CHAPTER X
HIGH-SPEED STEEL
For centuries the secret art of making tool steel was handed down from father to son. The manufacture of tool steel is still an art which, by the aid of science, has lost much of its secrecy; yet tool steel is today made by practical men skilled as melters, hammer-men, and rollers, each knowing his art. These practical men willingly accept guidance from the chemist and metallurgists.
A knowledge of conditions existing today in the manufacture of high-speed steel is essential to steel treaters. It is well for the manufacturer to have steel treaters understand some of his troubles and difficulties, so that they will better comprehend the necessity of certain trade customs and practices, and, realizing the manufacturer"s desire to cooperate with them, will reciprocate.
The manufacturer of high-speed steel knows and appreciates the troubles and difficulties that may sometimes arise in the heat-treating of his product. His aim is to make a uniform steel that will best meet the requirements of the average machine shop on general work, and at the same time allow the widest variation in heat treatment to give desired results.
High speed steel is one of the most complex alloys known. A representative steel contains approximately 24 per cent of alloying metals, namely, tungsten, chromium, vanadium, silicon, manganese, and in addition there is often found cobalt, molybdenum, uranium, nickel, tin, copper and a.r.s.enic.
STANDARD a.n.a.lYSIS
The selection of a standard a.n.a.lysis by the manufacturer is the result of a series of compromises between various properties imparted to the steel by the addition of different elements and there is a wide range of chemical a.n.a.lyses of various brands. The steel, to be within the range of generally accepted a.n.a.lysis, should contain over 16 per cent and under 20 per cent tungsten; if of lower tungsten content it should carry proportionately more chromium and vanadium.
The combined action of tungsten and chromium in steel gives to it the remarkable property of maintaining its cutting edge at relatively high temperature. This property is commonly spoken of as "red-hardness."
The percentages of tungsten and chromium present should bear a definite relationship to each other. Chromium imparts to steel a hardening property similar to that given by carbon, although to a less degree. The hardness imparted to steel by chromium is accompanied by brittleness. The chromium content should be between 3.5 and 5 per cent.
Vanadium was first introduced in high-speed steel as a "scavenger,"
thereby producing a more h.o.m.ogeneous product, of greater density and physical strength. It soon became evident that vanadium used in larger quant.i.ties than necessary as a scavenger imparted to the steel a much greater cutting efficiency. Recently, no less an authority than Prof. J. O. Arnold, of the University of Sheffield, England, stated that "high-speed steels containing vanadium have a mean efficiency of 108.9, as against a mean efficiency of 61.9 obtained from those without vanadium content." A wide range of vanadium content in steel, from 0.5 to 1.5 per cent, is permissible.
An ideal a.n.a.lysis for high-speed steel containing 18 per cent tungsten is a chromium content of approximately 3.85 per cent; vanadium, 0.85 to 1.10 per cent, and carbon, between 0.62 and 0.77 per cent.
DETRIMENTAL ELEMENTS.--Sulphur and phosphorus are two elements known to be detrimental to all steels. Sulphur causes "red-shortness"
and phosphorus causes "cold-shortness." The detrimental effects of these two elements counteract each other to some extent but the content should be not over 0.02 sulphur and 0.025 phosphorus.
The serious detrimental effect of small quant.i.ties of sulphur and phosphorus is due to their not being uniformly distributed, owing to their tendency to segregate.
The manganese and silicon contents are relatively unimportant in the percentages usually found in high-speed steel.
The detrimental effects of tin, copper and a.r.s.enic are not generally realized by the trade. Small quant.i.ties of these impurities are exceedingly harmful. These elements are very seldom determined in customers" chemical laboratories and it is somewhat difficult for public chemists to a.n.a.lyze for them.
In justice to the manufacturer, attention should be called to the variations in chemical a.n.a.lyses among the best of laboratories.
Generally speaking, a steel works" laboratory will obtain results more nearly true and accurate than is possible with a customer"s laboratory, or by a public chemist. This can reasonably be expected, for the steel works" chemist is a specialist, a.n.a.lyzing the same material for the same elements day in and day out.
The importance of the chemical laboratory to a tool-steel plant cannot be over-estimated. Every heat of steel is a.n.a.lyzed for each element, and check a.n.a.lyses obtained; also, every substance used in the mix is a.n.a.lyzed for all impurities. The importance of using pure base materials is known to all manufacturers despite chemical evidence that certain detrimental elements are removed in the process of manufacture.
The manufacture of high-speed steel represents the highest art in the making of steel by tool-steel practice. Some may say, on account of our increased knowledge of chemistry and metallurgy, that the making of such steel has ceased to be an art, but has become a science. It is, in fact an art; aided by science. The human element in its manufacture is a decided factor, as will be brought in the following remarks:
The heat treatment of steel in its broad aspect may be said to commence with the melting furnace and end with the hardening and tempering of the finished product. High-speed steel is melted by two general types of furnace, known as crucible and electric. Steel treaters, however, are more vitally interested in the changes that take place in the steel during the various processes of manufacture rather than a detailed description of those processes, which are more or less familiar to all.
In order that good high-speed steel may be furnished in finished bars, it must be of correct chemical a.n.a.lysis, properly melted and cast into solid ingots, free from blow-holes and surface defects.
Sudden changes of temperature are to be guarded against at every stage of its manufacture and subsequent treatment. The ingots are relatively weak, and the tendency to crack due to cooling strains is great. For this reason the hot ingots are not allowed to cool quickly, but are placed in furnaces which are of about the same temperature and are allowed to cool gradually before being placed in stock. Good steel can be made only from good ingots.
Steel treaters should be more vitally interested in the important changes which take place in high-speed steel during the hammering operations than that of any other working the steel receives in the course of its manufacture.
QUALITY AND STRUCTURE
The quality of high-speed steel is dependent to a very great extent upon its structure. The making of the structure begins under the hammer, and the beneficial effects produced in this stage persist through the subsequent operations, provided they are properly carried out. The ma.s.sive carbides and tungstides present in the ingot are broken down and uniformly distributed throughout the billet.
To accomplish this the reduction in area must be sufficient and the hammer blows should be heavy, so as to carry the compression into the center of the billet; otherwise, undesirable characteristics such as coa.r.s.e structure and carbide envelopes will exist and cause the steel treater much trouble. Surface defects invisible in the ingot may be opened up under the hammering operation, in which event they are chipped from the hot billet.
Ingots are first hammered into billets. These billets are carefully inspected and all surface defects ground or chipped. The hammered billets are again slowly heated and receive a second hammering, known as "cogging." The billet resulting therefrom is known as a "cogged" billet and is of the proper size for the rolling mill or for the finishing hammer.
Although it is not considered good mill practice, some manufacturers who have a large rolling mill perform the very important cogging operation in the rolling mill instead of under the hammer. Cogging in a rolling mill does not break up and distribute the carbides and tungstides as efficiently as cogging under the hammer; another objection to cogging in the rolling mill is that there is no opportunity to chip surface defects developed as they can be under the trained eye of a hammer-man, thereby eliminating such defects in the finished billet.
The rolling of high-speed steel is an art known to very few. The various factors governing the proper rolling are so numerous that it is necessary for each individual rolling mill to work out a practice that gives the best results upon the particular a.n.a.lysis of steel it makes. Important elements entering into the rolling are the heating and finishing temperatures, draft, and speed of the mill. In all of these the element of time must be considered.
High-speed steel should be delivered from the rolling mill to the annealing department free from scale, for scale promotes the formation of a decarbonized surface. In preparation of bars for annealing, they are packed in tubes with a mixture of charcoal, lime, and other material. The tubes are sealed and placed in the annealing furnace and the temperature is gradually raised to about 1,650F., and held there for a sufficient length of time, depending upon the size of the bars. After very slow cooling the bars are removed from the tubes. They should then show a Brinnell number of between 235 and 275.
The inspection department ranks with the chemical and metallurgical departments in safeguarding the quality of the product. It inspects all finished material from the standpoint of surface defects, hardness, size and fracture. It rejects such steel as is judged not to meet the manufacturer"s standard. The inspection and metallurgical departments work hand in hand, and if any department is not functioning properly it will soon become evident to the inspectors, enabling the management to remedy the trouble.
The successful manufacture of high-speed steel can only be obtained by those companies who have become specialists. The art and skill necessary in the successful working of such steel can be attained only by a man of natural ability in his chosen trade, and trained under the supervision of experts. To become an expert operator in any department of its manufacture, it is necessary that the operator work almost exclusively in the production of such steel.
As to the heat treatment, it is customary for the manufacturer to recommend to the user a procedure that will give to his steel a high degree of cutting efficiency. The recommendations of the manufacturer should be conservative, embracing fairly wide limits, as the tendency of the user is to adhere very closely to the manufacturer"s recommendations. Unless one of the manufacturer"s expert service men has made a detailed study of the customer"s problem, the manufacturer is not justified in laying down set rules, for if the customer does a little experimenting he can probably modify the practice so as to produce results that are particularly well adapted to his line of work.
The purpose of heat-treating is to produce a tool that will cut so as to give maximum productive efficiency. This cutting efficiency depends upon the thermal stability of the complex hardenites existing in the hardened and tempered steel. The writer finds it extremely difficult to convey the meaning of the word "hardenite" to those that do not have a clear conception of the term. The complex hardenites in high-speed steel may be described as that form of solid solution which gives to it its cutting efficiency. The complex hardenites are produced by heating the steel to a very high temperature, near the melting point, which throws into solution carbides and tungstides, provided they have been properly broken up in the hammering process and uniformly distributed throughout the steel. By quenching the steel at correct temperature this solid solution is retained at atmospheric temperature.
It is not the intention to make any definite recommendations as to heat-treating of high-speed steel by the users. It is recognized that such steel can be heat-treated to give satisfactory results by different methods. It is, however, believed that the American practice of hardening and tempering is becoming more uniform. This is due largely to the exchange of opinions in meetings and elsewhere.
The trend of American practice for hardening is toward the following:
_First_, slowly and carefully preheat the tool to a temperature of approximately 1,500F., taking care to prevent the formation of excessive scale.
_Second_, transfer to a furnace, the temperature of which is approximately 2,250 to 2,400F., and allow to remain in the furnace until the tool is heated uniformly to the above temperature.
_Third_, cool rapidly _in oil_, dry air blast, or lead bath.
_Fourth_, draw back to a temperature to meet the physical requirements of the tool, and allow to cool in air.
It was not very long ago that the desirability of drawing hardened high-speed steel to a temperature of 1,100 was pointed out, and it is indeed encouraging to learn that comparatively few treaters have failed to make use of this fact. Many treaters at first contended that the steel would be soft after drawing to this temperature and it is only recently, since numerous actual tests have demonstrated its value, that the old prejudice has been eliminated.
High-speed steel should be delivered only in the annealed condition because annealing relieves the internal strains inevitable in the manufacture and puts it in vastly improved physical condition. The manufacturer"s inspection after annealing also discloses defects not visible in the unannealed state.
The only true test for a brand of high-speed steel is the service that it gives by continued performance month in and month out under actual shop conditions. The average buyer is not justified in conducting a test, but can well continue to purchase his requirements from a reputable manufacturer of a brand that is nationally known. The manufacturer is always willing to cooperate with the trade in the conducting of a test and is much interested in the information received from a well conducted test. A test, to be valuable, should be conducted in a manner as nearly approaching actual working conditions in the plant in which the test is made as is practical. In conducting a test a few reputable brands should be allowed to enter. All tools entered should be of exactly the same size and shape. There is much difference of opinion as to the best practical method of conducting a test, and the decision as to how the test should be conducted should be left to the customer, who should cooperate with the manufacturers in devising a test which would give the best basis for conclusions as to how the particular brands would perform under actual shop conditions.
The value of the file test depends upon the quality of the file and the intelligence and experience of the person using it. The file test is not reliable, but in the hands of an experienced operator, gives some valuable information. Almost every steel treater knows of numerous instances where a lathe tool which could be touched with a file has shown wonderful results as to cutting efficiency.
Modern tool-steel practice has changed from that of the past, not by the use of labor-saving machinery, but by the use of scientific devices which aid and guide the skilled craftsman in producing a steel of higher quality and greater uniformity. It is upon the intelligence, experience, and skill of the individual that quality of tool steel depends.
HARDENING HIGH-SPEED STEELS
We will now take up the matter of hardening high-speed steels. The most ordinary tools used are for lathes and planers. The forging should be done at carbon-steel heat. Rough-grind while still hot and preheat to about carbon-steel hardening heat, then heat quickly in high-speed furnace to white heat, and quench in oil. If a very hard substance is to be cut, the point of tool may be quenched in kerosene or water and when nearly black, finish cooling in oil.
Tempering must be done to suit the material to be cut. For cutting cast iron, bra.s.s castings, or hard steel, tempering should be done merely to take strains out of steel.
On ordinary machinery steel or nickel steel the temper can be drawn to a dark blue or up to 900F. If the tool is of a special form or character, the risk of melting or scaling the point cannot be taken. In these cases the tool should be packed, but if there is no packing equipment, a tool can be heated to as high heat as is safe without risk to cutting edges, and cyanide or prussiate of potash can be sprinkled over the face and then quenched in oil.