US1698935A - High-speed alloy - Google Patents

Description

Patented Jan. 15, 1929.

UNITED STATES PATENT OFFICE.

PERCY C. CHESTERFIELD, OF DETROIT, MICHIGAN, ASSIGNOR TO CHESTERFIELD METAL COMPANY, OF DETROIT, MICHIGAN, A CORPORATION OF MICHIGAN.

men-sumo ALLOY.

10 Drawing.

This invention relates to alloys, more particularly those designed for use in the production of high speed cutting tools.

This application is a continuation in part of my application Serial No. 553,996, nled April 17, 1922.

It is necessary that alloys for such purposes have the property of red hardness so that a tool made therefrom may maintain its cutting edge after the same has become redhot.

In addition to heat resistance the alloy must also possess abrasive hardness and, for this purpose, should contain embedded in the metallic matrix, hard crystals, usually metallic carbides.

The principal object of this invention s to improve carbide containing alloys of this character to increase their strength and heat resistance and also reduce their liability to flake, crack or splinter during use. I

Other and further important ObJGCtS of the invention will hereinafter appear.

I have found that an alloy formed of hard carbides and a matrix composed of both cobalt and nickel as its basal components gives a much superior cutting tool to one made with either cobalt or nickel alone.

The metals of the chromium group form hard carbides which are suitable for the present purpose, although metals of other groups may also be used if desired.

The metals of the chromium group comprise cromium, tungsten, molybdenum and uranium. While these metals show many resemblances to each other both chemical and physical, they are far from being identical in properties. The same is true of the somewhat closely allied metals nickel and cobalt. To obtain the best results the individual characteristics of these metals should be blended, although the invention is not restricted to the use of a plurality of metals of the chromium group. Thus chromium gives strength rather than hardness as compared with molybdenum. Then alloys with nickel alone have a tendency to be hotshort while the cobalt alloys are coldshort Further, unless the quantity of nickel is properly limited the product will not be satisfactor especially as regards its heat resisting qualities.

Application filed December 1, 1824. serial No. 753,353.

The chief function of the cobalt and nickel appears to be that of producing a stron tough, heat resisting matrix for the carbides of the chromium group or other group of metals. Neither cobalt or nickel possesses the affinity for carbon that is possessed by chromium or molybdenum for example, so that it is probable that there is little or no carbide of either cobalt or nickel in my alloys.

The amount of carbon b weight in my alloys is comparatively slig t, say 1.50% but the proportion of carbide by volume may be over 1570 of the entire alloy. This follows from the great differences in specific gravity or carbon and the metals with which it combincs to form carbide. In view of this large content of non-metallic compounds the composition of the matrix is of the utmost importance. It will also be evident that since these alloys may be regarded as a mass of carbide crystals embedded in a strong, tough, heat resisting matrix, a variety of carbides may be used with a matrix having as its basal constituents both cobalt and nickel.

These carbides are soluble to a certain extent in the molten alloy so that unless the carbon content exceeds certain limits depending upon the nature and proportion of the metals forming the alloy, the latter on cooling will not contain free carbide crystals but only carbide in solid solution. While carbide in solid solution has a hardening effect it is not the desired abrasive hardness which results from the presence of free carbide crystals. The carbon content of the alloy should, therefore, be high enough to provide a substantial proportion of free carbide crystals in the alloy.

Ordinarily, alloys made in accordance with this invention Will consist of cobalt, nickel, chromium and molybdenum with a small amount of carbon.

The percentage of these metals will usually be within the following limits:

Per cent. Cobalt 25 to 45 Nickel 10 to 20 Chromium 25 to 40 Molybdenum 12to25 The total amount of cobalt and nickel should be between 30 and 65%.

In certain cases a wider range of proportions may be employed such as those lying within the following percentages.

Per cent. Cobalt 15 to 50 Nickel -L 7 to 30 Chromium 20 to 4-5 Molybdenum 7 to 35 Per cent.

Cobalt Nickel 12 Chromium 32 Molybdenum 16 If desired the quantity of cobalt may be decreased and the nickel correspondingly increased as in the following example:

Per cent. Cobalt 2 Nickel 23 Chromium 31 Molybdenum 19 Preferably, however, when a high nickel content is used the chromium group per centage should be increased either by increasing the chromium as in the following example Per cent.

Cobalt Nickel 24 Chromium 40 Molybdenum 16 or by the use of larger quantities of molyb- -denum-thus:

. Per cent.

Cobalt Nickel 28 Chromium 25 Molybdenum 26 The above high chromium alloy is particularly suited for rough castings where a large amount of scale has to be cut. The high molybdenum alloy is especially useful for purposes requiring a hard keen cutting edge. In some instances mo ybdenum may be the only metal of the chromium group present, as in the case of the following alloy:

Percent. Cobalt Nickel 30 Molybdenum 30 In addition to the metallic constituents mentioned above, a hardening element should be added. Usually this will be carbon, although other elements, more particularly silicon, have a hardening effect. Many metallic silicides are as hard if not harder than the corresponding carbides.

- east iron as a material for molds.

Usually the amount of carbon in the alloy will be between 1 and 2.5%, for example around 1.5%, although in some cases it may be as low as 0.5% or as high as 3.5%. It is desirable on the one hand to have enough carbon to produce free carbide crystals and on the other hand not enough to cause the formation of particles of graphitic carbon throughout the alloy, as the presence of graphitic carbon greatly reduces the strength of the alloy.

As a given weight of chromium, for example, will combine with a much larger weight of carbon than will the same weight of molybdenum, the amount of carbon which may be added before free graphitic carbon is formed in the alloy will depend upon the nature and proportions of the metals composing the alloy.

The carbon'is most readily and accurately added as a carbide, such as the carbide of one. of the metals forming the alloy as chromium.

In addition to a hardening element it is frequently advisable to use a tie-oxidizer such as aluminum or boron. Further, the hardening element and de-oxidizer may to advantage be added simultaneously in the form ofboron carbide.

While my alloys consist essentially of the above metallic and non-metallic ingredients it will be understood that the addition or presence as impurities of small quantities of other metals etc., such as iron, manganese or the like, will not change the general characteristics of my alloys.

In the process of forming the alloy the v several ingredients in proper proportion are placed in a crucible preferably together with some readily fusible material, such as glass,

which will form a protecting layer over thev alloy and so prevent oxidation.

The temperature employed for fusing the constituents may be from 1750 to 1950 C., according to conditions. As these alloys do not respond to heat treatment, as does steel, at least at a temperature below 1100 C., the alloy must be formed into the desired shape by casting and then grinding instead of by forging.

To obtain the best results molds made of sand should not be employed since, using such molds, even if brushed over with graphite powder, the bars are apt to be full of blow holes and too soft to make good lathe tools. Preferably the molds are constructed of graphite although cast iron may be used for this purpose if the surface is treated before use to prevent the hot metal adhering thereto. Such treatment may consist either in treatment with sulphuric acid or coating with carbon by the application of a smoky flame thereto.

Graphite is, however, much superior to In the first place it is much easierto machine graphite than cast iron so that molds for casting special sizes and shapes can be more readily made. Then again, cast. iron molds have to be repeatedly treated with sulphuric acid solution since the effect of the treatment soon wears off.

Further, cast iron molds, especially for small sizes of bars, chills the metal too rapidly. This chilling makes the bars hard, and, while hardness is a desideratum, it should be uniform throughout the bar and chilling makes the outer layers harder than the center.

Now graphite has a lower specific heat per unit volume and also a much lower heat conductivity than cast iron. Consequently the rate of abstraction of heat from the cooling metal is far less in the case of graphite than in the case of iron molds.

I have also found that the hardness of bars cast with the above alloys vary according to the rate at which they cool so that a small bar, which necessarily cools more rapidly than a lar e one, is, other conditions being the same, arder. On the other hand, increasing the carbon content of the alloy increases its hardness, It has further been found that heat treatment of the alloy after casting does not appreciably change its hardness so that the alloy may be termed selfhardening.

To secure the best .results it is necessary to hit the happy means between too great hardness, which means brittleness and liability to flake or chip, and too little hardness, which means that a tool made therefrom will be too soft to cut for the desired len th of time or to cut hard metals.

his -I accomplish by the present invention by varying the amount of hardenin element, such as boron carbide, added wit varying dimensions of the bar to be cast.

For example, for a inch bar 0.56% boron carbi e may be used to advantage; for a inch bar 0.85%; and for a inch bar 0.97%.

By so varying the content of boron carbide the sum of the hardness due to chilling and the hardness due to the hardening element is maintained substantially uniform irrespective of the size of the bar cast.

f on casting a trial bar from any given melt the alloy appears to be too soft, small additions of tungsten may be added'to the crucible to give the requisite hardness.

The above mentioned quantities of carbon, added as boron carbide, are considerably lower than the desired carbon contents of the bars for the reason that not only do the constituent commercial metals contain small amounts of carbon but also larger amounts of carbon are picked up from the crucible in which the alloy is made, if an unlined graphite crucible be employed.

As a. result of the picking up of carbon from the crucible it is desirable to avoid heating the metal to too high a temperature or for too long a time in the crucible. Further, when remelting scrap along with a proportion of new metal the quantity of boron carbide added should be-decreased to allow for the carbon already in the scrap.

'I am aware that the proportions of the constituents of the alloys and numerous details of the method of manuf alloys and tools therefrom may be varied acture of such.

through a wide range without departing from the spirit of th1s invention, and I do not desire limiting the patent granted, otherwise than as necessitated by the prior art.

I claim as my invention:

1. A high speed tool formed of an allo comprising 15 to 50% cobalt, 7 to 30% nicke 20 to 45%. chromium, and 7 to 35% molybdenum with a small amount of carbon.

2. A high speed tool formed of an alloy COIIIPIlSlIlg 25 to 45% cobalt, 10 to 20% nickel, 25 to 40% chromium, 12 to 25% molybdenum, with a small amount of carbon.

3. A high speed tool formed of an alloy comprising approximately 40% cobalt, 12% nickel, 32% chromium, and 16% molybdenum, and a small amount of carbon.

4. An alloy comprising 15 to 50% cobalt, 7 to 30% nickel, 20 to 45% chromium, and 7 to 35% molybdenum, with a small amount of carbon.

5. An alloy comprising approximately 40% cobalt, 12% nickel, 32% chromium, and 16% molybdenum, carbon.

In testimony whereof I have hereunto subscribed my name.

PERCY C. CHESTERFIELD.

and a small amount of