MXPA98000397A - High density sintered alloy and method for the formation of pre-aleac powder spheres - Google Patents

Abstract

The invention relates to a process for forming a sintered metal powder article, comprising mixing graphite and lubricant with a pre-alloy iron-based powder, pressing the mixture to form in a single compacting step, sintering the article, and then at high temperature sintering the article in a reducing atmosphere to produce a sintered article of a density greater than 7-4 g /

Description

ALLOY SINTERIZADA DK HIGH DENSITY AND METHOD FOR THE FORMATION OF SPHERES FOR PRE-ALLOY POWDERS DESCRIPTION OF THE INVENTION The invention relates to a method or process for forming a sintered article of powdered metal having a high density and in particular it relates to a process for forming a sintered article from powdered metal, using a powder that is has previously formed by alloy, as the base material and adding graphite and other additives and then, sintering at high temperature, the article in a reducing atmosphere to produce sintered parts having high density, which then form spheres. In particular, this invention relates to a process for forming a high density metal powder article by mixing graphite with an iron-based powder and another pre-alloy containing molybdenum followed by heat treatment to form spheres the carbides, in the micro structure to produce an article that combines high mechanical strength and impact. Powdered metal technology is well known to the skilled artisan, and generally involves the formation of metal powders that are compacted and subjected to high temperature to produce a sintered object. Still further, US Patent 2,289,569 relates generally to powder metallurgy, and more particularly to a low melting point alloy powder and to the use of low melting point alloy powders in the formation of sintered articles. The modulus of elasticity and tightening or resistance of conventional metallic articles of dust, are limited by the density. Conventional processes are limited to approximately 7.2g / cc, in a single press with simple sintering techniques. With an increased cost, double pressing and double sintering can be used to increase the density. In addition several procedures have been designed with the aim of producing sintered articles having high densities. Such processes include double pressing, sintering for densities typically up to 7.5 g / cc, as well as hot powder forging, where densities up to 7.8 g / cc can be achieved virtually. However, such procedures are expensive and time consuming. Recently developed methods include the hot pressing of powders, up to 7.35g / cc, as presented in US Patent 5,154,881 (RUTZ), however, there are disadvantages with hot pressing, such as keeping tool separations with hot systems, so hot pressing does not allow very high densities up to and above 7.5g / cc, to be easily achieved in commonly used alloy systems without double pressing and double sintering.

However, another process is presented in US 2,027,763 which relates to a process for making sintered hard metal and basically consists of stages connected with the process for the production of hard metal. In particular, this patent 2,027,763 relates to a process for manufacturing sintered hard metal comprising producing a spray of a finely powdered dry blend of meltable metals, and an auxiliary metal easily meltable under high pressure producing a spray of customary bonding agent for joining metals hard under high stress, and thus direct the jets of the spray of the metallic powder and the jet of adhesive liquid that will be found in the molds or inside it, where the mold will be filled with a compact moist mass of metallic powder finally completing the Hard metallic particle thus formed by sintering. U.S. Patent 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering, by means of special additives which simultaneously serve as sintering aids and increase the ductility of the finished structural product. In addition, the US patent 4, 464, 206 relates to a method of setting powdered metal for a powder with prior alloy, in particular this patent teaches a process comprising the steps of commonly joining pre-compacted non-compactable metal powders to flatten the particles thereof, By heating the small particles of powdered metal to an elevated temperature and the particles adhering and forming a mass during heating, crush the mass of the metal powder to compact the crushed mass of metal powder, sinter the metal powder and work hot metal powder to get a forged product. Other methods for increasing the wear resistance or density of the sintered iron based alloys, are presented in U.S. Patent 5,151,247 which relates to a method for densifying the powder of the metallurgical parts, while the patent 4,885,133 refers to a process for producing sintered parts resistant to wear. The processes described in the prior art have serious cost disadvantages to effectively produce the desired mechanical properties of the sintered product. It is another object of the invention to provide a process for producing sintered articles of densities greater than 7.4 g / cc, by a single compacting process and a sintering process. It is an object of this invention to provide an improved process for producing sintered articles with improved dynamic strength characteristics and an exact method for controlling them.

It is another object of the invention to provide a process for producing sintered articles having improved strength characteristics with carbon contents above 0.8%, and in particular between 0.8%, to 2.0%, of carbon and an exact method to control them. . It is another aspect of the invention to produce PM articles with high ductility by forming spheres. Historically, steels have been produced with carbon contents of less than 0.8%, however ultra high carbon steels have been produced, these are carbon steels containing 0.8% to 2.0% carbon. The process for producing ultra high carbon steels with fine-sphere carbides is presented in US Patent 3,951,697, as well as in the article by D.R. Lesver, C.K. Syn, A. Goldberg, J. Wads orth and OD, Sherby, entitled "The Case for Ultrahigh-Carbon Steels as Structural Materials," which appeared in the Journal of the Minerals, Metals and Materials Soc., August 1993. The applicant has submitted an application No. PCT / CA 94/00065, on February 7, 1994, as well as an application for the United States 08 / 193,578, on February 8, 1994, for an invention entitled HIGH-DENSITY SINTERED ALLOY, which is referred to to the process of forming sintered metal powdered articles by mixing finely ground ferrous alloys with iron powders to produce sintered parts in a reducing atmosphere having a high density. It is an object of this invention to provide another improvement by producing high density sintered parts using a pre-alloyed powder as the base material and adding graphite. The broader aspect of this invention relates to a process for forming a sintered metal powder article comprising a mixture of graphite and lubricant with a pre-alloyed iron-based powder and pressing the mixture to form in a single step. of compacting the sintering of the article, and then, sintering at high temperature in a reduced atmosphere to produce a sintered article having a density greater than 7.4g / cc. In another aspect of the invention, to provide a process for forming a sintered metal powder article comprising mixing graphite at about 0.8 to 2.0% by weight, and lubricating with a pre-alloyed iron powder containing 0.5 to 3.0% , of molybdenum, pressing the mixture to form in a single compaction step, sintering the article and then sieving it at high temperature in a reducing atmosphere to produce a sintered article having a high density. It is another aspect to provide a powdered metal composition comprising a mixture of pre-alloy iron based powder and graphite to give a mass having between 0.5 to 3.0% molybdenum; 0.8 to 2.0% graphite, the rest being iron and unavoidable impurities. DESCRIPTION OF THE DRAWINGS These and other objects and features of the invention will be described in relation to the following drawings: FIGURE 1 is a graph of tensile elongation with respect to carbon as a percentage: forged steels; FIGURE 2, is a module to the density graph; FIGURE 3 is a diagram of the grain boundary carbides in a sintered article; FIGURE 4A is a schematic diagram of the stages of the high density metal powder process; FIGURE 4B is a schematic diagram of another embodiment of the steps of the high density metal powder process; FIGURE 5 is a planar top view of the connecting rod made in accordance with the invention; FIGURE 6 is a flow chart; FIGURE 7 illustrates the eutectoid portion of the Fe-Fe3C phase diagram; FIGURE 8 is a graph of the size distribution of the connecting rod: spherical parts of width with respect to the parts in coins or stamped. The invention presented uses a high temperature sintering from 1,250 to 1,350 * C, and a reducing atmosphere of for example, hydrogen, hydrogen / nitrogen or vacuum. In addition, the reducing atmosphere in combination with the high sintering temperature reduces or removes surface oxides, allowing the particles to form good bonds and for the compacted article to develop the appropriate strength. The lubricant is added in a known way to the technicians, to help in the bonding of the powder, as well as helping to expel the product after the pressing. Zinc stearate can be used as a lubricant, the article is formed by pressure molding the mixture using the appropriate pressure, for example, from 25 to 50 tons per square inch. The heat treatment steps can be carried out after the sintering step. Secondary operations, such as stamping, resizing, machining and the like, can be introduced after sintering. In addition, the micro structure of the finished products are improved because they have: a) high density b) well-rounded pores c) homogeneous structure d) carbides in finely dispersed spheres e) a product that is more similar to forged steels in their properties that the conventional powder metal steels STEEL WITH ULTRA HIGH CARBON Typically, the percentage of carbon in steel, is in the margin of up to 0.8%. Ultra high carbon steels contain 0.8% to 2% carbon. It is known that the tensile utility decreases dramatically with increasing carbon content, and therefore steels with an ultra-high amount of carbon have historically been considered too fragile to be widely used. Fig. 1 shows the relationship between elongation or ductility with respect to carbon content. It is apparent from Fig. 1 that the higher the carbon content, the less ductile the steel. But by reducing the carbon in the steels, their tensile strength is also reduced. However, using the thermal treatments suitable for ultra high carbon steels, high ductilities as well as high resistances can be obtained. The invention here described, comprises mixing graphite and lubricant with a pre-alloy iron-based powder, as described and illustrated in FIG. 6. An example of graphite used consists of Asbury grade 3203 but may include other grades of graphite. The previous alloy powder used herein consists of metallic powder composed of two or more elements that are subjected to alloying in the process of manufacturing the powder and in which the particles have the same nominal composition. The method described herein can be adapted to produce a high density powder sintered product having an ultra high carbon content with the following composition: Mo 0.5 - 3.0% C in the form of graphite 0.8 to 2.0% Fe and other impurities unavoidable, is the rest The graphite is mixed with the lubricant and with the powder of the previous alloy iron containing molybdenum and then it is compacted by conventional pressing methods to a minimum of 6.8g / cc. The sintering takes place in vacuum or in vacuum with partial filling (mixture of argon or nitrogen), or pure hydrogen or a mixture of hydrogen / nitrogen at a temperature of 1,250 to 1,350, and in particular 1,270 to 1,310. The vacuum is typically 200 microns. In addition, the compaction in a single step is preferably carried out between 6.8 g / cc, at 7.lg / cc. It has been found that by using the aforementioned composition, articles of a density greater than 7.4g / cc can be produced in a single compression step instead of by double pressing and double sintering. Using the invention, articles having a sintering density of 7.4 to 7.7g / cc can be produced. Such high density sintered articles can be used in articles that need the following characteristics: high modulus (rigidity) high wear resistance high tensile properties high fatigue strength high impact strength good machining Fig. 2, shows the relationship between the density of a sintered article and the module. It is apparent from Fig. 2 that the higher the density, the higher the modulus. It should be noted, that tensile strengths of approximately 100-120 ksi, as well as resistances of approximately 50 ft. Pounds have been achieved using the high density sintered alloy method described herein. Adding the graphite to the powder with previous alloy and sintering in a vacuum or a vacuum with filling or pure hydrogen or nitrogen-hydrogen mixture at a temperature of 1.270 to 1,350 * C, a high density alloy can be produced by super solid sintering. With respect to the aforementioned composition, an alloy with a density of 7.6 g / cc, can be produced in a single stage of compacting and sintering at 1,280 to 1,310 * in vacuum, or in a reducing atmosphere containing hydrogen and nitrogen. Particularly good results have been achieved by using an alloy-based iron powder with 0.85% molybdenum, in the alloy mixed with 1.5% graphite, and a lubricant. More particularly a commercially available grade that is commercially available and sold under the trademark QMP AT 4401, which has the following physical and chemical properties: Bulk density 2.92 g / cm3 Flow 26 sec./50g Chemical Analysis C 0.003% O 0.08 S 0.007 P 0.01% Mn 0.15 Mo 0.85 Ni 0.07 Si 0.003 Cr 0.05 Cu 0.02 Fe greater than 98% The commercially available alloy consists of 0.85% molybdenum, in alloy with iron and unavoidable impurities. The existence of these is known to the technician. Other powder grades with pre-alloy can be used. Graphitization elements such as Ni, Si, except as trace elements can be avoided. The sintered high carbon steel article produced according to the method described herein, presents a high density although the article tends to be fragile for the reasons described. In particular, the brittleness is due to the boundary grain of the carbides 50, which are formed as shown in Fig. 3, the carbides 50, will precipitate during the transformation in the cooling of austenite to ferrite. It should be noted, that iron has a ferrite phase and an austenite phase. In addition, up to 0.02% of carbon can be dissolved in the alpha or ferrite phase and up to 2.0% in the gamma or austenite phase. The transition temperature between the ferrite and austenite phase is approximately 727 ° C, as illustrated in Fig. 7. The formation of spheres in any steel heating and cooling process produces a rounded or globular shape of the carbide, the form spheres, is the process of heat treatment that changes the carbide grains limits and other annular carbides to a rounded shape. In the prior art, the process of forming spheres is time consuming and not economical, since the carbides are transformed to a rounded shape very slowly. Typically, forming spheres requires a long time at a certain temperature. One method to accelerate the process is to use thermo mechanical treatments that combine mechanical work and heat to cause faster spheres formation. This process is not suitable for a high pricisión, perfectly formed parts and also has cost disadvantages. A method for forming spheres has been developed for high density sintered components with which the parts are sintered, cooled inside the sintering furnace, above the mAb of approximately 1,000 * C, and quickly brought to 200 * C, so that prevent or minimize the precipitation of grain border carbides. This process results in the formation of a stable meta micro structure, which consists largely of austenite and martensite. A subsequent heat treatment with which it rises to a temperature close to the Al temperature, from 700 to 800 *, results in a relatively rapid spheres formation in the carbides, and high strength and ductility are combined. Fig. 4a is a graph illustrating the method for this sphere formation. The process of Fig. 4a, also illustrated in Fig. 6, cooling or quenching can be performed with oil, as graphically illustrated in Fig. 4a. In another embodiment, the parts are sintered in the first stage, but allowed to cool to room temperature as shown in Fig. 4b, the sintered micro-structure will then contain the chebradisic carbides. The second stage is carried out in a separate heat treatment line, where the parts are austenitized to approximately 1,000", to dissolve the carbides, it is cooled with oil, followed by the formation of spheres, therefore, when forming the spheres, the steel with a high content of sintered coal, the process It produces a metal powder that has high ductility, typically 5-10%, tensile elongation and high strength of 100-120 ksi UTS.The rounding treatment causes the carbides to be less brittle spherical steel metallic powder with high carbon content In spheres, it produces a high density steel p / n, which has a good balance of properties with high strength and ductility.This ductility allows the parts to be stamped to maintain good dimensional accuracy.Such sintered parts can be used in the form of spheres or still be treated thermally, for very high strength components.In addition, the metallic powder that has been prepared can also be thermally treated after the formation of the spheres, dissolving the spherical carbides for very high strength and durability, such as: 1. austenite matrix 2. tempering for martensite 3. martensite tempering Various sintered articles can be made according to the invention, a particularly good application is concerned to the manufacture of connecting bars of a car engine. Although the sintered connecting rods have been manufactured in the prior art as mentioned in the article entitled "Fatigue Design of Sintered Connecting Rods", which appeared in Journal of the Minerals, Metal and Materials Soc., May 1988. However, such technique Previous of a single pressing and a single sintering for the connecting rods, which have not been produced on a commercial basis since these bars of a single pressing and a single sintering do not have high density or high modulus of elasticity. In addition, some designs require heat treatment for high strength and are difficult to machine. Fig. 5 illustrates a connecting rod. In particular, the high density sintered alloy connector bars can be produced with the method presented herein, as well as with the high carbon steel described herein. More particularly, automotive connecting rods can be manufactured having the following compositions: Mo 0.6% at 3.0% C 0.8% at 2.0% Fe balance plus unavoidable impurities. Such automobile connector bars have exhibited the following characteristics: With spheres: UTS (ultimate tensile stress) 120 ksi YS (performance) 95 Isi% elongation 8% impact resistance 40 foot pounds fatigue of inverse flex 40 ksi Reference to%, is refer to the weight. In the condition as spheres, the components may show some degree of distortion. For example, a sintered connecting rod may have a lateral (narrower) lift taper to the bottom (wider), and a variable part-to-side shrinkage. The formation of spheres as described, results in high ductility that allows the part to be stamped cold with precision reaching a good dimensional accuracy. For example, Fig. 8 illustrates variations in the dimension X, of Fig. 5, in the condition of spheres using composition QMO AT 4401, to which we have referred above, with 1.5% graphite, as well as the variation after stamping. In particular, Fig. 8 shows an approximate variation in the rounded or spherical condition of approximately 2.94 to 2.99 inches while the stamped variation is approximately 2.975 to 2.98. Other products such as high effort transmission gears can be made in accordance with the present invention.

Although the present embodiment, as well as the operation and use have been specifically described in relation to the drawings, it should be understood that there are numerous variations that can be made by the technician without departing from the spirit and scope of the present invention.

Claims (16)

1. REVI ND IC ACTIONS 1.- A process for forming a sintered article of metallic powder comprising mixing graphite and lubricant with a powder based on pre-alloy iron, pressing the mixture into a form in a single stage of compaction and sintering the article , and then at a high temperature sintering the article in a reducing atmosphere to produce a sintered article having a density greater than 7.4 g / cc.
2. 2. A procedure according to the claim 1, wherein the pre-alloy iron-based powder contains approximately 0.6% to 3.0% molybdenum.
3. 3. -A procedure according to the claim 2, where the graphite is in the range of 0.8% to 2.0%.
4. 4.- A procedure according to the claim 3, where the reducing atmosphere is hydrogen, hydrogen / nitrogen, vacuum or a vacuum with partial filling.
5. 5. A process according to claim 4, wherein the sintering is performed at a temperature between 1250 * C and 1350 * C in a single sintering process.
6. 6. A procedure according to the claim 4, where the sintering is conducted at a temperature between 1270 * C and 1310 * C in a single sintering process.
7. 7. A process according to claim 6 wherein the pre-alloy iron-based powder is mixed with about 1.5% by weight and a lubricant.
8. 8.- A procedure according to the claim 7, where the mixture is pressed at an approximate density of 6.8 g / cc before sintering.
9. 9. A procedure according to the claim 8, wherein the sintered article includes austenite grains and boundary or border grain carbides between the austenite grains and wherein the sintered article is heat treated to round or form the carbides into spheres and produce a sintered powder metal article with a density greater than 7.4 g / cc.
10. 10. A procedure according to the claim 9, characterized in that it includes: a) cooling the sintered article inside a sintering furnace to just above the Acm temperature; b) rapidly cooling the sintered article to less than 200 * C; c) then rapidly raise the temperature near the Al temperature to rapidly round off the carbides.
11. 11. A procedure according to the claim 9, characterized in that it includes a) cooling the sintered article inside the sintering furnace; b) austenizing the article in a heat treatment line at 1000 * C for thirty minutes; c) immerse or cool in oil; d) then raise the temperature to near the temperature Al to form spheres in the article.
12. 12. A process for forming or modeling a sintered powder metal article comprising mixing graphite in approximately 0.8 to 2.0% by weight and lubricating with an alloy-based iron powder containing approximately 0.5% to 3.0% by weight. molybdenum, pressing the mixture into a single stage compaction, sintering the article, and then at a high temperature sintering the article in a reducing atmosphere to produce a sintered article having a density greater than 7.4 g / cc.
13. 13. A metallic powder composition consisting essentially of: a) a powder based on pre-alloy iron having between 9.5 and 3.0% molybdenum; b) graphite that has between 0.8 and 2.0% carbon; c) the rest being iron and unavoidable impurities; d) a lubricant.
14. 14. A metallic powder composition according to claim 13 wherein the lubricant comprises zinc stearate.
15. 15. A metallic powder composition according to claim 14, wherein the pre-alloy iron-based powder consists essentially of 0.85% molybdenum.
16. 16. - A metallic powder or powdered metal composition according to claim 15, wherein the graphite consists of 1.5% carbon. SUM SUM The invention relates to a process for forming a sintered metal powder article, comprising mixing graphite and lubricant with a pre-alloyed iron-based powder, pressing the mixture to form in a single compacting step, sintering the article, and then at elevated temperature sinter the article in a reducing atmosphere to produce a sintered article of a density greater than 7.4 g / cc.