Method of preparing sintered products having high tensile strength and high impact strength

The invention concerns a method of preparing sintered products having high tensile strength and high impact strength comprising the steps of forming a mixture by mixing an iron powder including 1-4% by weight of Cr, 0.2-0.8% by weight of Mo 0.09-0.3% by weight % of Mn, less than 0.01% of C, less than 0.25% by weight of O, 0-1.2% of graphite, a high temperature lubricant and optionally an organic binder; preparing a heated powder composition by heating the mixture to a temperature above ambient temperature; transferring the heated powder composition to a preheated die; forming a compacted body by compacting the heated powder composition in the die at an elevated temperature; and forming a sintered product by sintering the compacted body at a temperature of at least 1220.degree. C.

TECHNICAL FIELD
 This invention relates to the art of powder metallurgy and more
 particularly to a method of preparing sintered products, which combine the
 two properties high strength and high toughness, which normally are not
 present in one and the same product.
 BACKGROUND ART
 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 of motor vehicles and office automation apparatus.
 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 impairs 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. Impact energy is, however,
 the property the most adversely affected.
 Despite their impaired impact energy, iron-based powder-metallurgical
 materials are, to a certain extent, used in components requiring high
 impact energy. Naturally, this necessitates high precision when
 manufacturing the components, the effect of the porosity on impact energy
 being well-known.
 The impact energy of sintered steel may be increased by alloying with Ni,
 which augments the strength and ductility of the material and,
 furthermore, causes shrinkage of the material, i.e. a density increase.
 There is, however, an increasing demand for powders which do not contain
 nickel since, inter alia, nickel is expensive, gives dusting problems
 during the processing of the powder, and causes allergic reactions in
 minor amounts. From an environmental point of view, the use of nickel
 should thus be avoided
 Sintered components having high impact strength without using Ni as
 alloying element are disclosed in U.S. Pat. No. 5,728,238. This patent
 discloses that it is possible to obtain impact strength of up to 100 J by
 using an iron-based powder which, in addition to Fe, contains Mo and P,
 and in which the content of other alloying elements is maintained on a low
 level. This material is, inter alia, characterised by the fact that
 sintering even below 1150.degree. C. results in an impact energy which is
 higher than that of powder-metallurgical materials sintered at higher
 temperatures. Further, the material has excellent compressibility and is
 capable of considerable shrinkage, giving a sintered material of high
 density. For one and the same density, this known material has a
 substantially higher impact energy than today's powder-metallurgical
 materials. A serious restriction, however, is that these sintered products
 have relatively low tensile strength of about 430 MPa.
 OBJECTS OF THE INVENTION
 An object of the invention is to provide sintered components combining high
 tensile strength and high impact strength. In this context the term high
 tensile strength means values above about 700 MPa and high impact strength
 values above about 45 J.
 Another object is to provide a simple process for the preparation Of such
 components by using commercially used powders.
 SUMMARY OF THE INVENTION
 Unexpectedly, it has now been found that when water-atomised powders
 including specified amounts of the components Cr and Mo are subjected to a
 combination of specific compacting and sintering conditions it is possible
 to obtain sintered products which have not only high tensile strength but
 also a high impact strength. In accordance with the invention it has thus
 been found that when the compacting is performed as warm compaction and
 the sintering as high temperature sintering, products having a tensile
 strength above about 700 MPa and an impact strength above about 45 J may
 be obtained.
 DETAILED DESCRIPTION OF THE INVENTION
 Specifically the method of preparing sintered products having high impact
 strength and high tensile strength according to the invention comprises
 the steps
 forming a mixture by mixing an iron powder including 1-4% by weight of Cr,
 0.2-0.8% by weight of Mo 0.09-0.3 by weight % of Mn, less than 0.01% of C,
 less than 0.25% by weight of O, 0.2-1.2% of graphite, a high temperature
 lubricant and optionally an organic binder;
 preparing a heated powder composition by heating the mixture to a
 temperature above ambient temperature;
 transferring the heated powder composition to a preheated die;
 forming a compacted body by compacting the heated powder composition in the
 die at an elevated temperature; and
 sintering the compacted body at a temperature of at least 1220.degree. C.
 powders having the same or similar composition as those used according to
 the present invention are previously disclosed in the EP publication 653
 262 and SE99/00092 PC. A commercially available powder is Astaloy CrM
 available from Hoganas AB, Sweden.
 The EP publication 653 262 discloses an alloy steel powder for sintered
 bodies, which is characterised by comprising, by wt %, not more than 0.1%
 of C, not more than 0.08% of Mn, 0.5-3% of Cr, 0.1-2% of Mo, not more than
 0.01% of S, not more than 0.01% of P, not more than 0.2% of O, optionally
 one or more of 0.2-2.5% of Ni, 0.5-2.5% of Cu, 0.001-0.004% of Nb and
 0.001-0.004% of V, and the balance being Inevitable impurities and Fe. A
 sintered body having high tensile strength, high fatigue strength and high
 toughness may be prepared from this powder when the sintering is performed
 at a temperature of 1100-1300.degree. C. and the obtained body is
 immediately cooled at a cooling rate of 10.degree.-200.degree. C./minute.
 The powder having the highest impact strength, 3.6 kgfm/mm.sup.2 or about
 35 Mpa, had the composition 0.03% by weight of Mn, 1% by weight of Cr and
 0.3% by weight of Mo (cf. table 6).
 The patent publication SE99/00092 discloses a powder composition which
 differs from the one known from the EP publication above in that the Cr
 content is limited to a value between 2.5 and 3.5, the Mo content is
 between 0.3 and 0.7 and the Mn content is limited to 0.09-0.3% by weight.
 This powder also includes less then 0.25% of O and less than 0.01% of C.
 An important feature is that sintered products having high tensile
 strength can be obtained without heat treatments also when the sintering
 is carried out at low temperatures i.e. temperatures lower than
 1220.degree. C. Thus for powders including graphite in amounts ranging
 from 0.3-0.6% according to this invention low temperature sintered
 products having tensile strength up to about 1000 MPa and an impact
 strength up to about 26 J are obtained. The figures of this publication
 clearly discloses that when the tensile strength increases the impact
 strength decreases.
 These two publications disclose sintered products including the two
 alloying elements Cr and Mo which products have high tensile strength. The
 impact strength obtained is, however, moderate.
 Preferably the powders used according to the present invention essentially
 consist of, in % by weight, Cr 2.5-3.5, Mo 0.3-0.7, Mn 0.09-0.15, Cu&lt;0.10,
 Ni&lt;0.15, P&lt;0.02, N&lt;0.01 V&lt;0.10, Si&lt;0.10, W&lt;0.10, the balance being iron
 and, an amount of not more than 0.5%, inevitable impurities.
 The graphite addition according to the present invention may vary between
 0.1 and 1.2, preferably between 0.2 and 0.7% by weight of the composition.
 Sintered products having the valuable combination of high tensile strength
 and high impact strength may also be obtained without graphite addition on
 the assumption that the sintering is performed during carburizing
 conditions, i e in an atmosphere including a carbon containing gas such as
 methane, propane. A combination of graphite addition and carburizing
 atmosphere might also be used. The carbon content of the sintered product
 should be above about 0.1% by weight, most preferably above about 0.2 and
 most preferably above 0.25% irrespective of the method of incorporation of
 carbon, i.e. graphite addition, carburization or combinations thereof. The
 upper limit for the C content of the sintered product is about 0.6.
 Preferably the sintered products should have a carbon content between 0.25
 and 0.5.
 The high temperature lubricant may be any of recently developed lubricants
 or mixtures thereof which are useful for warm compaction. Specific
 examples of suitable lubricants are disclosed in e.g. the U.S. Pat. Nos.
 5,484,469 and 5,744,433. The amount of lubricant may vary between 0.3 and
 1, preferably between 0.4 and 0.8% by weight of the composition to be
 compacted.
 The binder used in the metal-powder composition may consist of e.g.
 cellulose ester resins, hydroxyalkyl cellulose resins having 1-4 carbon
 atoms in the alkyl group, or thermoplastic phenolic resins.
 The mixture of the powder, lubricant and, optionally, binder is heated to a
 temperature above ambient temperature, preferably above 100.degree. and
 most preferably above 120.degree. C.
 The obtained preheated mixture is subsequently transferred to a preheated
 die and compacted at a pressure between 600 and 1200 MPa.
 After the compaction the released green compact is sintered at a
 temperature between 1220.degree. C. and 1300.degree. C. in hydrogen and/or
 nitrogen based atmosphere such as 90N.sub.2 /10H.sub.2.

The invention is further illustrated by the following example.
 Steel powders having Cr content of 3% by weight, an Mo content of 0.5% by
 weight and an Mn content of 0.11% by weight were water-atomised and
 annealed as described in the patent application PCT/SE 97/01292. Graphite
 (C-UF4) in amounts varying from 0.3 to 0.7% by weight was added as well as
 0.6% by weight of a lubricant, Advawax.RTM.. The powders were compacted at
 700 MPa and then sintered in an atmosphere of 95% N.sub.2 /5H.sub.2 for 60
 minutes at 1250.degree. C.
 The following table discloses the green density (GD), the tensile strength
 (TS), and the impact energy (Charpy) for the products prepared.