US3369892A - Heat-treatable nickel-containing refractory carbide tool steel - Google Patents

Description

United States Patent.

ABSTRACT OF THE DISCLOSURE A refractory carbide tool steel is disclosed comprising about 20 to 80% by volume of a carbide selected from the group consisting of VC, CbC and TaC distributed through a low carbon alloy steel matrix making up substantially the balance, the matrix being being characterized in the solution annealed state by a microstructure of soft martensite containing at least one age hardening element when air cooled from an austenitizing temperature. The matrix may contained by weight about 10 to 30% nickel, about 0.2 to 9% titanium, up to about 5% aluminum, the sum of the titanium and aluminum not exceeding about 9%, up to about 25% cobalt, up to about 10% molybdenum, substantially the balance of the matrix being at least 50% iron.

This invention relates to a nickel-containing refractory carbide tool steel and, in particular, to a nickel-containing refractory carbide tool steel bar stock capable of being precision machined into desired shapes in the marten sitic condition and of being thereafter heat treated to high hardness while maintaining close dimensional tolerances.

In US. Patent No. 2,828,202, dated Mar. 25, 1958,

a tool steel comprising titanium carbide is disclosed in which the amount of titanium employed is at least 10% of the total composition, the titanium being in the form of a primary titanium carbide. The titanium carbide is uniformly distributed through a heat treatable ferrous matrix comprising either carbon steel, or carbon-containing medium alloy or high alloy steel.

As set forth in the aforementioned patent, the composition is formed by employing titanium and carbon together in a combined form as titanium carbide as an alloying ingredient together with a steel matrix utilizing powder metallurgy methods of fabrication. The steel employed in forming the matrix contains iron as the major allowing element which generally comprises at least about 60% by weight of the steel matrix composition. The amount of titanium may range from about 10% to 70% by weight (about to 90% by volumn of titanium carbide or 12.5% to 87% by weight) and preferably about 20% to 58% by weight of titanium (about 40% to 80% by volume of titanium carbide or to 75% by weight), substantially the balance being formed of a carbon-containing steel matrix.

Another tool steel of the foregoing type is one conl taining at least one refractory carbide selected from the group consisting of VC, CbC and TaC, with the balance formed substantially of a carbon-containing steel matrix, for example a steel matrix constituting about 25% to 75%, or preferably to 60%, by weight of the total composition.

In producing titanium carbide tool steels in accordance with the teachings of US. Patent No. 2,828,202, a sintered product is first fabricated which is annealed by furnace cooling from about 1300 C. to room temperature, the microstructure of the matrix metal generally comprising pearlite. The annealed product is then machined into the desired shape by turning and/or grinding, and then subjected to a hardening heat treatment by austenitizing the "ice annealed tool steel at a temperature of about 950 C. for about one-quarter of an hour followed by quenching in oil or water. Hardnesses of up to about 70 Rockwell C are obtained for such titanium carbide tool steels. This class of materials has found commercial acceptance as specialty steels.

Generally speaking, in working with steels of the quench hardening type, care must be taken to minimize dimensional changes which may occur during quenching. For example, articles having slender or intricate portions may tend to warp, in which case it may be necessary to harden oversized pieces to take into account the amount of metal to be removed to insure precision dimensions in the final article. However, this is not too desirable in steels containing refractory carbides because of the machining and grinding costs involved, clue generally to the high carbide content. Moreover, articles of intricate shape may tend to crack during quenching.

Where size and shape of the article to be heat treated is such that warping does not occur, grinding may be still a problem. Generally, growth occurs when the steel matrix transforms from austenite to rnartensite, the growth being as high as 0.0004 inch per inch, wherein the excess metal must be removed by grinding. As stated above, this is expensive and time consuming due to the relatively high amounts of carbide present.

A sill further problem is the additional finish grinding which is sometimes required when the matrix of a heat treated titanium carbide steel has decarburized during heat treatment. While the decarburized matrix is generally softer, the presence of primary grains of refractory carbide makes grinding diflicult. Moreover the hardness of the decarburized matrix is adversely affected. To avoid this, strict percautions must be taken via atmosphere control to prevent decarburization.

It would be desirable, therefore, if a carbidic tool steel of the aforementioned type could be provided which would not require an oil or water quenching treatment to harden it so that all types of article shapes can be fully hardened to the desired hardness while closely maintaining dimensional tolerances and avoiding cracking, warping and the like.

It is an object of our invention to provide a new and improved nickel-containing high carbon tool steel composition capable of maintaining close dimensional tolerances after a hardening heat treatment.

Another object is to provide a new and improved refractory carbide tool steel capable of being hardened without quenching in oil, water or other liquid media.

A further object is to provide as an article of manufacture a tool steel bar stock containing substantially large amounts of a carbide selected from the group consisting of VC, CbC and TaC distributed as primary carbide grains through a nickel-containing steel matrix having an age hardenable martensitic microstructure.

These and other objects will more clearly appear from the disclosure which follows.

In producing the nickel-containing carbidic tool steel of the invention, we employ as a primary carbide one or more of the refractory carbides selected from the group consisting of VC, CbC and TaC. By primary carbide is meant the refractory carbide which is added to the composition as such and which is substantially insoluble in the matrix, whereby it is still recognizable under the microscope after the composition is subjected to fabrication and to normal steel heat treating practice.

We have found that by combining the foregoing carbide with a special nickel-containing steel composition, we are enabled to heat treat the resulting carbidic tool steel composition to relatively high hardness while maintaining close dimensional tolerances of the article made from the composition.

In its preferred embodiment, the nickel-containing carbidic tool steel comprises by volume about 20% to 80% of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a nickel-containing steel matrix making up substantially the balance, the matrix containing by weight of steel matrix about 10% to 30% nickel, about 0.2% to 9% titanium and up to about 5% aluminum, the sum of the titanium and aluminum content not exceeding about 9%, up to about 25% cobalt, up to about 10% molybdenum, with substantially the balance of the matrix by weight being at least 50% iron. The elements making up the matrix composition are proportioned such that when the nickel content ranges from about 10% to 22% and the sum of titanium and aluminum is less than about 1.5% or less than about 1.3%, the cobalt and molybdenum contents are each at least about 2% by weight; and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than about 2%, the sum of titanium and aluminum in the matrix exceeds about 1.5%.

A composition range which is particularly advantageous for our purpose is one comprising about 20% to 80% or 30% to 70% by volume of primary carbide grains based on a carbide selected from said group distributed through a matrix making up the balance containing by weight of the matrix about 18% to 30% nickel, about 1.5% to 9% of titanium, up to about 5% aluminum, the sum of titanium and aluminum not exceeding about 9%, substantially the balance of the matrix alloy being at least about 50% iron by weight. Other metals which may advantageously be present include up to about 20% cobalt, up to about 2% molybdenum and chromium total with substantially the balance of the matrix being at least 50% iron. A composition which is advantageous for our purpose comprises 24% to 30% nickel, 1.5% to 9% titanium and/or aluminum, the total titanium and aluminum not exceeding 9%, and low carbon in the matrix, for example below 0.15 carbon and more advantageously below 0.1 carbon. Where a high matrix hardness is desired, the titanium and/or aluminum content may range from about 5% to 9% by weight total.

Another composition range which is advantageous for our purpose is one comprising about 20% to 80% or 30% to 70% by volume of said primary carbide grains distributed through a nickel alloy steel matrix constituting the balance, the matrix containing by weight about 18% to 24% nickel, about 1.5% to 3% of a metal from the group titanium and aluminum, low carbon, e.g., not exceeding 0.1% C, and the balance of the matrix substantially iron.

A still further composition range which is advantageous for our purpose is one comprising about 20% to 80% or 30% to 70% by volume of said primary carbide grains distributed through a low carbon nickel alloy steel matrix containing by weight of the matrix about 10% to 22% nickel, about 0.2% to 1.5% of titanium, up to about 1.5% aluminum, with the sum of titanium and aluminum contents less than 1.5%, about 2% to 10% cobalt, about 2% to 8% molybdenum and the balance of the matrix substantially iron of at least about 50% by weight, the carbon content being maintained below 0.15% and more advantageously not exceeding 0.1%.

A matrix composition which is particularly advantageous when employed with the primary carbide over the foregoing ranges is one comprising about 16% to 22% nickel, about 6% to 10% cobalt and about 2% to 6% of molybdenum, about 0.2% to 1% titanium, up to about 0.4% aluminum and the balance of the matrix substantially iron of at least about 50% by weight. The foregoing compositions may contain up to 10% in the aggregate of up to about 5% Cr, up to about 7% W, up to about 3% Cb and/or Ta, up to about 6 Cu, up to about 0.5% Mn, up to about 1% Be, etc.

We find the foregoing matrix steel compositions particularly useful in that when the carbidic tool steel is produced, the resulting carbidic composition can be solution treated at an elevated temperature and air cooled to form a soft martensitic matrix having attributes that enable the resulting product to be easily machined and/ or ground to substantially precise dimensions prior to hardening. The hardening treatment employed after the solution treatment is unlike the high temperature austenitizing quenching treatment in that relatively low temperatures are employed while the matrix is in the martensitic condition, followed by air cooling. The heat treatment is in nature of an agehardening treatment due to the presence of age hardening elements, which heat treatment is carried out at a temperature in the range of about 265 C. to 655 C. (about 500 F. to 1200 F.).

As illustrative of one embodiment of the invention, the following example is given:

Example 1 A heat treatable carbidic tool steel containing columbiurn carbide and a matrix of a nickel-containing alloy steel was produced with the following composition: Primary carbide about 45 vol. percent Cbc Matrix steel about 55 vol. percent Nickelcontaining steel.

The matrix had the following composition by Weight:

Percent Ni 21.7 C0 8.49 M0 3.42 Ti 0.37 Fe Substantially the balance 1 The balance iron includes small amounts of other ingredients which do not adversely affect the novel characteristics of the alloy.

In producing the carbidic tool steel composition, about 800 grams of CbC powder is mixed with 1,000 grams of powdered steel-forming ingredients corresponding to the aforementioned matrix metal composition. The CbC powder has a total carbon content of about 11.4% which corresponds to a total of about 5% carbon in 1,800 grams of powder mixture. It is desirable for optimum results that the carbon content of the matrix metal not exceed 0.15% by weight of matrix and more advantageously not exceed about 0.1%. As CbC contains some free carbon and may also decompose during processing to form an additional small amount of free carbon which normally goes into the matrix, a strong carbide former other than Mo and Cr, and which is substantially insoluble in the matrix as the carbide, such as excess metallic titanium or other strong car-bide former is advantageously added to the mixture to combine with any excess free carbon to form a secondary carbide of titanium by reaction. This is achieved by adding the titanium in the form of TiH which yields active titanium for combining with free carbon. Another strong carbide former wihch may be added is zirconium in the form of zirconium hydride. The amount of titanium added is calculated to be at least 4 times the amount of free carbon to be combined as TiC plus excess titanium to leave about 0.2% to 0.4% free titanium to each matrix metal. Examples of other strong carbide formers which form substantially insoluble carbides are V, Cb, Ta, etc.

Thus, starting with about 45 vol. .percent (about 45 wt. percent) of CbC which contains about 11.4% combined carbon and some free carbon, suflicient TiH by weight of the mixture is added to yield enough active free titanium to combine with the free carbon to form a secondary carbide of titanium and provide an excess of free titanium (about 0.2% to 0.4%) for the matrix metal.

To the 1800 gram mixture (800 grams CbC and 1000 grams of steel-forming ingredients) is added one gram of parafiin wax for each grams of mix and the mixture ball milled for about 60 hours in a stainless steel ball mill half filled with stainless steel balls using hexane as a vehicle. After milling, the mixture is dried on a hot plate at 68 C. (150 F.) until all the hexane is driven off. The dry powder is then pressed into compacts or slugs at 15 tons per square inch.

The compacts thus produced are subjected to liquid phase sintering by heating'them to about 1425 C. in vacuum in 2 /2 hours and holding at temperature for three-quarters of an hour, followed by cooling to 1300 C. in 30 minutes and then furnace cooling from 1300 C. to room temperature. The sintering is advantageously carried out on a ceramic plate of previously fired Magnorite (a commercial MgO refractory).

The sintered alloy is solution annealed by heating to a temperature at which austenite prevails, for example, from about 760 C. (1400 F.) to 1165 C. (2150 F.) followed by air cooling. After heating the alloy at 815 C. (-1500" F.) for thirty minutes and air cooling to ambient temperature, it has a Rockwell C hardness not exceeding about 50, the microstructure of the matrix being soft martensite. In this condition, the alloy machines and/or I grinds easily to a precisely dimensioned shape. By cooling the alloy to ambient temperature in air from the solution temperature, transformation to soft martensite is effected. Thus, any growth that has ocurred due to transformation to soft martensite presents no problems since the carbidic alloy can be easily machined and hardened without any further growth taking place.

The alloy is hardened to values of 60 R and above by aging it at a temperature in the range of about 260 C. (500 F.) to 650 C. (1200 F.) for about three hours followed by air cooling. The advantage of the hardening heat treatment at the lower temperature .(e.g., 260 C. to 650 C.) is that substantially close dimensional tolerance can be maintained with intricate shapes and cracking greatly inhibited.

As stated above, it is important for the purposes of this invention that the amount of carbon in the nickel-containing steel matrix be maintained as low as possible, for example, below 0.15% by weight of the matrix. For example, in preparing a similar refractory carbide composition, TiH was omitted from the mixture as a result of which the as-sintered hardness was 57.8 R and the solution hardness after cooling from 1500 C. was 61.5 R Because the free carbon was not combined with the refractory metal, the solution hardness rose substantially above 50 R and was as high as 61.5 R

As examples of other compositions falling within the scope of the invention, the following are given:

Example 2 Primary carbide: about 30 vol. percent of CbC.

.Matrix steel: about 70 vol. percent nickel-containing steel.

The nominal composition of the matrix by weight is as follows:

The amount of CbC present also includes with it some excess titanium added to combine with any free carbon present and prevent it from entering the matrix metal.

Example 3 Primary carbide: about 65 vol. percent VC. 3 Matrix steel: about 35 vol. percent nickel-containing steel.

As in the previous examples, sufficient TiH was added to the mixture to combine with the free carbon during sintering to convert to it a secondary carbide of titanium.

Example 4 Primary carbide: about 25 vol. percent TaC.

Matrix steel: about 75 vol. percent nickel-containing steel.

The nominal composition of the matrix steel by weight is as follows:

Percent Ni 13 Co 8.5 Mo 4.5 Ti 0.35 A1 0.15 Mn 0.4 Si .1 Fe Substantially the balance Sufiicient excess of TiH was added to the mix to insure combining with free carbon prior to sintering and convert it to a secondary carbide .of titanium.

Example 5 Primary carbide: about 40 vol. percent CbC. Matrix steel: about 60 vol. percent of nickel-containing steel.

The nominal composition of the matrix by weight is as follows:

Percent Ni 18 C0 20 Mo 3 Ti 0.5 A1 0.1 Fe Substantially the balance The amount of free carbon present was compensated for by adding suflicient Til-I to the mixture prior to sintering.

Example 6 Primary carbide: about 75 vol. percent CbC. Matrix steel: about 25 vol. percent nickel-containing steel.

Nominal composition of matrix metal:

Percent Ni 18 Co 7.5 Mo 4.5 Ti 0.65 Al 0.15 Mn 0.4 Si 0.1 Fe Substantially the balance Free carbon in the system was compensated for by the addition of TiH to the powder mixture prior to sintering.

Example 7 Primary carbide: about 50 vol. percent TaC. Matrix steel: about 50 vol. percent nickel-containing steel.

Nominal composition of the matrix:

Percent Ni 21.5 C 6 A1 0.1 Fe Substantially the balanc Free carbon in the system is combined with titanium by adding Til-I to the powder mixture prior to sintering.

The iron given as the balance in the foregoing examples does not exclude the presence of amounts of other ingredients which do not adversely affect the novel characteristics of the carbide steel and small amounts of such other ingredients such as calcium, boron, zirconium as well as manganese and silicon and the like.

In producing the foregoing compositions, the powder metallurgy method of mixing the powdered ingredients and then compacting the mixture into a desired shape followed by liquid phase or solid state sintering at an elevated temperature to achieve full densification is found to be advantageous for our purposes. Broadly, this method comprises mixing the appropriate amount of steel-forming ingredients with the appropriate amount of the primary carbide, using a small amount of wax to give sufficient green strength to the resulting pressed compact, for example one gram of wax for each 100 grams of mixture. The mixture may be shaped a variety of ways. We find it advantageous to press the mixture to a density at least 50% of true density by pressing over the range of about t.s.i. to 75 t.s.i., preferably t.s.i. to 50 t.s.i., followed by sintering in a vacuum, preferably below 300 microns of mercury, generally at a temperature above the melting point of the steel matrix, depending on the alloying ingredients present, ranging from about 1300 C. to 1575 C. for a time sufficient for the primary carbide and the matrix to reach equilibrium and to obtain substantially complete densification, for example upwards of six hours.

When the liquid phase sintering is completed, the product is allowed to furnace cool to ambient temperature. If necessary, the as-sintered product is subjected to any mechanical cleaning and then solution treated over the range of about 760 C. (1400 F.) to 1165 C. (2150 F.) followed by air cooling. We have found the range of 760 C. (1400 F.) to 982 C. (1800 F.) to be particularly advantageous, The solution treatment may be carried out at a temperature for one-quarter hour or longer, for example, one hour.

As will be appreciated, other ingredients which may be present in the matrix metal besides the main constituents include up to about 1% Mn, up to about 0.5% Si, up to about 0.1% Ca, up to about 0.1% B, up to about 0.1% Zr, etc. Other alloying ingredients which may be present in the matrix steel in amounts which do not adversely affect the novel characteristics of the carbidic tool steel are Cr, Cu, W, V and Cb, among others.

In addition to the primary carbides, VC, CbC and TaC, other carbides may be present, in amounts which do not adversely affect the tool steel, such as up to about 25% zirconium carbide and the like, provided they are substantially insoluble in the matrix.

It is to be understood that the expression primary carbide based on a carbide comprising essentially a carbide from the group VC, CbC and TaC is meant to include the presence of other carbides of the aforementioned type, it being understood that such carbides may be present alone or as complete or partial solid solutions with said primary carbide.

It is to be understood that the invention provides a carbidic tool steel comprising about to 80% or 30% to 70% by volume of primary carbide grains comprising essentially the primary carbide distributed through a relatively low carbon alloy steel matrix making a substantially the balance, the matrix being characterized on slow cooling from its austenitizing temperature by a microstructure comprising substantially martensite in the relatively soft condition containing at least one age hardening element. The foregoing composition, despite the presence of substantial amounts of primary carbide, is advantageous in that after it is sintered, it can be solution treated by air cooling from a high temperature (i.e., the austenitizing temperature) to form a matrix of relatively soft martensite. In this condition, an article of this com position may then be precision machined to any shape, however intricate, and then hardened at a relatively low temperature without any substantial amount of warping, cracking, or volumetric change occurring. On the other hand, a carbidic tool steel having a quench-hardenable steel matrix generally requires re-finishing after hardening due to volumetric growth, warping, etc. Another advantage of the composition of the invention is that decarburization is not a problem since the matrix does not rely on the presence of carbon to achieve the requisite hardening. Thus, the usual precaution of strict atmosphere control during heat treatment is not necessary.

The invention provides a carbidic heat treatable ferrous alloy which in the form of bar stock, rounds, squares, blocks, ingots and other shapes can be utilized in the fabrication of cutting tools, blanking dies, forming dies, drawing dies, rolls, hot extrusion dies, forging dies, upsetting dies, broaching tools, and in general all types of wear and/or heat resisting elements, tools or machine parts.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What is claimed is:

1. A hardenable refractory carbide tool steel comprising about 20% to by volume of primary carbide grains comprising essentially a carbide selected from the group consisting of VC, CbC and TaC distributed through a low carbon alloy steel matrix making up substantially the balance, the matrix being characterized in the solution annealed state by a microstructure of soft martensite containing at least one age hardening element.

2. The tool steel of claim 1 wherein the amount of primary carbide ranges from about 30% to 70% by volume of the total composition.

3. A heat treatable carbidic tool steel comprising about 20% to 80% by volume of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by weight of matrix about 10 to 30% nickel, about 0.2 to 9% of titanium and up to about 5% aluminum, the sum of titanium and aluminum not exceeding about 9%, up to about 25% cobalt, up to about 10% molybdenum, substantially the balance of the matrix being at least 50% iron; the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of aluminum and titanium is less than about 1.5%, the cobalt and molybdenum contents are each at least about 2% by weight; and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than 2%, the sum of aluminum and titanium exceeds 1.5%, said matrix being also characterized in the solution treated condition by a microstructure of soft martensite.

4. A heat treatable carbidic tool steel comprising about 30% to 70% by volume of primary carbide grains based on a carbide selected from the group consisting of'VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by weight of matrix about 10 to 30% nickel, about 0.2 to 9% of titanium and up to about aluminum, the sum of titanium and aluminum not exceeding about 9%, up to about 25 cobalt, up to about 10% molybdenum, substantially the balance of the matrix being at least 50% iron, an efiective amount of a strong carbide former at least sufiicient to combine with carbon in excess of the carbon combined as primary carbide selected from the group consisting of canadium, columbium, tantalum, zirconium and titanium, the titanium of said group being that amount in addition to the titanium employed in producing the nickel alloy steel matrix; the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of aluminum and titanium is less than about 1.5%, the cobalt and molybdenum contents are each at least about 2% by Weight; and such that when the nickel ranges from about 18% to 30% and the molybdenum content is less than about 2%, the sum of aluminum and titanium exceeds 1.5 said matrix being also characterized in the solution treated condition by a microstructure of soft martensite.

5. A heat treatable carbidic tool steel comprising about 20% to 80% by volume of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by weight of matrix about 10 to 22% nickel, about 0.2 to 1.5% of titanium, up to about 1.5 aluminum, the sum of titanium and aluminum not exceeding about 1.5 about 2% to 10% cobalt, about 2% to 8% molybdenum, an effective amount of a strong carbide former at least sutficient to combine with carbon in excess of the carbon combined as primary carbide selected from the group consisting of vanadium, columbium, tantalum, zirconium and titanium, the titanium of said group being that amount in addition to the titanium employed in producing the nickel alloy steel matrix, substantially the balance of the matrix being at least 50% iron, and matrix being also characterized in the solution treated condition by a microstructure of soft martensite.

6. A heat treatable carbidic tool steel comprising about 30% to 70% by volume of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by Weight of matrix about 16% to 22% nickel, about 0.2% to 1% titanium, up to about 0.4% aluminum, about 6% to 10% cobalt, about 2% to 6% molybdenum, the amount of titanium present being at least sufiicient to combine with carbon in excess of the carbon combined as primary carbide and provide age hardening, substantially the balance of the matrix being at least 50% iron, said matrix being also characterized in the solution-treated condition by a microstructure of soft martensite.

7. A heat treatable high carbon high titanium tool steel comprising about 20% to 80% by volume of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by weight of matrix about 18 to 30% nickel, about 1.5% to 9% of a metal selected from the group consisting of titanium and aluminum, up to about 20% cobalt, less than about 2% of molybdenum, and an etfective amount of a strong carbide former at least sufiicient to combine with carbon present in excess of the carbon combined as primary carbide selected from the group consisting of vanadium, columbium, tantalum, zirconium and titanium, the titaniumpf said group being that amount in addition to the titanium employed in producing the nickel alloy steel matrix, substantially the balance of the matrix being at least 50% iron, said matrix being also characterized in the solution treated condition by a microstructure of soft martensite.

8. The tool steel of claim 7 wherein the primary carbide ranges by volume from about 30% to 70% of the tool steel composition.

9. A heat treatable high carbon tool steel comprising about 20% to 80% by volume of primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC distributed through a matrix of a high nickel alloy steel constituting the balance; said matrix nickel alloy steel containing by weight of matrix about 18 to 24% nickel, about 1.5% to 3% of a metal selected from the group consisting of titanium and aluminum, and an effective amount of a strong carbide former at least sufiicient to combine with carbon in excess of the carbon combined as primary carbide selected from the group consisting of vanadium, columbium, tantalum, zirconium and titanium, the titanium of said group being that amount in addition to the titanium employed in producing the nickel alloy steel matrix, substantially the balance of the matrix being at least 50% iron.

10. A method of producing a machinable refractory carbide tool steel which comprises providing a compact produced by compressing a powder mixture containing about 20% to 80% by volumeof primary carbide grains based on a carbide selected from the group consisting of VC, CbC and TaC dispersed through a matrix powder of nickel-containing steel-forming ingredients constituting the balance, said matrix powder containing by weight of matrix about 10 to 30% nickel, about 0.2 to 9% of titanium and up to about 5% aluminum, the sum of the titanium and aluminum not exceeding 9%, up to about 25% cobalt, up to 10% molybdenum, a strong carbide former in an amount at least sufiicient to combine with any carbon in excess of the carbon combined as primary carbide selected from the group consisting of vanadium, columbium, tantalum, Zirconium and titanium, the titanium of said group being that amount in addition to the titanium employed in producing the nickel alloy steel matrix, substantially the balance of the matrix being at least 50% iron; the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of aluminum and titanium is less than about 1.5 the cobalt and molybdenum contents are each at least about 2% by weight; and such that when the nickel content ranges from about 18 to 30% by weight and the molybdenum content is less than about 2%, the sum of aluminum and titanium exceeds 1.5 subjecting said compact to sintering at an elevated temperature, cooling said sintered compact to room temperature, and then subjecting said sintered compact to a solution annealing treatment by heating it to a temperature in the range of about 760 C. to 1165 C., and air cooling to ambient temperature, whereby to produce a solution treated matrix characterized by a microstructure of soft martensite.

11. The method of claim 10 wherein after solution annealing the tool steel alloy, the alloy is machined to a desired shape and dimension and then age hardened by heating at a temperature ranging from about 260 C to 650 C. and then air cooled.

References Cited UNITED STATES PATENTS 3,093,518 6/1963 Bicber 14831 3,093,519 6/1963 Decker et al l483l 3,132,937 5/1964 Sadowski et al. -124 3,183,127 5/1965 Gregory et al. l483l 3,303,066 2/1967 McGee 75123 X CHARLES N. LOVELL, Primary Examiner.