Title:
ATOMIZED STEEL POWDER HAVING IMPROVED HARDENABILITY
United States Patent 3725142
Abstract:
Atomized steel powder having improved hardenability by the addition of boron. The powder is produced by atomizing a molten stream of steel containing 0.06 to 0.12 percent carbon and 0.005 to 0.100 percent boron. Following the atomization, the particles are annealed at a temperature of about 1500° to 2100° F. to soften the steel, as well as reducing the carbon content to a value below 0.05 percent. The annealed steel contains about 0.001 to 0.012 percent boron. After annealing, the cake-like structure is broken up to restore the as-atomized particle size and the particles are subsequently compacted into the shape of the desired part, sintered and heat treated to develop the desired hardness. The addition of boron aids in improving the hardenability of the part during heat treatment.
US Patent References:
Steel for plastic mold dies and articles made therefrom
Payson - August 1949 - 2478420
Boron-treated molybdenum steel
Bardgett - July 1950 - 2513395
Wear resistant coating composition on a valve for internal combution engine
Rowady - September 1966 - 3275426
Method of making metal powder
Huseby - June 1967 - 3325277
METHOD OF MAKING STEEL POWDER
Huseby et al. - September 1970 - 3528081
Steel for plastic mold dies and articles made therefrom
Payson - August 1949 - 2478420
Boron-treated molybdenum steel
Bardgett - July 1950 - 2513395
Wear resistant coating composition on a valve for internal combution engine
Rowady - September 1966 - 3275426
Method of making metal powder
Huseby - June 1967 - 3325277
METHOD OF MAKING STEEL POWDER
Huseby et al. - September 1970 - 3528081
Application Number:
05/174159
Publication Date:
04/03/1973
Filing Date:
08/23/1971
View Patent Images:
Export Citation:
Assignee:
A. O. Smith-Inland Inc. (Milwaukee, WI)
Primary Class:
Other Classes:
420/106, 75/337, 75/244, 420/121, 419/12, 75/246
International Classes:
C22C33/02; B22F1/00; B22F3/24
Field of Search:
75/202,200,201,123,128,211 29/182,182.5 148/16,31,36,126
Primary Examiner:
Lovell, Charles N.
Claims:
I claim
1. Finely divided annealed steel powder having improved hardenability and to be used in powder metallurgy processes, comprising a plurality of agglomerated steel particles, said steel consisting essentially of .001 to .020% by weight of carbon, 0.001 to 0.012 percent by weight of boron, less than 0.8 percent manganese, less than 0.1 percent silicon, less than 0.01 percent aluminum, and the balance iron said steel particles having improved hardenability when subsequently heat treated after compaction and sintering.
2. Finely divided annealed steel powder having improved hardenability to be used in powder metallurgy processes, comprising a plurality of agglomerated steel particles consisting essentially by weight of 0.001 to 0.020 percent carbon; an element selected from the group consisting of 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0 percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese; 0.01 to 0.10 percent silicon; 0.001 to 0.002 percent boron; and the balance iron; said particles having improved hardenability on heat treatment after compaction and sintering.
3. The steel powder of claim 2, wherein said annealed steel has an oxygen content less than 0.40 percent by weight.
4. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of atomizing a stream of molten steel consisting essentially of 0.06 to 0.12 percent by weight of carbon, and 0.005 to 0.100 percent by weight of boron, less than 0.8 percent manganese, less than 0.1 percent Si, and the balance iron to thereby provide a plurality of agglomerated steel particles, annealing the agglomerated particles in a reducing atmosphere at a temperature of 1500° to 2100° F. for a period of time to soften the particles and reduce the carbon content to a value less than 0.05 percent by weight and reduce the oxygen content to a value less than 0.40 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part at a temperature in the range of 2000° to 2300° F., heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
5. The method of claim 4, wherein the annealed particles have a boron content in the range of 0.001 to 0.012 percent by weight.
6. The method of claim 4, wherein the steel consists essentially by weight of 0.001 to 0.020 percent carbon; an element selected from the group consisting of 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0 percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese; 0.01 to 0.10 percent silicon; 0.001 to 0.002 percent boron; and the balance iron.
7. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of forming a steel melt consisting essentially of from 0.06 to 0.12 percent by weight of carbon, less than 0.8 percent manganese, less than 0.1 percent silicon and the balance iron, adding a deoxidizing agent to only a portion of the melt to deoxidize said portion, said portion comprising from 5 to 25 percent by weight of the melt, adding boron to the deoxidized portion of the melt in an amount of 0.005 to 0.100 percent by weight of the entire melt and effecting solution of said boron in said portion, adding the remaining portion of the melt to said deoxidized portion to provide a blended melt, atomizing the blended melt to provide a plurality of agglomerated steel particles containing from 0.001 to 0.012 percent by weight of boron, annealing the particles to soften the particles and reduce the carbon content to a value in the range of 0.001 to 0.050 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part, heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
8. The method of claim 7, in which the deoxidizing agent is aluminum and is added to said portion of the melt in an amount sufficient to substantially completely deoxidize said portion, said aluminum comprising less than 0.010 percent by weight of the steel particles.
9. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of forming a steel melt consisting essentially of from 0.06 to 0.12 percent by weight of carbon, less than 0.8 percent Mn, less than 0.1 percent silicon, and the balance iron, adding a calcium alloy to the melt in an amount sufficient to deoxidize the melt, adding boron to the melt in an amount sufficient to provide an elemental boron content of 0.001 to 0.012 percent by weight of the melt, atomizing a stream of the melt to thereby provide a plurality of agglomerated steel particles, annealing the agglomerated particles in a reducing atmosphere at a temperature of 1500° F. for a period of time to soften the particles and reduce the carbon content to a value less than 0.05 percent by weight and reduce the oxygen content to a value less than 0.40 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part, heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
10. The method of claim 9, wherein the calcium alloy is selected from the group consisting of calcium-silicon, calcium-manganese-silicon, and mixtures thereof.
11. The method of claim 9, wherein the calcium alloy is selected from the group consisting by weight of (a) 60 to 65 percent silicon, 30 to 33 percent calcium and 1.5 to 3 percent iron, (b) 16 to 20 percent calcium, 14 to 18 percent manganese and 54 to 59 percent silicon, and (c) mixtures of (a) and (b).
1. Finely divided annealed steel powder having improved hardenability and to be used in powder metallurgy processes, comprising a plurality of agglomerated steel particles, said steel consisting essentially of .001 to .020% by weight of carbon, 0.001 to 0.012 percent by weight of boron, less than 0.8 percent manganese, less than 0.1 percent silicon, less than 0.01 percent aluminum, and the balance iron said steel particles having improved hardenability when subsequently heat treated after compaction and sintering.
2. Finely divided annealed steel powder having improved hardenability to be used in powder metallurgy processes, comprising a plurality of agglomerated steel particles consisting essentially by weight of 0.001 to 0.020 percent carbon; an element selected from the group consisting of 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0 percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese; 0.01 to 0.10 percent silicon; 0.001 to 0.002 percent boron; and the balance iron; said particles having improved hardenability on heat treatment after compaction and sintering.
3. The steel powder of claim 2, wherein said annealed steel has an oxygen content less than 0.40 percent by weight.
4. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of atomizing a stream of molten steel consisting essentially of 0.06 to 0.12 percent by weight of carbon, and 0.005 to 0.100 percent by weight of boron, less than 0.8 percent manganese, less than 0.1 percent Si, and the balance iron to thereby provide a plurality of agglomerated steel particles, annealing the agglomerated particles in a reducing atmosphere at a temperature of 1500° to 2100° F. for a period of time to soften the particles and reduce the carbon content to a value less than 0.05 percent by weight and reduce the oxygen content to a value less than 0.40 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part at a temperature in the range of 2000° to 2300° F., heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
5. The method of claim 4, wherein the annealed particles have a boron content in the range of 0.001 to 0.012 percent by weight.
6. The method of claim 4, wherein the steel consists essentially by weight of 0.001 to 0.020 percent carbon; an element selected from the group consisting of 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium, 0.20 to 1.0 percent molybdenum and mixtures thereof; 0.01 to 0.80 percent manganese; 0.01 to 0.10 percent silicon; 0.001 to 0.002 percent boron; and the balance iron.
7. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of forming a steel melt consisting essentially of from 0.06 to 0.12 percent by weight of carbon, less than 0.8 percent manganese, less than 0.1 percent silicon and the balance iron, adding a deoxidizing agent to only a portion of the melt to deoxidize said portion, said portion comprising from 5 to 25 percent by weight of the melt, adding boron to the deoxidized portion of the melt in an amount of 0.005 to 0.100 percent by weight of the entire melt and effecting solution of said boron in said portion, adding the remaining portion of the melt to said deoxidized portion to provide a blended melt, atomizing the blended melt to provide a plurality of agglomerated steel particles containing from 0.001 to 0.012 percent by weight of boron, annealing the particles to soften the particles and reduce the carbon content to a value in the range of 0.001 to 0.050 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part, heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
8. The method of claim 7, in which the deoxidizing agent is aluminum and is added to said portion of the melt in an amount sufficient to substantially completely deoxidize said portion, said aluminum comprising less than 0.010 percent by weight of the steel particles.
9. A method of forming a steel part from a plurality of agglomerated steel particles, comprising the steps of forming a steel melt consisting essentially of from 0.06 to 0.12 percent by weight of carbon, less than 0.8 percent Mn, less than 0.1 percent silicon, and the balance iron, adding a calcium alloy to the melt in an amount sufficient to deoxidize the melt, adding boron to the melt in an amount sufficient to provide an elemental boron content of 0.001 to 0.012 percent by weight of the melt, atomizing a stream of the melt to thereby provide a plurality of agglomerated steel particles, annealing the agglomerated particles in a reducing atmosphere at a temperature of 1500° F. for a period of time to soften the particles and reduce the carbon content to a value less than 0.05 percent by weight and reduce the oxygen content to a value less than 0.40 percent by weight, compacting the annealed particles into the desired shape of a part, sintering the part, heating the sintered part to a temperature in the range of 1475° to 1850° F., quenching the part, and tempering the part at a temperature in the range of 300° to 1000° F., the addition of boron improving the hardenability of the steel during the heat treatment following the compaction and sintering.
10. The method of claim 9, wherein the calcium alloy is selected from the group consisting of calcium-silicon, calcium-manganese-silicon, and mixtures thereof.
11. The method of claim 9, wherein the calcium alloy is selected from the group consisting by weight of (a) 60 to 65 percent silicon, 30 to 33 percent calcium and 1.5 to 3 percent iron, (b) 16 to 20 percent calcium, 14 to 18 percent manganese and 54 to 59 percent silicon, and (c) mixtures of (a) and (b).
Description:
BACKGROUND OF THE INVENTION
Steel powder to be used in powder metallurgy processes can be prepared by a number of different procedures, such as electrolytic processes, reduction processes, or by air or by water atomization processes as described in the U.S. Pat. No. 3,325,277 of Robert A. Huseby. According to the process of that patent, molten steel is fed by gravity in the form of a downwardly moving stream and a series of flat sheets of water are impinged against the stream of molten steel at an angle to thereby atomize the stream and produce a plurality of agglomerates of spheroidal steel particles. Subsequently, the particles are annealed in a reducing atmosphere for a period of time sufficient to soften the particles and reduce the carbon content. Following the annealing, the particles are subjected to hammer-milling to break up the cake-like structure formed during the anneal and restore the as-atomized particle size.
The steel powder formed according to the method of the aforementioned patent after annealing has a low carbon content, generally less than 0.05 percent and a relative high oxygen content, up to 0.40 percent.
In the past, atomized steel powder was normally used to fabricate small parts of relatively light section and hardenability was not a problem. More recently, steel powder has been used to form larger parts of heavy section, and with the heavier sections, hardenability is increasingly important. Hardness is determined by heat treatment and hardenability is the ability for the hardness to penetrate through the section. Steel powder formed by an atomization process is extremely fine grained, and metallurgically, any fine grained structure has inherently poor hardenability.
It is recognized that the addition of boron to steel will improve the hardenability of the steel. It is also known that boron has deoxidizing and denitrifying characteristics in the melt, and because of this, the attainment of a substantial boron content in the steel is difficult, particularly where a specific boron content is desired. To prevent the loss of boron in the melt through deoxidizing and denitrifying actions, steel has generally been degasified before the addition of boron, as described in U.S. Pat. No. 2,823,299, by the addition of deoxidizing agents, such as aluminum, silicon and manganese. These metals react with the oxygen and nitrogen to remove the same so that the loss of boron in the melt will be minimized.
While the addition of deoxidizing agents such as aluminum, silicon and manganese to the melt has been successful in minimizing the loss of boron, this procedure has not been utilized in steel powder atomization processes, for the aluminum killed steel will freeze the tundish so that the molten steel will not properly flow through the outlets for the atomization, and the addition of silicon and manganese substantially reduces the mechanical properties of the resulting compacted powder. In view of this, and the fact that the steel to be atomized is a low-carbon, high-oxygen type, boron has not been considered a prospect for increasing the hardenability of steel powder.
SUMMARY OF THE INVENTION
The invention relates to an atomized steel powder which has increased hardenability due to the addition of boron. According to the invention, steel powder is produced by atomizing a molten stream of steel containing 0.06 to 0.12 percent carbon and 0.005 to 0.015 percent boron. Following the atomization, the resulting particles are annealed at a temperature of about 1500° to 2100° F to soften the steel and reduce the carbon content to a value in the range of about 0.01 to 0.05 percent. The annealed steel contains about 0.001 to 0.002 percent boron.
After annealing, the cake-like structure is broken up by hammer-milling to restore the as-atomized particle size, and the annealed particles are subsequently compacted into the desired shape of the part to be formed and sintered at a temperature in the range of 2000° to 2300° F. Following the sintering, the part is heat treated to develop the hardness by heating to a temperature in the range of about 1475° to 1650° F., quenching and subsequently tempering at a temperature in the range of 300° to 1000° F.
The addition of boron increases the hardenability of the sintered part, yet retains the fine grain structure of the particles, thereby increasing the mechanical properties of the heat treated part.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The steel to be used in the process of the invention can be produced by one of the conventional steel making processes such as open-hearth, electric furnace, basic oxygen or the like. The steel contains from .001 to 0.20 percent carbon, generally in the range of 0.06 to 0.12 percent carbon and preferably in the range of 0.03 to 0.08 percent carbon. In addition, the steel can contain one or more of the following elements: 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium and 0.20 to 1.0 percent molybdenum.
The silicon and manganese should be maintained below certain limits. The silicon content of the steel should be maintained less than 0.10 percent by weight and in the range of 0.01 to 0.10 percent, while the manganese content should be less than 0.30 percent by weight and in the range of 0.05 to 0.30 percent, but can be as high as 0.80 percent by weight when the alloy contains substantial additions of chromium, nickel and/or molybdenum.
The titanium content of the alloy should be less than 0.05 percent by weight, the sulfur and phosphorus should be less than 0.04 percent and 0.035 percent, respectively, and the aluminum content should be less than 0.010 percent and preferably less than 0.005 percent.
According to the invention, boron is added to the melt in an amount of 0.005 to 0.100 percent by weight and preferably in the range of 0.0075 to 0.0500 percent by weight. The boron is preferably added in the form of ferro-boron which contains about 20 percent by weight of boron. Elemental boron can be added to the melt, but generally the loss of boron will be considerably greater than when using ferro-boron. Moreover, ferro-boron is readily available and relatively inexpensive.
The steel in the melt has a relatively low carbon content and inherently has a relatively high percentage of oxygen. If boron was added to the low-carbon, high-oxygen melt, one would normally expect the boron to be completely lost due to its deoxidizing and denitrifying characteristics. As previously mentioned, the conventional procedure in the past has been to substantially completely kill the steel prior to the addition of boron by use of a deoxidizing agent, such as aluminum, silicon or manganese. While small amounts of aluminum, up to 0.010 percent by weight, can be included in the steel without adverse effect, completely killing the steel with aluminum cannot be tolerated where the steel is to be utilized in an atomization process because the aluminum killed steel will be sluggish and will tend to freeze the tundish so that the molten steel will not adequately flow through the outlet slots for the atomization procedure. Similarly, killing the steel with silicon and manganese cannot be tolerated because silicon and manganese, during atomization and the subsequent annealing treatments, form oxides which are extremely refractive and are difficult to reduce during the anneal. This results in the powder having a high oxide content in the form of oxide inclusions which reduces the ductility, impact strength and fatigue strength of the resulting compacted powder.
According to the process of the invention, about 5 to 25 percent by weight of the melt, prior to the addition of boron, is initially killed with aluminum to substantially remove all oxygen and nitrogen from that portion of the melt. The entire quantity of the boron to be included in the alloy is then added to the killed portion of steel, and after solution of the boron, the remaining unkilled portion of the melt is added. As the boron is in solution in the small portion of the killed steel, reaction of the boron with the oxygen or nitrogen in the unkilled portion of the steel will be minimized, with the result that a substantial portion of the boron will be retained in the melt. The small amount of aluminum required to kill the minor portion of the melt will not adversely affect the characteristics of the melt in the tundish so that the molten steel will satisfactorily flow from the tundish in the form of molten streams.
Alternately, the steel in the melt can be substantially completely killed prior to the addition of the boron by use of a calcium alloy, such as calcium-silicon, which contains approximately 30 to 33 percent by weight of calcium, 60 to 65 percent silicon and 1.5 to 3 percent iron, or calcium-manganese-silicon which contains 16 to 20 percent of calcium, 14 to 18 percent manganese and 54 to 59 percent silicon. Any excess calcium, as well as the resulting oxides, will go off in the slag. Calcium is a strong deoxidizer, and as only a very small amount of the calcium alloy is required for killing the steel, the silicon or manganese present in the calcium alloy will not significantly contribute to the overall silicon or manganese content of the melt, so that the mechanical properties of the resulting compacted powder will not be seriously decreased. In some cases, small amounts of aluminum, below the maximum content set forth previously, can be incorporated with the calcium alloy.
The steel powder can be produced by an apparatus similar to that shown in U.S. Pat. No. 3,325,277. The molten steel is contained in a tundish at a temperature of about 3100° F. and flows by gravity from the tundish through a series of outlet slots or nozzles. A thin sheet or curtain of water is directed against the stream of molten steel at an angle greater than 5° with respect to the axis of the stream and generally at an angle of 15° to 55° from the vertical.
The temperature of the water employed in the atomization process is not critical and is generally less than 160° F. The water is under substantial pressure, usually above 500 psi and for most operations, above 1000 psi. There is no maximum pressure limit for the water and normally the maximum pressure is based on the pumping equipment used. In the atomization, the water pressure is correlated to the angle at which the water sheets are directed against the molten metal stream. As the angle is decreased and approaches the vertical, the water pressure must correspondingly increase. Generally, the horizontal component of water velocity should be above 105 feet per second to produce the desired agglomerated type of particles.
The water is preferably in the form of thin sheets having a thickness less than 0.075 inch and preferably less than 0.05 inch at the point of discharge from the nozzle. The nozzles are designed with respect to the molten streams so that the sheets of water do not flair out to any appreciable extent but maintain the thickness when impinging against the molten steel stream.
The thin sheets of water strike the molten steel stream and atomize or particalize the steel to produce chain-like agglomerates of generally spheroidal particles. The steel powder as-atomized has a particle size such that at least 85 percent will pass through an 80 mesh sieve and at least 75 percent will pass through a 100 mesh sieve.
Following the atomization, the steel powder is subjected to an annealing treatment which serves to soften the particles, reduce the oxide film and substantially decrease the carbon content. During the anneal, the powder is heated to a temperature in the range of 1500° to 2100° F. and preferably 1650° to 1850° F. in a reducing atmosphere such as disassociated ammonia, hydrogen or other conventional decarburizing reducing gases.
During the annealing, the particles are softened, the carbon content is reduced and the oxygen content is reduced. The annealed particles have a carbon content below 0.05 percent by weight and preferably in the range of 0.001 percent to 0.020 percent. The annealed powder has an oxygen content less than 0.40 percent by weight and in most cases in the range of 0.01 to 0.25 percent. If chromium is not used in the alloy steel, or if the chromium content is in the lower portion of its aforementioned range, the oxygen content will generally be below 0.25 percent. If the chromium content is in the upper portion of its aforementioned range, the oxygen content may be above 0.25 percent but below 0.40 percent.
To obtain the optimum ductility and subsequently obtain the maximum density for a given compaction pressure and increased physical properties in the sintered product, the powder should be maintained at the annealing temperature for a period of at least 1 1/2 hours and preferably about 2 hours.
As previously mentioned, a portion of the boron added to the melt is lost, but the annealed powder will generally have a boron content in the range of 0.001 to 0.012 percent and preferably in the range of 0.002 to 0.005 percent.
Following the anneal, the particles are generally caked together and are broken apart by hammer-milling process. The hammer-milling which is an impact process breaks the sintered cake while not breaking up the irregular agglomerated nature of the particles and serves to restore the as-atomized particle size.
The annealed powder has an apparent density, which is a non-compacted density as defined by test procedure ASTMB-212-48, in the range of 2.6 to 3.3 grams/cc. The steel powder has a pressed density of over 6.4 grams/cc and generally in the range of 6.4 to 6.8 grams/cc. The pressed density is based on a compaction pressure of 30 tons per square inch, as defined in the test procedure ASTMB-331-58T, except that 0.5 percent dry zinc stearate lubricant was mixed with the powder.
The steel powder can be used to form various parts or combination of parts of both light and heavy section by conventional powder metallurgy procedures. A conventional lubricant such as zinc stearate and additional carbon, if desired, can be blended with the steel powder by suitable blending equipment. The powder is then compacted into the desired shape by a compaction pressure generally above 15 tons per square inch and preferably about 30 tons per square inch or more.
Following the compaction, the steel powder is sintered in a reducing atmosphere at a temperature in the range of 2000° F. to 2300° F. for a period of 10 minutes to 1 hour, depending on the composition and the final density desired.
After the sintering, the sintered part is subjected to a heat treatment to develop hardness throughout the section thickness and improve its physical properties. In the heat treatment, the part is initially heated to a temperature in the range of about 1475° to 1850° F. for a period of time sufficient to permit the entire depth of the section to be at temperature. The part is then quenched, either by water or oil, and subsequently tempered at a temperature of 300° to 1000° F. The quench develops the hardness in the part, while the tempering improves the elongation and impact strength.
The heat treated part can have a hardness up to about 65 Rickwell-C and the specific hardness is determined primarily by the carbon content of the steel. The addition of boron does not, in itself, increase the hardness of the part, but improves the hardenability which is the ability to harden throughout the entire section. The boron has the ability to increase the hardenability at low cost, as compared to other alloying elements.
The addition of boron to the steel powder provides a substantial increase in hardenability for the steel powder while maintaining the fine grained structure which is necessary for optimum mechanical properties. The boron addition is obtained through the invention without substantially completely killing the steel in the melt with aluminum and without any increase in the content of silicon and manganese which are detrimental to the properties of the steel powder.
The following examples illustrate the process of preparing the steel powder of the invention: Rockwell-C
EXAMPLE I
10 percent by weight of a steel melt having the following composition in weight per cent was supplied to the ladle:
Carbon 0.100 Manganese 0.150 Phosphorus 0.010 Sulfur 0.018 Silicon 0.022 Iron balance
Aluminum powder was added to the ladle in sufficient quantity to substantially kill the steel. Ferro-boron containing approximately 18 percent by weight of boron was then added to the ladle in an amount such that elemental boron composed 0.0050 percent of the total melt. After solution of the ferro-boron, the remaining 90 percent of the melt was added to the ladle.
The molten steel was then supplied to a tundish and with the temperature of the steel at approximately 3100° F., the steel flowed downwardly by gravity throughout outlet nozzles having an internal diameter of seven-sixteenths inch. Two oppositely directed streams or curtains of water positioned at a downward angle of 33° with respect to the axis of the molten steel streams impinged against the streams to atomize the steel. The temperature of the water was initially 68° F. and had a final temperature of 138° F. The water was under pressure of 1050 psi and at a flow rate of 860 gallons per minute. The water streams were discharged through slots 3 inches long and 0.04 inches wide.
The resulting as-atomized steel powder had the following Tyler screen analysis:
Tyler Screen Analysis - %
+80 -80 +100 +150 +200 +250 -325 +100 +150 +200 +250 +325 trace 2.6 16.8 26.8 5.7 23.3 24.8
The steel powder was then annealed in dissociated ammonia at a temperature of 1,700° F. for 2 hours, subsequently cooled in a dissociated ammonia atmosphere to 140° F. and then air cooled to toom temperature.
The annealed steel powder had the following analysis in weight per cent:
Carbon 0.014 Manganese 0.150 Phosphorus 0.010 Sulfur 0.012 Silicon 0.022 Oxygen 0.188 Boron 0.0016 Iron balance
The steel powder was then broken up by hammer-milling to as-atomized size and the powder had an apparent density of 3.08 grams per cc, a green density at a compaction pressure of 30 tons per square inch with 0.75 percent zinc stearate lubricant of 6.70 grams per cc and a green strength of 1500 per square inch with 0.75 percent zinc stearate lubricant after pressing at 30 tons per square inch.
A sample of the annealed steel powder was compacted into the shape of a test bar having dimensions of 0.25 × 0.50 × 1.25 inches at a pressure of 32 tons per square inch. The compressed part was then sintered at a temperature of 2050° F. for a period of 1/2 hour. Subsequently, the sintered part was heated to a temperature of 1650° F. for a period of one-half hour, quenched in oil and tempered at a temperature of 600° F. for 1 hour.
The resulting heat treated part had a hardness of 26 Rockwell-C which penetrated throughout the section thickness.
EXAMPLE II
Ten lbs. of aluminum and 18 lbs. of calcium silicon were added to 48,000 lbs. of molten alloy steel in the ladle and boron in the form of ferro-boron was added to the steel in the ladle in an amount such that the melt contained 0.0075 percent of elemental boron.
The final tap analysis of the steel was as follows in weight per cent:
Carbon 0.05 Manganese 0.37 Molybdenum 0.61 Nickel 0.51 Boron 0.0010 Silicon 0.012 Iron balance
The molten steel was then atomized, annealed and hammer-milled according to the procedure outlined in Example I. The annealed steel powder had the following analysis in weight per cent:
Carbon 0.01 Manganese 0.35 Nickel 0.51 Molybdenum 0.61 Silicon 0.02 Boron 0.0010 Oxygen 0.15 Iron balance
The annealed steel powder had an apparent density of 2.89 gr/cc, a green density at a compaction pressure of 30 tsi and with 0.75 percent zinc stearate lubricant of 6.74 gr/cc and a green strength of 1189 psi with a 0.75 percent addition of zinc stearate lubricant after pressing at 30 tsi.
Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.
Steel powder to be used in powder metallurgy processes can be prepared by a number of different procedures, such as electrolytic processes, reduction processes, or by air or by water atomization processes as described in the U.S. Pat. No. 3,325,277 of Robert A. Huseby. According to the process of that patent, molten steel is fed by gravity in the form of a downwardly moving stream and a series of flat sheets of water are impinged against the stream of molten steel at an angle to thereby atomize the stream and produce a plurality of agglomerates of spheroidal steel particles. Subsequently, the particles are annealed in a reducing atmosphere for a period of time sufficient to soften the particles and reduce the carbon content. Following the annealing, the particles are subjected to hammer-milling to break up the cake-like structure formed during the anneal and restore the as-atomized particle size.
The steel powder formed according to the method of the aforementioned patent after annealing has a low carbon content, generally less than 0.05 percent and a relative high oxygen content, up to 0.40 percent.
In the past, atomized steel powder was normally used to fabricate small parts of relatively light section and hardenability was not a problem. More recently, steel powder has been used to form larger parts of heavy section, and with the heavier sections, hardenability is increasingly important. Hardness is determined by heat treatment and hardenability is the ability for the hardness to penetrate through the section. Steel powder formed by an atomization process is extremely fine grained, and metallurgically, any fine grained structure has inherently poor hardenability.
It is recognized that the addition of boron to steel will improve the hardenability of the steel. It is also known that boron has deoxidizing and denitrifying characteristics in the melt, and because of this, the attainment of a substantial boron content in the steel is difficult, particularly where a specific boron content is desired. To prevent the loss of boron in the melt through deoxidizing and denitrifying actions, steel has generally been degasified before the addition of boron, as described in U.S. Pat. No. 2,823,299, by the addition of deoxidizing agents, such as aluminum, silicon and manganese. These metals react with the oxygen and nitrogen to remove the same so that the loss of boron in the melt will be minimized.
While the addition of deoxidizing agents such as aluminum, silicon and manganese to the melt has been successful in minimizing the loss of boron, this procedure has not been utilized in steel powder atomization processes, for the aluminum killed steel will freeze the tundish so that the molten steel will not properly flow through the outlets for the atomization, and the addition of silicon and manganese substantially reduces the mechanical properties of the resulting compacted powder. In view of this, and the fact that the steel to be atomized is a low-carbon, high-oxygen type, boron has not been considered a prospect for increasing the hardenability of steel powder.
SUMMARY OF THE INVENTION
The invention relates to an atomized steel powder which has increased hardenability due to the addition of boron. According to the invention, steel powder is produced by atomizing a molten stream of steel containing 0.06 to 0.12 percent carbon and 0.005 to 0.015 percent boron. Following the atomization, the resulting particles are annealed at a temperature of about 1500° to 2100° F to soften the steel and reduce the carbon content to a value in the range of about 0.01 to 0.05 percent. The annealed steel contains about 0.001 to 0.002 percent boron.
After annealing, the cake-like structure is broken up by hammer-milling to restore the as-atomized particle size, and the annealed particles are subsequently compacted into the desired shape of the part to be formed and sintered at a temperature in the range of 2000° to 2300° F. Following the sintering, the part is heat treated to develop the hardness by heating to a temperature in the range of about 1475° to 1650° F., quenching and subsequently tempering at a temperature in the range of 300° to 1000° F.
The addition of boron increases the hardenability of the sintered part, yet retains the fine grain structure of the particles, thereby increasing the mechanical properties of the heat treated part.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The steel to be used in the process of the invention can be produced by one of the conventional steel making processes such as open-hearth, electric furnace, basic oxygen or the like. The steel contains from .001 to 0.20 percent carbon, generally in the range of 0.06 to 0.12 percent carbon and preferably in the range of 0.03 to 0.08 percent carbon. In addition, the steel can contain one or more of the following elements: 0.20 to 3.0 percent nickel, 0.20 to 1.0 percent chromium and 0.20 to 1.0 percent molybdenum.
The silicon and manganese should be maintained below certain limits. The silicon content of the steel should be maintained less than 0.10 percent by weight and in the range of 0.01 to 0.10 percent, while the manganese content should be less than 0.30 percent by weight and in the range of 0.05 to 0.30 percent, but can be as high as 0.80 percent by weight when the alloy contains substantial additions of chromium, nickel and/or molybdenum.
The titanium content of the alloy should be less than 0.05 percent by weight, the sulfur and phosphorus should be less than 0.04 percent and 0.035 percent, respectively, and the aluminum content should be less than 0.010 percent and preferably less than 0.005 percent.
According to the invention, boron is added to the melt in an amount of 0.005 to 0.100 percent by weight and preferably in the range of 0.0075 to 0.0500 percent by weight. The boron is preferably added in the form of ferro-boron which contains about 20 percent by weight of boron. Elemental boron can be added to the melt, but generally the loss of boron will be considerably greater than when using ferro-boron. Moreover, ferro-boron is readily available and relatively inexpensive.
The steel in the melt has a relatively low carbon content and inherently has a relatively high percentage of oxygen. If boron was added to the low-carbon, high-oxygen melt, one would normally expect the boron to be completely lost due to its deoxidizing and denitrifying characteristics. As previously mentioned, the conventional procedure in the past has been to substantially completely kill the steel prior to the addition of boron by use of a deoxidizing agent, such as aluminum, silicon or manganese. While small amounts of aluminum, up to 0.010 percent by weight, can be included in the steel without adverse effect, completely killing the steel with aluminum cannot be tolerated where the steel is to be utilized in an atomization process because the aluminum killed steel will be sluggish and will tend to freeze the tundish so that the molten steel will not adequately flow through the outlet slots for the atomization procedure. Similarly, killing the steel with silicon and manganese cannot be tolerated because silicon and manganese, during atomization and the subsequent annealing treatments, form oxides which are extremely refractive and are difficult to reduce during the anneal. This results in the powder having a high oxide content in the form of oxide inclusions which reduces the ductility, impact strength and fatigue strength of the resulting compacted powder.
According to the process of the invention, about 5 to 25 percent by weight of the melt, prior to the addition of boron, is initially killed with aluminum to substantially remove all oxygen and nitrogen from that portion of the melt. The entire quantity of the boron to be included in the alloy is then added to the killed portion of steel, and after solution of the boron, the remaining unkilled portion of the melt is added. As the boron is in solution in the small portion of the killed steel, reaction of the boron with the oxygen or nitrogen in the unkilled portion of the steel will be minimized, with the result that a substantial portion of the boron will be retained in the melt. The small amount of aluminum required to kill the minor portion of the melt will not adversely affect the characteristics of the melt in the tundish so that the molten steel will satisfactorily flow from the tundish in the form of molten streams.
Alternately, the steel in the melt can be substantially completely killed prior to the addition of the boron by use of a calcium alloy, such as calcium-silicon, which contains approximately 30 to 33 percent by weight of calcium, 60 to 65 percent silicon and 1.5 to 3 percent iron, or calcium-manganese-silicon which contains 16 to 20 percent of calcium, 14 to 18 percent manganese and 54 to 59 percent silicon. Any excess calcium, as well as the resulting oxides, will go off in the slag. Calcium is a strong deoxidizer, and as only a very small amount of the calcium alloy is required for killing the steel, the silicon or manganese present in the calcium alloy will not significantly contribute to the overall silicon or manganese content of the melt, so that the mechanical properties of the resulting compacted powder will not be seriously decreased. In some cases, small amounts of aluminum, below the maximum content set forth previously, can be incorporated with the calcium alloy.
The steel powder can be produced by an apparatus similar to that shown in U.S. Pat. No. 3,325,277. The molten steel is contained in a tundish at a temperature of about 3100° F. and flows by gravity from the tundish through a series of outlet slots or nozzles. A thin sheet or curtain of water is directed against the stream of molten steel at an angle greater than 5° with respect to the axis of the stream and generally at an angle of 15° to 55° from the vertical.
The temperature of the water employed in the atomization process is not critical and is generally less than 160° F. The water is under substantial pressure, usually above 500 psi and for most operations, above 1000 psi. There is no maximum pressure limit for the water and normally the maximum pressure is based on the pumping equipment used. In the atomization, the water pressure is correlated to the angle at which the water sheets are directed against the molten metal stream. As the angle is decreased and approaches the vertical, the water pressure must correspondingly increase. Generally, the horizontal component of water velocity should be above 105 feet per second to produce the desired agglomerated type of particles.
The water is preferably in the form of thin sheets having a thickness less than 0.075 inch and preferably less than 0.05 inch at the point of discharge from the nozzle. The nozzles are designed with respect to the molten streams so that the sheets of water do not flair out to any appreciable extent but maintain the thickness when impinging against the molten steel stream.
The thin sheets of water strike the molten steel stream and atomize or particalize the steel to produce chain-like agglomerates of generally spheroidal particles. The steel powder as-atomized has a particle size such that at least 85 percent will pass through an 80 mesh sieve and at least 75 percent will pass through a 100 mesh sieve.
Following the atomization, the steel powder is subjected to an annealing treatment which serves to soften the particles, reduce the oxide film and substantially decrease the carbon content. During the anneal, the powder is heated to a temperature in the range of 1500° to 2100° F. and preferably 1650° to 1850° F. in a reducing atmosphere such as disassociated ammonia, hydrogen or other conventional decarburizing reducing gases.
During the annealing, the particles are softened, the carbon content is reduced and the oxygen content is reduced. The annealed particles have a carbon content below 0.05 percent by weight and preferably in the range of 0.001 percent to 0.020 percent. The annealed powder has an oxygen content less than 0.40 percent by weight and in most cases in the range of 0.01 to 0.25 percent. If chromium is not used in the alloy steel, or if the chromium content is in the lower portion of its aforementioned range, the oxygen content will generally be below 0.25 percent. If the chromium content is in the upper portion of its aforementioned range, the oxygen content may be above 0.25 percent but below 0.40 percent.
To obtain the optimum ductility and subsequently obtain the maximum density for a given compaction pressure and increased physical properties in the sintered product, the powder should be maintained at the annealing temperature for a period of at least 1 1/2 hours and preferably about 2 hours.
As previously mentioned, a portion of the boron added to the melt is lost, but the annealed powder will generally have a boron content in the range of 0.001 to 0.012 percent and preferably in the range of 0.002 to 0.005 percent.
Following the anneal, the particles are generally caked together and are broken apart by hammer-milling process. The hammer-milling which is an impact process breaks the sintered cake while not breaking up the irregular agglomerated nature of the particles and serves to restore the as-atomized particle size.
The annealed powder has an apparent density, which is a non-compacted density as defined by test procedure ASTMB-212-48, in the range of 2.6 to 3.3 grams/cc. The steel powder has a pressed density of over 6.4 grams/cc and generally in the range of 6.4 to 6.8 grams/cc. The pressed density is based on a compaction pressure of 30 tons per square inch, as defined in the test procedure ASTMB-331-58T, except that 0.5 percent dry zinc stearate lubricant was mixed with the powder.
The steel powder can be used to form various parts or combination of parts of both light and heavy section by conventional powder metallurgy procedures. A conventional lubricant such as zinc stearate and additional carbon, if desired, can be blended with the steel powder by suitable blending equipment. The powder is then compacted into the desired shape by a compaction pressure generally above 15 tons per square inch and preferably about 30 tons per square inch or more.
Following the compaction, the steel powder is sintered in a reducing atmosphere at a temperature in the range of 2000° F. to 2300° F. for a period of 10 minutes to 1 hour, depending on the composition and the final density desired.
After the sintering, the sintered part is subjected to a heat treatment to develop hardness throughout the section thickness and improve its physical properties. In the heat treatment, the part is initially heated to a temperature in the range of about 1475° to 1850° F. for a period of time sufficient to permit the entire depth of the section to be at temperature. The part is then quenched, either by water or oil, and subsequently tempered at a temperature of 300° to 1000° F. The quench develops the hardness in the part, while the tempering improves the elongation and impact strength.
The heat treated part can have a hardness up to about 65 Rickwell-C and the specific hardness is determined primarily by the carbon content of the steel. The addition of boron does not, in itself, increase the hardness of the part, but improves the hardenability which is the ability to harden throughout the entire section. The boron has the ability to increase the hardenability at low cost, as compared to other alloying elements.
The addition of boron to the steel powder provides a substantial increase in hardenability for the steel powder while maintaining the fine grained structure which is necessary for optimum mechanical properties. The boron addition is obtained through the invention without substantially completely killing the steel in the melt with aluminum and without any increase in the content of silicon and manganese which are detrimental to the properties of the steel powder.
The following examples illustrate the process of preparing the steel powder of the invention: Rockwell-C
EXAMPLE I
10 percent by weight of a steel melt having the following composition in weight per cent was supplied to the ladle:
Carbon 0.100 Manganese 0.150 Phosphorus 0.010 Sulfur 0.018 Silicon 0.022 Iron balance
Aluminum powder was added to the ladle in sufficient quantity to substantially kill the steel. Ferro-boron containing approximately 18 percent by weight of boron was then added to the ladle in an amount such that elemental boron composed 0.0050 percent of the total melt. After solution of the ferro-boron, the remaining 90 percent of the melt was added to the ladle.
The molten steel was then supplied to a tundish and with the temperature of the steel at approximately 3100° F., the steel flowed downwardly by gravity throughout outlet nozzles having an internal diameter of seven-sixteenths inch. Two oppositely directed streams or curtains of water positioned at a downward angle of 33° with respect to the axis of the molten steel streams impinged against the streams to atomize the steel. The temperature of the water was initially 68° F. and had a final temperature of 138° F. The water was under pressure of 1050 psi and at a flow rate of 860 gallons per minute. The water streams were discharged through slots 3 inches long and 0.04 inches wide.
The resulting as-atomized steel powder had the following Tyler screen analysis:
Tyler Screen Analysis - %
+80 -80 +100 +150 +200 +250 -325 +100 +150 +200 +250 +325 trace 2.6 16.8 26.8 5.7 23.3 24.8
The steel powder was then annealed in dissociated ammonia at a temperature of 1,700° F. for 2 hours, subsequently cooled in a dissociated ammonia atmosphere to 140° F. and then air cooled to toom temperature.
The annealed steel powder had the following analysis in weight per cent:
Carbon 0.014 Manganese 0.150 Phosphorus 0.010 Sulfur 0.012 Silicon 0.022 Oxygen 0.188 Boron 0.0016 Iron balance
The steel powder was then broken up by hammer-milling to as-atomized size and the powder had an apparent density of 3.08 grams per cc, a green density at a compaction pressure of 30 tons per square inch with 0.75 percent zinc stearate lubricant of 6.70 grams per cc and a green strength of 1500 per square inch with 0.75 percent zinc stearate lubricant after pressing at 30 tons per square inch.
A sample of the annealed steel powder was compacted into the shape of a test bar having dimensions of 0.25 × 0.50 × 1.25 inches at a pressure of 32 tons per square inch. The compressed part was then sintered at a temperature of 2050° F. for a period of 1/2 hour. Subsequently, the sintered part was heated to a temperature of 1650° F. for a period of one-half hour, quenched in oil and tempered at a temperature of 600° F. for 1 hour.
The resulting heat treated part had a hardness of 26 Rockwell-C which penetrated throughout the section thickness.
EXAMPLE II
Ten lbs. of aluminum and 18 lbs. of calcium silicon were added to 48,000 lbs. of molten alloy steel in the ladle and boron in the form of ferro-boron was added to the steel in the ladle in an amount such that the melt contained 0.0075 percent of elemental boron.
The final tap analysis of the steel was as follows in weight per cent:
Carbon 0.05 Manganese 0.37 Molybdenum 0.61 Nickel 0.51 Boron 0.0010 Silicon 0.012 Iron balance
The molten steel was then atomized, annealed and hammer-milled according to the procedure outlined in Example I. The annealed steel powder had the following analysis in weight per cent:
Carbon 0.01 Manganese 0.35 Nickel 0.51 Molybdenum 0.61 Silicon 0.02 Boron 0.0010 Oxygen 0.15 Iron balance
The annealed steel powder had an apparent density of 2.89 gr/cc, a green density at a compaction pressure of 30 tsi and with 0.75 percent zinc stearate lubricant of 6.74 gr/cc and a green strength of 1189 psi with a 0.75 percent addition of zinc stearate lubricant after pressing at 30 tsi.
Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.