US20130136646A1 - Nitrided sintered steels - Google Patents

Nitrided sintered steels Download PDF

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US20130136646A1
US20130136646A1 US13/699,032 US201113699032A US2013136646A1 US 20130136646 A1 US20130136646 A1 US 20130136646A1 US 201113699032 A US201113699032 A US 201113699032A US 2013136646 A1 US2013136646 A1 US 2013136646A1
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weight
steel powder
sintered
based steel
wear
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Sigurd Berg
Senad Dizdar
Ulf Engstrom
Ola Litstrom
Eckart Schneider
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Hoganas AB
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Hoganas AB
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/12—Metallic powder containing non-metallic particles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/24—After-treatment of workpieces or articles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/24—After-treatment of workpieces or articles
    • B22F3/26—Impregnating
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C22C33/0264—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/48—Nitriding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

  • the present invention concerns a method of producing sintered components by single press and single sintering, and sintered components produced by the method.
  • the method provides a cost effective production of sintered steel components having wear resistance properties comparable to components made from chilled cast iron.
  • WO2006/045000 concerns carburized sintered alloys for cam lobes and other high wear articles fabricated from iron-based powder metal mixtures consisting of 0.5-3.0% Mo, 1-6.5% Cr, 1-5% V, and the balance Fe and impurities. However the wear resistance does not reach the same levels as that of CCI components.
  • Components produced according to the method demonstrate wear resistance properties similar to those of CCI-components.
  • the components have a hard case with softer core, and are thus not through-hardened.
  • a through-hardened component can make assembly more difficult compared to a case hardened component having a softer core.
  • the method is particularly suitable for automotive components working in oil lubricated environments, where the working temperature is below 250° C., and which components have functions that rely on sliding movements. For instance cam lobes, sprockets, CVT, and other power train, valve train and engine components. Of course the method can also be suitable to produce components for other applications where good wear properties are desirable.
  • the prealloyed iron-based steel powder provided in step a) of the method is preferably produced by water atomization of an iron melt including the alloying elements.
  • the atomized powder can further be subjected to a reduction annealing process.
  • the particle size of the prealloyed powder alloy could be any size as long as it is compatible with the press and sintering processes. Examples of typical particle size is the particle size of the known powder ASC100.29 available from Höganäs AB, Sweden, having maximum 2.0% by weight above 180 ⁇ M and 15-30% by weight below 45 ⁇ m. However, coarser as well as finer grained powders may be used.
  • Example of such powders are iron-based powders having an average particle size between 75 and 300 ⁇ m, wherein less than 10% of the powder particles have a size below 45 ⁇ m and the amount of particles above 212 ⁇ m is above 20%.
  • Finer iron-based steel powders could also be used. When using fine powders, it is preferred that they are bonded with binding agent(s) and/or flow agent(s), in order to provide better powder properties and compressibility. Such powders could e.g. have a average particle size in the range of 20-60 ⁇ m.
  • the prealloyed steel powder provided in step a) of the method is iron-based and comprises Mn and at least one element selected from the group of Cr, Mo and V.
  • the prealloyed steel powder may optionally further comprise Ni and/or additional strong nitride forming element(s), such as tungsten, titanium, niobium and/or aluminium.
  • Manganese, Mn is present in amounts between 0.02-0.3% by weight. In practice, it is very hard to achieve contents below 0.02% by weight when using recycled scrap unless a specific treatment for the reduction during the course of the steel manufacturing is carried out, which increases costs. Furthermore, Manganese increases strength, hardness, and hardenability of the steel powder and it is therefore preferred to have a manganese content above 0.05% by weigh, ore preferably above 0.9% by weight. A Mn content above 0.3% by weight will increase the formation of manganese containing inclusions in the steel powder and will also have a negative effect on the compressibility due to solid solution hardening and increased ferrite hardness. Therefore the Mn content should not exceed 0.3% by weight. The most preferred range for Mn is 0.1-0.3% by weight.
  • Chromium, Cr, as an alloying element serves to strengthen the matrix by solid solution hardening. Chromium also increases hardenability and abrasion resistance of a sintered body. Furthermore, Cr is a very strong nitride former and thus promotes nitriding. If chromium is added, it should be added in an amount of at least 0.2% by weight to have desired impact on the properties of the sintered component, preferably at least 0.4% by weight, and more preferably at least 1.3% by weight. However, with increasing addition of chromium the requirements of controlled atmospheres during sintering increase, making components more costly to manufacture. Therefore, if chromium is added it should be at most 3.5% by weight of Cr, preferably at most 3.2% by weight. In a preferred embodiment the chromium content is 0.4-2.0% by weight, more preferably 1.3-1.9% by weight. In another preferred embodiment the chromium content is 2.8-3.2% by weight.
  • Molybdenum, Mo stabilizes ferrite after sintering. If molybdenum is added, it should be added in an amount of at least 0.1% by weight to have desired impact on the properties of the sintered component, preferably in an amount of at least 0.15% by weight. It is not desired to have a too high Mo-content as it will not contribute enough to the performance. Therefore, if molybdenum is added it should be at most 1.2% by weight of Mo, preferably at most 0.6% by weight. In some embodiments the steel may be essentially free from Mo, having contents of Mo below 0.1% by weight, preferably below 0.05% by weight.
  • Vanadium, V increases the strength by precipitation hardening. Vanadium has also a grain size refining effect and is a strong nitride forming element. If vanadium is added, it should be added in an amount of at least 0.05% by weight to have desired impact on the properties of the sintered component, preferably in an amount of at least 0.1% by weight, more preferably in an amount of at least 0.25% by weight. However, high vanadium contents facilitate oxygen pickup, thereby increasing the oxygen level in a component produced by the powder, which is not desired in too high amounts. Therefore the vanadium content should be at most 0.4% by weight, preferably at most 0.35% by weight.
  • the prealloyed steel powder may optionally further comprise additional strong nitride forming element(s) as known in the art, such as one or more of element(s) selected from the group of tungsten (W), titanium (Ti), niobium (Nb) and aluminium (Al). If added, the total amount of said optional additional strong nitride forming element(s) should be between 0.05% and 0.50% by weight, preferably between 0.1% and 0.4%, and more preferably 0.15% to 0.30% by weight.
  • additional strong nitride forming element(s) should be between 0.05% and 0.50% by weight, preferably between 0.1% and 0.4%, and more preferably 0.15% to 0.30% by weight.
  • the prealloyed steel powder may optionally comprise Ni in an amount of 0.1-1.0% by weight, preferably 0.1-0.5% by weight. In a preferred embodiment the prealloyed steel powder is essentially free from nickel, and thus contains below 0.1% by weight, preferably below 0.05% by weight.
  • Oxygen, O is at most 0.25% by weight. Too high content of oxygen impairs strength of the sintered component, and impairs the compressibility of the powder. For these reasons, O is preferably at most 0.18% by weight. In practice, when using water atomization techniques, it is difficult to reach oxygen contents below 0.1% by weight. The oxygen content in water atomized and annealed powders are therefore normally in the range of 0.10-0.18% by weight.
  • Carbon, C, in the steel powder should be at most 0.1% by weight, preferably less than 0.05% by weight, more preferably less than 0.02% by weight, and nitrogen, N, should be at most 0.1% by weight, preferably less than 0.05% by weight, more preferably less than 0.02% by weight. Higher contents of carbon and nitrogen will unacceptably reduce the compressibility of the powder.
  • each incidental impurity element such as any element selected from the group consisting of copper (Cu), phosphorous (P), silicon (Si), sulphur (S), and any other element not intentionally added to the alloy, should be less than 0.15%, preferably less than 0.10%, more preferably less than 0.05%, and most preferably less than 0.03% by weight of each element, in order not to deteriorate the compressibility of the steel powder or act as formers of detrimental inclusions.
  • the total sum of all incidental impurities should be less than 0.5% by weight, preferably less than 0.3% by weight, more preferably less than 0.2% by weight.
  • the prealloyed steel powder according to the invention consists of (in % by weight):
  • the prealloyed steel powder according to the invention consists of (in % by weight):
  • the prealloyed steel powder according to the invention consists of (in % by weight):
  • the prealloyed steel powder according to the invention consists of (in % by weight):
  • prealloyed steel powder according to the invention consists of (in % by weight):
  • V 0.05-0.4
  • the prealloyed steel powder is mixed with lubricants, graphite, optionally one or more machining enhancing agent(s) and optionally other conventional additives, such as hard phase materials.
  • carbon is introduced in the matrix.
  • Carbon is added as graphite to the composition in amount between 0.15-1.0% by weight of the composition.
  • An amount less than 0.15% by weight will result in a too low strength and an amount above 1.0% by weight will result in an excessive formation of carbides, affecting the nitride formation properties negatively.
  • graphite is added in an amount between 0.20-0.80% by weight, and more preferably in an amount of 0.30-0.60% by weight.
  • Lubricants are added to the composition in order to facilitate the compaction and ejection of the compacted component.
  • the addition of less than 0.05% by weight of the composition of lubricants will have insignificant effect and the addition of above 2% by weight of the composition will result in a too low density of the compacted body.
  • the amount of added lubricant is between 0.3-0.8% by weight of the composition, more preferably 0.4-0.6% by weight of the composition.
  • Any type of lubricant suitable for compaction may be used.
  • Lubricants may be chosen from the group of metal stearates, waxes, fatty acids and derivates thereof, oligomers, polymers and other organic substances having lubricating effect.
  • composite lubricant particles suitable for compacting with a heated die are chosen, such as composite lubricant particles comprising a core of 10-60% by weight of at least one primary fatty acid amide having more than 18 and not more than 24 carbon atoms and 40-90% by weight of at least one fatty acid bisamide, said lubricant particles also comprising nanoparticles of at least one metal oxide adhered on the core.
  • the composite lubricant particles suitable for compacting with a heated die comprise 10-30% by weight of the at least one primary fatty acid amide and 70-90% by weight of the at least one fatty acid bisamide.
  • the at least one fatty acid bisamide is preferably selected from the group consisting of methylene bisoleamide, methylene bisstearamide, ethylene bisoleamide, hexylene bisstearamide and ethylene bisstearamide.
  • the nanoparticles of the at least one metal oxide are preferably selected from the group consisting of TiO2, Al2O3, SnO2, SiO2, CeO2 and indium titanium oxide.
  • Copper is a commonly used alloying element in the powder metallurgical technique. Cu will enhance the strength and hardness through solid solution hardening. Cu, will also facilitate the formation of sintering necks during sintering as copper melts before the sintering temperature is reached providing so called liquid phase sintering.
  • the powder may optionally be admixed with Cu, preferably in an amount of 0.2-3% by weight Cu. In a preferred embodiment no copper is admixed to the composition.
  • Nickel, Ni increases strength and hardness while providing good ductility properties. However, contents above 1.5% by weight will tend to form Ni-rich austenite during heat treatment conditions, which will lower the strength of the material.
  • the powder may optionally be admixed with Ni in an amount of 0.1-1.5% by weight. In a preferred embodiment no nickel is admixed to the composition.
  • Machinability enhancing agent(s) can optionally be admixed to the composition in an amount of 0.1-1.0% by weight of the composition. Below 0.1% the effect is not good enough and above 1.0% no additional improvement is added.
  • the machinability enhancing agent(s) is in an amount of 0.2-0.8% by weight of the composition, more preferably 0.3-0.7% by weight of the composition.
  • the machinability enhancing agent(s) are preferably selected from the group consisting of MnS, MoS 2 , CaF 2 , and/or phyllosilicates, such as kaolinites, smectites, bentonites, and micas (such as muscovite or phlogopite). In working conditions said machinability enhancing agent(s) also work as solid lubricants and thus help to increase the wear resistance of the components.
  • sintering additives such as hard phase materials
  • the iron-based powder composition is transferred into a press mould and subjected to a compaction pressure of between 400-2000 MPa, preferably 500-1200MPa.
  • the die in the press is heated to a temperature of 40-100° C., preferably 50-80° C., before and during compaction. This technique is referred to as “warm die compaction” or “heated die compaction”.
  • the component is preferably compacted to a green density of at least 7.10 g/cm 3 , preferably at least 7.15 g/cm 3 , more preferably at least 7.20 g/cm 3 .
  • the obtained green component is further subjected to sintering in a reducing atmosphere at a temperature of about 1000-1400° C.
  • the component is sintered at regular sintering temperatures, in the range of at 1000-1200° C., preferably 1050-1180° C., most preferably 1080-1160° C.
  • the component could also be sintered at higher temperatures, e.g. in the range of 1200-1400° C., preferably 1200-1300° C., and most preferably 1220-1280° C.
  • the component is sintered to a density in the range of 7.1 to 7.6 g/cm 3 , preferably 7.15 to 7.50 g/cm 3 , more preferably 7.20 to 7.45 g/cm 3 .
  • sinter to higher densities than 7.6 g/cm 3 .
  • the sintered component is then subjected to a nitriding process, for obtaining the desired microstructure.
  • the nitriding process is performed in a nitrogen containing atmosphere in temperatures around 500° C.
  • the nitriding process is performed in a mixture of nitrogen and hydrogen gas at a temperature of 400-600° C., preferably 470° -580° C., with a soaking time of less than 3 hours, preferably less than 2 hours time, more preferably less than 1 hour.
  • the soaking time during nitriding is preferably at least 10 minutes, more preferably at least 20 minutes.
  • nitriding process can be used, such as (but not limited to) carbonitriding and nitrocarburizing.
  • the sintered components need to be steam-treated first in order to close the pores and enable control of nitrogen penetration, since an excessive nitrogen penetration into the component may lead to brittle structure.
  • this step is not necessary when providing components according to the invention since the achieved sintered density is high enough to ensure a closed porosity. The components can thus be case nitrided in a controlled manner without the prior step of steam-treatment.
  • the surface of the component comprises a nitride rich so-called white layer or compound layer of 1 to 20 ⁇ m, preferably 5 to 15 ⁇ m in thickness and a nitride enriched hardened zone down to approx. 1-6 mm in depth, preferably 1-4mm.
  • the sintered components have closed porosity directly after sintering, eliminating the need of steam treatment prior to gas nitriding.
  • the components made by the claimed method includes a deeper surface porosity in comparison with CCI-components, which during working conditions, without being bound to any specific theory, seems to provide a lubricating effect as lubricating oil and the machining enhancing agent become present inside these pores.
  • the nitrided finished component has a hardness of more than double that of the core at 0.5 to 1 mm depth, preferably above 600 MHV 0.05 , more preferably above 700 MHV 0.05 when the core hardness is around 300 MHV 0.05 or above 700 MHV 0.05 , preferably above 800 MHV 0.05 when the core hardness is around 350 MHV 0.05 .
  • the total case depth should be between 0.5-4.0 mm, preferably 1.0-3.0 mm, more preferably 1.5-2.5 mm.
  • core hardness is to be interpreted as the hardness value in the center of the component before nitriding.
  • total case depth is to be interpreted as the distance from the surface of the component, where the hardness value is the same as the core hardness value.
  • the finished component should demonstrate a good wear resistance in lubricating sliding contact.
  • the component When tested at a sliding velocity of 2.5 m/s during 100 seconds, the component should show safe wear for herzian pressures up to at least 800 MPa, preferably up to at least 900 MPa, and more preferably up to at least 1000 MPa.
  • the IRG wear transitions diagram ( FIG. 1 ) shows three main wear regions, mild (safe) wear, limited wear and scuffing (severe adhesive wear).
  • the wear depends mainly on relative sliding velocity between the contact surfaces but also on other factors such as lubrication mode, lubricant chemistry, surface roughness—topography, surface metallurgy and geometry of the contacting bodies.
  • Different alloys will have similar curves at different pressures and FIG. 1 is only shown as an illustrative example.
  • Automotive cam lobe to cam follower sliding contact is a good example of a component subjected to sliding velocities of about 0.1 m/s over 3 m/s when in use.
  • Chatterley T. C. Chatterley, “Cam and Cam follower Reliability”, SAE Paper No. 885033, 1988] summarized MIRA engine test bench testing of a number of chilled cast iron (CCI) cam lobes to CCI, coated, boronized and ceramic followers.
  • CCI chilled cast iron
  • Wear testing was done by using a commercial tribometer, a multipurpose friction and wear measuring machine with crossed cylinders test set-up ( FIG. 2 ).
  • the tribometer applies normal load on the cylinder specimen holder by dead weights/load arm while an AC thyristor controlled motor drives the counter ring.
  • the counter ring is immersed in an oil bath with approx. 25 ml oil and option for heating up to 150° C.
  • a PC controls the test and logs linear displacement in the contact, wear, friction force and oil temperature.
  • the linear displacement acquired is about three times larger than the linear wear over the wear track, since the displacement transducer is placed not over the test cylinder but on the load arm lever.
  • Hertzian pressure is proportional to the linear wear h of the cylinder sample, which in turn is proportional to the length a of the wear track. The length a and can be visually determined by using a light optical microscope, as indicated by FIG. 3 .
  • Table 1 lists the properties of the lubricating oil used during wear testing.
  • Table 2 lists the prealloyed steel powders used in the testing
  • DistaloyTM DC-1, AstaloyTM CrL and AstaloyTM 85 Mo are well known powder metallurgy prealloyed steel powders available from Höganäs AB (www.hoganas.com). Powder C is produced in the same manner as AstaloyTM 85 Mo and AstaloyTM CrL.
  • Test specimens for this investigation were sintered test specimens and reference cast iron specimens as overviewed in table 3 and 4.
  • MnS is a machining agent available from Höganäs AB (www.hoganas.com)
  • KenolubeTM is a compaction lubricant available from Höganäs AB
  • C-UF4 is a graphite product available from Graphit Kropfmühl AG (www.graphite.de).
  • FIG. 4 represents the results from the evaluation of the test specimens at 2.5 m/s. It can be seen that all specimens produced according to the invention surprisingly reach a level comparable to that of the reference R1 and R2, i.e. the chilled cast iron references.
  • R1 and R2 i.e. the chilled cast iron references.
  • the nitriding step is essential for the properties of the material.
  • the component which had only been subjected to step a)-d) of the claimed method and not to the nitriding step e) showed severe wear.
  • the component subjected to step a)- to e) on the other hand firstly showed severe wear on a Hertzian level of 1100 MPa, i.e. considerably better.
  • the results of table 5 are illustrated in FIG. 5 .
  • FIG. 6 shows a metallographic image of nitrided specimen C-A.
  • the white nitride enriched layer can be seen at the sintered surface, which provides high adhesive wear resistance as seen in the results above.
  • FIG. 7 shows the hardness profile as measured in Vickers (according to ISO 4498:2005 and ISO 4507:2000) of the specimen C-A. As can be seen in this figure the hardness is above 700 MHV 0.05 at 1 mm depth, and thus a case has been formed with hardness more than double that of the core.

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