POWDERED METAL ALLOY COMPOSITION FOR WEAR AND TEMPERATURE RESISTANCE APPLICATIONS AND METHOD OF PRODUCING SAME

Abstract

A powder metal steel alloy composition for high wear and temperature applications is made by water atomizing a molten steel alloy composition containing C in an amount of at least 3.0 wt %; at least one carbide-forming alloy element selected from the group consisting of: Cr, V, Mo or W; an O content less than about 0.5 wt %, and the balance comprising essentially Fe apart from incidental impurities. The high carbon content reduces the solubility of oxygen in the melt and thus lowers the oxygen content to a level below which would cause the carbide-forming element(s) to oxidize during water atomization. The alloy elements are thus not tied up as oxides and are available to rapidly and readily form carbides in a subsequent sintering stage. The carbon, present in excess, is also available for diffusing into one or more other admixed powders that may be added to the prealloyed powder during sintering to control microstructure and properties of the final part.

Claims

1. A method of making powdered metal, comprising: preparing a molten steel alloy composition containing C, at least one carbide-forming alloy element selected from the group consisting of Cr, V, Mo or W, and the balance comprising essentially Fe apart from incidental impurities; water atomizing the molten alloy to yield prealloyed powder metal particles; and during the preparation of the molten steel alloy, controlling the amount of carbon added so that the carbon content exceeds that required to combine with the at least one carbide forming alloy element to produce carbides during a subsequent sintering stage, and thereby defining an excess carbon constituent which has the effect of decreasing the solubility of oxygen in the molten steel alloy and protecting the at least one carbide-forming alloy element from substantially oxidizing during the water atomization.

2. The method of claim 1, wherein the carbon content is at least 3.0 wt % and having an O content less than about 0.5 wt %.

3. The method of claim 1 including selecting at least Cr as the carbide-forming alloy clement in an amount greater than 10 wt %.

4. The method of claim 3, including selecting the Cr content to be about 13 wt %.

5. The method of claim 1 including selecting at least Mo as the carbide-forming alloy element in an amount below 5 wt %.

6. The method of claim 1 wherein the at least one carbide-forming alloy element is supersaturated in the water-atomized powder.

7. The method of claim 1, including compacting and sintering the powder metal and causing the carbon to combine with the at least one carbide-forming alloy element to form carbides.

8. The method of claim 7, including admixing the prealloyed powder with another powder and causing at least some of the carbon in the prealloyed powder to diffuse into the admixed powder during sintering.

9. The method of claim 1, including mechanically grinding the prealloyed powder prior to sintering.

10. The method of claim 1, including annealing the prealloyed powder prior to sintering, wherein at least a fraction of the at least one carbide-forming alloy clement is present in a supersaturated state.

11. The method of claim 1, wherein the prealloyed powder is unannealed and unground before sintering.

12. The method of claim 1, wherein Cr is at about 13 wt %, Mo is at about 1,5 and further including V at about 4 wt% and W at about 2.5 wt%.

13. A method for making a sintered comprising: preparing a molten steel alloy composition containing C in an amount of at least 3.0 wt%; at least one carbide-forming alloy element selected from the group consisting of: Cr, V, Mo or W; an O content less than about 0.5 wt %, and the balance comprising essentially Fe apart from incidental impurities; water atomizing the molten steel alloy to produce prealloyed powder; compacting and sintering the prealloyed powder either alone or admixed with another powder to cause the carbon to combine with the at least one carbide-forming alloy element to produce carbides.

14. The method of claim 13, wherein following water atomization, the at least one carbide-forming alloy element is supersaturated.

15. The method of claim 13, wherein the prealloyed powder is admixed with another powder and during sintering, some of the carbon diffuses from the prealloyed powder into the admixed powder.

16. The method of claim 13, wherein during sintering, the carbon in the prealloyed powder combines with the at least one carbide-forming alloy element to form carbides.

17. The method of claim 14, wherein the sintered prealloyed particles have a volume fraction of chromium rich carbides of at least 40 vol %.

18. The method of claim 15, wherein the sintered prealloyed particles have a volume fraction of chromium-rich carbides of about 45 vol %.

19. The method of claim 15, wherein the sintered prealloyed particles have a volume fraction of vanadium-rich carbides of about 7 vol %.

20. The method of claim 16, wherein the sintered prealloyed particles have a volume fraction of vanadium-rich carbides of about 7 vol %.

21. The method of claim 14, wherein the sintered prealloyed particles have a volume fraction of carbides of at least 47 vol %.

22. The method of claim 19, wherein the carbides have a size of about 1-2 m.

23. The method of claim 13, wherein Cr is present at about 13 wt %, Mo is present at about 1.5 wt %, V is present at about 4 wt % and W is present at about 2.5 wt %.

24. The method of claim 13, wherein Cr is present above 10 wt % and Mo is present below 5 wt%.

25. The method of claim 13, wherein the sintered prealloyed powder has a microhardness of 1000-1200 Hv.sub.50. 26, A sintered powder metal composition, comprising: at least a fraction of prealloyed water-atomized steel powder containing C in an amount of at least 3.0 wt %; Cr in an amount of about 13 wt %; Mo in an amount of about 1.5 wt %; V in an amount of about 4 wt %; W in an amount of about 2.5 wt %; an O content of less than about 0.5 wt %, and the balance comprising essentially Fe apart from incidental impurities.

27. The composition of claim 26, wherein the prealloyed water-atomized steel powder contains C in an amount of about 3.8 wt %.

28. The composition of claim 27 including chromium-rich carbides in an amount of about 40-45 vol. %.

29. The composition of claim 28 including vanadium rich carbides in an amount of about 7 vol. %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other features and advantages of the invention will become more apparent to those skilled in the art from the detailed description and accompanying drawing which schematically illustrates the process used to produce the powder.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0016] A process for producing high carbon, high alloy steel powder is schematically illustrated in the sole drawing FIG. 1.

[0017] A molten hatch 10 of the fully alloyed steel is prepared and fed to a water atomizer 12, where a stream of the molten metal 10 is impacted by a flow of high-pressure water which disperses and rapidly solidifies the molten metal stream into fully alloyed metal droplets or particles of irregular shape. The outer surface of the particles may become oxidized due to exposure to the water and unprotected atmosphere. The atomized powder is passed through a dryer 14 and then onto a grinder 16 where the powder is mechanically ground or crushed. A ball mill or other mechanical reducing device may be employed. The mechanical grinding of the particles fractures and separates the outer oxide skin from the particles. The particles themselves may also fracture and thus be reduced in size. The ground particles are then separated from the oxide to yield water-atomized powder 18 and oxide particles 20. The powder 18 may be further sorted for size, shape and other characteristics normally associated with powder metal.

[0018] The batch 10 of alloy steel is one that has a high alloy content and a high carbon content and a low oxygen content. The alloy content includes carbide-forming elements characteristic of those employed in tool steel grade of steels, namely at least one of chromium, molybdenum, vanadium or tungsten. The high content of carbon is defined as that in excess of the amount which is stoichiometrically needed to develop the desired type and volume of carbides in the particles. The low oxygen content means oxygen levels below about 0.5 wt%.

[0019] One reason for adding the excess carbon in the melt is to protect the alloy from oxidizing during the melt and during atomization. The increased carbon content of the steel decreases the solubility of oxygen in the melt. Depleting the oxygen level in the melt has the benefit of shielding the carbide-forming alloy constituents from oxidizing during the melt or during water atomization, and thus being free to combine with the carbon to form the desired carbides during sintering. Another reason for the high level of carbon is to ensure that the matrix in which the carbides precipitate reside is one of essentially martensite and/or austenite, particularly when the levels of Cr and/or V are high.

[0020] For at least cost reasons, there is a desire to increase the amount of some of the carbide-forming alloy elements over others. Thus, while Mo is an excellent choice for forming very hard carbides with a high carbide density, it is presently very costly as compared, to say, Cr, So, to develop a low cost tool grade quality of steel that is at least comparable in performance to a more costly and conventional M2 grade of tool steel, it is proposed to replace more expensive forming elements with less expensive elements while increasing the carbon content to achieve the desired end result by way of properties and cost structure. This is done by adding to the steel alloy Cr at an amount of at least 5 wt. %, reducing the Mo to less than 1.5 wt. % and increasing the amount of C to above 3 wt %. Additions of V, W can vary depending upon the desired carbides to be formed. Table 1 below shows an example of a specific alloy composition LA prepared in connection with the present invention, along with the composition of commercial grade of M2 tool steel for comparison.

TABLE-US-00001 TABLE 1 Alloy compositions (in wt. %) Powder Cr V Mo W C Fe LA 13 4 1.5 2.5 3.8 bal. M2 4 7 5 6 0.85 bal.

[0021] Inventive powder LA was prepared according to the process described above and schematically illustrated in the drawing figure. It was shown to have a very high volume % of chromium-rich carbides, on the order of about 40-45 vol. %, and vanadium-rich carbides on the order of about 7 vol. %. The chromium-rich carbides have a size of about 1-2 m and the V-rich carbides have a size of about 1 m. The surrounding matrix of the particles in which the carbides were precipitated was essentially rnartensitic with essentially no ferrite. Austenite may be permissible. The microhardness of the LA particles was measured to be in the range of about 1000 -1200 Hv.sub.50 in the sintered condition. The hardness was maintained above a 1000 Hv.sub.50 after compacting, sintering and tempering when the LA particles were admixed as hard particles at 15 and 30 vol. % with a primary low carbon, low alloy powder composition. Some of the carbon from the hard particles was shown to have diffused into the neighboring lower carbon content primary powder matrix material of the admix. Controlling the sintering and tempering cycles allows one to control the properties of the primary matrix, including varying amounts of ferrite, perlite, bainite and/or martensite. Additions, such as MnS and/or other compounds may be added to the admix to alter the properties of the admix, for example to improve machinability. The LA hard particles remain essentially stable and their properties essentially uninhibited by subsequent heat treatments employed to develop the properties of the primary matrix material.

[0022] The invention has been described in connection with presently preferred embodiments, and thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of invention is not to be limited to these specific embodiments, but is defined by any ultimately allowed patent claims.