Powder Metallurgy Sintered Corrosion And Wear Resistant High Chromium Refractory Carbide Alloy

Abstract

A corrosion and wear resistant high chromium refractory carbide alloy is provided by powder metallurgy suitable for use as a seaming tool in the food canning industry comprising primary carbide grains of at least one refractory carbide selected from the group consisting of TiC, CbC, VC and TaC dispersed through a high chromium alloy matrix consisting essentially by weight of about 14 to 24 percent chromium, about 0.4 to 1.2 percent carbon, up to about 3 percent nickel, up to about 5 percent molybdenum, and the balance essentially iron.

Description

This invention relates to a powder metallurgy sintered corrosion and wear resistant, high chromium refractory carbide alloy and, in particular, to a hardened sintered tool element formed of said alloy.

RELATED U.S. PATENTS

In U.S. Pat. Nos. 2,828,202 dated Mar. 25, 1958 and No. 3,416,976, dated Dec. 17, 1968 and issued to the same assignee, a tool steel of high carbon content based on titanium carbide is disclosed in which the amount of titanium employed by weight is at least 10 percent (U.S. Pat. No. 2,828,202) substantially all combined in the form of primary carbide grains, the titanium carbide grains being dispersed through a heat treatable steel matrix.

As pointed out in the aforementioned patents, the composition is formed by employing titanium and carbon together in the combined form as primary grains of titanium carbide as an alloying ingredient together with a steel matrix which reacts with the carbide to a certain extent in producing the desired composition. The steel employed in forming the matrix contains at least about 60 percent iron by weight of the steel matrix composition.

Powder metallurgy is employed as the preferred method in producing the desired composition which comprises broadly mixing powdered steel-forming ingredients and forming a compact by pressing the mixture in a mold, followed by subjecting the compact to liquid phase sintering under non-oxidizing conditions, such as in vacuum. A steel matrix found particularly useful in combination with titanium carbide is one containing about 0.5 percent carbon, about 3 percent chromium, about 3 percent molybdenum and the balance iron.

In producing a titanium carbide tool steel composition containing for example 33 percent by weight of TiC (approximately 45 volume percent) and substantially the balance the aforementioned steel matrix, about 500 grams of powdered TiC (of about 5 to 7 microns in average size) are mixed with about 1,000 grams of steel-forming ingredients in a mill half filled with stainless steel balls. To the powder ingredients is added one gram of paraffin wax for 100 grams of mix. The milling is conducted for about 40 hours using hexane as a vehicle.

After completion of the milling, the mix is removed and dried and compacts of a desired shape pressed at about 15 t.s.i. and the compacts then subjected to liquid phase sintering in vacuum at a temperature of about 2,640.degree.F (1,450.degree.C) for about one-half hour at a vacuum corresponding to 20 microns of mercury or better. After completion of the sintering, the compacts are cooled and then annealed by heating to 900.degree.C for 2 hours followed by cooling at a rate of about 60.degree.F (33.degree.C or 35.degree.C) per hour to about 1,000.degree.F (538.degree.C) and thereafter furnace cooled to room temperature to produce an annealed structure containing spheroidite. The annealed hardness is in the neighborhood of about 45 R.sub.c and the high carbon tool steel is capable of being machined and/or ground into a desired tool shape or machine part prior to hardening.

The hardening treatment employed comprises heating the machined piece to an austenitizing temperature of about 1,750.degree.F (about 955.degree.C) for about one-quarter hour followed by quenching in oil to produce a hardness in the neighborhood of about 70 R.sub.c.

THE PROBLEM CONFRONTING THE ART

The aforementioned titanium carbide tool steel containing by volume about 45 percent titanium carbide and the balance a low chromium-molybdenum steel containing by weight of about 0.3 to 0.8 percent C, 1 to 6 percent Cr, 0.3 to 6 percent Mo and the balance essentially iron has been found very useful in the manufacture of tools, dies and many wear parts; particularly for use under generally normal environmental conditions.

However, in certain special environments, such as prevail in the food canning industry, among others, including corrosive media, the foregoing composition presents certain problems, insofar as tool life and overall tool efficiency are concerned. For example, where the tool is a pair of seaming rolls or hammers employed in the manufacture of cans involving the use of chloride soldering fluxes (for example, a mixture of ammonium and zinc chlorides), the tool does not exhibit adequate corrosion resistance to the fluxes, whereby the steel matrix relative to the titanium carbide grains is selectively corroded. When this occurs, titanium carbide grains are dislodged due to the lack of support in the matrix. This leads to an accelerated wearing of the seaming rolls, which results in a loss in tool life and tool efficiency. Similarly, where the aforementioned tool composition is employed as a tool or wear part in food canning apparatus, the acid media which normally prevail in the canning of foods, such as, by way of example, citric acid, carbonic acid and the like, will generally have a corrosive effect on the tool or wear part and, as described hereinabove, adversely affect the life of the tool or wear resistant part.

It would be desirable to provide a corrosion and wear resistant composition capable of being heat treated to a substantially high hardness and which will provide a long life under conditions of use ranging from room temperature up to as high as 800.degree.F (427.degree.C).

OBJECTS OF THE INVENTION

It is thus an object of this invention to provide a corrosion and wear resistant high chromium refractory carbide alloy.

Another object is to provide a hardened sintered corrosion and wear resistant tool element formed of a high chromium refractory carbide alloy.

These and other objects will more clearly appear from the following disclosure and the appended claims.

STATEMENT OF THE INVENTION

Broadly stated, the invention is directed to a powder metallurgy sintered corrosion and wear resistant high chromium containing refractory carbide alloy comprising primary grains of at least one refractory carbide selected from the group consisting of TiC, CbC, VC and TaC dispersed or distributed through a high chromium alloy matrix consisting essentially by weight of about 14 to 24 percent chromium, about 0.4 to 1.2 percent carbon and the balance essentially iron. The term "balance essentially iron" does not exclude the presence of other elements in amounts which do not adversely affect the basic characteristics of the matrix alloy. Thus, the high chromium ferrous matrix may contain other elements, such as small amounts of one or more of the elements silicon, manganese, vanadium, molybdenum, and the like.

A composition range which is particularly advantageous is one in which the refractory carbide ranges by volume from about 30 to 75 percent, with the balance substantially the aforementioned high chromium matrix alloy.

A more advantageous composition is one in which the refractory carbide (e.g. TiC) ranges by volume from about 35 to 55 percent, and wherein the matrix alloy making up substantially the balance consists essentially by weight of about 16 to 20 percent chromium, about 0.5 to 0.9 percent carbon, and the balance essentially iron.

The foregoing composite refractory carbide alloy is capable of being annealed to a hardness as low as 50 R.sub.c and hardened to as high as 69 R.sub.c to provide markedly improved resistance to wear and corrosion. By controlling the carbon content of the matrix alloy over the broad range of 0.4 to 1.2 percent by weight of the matrix and, more preferably, over the range of about 0.5 to 0.9 percent, a substantially martensitic matrix is assured by heat treatment, including a dispersion in the matrix of a secondary carbide containing chromium, probably an iron-chromium carbide. The secondary carbide together with the primary carbide provides improved wear resistance while the chromium dissolved in the matrix assures resistance to corrosion.

DETAIL ASPECTS OF THE INVENTION

As illustrative of the various embodiments of the invention, the following examples are given:

EXAMPLE 1

A heat treatable high chromium refractory carbide alloy comprised of titanium carbide and a matrix of a high chromium ferrous alloy was produced with the following composition:

Primary Carbide about 45 vol.% TiC Matrix about 55 vol.%

The matrix metal had the following nominal composition by weight:

Percent Carbon 0.8 Chromium 20.0 Iron.sup.1 balance 1. The balance iron may include the presence of amounts of other ingredients which do not adversely affect the basic characteristics of the alloy.

In producing the composition, 500 grams of TiC powder (about 45 vol. percent) of average particle size of about 3 to 7 microns are mixed with 1,000 grams of powdered steel-forming ingredients corresponding to the aforementioned matrix metal composition. The 1,000 grams of steel-forming ingredients include 200 grams of minus 100 mesh high purity electrolytic chromium powder, an amount of carbon equivalent to 8 grams (0.8 percent) taking into account any free carbon available through titanium carbide and the balance about 792 grams of iron powder of approximately 20 microns average size. The powder mixture (TiC and the steel-forming ingredients) also contains 1 gram of paraffin (1 percent) for each 100 grams of mix. The mix is placed in a stainless steel ball mill half filled with stainless steel balls, using hexane as the vehicle. The milling is conducted for about 40 hours.

After completion of the milling, the mix is removed and vacuum dried. A proportion of the mixed product is compressed in a die at 15 tons/sq. inch to the desired shape. The shape is liquid phase sintered at a temperature of about 1,350.degree.C for one-half hour (after reaching the temperature) at a vacuum corresponding to 20 microns or better. After completion of sintering, the shape is cooled and then annealed by heating to 900.degree.C for 2 hours followed by cooling at a rate of about 25.degree.C/hour to about 550.degree.C and thereafter furnace cooled to room temperature to produce an annealed microstructure containing mainly sphereoidite, the hardness being about 50 R.sub.c. The sintered shape is then machined into a tool element, e.g., seaming rolls or hammers for the canning industry, and thereafter hardened by heating to an austenitizing temperature of about 1,875.degree.F (about 1,025.degree.C) for about one-quarter hour at temperature and then air or oil quenched to form a hard microstructure consisting essentially of martensite. Following hardening, the tool element is tempered at a temperature within the range of about 400.degree.F (205.degree.C) to 800.degree.F (427.degree.C) for about 1 to 2 hours and thereafter cooled in air. The final hardness is in the neighborhood of about 68 R.sub.c. Following hardening, the tool element which is slightly oversize is then precision ground.

EXAMPLE 2

Primary carbide about 30 vol.% CbC Matrix about 70 vol.%

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

Percent Carbon 0.6 Chromium 16.0 Iron balance

The method of formulation, the sintering procedure and the heat treatment are similar to those described for Example 1.

Similar procedures are employed in the following examples.

EXAMPLE 3

Primary carbide about 40 vol.% VC Matrix about 60 vol.%

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

Percent Carbon 1.0 Chromium 18.0 Iron balance

EXAMPLE 4

Primary Carbide about 55 vol.% Tac Matrix about 45 vol.%

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

Percent Carbon 1.1 Chromium 22.0 Nickel 1.0 Iron balance

EXAMPLE 5

Primary Carbide about 65 vol.% TiC Matrix about 35 vol.%

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

Percent Carbon 1.2 Chromium 23.0 Nickel 1.5 Iron balance

EXAMPLE 6

Primary carbide about 70 vol.% TiC Matrix about 30 vol.%

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

Percent Carbon 0.5 Chromium 15.0 Molybdenum 2.0 Iron balance

Broadly, in producing the various compositions by powder metallurgy, the appropriate amount of steel-forming ingredients is mixed with an appropriate amount of primary carbide in a ball mill. The mixture may be shaped a variety of ways. It is preferred to press the mixture to a density of at least about 50 percent of true density by pressing over the range of about 10 t.s.i. to 75 t.s.i., preferably 15 t.s.i. to 50 t.s.i., followed by sintering under substantially inert conditions, e.g., in a vacuum or an inert atmosphere. Advantageously, the temperature employed is above the melting point of the chromium steel matrix, for example, at a temperature up to about 100.degree.C above the melting point for a time sufficient for the primary carbide and the matrix to reach equilibrium and to obtain substantially complete densification, for example, for about one minute to six hours.

When the liquid phase sintering is completed, the product is allowed to furnace cool to room temperature. If necessary, the as-sintered product is subjected to mechanical cleaning. If the as-sintered product requires annealing, it is heated to a temperature of about 1,550.degree.F (845.degree.C) to 1,700.degree.F (926.degree.C) for about 2 to 5 hours and then slowly cooled at a rate not exceeding 25.degree.C/hour.

For hardening, the austenitizing temperature may range from about 1,700.degree.F (926.degree.C) to 2,000.degree.F (1,093.degree.C) for about 30 minutes to 2 hours followed by air cooling. Thereafter, the hardened composition may be tempered at a temperature ranging from about 400.degree.F (205.degree.C) to 800.degree.F (427.degree.C) for about 1 to 2 hours. For the compositions given hereinbefore, the hardness after tempering may range from about 65 R.sub.c to 71 R.sub.c.

Corrosion studies have indicated that the alloy compositions of the invention exhibit good resistance to corrosion in such acid media as concentrated nitric acid and dilute (about 10 vol. percent) sulfuric acid.

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 the appended claims.

Claims

What is claimed is:

1. A powder metallurgy sintered corrosion and wear resistant high chromium refractory carbide alloy comprising about 30 to 75 percent by volume of primary grains of at least one refractory carbide selected from the group consisting of TiC, CbC, VC and TaC dispersed through a high chromium alloy matrix making up the balance, said alloy matrix consisting essentially by weight of about 14 to 24 percent chromium, about 0.4 to 1.2 percent carbon, up to about 3 percent nickel, up to about 5 percent molybdenum, and the balance essentially iron.

2. The sintered corrosion and wear resistant high chromium refractory carbide alloy of claim 1, wherein the refractory carbide ranges by volume from about 35 to 55 percent TiC, and wherein the matrix alloy making up substantially the balance consists essentially by weight of about 16 to 20 percent chromium, about 0.5 to 0.9 percent carbon, and the balance essentially iron.

3. A hardened sintered corrosion and wear resistant tool element formed of a high chromium refractory carbide alloy comprising about 30 to 75 percent by volume of primary grains of at least one refractory carbide selected from the group consisting of TiC, CbC, VC and TaC dispersed through a high chromium alloy matrix consisting essentially by weight of about 14 to 24 percent chromium, up to about 3 percent nickel, up to about 5 percent molybdenum, about 0.4 to 1.2 percent carbon and the balance essentially iron, the metallographic structure of the alloy matrix consisting essentially of martensite.

4. The hardened tool element of claim 3, wherein the refractory carbide ranges by volume from about 35 to 55 percent TiC and wherein the matrix alloy making up substantially the balance consists essentially by weight of about 16 to 20 percent chromium, about 0.5 to 0.9 percent carbon, and the balance essentially iron.