Modern Machine-Shop Practice Part 264

"The f.l.a.n.g.es of the furnaces should always be examined in the bends, for flaws, for such defects, although not very common, do at times unexpectedly make their appearance, and might, if not detected, be the means of breaking the boiler down at sea. This part of the inspection being made, any drilling that is to be done to ascertain the thickness of suspected plates may be proceeded with before the rest of the inspection is made.

"It may, however, be well to remark that a very common defect is the wasting away of the combustion box plates around the necks of the stays or the internal surface of the plates, and it is a usual thing for deposits to acc.u.mulate around these necks, hence, unless these deposits have been removed (particularly in the case of boilers about three years old), the true condition of the boiler may not be known.

"The plate around the man hole door should next be examined, a great defect from waste at the surface that makes the water tight joint. Next comes the man hole door itself, which should have the rubber or other material used to make the joint cleaned off, for cases have occurred where the surface beneath was found apparently sound, whereas the application of a chisel showed that the iron was so corroded that but little iron was left in the f.l.a.n.g.e, causing great surprise that the whole door had not blown out. This defect may generally be looked for in old boilers, and serves to emphasize the necessity for strong wrought iron doors.

"The outside surfaces of the end plate in the vicinity of the furnace fronts are a great source of trouble in some boilers, particularly where plane furnaces are fitted and flush rivets used for connecting them to the end sh.e.l.l plates.

"The insides of the furnaces and combustion boxes next require attention. The most common defects here are lamination of the furnace plate (if of iron), slight collapsing of furnaces, wasting of the furnace plates (particularly when anthracite coal has been used), and wasting when the fire bar bearers or bridges have rested against the plate.

"In the combustion box the buckling of flat plates may have occurred; plates may have wasted from leaks, distortion of the crown sheet from shortness of water may have occurred, or tubes may leak, and whenever, after sounding with the hammer, doubt exists as to the strength of the plate, a hole should be drilled through to test the thickness.

"The wing sides of the furnace may next be examined (through the usual peep holes or by having a boiler mounting taken off for the purpose), and the sh.e.l.l plating on the sides of the boiler, paying special attention to the plates where the feed water enters.

"We may next examine the outside of the bottom of the boiler, which should never be totally inaccessible to the eye, and should always be capable of being reached by a long-handled paint brush, for if kept well painted, the bottom of the boiler is, so far as the exterior is concerned, as durable as the other parts of the sh.e.l.l.

"If, however, the bottom is not kept painted and gets damp, and more particularly from bilge water, it will corrode rapidly, and the boiler must be lifted for examination. Under these circ.u.mstances a new boiler _must_ at five years, at the very most, be lifted for examination, and if found comparatively good it should not be taken as an indication of the probable condition of any other boiler working under similar conditions, for the only means of avoiding a great risk in this matter is to rigidly inspect.

"In the case of flat bottomed boilers in small vessels a good result has obtained by placing them on a bed of cement, which if properly done excludes the bilge water from approaching the plate; but even this precaution would scarcely be sufficient to justify an engineer in neglecting to lift the boiler at reasonable periods for examination of the bottom.

"The internal examination of the boiler is continued from the top by examining the stays in the steam s.p.a.ce, the tube and tube plates, getting down between the nests of tubes and reaching the crowns of the furnaces. The surface of the sh.e.l.l plates should also be examined, more particularly if the boiler contains plates subject to heat on the outside and steam on the other (as in the case of wet up take boilers), for under these conditions a steel plate may become as weak and unreliable as a piece of cast iron.

"If the boiler is fitted with the superheater, the examination of the latter is of the utmost importance, as rapid destruction is here a common occurrence. In the case of a circular marine boiler of any size, nothing need be taken for granted, even though an hydraulic test be made up to twice the working pressure, because there is room for a thorough internal inspection which may disclose defects that would not be shown from the hydraulic test. The proper proportions of fire grate surface, heating surface, steam s.p.a.ce, etc., in a marine boiler differ with the type of boiler and engine, and the steam pressure and degree of expansion employed.

"Upon the question of steam s.p.a.ce, for example, it is a.s.serted by many that marine boilers are not so liable to prime under the higher pressures, and as a result of this a.s.serted fact the steam receiver is in some cases being dispensed with.

"It may be observed, however, that priming to any extent is so costly and detrimental that much consideration needs to be exercised before dispensing with the provisions ordinarily made to prevent it.

"For circular tubular boilers, having a working pressure of from 60 to 80 lbs. per square inch and to be used for compound engines, the following proportions represent current practice.

"1st. One square foot of fire grate area to every indicated horse power of the engine.

"2d. 28 square feet of heating surface[73] to 1 square foot of fire grate area.

[73] The heating surface here referred to includes the total interior surface of the tubes, the sides, backs, crowns and tube plates of the combustion boxes, and that part of the furnace that is above the level of the fire bars, but does not include the front tube plate (_i. e._, the tube plate in the smoke box).

"3d. 6-1/2 to 8 cubic feet of steam s.p.a.ce to each square foot of fire grate area.

"4th. 8 to 10 square feet of tube surface to the total heating surface in single ended boilers.

"5th. 8-1/2 to 10 is about the ratio of tube surface to the total heating surface in double ended boilers.

"6th. The diameters of boiler tubes should be about one-half inch for each foot of length of tube. If less, the tube is liable to choke. About 14 cubic feet of steam (of from 60 to 80 lbs. pressure) should be made for each square foot of fire grate area.

"Each square foot of fire grate will burn from 13 to 18 lbs. of steam coal per hour. About 1-1/2 cubic feet of live steam (of the above pressure) is required for each indicated horse power."

CHAPTER XLIV.--HARDENING AND TEMPERING.

Hardening and tempering processes are performed upon steel for three purposes:

1st. To enable it to resist abrasion and wear.

2nd. To increase its elasticity.

3rd. To enable it to cut hard substances and increase the durability of the cutting edge.

Of these, the first is the simplest, because the precise degree of hardness imparted is not of vital importance.

The second is more difficult, because the quality of the steel employed for such purposes is variable, and hence the tempering process must be varied to suit the steel. The third is of the greatest importance, because the articles to be tempered are the most expensive to make, the duty obtained is of the greatest consequence to manufacturing pursuits, and the fine grade of steel employed renders it more liable to crack in the hardening process.

In those mechanical parts of machines which are hardened to resist abrasion and wear, the quality or grade of the steel is very often selected with a view to obtain strength in the parts and ease of mechanical manipulation in cutting them to the required shape, rather than to the capacity of the steel to harden. Hence, tougher and more fibrous grades of soft steel termed "Machine" steel, are employed, meaning that the steel is especially suitable for the working parts of machines. This cla.s.s of steel is of a lower grade than that known as "tool" steel. It is softer, works, both on the anvil and in the lathe, more easily, and will bear heating to a higher temperature without deteriorating. It approaches more nearly to wrought iron, and is sometimes made of so low a grade as to be scarcely distinguishable therefrom.

The kinds of steel used where elasticity is desired are known as spring steel, blister steel, and shear or double-shear steel, although, for small springs, steel of the tool-steel cla.s.s is often employed.

The word _temper_, as used by the manufacturer of steel, means the percentage of carbon it contains, the following being the most useful tempers of cast steel.

_Razor Temper_ (1-1/2 per cent. carbon).--This steel is so easily burnt by being overheated that it can only be placed in the hands of a very skilful workman. When properly treated it will do twice the work of ordinary tool steel for turning chilled rolls, &c.

_Saw-file Temper_ (1-3/8 per cent. carbon).--This steel requires careful treatment, and although it will stand more fire than the preceding temper should not be heated above a cherry red.

_Tool Temper_ (1-1/4 per cent. carbon).--The most useful temper for turning tools, drills, and planing-machine tools in the hands of ordinary workmen. It is possible to weld cast steel of this temper, but not without care and skill.

_Spindle Temper_ (1-1/8 per cent. carbon).--A very useful temper for mill picks, circular cutters, very large turning tools, taps, s.c.r.e.w.i.n.g dies, &c. This temper requires considerable care in welding.

_Chisel Temper_ (1 per cent. carbon).--An extremely useful temper, combining, as it does, great toughness in the unhardened state, with the capacity of hardening at a low heat. It may also be welded without much difficulty. It is, consequently, well adapted for tools, where the unhardened part is required to stand the blow of a hammer without snipping, and where a hard cutting edge is required, such as cold chisels, hot salts, &c.

_Set Temper_ (7/8 per cent. carbon).--This temper is adapted for tools where the chief punishment is on the unhardened part, such as cold sets, which have to stand the blows of a very heavy hammer.

_Die Temper_ (3/4 per cent. carbon).--The most suitable temper for tools where the surface only is required to be hard, and where the capacity to withstand great pressure is of importance, such as stamping or pressing dies, boiler cups, &c. Both the last two tempers may be easily welded by a mechanic accustomed to weld cast steel.

The preference of an expert temperer for a particular brand of steel is, by no means, to be taken as proof of the superiority of that steel for the specific purpose. It may be that, under his conditions of manipulation, it is the best, but it may also be that, under a slight variation of treatment, other brands would be equal or even superior. It may be accepted as a rule that the reputation of a steel for a particular purpose is a sufficient guarantee of its adaptability to that purpose, and all that is necessary to a practical man is to be guided by the reputation of the brand of steel, and only change when he finds defects in the results, or ascertains that others are using a different steel with superior results.

Where large quant.i.ties of steel are used the steel manufacturers in many cases request customers to state for what particular purposes the steel is required, their experience teaching them what special grade of their make of steel is most suitable.

To harden steel it is heated to what is termed a "cherry red" and then dipped into water and held there until its temperature is reduced to that of the water.

_Tempering_ steel as the blacksmith practises it consists in modifying, lowering, or tempering the degree of hardness obtained by hardening. The hardening of steel makes it brittle and weak in proportion as it is hardened, but this brittleness and weakness are removed and the steel recovers the strength and toughness due to its soft state in proportion as it is lowered or tempered.

When therefore a tool requires more strength than it possesses when hardened, it is strengthened by tempering it. Tempering proceeds in precise proportion as the temperature of the hardened steel is raised.

When the steel is heated to redness the effects of the hardening are entirely removed, and the steel, if allowed to cool slowly, is softened or annealed. To distinguish maximum hardness from any lesser degree the terms to give the steel "all the water," or to harden it "right out" are employed, both signifying that the steel was heated to at least a clear red, was cooled off in the water before being removed from the same, and was not subsequently tempered or modified in its hardness. If a piece of steel has its surface bright and is slowly heated, that surface will a.s.sume various colors, beginning with a pale straw color (which begins when the steel is heated to about 430) and proceeds as in the following table:--

Fahr.

Very pale yellow 430 Straw yellow 460 Brown yellow 500 Light purple 530 Dark purple 550 Clear blue 570 Pale blue 610 Blue tinged with green 630

It happens that between the degree of hardness of hardened steel and the temper due to reheating it up to about 600 Fahr. lie all the degrees of hardness which experience has taught us are necessary for all steel-cutting tools. Hence we may use the appearance of colors as equivalent to a thermometer, and this is called color-tempering. The presence of these colors or of any one of the tints of color, however, is no guarantee that the steel has been tempered or possesses any degree of hardness above the normal condition, because they appear upon steel that is soft or has not been hardened. To obtain exact results by color tempering, therefore, the steel must first be thoroughly hardened, and this is known in practice by the whiteness of the hardened surface.