Steel

BURNING, OVERHEATING, RESTORING.

From what has been said already about the effects of heat it follows without further argument that heating is one of the most important, or perhaps more properly the most important of all, of the operations to which steel has to be subjected.

The first and vital thing to be borne in mind is that all heating should be uniform throughout the mass. It has been shown that heat affects the grain, the structure, as surely as it moves the mercury-column, and such being the case it is plain that as perfect uniformity as it is possible to attain is the first essential for all heating, no matter what the ultimate object may be.

In heating for forging the limit lies between the point of recalescence, the beginning of true plasticity, and the granular condition, the end of plasticity; these temperatures lie between dark or medium orange for all steels and medium or light lemon on the upper limit, depending on the carbon content, or lower if it be an alloy steel.

If there is much work to be done upon a piece of steel, it is well to heat at first to as high a temperature as is safe, and then to forge or work heavily at the higher heat, reducing the blows or passes as the piece is reduced and the temperature falls. Although this high heating will raise the grain of the steel, the heavy working will bring it back to a fine, compact structure.

If little work is to be done, then it is better to heat as low as may be safe, and allow the work to be done without letting the heat down below orange red, so that the steel may not be crushed in the grain.

Below orange red, the so called “dark cherry,” steel should not be forged, except that in forging for fine tools it is well to give many light and rapid blows until black begins to show in order to hammer-refine it; this must be done with extreme care so as not to crush the steel and cause cracking in the subsequent hardening, or crumbling in the hardened tool.

HEATING FOR HARDENING.

When a piece of steel is to be hardened by quenching in water or any quick-cooling medium, it should be heated with great care to the exact temperature to produce the required hardness.

After forging, no piece of steel should be quenched without first being heated uniformly to the proper temperature. Ede in his book recommends quenching immediately after forging in some cases. The so-called Harvey patent recommends cooling from a high heat down to the required heat and then quenching.

Both practices are bad. In the Ede case this is believed to be the only bad piece of advice in his very valuable book—in every other respect the most practical and useful book upon the manipulation of steel known to the author.

The reason for objecting to the quenching after forging without re-heating is that forging always sets up uneven strains in the mass; the flow is easier from the sides than from the middle of the piece, and therefore the amount of work done upon one part is greater than upon another; also it is impossible to hammer or press a piece of steel with exact uniformity throughout, so that it follows that after forging there is never exact uniformity of texture or temperature, and such uniformity is the one essential thing to insure good and even hardening.

The practice of allowing a highly heated piece to cool down to a given color and then quenching is objectionable, because it produces a coarse and brittle grain due to the higher heat.

Referring to the illustration on page 67 of the squares representing grains due to different temperatures: Assume that square No. 3 represents the heat at which quenching is to take place, and No. 6 is the heat to which the piece has been subjected; then the piece when it has cooled to No. 3 will not have the grain due to No. 3 heat: it will have a larger, coarser grain that formed as the piece cooled from No. 6. If now it be quenched, it will have only the hardness due to No. 3, with a much coarser and more brittle grain than No. 3 heat should give. The way to manage such a case is to let the piece cool completely and assume the No. 6 grain; then re-heat carefully to exactly No. 3 and no hotter; keep the piece at that heat for a few minutes, or moments, according to its size, to allow for lag; then it will have the finer grain due to No. 3 heat, and when quenched it will be as hard as under the other method, and it will be much finer and stronger.

The same rule applies to any two temperatures.

As an expression of exactness as to evenness of heat, it may be said that the piece should be as uniform in color as if it had been dipped into a pot of paint. When such uniformity is attained, a break from quenching is rare, unless the piece has been shamefully overheated so that the strains of quenching are greater than the tenacity of the steel.

HEATING FOR WELDING.

When an ingot is to be forged or rolled, it is well to take the highest heat possible—that immediately below the heat of granulation. Such a heat may be taken safely by keeping the steel covered with a surface flux to protect it from the flame. Ordinary red clay, dried and powdered, is an excellent flux for the purpose, and the cheapest known. Melted and powdered borax is the best of known fluxes, but it is so expensive that, as a rule, it is used only on the finest tool-steel, or on some of the alloy steels where the highest heat possible is not above a bright orange color, or hardly so high.

A good flux, intermediate in cost between common red clay and powdered borax, is an earth or mineral barite, or heavy spar. This material fuses more readily than red clay and not quite so easily as borax. It forms a good protective covering on the steel, and it is nearly or quite as efficient as borax.

The object in heating so high is to make the steel as soft and plastic as it may be, so that the subsequent working will close up all porosity as far as possible. Nearly all ingots have in them a greater or less number of cavities, commonly called blow-holes, that are caused by the separation of occluded gases during cooling. If such porosities are not oxidized on the surface they will disappear under heavy working at a high heat. It is probable that under the compression of the work the gases are redisseminated in the mass and the walls of the cavities are reunited. If there be the slightest oxidation of the surface of a cavity the walls will not reunite: there will be left in the mass a little flat film of oxide which will prevent the union.

In mild steels used for machinery or structural purposes these little films may do no harm, the factor of safety being sufficient to more than cover any weakening effect. In tool-steel that is to be hardened such little films are almost certain to cause fracture. Dies as large as twelve inches square and six to eight inches thick, having been heated and quenched with the greatest care, have split fairly in two, and have revealed in the fracture a little film no larger than half an inch in diameter and of inappreciable thickness. At the same time the perfectly uniform grain and hardness showed that the highest skill had been used. This is only one illustration of the fact that every break in the continuity of the grain in steel forms a starting-point for fracture under heavy stress.

From what has been said it is plain that to weld two pieces of steel together is a difficult matter; still it can be done if great care be used. In general it is better to avoid such welding except in cases of necessity. The welding of steel tubing, and the electric welding of rails, frogs, switches, etc., is done on a large scale and satisfactorily, so that it will not do to say that steel cannot be welded. It can be welded or pasted together, and it is a good operation to avoid in all high steel. In case steel is to be hardened a weld will reveal itself almost certainly.

BURNING IN HEATING.

When a piece of steel breaks and shows a coarse, fiery fracture, it is common to say that it is burned. This is not necessarily the case. There are several degrees in the effects of heat. The first is the raising of the grain; the second, in high steel, is the decarbonizing or burning out of carbon from the surface in, the depth of the decarbonizing depending upon time and temperature; the third is oxidizing, or actual burning in the common acceptance of the term.

All of these operations go on to a slight extent every time a piece of steel is heated, but when the heating is done carefully there is only a small film of steel that is decarbonized and oxidized, and this film flies off when the piece is quenched for hardening. When the steel is forged or rolled this skin will be united firmly to the steel, and it will be thinner or thicker, according to the number of heatings and the time of exposure to the fire. In tool-making this skin must always be removed. Many an expensive tool is made perfectly worthless by not having this skin all removed, owing usually to mistaken economy. The steel is expensive, and the tool-maker does not wish to cut it up into worthless chips.

When a tool costing, say, twenty-five dollars is made useless by failure to cut away twenty-five cents’ worth of useless skin, the economy of such an operation requires no discussion. It is impossible to forge a piece of steel without producing such a skin, and it is well known that decarbonized iron will not harden.

Ordinarily a cut of ¹/₁₆ of an inch should remove such a skin on straight rolled or hammered bars. In the case of a shaped forging where many re-heatings have been required the forgeman will have done good work if the cutting away of ⅛ of an inch will present a good surface: tool-makers should consider this and allow for it. On the other hand, if a tool-maker finds that the removal of ⅛ of an inch from a bar, or ¼ of an inch from a forging will not yield him a good, hard surface, he should hold the steel-maker responsible for bad work.

Actual burning reveals itself in rough tears, and cracks at the surface and corners of the piece. Such a piece should go to the scrap heap.

Overheated steel of coarse, fiery grain has been injured, and not necessarily destroyed. Such a piece may be restored to any fineness of grain by heating to the right temperature—medium orange for the best grain—keeping it at that heat for, say, one minute for a little piece, and five to ten or fifteen minutes for a large piece. The heat should penetrate the whole mass, and it should not be allowed to run above the given color in any part, not even for a moment. It should then be allowed to cool in a dry place, without disturbance. The grain will now be fine and uniform, and the steel may be worked in the ordinary way.

This simple operation is all that is necessary to restore to a fine grain any piece of steel that has been overheated, provided that the piece has not been actually burned nor ruptured.