FIELD OF THE INVENTION

The present invention refers to powder metallurgy and more specifically to pre-alloyed chromium powder metal parts with improved fatigue properties.

BACKGROUND OF THE INVENTION

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 prod¬ uct has pores which are inevitably formed during the course of its fabrication. These remaining pores of the sintered powder-metallurgical materials impair the me¬ chanical 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 de¬ crease as the porosity increases.

Despite their comparatively low fatigue strength , iron- based powder-metallurgical materials are, to a certain extent, used in components requiring high fatigue strength. Distaloy® HP, available from Hδganas AB®, Swe¬ den, is a steel powder possible for use in high perform- ing purposes. In this Distaloy®-powder the base-powder is alloyed with nickel, which is an expensive alloying ele¬ ment. 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. One route to improve the fatigue performance of powder metallurgical steels are secondary operations. Through hardening, case hardening or shot peening (or a combina- tion) are possible processes to get highest possible fa¬ tigue resistance of 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.

Shot peening of compacted parts is disclosed in e.g. the US patent 6 171 546. According to this patent 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 de¬ mand 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. Also the US patent application 2004/0177719 relates to a method including shot peening. More specifically, this application discloses a method, wherein a portion of the surface of a compacted part is subjected to shot peening after sintering. According to this application a densifying process involving powder forging or sizing is necessary in order improve the prop- erties of the final compacted part.

An object of the present invention is to provide a cost effective process for the preparation of powder metallur¬ gical components with high fatigue strength without any steps for achieving core densification. Another object is to provide a process involving powder materials, which are free from nickel. SUMMARY OF THE INVENTION

It has unexpectedly been found that components having high fatigue strength can be obtained by shot peening of sintered components prepared from iron based powders dis¬ tinguished by low levels of chromium and molybdenum.

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 pow- der may also contain small amounts, 0.09 to 0.3% by weight, of manganese and inevitable impurities. Such pow¬ ders are known 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- 1.0%, preferably 0.15-0.85%. The powder mixture is com¬ pacted 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 be per¬ formed by cold compaction or warm compaction. After the compaction the obtained green part is sintered at a sin¬ tering temperature above 11000C, preferably above 12200C. The sintering atmosphere is preferably a mix of nitrogen and hydrogen. A normal cooling rate in the sintering process is 0.8°C/s, a range between 0.5°C/s and 1.0°C/s is preferred. The sintered density is preferably above 7.15 g/cm3, more preferably above 7.3 g/cm3. The obtained microstructure in the as-sinterd material is mainly fine- pearlitic with a lower chromium and molybdenum content and martensitic or lower bainitic for slightly higher chromium and molybdenum content.

It has now unexpectedly been found that a remarkable in- crease 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 follow- ing 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 compres¬ sive stresses, which is beneficial for the fatigue limit.

Sinterhardening is an alternative process which is ap¬ plied 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. The invention is illustrated by the following non- limiting 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 diffusion- alloyed 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 1.5 Astaloy CrM 0.5 3.0 Distaloy HP 4.0 2.0 1.5

Detailed information on process parameters, density and carbon levels will be given below. In table 2 plane bend¬ ing fatigue performance of un-notched specimen is shown for different alloys which are sintered 30 min in 90/10 N2/H2 with cooling rate about 0.8C/sec. Fatigue tests on un-notched specimens are performed using 5 mm ISO3928 samples with chamfered edges. The tests are made in four- point plane bending at load ratio R=-1. 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 de¬ viation) is made according to the MPIF 56 standard. Test frequency is 27 - 30 Hz. Table 2

The microstructure of Astaloy CrL with sintered carbon below 0.6% and cooling rate about 0.8°C/s is upper bain- ite. Increased carbon above 0.74% changes the microstruc- ture to fine pearlite.

Microstructure analysis of 11200C sintered Astaloy CrM materials and cooling rate 0.8°C/s and with sintered car¬ bon 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 irregu¬ lar mix of ferrite and cementite. The differences are the smaller distances between carbides and sizes of the car¬ bides. Increased sintered carbon shifts the microstruc- ture 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 11200C for 30 min. in 90/10 N2/H2. In table 3 a summary of plane bending fatigue performance of Astaloy CrL at two compaction pressures and two levels of addi¬ tional 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. Table 3

Influence of sintering temperature on the fatigue per¬ formance with un-notched specimen is shown in Table 4. The microstructures of the materials in table 4 are char¬ acterized by mainly upper bainite (112O0C 0.58%C) and fine pearlite (11200C, 0.77%C and 1250°C, 0.74%C) .

Table 4

Example 2

Influence of shot peening and the combination of heat treatment and shot peening has been investigated on Asta- loy CrL 3 mm edge-notched specimens. The notch is in¬ cluded 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 12800C for 30 min in E2- Cooling rate is 0.8°C/s.

The shot peening is performed to obtain an Almen A inten- sity of 0.32 mm. Estimated plane bending fatigue performance of as sin¬ tered and as-sintered plus shot peened samples is shown in table 5.

Table 5

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 austeniti- zation temperature at 8800C. The cooling rate after aus- tenitization is made at 8°C/s. Finally the specimen are tempered at 2500C for 1 hour .

Table 6

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.