OPEN ANNEALING.

Open annealing, or annealing without boxes or pipes, is practised wherever there are comparatively few pieces to anneal and where a regular annealing-plant would not pay, or in a specially arranged annealing-furnace where drill-wire, clock-spring steel, etc., are to be annealed.

For ordinary work a blacksmith has near his fire a box of dry lime or of powdered charcoal. He brings his piece up to the right heat and buries it in the box, where it may cool slowly. In annealing in this way it is well not to use blast, because it is liable to force all edges up to too high a heat and to make a very heavy scale all over the surface. With a little common-sense and by the use of a little care this way of annealing is admirable.

It is a common practice where there is a furnace in use in daytime and allowed to go cold at night to charge the furnace in the evening, after the fire is drawn, with steel to be annealed, close the doors and damper, and leave the whole until morning. The furnace does not look too hot when it is closed up, but no one knows how hot it will make the steel by radiation: the steel is almost always made too hot, it is kept hot too long, and so converted into cast iron, and there is an excessively heavy scale on it.

Many thousands of dollars worth of good steel are ruined annually in this way, and it is in every way about the worst method of annealing that was ever devised.

To anneal wire or thin strands in an open furnace the furnace should be built with vertical walls about two feet high and then arched to a half circle. The inports for flame should be vertical and open into the furnace at the top of the vertical wall; the outports for the gases of combustion should be vertical and at the same level as the inports and on the opposite side of the furnace from the inports. These outflues may be carried under the floor of the furnace to keep it hot.

The bottom of the door should be at the level of the ports to keep indraught air away from the steel. The annealing-pot is then the whole size of the furnace—two feet deep—and closed all around.

The draught should be regulated so that the flame will pass around the roof, or so nearly so as to never touch the steel, not even in momentary eddies.

In such a furnace clock-spring wire not more than .01 inch in diameter, or clock-spring strands not more than .006 to .008 inch thick and several hundred feet long, may be annealed perfectly. The steel is scaled of course, but the operation is so quick and so complete that there is no decarbonized surface under the scale.

This plan is better than the Jones method or any closed method, because the big boxes necessary to hold the strands or coils cannot be heated up without in some parts overheating the steel; all of which is avoided in the open furnace, because by means of peep-holes the operator can see what he is about, and after a little practice he can anneal large quantities of steel uniformly and efficiently.

VIII.
HARDENING AND TEMPERING.

For nearly all structural and machinery purposes steel is used in the condition in which it comes from the rolls or the forge; in exceptional cases it is annealed, and in some cases such as for wire in cables or for bearings in machinery, it is hardened and tempered.

For all uses for tools steel must be hardened, or hardened and tempered. The operations of hardening and tempering, including the necessary heating, are the most important, the most delicate, and the most difficult of all of the manipulations to which steel is subjected; these operations form an art in themselves where skill, care, good judgment, and experience are required to produce reliable and satisfactory results. It is a common idea that all that is necessary is to heat a piece of steel, quench it in water, brine, or some pet nostrum, and then warm it to a certain color; these are indeed the only operations that are necessary, but the way in which they are done are all-important.

An experienced steel-maker is often amazed at the confidence with which an ignorant person will put a valuable tool in the fire, rush the heat up to some bright color, or half a dozen colors at once, and souse it into the cooling-bath without regard to consequences. That such work does not always result in disastrous fractures shows that steel does possess marvellous strength to resist even the worst disregard of rules and facts.

On the other hand, the beautiful work upon the most delicate and difficult shapes that is done by one skilled in the art cannot but excite the surprise and admiration of the onlooker who is familiar with the physics of steel, and who can appreciate the delicacy of handling required in the operation.

There are a few simple laws to observe and rules to follow which will lead to success; they will be stated in this chapter as clearly as may be, in the hope of giving the reader a good starting-point and a plain path to follow; but he who would become an expert can do so only by travelling the road carefully step by step. The hair-spring of a watch, or a little pinion or pivot, so small that it can only be seen through a magnifying-glass, the exquisitely engraved die costing hundreds or thousands of dollars, and the huge armor-plate weighing many tons, must all be hardened and tempered under precisely the same laws and in exactly the same way; the only difference is in the means of getting at it in each case.

Referring now to properties mentioned in the previous chapters, we have first to heat the piece to the right temperature and then to cool it in the quickest possible way in order to secure the greatest hardness and the best grain. In doing this we subject the steel to the greatest shocks or strains, and great care must be used.

The importance of uniformity in heating for forging and for annealing has been stated, and it has been shown how an error in this may be rectified by another and a more careful heating; when it comes to hardening, this uniformity must be insisted upon and emphasized, for as a rule an error here has no remedy.

There may be cases of bad work that do not cause actual fracture that can be remedied by re-heating and hardening, but these are rare, because even if incurable fracture does not occur the error is not discovered until the piece has been put to work and its failure develops the errors of the temperer.

If the error is one of merely too low heat, not producing thorough hardening, it will generally be discovered by the operator, who will then try again and possibly succeed; but if the error be of uneven heat, or too much heat, the probabilities are that it will not be discovered until the piece fails in work, when it will be too late to apply any remedy.

Referring to [Table I, Chap. V], treating of specific gravities, it is clear that all steel possesses different specific gravities, due to differences of temperature, and that these differences of specific gravity increase as the carbon content increases; it follows that if a piece of steel be heated unevenly, internal strains must be set up in the mass, and it is certain that if steel be quenched in this condition violent strains will be set up, even to the causing of fractures.

The theory of this action, as of all hardening, is involved in discussion which will be considered later; in this chapter the facts will be dealt with. When a piece of steel is heated, no matter how unevenly or to what temperature below actual granulation, and is allowed to cool slowly and without disturbance, it will not break or crack under the operation. If a piece be heated as unevenly as, say, medium orange in one part and medium lemon in another, and is then quenched, it will be almost certain to crack if it contains enough carbon to harden at all in the common acceptance of the term, that is to say, file hard or having carbon 40 or higher.

This fact is too well known to be open to discussion; therefore the quenching of hot steel, the operation of hardening, does set up violent strains in steel, no matter what the true theory of hardening may be.

Referring to [Chap. V], to the series of squares representing the apparent sizes of grain due to different temperatures, similar results follow from hardening, with the exceptions that the different structures are far more plainly marked, and the squares should be arranged a little differently; they are shown as continuously larger in [Chap. V], from the grain of the cold bar up to the highest temperature; this is true if a bar has been rolled or hammered properly into a fine condition of grain. Of course if a bar be finished at, say, medium orange it will have a grain due to that heat—No. 3 in the series of squares. Then if it be heated to dark orange and cooled from that heat it will take on a grain corresponding to square No. 2, and No. 1 square will be eliminated.

The series of squares to represent hardened grain will be as follows:

The heat colors being the same as before, viz.:

1. The natural bar—untreated.
2. Quenched at dark orange or orange red.
3.““medium orange—refined.
4.““bright orange.
5.““dark lemon.
6.““medium lemon.
7.““bright lemon.
8.““very bright lemon or creamy.
Heats 6, 7, 8 will almost invariably produce
cracks although the pieces be evenly heated.

These squares do not represent absolute structures with marked divisions; they are only the steps on an incline, like the temper numbers in the carbon series; thus, the carbon-line is continuous, but the temper divisions represent steps up the incline. So with the series of squares, the changes of grain or structure are continuous, as represented by the doubly inclined line; the squares being only the steps to indicate easily observed divisions. The minuteness of the changes is illustrated by the fact that in a piece heated continuously from creamy to dark orange and quenched, differences of grain have been observed unmistakably on opposite sides of pieces broken off not more than ⅛ inch thick.

In practice the differences due to the colors given in the list above are as plain and surely marked as are the differences in the structure of ingots due to the different temper carbons already described.

In this hardened series each carbon temper gives its own peculiar grain; in low steel, say 40 carbon compared to 1.00 carbon or higher, No. 3 will be larger and No. 8 will be smaller in the low temper than in the high—another illustration of the fact that low steel is more inert to the action of heat than high steel. All grades and all tempers go through the same changes, but they are more marked in the high than in the low steel.

The grain of hardened steel is affected by the presence of silicon, phosphorus, and manganese, and doubtless by any other ingredients, these three being the most common.

It is in the grain of hardened steel that the conditions described in [Chap. V] as “sappy,” “dry,” and “fiery” are the most easily and frequently observed, although the same conditions obtain in unhardened steel in a manner that is useful to an observing steel-user. But it is in this hardened condition that the excellences or defects of steel are brought out and emphasized.

When a piece of steel is heated continuously from “creamy,” or scintillating, down to black, or unheated, and is then quenched, the grain will be found to be coarsest, hardest, and most brittle at the hottest end, and with the brightest lustre, even to brilliancy, and to become finer down to a certain point, noted as No. 3 in the series of squares, or at a heat which shows about a medium orange color; here the grain becomes exceedingly fine, and here the steel is found to be the strongest and to be without lustre. Below this heat the grain appears coarser and the steel is less hard, until the grain and condition of the unheated part are reached. This fine condition, known as the refined condition, is very remarkable. It is the condition to be aimed at in all hardening operations, with one or two exceptions which will be noted, because in this state steel is at its best; it is strongest then, and it would seem to be clear without argument that the finest grain and the strongest will hold the best at a fine cutting-edge, and will do the most work with the least wear, although a coarser grain may be a little harder, the coarser and more brittle condition of the latter more than counterbalancing its superior hardness.

The advantages of this refined condition are so great that it is found to be well to harden and refine mild-steel dies, and battering- and cutting-tools that are to be used for hot work, although the heat will draw out all of the temper in the first few minutes, because the superior strength of the fine grain will enable the tool to do twice to twenty times more work than an unhardened tool.

The refining-heat, like most other properties, varies with the carbon; the medium orange given is the proper heat for normal tool-steel of from about 90 to 110 carbon. Steel of 150 carbon will refine at about a dark orange, and steel of 50 to 60 carbon will require about a bright orange to refine it.

This range is small, but it must be observed and worked to if the best results are desired.

A color-blind person can never learn to harden steel properly.

In studying this phenomenon of refining, the conclusion was reached that it occurred at or immediately above the temperature that broke up the crystalline condition of cold steel and brought it fairly into the second, the plastic condition. Farther observation led to the conclusion that the coarser grain and greater hardness caused by higher heats were due to the gradual change from plastic toward granular condition that takes place as the heat increases. Later investigations have given no reason for changing these conclusions.

When the phenomenon of recalescence was observed and investigated by Osmond and others, different theories were advanced in explanation.

Langley concluded that if recalescence occurred at the change from a plastic to a crystalline condition, then the heat absorbed and again set free during such changes would account for the visible phenomenon of recalescence.

Again, if it should prove that recalescence occurred at the refining point, the conjunction of these phenomena would indicate strongly, first, that refining does occur at the point where this change of structure is complete in the reverse order, from crystalline to plastic; and second, the first being true, recalescence would be explained as stated, as indicating the inevitable absorption and emission of heat due to such a change.

Langley fitted up an electric apparatus for heating steel, in a box so placed that the light was practically uniform, that is, so that bright sunlight, or a cloudy sky, or passing clouds would not affect seriously the observation of heat-colors.

Pieces of steel were heated far above recalescence, up to bright lemon, and then allowed to cool slowly; in this way recalescence was shown clearly.

It was found to occur at the refining heat in every case, shifting for different carbons just as the refining heat shifts.

Immediately under the pieces being observed was a vessel of water into which the pieces could be dropped and quenched. After observing the heating and cooling until the eye was well trained, pieces were quenched at different heats and the results were noted. It was found that in the ascending heats no great hardness was produced until the recalescence heat was reached or passed slightly; and in the descending heat excessive hardening occurred at a little below the recalescent heat, although no such hardening occurred at that color during ascending heats. This apparent anomaly is due simply to lag. If, in ascending, the piece be held for a few moments at the recalescent point, no increase being allowed, and then it be quenched, it will harden thoroughly and be refined. If, in descending, the cooling be arrested at a little below the recalescence for a few moments, neither increase nor decrease being allowed, and then the piece be quenched, it will not harden any better than if it be quenched immediately upon reaching the same heat in ascending.

Time must be allowed for the changes to take place, and lag must be provided for.

These experiments show that refining and recalescence take place at the same temperature.