Non-technical chats on iron and steel, and their application to modern industry

by La Verne W. Spring

Chrome-Vanadium Steels

Within a comparatively short time the chrome-vanadium steels have come to be very largely used, often in place of the chrome and nickel-chrome steels. As vanadium is a “deoxidizer,” whereas nickel is not, the chrome-vanadium steels show fewer imperfections than the nickel-chrome steels and they also roll, forge and machine better.

They are used for automobile frames, shafts, for miscellaneous forged and rolled articles and for heat-treated armor plates. Of this comparatively new material about 90,000 tons were made during 1913, according to a recently issued bulletin of the Department of the Interior.

It is impossible, of course, to even begin to impart any adequate conception of the qualities and great importance of the alloy steels for purposes of construction. As has been shown they are special steels for special purposes and their application is wide. Incorporation of the new element in the alloy imparts peculiar and valuable properties: for example, 12 per cent of manganese, great hardness and toughness; 23 per cent of nickel, non-corrosive properties and great strength; chromium, nickel with chromium or chromium with vanadium, strength and high elastic limit (resistance to distortion) as well as great hardening power when desired, this, of course, the usual hardening through quenching from a cherry-red heat.

Very often instead of the single defining element, a combination of two, three or even four of them is used. Such, of course, are rather complicated steels having combinations of properties as might naturally be expected, though very often these resulting properties are not those which are expected. In fact no one can tell in advance what properties any new combination of metals in an alloy will produce and often new proportions of the same constituent metals give entirely different and unique results.

The only certain method of ascertaining what characteristics and properties a new alloy will have is to develop it and in that way find out.

Description of a special and extremely important class of these alloys, the “high-speed steels,” will serve to show how laborious, slow and expensive a process development of new alloys may be and what unlooked-for results are sometimes obtained.

CHAPTER XV
THE HIGH-SPEED STEELS

During early centuries the art of metal cutting made little progress except in-so-far as the application of greater driving power and the use of better machines were concerned. Lathes had been known since the sixth century B. C., at least, but, of course, little was or could be accomplished, comparatively speaking, before the invention of the steam engine by Watt, which was the first contrivance to give sufficient power for machining purposes. During the century which followed Huntsman’s time, steel makers and smiths became very expert in the manufacture and tempering of carbon steels. Lathe tools were made of these carbon steels, the only available material, but even with the better machine shop practice of the 19th century they were capable only of what we now consider to be inefficient results. The trouble was that since carbon tool steel gets its hardness from quenching from a red heat and then “drawing” the temper by reheating and slowly cooling from 400° or 500° F., tools made from it could not retain their hardness if their cutting points became much heated, as occurred if the lathe was run too fast. The usual cutting speed, therefore, was 20 or 30 linear feet per minute. Speeds in excess of this took the temper out of the tools and soon made them useless.

About 1868 Robert F. Mushet, a metallurgist of Sheffield, England, made a momentous discovery. He found that a piece of tool steel which had cooled in the air was as hard as some of those which he had quenched. Being an investigator he set about discovering the reason for this experience which was without precedent. Analysis showed that beside the usual constituents of tool steel this particular bar contained tungsten, a comparatively new metal. He experimented with some hundreds of mixtures and evolved an alloy, which, in tools, would stand up under machine speeds double those which could be used with carbon steels. These new alloys became known as “air-hardening” or self-hardening tool steels because they required no quenching.

The principal application of these new steels was in the cutting of harder metal than it had before been possible to cut, and little attention, apparently, was paid to the getting of greater outputs by increase of machine speeds.

At this point Frederick W. Taylor of “efficiency” fame appears upon the scene. While manager of the Bethlehem Steel Works in the nineties of the last century, he was working upon the efficiency investigations for which he later became so famous. During his investigations, with Maunsel White he experimented with many air-hardening steels to determine the best grades to use for their shop work. Getting some inconsistent results they determined upon and made what was a most extended and systematic investigation, one so thorough and complete that Taylor and White have become part of the history of high-speed steels.

They produced new compositions the quenching of which could be from temperatures greatly in excess of those which tool makers for centuries had held to be ruinous and which really are ruinous to carbon tool steels. The best results were obtained when the new steels were quenched by plunging in oil from close to their melting points—a dripping or “sweating” heat as it is called. This was something entirely new but it developed that after proper drawing, steels quenched from these extreme temperatures, would stand up under lathe speeds as great as 200 or 300 feet per minute.

Compare these with the miserable speeds of 20 or 30 feet per minute, which were the average performances of the best carbon steels.

The secret of Taylor and White’s treatment was not long in coming out and soon high-speed steel makers in Europe and America were vying with each other in production of finer and finer high-speed steels.

The progress which has been made during the last twenty years and particularly during the past ten has been astounding. Improvement has followed improvement in composition, manufacture and heat treatment, so that to-day, instead of the cut at 30 feet per minute, which was a high figure with carbon steel tools, the modern lathe or shaper tool often works at 300 or 400 or more feet per minute, and, with sufficient power behind it, at somewhat lower speeds plows out ½ inch deep and ¾ inch wide chips so fast that their removal to keep the machine clear is no mean problem. Often 2,000 pounds of the material per hour can be thus cut away with one tool.

As suggested, the new steels do not suffer such loss of temper from the heat generated by the friction of the tool in the metal as occurs with carbon steel tools. In fact, tools of high-speed steel work best after “warming up” and they can run for a considerable period of time with the point of the tool red-hot, though such is not advisable.

As is readily seen the essential property of the high-speed steels is the so-called “red hardness” which is the ability to retain hardness at red heat. This is several hundred degrees in excess of the temperatures at which the carbon tool steels quickly lose their “temper.”

As forecast by Mushet, the essential constituent of the new steels is the metal, tungsten. But tungsten alone cannot give the desired property. Mushet, it will be remembered, was the metallurgist whose patents for the use of manganese in steel Bessemer was obliged to recognize to make his process a success, though the metal had earlier been used in crucible steel. The air-hardening property of Mushet’s steel was contributed by a happenstance combination of tungsten and this same metal, manganese. It later developed that tungsten and chromium were the best hardening elements and these have maintained their place, though refinements of the past few years have made use of vanadium, and, more recently, cobalt in addition. Usual amounts may be said to be tungsten 14 to 25 per cent, chromium 2 to 7 per cent, with vanadium ½ to 1½ per cent, and cobalt up to 4 per cent, perhaps. The carbon content is usually .6 to .8 per cent. Sometimes another comparatively rare metal, molybdenum, is used in high-speed steels in place of part of the tungsten, but its use does not seem to be on the increase.

Manufacturers differ considerably in formulas.

It will be noticed that at best there is left room for only 70 or 80 per cent of iron in the alloy. From certain standpoints, the high-speed steels might not at first thought be called “steels” at all since carbon seems to be of so little importance. They might be considered to be low carbon alloys somewhat similar to the newer “stellite” (an alloy from which tools are made), which contains little or no carbon and no iron but is made up mainly of cobalt and chromium. They fit in, however, with the general and very comprehensive scheme of classification of the iron-carbon alloys which has been developing over a period of twenty years and there is no doubt among metallurgists and metallographists that, as is the case with the alloy steels described above, they are iron-carbon alloys—in other words, steels—the properties of which have been greatly modified through the presence of the other elements. Carbon, therefore, is an essential, though it is much less in amount than in the carbon tool steels. The hardening and softening properties, also, very definitely classify these alloys with the “tool steels.” Stellite cannot be softened.

As with the carbon tool steels, most of the high-speed steels are made by the crucible method, though a small but increasing amount is of late being produced in the electric furnace. After careful pouring into small ingots and cooling, the ingots are removed from the iron molds and “topped” to remove any “pipe” or unsound portion. Then, if without defect and satisfactory as to analysis, they are slowly and carefully heated to forging temperature and are hammered out into bars. By this method they are taken nearly down to the final size desired. The bars are finished by rolling to size. After careful annealing they are ready for shipment to the tool maker.

When taking heavy cuts a tool of to-day may exert as much as ten tons’ pressure against the metal it is cutting and the advent of this wonderful material for tools necessitated the building of immensely heavier and stronger lathes and other machines, which, alone, were capable of giving them power to do their work. The high-speed steels, therefore, have revolutionized metal-cutting practice and shop methods and have very largely aided efficiency.

CHAPTER XVI
THE MECHANICAL TREATMENT OF STEEL

Molten steel is practically always poured into upright molds of cast iron which shape it into long slightly tapering blocks of metal of square or rectangular cross-section. After the ingot mold has been stripped off, the still red-hot ingot cannot well be taken directly to the rolls, for, while the exterior parts may have the proper temperature for rolling, the interior of the ingot may still be liquid. The ingot, throughout, should be uniform in temperature when it is rolled. It is therefore put into a closed pit or furnace of proper temperature where the center of the ingot can be cooling while the outer portions are kept hot or are reheated if necessary, until all is ready for the rolling operation.

It would take a “steel man” a long time to tell you all of the unfortunate things that can and do happen to such blocks or ingots of steel which influence their applicability to the purposes for which they are intended. You must have learned of the most serious of these—“pipes,” “cracks,” “segregation,” etc., through reports of investigations of broken railroad rails and accidents caused thereby. A word or two regarding these:

In the ingot mold the outside of the steel ingot is, of course, the first to solidify. It may be hours after the freezing of the outer crust before the interior is able to cool sufficiently that it, too, can set. As steel, like most other metals and alloys, occupies less space when “frozen” than it does when molten, there must occur a hollow space in the interior since the crust is solid and cannot contract much. This hollow space usually takes the form of a more or less elongated cavity extending along the axis of the upper quarter of the ingot. It is called a “pipe.”

Pipe and Blowholes in an Ingot of Steel

Then, too, the metalloids of the steel do not always stay where they belong. Even if the steel has been of a uniform chemical composition when poured, the interior portions of the ingot after cooling will be found to have a greater amount of sulphur, phosphorus and carbon than parts which are nearer the surface. Such gathering together of constituents of the steel is known as “segregation.”

With the development of the steel industry and the demand for greater and greater tonnages, ingots have been made larger and larger. Piping, segregation, etc., are very naturally accentuated in the large masses of steel.

Much “gray matter” has been expended in attempting to overcome these and other defects to which large steel ingots are liable. Covering the molten ingot top with charcoal; filling in before complete solidification with additional molten metal; and keeping the ingot top molten by application of powerful gas flames have been, perhaps, the most useful methods.

But, even so, piping and segregation have not been completely prevented, though great improvement has resulted.

The usual way around the difficulty is to make certain that only the bottom (or best half) of each ingot is used for the most important products, such as locomotive and car axles, firebox and boiler plates, rails, etc. The next or third quarter or a little more is utilized for products which go into less exacting service. These may be plates for ordinary water tanks, for flooring, for ship plates, etc. The top part which contains the pipe is cut off and goes back to the furnace to be remelted. It is termed “discard.”

The big steel makers themselves shape most of their steel into such finished products as rails, plates, rods, and wire. Some of it is by them reduced from the ingot into intermediate “blooms,” “billets,” “bars,” etc., and sold in this form for the manufacture of axles, drop forgings and the hundreds of products which we each day see.

It is a very fortunate circumstance that at a cherry-red or white heat the carbonless irons and most of the steels can be quite easily fashioned into products. As is well known to us the most usual methods of mechanically shaping these metals while hot are by hammering, by rolling and by forging in a press.

With sufficient power and proper appliances, soft and medium steel to a considerable extent can be fashioned cold, but, of course, in this condition its resistance to reshaping is immensely greater. The cold treatment of these metals is usually some form of tube or wire drawing.

Certain other methods such as extrusion, spinning, etc., are also in use, and, through them, some otherwise difficultly formed products are made.

In one of the earlier chapters we saw that annealing refines (make finer) the grain of a steel casting and improves its physical properties. Annealing for refining purposes is practiced with other steel products also, and with just as effective results.

However, the mechanical shaping of steel while at cherry-red or at a white heat much more materially refines the grain while helping the strength and greatly increasing the ductility of the alloy. Steel which has been hot-forged or rolled is said to have been “hot worked.” Steel usually is “hot worked,” for “cold-working” methods are not so generally applicable and the product is more liable to suffer under the more drastic treatment. The amount of “hot-work,” at proper temperatures, that low and medium carbon steels will stand with improvement of the grain and physical properties is considerable.

As we must anyway shape the metal into useful implements and other products, it is fortunate that the quality of the metal is benefited by the process.

No. 69a. Photomicrograph of Cold-Drawn Steel Wire Showing Distortion of the Crystals from Cold Working
Hot Working does not produce distortion
but makes the grain finer. Annealing relieves
this distortion to a great extent.
(Magnification 70 Diameters.)