DIFFERENCE BETWEEN CRACKS AND SEAMS.
When temperers find that their tools are cracking under their treatment, they are apt to assume that, as they are working in their ordinary way, there must be something wrong with the steel. It is either seamy, or harder than usual, or not uniform in temper, or it is of inferior quality.
All or any of these conditions may exist and be the cause of the trouble; but every man should bear in mind that he is also a variable quantity; he may be unwell and not see and observe as closely as usual; there may be a long spell of unusual weather giving him a light differing from that to which he is accustomed; or, as is often the case, he may simply have unconsciously departed from the even track by not having his mind carefully intent upon the routine which has become a sort of second nature to him, so that for a time he ceases to think, makes of himself an animated machine, and the machine left to itself does not run with perfect regularity.
If personal pride, egotism, or ill temper be set aside, it is always easy to find out whether the fault is in the steel or in the man; that once determined the remedy is easily applied, and the sooner the better for all parties.
How to Break a Tool. Let an ordinary axe be considered.
If the axe be cracked as shown in [Fig. 1], the corners have been hotter than the middle of the blade; probably by snipping the corners and the middle and comparing the fractures the coarser grain at the corners will tell the tale.
If the crack be as shown in [No. 2], the middle of the blade has been hotter than the corners: snipping and comparing the grains will tell the story.
If the crack be more nearly a straight line, as shown in [number 3], the chances are that there is a seam there and the steel is at fault.
How to Tell a Seam from a Water-crack.—A seam is caused by a gas-bubble in the ingot which has not been closed up by hammering or rolling; it always runs in the direction of the work; in bars it is parallel to the axis.
The walls of a seam are always more or less smooth, the surfaces having been rubbed together under heavy pressure during hammering or rolling, and they are black usually, being coated with oxide.
The walls of a water-crack are never smooth, they are rough and gritty, and they may have any of the temper colors caused by the action of water and heat.
There need never be any question as to which is which.
If a long tool cracks down the middle, it may be from too much heat, from seams, or from a lap.
A lap is caused by careless working under a hammer, or by bad draughts in the rolls, folding part of the steel over on itself. Laps, like seams, run parallel to the axis of a bar, and usually in very straight lines.
Any long piece of steel may be split in hardening by too much heat. In making the experiment of heating a piece continuously from scintillating, or creamy color, down to black, to show the differences of grain due to the different heats, the sample almost invariably splits down the middle as far as the strong, refined grain, or nearly that far.
As stated before, a round bar will be almost certain to split if it be heated up to medium lemon, although a square bar may endure the same heat without cracking.
An examination of the walls of a split will settle at once whether it is a seam, a lap, or a water-crack.
A seam will not necessarily be long; its walls will be smooth.
A lap usually runs the whole length of the bar, and the walls are smooth.
By smooth walls of seams and laps comparative smoothness is meant; they are sometimes polished, but not always, and they are never granular like the walls of water-cracks.
If the split be a water-crack, the walls will be rough and granular.
After a temperer has straightened himself out, and brought his work to usual accuracy and uniformity, if his tools continue to crack and indicate weakness in the steel, it is time for him to suspect the character of his material and to require the steel-maker to either show up the faults in tempering, or improve the quality of his product.