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chisel steel
Значение термина chisel steel в knolik
chisel steel - chisel steel
chisel steel - The following is extracted from a paper read before the National Eailroad Blacksmiths' Association of America.
The operation of making and dressing chisels appears so easy and simple that it is scarcely given any attention, and many are of the opinion that a cheap grade of steel is perfectly suitable for the purpose. There are various reasons why good steel should be used for this purpose, viz. they must hold a good cutting edge, they undergo many redressings, they are subjected to impact, and they are often used by inexperienced workmen. Cheap steel, when reheated and retempered many times, deteriorates very rapidly when compared with good steel. This cheap steel has a very open and loose structure; also, it contains a greater percentage of the impurities; these impurities envelop the grains and prevent the necessary cohesion from taking place between the grains. These vibrations which take place when a chisel is struck a blow from the hammer are very conducive to fatigue in poor steel, and in this condition a chisel will break in a very short time. They do not exert such a marked influence on good steel, and, furthermore, this rule will also apply to iron in the same respect.
In making chisels care should be taken to clip off the corners; if not, they will draw over and overlap the interior metal, which will produce a split point. I believe in edging-up, or, in other words, upsetting edgewise, when the point of the chisel is very thin, and it being at a dull-red heat is the cause of more chisels breaking than any other treatment it receives, unless it be overheating for hardening. The smith should aim to do most of his edging-up before the chisel is drawn down too thin; if it should spread a little wider than the width of the steel, it would be better to leave it in this shape than to edge it up when it is very thin.
In order to obtain the best results a good hammering to pack the steel is very essential, but it should be properly done. The chisel should be evenly heated, and the process of packing should commence at the thicker part of the chisel first, gradually increasing the amount of hammering on reaching the point, and aiming to give an equal amount of it on each side. At times we have a difficulty with chisel points snapping off; there are good reasons for these failures. First, when a chisel is unevenly heated, and quenched in this condition, it is left in a state of unequal tension; then we find areas with different degrees of hardness; also the transition from the hard areas to the soft being so abrupt, the chisel is left in a state of great weakness.
Secondly, the point of the chisel is heated to the proper temperature, but just back of this (say about 5/8- in.) the colour is scarcely visible; it is quenched in this condition. This chisel will break at the junction between the hardened and the unhardened parts. The smith will then test the fracture with his file; finding it very soft, he wonders why it broke.
When steel is quenched between what is called the neutral and hardening zone, or, in other words, just before it arrives at the true hardening heat, it is in its weakest condition, and this accounts for the point jumping off. This can be remedied by hardening the chisel farther up where it is thicker and stronger, and then drawing the temper accordingly.
A practice which should not be tolerated is when the chisel point is heating too fast and it is checked by dipping it into the water (just for an instant) and then placed in the fire again.
In dressing chisels many are returned having considerable temper remaining; generally they are thrust into the centre of the fire; here the change is so sudden that the tenacity of the steel is impaired, and at times will cause surface cracks.
The use of sulphurous coal is also quite a factor in causing unsatisfactory results.
It seems also unnecessary to mention that quick heating and overheating in any part of the treatment is the cause of many failures.
We will make a number of the chisels from the same bar of steel, and will declare they all received precisely the same treatment; but will the final results support us in this claim ? I believe not, for there must have been a variation in the treatment somewhere for the machinist (who is a careful man) will state that some broke very easily, while others were exceptionally good; until we can explain why this is so, we should refrain from upbraiding our friend when he returns with these chisels broken. Many efforts have been made for the purpose of taking a short cut on this undesirable task of chisel dressing, but the old method still prevails to a great extent.
Some have recommended the use of lead, others cyanide of potash, which is heated in a ladle or pot to the proper temperature, and the chisel points are placed in this till they attain the desired heat, but if these mediums are not kept at the proper temperature the results will be very unsatisfactory. Some advise heating the points in the fire and then quenching in oil or a mixture of tallow, prussiate of potash, and resin; then, again, tin that is just brought to the melting-point is used. It is claimed that when quenched in any of these mediums the temper need not be drawn; it will be ready for use. For my part, I do not believe that they are worth while considering.
Some practise drawing the temper very slowly in oil or sand which is heated to the proper temperature to give the required hardness to the chisel.
It would be well to give a little attention to the water emery-wheels in use at so many places at the present time; they are usually too fine for the purpose intended, which causes them to glaze very quickly when used on hardened steel. They are the cause of many surface cracks which we see on the cutting edge of the tools, especially those made from alloyed steel. The tools are thrust against this glazed surface of the emery-wheel with considerable pressure; the wheel will not cut, but glide over the surface of the tool; this friction generates heat so quickly that it exceeds the conduction power of the steel. Consequently only a thin shell of the steel is heated; expansion must take place but the internal condition of the steel being cold and unyielding, this thin shell relieves itself by cracking. Then again, tools ground on wheels in this condition will become soft as well as glazed, and thus will require hardening again; but this glazed film prevents hardening from taking place, and then we will blame the steel for being deficient in carbon. These emery-wheels will often take the temper out of the extreme cutting edge, which will not penetrate more than 0.001 of an inch beyond the surface; but it is enough, for the tool gives down very quickly, and on such tools as mills it is liable to break out the teeth.
In some places when tools are to be annealed they are placed in a furnace that is heated to a very high temperature. This is a bad practice, and should not be continued on such tools as mill, hobs, reamers, etc., for the small teeth are heated so quickly that it will cause a strain at the base of the teeth. Then if the old teeth are not entirely cut away (which is often the case), it will be disposed to crash at these strains when tempered. It is well known that steel on being hardened will change from its original size when cold. Generally an expansion will take place; but it is not unusual to have a piece that will show a slight shrinkage.
At times we will notice pieces of steel which conform exactly to the same size and shape, made in the same manner, and from the same bar of steel, when hardened, they will show a slight difference in the expansion, and perhaps a piece or two will show a slight contraction. We feel confident that we heated these pieces the same temperature; but the eye is very easily deceived and every little increment or decrement in temperature of water will have an influence in producing different results.
These variations, being very small, would not count on many pieces to be hardened for ordinary work, but on such tools as master-taps and dies it would probably render them worthless.
Some blame the steel for these variations, but I believe it is due partly to our method of hardening. The following I have copied from a little book, which will partly explain the difficulty: "In pieces of steel above a certain size the hardness does not extend right through to the centre. The surface, when it is suddenly cooled, contracts to a certain extent and exerts a considerable com-pressive force on the metal in the interior, which, as it slowly cools, is forced to occupy a smaller volume than it did originally; whilst the hardened portion, which is in a state of tension, owing to it having been cooled suddenly, occupies a greater. If, then, the contraction of the interior be greater than the expansion of the exterior, the piece of steel, as a whole, will be smaller after hardening than it was before, and vice versa. The whole question turns on the relation of the volume of the hardened portion to that which has been only partially hardened."
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