The forging of shafts for steam vessels is an important piece of work and demands close attention to every detail. A most important thing for the smith to know before beginning work is the location of the bearings. He should then select his material and put it together in such a manner that no joints or welds shall occur at these points. The iron selected should be of new puddled metal, or if of steel, from an ingot newly made. If the latter is used the portion for the shaft should be cut from the lower end of the ingot. This avoids the laminations due to the piping that usually occurs in the upper portions of all large ingots. Where steel of this character is used the work is simplified by the fact that it consists of drawing the ingot down to the desired diameter and length. When iron is used the first pile built up should be about three feet long, bound together at the ends with soft iron binders and with a porter bar set in at one end. If the furnace is large enough, the whole of the pile may be heated at once. If not, then a sufficient length to reach beyond the first bearing is welded. Three piles of this sort will suffice for a shaft 12 feet long. If the bearings are at or near the ends, this will put the welds in the central section. A suitable form of scarfing for the weld of such a shaft is that shown in Fig. 51. It is best when welding such a piece to close the joint by driving the pieces endwise. Then reheat the whole and finish the weld under the steam hammer in the ordinary manner. It is important, however, to forge the whole shaft, if possible, of one large piece. This avoids all necessity for any welds. The progressive appearances of such a shaft from the rough piece to the finished product are shown in Fig. 82. In this, A is the fagot that is first heated and to which the porter bar a has been welded. B represents the piece after the first hammering. It has been reduced in section in the center and made round. The ragged ends are still left in an enlarged shape. C shows the results of further hammering with a portion of the enlarged ends worked down into the body of the shaft. D illustrates the completed shaft with the crop ends cut off, the ends smooth and the whole ready for the machine shop.
The working down of such a shaft as this requires constant care during each step of the process. In the first place the heating must receive close attention. The metal should he heated to a dark straw color. This should be done slowly. If the metal is taken out of the furnace after a too rapid heating, it will he found that the center is at a much lower temperature than the outside. The result of a blow delivered to metal in such a condition will cause the outside to flow over the coole, core. Such a movement will be apt to cause the metal to crack and leave cold-shuts in the body of the shaft. A cold-shut is a crack or space over whose sup-face the metal is not united. They usually occur where an imperfect weld has been formed. The action above described may produce a condition resembling a cold-shut.
While the metal is at such high temperatures, scale forms with great rapidity. Scale, as already stated, is an oxide of iron. It can usually be brushed from the surface of the metal with perfect ease. During the process of hammering, a man should be stationed at the hammer with a longhandled brush with which he should remove the scale as rapidly as it is formed. If it is allowed to remain upon this soft and plastic material, the hammer is apt to imbed it in the body of the metal. Succeeding blows may cause the metal to flow over it, so that cold-shuts are formed.
When possible, the whole piece should be heated in the furnace. Under these circumstances the hammering should be over the whole surface and not confined to any one place. If the work is evenly distributed, the metal flows evenly, and there are no weaknesses developed by distortion. But if, on the other hand, an excessive reduction is made at any one point, the hammering that must afterwards be done on the untouched section will cause a weakness, due to a distortion of the metal. If such hammering is done at the same heat the metal in the larger is worked at a lower temperature than in the first section. If the piece is reheated, the smaller section will be raised to a higher temperature than the larger and the metal at the junction of the two will probably be weakened by distortion and a flow at such unequal temperatures. In many cases the furnace can heat but one end at a time. When working metal heated in this manner, only a small amount of hammering should l>e done each time. The piece should be reduced gradually, first at one end and then at the other. This will prevent the formation of deep shoulders and the consequent weakening of the shaft at that point. It also avoids the wide variation in the temperatures for the different sections.
We have seen that, at high temperatures, the formation of scale is very rapid. When of a straw or light red color, the scale forms in flakes that may be easily brushed away. It is also thick and heavy. During the early stages of the forging the tempera-ture of a heavy shaft should not be allowed to fall below a light red. As soon as it comes down to this point hammering should cease and the piece be put back into the furnace for reheating. During this early period, it is therefore, quite sufficient to brush the scale away as previously directed. As the shaft approaches completion, it is however, desirable to do the finishing work at a lower temperature. The formation of scale then becomes troublesome, as it is more and more adhesive. If allowed to remain upon the metal it may so cover the latter with an opaque coating that the piece will appear black and cold, whereas, it may actually be at a light red heat. The adhesion of the scale prevents it from being brushed away. It is, therefore, customary to turn a small stream of cold water upon the metal during the last or finishing hammering. This treatment loosens the scale, and is continued until the metal has cooled to a dark red, when the final coating of scale will form. This completely encases the body metal and serves as a tolerably efficient protection against rust.
The metal should be worked until it has been cooled to a dark red. There is a two-fold object in this. The metal is thoroughly compact and the scale is of the dark blue color that affords the most efficient protection. Care should, however, be exercised that the hammering is not prolonged until the temperature is too low. Great injury may be done to metal if this occurs. This is especially true of steel. After the red has disappeared the metal conies into what is known as the critical temperature, noticeable from the blue color. As an example of the effect of this blue temperature, which ranges from about 550° to 800° Fahr., that of boiler steel may be cited. It should be possible to bend a plate of good boiler steel double either when cold or at a red heat, without having it show any crack or flaw. If, however, an attempt is made to bend the same piece of steel when it is at a blue heat, it will crack at an angle of from 90° to 120°. Therefore steel should not be worked below a dull red heat.
The quality of a forging depends to a great extent upon the amount of work that is put upon it. Two pieces of metal may have exactly the same chemical properties and yet be entirely different in their physical characteristics. If one is rapidly forged in a single heat and at high temperature down to the finished size, it will be much weaker than another where the work has been done more slowly and by repeated heating. The work done on the metal condenses the material, adds to its strength and increases its wearing qualities. It is the investment of labor for which durability and strength are returned as a dividend.