Method for Manufacturing Iron-based Powder Metallurgical Parts

Abstract

A method for manufacturing iron-based metallurgical parts, the method comprising: mixing graphite powder; pressing; presintering; oxidizing the presintered metallurgical part to form an oxide layer having a thickness of 1 m to 50 m on its surface to form an oxidized presintered metallurgical part; sintering; machining; carburizing; quenching and tempering. An oxide layer is formed on the surface of a part by oxidization, oxygen in the oxide layer is chemically reacted with the carbon in the surface layer of the product during the sintering, and the resulting product enters a sintering atmosphere in the form of gas to form a decarburized layer having a certain thickness on the surface of the part, so that the decarburization is realized.

Claims

1. A method for manufacturing iron-based metallurgical parts, the method comprising: (a) mixing graphite powder, iron powder, alloy element powder, and lubricant powder to obtain a mixed powder; (b) pressing the mixed powder on a powder metallurgical molding press to obtain a molded green compact; (c) presintering the molded green compacts in a non-oxidizing atmosphere at 600 C. to 1050 C. for 10 min to 300 min to obtain a presintered metallurgical part; (d) oxidizing the presintered metallurgical part to form an oxide layer having a thickness of 1 m to 50 m on its surface to form an oxidized presintered metallurgical part; (e) performing a secondary sintering of the oxidized presintered metallurgical part in an non-oxidizing atmosphere at 1050 C. to 1350 C. for 10 min to 200 min to obtain a sintered metallurgical part; (f) performing a process to increase the density and/or surface compactness of the sintered part to form a compact metallurgical part; (g) carburizing the compact metallurgical part in an atmosphere having a carbon potential of 0.3% to 2.0% at 700 C. to 1200 C. for 5 min to 400 min to form a carburized metallurgical part, and cooling the carburized metallurgical part to a temperature suitable for quenching; and (h) quenching and tempering the carburized metallurgical part to achieve the desired mechanical properties, wherein the iron-based metallurgical part comprises iron, 0.2% to 1.5% by mass percentage carbon, the alloy element and the lubricant.

2. The method of claim 1, wherein the oxidizing in the step (d) is steam treatment carried out at 350 C. to 700 C. for 20 min to 300 min.

3. The method of claim 1, wherein the oxidizing in the step (d) is treating the presintered part in an atmosphere containing 0.5 vol % to 100 vol % of oxygen at 200 C. to 600 C. for 20 min to 300 min.

4. The method of claim 1, wherein when the temperature of the part in the cooling stage of the step (c) is 400 C. to 600 C., 10 vol % to 100 vol % of steam is supplied for a steam treatment to carry out the oxidation of step (d).

5. The method of claim 1, wherein 0.5 vol % to 50vol % of air or oxygen is supplied into the non-oxidizing atmosphere after the presintering of step (c) to carry out the oxidation of step (d).

6. The method of claim 1, wherein a continuous sintering furnace is used such that the step (c) is carried out in a presintering region, the step (d) is carried out by maintaining 0.1 vol % to 10 vol % of oxygen or 0.5 vol % to 20 vol % of steam in the non-oxidizing atmosphere, and the oxidized presintered metallurgical part is delivered to a sintering region for carrying out the step (e).

7. The method of claim 1, wherein in the step (c), the presintering is carried out at 750 C. to 1000 C. for 20 min to 120 min.

8. The method of claim 1, the process for increasing the density and/or surface compactness of the sintered part in step (f) is at least one of extrusion molding, reshaping, repressing, surface rolling and cross rolling.

9. The method of claim 1, wherein the alloy element is at least one of Ni, Cu, Mn, Cr and Mo.

10. The method of claim 1, further comprising a shot blasting treatment after step (h).

11. The method of claim 1, wherein the non-oxidizing atmosphere in steps (c) and (e) is a nitrogen-based atmosphere or vacuum.

12. The method of claim 1, wherein the non-oxidizing atmosphere is a nitrogen hydrogen atmosphere containing 5 vol % of hydrogen.

13. The method of claim 1, wherein the iron-based metallurgical part comprises the following components in percentage of weight: 0.41.5% C, 04% Cu, 05% Ni, 02% Mo, 06% Cr, 05% Mn, and a remainder component being Fe and less than or equal to 2% unavoidable impurities.

14. The method of claim 1, wherein the lubricant is 0.11% (in percentage of weight) powder lubricant, and the lubricant is polyamide wax or stearate or polyacrylamide wax.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 shows a metallographic structure after presintering in Embodiment 1;

[0032] FIG. 2 shows a metallographic structure after oxidization in Embodiment 1;

[0033] FIG. 3 shows a metallographic structure of the tooth portion after surface densification in Embodiment 1;

[0034] FIG. 4 shows a metallographic structure of the root portion after surface densification in Embodiment 1;

[0035] FIG. 5 shows pores of the tooth portion after surface densification in Embodiment 1; and

[0036] FIG. 6 shows a tooth structure after tempering in Embodiment 1.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The specific implementations of the present invention will be further described in detail by embodiments with reference to the accompanying drawings.

[0038] In the material composition in the following embodiments, Ni, Cu, Mn, Cr, Mo or other alloy elements may be selected as desired. In the sintering or presintering in the following embodiments, a mesh-belt furnace, a push-rod furnace, a bell-type furnace or the like may be selected.

Embodiment 1

[0039] Raw materials were prepared in the following proportion (mass percentage): 96.8% of atomized iron powder, 0.70% of carbon, 2% of copper powder and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.10 g/cm.sup.3.

[0040] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere at 900 C. for 60 min.

[0041] Oxidizing: the presintered part was placed in a batch steam treatment furnace and then treated with steam at 550 C. for 60 min, wherein the thickness of the steam-treated oxide layer was 6 m.

[0042] Sintering: secondary sintering was performed in nitrogen containing 3 vol % of hydrogen at 1120 C. for 30 min.

[0043] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.3 mm, without damaging the surface.

[0044] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.9% at 800 C. for 60 min, and the carburized part was cooled to a temperature suitable for quenching.

[0045] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0046] Shot blasting: Shot blasting: the fatigue resistance of the part was improved by shot blasting.

[0047] FIG. 1 shows a metallographic structure after presintering in Embodiment 1, where it can be known that there are many ferrites and unmelted copper particles A; FIG. 2 shows a metallographic structure after oxidization in Embodiment 1, with an oxide layer in 6.9 m thickness on the surface; FIG. 3 shows a metallographic structure of the tooth portion after surface densification in Embodiment 1, where the decarburized layer can be seen clearly; FIG. 4 shows a metallographic structure of the root portion after surface densification in Embodiment 1, where it can be known that the root portion has an obvious deformed structure and there are many ferrites; FIG. 5 shows pores of the tooth portion after surface densification in Embodiment 1, where it can be known that the flank has been completely densified; and, FIG. 6 shows a tooth structure after tempering in Embodiment 1, where the tooth portion is completely transformed into martensites and the surface hardness HV5 of the teeth reaches 670.

[0048] Embodiment 2

[0049] Raw materials were prepared in the following proportion: 50% of iron alloy powder (3.0% of chromium, 0.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 48.7% of pure ion powder, 0.8% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.2 g/cm3.

[0050] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere at 1000 C. for 50 min.

[0051] Oxidizing: the presintered part was placed in a batch steam treatment furnace and then treated with steam at 450 C. for 60 min, wherein the thickness of the steam-treated oxide layer was 5 m.

[0052] Sintering: secondary sintering was performed in nitrogen containing 6 vol % of hydrogen at 1200 C. for 20 min.

[0053] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.25 mm, without damaging the surface.

[0054] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.8% at 800 C. for 50 min.

[0055] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0056] Shot blasting: the fatigue resistance of the part was improved by shot blasting.

[0057] The surface hardness after the presintering was HRB35 to HRB42, the surface hardness after the oxidization was HRB50 to HRB55, the surface hardness after the sintering was HRB35 to HRB40, and the surface hardness HV5 of teeth after the quenching and tempering was 710.

Embodiment 3

[0058] Raw materials were prepared in the following proportion: 98.5% of iron alloy powder (1.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 1.0% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 6.9 g/cm3.

[0059] Presintering and oxidization were combined together: the molded green compacts were presintered in a nitrogen atmosphere containing 1% of air at 1050 C. for 70 min to obtain an oxide layer having a thickness of 6 m.

[0060] Sintering: secondary sintering was performed in nitrogen containing 6 vol % of hydrogen at 1250 C. for 40 min.

[0061] Machining: extrusion was performed to obtain a densified layer having a thickness of 0.50 mm, without damaging the surface.

[0062] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 1.5% at 900 C. for 70 min.

[0063] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0064] The surface hardness after the combination of the presintering and oxidization was HRB60 to HRB70, the surface hardness after the sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 740.

Embodiment 4

[0065] Raw materials were prepared in the following proportion: 50% of iron alloy powder (3.0% of chromium, 0.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 48.5% of pure ion powder, 1.0% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.2 g/cm3.

[0066] Presintering and oxidization were combined together: the molded green compacts were presintered in a nitrogen atmosphere containing 1 vol % of oxygen at 850 C. for 20 min to obtain an oxide layer having a thickness of 6 m.

[0067] Sintering: secondary sintering was performed in nitrogen containing 6 vol % of hydrogen at 1150 C. for 70 min.

[0068] Machining: extrusion was performed to obtain a densified layer having a thickness of 0.60 mm, without damaging the surface.

[0069] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 1.0% at 900 C. for 70 min.

[0070] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0071] The surface hardness after the presintering and oxidization was HRB45 to HRB50. The surface hardness after the sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 730.

Embodiment5

[0072] Raw materials were prepared in the following proportion: 98.7% of iron alloy powder (0.5% of molybdenum, 1.5% of copper, 1.75% of nickel, less than 1% of other inevitable substances, and the remaining of ion), 0.8% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.05 g/cm3.

[0073] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere containing 5 vol % of hydrogen at 900 C. for 40 min.

[0074] Oxidizing: the presintered part was placed in a mesh-belt furnace in an air atmosphere and then treated at 300 C. for 60 min to obtain an oxide layer having a thickness of 4 m.

[0075] Sintering: secondary sintering was performed in nitrogen containing 3% of hydrogen at 1120 C. for 30 min.

[0076] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.3 mm, without damaging the surface.

[0077] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.8% at 800 C. for 60 min.

[0078] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0079] The surface hardness after the presintering was HRB40 to HRB50, the surface hardness after the oxidization was HRB50to HRB65, the surface hardness after the sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 690.

Embodiment 6

[0080] Raw materials (i.e., mixed powder of iron chromium molybdenum and carbon) were prepared in the following proportion: 96.8% of atomized iron powder, 0.70% of carbon, 2% of nickel powder and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.10 g/cm3.

[0081] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere containing 5 vol % of hydrogen at 900 C. for 40 min.

[0082] Oxidizing: the presintered part was placed in a mesh-belt furnace in a nitrogen atmosphere containing 5% of air and then treated at 300 C. for 60 min to obtain an oxide layer having a thickness of 3 m.

[0083] Sintering: secondary sintering was performed in nitrogen containing 3% of hydrogen at 1120 C. for 30 min.

[0084] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.3 mm, without damaging the surface.

[0085] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.7% at 800 C. for 60 min.

[0086] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0087] The surface hardness after the presintering was HRB40 to HRB50, the surface hardness after the oxidization was HRB50 to HRB65,the surface hardness after the sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 690.

Embodiment 7

[0088] Raw materials (i.e., mixed powder of iron molybdenum and carbon) were prepared in the following proportion: 98.5% of iron molybdenum alloy powder (0.85% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 1.0% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.0 g/cm3.

[0089] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere containing 5% of hydrogen at 900 C. for 40 min.

[0090] Oxidizing: the presintered part was placed in a mesh-belt furnace in an atmosphere containing 100 vol % of steam and then treated at 500 C. for 60 min to obtain an oxide layer having a thickness of 5 m.

[0091] Sintering: secondary sintering was performed in nitrogen containing 3% of hydrogen at 1120 C. for 30 min.

[0092] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.5 mm, without damaging the surface.

[0093] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.7% at 800 C. for 60 min.

[0094] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0095] The surface hardness after the presintering was HRB30 to HRB40, the surface hardness after the oxidization was HRB35 to HRB45, the surface hardness after the sintering was HRB30 to HRB40, and the surface hardness HV5 of teeth after the quenching and tempering was 700.

Embodiment 8

[0096] Raw materials were prepared in the following proportion: 99.1% of iron alloy powder (1.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 0.4% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain sprocket green compacts having a density of 7.0 g/cm3.

[0097] Presintering, oxidization and sintering were combined together: a mesh-belt furnace was used as the continuous sintering furnace, the used atmosphere was nitrogen containing 5 vol % of hydrogen, 3 vol % of steam was fed into a presintering region, the presintering was performed at 700 C. for 20 min, and the sintering was performed at 1120 C. for 30 min. At the end of sintering, the sintered part was directly cooled to the room temperature by a water jacket. In this embodiment, the thickness of the oxide layer was 4 m .

[0098] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.4 mm, without damaging the surface.

[0099] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.6% at 800 C. for 120 min.

[0100] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0101] The surface hardness after the combination of the presintering, oxidization and sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 690.

Embodiment 9

[0102] Raw materials were prepared in the following proportion: 99.1% of iron alloy powder (1.5% of Mn, less than 1% of other inevitable substances, and the remaining of ion), 0.4% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.1 g/cm.sup.3.

[0103] Presintering and oxidization were combined together: the molded green compacts were presintered in a nitrogen atmosphere at 600 C. for 10 min; when the temperature of the part was 400 C. to 600 C. in a cooling stage, 10 vol % of steam was fed for steam treatment to obtain an oxide layer having a thickness of 1 m; or, during the presintering, a nitrogen atmosphere containing 0.5 vol % of air was used to obtain an oxide layer having a thickness of 1 m.

[0104] Sintering: secondary sintering was performed in nitrogen containing 3% of hydrogen at 1350 C. for 10 min.

[0105] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.1 mm, without damaging the surface.

[0106] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 2% at 700 C. for 5 min.

[0107] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0108] The surface hardness after the combination of the presintering and oxidization was HRB25 to HRB35. The surface hardness after the sintering was HRB20 to HRB30, and the surface hardness HV5 of teeth after the quenching and tempering was 690.

Embodiment 10

[0109] Raw materials were prepared in the following proportion (mass percentage): 96.8% of atomized iron powder, 0.70% of carbon, 2% of copper powder and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.10 g/cm.sup.3.

[0110] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere at 1050 C. for 300 min.

[0111] Oxidizing: the presintered part was placed in a batch steam treatment furnace and then treated with steam at 700 C. for 300 min, wherein the thickness of the steam-treated oxide layer was 50 m. Or, the presintered part was placed in a mesh-belt furnace in a nitrogen atmosphere containing 20 vol % of oxygen and then treated at 600 C. for 300 min to obtain an oxide layer having a thickness of 50 m.

[0112] Sintering: secondary sintering was performed in nitrogen containing 3 vol % of hydrogen at 1300 C. for 200 min.

[0113] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 5 mm, without damaging the surface.

[0114] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.3% at 1200 C. for 400 min, and the carburized part was cooled to a temperature suitable for quenching.

[0115] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0116] Shot blasting: the fatigue resistance of the part was improved by shot blasting.

[0117] The surface hardness after the presintering was HRB60 to HRB75, and the surface hardness after the oxidization was HRB75 to HRB90. The surface hardness after the sintering was HRB40 to HRB55, and the surface hardness HV5 of teeth after the quenching and tempering was 680.

Embodiment 11

[0118] Raw materials were prepared in the following proportion: 98.7% of iron alloy powder (0.5% of molybdenum, 1.5% of copper, 1.75% of nickel, less than 1% of other inevitable substances, and the remaining of ion), 0.8% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.05 g/cm.sup.3.

[0119] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere containing 5 vol % of hydrogen at 750 C. for 120 min.

[0120] Oxidizing: the presintered part was placed in a mesh-belt furnace in an atmosphere of oxygen and then treated at 200 C. for 20 min to obtain an oxide layer having a thickness of 5 m.

[0121] Sintering: secondary sintering was performed in nitrogen containing 3% of hydrogen at 1050 C. for 30 min.

[0122] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.3 mm, without damaging the surface.

[0123] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.8% at 800 C. for 60 min.

[0124] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0125] The surface hardness after the presintering was HRB30 to HRB45, the surface hardness after the oxidization was HRB35 to HRB50, the surface hardness after the sintering was HRB30 to HRB45, and the surface hardness HV5 of teeth after the quenching and tempering was 740.

Embodiment 12

[0126] Raw materials were prepared in the following proportion: 50% of iron alloy powder (3.0% of chromium, 0.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 48.7% of pure ion powder, 0.8% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.2 g/cm.sup.3.

[0127] Presintering: the molded green compacts were presintered in a pure nitrogen atmosphere at 1000 C. for 50 min.

[0128] Oxidizing: the presintered part was placed in a batch steam treatment furnace and then treated with steam at 350 C. for 20 min to obtain a steam-treated oxide layer having a thickness of 1 m; or, the presintered part was placed in a mesh-belt furnace in a nitrogen atmosphere containing 0.5 vol % of oxygen and then treated at 200 C. for 20 min to obtain an oxide layer having a thickness of 1 m.

[0129] Sintering: secondary sintering was performed in nitrogen containing 6 vol % of hydrogen at 1200 C. for 20 min.

[0130] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.25 mm, without damaging the surface.

[0131] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.8% at 800 C. for 50 min.

[0132] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0133] Shot blasting: the fatigue resistance of the part was improved by shot blasting.

[0134] The surface hardness after the presintering was HRB25 to HRB35, the surface hardness after the oxidization was HRB35 to HRB45, the surface hardness after the sintering was HRB25 to HRB35, and the surface hardness HV5 of teeth after the quenching and tempering was 680.

Embodiment 13

[0135] Raw materials were prepared in the following proportion: 50% of iron alloy powder (3.0% of chromium, 0.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 48.5% of pure ion powder, 1.0% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain gear (sprocket) green compacts having a density of 7.2 g/cm3.

[0136] Presintering an oxidization were combined together: the molded green compacts were presintered in a nitrogen atmosphere containing 50 vol % of oxygen at 850 C. for 20 min to obtain an oxide layer having a thickness of 10 m; or, when the temperature of the part was 400 C. to 600 C. in a cooling stage, 100 vol % of steam was fed for steam treatment to obtain an oxide layer having a thickness of 10 m .

[0137] Sintering: secondary sintering was performed in nitrogen containing 6 vol % of hydrogen at 1150 C. for 70 min.

[0138] Machining: extrusion was performed to obtain a densified layer having a thickness of 0.60 mm, without damaging the surface.

[0139] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 1.0% at 900 C. for 70 min.

[0140] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0141] The surface hardness after the presintering and oxidization was HRB45 to HRB50. The surface hardness after the sintering was HRB40 to HRB50, and the surface hardness HV5 of teeth after the quenching and tempering was 730.

Embodiment 14

[0142] Raw materials were prepared in the following proportion: 99.1% of iron alloy powder (1.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 0.4% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain sprocket green compacts having a density of 7.0 g/cm.sup.3.

[0143] Presintering, oxidization and sintering were combined together: a mesh-belt furnace was used as the continuous sintering furnace, the used atmosphere was nitrogen containing 5 vol % of hydrogen, 0.5 vol % of steam or 0.1 vol % of oxygen was fed into a presintering region, the presintering was performed at 700 C. for 20 min, and the sintering was performed at 1120 C. for 30 min. At the end of sintering, the sintered part was directly cooled to the room temperature by a water jacket. In this embodiment, the thickness of the oxide layer was 1 m.

[0144] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 0.1 mm, without damaging the surface.

[0145] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.6% at 800 C. for 120 min.

[0146] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0147] The change in hardness of the part in this embodiment was similar to that in Embodiment 8.

Embodiment 15

[0148] Raw materials were prepared in the following proportion: 99.1% of iron alloy powder (1.5% of molybdenum, less than 1% of other inevitable substances, and the remaining of ion), 0.4% of carbon, and 0.5% of a lubricant. The mixed powder was pressed at 600 MPa to obtain sprocket green compacts having a density of 7.0 g/cm.sup.3.

[0149] Presintering, oxidization and sintering were combined together: a mesh-belt furnace was used as the continuous sintering furnace, the used atmosphere was nitrogen containing 5 vol % of hydrogen, 10 vol % of steam or 20 vol % of oxygen was fed into a presintering region, the presintering was performed at 700 C. for 20 min, and the sintering was performed at 1120 C. for 30 min. At the end of sintering, the sintered part was directly cooled to the room temperature by a water jacket. In this embodiment, the thickness of the oxide layer was 20 m.

[0150] Machining: densification was performed by surface rolling to obtain a densified layer having a thickness of 2 mm, without damaging the surface.

[0151] Carburizing: according to the requirements, carburization was performed in a carburizing atmosphere having a carbon potential of 0.6% at 800 C. for 120 min.

[0152] Quenching and tempering: the carburized part was quenched and tempered to achieve the desired mechanical properties.

[0153] The change in hardness of the part in this embodiment was similar to that in

Embodiment 8

[0154] It can be known from the above embodiments that, in the present invention, the surface hardness of the part after the presintering is almost the same as the surface hardness of the part after the sintering, but less than the surface hardness after the oxidization; and, when machining or a process for increasing the density and/or surface compactness of the sintered part, for example, extrusion molding, reshaping, repressing, surface rolling or cross rolling, is performed after the sintering, the surface of the part is less likely damaged, and the wear to cutters or dies is low.