Method of manufacturing of a mechanical face seal

The present invention discloses a method of manufacturing a face seal wherein an annular groove on the face seal substrate is filled with a mixture of a metal binder and refractory metal carbide and pressed. A second metal binder layer is then placed upon the pressed mixture and a second pressing step is performed to form an integral green body of the first pressed mixture and the second metal binder layer. The integral green body is then heated to consolidate and adhere the green body to the substrate.

FIELD OF THE INVENTION
 The present invention relates to the field of powder metallurgy and more
 particularly to manufacturing of mechanical face seals from cemented
 carbides.
 Such face seals usually are configured as disc shaped substrate made of an
 iron based alloy and provided with a ring like annular insert made of a
 wear resistant material. For the sake of brevity these seals will be
 further referred to as seals provided with working ring and the seals
 which are fully made of cemented carbide will be further referred to as
 solid carbide seals.
 The ring insert is disposed within a corresponding groove previously cut in
 the seal substrate and is firmly adhered thereto. By virtue of high
 wear-resistance such face seals are widely used in various industrial
 fields and in particular in chemical industry for sealing shafts of
 mechanical rotary/centrifugal pumps. It should be understood however that
 the present invention is not limited strictly to manufacturing of
 mechanical face seals for pumps. The present method can be employed for
 manufacturing of face seals suitable for exploitation in any other heavy
 duty, abusive application where high abrasive resistance in combination
 with good mechanical properties is required. Among these applications can
 be mentioned seals for submersible electric motors for oil production from
 wells, seals for electric drills for oil well drilling, seals for turbo
 generators and compressors etc.
 BACKGROUND OF THE INVENTION
 Mechanical face seals provided with annular working ring made of cemented
 carbide are known and their advantages in comparison with mechanical solid
 carbide seals are explained for example in U.S. Pat. No. 4,280,841
 assigned to Nippon Tungsten Co.
 In this patent is described manufacturing of mechanical seal provided with
 a cemented carbide hardened layer or ring which is firmly bound to the
 seal substrate. According to one of the embodiments disclosed in this
 patent powder of tungsten carbide mixed with an addition of 6.5 percent of
 Co is placed within a groove formed in stainless steel substrate and is
 compressed therein to produce a green compact. The compact is then
 presintered by heating in vacuum to obtain carbide layer with the
 thickness 1.3 mm. Then a paste which is a mixture of Ni--P alloy is coated
 or sprayed onto the presintered compact and finally the presintered
 carbide compact is heated in a non-oxidizing atmosphere to obtain cemented
 carbide hardened layer strongly bonded with the substrate by virtue of
 diffusion-bonding effect.
 The disadvantage of this method lies in the fact that use of low melting
 Ni--P alloy does not allow obtaining cemented carbide working ring with
 sufficient hardness. The function of Ni--P contained in the upper layer is
 merely to infiltrate into the bulk of the tungsten carbide and to provide
 for soldering with the substrate. The reported hardness in the '841 patent
 of the working ring lies between H.sub.v 720-850 which is almost two times
 less than that of the solid carbide. Despite it is stated in the patent
 that the above hardness showed improved wear resistance one can assume
 that it might be still insufficient for many heavy duty applications.
 Moreover, due to relatively low hardness one can expect that mechanical
 end seal rings manufactured by this method have high coefficient of
 friction and are prone to seizure at relatively low loads.
 The further disadvantage of the known method is associated with the fact
 that the thickness of the obtained working ring is limited to only 1.3 mm
 which shortens the service life of a seal provided with such a ring.
 In GB1290980 is disclosed method of obtaining a wear-resistant surface on a
 steel part. This method comprises coating of at least the bottom of the
 groove made in the part by a copper layer, placing thereon a layer of
 powdered tungsten carbide, pressing this layer within the groove,
 superimposing an upper layer of copper powder on the layer of tungsten
 carbide, pressing the copper layer and then heating the whole part in a
 neutral atmosphere which is sufficient to melt the copper. The copper
 residing in the upper layer melts, impregnates the bulk of tungsten
 carbide portion and binds thereof, while the copper layer placed on the
 bottom of the groove provides reliable soldering of the bulk tungsten
 carbide portion to the substrate.
 Unfortunately since copper is very prone to corrosion in presence of many
 industrial gases and liquids, especially H.sub.2 S, organic acids,
 sulfuric acid, ammonia, sodium hydroxide, distilled water or natural gas
 the face seals manufactured by the above method are not suitable for use
 in those industrial applications where such gases or liquids might be
 expected.
 Furthermore, the necessity in coating the groove walls by the copper layer
 renders this method technologically complicated.
 In RU2021078 is described method of production of wear-resistant layer on
 working end surface of a face seal. This method involves filling circular
 groove made in a steel based substrate by a powdered tungsten carbide,
 pressing thereof within the groove, placing on the green compact of a
 layer of powdered Cu--P alloy, pressing this layer and then heating the
 substrate in vacuum until the upper layer melts and impregnates the
 tungsten carbide portion. It is mentioned that Cu--P effects reliable
 soldering between the substrate and tungsten carbide and thus eliminates
 the necessity in preliminary coating the groove by a copper layer.
 Unfortunately since this method also utilizes copper it is inevitably
 associated with the same disadvantage, i.e. the seals manufactured
 according to this method are insufficiently chemically resistant in the
 presence of industrial or natural fluids.
 In conclusion it should be emphasized that despite the fact that different
 methods for manufacturing of mechanical end seals employing working
 cemented carbide ring have been devised there is still a need for a new
 method ensuring producing of such face seals with improved performances
 comparable with the seals made of solid carbide, but being cheaper than
 solid carbide face seals.
 OBJECT OF THE INVENTION
 The object of the present invention is to provide a new and improved method
 for manufacturing of mechanical end seals enabling sufficiently reduce or
 overcome the above mentioned drawbacks of the known in the art methods.
 In particular the main object of the present invention is to provide a new
 and improved method of manufacturing of mechanical face seals having
 improved hardness comparable with that of the solid carbide.
 The further object of the present invention is to provide a new and simple
 method of manufacturing of mechanical end seals having improved friction
 properties and wear resistance by virtue of low coefficient of friction
 and high resistance to seizure.
 The third object of the present invention is to provide new method of
 manufacturing of mechanical seals ensuring high strength of the working
 ring and reliable adhesion thereof to the substrate.
 Still further object of the invention is to provide a new and improved
 method of manufacturing of mechanical face seals with improved chemical
 resistance by virtue of eliminating copper in their composition.
 The above and other objects and advantages of the present invention can be
 achieved in accordance with the following combination of its essential
 features, referring to different embodiments thereof.
 According to one of the preferred embodiments the present method is
 suitable for manufacturing of mechanical face seals, said seals configured
 as an iron based alloy substrate provided with annular groove. The groove
 is filled with a wear resistant material comprising at least one
 refractory metal carbide and a binder. The method comprises the following
 main sequence of steps:
 filling the annular groove formed in the substrate with a powdered mixture
 of refractory metal carbide and the binder,
 applying first external pressure onto said powdered mixture so as to
 produce a compacted bulk portion thereof, said compacted bulk portion
 being disposed within the annular groove,
 placing a powder of the binder onto the upper surface of the compacted bulk
 portion so as to obtain thereon an upper layer of the powdered binder,
 applying second external pressure onto said upper layer so as to press
 thereof together with the compacted bulk portion and to produce an
 integral green body,
 heating said substrate in a non oxidizing atmosphere at a temperature
 sufficient to induce infiltrate and diffusion of the binder from the upper
 layer portion into the bulk portion, said diffusion resulting in sintering
 the integral green body is accompanied by reliable adhesion of the green
 body to the substrate.
 In accordance with the further embodiment the amount of refractory metal
 carbide in said powdered mixture is at least 80 weight percent with the
 reminder being the binder.
 In the other embodiment said refractory metal carbide is tungsten carbide
 and said binder is nickel based alloy.
 In yet another embodiment metal carbide in said powdered mixture has
 particle size of not less than 50 microns.
 In accordance with still further embodiment said filling step is carried
 out to fill the annular groove up to at least the half of the depth
 thereof.
 As per other embodiment said heating step is carried out in vacuum
 10.sup.-2 -10.sup.-3 mm Hg, at 1200-1300 degree C., during at least 2
 hours.
 According to the further embodiment the nickel based alloy contains at
 least 70% of nickel and has hardness of at least Hc 58.
 And in accordance with still further embodiment said refractory metal
 carbide is at least one carbide of a metal selected from the group IVb,
 Vb, VIb of the periodical table or their mixtures.
 The present invention in its various embodiments has only been summarized
 briefly. For better understanding of the present invention as well of its
 advantages, reference will now be made to the following description of its
 embodiments.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
 The present invention basically employs similar steps which were mentioned
 in connection with RU2021078. These steps comprise filling of a groove
 made in a steel based substrate with powdered tungsten carbide, pressing
 thereof to obtain green bulk portion, placing on the upper surface of the
 bulk portion a layer of powdered binder, pressing the binder together with
 the compact and then sintering the substrate in a non oxidizing
 atmosphere. However in contrast to the known method which employs solely
 powdered tungsten carbide within the bulk portion it has been unexpectedly
 revealed that very good mechanical properties of the working ring in
 combination with reliable adhesion thereof to the substrate can be
 achieved if the groove is filled with a mixture consisting of powdered
 tungsten carbide and the same binder which is placed further on the upper
 surface of the compacted bulk portion. It has been empirically found that
 it is very advantageous to use as a suitable binder nickel based alloy
 with hardness of at least 50 Hc and with melting point which is lower than
 the softening point of the substrate material.
 During the sintering step the binder contained in the powdered mixture
 ensures good adhesion of the bulk portion to the substrate while the
 binder residing in the upper layer penetrates into the bulk portion and
 firmly consolidates thereof. Since the binder itself has high hardness its
 presence does not deteriorate the hardness of the sintered bulk portion.
 By virtue of the above provisions it was possible to manufacture mechanical
 face seals with thickness of several millimeters and having excellent
 mechanical properties in terms of coefficient of friction, hardness,
 seizure and wear resistance. The mechanical properties of face seals
 manufactured in accordance with the invention were at least comparable
 with those of solid tungsten carbide seals and by far superior in
 comparison with the conventional working ring face seals.
 Furthermore, since the present method does not employ copper or its alloys
 it might be expected that chemical resistance of the seals should be also
 improved.
 Having explained the underlying idea of the present invention it will be
 now disclosed in more details with reference to the following non limiting
 example 1.
 EXAMPLE 1
 On the upper face of a substrate made of stainless steel AISI 316 annular
 groove is cut having rectangular cross section. The groove's depth is
 about 5-6 mm. The groove is filled with powdered mixture of tungsten
 carbide and nickel based binder. The mixture consists of 90 weight percent
 of tungsten carbide powder having particle size 0.1-0.25 mm and 10 weight
 percent of binder having approximately the same powder size. As suitable
 row materials one can employ commercially available cast tungsten carbide
 powder, e.g. of the grade FTC, manufactured by company H.C. Starck GmbH &
 Co. KG. As a binder can be used commercially available powdered nickel
 alloy, e.g. of the grade N62SA, manufactured by company Wall Colmonoy
 Corp. The tungsten carbide powder has microhardness H.sub.v --2100-2400
 kg/mm.sup.2 and the binder powder has hardness Hc--58-63. The groove is
 filled with the powdered mixture up to at least 0.5-0.6 of its depth. The
 mixture is then subjected to the first uniaxial pressure of 300-400 MPa so
 as to achieve dense compact which will constitute the bulk portion of the
 seal. On the upper surface of the bulk portion is homogeneously
 distributed powder of the same binder which is contained in the bulk
 portion. The amount of the binder placed on the bulk portion should be
 sufficient to fill the remainder of the groove at least up to the upper
 face of the substrate. The binder superimposed on the bulk portion
 constitutes the upper layer portion of the seal.
 Then the upper portion is subjected to second uniaxial pressure preferably
 of the same magnitude as the first pressure so as to produce integral
 compacted green body constituting the body of the seal. The substrate and
 the green body residing in the groove are subjected to thermal treatment
 in non oxidizing atmosphere so as to induce melting and infiltration of
 the binder from the upper portion into the bulk portion accompanied by
 consolidation thereof. The binder residing in the bulk portion causes
 adhesion of the consolidated tungsten carbide to the groove. In practice
 the heat treatment step is carried out in vacuum furnaces capable to
 maintain vacuum of at least 10.sup.-2 -10.sup.-3 mm. The heat treatment is
 carried out at 1200-1300 degree C. during 2-3 hours.
 After completing the thermal treatment the face side of the seal was
 subjected to flat grinding by diamond wheel to expose the tungsten carbide
 ring portion. Then the face side was lapped by diamond paste or powder to
 impart thereto required roughness and flatness. The height of the tungsten
 carbide insert in the final product was about 3 mm. Properties and
 performances of end seals manufactured in accordance with the above
 procedure were tested.
 The measured microhardness of new seals was H.sub.v --2000-2300 kg/mm.sup.2
 which is by far higher than H.sub.v --1500 kg/mm.sup.2 of the solid
 tungsten carbide.
 The performances test included measurements of the friction between two
 seal rings lubricated with water under gradually increasing axial load.
 The velocity of rotation was 4000 RPM. The test was terminated at an axial
 load of 360 N or when seizure occurred and the friction increased
 substantially. The outside and inside diameters of the working insert
 (dam) were correspondingly as follows: D.sub.out --35 mm, D.sub.ins =27
 mm.
 The average friction coefficient of conventional solid carbide face seals
 and face seals manufactured according to the invention are summarized in
 table 1.
 TABLE I
 Conventional face seals Working ring face seals
 Axial load in N made of solid carbide according to the invention
 15 0.073 0.078
 330 0.0083 0.0096
 It can be seen that friction coefficient of face seals manufactured in
 accordance with the present method is very close to that of the face seals
 made of the solid tungsten carbide. The resistance to seizure or maximum
 axial load at seizure inception was monitored and found that in one test
 the solid carbide face seals exhibited seizure at 180 N, while the seals
 manufactured in accordance with the invention exhibited seizure between
 210 and 345 N.
 It has been also found that face seals manufactured by the present
 invention have additional significant advantages like high resistance to
 wear and vibration due to absence of thermal cracking and low brittleness
 and increased service life due to possibility to achieve larger height of
 the working insert.
 It should be appreciated that the present invention is not limited to the
 above-described embodiments and that changes and modifications can be made
 by one ordinarily skilled in the art without deviation from the scope of
 the invention, as will be defined in the appended claims.
 For example instead of tungsten carbide one can use in the mixture other
 carbides of refractory metals of groups IVb, Vb, VIb of the periodical
 table, e.g. titanium carbide, tantalum carbide etc.
 The amount of the carbide component in the powdered mixture can vary
 between 80 and 95 weight percent and the particle size of the powder
 mixture preferably varies between 50-250 micron.
 The nickel-based alloy of the binder can contain lesser amount of nickel as
 far the binder hardness and melting point match the above mentioned
 requirements.
 It should also be appreciated that the features disclosed in the foregoing
 description, and/or in the following claims, and/or in the accompanying
 example and rable may, both separately and in any combination thereof, be
 material for realizing the present invention in diverse forms thereof.