PRE-ALLOYED IRON- BASED POWDER, AN IRON-BASED POWDER MIXTURE CONTAINING THE PRE-ALLOYED IRON-BASED POWDER AND A METHOD FOR MAKING PRESSED AND SINTERED COMPONENTS FROM THE IRON-BASED POWDER MIXTURE

Abstract

The present invention provides a low cost pre-alloyed iron based powder which has high compressibility, capable of rendering a compacted and sintered component high green density, (GD), and high sintered density, (SD). Also, a method or process for producing components, especially gears, including compaction of powder mixture containing the pre-alloyed iron-based powder, sintering of the compacted component, Low Pressure carburizing, (LPC), High Pressure Gas Quenching, (HPGQ), and tempering, is provided. In one embodiment, the process includes high temperature sintering. Other aspects of the present invention include a powder mixture containing the pre-alloyed iron based powder and components produced by the new process from the powder mixture. Such carburized components exhibit a hard surface combined with a softer and tougher core, necessary properties for e.g. automotive gears subjected to harsh environment.

Claims

1. A pre-alloyed iron based powder consisting of; 0.7-0.9% by weight of chromium (Cr); 0.2-0.4% by weight of molybdenum (Mo); 0.01-0.15% by weight of manganese (Mn); at most 0.20% by weight of oxygen (O); at most 0.05% by weight of carbon (C) less than 0.05% by weight of nitrogen (N) at most 0.3 of other inevitable impurities; and balance iron (Fe).

2. A pre-alloyed iron base powder according to claim 1 wherein the amount of Mn is 0.09-0.15% by weight.

3. A pre-alloyed iron base powder according to claim 1 wherein the amount of Mn is 0.01-0.009% by weight.

4. A pre-alloyed iron-based powder according to claim 1 wherein the amount of O is less than 0.15% by weight.

5. A pre-alloyed iron-based powder according to claim 1 wherein the amount of inevitable impurities beside O, C and N is at most less than 0.3% by weight.

6. A pre-alloyed iron-based powder according to claim 1, wherein the number of inclusions having its longest extension longer than 100 μm is at most 1.0/cm.sup.2 as measured according to ASTM B796-02.

7. A pre-alloyed iron-based powder according to claim 1 wherein the number of inclusions having its longest extension longer than 150 μm is at most 0.0/cm.sup.2 as measured according to ASTM B796-02.

8. An iron-based powder mixture comprising or containing; a pre-alloyed Iron-based powder according to claim 1; graphite in an amount of 0.2-0.7% by weight of the iron based powder mixture; optionally lubricant(s) in an amount of up to 1% by weight of the Iron based powder mixture; optionally machinability enhancing agent(s) in an amount of up to 1% by weight of the iron based powder mixture; and, optionally hard phase materials.

9. A method for making a sintered and carburized component comprising the steps of; a) providing an iron-based powder mixture according to claim 8; b) transferring the iron-based powder mixture into a compaction mold; c) compacting the iron-based powder mixture at a compaction; pressure of at least 600 MPa into a green compact; d) ejecting the green compact from the mold; e) subjecting the green compact to a sintering step; f) optionally, further densifying the sintered component; g) subjecting the sintered component to Low Pressure Carburizing; (LPC), in a carbon containing atmosphere at a pressure of at most 40 mbar; h) subjecting the carburized component to High Pressure Gas Quenching, HPGQ, at a pressure between 10 and 30 bar and at a cooling rate of at least 5° C. from a temperature of about 850-1000° C. down to at least below about 300° C.; and i) optionally subjecting the quenched component to tempering in air at a temperature between 150-300° C.

10. A method according to claim 9 wherein the green compact after ejection has a green density of at least 7.10 g/cm.sup.3.

11. A method according to claim 9 wherein the sintering step comprising sintering at a temperature between 1000° C. and 1350° C., in a reducing atmosphere or in vacuum at a pressure less than 20 mbar.

12. A method according to claim 9 wherein the Low Pressure Carburizing step comprises carburizing in an atmosphere containing at least one of C.sub.2H.sub.2, CH.sub.4 and C.sub.3H.sub.8.

13. A method according to claim 9 wherein the Low pressure Carburizing step further includes carbonitriding in an atmosphere containing ammonia.

14. A sintered component consisting of; 0.7-0.9% by weight of chromium (Cr); 0.2-0.4% by weight of molybdenum (Mo); 0.01-0.15% by weight of manganese (Mn); 0.2-1.0% by weight of carbon (C); at most 0.15% by weight of oxygen (O); at most 1.0% of inevitable impurities; and, balance iron (Fe).

15. A sintered component according to claim 14 characterized in that the component is a gear wherein gear teeth surface microhardness is minimum 700 HV0.1 and the gear teeth core hardness is between 300-550 HV0.1.

Description

FIGURE LEGENDS

[0082] FIG. 1 shows ultimate tensile strength (UTS) versus carbon content for the investigated materials in Example 1.

[0083] FIG. 2 shows microhardness (HV0.1) versus carbon content for the investigated materials in Example 1.

[0084] FIG. 3 shows a PM gear specimen used in Example 2 (measures in mm).

[0085] FIG. 4 shows a metallographic image of gear tooth cross section of heat treated test sample in Example 2.

[0086] FIG. 5 shows microhardness (HV0.1) profiles measured on gear teeth of heat treated test sample in Example 2.

[0087] FIG. 6 shows green density (GD), (compressibility) of test specimens (after uniaxial compaction with 700 MPa compaction pressure) versus Cr-content of the pre-alloyed steel powders used in the test mixes in Example 3.

EXAMPLES

Example 1

[0088] A pre-alloyed steel powder according to the invention, A1, was produced by water-atomization followed by a subsequent reduction annealing process. Atomization was done in protective N2 atmosphere in a small-scale (15 kg melt size) water-atomization unit. Annealing was done in a lab-scale belt furnace in H2 atmosphere at a temperature in the range of 1000-1100° C. Milling and sieving (−212 □m) of the powders was done after annealing. The chemical composition of the powder is presented in Table 1 together with the compositions of two other pre-alloyed steel powders which are commercial grades, B=Astaloy® 85Mo and C=Astaloy@CrA, available from Höganäs AB, Sweden, and used as reference materials. All three powders have standard particle size distribution for PM and are sieved with a −212 μm mesh sieve size.

TABLE-US-00001 TABLE 1 Chemical composition (in wt %). Powder Fe (%) Cr (%) Mo (%) Mn (%) O (%) C (%) A1 Base 0.90 0.34 0.03 0.03 <0.01 B Base 0.03 0.85 0.09 0.07 <0.01 C Base 1.80 0.04 0.09 0.14 <0.01

[0089] The compressibility of the steel powders was evaluated by uniaxial compaction of cylindrical test specimens (diameter 25 mm, height 20 mm) in a lubricated die with a compaction pressure of 600 MPa. The green density (GD) of each specimen was measured by weighing the specimen in air and water in accordance with Archimedes principle. The results are given in Table 2 and show that powder A1 has considerably better compressibility than powder C and comparable compressibility to powder B.

TABLE-US-00002 TABLE 2 Compressibility (600 MPa compaction pressure, lubricated die). Powder GD (g/cm.sup.3) A1 7.13 B 7.15 C 7.05

[0090] The steel powders were mixed with 0.25-0.35 wt % graphite (Kropfühl UF4) and 0.60 wt % lubricant (Lube E, available from Höganäs AB, Sweden). Standard tensile test bars according to ISO 2740 were produced from the powder mixes by uniaxial compaction with a compaction pressure of 700 MPa. Green density of the test bars was around 7.25 g/cm.sup.3.

[0091] The test bars were sintered at 1120° C. for 30 min in N.sub.2/H.sub.2 (95/5) atmosphere. Heat treatment of the sintered specimens was done at 920° C. for 60 min in vacuum (10 mbar) followed by high pressure gas quenching with 20 bar N.sub.2. No carburizing was done in this heat treatment operation, since the aim of the experiment was to evaluate the hardenability of the alloys at the carbon contents given by the graphite additions to the powder mixes. Subsequent tempering was done at 200° C. for 60 min in air.

[0092] Tensile testing was done on the heat treated test specimens. The test results show that A1 and C have similar ultimate tensile strength (UTS) values of about 750-1130 MPa over the investigated carbon content range; see FIG. 1. Material B has significantly lower UTS values of below 600 MPa for all carbon contents. Microhardness measurements (HV0.1 according to the Vickers method) were also performed on polished cross sections of the heat treated test specimens; see results in FIG. 2. Material A1 has microhardness values of 310-510 HV0.1 for carbon contents in the range of 0.25-0.31% C. Material B has relatively low microhardness of below 300 HV0.1 even at the highest evaluated carbon content (0.30% C). The microhardness values of material C are relatively comparable to those of material A1.

[0093] This example demonstrates that powder A1 has an attractive combination of properties for a PM gear material. The high compressibility enables compaction to high density and the hardenability is sufficient to provide microhardness values in the range of 300-550 HV0.1. This is the desired hardness range for core hardness of gear teeth after case hardening in the manufacture of gears for highly loaded transmission applications. The evaluated carbon contents correspond to typical carbon levels in the core areas of gear teeth.

Example 2

[0094] A pre-alloyed steel powder A2, according to the invention, was produced by water-atomization followed by a subsequent reduction annealing process. Atomization was done in protective N2 atmosphere in a small-scale (15 kg melt size) water-atomization unit. Annealing was done in a lab-scale belt furnace in H2 atmosphere at a temperature in the range of 1000-1100° C. Milling and sieving (−212 □m) of the powders was done after annealing. The chemical composition of the powder is presented in Table 2. The powder has standard particle size distribution for PM and is sieved with a −212 μm mesh sieve size.

TABLE-US-00003 TABLE 2 Chemical composition (in wt %). Powder Fe (%) Cr (%) Mo (%) Mn (%) O (%) C (%) A2 Base 0.85 0.30 0.04 0.06 <0.01

[0095] Powder A2 was mixed with 0.40 wt % graphite (C-UF) and 0.60 wt % lubricant (Lube E). Large gear specimens (see dimensions in FIG. 3) were compacted from the powder mix by uniaxial compaction with a compaction pressure of 700 MPa. Green density of the gear specimens was 7.20 g/cm.sup.3.

[0096] The gear specimens were sintered at 1250° C. for 30 min in N.sub.2/H.sub.2 (95/5) atmosphere. Case hardening of the sintered gears was done by low pressure carburizing (LPC) at 965° C. followed by high pressure gas quenching with 20 bar N.sub.2. Base atmosphere in the LPC process was N.sub.2 (8 mbar pressure) and the carburizing gas was C.sub.2H.sub.2/N.sub.2 (50/50). Four carburizing boost cycles were applied with a length of each boost cycle of 37-65 seconds. The diffusion time after each boost cycle varied between 312-3550 seconds. The total time at 965° C. was 96 minutes. Subsequent tempering after gas quenching was done at 200° C. for 60 minutes in air.

[0097] A metallographic examination performed on polished and etched cross sections of the heat treated gear specimens shows that the gear teeth have a martensitic surface layer and a bainitic core structure; see FIG. 4. Microhardness measurements (HV0.1 according to the Vickers method) were also done on the polished cross sections to investigate the hardness profiles of the gear teeth, see results in FIG. 5. These measurements show that surface hardness is above 800 HV0.1 and that the core hardness is 320-340 HV0.1, with somewhat lower hardness levels at the root of the teeth than at the flank. The case depth (where the hardness is 550 HV0.1) is 0.8 mm at the flank and 0.6 mm at the root.

[0098] This example demonstrates that powder A2 is suitable for the manufacture of high strength PM gears in a process where case hardening is done by the LPC-HPGQ method. A graphite content of 0.40 wt % of the iron-based powder mixture was used in the powder mix in order to provide sufficient hardenability to the alloy at the cooling rates obtained inside large gear components when HPGQ is applied. The high compressibility of the powder enables compaction to high density of the gear, and desired levels of hardness values after heat treatment are obtained, both at the surface and in the core areas of the gear teeth. Well-defined cased depths were also accomplished.

Example 3

[0099] Pre-alloyed steel powders with different contents of Cr (0.5-1.0%) and the same content of Mo (0.3%) were produced by water-atomization followed by a subsequent reduction annealing process. Atomization was done in protective N2 atmosphere in a small-scale (15 kg melt size) water-atomization unit. Annealing was done in a lab-scale belt furnace in H2 atmosphere at a temperature in the range of 1000-1100° C. The same annealing parameters were used for all powders. Milling and sieving (−212□m) of the powders was done after annealing. Chemical composition of the powders is presented in Table 3.

TABLE-US-00004 TABLE 3 Chemical composition (in wt %). Powder Fe Cr (%) Mo (%) Mn (%) O (%) C (%) X1 Base 0.57 0.30 0.04 0.11 <0.01 X2 Base 0.76 0.32 0.03 0.13 <0.01 X3 Base 0.83 0.32 0.04 0.13 <0.01 X4 Base 0.92 0.33 0.03 0.12 <0.01 X5 Base 1.00 0.32 0.03 0.11 <0.01

[0100] The steel powders were mixed with 0.25/0.35 wt % graphite (Kropfmühl UF4) and 0.60 wt % lubricant (Lube E, available from Höganás AB, Sweden). The compressibility of the powder mixes was evaluated by uniaxial compaction of cylindrical test specimens (diameter 25 mm, height 20 mm) with a compaction pressure of 700 MPa. The green density (GD) of each specimen was measured by weighing the specimen in air and water in accordance with Archimedes principle. The results are presented in FIG. 6 and demonstrates that a pre-alloyed iron based powder with an alloying content of 0.7-0.9 wt % Cr and 0.3 wt % Mo (in accordance with the claimed invention) yields high compressibility and that Cr content shall be at most 0.9 wt %. Cr-content below 0.7 wt % does not significantly increase the compressibility, i.e. yields higher green density (GD).