AU2005252150B2

AU2005252150B2 - Sintered metal parts and method for the manufacturing thereof

Info

Publication number: AU2005252150B2
Application number: AU2005252150A
Authority: AU
Inventors: Anders Bergmark; Koki Kanno
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 2004-06-14
Filing date: 2005-06-13
Publication date: 2009-01-08
Anticipated expiration: 2025-06-13
Also published as: EP1771268A1; TWI290073B; SE0401535D0; JP2008502803A; ZA200610348B; BRPI0512041A; RU2345867C2; JP4825200B2; UA85245C2; WO2005120749A1; TW200610599A; CN1968775A; MXPA06014234A; CN100475389C; CA2570236A1; RU2007101313A; AU2005252150A1

Description

SSINTERED METAL PARTS AND METHOD FOR THE MANUFACTURING 0THEREOF FIELD OF THE INVENTION c The present invention refers to powder metallurgy and more specifically to prealloyed chromium powder metal parts with improved fatigue properties.

In in BACKGROUND OF THE INVENTION Ic A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

In general, sintered products made by powder metallurgy are advantageous in cost over ingot steels obtained through forging and rolling steps and has wide utility as parts in e. g. motor vehicles. However, the sintered product has pores which are inevitably formed during the course of its fabrication. These remaining pores of the sintered powder-metallurgical materials impair the mechanical properties of the materials, as compared with completely dense materials. This is a result of the pores acting as stress concentrations and also because the pores reduce the effective volume under stress. Thus, strength, ductility, fatigue strength, macro-hardness etc. in iron-based powder-metallurgical materials decrease as the porosity increases.

Despite their comparatively low fatigue strength, iron-based powdermetallurgical materials are, to a certain extent, used in components requiring high fatigue strength. Distaloy® HP, available from Hoganas AB®, Sweden, is a steel powder possible for use in high performing purposes. In this Distaloy®powder the base-powder is alloyed with nickel, which is an expensive alloying element. This high performing material is therefore rather costly and there is a need for less expensive materials, which have at least as good fatigue strength.

U :ECPaPtem Spec1tons Amended spealications1RN178875 -retyped pages (7 12 0) doc IDOne route to improve the fatigue performance of powder metallurgical steels are

C

C secondary operations. Through hardening, case hardening or shot peening (or U a combination) are possible processes to get highest possible fatigue resistance Sof a component. Shot peening is normally performed in order to utilize the beneficial influence of compressive residual stresses in the surface. Pores open to the surface are weak points in powder metallurgical materials. These pores are at least partly neutralized by introduction of surface compression residual stresses.

'C 10 In one shot peening method of compacted parts known to the applicant the shot peening is followed by a final sintering step. An iron-based powder containing i.

a. nickel is used as starting material. As indicated above there is an increasing demand for powders, which do not contain nickel, as nickel is expensive. Other disadvantages with nickel containing powders are dusting problems which may occur during the processing of the powder, and which may cause allergic reactions also in minor amounts. The use of nickel should thus be avoided.

Another method including a shot peening process known to the applicant involves a method, wherein a portion of the surface of a compacted part is subjected to shot peening after sintering. According to this method, a densifying process involving powder forging or sizing is necessary in order improve the properties of the final compacted part.

It would therefore be desirable to provide an alternate process to those known to the applicant. Preferably this process would provide a cost effective process for the preparation of powder metallurgical components with high fatigue strength without any steps for achieving core densification. Preferably this process could also involve powder materials, which are free from nickel.

Throughout the description of the specification, the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

UAECPPatent Speicattonsmeded speafiCcatonsRN787S -retyped pages (7.12.O)do c 00

O

O SUMMARY OF THE INVENTION It has unexpectedly been found that components having high fatigue strength C 5 can be obtained by shot peening of sintered components prepared from iron based powders distinguished by low levels of chromium and molybdenum.

SAccording to one aspect of the present invention, there is provided a method for tt producing powder metallurgical parts with improved fatigue strength including 0n 10 the steps of: providing a pre-alloyed iron-based metal powder including at least 1.3by weight of chromium, 0.15-0.7% by weight of molybdenum; mixing said powder with 0.1-1.0% by weight of graphite; compacting the obtained mixture at a pressure of at least 600 MPa; sintering the compacted part in a single step at a temperature above 1100 0 C; and shot-peening the part; wherein said parts are notched and have a strength concentration factor above 1.3.

Preferably, the increase of the fatigue strength is at least Preferably, the compacted and sintered part is subjected to hardening and tempering prior to shot peening.

In one embodiment, the powder metallurgical part has a mainly fine pearlitic microstructure. In another embodiment the powder metallic part has a mainly fine pearlitic microstructure. In yet another embodiment, the powder metallurgical part has a martensitic and lower bainitic microstructure. In yet another embodiment, the powder metallurgical part has a mainly tempered martensitic microstructure.

In one embodiment, the powdered metallurgical part has a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm 3 Y:\BEFA7805k1RR Amondmennta_Aug08 doc LEL 2a oo In another embodiment, the powder metallurgical part has a bending fatigue 0 limit of at least 400 MPa at a sintered density of 7.3 g/cm 3

O

z In yet another embodiment, the powder metallurgical part has a bending fatigue S 5 limit of at least 340 Mpa at a sintered density of 7.15 g/cm 3 0The Applicant has also considered use of a low chromium prealloyed powder for preparing notched sintered parts having a bending fatigue limit of at least n 340 MPa at a sintered density of 7.15 g/cm 3 wherein said powder is 10 compacted, sintered and is subjected to shot peening.

Preferably, said powder is compacted, sintered and tempered and annealed and is subjected to shot peening.

Preferably, said parts prepared by the use of the low chromium prealloyed powder as described above have a bending fatigue limit of at least 400 MPa at a sintered density of 7.3 g/cm 3 Preferably, said parts prepared by the use of the low chromium prealloyed powder as described above have a stress concentration factor above 1.3.

DETAILED DESCRIPTION OF THE INVENTION The powders used in the present invention are pre-alloyed iron-base powders comprising low amounts of chromium and molybdenum. A preferred amount is 1.3-3.5% by weight of chromium and 0.15-0.7% by weight of molybdenum. The powder may also contain small amounts, 0.09 to 0.3% by weight, of manganese and inevitable impurities. Such powders are known to the Applicant from the US patent 6 348 080 and WO 03/106079.

The base powder is further mixed with graphite to obtain the desired strength in the material. The amount of graphite which is mixed with the iron-base powder is 0.1- preferably 0.15-0.85%. The powder mixture is compacted in a die to produce a green body. The compaction pressure is at least 600 MPa, preferably at least 700 MPa and more preferably 800 MPa. The compaction can Y: BEh78685\RR_Amendment_Aug08 doc LEL 0 be performed by cold compaction or warm compaction. After the compaction 0 the obtained green part is preferably sintered at a sintering temperature above b 11000C, and more preferably above 12200C.

5 The sintering atmosphere is preferably a mix of nitrogen and hydrogen. A normal cooling rate in the sintering process is preferably 0.8 0 C/s, and a range Sbetween 0.5°C/s and 1.0°C/s is more preferred. The sintered density is Spreferably above 7.15g/cm 3 more preferably above 7.3 g/cm 3 The obtained Smicrostructure in the as-sinterd material is mainly fine-pearlitic with a lower chromium and molybdenum content Y:'BEH(78Ma55IRRAm.msnsAugQO.doc LEL WO 2005/120749 PCT/SE2005/000908 4 and martensitic or lower bainitic for slightly higher chromium and molybdenum content.

It has now unexpectedly been found that a remarkable increase in the bending fatigue limit can be obtained by shot peening the sintered low chromium powder materials.

Especially remarkable increase is obtained for notched parts, where an increase of more than 50% and even more than 70% can be obtained as can be seen from the following examples. The degree of shot peening as defined by Almen A intensity, is preferably between 0.20 and 0.37 mm.

Secondary operations e.g. through hardening and case hardening, can be performed before the shot peening in order to improve the properties even more. Thus, after through hardening followed by tempering the material is mainly martensitic and the fatigue limit is raised by shotpeening. The martensite in the surface which is formed during case hardening is believed to form compressive stresses, which is beneficial for the fatigue limit.

Sinterhardening is an alternative process which is applied in the sintering process. Sinterhardening uses forced cooling at the end of the sintering process of the components which results in a hardened structure.

The fatigue tests have been performed on notched specimen with a stress concentration factor, Kt, of 1.38 and on un-notched specimen. The tests show a greater increase in bending fatigue limit when shot peening notched specimen than when the shot peening is performed on un-notched specimen. The expression "notched" in this context refers to a specimen or component having a stress concentration factor above 1.3.

WO 2005/120749 PCT/SE2005/000908 The invention is illustrated by the following nonlimiting examples.

Example 1 Two pre-alloyed base-powders, Astaloy® CrL and Astaloy® CrM, and one diffusion-alloyed base powder, Distaloy® HP, are included in the study. Distaloy® HP is diffusionalloyed with Ni and Cu and pre-alloyed with Mo. The three materials included in this study are shown in Table 1.

Table 1 Material Ni Cu Mo Cr Astaloy CrL 0.2 Astaloy CrM 0.5 Distaloy HP 4.0 2.0 Detailed information on process parameters, density and carbon levels will be given below. In table 2 plane bending fatigue performance of un-notched specimen is shown for different alloys which are sintered 30 min in 90/10

N

2

/H

2 with cooling rate about 0.8C/sec. Fatigue tests on un-notched specimens are performed using 5 mm IS03928 samples with chamfered edges. The tests are made in fourpoint plane bending at load ratio The staircase method is used with 13 18 samples in the staircase and 2 million cycles as run-out limit. Evaluation of the staircase (50% probability fatigue limit and standard deviation) is made according to the MPIF 56 standard. Test frequency is 27 30 Hz.

WO 2005/120749 PCT/SE2005/000908 6 Table 2 Powder Density Carbon OY,5o0 Std Dev [g/cm 3 As-Sint [MPa] [MPa] [MPa] AstaloyCrL 7.17 0.60 244 7 234 7.16 0.80 267 5 260 AstaloyCrM 7.06 0.35 284 7.0 274 7.04 0.56 316 8.4 300 Distaloy HP 7.13 0.65 295 22.5 261 7.13 0.85 330 <5 >322 The microstructure of Astaloy CrL with sintered carbon below 0.6% and cooling rate about 0.8 0 C/s is upper bainite. Increased carbon above 0.74% changes the microstructure to fine pearlite.

Microstructure analysis of 1120'C sintered Astaloy CrM materials and cooling rate 0.8 0 C/s and with sintered carbon levels between 0.32% and 0.49% show a dense upper bainitic microstructure. Dense upper bainite has the same characteristics as regular upper bainite, i.e. an irregular mix of ferrite and cementite. The differences are the smaller distances between carbides and sizes of the carbides. Increased sintered carbon shifts the microstructure to a mix of martensite and lower bainite.

Table 3 shows influence of compaction pressure and carbon level for cold compacted Astaloy CrL. All materials were sintered at 1120 0 C for 30 min. in 90/10 N 2

/H

2 In table 3 a summary of plane bending fatigue performance of Astaloy CrL at two compaction pressures and two levels of additional graphite. Std.dev. <5 indicates that the scatter is small and the MPIF standard 56 evaluation of standard deviation cannot be applied. The specimen in table 3 are un-notched.

WO 2005/120749 WO 205/10749PCT/SE2005/000908 Table 3 Material Graphite Compacting Density Carbon oA,5o% Std GA, C-UP4 Pressure As-sint As- Dev [M'Pa] [q/cm3] sint [MPal [MPa] MPa] AstaloyorL 0,6 600 6,94 0,5 6 224 11.5 205 1120'C, 0,8 600 6,93 0,75 233 9.5 218 0,6 1 800 7,13 0,55 236 8.5 222 90/10 N 2

/H

2 0880 70 ,4 22 0.800/3 1, 80 >244~ Influence of sintering temperature on the fatigue performance with un-notched specimen is shown in Table 4.

The microstructures of the materials in table 4 are characterized by mainly upper bainite (1120'C 0.58%C) and fine pearlite (1120'C, 0.77%C and 1250'C, 0.74%C).

TableP 4 Powder Sint. Density -Carbon GA, 50% StdDev YA, temnp As-Sint As-Sint [MPal [MPa] [MPa] 1120C [g/CM 3 AstaloyCrL 7.10 -0.58 220 11 203 1120"C 7.08 -0.77 236 9.7 222 0 C 7.02 -0.74 290 18 264 Example 2 Influence of shot peening and the combination of heat treatment and shot peening has been investigated on Astaicy CrL 3 mm. edge-notched specimens. The notch is included in the press tool and no machining is performed.

The stress concentration factor in bending is obtained by FEM to Kt=l.38. Test frequency is 27-30 Hz.

The materials are sintered at 1280'C for 30 min in H 2 Cooling rate is 0.8'C/s.

The shot peening is performed to obtain an Almen A intensity of 0.32 mm.

WO 2005/120749 PCT/SE2005/000908 8 Estimated plane bending fatigue performance of as sintered and as-sintered plus shot peened samples is shown in table Table Powder Carbon Density Secondary op- Bending Fa- Increase As-Sint As-Sint eration tigue Limit after shot [g/cm 3 Shot peening [MPa] peening AstaloyCrL 0.70 7.30 NO 235 notched YES 420 +79% AstaloyCrL 0.85 7.30 NO 340 un-notched YES 450 +32% In table 6 an estimated plane bending fatigue performance of through hardened tempered and shot peened samples is shown. Through hardening is performed with an austenitization temperature at 880 0 C. The cooling rate after austenitization is made at 8°C/s. Finally the specimen are tempered at 250 0 C for 1 hour Table 6 Powder Carbon Density Secondary operations Bending Increase As- As-Sint Fatigue after Sint [g/cm 3 Through Tempering Shot Limit shot hardening 250 0 C Ih peening [MPa] peening AstaloyCrL 0.50 7.30 YES YES NO 295 notch 0.50 7.30 YES YES YES 490 +73% AstaloyCrL 0.50 7.30 YES YES NO 370 un-notch 0.50 7.30 YES YES YES 520 +41% From the tables 5 and 6 it can be found that by shot peening the materials containing chromium and molybdenum a great increase of the bending fatigue limit is achieved.

Claims

1. A method for producing powder metallurgical parts with improved fatigue strength including the steps of: providing a pre-alloyed iron-based metal powder including at least 1.3- by weight of chromium, 0.15-0.7% by weight of molybdenum; mixing said powder with 0.1-1.0% by weight of graphite; compacting the obtained mixture at a pressure of at least 600 MPa; sintering the compacted part in a single step at a temperature above 1100°C;and shot-peening the part; wherein said parts are notched and have a stress concentration factor above 1.3.

2. The method according to claim 1 wherein the increase of the fatigue strength is at least

3. The method according to claim 1 or 2, wherein the compacted and sintered part is subjected to hardening and tempering prior to shot peening.

4. A powder metallurgical part manufactured using a method according to any one of claims 1 to 3.

5. A powder metallurgical part manufactured according to any one of the claims 1 to 3 having a mainly pearlitic microstructure.

6. The powder metallurgical part manufactured according to any one of the claims 1 to 3 having a mainly fine pearlitic microstructure.

7. The powder metallurgical part manufactured according to any one of the claims 1 to 3 having a martensitic and lower bainitic microstructure.

8. The powder metallurgical part manufactured according to any one of the claim 1 to 3 having a mainly tempered martensitic microstructure.

9. The powder metallurgical part according to any one of the claims 1 to 8 having a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm 3 The powder metallurgical part according to any one of the claims 1 to 8 having a bending fatigue limit of at least 400 MPa at a sintered density of 7.3 g/cm 3 Y:IBE 781 W1RR~_Amendmentat_Aug08.dc LEL 0 o 11. A method for producing powder metallurgical parts with improved fatigue Sstrength as herein described with reference to any one of the examples.

12. A powder metallurgical part manufactured using a method substantially O Z as herein described with reference to any one of the examples. t i O- Y:\BEHW7885\1RRAmendmens_Aug08 doc LEL