Patent Publication Number: US-2006002812-A1

Title: Sintered metal parts and method for the manufacturing thereof

Description:
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 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 powder-metallurgical materials are, to a certain extent, used in components requiring high fatigue strength. Distaloy® HP, available from Höganäs 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.  
      One route to improve the fatigue performance of powder metallurgical steels are secondary operations. Through hardening, case hardening or shot peening (or a combination) are possible processes to get highest possible fatigue 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 U.S. Pat. No. 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 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. Also the U.S. 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 properties of the final compacted part.  
      An object of the present invention is to provide a cost effective process for the preparation of powder metallurgical 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 distinguished 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 powder may also contain small amounts, 0.09 to 0.3% by weight, of manganese and inevitable impurities. Such powders are known from the U.S. Pat. No. 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 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 be performed by cold compaction or warm compaction. After the compaction the obtained green part is sintered at a sintering temperature above 1100° C., preferably above 1220° C. 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/cm 3 , more preferably above 7.3 g/cm 3 . 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 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, K t , 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 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 0.8 C/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 F=−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 deviation) is made according to the MPIF 56 standard. Test frequency is 27-30 Hz.  
                                   TABLE 2                           Density   Carbon   σ A, 50%     Std Dev   σ A, 90         Powder   [g/cm 3 ]   As-Sint [%]   [MPa]   [MPa]   [MPa]                                                        Astaloy CrL   7.17   0.60   244   7   234           7.16   0.80   267   5   260       Astaloy CrM   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   &lt;5   &gt;322                  
 
      The microstructure of Astaloy CrL with sintered carbon below 0.6% and cooling rate about 0.8° 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° C./s and with sintered carbon levels between 0.32% and 0.49% show a dense upper bairitic 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° 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. &lt;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                                       Carbon                       Graphite   Compacting   Density   As-       Std           C-UF4   Pressure   As-sint   sint   σ A, 50%     Dev   σ A, 90         Material   [%]   [MPa]   [g/cm 3 ]   [%]   [MPa]   [MPa]   [MPa]                                                                Astaloy CrL   0.6   600   6.94   0.56   224   11.6   205       1120° C.,   0.8   600   6.93   0.75   233   9.5   218       30 min,   0.6   800   7.13   0.55   236   8.5   222       90/10 N 2 /H 2     0.8   800   7.09   0.74   252   &lt;5   &gt;244       0.8° C./5                  
 
      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).  
                                       TABLE 4                               Density   Carbon                       Sint.   As-Sint   As-Sint   σ A, 50%     StdDev   σ A, 90         Powder   temp   [g/cm 3 ]   [%]   [MPa]   [MPa]   [MPa]                                                            Astaloy   1120° C.   7.10   0.58   220   11   203       CrL   1120° C.   7.08   0.77   236   9.7   222           1250° 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 Astaloy 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 K t =1.38. 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 nm.  
      Estimated plane bending fatigue performance of as sintered and as-sintered plus shot peened samples is shown in table 5.  
                                   TABLE 5                                       Bending               Carbon   Density   Secondary   Fatigue   Increase           As-Sint   As-Sint   operation   Limit   after shot       Powder   [%]   [g/cm 3 ]   Shot peening   [MPa]   peening                  Astaloy CrL   0.70   7.30   NO   235           notched           YES   420   +79%       Astaloy CrL   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° C. The cooling rate after austenitization is made at 8° C./s. Finally the specimen are tempered at 250° C. for 1 hour  
                                       TABLE 6                                      Carbon           Bending               As-   Density   Secondary operations   Fatigue   Increase                                                 Sint   As-Sint   Through   Tempering   Shot   Limit   after shot       Powder   [%]   [g/cm 3 ]   hardening   250° C. 1 h   peening   [MPa]   peening               Astaloy CrL   0.50   7.30   YES   YES   NO   285           notch   0.50   7.30   YES   YES   YES   490   +73%       Astaloy CrL   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.