JP4825200B2 - Powder metallurgy parts and manufacturing method thereof - Google Patents

Description

  The present invention relates to powder metallurgy, and more particularly to alloyed chromium powder metal parts having improved fatigue properties.

  In general, a sintered product made by powder metallurgy is advantageous in terms of cost compared to an ingot steel obtained through a forging or rolling process, and has wide utility as, for example, an automobile part. However, sintered products have pores that are inevitably formed during the manufacturing process. These residual pores of the sintered powder metallurgy material impair the mechanical properties of the material when compared to a fully dense material. This is a result of the pores acting as stress concentrations, and the pores reduce the effective volume under stress. Thus, the strength, ductility, fatigue strength, macro hardness, etc. of the iron-based powder metallurgy material decrease with increasing porosity.

  Iron-based powder metallurgy materials are used in parts that require high fatigue strength to some extent despite their relatively low fatigue strength. Distalloy® HP, available from Hoganas AB®, Sweden, is a steel powder that can be used for high performance purposes. In this Distalloy® powder, the iron-based powder is alloyed with nickel, which is an expensive alloying element. Therefore, since this high-performance material is rather expensive, an inexpensive material having at least as good fatigue strength is required.

  One way to improve the fatigue performance of powder metallurgical steel is a secondary operation. Total cure, surface cure or shot peening (or combination) is a possible process to obtain the highest possible fatigue resistance of the part. Shot peening is usually performed to take advantage of the advantageous effects of surface residual compressive stress. The pores having openings on the surface are weak points of the powder metallurgy material. These pores are at least partially invalidated by the addition of surface residual compressive stress.

  Shot peening of consolidated parts is disclosed, for example, in US Pat. No. 6,171,546. According to this patent, the final sintering step is performed after shot peening. An iron-based powder containing nickel is used as the starting material. As described above, since nickel is expensive, the demand for powder containing no nickel is increasing. Another disadvantage of nickel containing powders is the dust problem, which can occur during powder processing, and even a small amount will cause an allergic reaction. Therefore, the use of nickel should be avoided. US patent application 2004/0177719 also relates to a method involving shot peening. In particular, this application relates to a method in which a part of the surface of a consolidated part is subjected to shot peening after sintering. According to this application, a densification step involving forging or sizing of the powder is necessary to improve the properties of the final consolidated part.

The object of the present invention is to provide a cost-effective method for preparing powder metallurgy parts having high fatigue strength without the treatment of densifying the core.
Another object is to provide a method using a powder material that does not contain nickel.

  We have unexpectedly found that parts with high fatigue strength can be obtained by shot peening of sintered parts made with iron-based powders characterized by low levels of chromium and molybdenum.

  The powder used in the present invention is an alloyed iron-based powder containing small amounts of chromium and molybdenum. Preferable amounts are chromium: 1.3 to 3.5% by weight and molybdenum: 0.15 to 0.7% by weight. Further, this powder contains a small amount of manganese: 0.09 to 0.3% by weight and inevitable impurities. Such powders are known from US Pat. No. 6,348,080 and WO 03/106079.

Furthermore, the iron-based powder is mixed with graphite in order to obtain the desired strength of the material. The amount of graphite mixed with the iron-based powder is 0.1 to 1.0%, preferably 0.15 to 0.85%. The powder mixture is consolidated in a die to form a green compact. The consolidation pressure is at least 600 MPa, preferably at least 700 MPa, and more preferably 800 MPa. Consolidation is performed by cold consolidation or warm consolidation. After consolidation, the resulting green compact is sintered at a sintering temperature above 1100 ° C. (preferably above 1220 ° C.). The sintering atmosphere is preferably a mixture of nitrogen and hydrogen. A normal cooling rate in the sintering process is 0.8 ° C./second, and a range between 0.5 ° C./second and 1.0 ° C./second is suitable. The sintered density is preferably 7.15 g / cm ³ , more preferably 7.3 g / cm ³ . The microstructure obtained in the as-sintered material is the main fine pearlite with low chromium and molybdenum content and martensite or lower bainite with slightly higher chromium and molybdenum content.

  It has now unexpectedly been found that a significant increase in the bending fatigue limit is obtained by shot peening a sintered low chromium powder material. A particularly significant increase was obtained with the notched parts. In this case, as can be seen from the examples below, an increase of over 50% and rather of over 70% can be obtained. The degree of shot peening defined by the Almen A intensity is preferably 0.20 to 0.37 mm.

  Secondary operations (eg, overall cure and surface cure) can be performed prior to shot peening to further improve the properties. Therefore, after total curing and subsequent tempering treatment, the material mainly becomes martensite and the fatigue limit is improved by shot peening. It is believed that the surface martensite formed during surface hardening creates compressive stress, which is advantageous for the fatigue limit.

  Sinter hardening is an alternative method applied to the sintering process. Sinter hardening uses forced cooling at the end of the sintering process of the components resulting in a hardened structure.

  Fatigue tests were performed on notched and non-notched samples having a stress concentration factor of 1.38, Kt. These tests show a greater increase in the bending fatigue limit when shot peening a notched sample than when shot peening is performed on a notched sample. In this context, the expression “notch” refers to a specimen or part having a stress concentration factor greater than 1.3.

  Hereinafter, non-limiting examples of the present invention will be described.

[Example 1]
Two alloyed base powders, Astaroy® CrL and Astaroy® CrM, and one diffusion-alloyed base powder, Distaloy® HP, are included in this study It is. Distalloy® HP is diffusion alloyed with Ni and Cu and alloyed with Mo. The three materials involved in this study are shown in Table 1.

Details about process parameters, density, and carbon content are given below. Table 2 shows the flat bending fatigue performance of non-notched samples for various alloys when sintered for 30 minutes at a cooling rate of about 0.8 ° C./second and 90/10 N ₂ / H ₂ . . Fatigue tests on non-notched samples have been performed using 5 mm ISO 3928 samples with chamfered edges. This test was performed with a four-point plane bend with a load ratio R = -1. In the staircase method, 13 to 18 samples by the staircase method and 2 million cycles as the run-out limit are used. Evaluation of this staircase method (50% probability fatigue limit and standard deviation) is made according to the MPIF56 standard. The test frequency is 27-30 Hz.

０．６％未満の焼結炭素および約０．８℃／秒の冷却速度でのＡｓｔａｌｏｙ ＣｒＬのマイクロ組織は、上部ベイナイトである。０．７４％を超える増大した炭素は、マイクロ組織を微細パーライトに変える。

The microstructure of Astaroy CrL at less than 0.6% sintered carbon and a cooling rate of about 0.8 ° C./sec is upper bainite. Increased carbon above 0.74% turns the microstructure into fine pearlite.

  Analysis of Astroy CrM material sintered at 1120 ° C. and microstructure with cooling rate of 0.8 ° C./sec and sintered carbon level of 0.32% to 0.4% is a dense upper bainite microstructure Indicates. The dense upper bainite has the same properties as regular upper bainite, i.e. has an irregular mixture of ferrite and cementite. The difference is that the distance between carbides and the size of the carbides are smaller. The increased sintered carbon turns the microstructure into a mixture of martensite and lower bainite.

Table 3 shows the effect of consolidation pressure and carbon level on cold consolidated Astloy CrM. All materials were sintered for 30 minutes at 1120 ° C. in 90/10 N ₂ / H ₂ . Table 3 outlines the plane bending fatigue performance of Astaroy CrL at two consolidation pressures and at two levels of additional graphite. Std. dev. <5 indicates that scattering is small and the MPIF standard 56 rating of standard deviation cannot be applied. The samples in Table 3 are not cut out.

  Table 4 shows the effect of sintering temperature on fatigue performance for non-notched samples. The microstructure of the materials in Table 4 is mainly characterized by upper bainite (1120 ° C, 0.58% C) and fine pearlite (1120 ° C, 0.77% C and 1250 ° C, 0.74% C). It is done.

Example 2 The effect of shot peening and the combination of heat treatment and shot peening was examined on an edge-cut sample of Astaroy CrL 3 mm. Notches are included in the press tool and are not machined. The stress concentration factor in bending is given by FEM vs. Kt = 1.38. The test frequency is 27-30 Hz.

The material is sintered at 1280 ° C. in H ₂ for 30 minutes. The cooling rate is 0.8 ° C./second.

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

  Table 5 shows the evaluated plane bending fatigue performance of the as-sintered and as-sintered samples that were shot peened.

  Table 6 shows the evaluated plane bending fatigue performance of the samples that have been subjected to the overall curing process, the tempering process, and the shot peening process. Overall curing takes place at an austenitizing temperature of 880 ° C. The cooling rate after austenitization is 8 ° C./second. Finally, the sample is tempered at 250 ° C. for 1 hour.

  From Tables 5 and 6, it can be seen that by shot peening a material containing chromium and molybdenum, the bending fatigue limit is greatly increased.

Claims (4)

A method for producing a notched powder metallurgy part having improved bending fatigue strength, comprising:
Alloyed with 1.3-3.5 wt% chromium, 0.15-0.7 wt% molybdenum, 0.09-0.3 wt% manganese, and the balance iron and inevitable impurities Prepared iron-based metal powder,
Mixing the iron-based metal powder with 0.1 to 1.0% by weight of graphite,
Consolidating the resulting mixture at a pressure of at least 600 MPa,
Sintering the consolidated part in a single step at a temperature above 1100 ° C,
The parts are cured and tempered,
Subject the part to shot peening,
A method for producing a powder metallurgy part having improved fatigue strength, wherein the resulting notched powder metallurgy part comprising the steps described above has a stress concentration factor greater than 1.3.   The method of manufacturing a powder metallurgical part having improved fatigue strength according to claim 1, wherein the shot peening treatment increases bending fatigue strength by at least 50% before the shot peening treatment.   A powder metallurgy part manufactured according to the manufacturing method according to claim 1 or 2, wherein the powder metallurgy part has a martensite microstructure which is mainly tempered. Powder metallurgy parts according to 請 Motomeko 3 that have a bending fatigue limit of at least 400MPa with sintered density of 7.3 g / cm ^3.