Alloy steel for use in injection molded sinterings produced by powder metallurgy

Abstract: An alloy steel for use in injection-molded sinterings produced by power metallurgy which comprises by weight, from 0.5 to 3 % of Cr and/or Mn, from 0.3 to 1% of C, and balance Fe, is claimed. The alloy steel according to the present invention provides injection-molded sinterings having favorable post workability well-comparable to that of Fe-Ni-C alloys, and further improved in abrasion resistance when hardened and tempered to give a high Vickers hardness of over Hv 700.

Description: INDUSTRIAL FIELD OF INVENTION The present invention relates to an method of producing injection-moulded alloy steel products having improved hardness. BACKGROUND OF THE INVENTION Sinterings having three-dimensionally complicated shapes are currently manufactured by powder metallurgy using an injection molding process. This process comprises steps as follows: first kneading a binder with a powder of metals such as pure iron, an Fe-Ni system alloy, an Fe-Ni-C system alloy, high speed steel, precipitation-hardened steel, stainless steel, and sintered carbide; injection-molding the kneaded mixture; and sintering the debindered molding. Sintered alloys produced by this method are in general, subjected to post treatment or working. To begin with, sizing, followed by treatments such as milling, swaging or punching, tapping, barrel-polishing, and the like, as well as heat treatments such as hardening-tempering, softening, magnetic annealing, aging, and HIPping (hot isostatic pressing), to thereby obtain the final products. There have been, however, increasing demands from a wide field these days, that the as-sintered products have excellent post workability and that they possess favorable abrasion resistance, which should result from favorable surface hardenability upon hardening and tempering. It has been used, consequently, Fe-Ni-C alloys as a material which most satisfies the demands above. The Fe-Ni-C alloys, as-sintered, have good post workability indeed, however, the hardenability is yet to be improved. That is, it is not possible to obtain an oil-hardened and tempered product therefrom which yields a hardness (Hv) as high as exceeding 700, and therefore the abrasion resistance was still a disadvantage. FR-A-2333052 discloses an alloy steel powder containing 0.5% - 3.0% by weight of Cr and/or Mn, and 0.3 - 1.0% by weight of C; the powder being sintered to form a product. DE 2432338 discloses low alloy steel powders containing 0.9 - 2.5% Mn, 0.35 - 1.5% Cr, 0.1 - 1.0% Mo, 1.7 - 3.1% Mn + Cr, 0.1% maximum Si, and the balance Fe plus impurities. These are used to make abrasion resistant sinter forged products. SUMMARY OF THE INVENTION The invention provides a method of producing an injection-moulded steel product having a hardness (Hv) exceeding 700 and comprising by weight, from 0.5 to 3.0% of Cr and/or Mn, from 0.3 to 1% of C, and the balance Fe comprising the steps of: injection moulding kneaded steel powder and binder to form a product; removing binder from the product; sintering the product; post-working the product to a finished shape; and hardening the product. In a preferred embodiment a powder is provided for use in the method of the invention consisting of a mixture of an Fe-Cr alloy containing 30% by weight Cr, and carbonyl iron powder containing 0.9% by weight of carbon and natural graphite powder, the Fe-Cr alloy having an average particle diameter of 30 µm, the iron powder having an average particle diameter of 5 µm, and the carbon and natural graphite powder having an average particle diameter of 22 µm. Alternatively the powder for use in the method of the invention may consist of a mixture of an Fe-Mn alloy containing 77% by weight Mn, and a carbonyl iron powder containing 0.5% or 0.9% by weight of carbon and natural graphite powder, the Fe-Mn alloy having an average particle diameter of 8 µm, the carbonyl iron powder having an average particle diameter of 5 µm, and the carbon and natural graphite powder having an average particle diameter of 22 µm DETAILED DESCRIPTION OF THE INVENTION The alloy formed by the method of the present invention comprises Cr and/or Mn as essential elements for improving hardenability, and C also as an essential element to maintain favorable hardenability. When the Cr and/or Mn accounts for less than 0.5% by weight, and/or C for less than 0.3% by weight, the hardenability of the resulting alloy remains still unsatisfactory; when the amount of Cr and/or Mn exceeds 3% by weight, and/or that of C exceeds 1% by weight, the post-workability is impaired since the resulting as-sintered product becomes too hard. Accordingly, the Cr and/or Mn content is set to a range of from 0.5 to 3% by weight and C content is confined in the range of from 0.3 to 1% by weight. EXAMPLES Now the invention is described in further detail with reference to non-limiting Examples. Example 1 A water-atomized fine powder (30 µm in average particle diameter) of an Fe-Cr alloy containing 30 % by weight of Cr (hereinafter Fe-30wt.%Cr alloy) as the mother alloy was mixed with carbonyl iron powder (5 µm in average particle diameter) containing 0.9% by weight of carbon and natural graphite powder (22 µm in average particle diameter) at ratios as shown in Table 1, and to the mixture was further added an organic binder to make a total of 10 kg. The resulting mixture was kneaded, and was injection-molded in a metal mold to obtain a test piece 10 mm in width, 10 mm in thickness, and 55 mm in length. Thus were obtained test pieces No.1 to No.7. The molded test pieces were debindered in nitrogen atmosphere at 300 °C, and subjected to sintering in a semi-continuous vacuum sintering furnace at 1250 °C under vacuum of 5 x 10⁻² Torr to obtain sound sinterings. The sinterings had a relative density ranging from 93% to 95%, depending on the composition. Vickers hardness of the sintering was measured applying a load of 10 kg. The sinterings thereafter were subjected to oil-quenching and tempering. Quenching was carried out by oil-quenching a sintering maintained at 830 °C for 30 minutes. Tempering comprised air-cooling a sintering maintained at 170 °C for 60 minutes. Vickers hardness under 10-kg load was then measured again on each of the heat-treated sintering Test piece No. 8 was then prepared in the same manner as described above, except for using a carbonyl iron powder (5 µm in average particle diameter) containing 0.9 % by weight of carbon and carbonyl nickel powder (7 µm in average particle diameter) at amounts shown in Table 1. Vickers hardness was also measured on this sintering having a relative density of 95%. The measured hardness for the sintering and the heat-treated products are given in Table 1. Chemical composition (weight %) Vickers Hardness (Hv) Cr Ni C Fe as-sintered heat-treated Invention 1 0.5 - 0.5 bal. 210.5 705.4 Invention 2 1.0 - 0.5 bal. 236.4 720.0 Invention 3 2.5 - 0.5 bal. 258.2 760.2 Invention 4 1.0 - 0.9 bal. 252.3 743.1 Comparative 5 0.3 - 0.5 bal. 182.1 606.3 Comparative 6 3.5 - 0.5 bal. 350.6 780.3 Comparative 7 1.0 - 1.2 bal. 290.6 725.4 Prior Art 8 - 2.0 0.5 bal. 190.4 635.5 Table 1 reads that the as-sintered alloys according to the present invention have low Hv of 260 or less. This signifies that the post workability of the alloys according to the present invention is well comparable to that of the prior art alloy. Concerning the heat-treated alloys of the present invention, the hardness thereof are as high as Hv exceeding 700, clearly indicating the superiority in hardenability. Example 2 A mechanically crushed fine powder (8 µm in average particle diameter) of an Fe-Mn alloy containing 77 % by weight of Mn (hereinafter Fe-77wt.%Mn alloy) as the mother alloy was mixed with carbonyl iron powder (5 µm in average particle diameter) containing 0.5% or 0.9% by weight of carbon and natural graphite powder (22 µm in average particle diameter) at ratios as shown in Table 2, and to the mixture was further added an organic binder to make a total of 10 kg. The resulting mixture was kneaded, and the kneaded product was injection-molded in a metal mold to obtain a test piece 10 mm in width, 10 mm in thickness, and 55 mm in length. Thus were obtained test pieces No.9 to No.15. The molded test pieces were sintered in the same manner as in Example 1, to obtain sinterings having a relative density ranging from 92% to 95%, depending on the composition. Hardness of the sinterings was measured in the same manner as in Example 1. Subsequent heat treatment and the hardness measurement on the heat-treated sinterings were carried out in accordance with the method described in Example 1. The measured hardness for the sinterings and the heat-treated products are given in Table 2. Chemical composition (weight %) Vickers Hardness (Hv) Mn Ni C Fe as-sintered heat-treated Invention 9 0.5 - 0.5 bal. 180.6 706.2 Invention 10 1.0 - 0.5 bal. 210.3 719.8 Invention 11 2.5 - 0.5 bal. 265.2 748.3 Invention 12 1.0 - 0.9 bal. 236.4 732.8 Comparative 13 0.3 - 0.5 bal. 175.4 652.7 Comparative 14 3.5 - 0.5 bal. 335.3 792.4 Comparative 15 1.0 - 1.2 bal. 275.3 724.5 Table 2 reads that the as-sintered alloys according to the present invention have low Hv of 270 or less. This signifies that the post workability of the alloys according to the present invention is well comparable to that of the prior art alloy. Concerning the heat-treated alloys of the present invention, the hardness thereof are as high as Hv exceeding 700, clearly indicating the superiority in hardenability.

Claims: A method of producing an injection-moulded steel product having a hardness (Hv) exceeding 700 and comprising by weight, from 0.5 to 3.0% of Cr and/or Mn, from 0.3 to 1% of C, and the balance Fe comprising the steps of: injection moulding kneaded steel powder and binder to form a product; removing binder from the product; sintering the product; post-working the product to a finished shape; and hardening the product. A powder for use in the method of claim 1 and consisting of a mixture of an Fe-Cr alloy containing 30% by weight Cr, and carbonyl iron powder containing 0.9% by weight of carbon and natural graphite powder, the Fe-Cr alloy having an average particle diameter of 30 µm, the iron powder having an average particle diameter of 5 µm, and the carbon and natural graphite powder having an average particle diameter of 22 µm. A powder for use in the method of claim 1 and consisting of a mixture of an Fe-Mn alloy containing 77% by weight Mn, and a carbonyl iron powder containing 0.5% or 0.9% by weight of carbon and natural graphite powder, the Fe-Mn alloy having an average particle diameter of 8 µm, the carbonyl iron powder having an average particle diameter of 5 µm, and the carbon and natural graphite powder having an average particle diameter of 22 µm.

Inventors: KIJIMA YOSHIO, KIJIMA, YOSHIO
Applicants: SUMITOMO METAL MINING CO, SUMITOMO METAL MINING COMPANY LIMITED
Application year: 1990
Main classification: C22C33
IPC codes: C22C38, C22C33, B22F3