US3598567A - Stainless steel powder product - Google Patents

Abstract

A METHOD IS PROVIDED FOR PRODUCING HOT WORKABLE METAL POWDER FROM COMPOSITIONS NORMALLY DIFFICULT OR IMPOSSIBLE TO WORK. A MOLTEN METAL BATH IS ESTABLISHED OF A METAL COMPOSITION OF MELTING POINT ABOVE 1000* C. CONTAINING SUBSTANTIAL AMOUNTS OF AT LEAST ONE PHASEFORMING CONSTITUENT WHICH NORMALLY FORMS A SEGRAGATABLE PHASE ON COOLING. THE BATH IS SUBDIVIDED INTO MEDIUM TO SMALL METAL DROPLETS AND IS RAPIDLY COOLED TO A TEMPERATURE BELOW THE FREEZING POINT AT A COOLING RATE OF AT LEAST ABOUT 100* C./SEC., AND PREFERABLY FURTHER RAPIDLY COOLED TO OR NEAR ROOM TEMPERATURE. THE POWDER THUS PRODUCED, BECAUSE OF A FINE DENDRITIC GRAIN SIZE, IS ONLY SLIGHTLY IF AT ALL SEGREGATED AND ALL HARD BRITTLE PHASES ARE DISITRIBUTED AS FINE PARTICLES, MAKING THE ALLOY READILY HOT WORKABLE. EXCESS SOFT PHASES ALSO FOLLOW THE SAME HIGHLY DISPERSED DIESTRIBUTION. THUS, A PRODUCT IS PROVIDED CHARACTERIZED BY A FINE NEAR MICRON DISPERSION OF EXCESS PHASES WHICH NORMALLY SEGREGATE AND FORM AS COARSE PHASES, FOR EXAMPLE AT GRAIN BOUNDARIES, WITH RESULTANT POOR HOT WORKING PROPERTIES.

Description

g- 10, 1971 N. J. GRANT 3,598,567

STAINLESS STEEL POWDER PRODUCT Filed July 1, 1968 f NICHOLAS J'- GRANT United States Patent Oifice US. Cl. 75---.5BA 2 Claims ABSTRACT OF THE DISCLOSURE A method is provided for producing hot workable metal powder from compositions normally difficult or impossible to work. A molten metal bath is established of a metal composition of melting point above 1000 C. containing substantial amounts of at least one phaseforming constituent which normally forms a segregatable phase on cooling. The bath is subdivided into medium to small metal droplets and is rapidly cooled to a temperature below the freezing point at a cooling rate of at least about 100 C./sec., and preferably further rapidly cooled to or near room temperature. The powder thus produced, because of a fine dendritic grain size, is only slightly if at all segregated and all hard brittle phases are distributed as fine particles, making the alloy readily hot workable. Excess soft phases also follow the same highly dispersed distribution. Thus, a product is provided characterized by a fine near micron dispersion of excess phases which normally segregate and form as coarse phases, for example at grain boundaries, with resultant poor hot working properties.

This invention relates to a powder metal product and to a process of producing metal powder from metal compositions normally difficult to hot or cold work and, in particular, to a wrought metal product and to a process for producing wrought metal shapes from normally difi'icult-to-hot-work compositions, with resultant superior properties.

The production of wrought metal shapes by conventional metallurgical techniques generally involves melt ing, casting of the molten metal into an ingot of large cross section and then hot working of the ingot by stages to a desired shape. Depending on the kinds of metals or alloys being produced and their compositions, manufacturing difficulties may arise during hot working due to the lack of ingot inhomogeneity as a result of very slow solidification and subsequent cooling.

Ingots suffer from three major kinds of segregation: that which occurs on a dendritic or finer scale, that which occurs on a grain size scale and finally composition segregation on an ingot length or radius scale. Longtime soaking at high temperatures may improve on a small scale the dendrite segregation if the excess phases are soluble at the chosen temperature; however, it is found that long-time high temperature soaking can do more damage than good by leading to grain growth and coarsening of partially soluble phases, which adversely affect hot forgeability, extrusion, or rolling.

Generally speaking, alloy compositions in the molten condition are homogeneous but become inhomogeneous and highly segregated, preferentially, on a dendritic, grain size or ingot diameter basis due to temperaturesolubility laws. Because of extremely slow cooling which generally prevails in the casting of large ingots, at rates as low as 0.01 to 1 C./sec., segregation of both metallic and non-metallic constituents occurs. Inclusions, hard particles of oxides, sulfides, borides, carbides, etc., which form before and during solidification are therefore coarse and tend to be trapped at both the coarse dendrite boundaries and grain boundaries. The inclusions generally get coarser Patented Aug. 10, 1971 towards the center and towards the top of the ingot. Forexample, even at oxygen levels as low as 0.01% in stainless steel, oxide inclusion resulting therefrom can be large (100 to 1000 microns) and provide stringers in the form of seams in wrought material and adversely affect the surface appearance of the final product, its resistance to fatigue and to impact.

The effect of sulfur may be bad. at content as low as 0.02% by weight and severe above 0.04%. The casting morphology of complex allows, e.g.. superalloys, stainless steel, complex alloy tool steels, and the like, is not very easy to control, especially those compositions containing substantial amounts of segregatable phase-forming constituents. It is known that ingots cooled in the conventional manner usually suffer from the formation of coarse secondary phase particles, dendrites, aggregates, etc., such as carbides, sulfides, oxides, borides, silicides, nitrides, and certain intermetallics such as Ni Al1, CuAl and the like. Sometimes these particles are so coarse after solidification that normal soaking at elevated temperatures to promote homogenization does not effectively dissolve the phases but tends to lead to coarsening of the particles, whereby processing difficulties arise in hot working and moreover the physical properties of the final product, such as fatigue and impact resistance, machinability and other properties are adversely affected.

In such alloys as chromium-irons, stainless steel, and the like, while it is generally desirable to maintain the sulfur content as low as possible, it is not uncommon to deliberately add sulfur to improve: the machinability of such metals. It has been found that as much as 0.12%

sulfur improves the machining properties, but that more than 0.2% sulfur adversely affects the ingot and bar hot workability to the extent as to render the material commercially unworkable.

It would be desirable to provide a method of further increasing the tolerance level of such phase-forming ingredients, especially sulfur, as increased amounts of sulfur are known to improve further the machining and nonseizing properties of complex alloys. In particular, while nickel-base alloys are sensitive to die presence of very small amounts of sulfur, it would be advantageous if sulfur could be added to such alloys so as to render them easily machinable for the production of high temperature nut-s, bolts and other useful articles.

It is thus the object of this invention to provide a method of improving the hot workability of difficultto-work metals and alloys, and to .make possible the hot workability of non-hot workable alloys.

Another object is to provide a workable metal powder of composition normally diflicult or impossible to work.

A further object is to provide a hot workable alloy containing substantial amounts of sulfur and characterized by improving machinability and improved physical properties.

A still further object is to provide a wrought metal product containing substantial amounts of phase-forming constituents, the phases of which are refined and uniformly dispersed throughout the metal matrix.

Other objects are to improve the hot workability and the resultant properties of presently diflicult to hot wonk alloys; to provide hot workability in alloys which are today found to be incapable of hot working by known practices; to provide structures in still more highly alloyed compositions than now exist which will have adequate hot workability but which are known today not to be hot workable; and to produce alloy powders containing substantially larger amounts of carbides, oxides, sulfides, borides, and the like, and/or intermetallic compounds than are now presently feasible, and which would be hot workable to provide improved properties, such as machinability, mechanical strength and ductility, temperature stability, hardness and the like properties.

These and other objects will more clearly appear from the following disclosure and the accompanying drawing, wherein:

FIG. 1 is a representation of a macrograph taken at five times magnification of atomized stainless steel particles;

FIG. 2 is a representation of a mricrograph taken at 100 times magnification of a polished, unetched cross section of stainless steel powder of the type shown in FIG. 1 containing 0.39% sulfur; and

FIG. 3 is a representation of a micrograph of an etched, atomized, stainless steel powder taken at 500 times magnification, the amount of sulfur being about 0.52%}.

Stating it broadly, a method is provided for producing a metal powder and a wrought metal product thereof containing substantial amounts of excess phase-forming constituents, for example, sulfur, that normally form injurious segregatable phases when produced by melting and conventional ingot casting techniques. Briefiy, one embodiment of the invention comprises establishing a molten bath of a metal composition of melting point above 1000 C. containing at least one hard phase-forming constituent in an amount which normally forms, on casting said bath into an ingot, a segregatable phase selected from the group consisting of metal carbides, borides, silicides, nitrides, oxides, phosphides, silicates, sulfides, tellurides, selenides and intermetallic compounds, continuously subdividing the molten bath into small metal droplets, and rapidly solidifying the metal droplets to below their freez ing point at a cooling rate of at least about 100 C./ sec. The metal powder produced in this manner will have a highly refined structure, will be substantially more free from segregation and will be capable of being hot worked into a wrought metal shape by hot consolidating the powder mass, e.g. by hot extrusion, at a temperature above the recrystallization temperature of the alloy composition.

The invention is particularly applicable to the production of free machining steels, such as carbon steel, stainless steel, nickel and cobalt-base alloys, copper-base alloys, and the like, containing substantial amounts of sulfur, for example, sulfur ranging from about 0.1% to as high as about 1% and, more advantageously, from about 0.2% to 0.6% sulfur by weightJIn addition to sulfurcontaining steels, the invention is also applicable to sulfurcontaining alloys having melting points above about 1350 C., such as heat resistant nickel-base alloys. By means of the invention, a heat resistant alloy containing about chromium, 8% iron and the balance. essentially nickel can be made free machining by using levels of sulfur over the foregoing range which normally render such alloys almost impossible to hot work or which normally exhibit poor properties, such as low strength, low ductility, low

resist to impact and fatigue, etc.

The term phase-forming constitutents" includes such constituents as oxygen, nitrogen, carbon, silicon, phosphorous, sulfur, tellurium, selenium, boron, and the like, which when present in a molten bath form reaction products. In the case of oxygen, the reaction product may be a metal oxide, such as A1 0 or by reaction with silicon, a metal silicate (e.g. MnO.SO In the case of nitrogen, the reaction product may be a nitride, or, if carbon is also present, a cyano-nitride. In the case of carbon, the reaction product may be a dendritically dispersed carbide. With respect to phosphorous and sulfur, the reaction products in the metal bath on cooling may be a metal phosphide or sulfide, respectively. Phase-forming constituents which behave like sulfur are selenium and tellurium.

Other phase-forming constituents include those constituents which form intermetallic compounds. In the case of nickel-base alloys containing aluminum, intermetallic compounds which tend to form, depending upon the amount of aluminum present, are Ni Al, NiAl, etc. These compounds have desirable properties and are hard and heat resistant. However, when large amounts of such compounds are present in segregated forms in a nickelbase alloy produced by conventional methods of melting and ingot casting, the alloy is diflicult to hot forge and tends to crack. The method of the invention is particularly applicable for producing alloys containing such compounds which tend to segregate and/or form coarse particles or phases by conventional methods of production. Examples of other intermetallic compounds which can be refined in size and be uniformly dispersed throughout a metal matrix utilizing the advantages of the invention are: a nickel-base alloy containing large amounts of columbium, e.g. about 2% to 6% Cb, which may form the compound Ni Cb; a nickel-base alloy containing titanium and aluminum which may contain the compound Ni (Al, Ti); a cobalt-base alloy containing about 6% to 8% W which may form CoW a cobalt-base alloy containing about 1% to 4% Zr which may form Co Zr; a copper-base alloy containing Be which forms Cu Be or even brittle phaseforrning constituents such as arsenic; and a nickel alloy containing, for example, 2% Hf or Zr which forms Ni Hf or Ni Zr, etc.

The invention is also applicable to the production of complex alloy tool steels containing substantial amounts of carbon, such as the 18-4-1 (W-Cr-V) variety containing upwards of about 1% carbon or more. Such steels when cast as an ingot by normal methods tend to form large dendritic carbides and carbide aggregates and segregates which are diflicult to remove by long time high temperature soaking, and, consequently, such steels are dilficult to forge. Because of the amount of carbon which is usually present, such steels: may contain up to about 25 to 30% by volume of metal carbides as a reaction product which generally form coarse dendritic structures. Further, because of the vastly improved hot workability of the refined powder structures, alloys such as 18-4-1 high speed steels can contain larger amounts of carbon (1.25%) and carbide-forming elements, such as 20% vs.-18% .W, 6% vs. 4% Cr and 2 or 3% vs. 1% V, and still be hot workable; except that now the alloy would be both harder and stronger and be a 'better machining tool steel.

-It is advantageous in producing free-machining steels and non-ferrous alloys, that the metals contain strong sulfide formers. Examples of strong sulfide formers are manganese, chromium, cerium, lanthanum and other rare earths and similar sulfide formers.

Likewise, in producing steels and alloys containing substantial amounts of carbon, it is advantageous in carrying out the invention that the steels and alloys contain strong carbide formers, such as tungsten, chromium, molybdenum, titanium, columbium, vanadium, tantalum, zirconium, hafnium, etc. As stated hereinbefore, in producing free-machining steels and alloys containing S, Se or Te, these elements may range from about. 0.1 to 1% by weight. In the case of carbon-containing steels and alloys, the amount of carbon may be sufficient to provide an amount of 'finely dispersed metal carbide particles ranging from about 5 to about 50% by volume. The same is true for alloys containing other'kinds of hard phases in amounts ranging up to about 50% by volume or higher.

In producing metals and alloys in. accordance with the invention, metal powders are advantageously employed which are produced by atomization from a molten metal bath, the atomized particles or droplets being rapidly solidified, and then advantageously rapidly quenched to low temperatures to avoid coarse particles precipitation and/or growth. As the liquid metal particles are pro duced, they are delivered to a quenching medium, such as refrigerated air, nitrogen or argon and, more advantageously, wet steam, water, brine or even a cold metal substrate of high heat conductivity metal, such as copper, silver, steel and the like. The rate of cooling to achieve a fine dendritic spacing of the phases should be at least about C./sec. and, where cooling on a metal substrate is employed, range up to about 10 or 10 C./sec. With regard to the latter, the high rate of cooling is achieved by projecting the finely divided liquid droplets of metal at high velocity against the metal substrate.

One of the advantages of rapid quenching is that there is a tendency to increase the solubility of solute elements in the matrix metal, of more uniformly distribting the solute element, thereby modifying the aging reaction of age-hardenable alloys, or to produce metastable compounds or unusual mixtures of crystals as compared to eutectic or peritectic phase distribution. Of great practical importance is the refining of the dendrite size and grainsize and of sharply decreasing segregation.

Atomization using wet steam as the cooling agent has been found advantageous in producing cooling rates of at least about 100 C./sec., particularly in producing coarse metal powders. Thus, particle sizes of up to 3 to 5 mm. in diameter can be produced using a flow of pulsed wet steam as the source of atomization, that is, steam which is controlled to condense rapidly in the vicinity of the metal stream, whereby the stream is shattered into substantially uniform droplets. By using wet, low temperature steam atomization followed by water quenching, the dendrite size in the solidified particle is small, with oxide and sulfide inclusions maintained at a size of less than two microns and about one micron or finer. Particles of 18/8 stainless steel (about 18% Cr, about 8% Ni and the balance substantially iron) have been produced containing about 0.4% and 0.6% sulfur successfiully and easily. Hot extrusions of these high sulfur steels were completely crack free. The size of the dendrites was less than about 10 microns and the sulfide particles were unusually fine and uniformly distributed. Unlike the sulfides obtained by ordinary casting methods which generally appear as large globules at grain boundaries and develop into stringers during hot working, the sulfide particles were small, isolated from each other and well dispersed in the atomized powders.

As illustrative of the invention as applied to the production of free-machining, high sulfur stainless steel of the AISI 304 type, the following example is given:

EXAMPLE 1 A sulfur-containing coarse metal powder of type 304 stainless steel was produced by steam atomization in which a stream of liquid metal is disintegrated by pulsed blasts of wet, supersaturated, low temperature steam. The pulses are achieved by the continuous and rapid condensation of the steam as it passes through the cooler surrounding air whereby the metal stream is subdivided into metal droplets. The droplets thus formed are solidified rapidly by the surrounding wet stream at a cooling rate of at least 100 C./sec. and then fall into a bath of cold water. The particles consequently have a very fine dendritic structure. The powder particles had shapes ranging from tear drops to substantially spheroidal as shown in FIG. 1 which is a representation of the particles taken at a magnification of 5 times diameter. The composition of the powder was as follows: 16.28% Cr, 9.26% Ni, 0.05% Mo, 0.39% S, 0.05% C and the balance essentially iron. The powder was mounted in plastic, polished and examined in the unetched condition at 100 times diameter (note FIG. 2).

Prior to extrusion, the coarse stainless steel powder was cleaned by self-milling followed by acid leaching to remove any oxide film. Self-milling is particularly advantageous when applied to rounded coarse powders ranging from about 0.4 to 5 mm. in diameter. About 5 pounds of powder were packed in a mild steel can having an outside diameter of about 3.5 inches and a length of 10 inches. The powder in the can was reduced with hydrogen at about 1850 F. and then sealed at about 1830 F. after being evacuated to a pressure of 10- millimeters of mercury. The can was extruded at a temperature of 2000 F. at a reduction rate of 10 to 1. Despite the high sulfur content, the composition extruded very well. The steel skin remaining from the case was removed from the extruded product by acid leaching.

The extruded bar was cold worked by swaging to a smaller diameter. Samples of both the swaged and unswaged bars were mounted for metallographic examination and examined for grain size and inclusions. .Also, samples of both the swaged and unswaged bars were subjected to a one hour heat treatment of 1950 F., water quenched, and examined metallographically. Physical properties were obtained on the samples.

Metallographic examination showed very uniform dispersion of the sulfide particles. Etching of the samples with Marbles Reagent revealed a fine dendritic structure .with a dentrite spacing of about 6. microns. Room temperature tensile tests, using a constant strain rate of 0.2 inch per minute, were performed on specimens of one inch gauge length and 0.250 inch diameter machined from extruded bars. The cold worked stainless steel bars exhibited a yield stress of about 191,000 p.s.i., an ultimate stress of about 192,500 p.s.i., an elongation of about 12.5% and a reduction in area of about 24.8%. On the other hand, the cold worked bars annealed at 1950 F. for one hour exhibited a yield stress of about 80,750 p.s.i., an ultimate stress of about 120,250 p.s.i., an elongation of about 43.6% and a reduction in. area of about 38.1%.

An ingot cast type 304 stainless, without sulfur additions, exhibited a yield stress of only. 38,000 p.s.i., a lower ultimate stress of 85,000 p.s.i., elongation of 63% and a reduction area of 76%. Had a sulfur content of about 0.25% been added and the ingot then produced by conventional casting, the strength and ductility would have been much lower.

As will be noted, the mechanical properties show that the fine sulfide distribution resulted in a large improvement in strength (dispersion strengthening) contrary to what might be expected. Thus, the annealed bar gave a yield stress of about. 80,750 p.s.i. compared to the much lower figure of 38,000 p.s.i. for Type 304 produced from a conventionally cast ingot. In addition, the ultimate stress of high sulfur material is 35,000 p.s.i. greater than that of the ingot cast material. The important feature of the material produced by the invention is that even with the high sulfur content, high strength and good ductility were obtained and, in addition, the material was readily cold workable, achieving as much as 70% reduction of area without intermediate anneals.

The foregoing alloy machines extremely well; the chip is fine and well fragmented and is friable. The machineability of the high sulfur steel compares very favorably with free-machining brass, one of the best machining alloys known. Further, annealing treatments up to 2150 F. for 1 hour failed to produce grain coarsening, due to the blocking effect of the fine sulfides on grain boundary migration. The retention of a fine grain size is of particular importance in connection with possible loss of properties during any form of welding.

EXAMPLE 2 The procedure of Example 1 was employed in producing another sulfur-containing Type 304 stainless steel comprising 16.16% Cr, 9.31% Ni, 0.58% S, 0.08% C and the balance substantially iron. The metal powder had substantially the same shape and size as that shown in FIG. 1. Particles of the foregoing composition exhibited very closed spaced dendrites as depicted in FIG. 3 which is an etch metallographic representation of an atomized particle (Marbles Reagent) taken at 500 times diameter, the dendrite spacing being about 6 microns, and the sulfide particles being about 1 micron in average size, ranging from about 0.5 to 2 microns and, therefore, of great uniformity.

As in Example 1, the atomized particles were canned and hot extruded into rods at 2000" F., specimens of the hot extruded rods being thereafter cold swaged, with a portion of the cold swaged rods annealed for one hour at 1950 F. The cold swaged rods exhibited a yield stress of- 162,000 p.s.i., an ultimate stress of 163,000 p.s.i., an elongation of about 15.5% and a reduction of area of 37.2%. After annealing, the yield stress was 73,900 p.s.i., the ultimate stress 114,700 p.s.i., the elongation 44.7% and the reduction of area 33.7%. As in Example 1, it was apparent that the finely dispersed sulfide particles had a marked dispersion strengthening effect on the composition, the wrought Type 304 steel (produced from the ingot) exhibiting in the annealed condition a yield stress of 38,000 p.s.i. and an ultimate stress of 85,000 p.s.i. As in Example 1, the alloy produced in accordance with the invention exhibited good machinability.

It is apparent from Examples 1 and 2 that it is possible to work at much higher levels of sulfur with significant increases in strengths and without important losses in ductility compared to the properties of wrought materials made from ingot cast materials. The results indicate that coarse particles can be employed with greatly enhanced beneficial effects so long as the atomized powders are produced by cooling rapidly at a rate of at least about 100 C./sec.

EXAMPLE 3 An AISI Type 4140 chromium-molybdenum-carbon steel powder was provided in the rapidly cooled state in accordance with Example 1, the average particle size of the particles ranging from about 0.5 to 4 mm. The average composition was 1.07% Cr, 0.23% Mo, 0.46% C, 0.45% Mn, 0.51% Si and the balance substantially iron. The powder which had a shape ranging from substantially tear drop to spheroidal was cleaned by self-milling and canned in a mild steel can as described hereinbefore in Example 1 and the canned powder extruded at a temperature of 1900 F. at an extrusion ratio of 12 to 1.

The extruded steel was austenitized for one hour at 1550 F. and oil quenched followed by tempering for one hour at 600 F. and air cooling to room temperature. The Rockwell C hardness was measured in the transverse direction in the as-quenched and as-tempered condition. The bars were machined into test specimens as described in Example 1. The as-quenched hardness for the extruded bar was about 54 R and the as-tempered hardness about 52 R These hardnesses are comparable with those of a rod produced from ingot cast AISI 4140 which exhibited an as-quenched hardness of about 57 R and a tempered hardness (600 F.) of 50 R Metallographic examination of the steel produced in accordance with the invention showed in the unetched condition at 1000 times magnification a fine dispersion of oxide inclusions of average size ranging up to one micron at an average interparticle spacing exceeding 10 microns. In a conventionally cast ingot of the same composition, the oxide inclusions tend to be very much larger and tend to segregate during solidfication of the ingot, leading to oxide seams and stringers and resulting in poor transverse properties, poor fatigue, etc.

The material of the invention at a Rockwell C hardness of 52 exhibited a yield stress under the same testing conditions of Example 1 of 262,000 p.s.i. and an ultimate stress of 274,000 p.s.i. By comparison, the material having the same hardness produced from ingot cast AISI 4140 exhibited a lower yield stress of 233,000 p.s.i. and a lower ultimate stress of 255,000 p.s.i. While the percent elongation of both materials was substantially the same, the steel of the invention exhibited a lower reduction of area.

EXAMPLE 4 In producing a nickel-chromium alloy containing by weight 76% Ni, Cr, 3.4% Tiand 0.6% C, a bath is established which is wet steamed atomized into particles 8 of about 0.5 to 4 mm. in diameter and then immediately water quenched to room temperature. The product is selfmilled within a rotating cylinder to remove fine surface oxides, cleaned in acid, packed in a mild steel can, .evacu ated and hermetically sealed, heated to 1900 F. and then extruded at a ratio of 16 to 1. Extrusions produced' in accordance with the foregoing exhibited sub-micron particles of TiC uniformly dispersed in an austenitic matrix; the interparticle spacing being less than one micron. The room temperature yield strength was 160,000 p.s.i., an unusually high value of strength for such a simple compo*' sition, with a ductility of about 14% elongation. The creep properties at 1500 F. for 100 hour life was about 25,000 p.s.i. which is considered quite high for a composition ofthis type.

EXAMPLE 5 An experimental alloy containing by weight 78% Ni,- 20% Cr, 2% Zr was induction melted, wet steam atomized to produce substantially spheroidal coarse powder, and immediately water quenched to room temperature. The selected powder fraction was 0.4 to 4 mm. in diameter, was self-milled to remove surface oxides and then acid cleaned. The powders were packed into a mild steel can, evacuated and sealed. The can assembly was heated to 1800 F. and extruded at a ratio of 16 to 1. As a result of the rapid quench, which was about 1000 C./see., the intermetallic phase Ni Zr was of sub-micron size and dispersed on a sub-micron scale. The resultant properties were excellent: 165,000 p.s.i. yield strength, 170,000 p.s.i. ultimate strength and 12% elongation. In short time creep tests at 1500 F., the alloy exhibited a stress of 22,000 p.s.i. for a 100 hour life.

EXAMPLE 6 A modified high speed tool steel composition by weight of 20% W, 14% Co, 4% Cr, 2% V and 1.3% C and the balanceiron was induction melted, then rapidly wet steam atomized and immediately further quenched in cold water to produce a yield of 80% of useful powder in the size range of 0.4 to 4 mm. The powders were self-milled, then placed in a mild steel can which was sealed, leaving a tubular means for flushing out the container. The can was filled with hydrogen, heated to 2000 F. and the tubular means sealed off by weld-crimping. This assured the reduc.- tion of any oxides on the surface. The can was held. at temperature for about 20 minutes to allow carbon to diffuse to the outer layers of the powders. The material was further heated to 2100" F. and extruded at a reduc tion ratio of 12 to 1.

In an unetched polished section, the oxides were less than about 1 micron in diameter and well scattered; the sulfides were less than 1 micron and also well scattered. After heat treatment, an unusually fine carbide dispersion was observed, with no carbide being coarser than 2 microns and all of extreme uniformity. The hardness was 67 to 68 Rockwell C.

As stated hereinbefore, the invention enables the provision of hot workable free-machining alloys containing about 0.1% to 1% by weight of sulfur, and, more advantageously, 0.2% to 0.6% S. Other free-machining aids, such as Se and Te, can be added in similar amounts. Examples of alloys capable of being made free-machining-by means of the invention are plain carbon steels, low alloy steels, high alloy steels, tool steels, hot work diesteels such as those referred to in the trade as 4130, 52100, and chromium-molybdenum steels such as 5% Cr, 1% Mo, 0.55% V, 0.5% carbon and the balance iron. Other steels include high carbon, high chromium alloy steels referred to as Types 430 (AISI D2), 431 (AISI D4), 432 (AISI D3), 433 (AISI D6), 434 (AISI D5) and 435 (AISI D1); the high speed steels of the 18-4-1 (W-Cr-V) variety and similar steels; ferritic stainless steels referred to as Types 410, 417, 430, 427, etc.; and austenitic 'stainless steels bearing the type designations 301, 302, 303, 304, 321, 347, 316, 317, 309 and 320.

Precipitation hardening steels may also be rendered free machining, such as those referred to as 17-7 PH, and the like. Examples of high temperature alloys which may be made free machining are the well known heat resistant metals of the iron-group nickel, cobalt and iron containing one or more of the metals chromium, tungsten and molybdenum with or without age hardening elements, such as titanium, aluminum, zirconium, columbium and the like. Generally speaking, the foregoing steels and alloys include those having a melting point of at least about 1000 C.

Stating it broadly, the element from the group S, Se and Te may be added to a broad range of steels and alloys falling within the following composition range: up to about 30% of at least one metal from the group consisting of chromium, tungsten and molybdenum, the total amount of these metals not exceeding about 40%, up to about 2% carbon, up to about columbium, up to about 10% tantalum, up to about 6% vanadium, up to about 14% manganese, up to about 6%v silicon, up to about 0.4% boron, up to about 4% zirconium and substantially the balance at least about 40% of one or more metals from the group iron, nickel and cobalt. As will be apparent, the foregoing range of compositions includes low, medium and high alloy steels, high carbon steels, high speed steels, ferritic stainless steels, austenitic stainless steels, nickel-and-cobalt-base high temperature alloys and many other alloy compositions. As is further apparent, the foregoing composition range includes rather complex alloys which are normally difficult to work, or not workable at all, but which are capable of being worked when-produced in the form of rapidly cooled atomized powders at cooling rates of at least about 100 C./ sec.

The average particle size of the atomized powder may range from about 0.2 m. to as high as 5 mm. Coarse particles of particle size ranging from about 0.4 m. to 4 mm. are particularly advantageous as such powders are easy to handle without contamination and, moreover, have good packing density.

Hot extrusion is preferred as the method of fabrication by packing the powder in a mild steel or stainless steel can hermetically sealed against atmospheric contamination. Depending upon the situation, the extrusion pressure may range from about 50 to 250 p.s.i. over a temperature 10 range of about 800 C. to 1300 C. at extrusion ratios of about 8 to 1 to 30 to 1.

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.

What is claimed is:

1. Rapidly cooled hot workable atomized coarse particles of stainless steel of average particle of about 0.2 to 5 mm. characterized by a fine cast dendritic structure of a disperse phase selected from the group consisting of metal sulfides, tellurides and selendides corresponding by weight to about 0.1 to 1% of an element selected from the group consisting of sulfur, tellurium and selenium, the average size of the dendrites being less than about 10 microns, the average size of the disperse phase being less than 5 microns, said coarse stainless particles being further characterized in that hot consolidated into a wrought stainless product, said wrought product has improved machinability and markedly improved strength properties due to the dispersion effect of the disperse phase.

2. The atomized coarse particles of claim 1, wherein the disperse phase is a sulfide based on a sulfur content of about 0.2% to 0.6% by weight, and wherein the average size of the disperse sulfide phase ranges up to about 2 microns.

References Cited UNITED STATES PATENTS 2,372,696 4/ 1945 Tholand -211 3,150,444 9/1964 Reen 29-4205 3,244,506 4/1966 Reen 75.5 3,502,463 3/1970 Holtz, Jr. 75--.5 3,502,464 3/1970 Holtz, Jr. 75-.5

L. DEWAYNE RUT'LEDGE, Frimary Examiner W. W. STALLARD, Assistant Examiner 1 US. Cl. X.R.

Images