FIELD OF THE INVENTION

The present invention concerns a powder metallurgical produced component and the manufacturing process thereof. In particular, the component is a belt pulley to be used in a combustion engine, the component having enhanced resistance against wear and sufficient strength. The production method utilise comparably non expensive powder materials and production steps, hence being cost effective.

BACKGROUND OF THE INVENTION

In industries the use of metal products manufacturing by compaction and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced and the quality

requirements are continuously raised at the same time as it is desired to reduce the cost. Through pressing and sintering of iron powder compositions net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained. The pressing and sintering technique is also characterised by high degree of material utilisation, thus the technique has great advantage over conventional techniques for forming metal parts such as moulding or machining from bar stock or forgings.

One problem connected to the press and sintering method is however that the sintered component contains a certain amount of pores decreasing the strength of the component. There are several ways to overcome the negative effect on mechanical properties caused by the component porosity. The strength of the sintered component may be increased by introducing alloying elements such as carbon, copper, nickel, molybdenum etc. The porosity of the sintered component may be reduced by increasing the compressibility of the powder composition, and/or increasing the compaction pressure for a higher green density, or increasing the shrinkage of the component during sintering. By locally eliminating the porosity where the presence of pores is most detrimental, i.e. at the surface, the negative effect of porosity may be overcome and it may even be possible to benefit from the presence of pores in the centre of a body, for example lower weight. In practise, a combination of strengthening the component by addition of alloying elements and minimising the porosity is applied. Thus, various compositions of low- alloyed steel powders, methods for compaction of these powders and methods for elimination surface porosity are known for production of PM components.

Automotive components of small and medium size are very suitable to be produced by the powder metallurgical route as these components often are made in large series and often have suitable geometries. Examples of such components are valve seats, cam lobes, connecting rods, sprockets, gears and belt pulleys. However, in order to be competitive against conventional production methods, the use of too much costly raw materials and costly processes must be avoided.

Belt pulleys are components which are produced today by the powder metallurgical production method and it should be possible to increase the production of automotive belt pulleys provided enough wear resistance could be obtained. A problem with automotive belt pulleys is that when used in severe conditions where the engine is subjected to excessive dust or hard dust particles, particles may be trapped between the belt and the pulley causing wear on the pulley which may lead to slipping of the belt and possible severe damages on the motor. Such particles may also get into the surface pores where they may be stuck and causing excessive wear on the belt. Engines subjected to such conditions are therefore equipped with pulleys having high wear resistance and being produced by alternative methods to press and sintering. It would however be beneficial if the same type of cost effectively powder metallurgical produced pulleys could be used independently on the environmental conditions for the engine. SUMMARY OF THE INVENTION

The present invention provides a solution to the problem described above. By utilising relatively inexpensive low chromium content and low molybdenum content prealloyed or admixed iron- based powders, compacting such powders together with graphite and lubricants followed by sintering and a specific post- sinter heat treatment procedure, cost effective belt pulleys having improved wear resistance and sufficient mechanical strength can be obtained. The heat treatment will also close the surface pores and hinder particle entrapment. DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the present invention, there is provided a method for producing a sintered and heat-treated component, in particular a belt pulley, having enhanced wear resistance. The method comprises the steps of; a) providing an iron based powder composition containing a prealloyed iron- based powder or an admixed iron- based powder, 0.20-0.90% by weight of graphite and, 0.20-1.5%) by weight of lubricant, the prealloyed iron- based powder or the admixed iron- based powder containing 1-3.5% by weight of chromium combined with 0.04-0.3 %, preferably 0.04-0.25%), by weight of manganese, inevitable impurities up to 1.0% and balance iron, b) compacting the iron- based powder composition into a green component having a density of 6.7-7.2 g/cm ³ , preferably 6.7-7.1 g/cm ³ , and most preferably 6.7-7.05 g/cm ³ c) sintering the green component in a reducing atmosphere at a temperature between 1000°C and 1400°C for a period between 15 minutes and 1 hour, thereby producing a sintered component, d) subjecting the sintered component to steam treatement in an atmosphere containing water vapour at a temperature between 400°C and 570°C for a period between 30 minutes and 4 hours, e) subjecting the sintered and steam treated component to a nitriding step in an atmosphere containing nitrogen at a temperature between 460°C and 550°C for a period of 15 minutes to 4 hours, or;

subjecting the sintered component to a nitrocarburizing step in an atmosphere containing nitrogen and carbon at a temperature between 525°C and 625°C for a period of 15 minutes to 4 hours, f) recovering the sintered and heat treated component. In another aspect of the invention there is provided a method for producing a sintered and heat-treated component, in particular a belt pulley, having enhanced wear resistance. The method comprises the steps of; a) providing an iron based powder composition containing a prealloyed iron- based powder or an admixed iron- based powder, 0.20-0.90% by weight of graphite and, 0.20-1.5%) by weight of lubricant, the prealloyed iron- based powder or the admixed iron- based powder containing 1 -3.5 % by weight of chromium, 0.15 - 1 % by weight of molybdenum, combined with 0.04-0.3%), preferably 0.04-0.25%), by weight of manganese, inevitable impurities up to 1.0% and balance iron, b) compacting the iron- based powder composition into a green component having a density of 6.7-7.2 g/cm ³ , preferably 6.7-7.1 g/cm ³ , and most preferably 6.7-7.05 g/cm ³ c) sintering the green component in a reducing atmosphere at a temperature between 1000°C and 1400°C for a period between 15 minutes and 1 hour, thereby producing a sintered component, d) subjecting the sintered component to steam treated in an atmosphere

containing water vapour at a temperature between 400°C and 570°C for a period between 30 minutes and 4 hours, e) subjecting the sintered and steam treated component and to a nitriding step in an atmosphere containing nitrogen at a temperature between 460°C and 550°C for a period of 15 minutes to 4 hours, or;

subjecting the sintered and steam treated component to a nitrocarburizing step in an atmosphere containing nitrogen and carbon at a temperature between 525°C and 625°C for a period of 15 minutes to 4 hours, f) recovering the sintered and heat treated component. Preparation of the iron-based alloyed steel powder.

The iron- based powder used for producing the belt pulley may be produced through atomization of a melt containing the appropriate amount of chromium (Cr) and manganese (Mn), i.e. the alloying elements being present in pre- alloyed form. If higher strength of the core of the produced component is desired appropriate amount of molybdenum (Mo) may optionally be added to the melt. Alternatively the iron- based powder may be produced by admixing all or part of the alloying element(s) to an iron powder. Preferably the alloying elements being pre- alloyed.

The atomized powder is further subjected to a reduction annealing process. The particle size of the steel powder could be any size as long as it is compatible with the press and sintering process. Examples of suitable particle size is the particle size of the known powder ABC 100.30 available from Hoganas AB, Sweden, having about 10% by weight above 150 um and about 20 % by weight below 45 μιη.

Cr

Cr serves to strengthen the matrix by solid solution hardening, Cr also increases hardenability. At the nitriding or nitrocarburizing processes Cr will form nitrides or nitrides and carbides enhancing oxidation resistance and abrasion resistance of the sintered body. A content of Cr above 3.5% by weight will however reduce the compressibility of the steel powder and render the formation of a desired

ferritic/pearlitic or pearlitic microstructure more difficult. Preferably from the viewpoint of compressibility the content is below 3% by weight, even more preferred below 2% by weight. Cr content below 1% by weight will have insignificant effect on desired properties.

Mo

Mo increases hardenability and strength. Mo will as Cr form nitrides at the nitriding process. Mo may be present in a content of 0.15-1%) by weight, preferably 0.2-0.6%) by weight. A content of Mo less than 0.15% by weight will have insignificant effect on the properties and above 1% by weight the cost/effect ratio will become disadvantageous. Mo may preferably be prealloyed as Mo has a very limited negative effect on compressibility. In some case it may be interesting to diffusion- bond some or all of the Mo content to the surface of the iron or iron- based powder. In such case lower amounts of Mo may be used. The reason being that the limited Mo content located in the particle boundary region contributes more to the hardenability than that in the matrix. Mn

Mn will, as for chromium, increase the strength, hardness and hardenability. Content above 0.3% by weight will increase the risk of formation of manganese containing oxides and inclusions in the steel powder and will also have a negative effect on the compressibility due to solid solution hardening and increased ferrite hardness.

Preferably, the manganese content is up to 0.25% by weight, even more preferably the manganese content is up to 0.2% by weight. If the manganese content is below 0.04% by weight it will not be possible to use recycled scrap unless a specific treatment for the reduction the Mn content during the course of the steel manufacturing is carried out.

Oxygen (O) is suitably at most 0.25% by weight to limit formation of oxides with chromium and manganese that impairs strength and compressibility of the powder. For these reasons oxygen preferably is at most 0.18% by weight. Carbon (C ) in the steel powder shall be at most 0.1% by weight. Higher contents will unacceptably reduce the compressibility of the powder. For the same reason nitrogen shall be kept less than 0.1% by weight.

The total amount of inevitable impurities should be less than 1% by weight in order not to impair the compressibility of the steel powder or act as formers of detrimental inclusions.

Iron-based powder composition Before compaction the iron-based steel powder is mixed with graphite and lubricant(s). Graphite is added in an amount between 0.20-0.9% by weight of the composition and lubricant(s) are added in an amount between 0.2-1.5% by weight of the composition. The iron-base powder composition can also be prepared as a "bonded mix" where finer particles in the iron- based powder composition are adhered to the larger particles by means of a binder.

Graphite

In order to enhance strength and hardness of the sintered component carbon is introduced in the matrix. Carbon is added as graphite in amount between 0.20-0.9% by weight of the composition. An amount less than 0.20% by weight will result in too low hardness and strength and an amount above 0.9% will result in an excessive formation of carbides yielding too high hardness, insufficient elongation and impair the

machinability properties.

Lubricant(s)

Lubricant(s) are added to the composition in order to facilitate compaction of the composition and ejection of the compacted component. The addition of less than 0.20% by weight of the composition will have insignificant effect and the addition of above 1.5% by weight of the composition will result in too low density of the compacted body. 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.

Other substances

Other substances such as hard phase materials and machinability enhancing agents, such as MnS, MoS ₂ , CaF ₂ , bentonites and other kinds of minerals etc. may be added at an amount of 0.1-1%> by weight of the iron- based powder composition.

Compaction and Sintering The iron-based powder composition is transferred into a mould and subjected to a compaction pressure of about 400-1300 MPa to a green density between 6.7 and 7.2 g/cm ³ , preferably between 6.7 and 7.1 g/cm ³ , and most preferably between 6.7 and 7.05 g/cm ³ . The obtained green component is further subjected to sintering in a reducing atmosphere at a temperature of about 1000-1400° C, preferably between about 1100- 1300° C

Post sintering treatments

Sintering is followed by a steam treatment process in order to close the porosity and improve the wear resistance. The closure of the porosity at the surface is also beneficial for the ability of controlling the formation of the nitride containing compound layer.

Thus, at the steam treatment an oxide layer, mainly free from surface pores, is created having a thickness of 5-15 μιη, preferably 5-10 μιη. The steam treatment process may be performed by subjecting sintered component to steam at atmospheric pressure or above at a temperature between 400°C and 570°C, preferably 500°C -550°C, most preferably 500°C -520°C for a period between 30 minutes and 4 hours, preferably for a period between 60 minutes and 1.5 hours. After the steam treatment step, the component is subjected to a to a nitriding step in an atmosphere containing nitrogen. The nitriding step is performed at a temperature between 460°C and 550°C, preferably between 470°C and 490°C for a period of 15 minutes to 5 hours, preferably 30 minutes to 1.5 hours.

Alternatively the steam treated component is subjected to a nitrocarburizing step in an atmosphere containing nitrogen and carbon. The nitrocarburizing step is performed at a temperature between 525 °C and 625°C, preferably between 550°C and 590°C, more preferably between 570°C and 590°C for a period of 15 minutes to 4 hours, preferably 30 minutes to 1.5 hours.

Nitriding or nitrocarburizing are classified as low temperature heat treatments in contrast to case hardening which is performed at higher temperatures above 850°C. By performing the heat treatments at low temperatures less dimensional change and distortion of the heat treated body will occur, compared to case hardening, as the material is not subjected to phase transformations.

During the nitriding or nitrocarburizing the oxide layer is gradually reduced and transformed into a compound layer containing Cr- or Cr-Mo- containing nitrides and carbides. In one embodiment of the invention the process for formation of the compound layer is stopped before the entire oxide layer at the surface of the component has been reduced and transformed into a compound layer, thus a component having remaining layer of oxides combined with a compound layer is obtained. In another embodiment of the invention all oxides at the surface are reduced and transformed into the compound layer.

The nitriding or nitrocarburizing processing is controlled through adjusting temperature and/or nitriding or nitrocarburizing time and /or the composition of the nitriding or nitrocarburizing atmosphere in order to obtain a compound layer having a thickness of 1-25 μιη, preferably 5-20 μιη, more preferably 5-15 μιη and even more preferably 8-13 μιη. A too thick compound layer increases the risk of peeling, whereas a compound layer thinner than 5 μιη and definitely thinner than 1 μιη will probably render the component insufficient wear properties. The hardness of the compound layer is above 700 mhv (microvikers) measured on the component surface. Beneath the compound layer a diffusion zone is obtained giving a good support to the surface layer(s). The hardness of this diffusion layer is above 500 mhv and preferably above 550 and most preferably 600 mvh just below the compound layer and decreased gradually to the centre. The hardening depth, where the hardness is 450 mhv, shall be between 200-700 μιη from the surface.

The hardness of the core is mainly determined by the carbon content of the material. The hardness of the core should however be below 400 mhv, preferably below 350 mhv in order to render the core enough ductility.

Sintered and heat treated component

According to another aspect of the present invention there is provided a sintered and heat-treated component, in particular a belt pulley, having enhanced wear resistance. The density of the sintered component, average density, shall be between 6.7 and 7.2 g/cm ³ , preferably between 6.7 and 7.1 g/cm ³ , and most preferably between 6.7 and 7.05 g/cm ³ . Although a sintered density above 7.2 g/cm ³ may be beneficial in terms of strength and ductility of the component such density it is not regarded as cost effective for applications which the present invention is directed to, i.e. components subjected to wear from hard dust particles, e.g. belt pulleys. In order to reach higher sintered density above 7.2 g/cm ³ costly process steps such as high compaction pressures and/or elevated and/or prolonged sintering times have to be applied. In order to control the formation of the subsequently formed layers it is also of outmost importance that the inter component variation in sintered density is kept at a minimum. Thus the inter component variation in sintered density shall be at most 0.4 g/cm ³ , preferably at most 0.3 g/cm ³ and most preferably at most 0.2 g/cm ³ . If this variation is too high the different layers will not be uniformly formed and have an unacceptable variation in thickness leading to an unacceptable variation in hardness profile. The component is further characterised by;

- having a pearlitic and/or bainitic microstructure, alternatively pearlitic and/or bainitic microstructure combined with ferrite, and at least one specified wear resistant surface layer and a diffusion zone,

-containing Cr combined with Mn and C as alloying elements at contents of;

Cr; 1-3.5% by weight, Mn; 0.04-0.3% by weight,

C; 0.20-0.9% by weight, preferably 0.3-0.6% by weight,

the balance being iron and inevitable impurities up to 1.0% by weight, -the wear resistant surface layer(s) consisting of;

optionally an oxide layer having a thickness of 0-15 μιη, preferably 0-10 μιη, more preferably 0-5 μιη,

a compound layer having a thickness of 1-25 μιη, preferably 5-20 μιη, more preferably 5-15 μιη, and containing nitrides or nitrides and carbides of Cr,

-the diffusion zone having hardness above 500 mhv, preferably above 550 mhv and most preferable above 600 mvh just below the compound layer and decreasing gradually towards the centre. The hardening depth or hardness profile shall be 450 mvh at a depth between 200 and 700 μιη from the surface.

In specific embodiments of the invention the Cr content is limited to 1-3%, preferably 1-2%) by weight.

In specific embodiments of the invention the Mn content is limited to 0.04-0.25% or preferably 0.04-0.2% by weight.

In a specific embodiment the oxide layer is excluded, hence wear resistance is solely provided by the compound layer and diffusion layer. In an aspect of the invention the component is further characterised by;

- having a pearlitic and/or bainitic microstructure, alternatively pearlitic and/or bainitic microstructure combined with ferrite, and at least one specified wear resistant surface layer and a diffusion zone,

-containing Cr combined with Mn and C as alloying elements at contents of;

Cr; 1-3.5% by weight,

Mo; 0.15-1% by weight,

Mn; 0.04-0.3% by weight,

C; 0.20-0.9% by weight, preferably 0.3-0.6% by weight,

the balance being iron and inevitable impurities up to 1.0 % by weight, -the wear resistant surface layer(s) consisting of;

optionally an oxide layer having a thickness of 0-15 μηι, preferably 0-10 μηι, more preferably 0-5 μιη,

a compound layer having a thickness of 1-25 μιη, preferably 5-20 μιη, more preferably 5-15 μιη, and containing nitrides or nitrides and carbides of Cr and Mo,

-the diffusion zone having hardness above 500 mhv, preferably above 550 mhv and most preferable above 600 mvh just below the compound layer and decreasing gradually towards the centre. The hardening depth or hardness profile shall be 450 mvh at a depth between 200 and 700 μιη from the surface.

In specific embodiments of the invention the Cr content is limited to 1-3%, preferably 1-2%) by weight. In specific embodiments of the invention the Mo content is limited to 0.2-0.6% by weight.

In specific embodiments of the invention the Mn content is limited to 0.04-0.25% or preferably 0.04-0.2%> by weight.

In a specific embodiment the oxide layer is excluded, hence wear resistance is solely provided by the compound layer and diffusion layer.

EXAMPLES;

The following non limiting examples illustrate the invention. An iron- based prealloyed powder Astaloy®CrA, available from Hoganas AB Sweden , having a content of Cr of 1.8% by weight and a content of Mn of 0.12% by weight, was mixed with 0.35% of graphite and 0.7% of a lubricant Kenolube®, available from Hoganas AB Sweden.

The obtained iron- based powder composition was further compacted into test samples having a diameter of 65 mm and height of 32 mm to a density of 6.9 g/cm ³ .

The green samples were thereafter sintered in an atmosphere of 90 % nitrogen gas/ 10 % hydrogen gas at 1120°C for 30 minutes.

The sintering step was followed by steam treatment in a water vapour containing atmosphere at 510°C for 90 minutes.

After steam treatment the components were subjected to a nitrocarburizing step at 580°C for 90 minutes.

In a comparative example an iron powder having 0.5% by weight of Mo and 0.5% by weight of Ni diffusion bonded to the surface thereof was used. This powder was also mixed with 0.35% of graphite and 0.7% of a lubricant Kenolube®, available from

Hoganas AB Sweden. Compaction, sintering, steam treatment and nitrocarburizing were performed as described above.

Samples were cross sectioned for metallographic examination. The examination of the sample according to the invention reveals an oxide layer having a thickness of 2-3 μιη, a compound layer having a thickness of 10-15 μιη and a diffusion zone beneath the compound layer. Figure 1 shows the cross section of the sample according to the invention revealing the well-defined coherent wear resistant surface layers without any surface porosity. Microvickers hardness, mhv, HVo.i , was determined according to ISO 6507-1 at various depths from the surface. The results are shown in figure 3.

For the sample according to the invention 650-600 HVo.i was measured at a position just below the compound layer, and the case depth with a hardness higher than 450 HVo.i was in the range of 500 mm.

The metallographic structure of the cross section was perlitic/ferrititc. Figure 2 shows the cross section of the comparative example. The figure showing less well-defined surface layer having varying depth and some surface pores. Microvickers hardness for the comparative example was measured to 300-350 HVo.i at a position just below the surface.

Figure 2 also shows the compound layer formed as a network in the grain boundaries as well as on the surfaces off the inner pores which may cause brittleness.

Thus, the cross section of the sample according to the invention exhibits a well-defined compound layer and a relatively thick diffusion zone having higher hardness.