, and as there were no proofs one had as much right to consideration as another, and none seemed to be altogether satisfactory.
Since science has taken up the question the theories are about as numerous as the investigators, and while no one can claim as yet to have settled the matter definitely, each one has an apparent basis of reason deduced from observed facts.