NITRIDED SINTERED STEELS

Abstract

The present invention concerns a method of producing sintered components, and sintered components by the method. The method provides a cost effective production of sintered steel parts with wear resistance properties comparable to those of components made from chilled cast iron.

Description

FIELD OF THE INVENTION

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.

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. As net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained by press and sintering of iron based powder compositions in combination with a high degree of material utilisation, this technique has a great advantage over conventional techniques for forming metal parts such as moulding or machining from bar stock, casting or forgings.

It is desirable to increase the performance of sintered parts so that more parts can be substituted to this cost effective technique. Various industrial steel components, for instance in the automotive industry, have successfully been produced by the press and sintering technique. Automotive parts are manufactured in high volume for applications having strict performance, design and durability requirements. The single press and single sintering technique is therefore very suitable for production of such parts, provided that the overall quality requirements can be met.

For certain power train and valve train components in the automotive industry, such as cam lobes, requirements on wear resistance have so far made it very difficult to convert to conventionally sintered products. The predominant production techniques for such components are today machining from bar stocks, or casting with chilled cast iron (CCI). In the case of cam lobes for small cars, where the wear resistance requirements are somewhat lower; the parts have been produced successfully using double press/double sintering. So far however, no manufacturing technique involving single pressing and single sintering has proven to provide wear properties comparable to those of components manufactured using CCI.

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.

SUMMARY OF THE INVENTION

It has surprisingly been found that by using certain iron-based powder alloy compositions in combination with warm die compaction and a short nitriding process, components with wear resistance comparable to that of components made with CCI can be manufactured.

More specifically this can be achieved by a method of producing sintered components by single press and single sintering comprising the following steps:

• • a) Providing pre-alloyed iron-based steel powder comprising less than 0.3% by weight of Mn and at least one of Cr in an amount between 0.2-3.5% by weight, Mo in an amount between 0.05-1.20% by weight and V in an amount between 0.05-0.4% by weight, and maximum 0.5% incidental impurities, the balance being iron,
  • b) Mixing said pre-alloyed iron-based steel powder with lubricant and graphite, and optionally machining enhancing agent(s) and other conventional sintering additives,
  • c) Subjecting the mixed composition of step b) to compaction at pressures of 400-2000 MPa, thereby providing a compact,
  • d) Sintering said compact from step c) in a reducing atmosphere at a temperature between 1000-1400° C., thereby providing a sintered component,
  • e) Nitriding said sintered component of step d) in a nitrogen containing atmosphere, at a temperature of 400-600° C., with a soaking time of less than 3 hours

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.

DETAILED DESCRIPTION OF THE INVENTION

Preparation of the Iron-Based Alloyed Steel Powder.

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 μM and 15-30% by weight below 45 μm. However, coarser as well as finer grained powders may be used.

The use of coarse iron-based steel powders is increasingly popular in the field of powder metallurgy. Example of such powders are iron-based powders having an average particle size between 75 and 300 μm, wherein less than 10% of the powder particles have a size below 45 μm and the amount of particles above 212 μm 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 μm.

Contents of the Prealloyed Steel Powder

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.

Nickel, Ni, increases strength and hardness while providing good ductility properties. However, nickel is an expensive element and is avoided if possible. If added, contents are kept low. 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.

The amount of 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.

Preferred Embodiments of the Prealloyed Steel Powder

In a preferred embodiment the prealloyed steel powder according to the invention consists of (in % by weight):

Fe: Bal.

Mn: 0.09-0.3

Cr: 1.3-1.9

Mo: 0-0.3

and max 0.3 incidental impurities

In another preferred embodiment the prealloyed steel powder according to the invention consists of (in % by weight):

Fe: Bal.

Mn: 0.09-0.3

Cr: 1.3-1.6

Mo: 0.15-0.3

and max 0.3 incidental impurities

In yet another preferred embodiment the prealloyed steel powder according to the invention consists of (in % by weight):

Fe: Bal.

Mn: 0.09-0.3

Cr: 1.5-1.9

Mo: 0-0.1

and max 0.3 incidental impurities.

In yet another preferred embodiment the prealloyed steel powder according to the invention consists of (in % by weight):

Fe: Bal.

Mn: 0.09-0.3

Cr: 2.8-3.2

Mo: 0.4-0.6

and max 0.3 incidental impurities

In yet another preferred embodiement prealloyed steel powder according to the invention consists of (in % by weight):

Fe: Bal.

Mn: 0.09-0.3

V: 0.05-0.4

Mo: 0-0.1

and max incidental 0.3 impurities

Powder Composition

Before compaction, 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.

In order to enhance strength and hardness of the sintered component 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. Preferably, 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. Preferably, 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.

In one embodiment 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.

In a preferred embodiment 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, Cu, 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. Preferably, if admixed, 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, MoS₂, CaF₂, 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.

Other conventional sintering additives, such as hard phase materials, may optionally be admixed to the composition.

Compaction

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. In a preferred embodiment 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/cm³, preferably at least 7.15 g/cm³, more preferably at least 7.20 g/cm³.

Thanks to the choice of lubricant and compaction process, high green densities can be reached, ensuring high sintered densities without excessive dimensional changes. This provides good tolerances and closed porosity of the sintered component.

Sintering

The obtained green component is further subjected to sintering in a reducing atmosphere at a temperature of about 1000-1400° C. In a preferred embodiment 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. However, depending on requirements, 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/cm³, preferably 7.15 to 7.50 g/cm³, more preferably 7.20 to 7.45 g/cm³. However it is also possible to sinter to higher densities than 7.6 g/cm³.

Post Sintering Treatments

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. In a preferred embodiment, 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. However, the soaking time during nitriding is preferably at least 10 minutes, more preferably at least 20 minutes.

Optionally, other common types of nitriding process can be used, such as (but not limited to) carbonitriding and nitrocarburizing.

Usually, when gas nitriding sintered components, 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. However, 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.

Using the inventive method the surface of the component comprises a nitride rich so-called white layer or compound layer of 1 to 20 μm, preferably 5 to 15 μm in thickness and a nitride enriched hardened zone down to approx. 1-6 mm in depth, preferably 1-4mm.

Properties of the Finished Component Components manufactured according to the invention achieve high wear resistance in sliding lubricated contact. The wear resistance achieved is comparable to components made with chilled cast iron.

The sintered components have closed porosity directly after sintering, eliminating the need of steam treatment prior to gas nitriding.

Furthermore, 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.

In a preferred embodiment 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.

The term core hardness is to be interpreted as the hardness value in the center of the component before nitriding. The term 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.

According to the test method described in the example section, the finished component should demonstrate a good wear resistance in lubricating sliding contact. 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.

EXAMPLES

Testing Method

A general characterization of wear in lubricated sliding contacts was done by researchers at international plane joined in informal IRG-WOEM group supported by OECD in 1980'. The several co-coordinated investigations gave a severity of valuable results of which the IRG-wear transitions diagram may be the most important one, see FIG. 1.

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. In 1988, 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. A Hertzian level of 800 MPa was failure-free for a majority of test runs, while 1000 MPa level passed only CCI to SiN ceramic test combination.

Based on the above, wear testing in the investigation was performed at three sliding velocities, 0.1, 0.5 and 2.5 m/s, having standard engine oil (see table 1 for specification) at 90° C. as lubricant. At 2.5 m/s, testing was performed by stepwise increasing Hertzian pressure until scuffing occurred.

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 1
Lubricating oil used in wear testing
SAE class/API grade              10W40/API SJ
Oil base                         Semi-synthetic
AW additive                      ZnDDP
Density in g/ml at 15° C. (ASTM D4052) 0.875
Kin. viscosity in mm2/s (ASTM D445)
40° C.                           88
100° C.                          13.5
Viscosity index (ASTM D2270)     150

Table 2 lists the prealloyed steel powders used in the testing

TABLE 2
Prealloyed steel powders used
ID	Name	Fe	Mn %	Cr %	Mo %	Ni %
R0	Distaloy ™ DC-1	Bal.	<0.3	—	1.4	2
[Reference
sample]
A	Astaloy ™ CrL	Bal.	0.12	1.5	0.2	—
B	Astaloy ™ 85 Mo	Bal.	0.11	<0.1	0.9	—
C	—	Bal.	0.13	1.8	<0.1	—

Distaloy™ DC-1, Astaloy™ CrL and Astaloy™ 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 Astaloy™ 85 Mo and Astaloy™ CrL.

Test specimens for this investigation were sintered test specimens and reference cast iron specimens as overviewed in table 3 and 4.

TABLE 3
Reference specimens
ID	Type	Manufacturing method
R1	DIN GJL-350	Chilled cast iron,
	(Fe—3C—2Si—0.5Mn—0.3Cr—0.6Cu)	ground, nitrited
R2	DIN GJL-350	Chilled cast iron, ground
	(Fe—3C—2Si—0.5Mn—0.3Cr—0.6Cu)

TABLE 4
Specimens manufactured by powder metallurgy
ID	Composition*	Manufacturing method
C-R	Powder R + 0.65% C-	Double press/Double sinter
[reference]	UF4 + 0.5% MnS +	1st pressing at 800 MPa followed by
	0.6% Kenolube ™	sintering to approximately 7.1 g/cm3
		density.
		2nd pressing at 1000 MPa followed by
		sintering to approximately 7.5 g/cm3
		density.
C-A	Powder A + 0.45% C-	Single press/Single sinter + Nitriding
	UF4 + 0.5% lubricant	Compaction with heated die,
	suitable for	followed by sintering at 1120° C. for
	compacting with a	30 mins in 90% N2/10% H2
	heated die	atmosphere, to a sintered density
		of 7.25 g/cm3.
		Gas nitriding at 510° C., in
		75NH3/25N2 atmosphere, with
		soaking time of 1 h.
C-B	Powder B + 0.45% C-	Single press/Single sinter + Nitriding
	UF4 + 0.5% lubricant	Compaction with heated die,
	suitable for	followed by sintering at 1120° C. for
	compacting with a	30 mins in 90% N2/10% H2
	heated die	atmosphere, to a sintered density
		of 7.25 g/cm3.
		Gas nitriding at 510° C., in
		75NH3/25N2 atmosphere, with
		soaking time of 1 h.
C-C	Powder C + 0.45% C-	Single press/Single sinter + Nitriding
	UF4 + 0.5% lubricant	Compaction with heated die,
	suitable for	followed by sintering at 1120° C. for
	compacting with a	30 mins in 90% N2/10% H2
	heated die	atmosphere, to a sintered density
		of 7.25 g/cm3.
		Gas nitriding at 510° C., in
		75NH3/25N2 atmosphere, with
		soaking time of 1 h.

*) MnS is a machining agent available from Höganäs AB (www.hoganas.com), Kenolube™ is a compaction lubricant available from Höganäs AB, and 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. When comparing the reference C-R to C-A, C-B and C-C of the invention, it becomes clear how efficient the new method of producing sintered components by single press/single sintering really is.

Moreover, a comparison was made for composition C-A, before and after the nitriding step at three velocities. The results can be seen in table 5.

TABLE 5
Results of wear testing for C-A
		Hertzian
	Velocity/	pressure
	Time	(MPa)	As-sintered	Nitrided
	2.5 m/s	1100		Severe wear/scuff.
	for 100	1000		Safe/mild wear
	sec.	900		Safe/mild wear
		600	Severe wear
		500	Severe wear
		380	Severe wear
		320	Severe wear
	Linear wear/Wear coefficient k
	h (μm)/(mm3/[Nm])
	0.5 m/s	800	50/71	7/10
	for 23 h	500	36/20	5/2
	0.1 m/s	800	19/2	6/2
	for 23 h	500	14/2	4/3

It can be seen in table 5 that the nitriding step is essential for the properties of the material. Already at a Hertzian level of 320 MPa 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.

Claims

1. A method of producing sintered components by single press/single sintering comprising the steps of:

a) providing pre-alloyed iron-based steel powder comprising less than 0.3% by weight of Mn and at least one of Cr in an amount between 0.2-3.5% by weight, Mo in an amount between 0.05-1.20% by weight and V in an amount between 0.05-0.4% by weight, and maximum 0.5% incidental impurities, the balance being iron,

b) mixing said pre-alloyed iron-based steel powder with lubricant and graphite, and optionally machining enhancing agent(s) and other conventional sintering additives,

c) subjecting the mixed composition of step b) to compaction at pressures of 400-2000 MPa, thereby providing a compact,

d) sintering said compact from step c) in a reducing atmosphere at a temperature between 1000-1400° C., thereby providing a sintered component,

e) nitriding said sintered component of step d) in a nitrogen containing atmosphere, at a temperature of 400-600° C., with a soaking time of less than 3 hours,

2. A method according to claim 1, wherein the lubricant consists of 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.

3. A method according to claim 1, wherein the compact is not steam treated before nitriding in step e).

4. A method according to claim 1, wherein in step c) the compact is compacted to a green density of at least 7.10 g/cm3.

5. A method according to claim 1, wherein in step d) the sintered component is sintered to a density between 7.1-7.6 g/cm3.

6. A method according to claim 1, wherein the pre-alloyed iron-based steel powder further comprises between 0.1-1.0% by weight of Ni.

7. A method according to claim 1, wherein the pre-alloyed iron-based steel powder is essentially free from Ni.

8. A method according to claim 1, wherein the pre-alloyed iron-based steel powder further comprises between 0.05% and 0.50% by weight of one or more of element(s) selected from the group of tungsten (W), titanium (Ti), niobium (Nb) and aluminium (Al).

9. A method according to claim 1, wherein the pre-alloyed iron-based steel powder consists of, in percentage by weight:

Fe: Bal.

Mn: 0.09-0.3

Cr: 1.3-1.6

Mo: 0.15-0.3

and max 0.3 incidental impurities.

10. A method according to any one of claims 1 5, claim 1, wherein the pre-alloyed iron-based steel powder consists of, in percentage by weight:

Fe: Bal.

Mn: 0.09-0.3

Cr: 1.5-1.9

Mo: max 0.1

and max 0.3 incidental impurities.

11. A method according to claim 1, wherein the pre-alloyed iron-based steel powder consists of, in percentage by weight:

Fe: Bal.

Mn: 0.09-0.3

Cr: 2.8-3.2

Mo: 0.4-0.6

and max 0.3 incidental impurities.

12. A method according to claim 1, wherein the pre-alloyed iron-based steel powder consists of, in percentage by weight:

Fe: Bal.

Mn: 0.09-0.3

V: 0.05-0.4

Mo: max 0.1

and max 0.3 incidental impurities.

13. A nitrated, sintered component, produced according to claim 1, and having a wear resistance in lubricating sliding contact that provides safe wear for hertzian pressures up to at least 800 MPa when tested at a sliding velocity of 2.5 m/s during 100 seconds.

14. A nitrated, sintered component, produced according to claim 1, and having a wear resistance in lubricating sliding contact that provides safe wear for hertzian pressures up to at least 900 MPa, when tested at a sliding velocity of 2.5 m/s during 100 seconds.

15. A nitrated, sintered component, produced according to claim 1, and having a wear resistance in lubricating sliding contact that provides safe wear for hertzian pressures up to at least 1000 MPa, when tested at a sliding velocity of 2.5 m/s during 100 seconds.