A POWDER METALLURGICAL COMPOSITION AND SINTERED COMPONENT

FIELD OF THE INVENTION

The present invention concerns a method of making a sintered components and the component made by the method. Examples of such components are parts in

turbochargers for internal combustion engines.

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 being raised. At the same time it is desired to reduce the costs. Since net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained by pressing and sintering of iron powder compositions, which implies a high degree of material utilisation, this technique has a great advantage over conventional techniques for forming metal parts such as casting, 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. Basically there are two ways to overcome the negative effect on mechanical properties caused by the component porosity:

1) The strength of the sintered component may be increased by introducing alloying elements such as carbon, copper, nickel molybdenum etc.

2) 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. In practise a combination of strengthening the component by addition of alloying elements and minimising the porosity is applied.

Iron-based powder compositions for powder metallurgical production of pressed and sintered component are often alloyed with carbon and copper. Other alloying elements are also commonly used. The alloying elements may be added according to various methods. For example, the alloying elements may be mixed with the iron-powder, carbon in the form of graphite is commonly added in this way. Such compositions may be called premixes. The alloying elements may also be added to the melt prior to the atomisation process. Iron-based powders produced according to this method are commonly referred to as pre-alloyed iron-based powders. The alloying elements may also be attached to the surface of the iron-based powder by a thermal diffusion bonding process. Such powders are referred to as diffusion bonded powders. Alternatively, the alloying elements may be attached to the surface of the iron-based powder by various binders. Such powders are commonly referred to as bonded powders or bonded mixtures.

During sintering, metal powder particles of the compacted or pressed component, the green component, will diffuse together in solid state forming strong bonds, so called sintering necks. The result is a relatively high dense net shape, or near net shape, part suitable for low or medium performance applications. Typically, sintered articles are manufactured from iron powder mixed with copper and graphite powders. Other types of materials suggested include iron powder prealloyed with nickel, chromium and molybdenum and small amounts of manganese to enhance hardenability without developing stable oxides. Sometimes machining of the sintered component is unavoidable. In order to facilitate the machining process, machinability enhancing agents such as MnS may be added to the iron-based powder composition.

For iron-based pressed and sintered components which are subjected to wear and corrosion and elevated temperature a prerequisite in order to withstand such conditions is that the components contain suitable alloying elements. High sintered density, i.e. low porosity is also necessary. Examples of such components are components in

turbochargers, such as unison or nozzle rings and sliding nozzles. In these cases a porosity less than 10 %, preferably less than 7% is needed. For some applications the components have to be gas tight. Today, unison or nozzle rings and sliding nozzles are often made from castings of alloyed iron-based material containing MnS phases, the presence thereof rendering a machinability enhancing effect. The powder metallurgical production route is very suitable for producing such components as they are often produced in large quantities and the components have a suitable size. In order to fully utilise the benefits of the powder metallurgical production route, the dimensional scatter between components have to be small and within a certain tolerance level, in order to avoid costly machining processes. In practise, machining can not always be avoided why the addition of machining enhancing agents to the iron-based powder composition may be needed, however, such additions should not to any larger extent negatively influence other properties such as strength, wear -, creep- and corrosion resistance of the sintered part. Different elements may also be added to the melt, prior to the atomisation process, which in the following atomisation process and/or at sintering of the component will form substances enhancing the machinability. An example of this is the formation of MnS phase in the atomised and annealed powder, and/or in the sintered component.

A problem when compacting alloyed iron-based powders is that such powders are harder and have less compressibility. By using premixed powder or diffusion bonded compositions instead of corresponding pre-alloyed compositions, the compressibility can to a certain extent be enhanced. A drawback with premixed powder compositions is however that segregation of the finer alloying elements within the coarser iron powder bulk may occur. Segregation can be reduced by using so called bonded compositions, i.e. the finer alloying elements are bonded to the surface of the coarser iron powders by a binder, or the finer alloying elements are bonded to the surface of the iron powder through applying a thermal diffusion binding process.

Pressed and sintered components made from inhomogeneous iron based powder compositions such as premixed compositions, bonded compositions or diffusion alloyed compositions will all exhibit an inhomogeneous micro structure which for certain applications is less desirable. The risk of obtain inhomogeneous density distribution, leading to non uniform shrinkage during sintering, may be higher compared to using a pre-alloyed powder.

By using finer particle size of the iron-based powder composition the green component will shrink more during sintering as such powders have higher specific surface, more active surface, thus yielding a higher sintered density and less porosity. A drawback with using fine powders is however the increased costs for its production.

Metal Injection Moulding, MIM, is a technique where very fine metal powders, typically having a value D ₅₀ below 10 μπι, are used (D ₅₀ ; 50 % by weight of the particles have a diameter less than D ₅₀ , 50 % by weight have a diameter above D ₅₀ ). The powder is mixed with high amounts of organic binders and lubricants in order to form a paste suitable to be injected in a die. The injected component is released from the die and is subsequently subjected to a de-binding process for removing the organic material followed by a sintering process. Small complex shaped components having low porosity can be produced by this method. A drawback is, however, a low production speed. The use of costly organic material also contributes to a relatively high production cost. The patent application DE10 2009 004 881 Al describes the production of a turbocharger component by this method.

In the uniaxially pressing technique, coarser iron-based powders are normally used, typically the particle size of the iron-based powder is below 200 μιη with about less than 25 % below 45 μιη. By using finer iron-based powder in the powder composition components having higher sintered density may be produced. Such compositions, however, normally suffer from poor flowability i.e. the ability of uniformly filling different portions of the die with the powder with uniform apparent density, AD.

Uniform filling with as small variation as possible of AD of the powder in different portions of the die is essential in order to obtain a sintered component having small variations of the sintered density in different portions. Further, a uniform and consistent filling ensures also that the weight and dimensional variations between the pressed and sintered components can be minimized.

Several production methods for obtaining components having high density have been suggested. For example in US patent US6503443B 1 is suggested comprsing blending an iron - based powder with graphite, compacting the powder blend to a preform having a density of not less than 7.3 g/cm ³ , provisional sintering the preform at a temperature between 700-1000°C, the obtained provisional sintered body having a structure in which graphite remains along a grain boundary of the metal powder, recompacting and resintering the body. Cold forging by backward extrusion is recommended in the recompaction step.

US patent application US2008/0202651 discloses a method for manufacturing a high density iron- based compact including the steps of mixing an iron- base powder with graphite, subjecting the mixture to precompaction and thereafter to presintering at a temperature of 1000-1300°C, the obtained presintered body having a content of C between 0.10-0.50%, oxygen 0.3 % or less, nitrogen 0.010 or less, the density being 7.2 g/cm ³ or above.- The presintered being thereafter subjected to high velocity compaction and optionally to resinteritng or heat treatment.

The composition must also flow fast enough during the filling stage to obtain an economical production speed. Apparent density, flowability and flow rate are commonly referred to as powder properties. Various methods for agglomeration of fine powders to coarser agglomerates having sufficient powder properties and still enhancing shrinkage during sintering have been suggested in order to overcome the above mentioned problems. JP3527337B2 describes a method for producing agglomerated spray dried powder from fine metal powder or pre alloyed powder.

Components for turbocharger, such as unison or nozzle rings and sliding nozzles, usually contain hard phases in order to withstand wear at elevated temperature. Such hard phases may be carbides or nitrides. Such components also contain various alloying elements in order to provide enough strength at elevated temperatures above 700°C. The presence of hard phases in combination with alloying elements has however normally a negative influence of compressibility of the iron-based powder composition and of the machinability of the sintered components. Further, in order to reach a low porosity, high sintered density, of the sintered component such components have to undergo a high degree of shrinkage during sintering, increasing the risk of obtaining components having an unacceptable variation in dimensions, both within as well as between components produced. Thus, there is a need of a method for producing components by uniaxial compaction of an austenitic stainless steel powder composition wherein the stainless steel powder has a particle size distribution which not negatively influence the powder properties. Such method especially suitable for producing components for turbocharger having high sintered density, low dimensional scatter between components produced as well low variation in dimensions within the component. The components produced should also be easy to machine despite the presence of hard phases.

SUMMARY OF THE INVENTION The object of the present invention is to eliminate or at least to minimize the problems described above. This is achieved through a method of producing a sintered component by uniaxial compaction according to claim 1 and through a sintered component according to claim 9.

DETAILED DESCRIPTION Stainless steel powder

The stainless steel powder is preferably produced by water or gas atomisation of a melt containing iron and alloying elements. The stainless steel powder is preferably austenitic. The powder may also be produced by mixing alloying elements with an iron- or iron- based powder. The particle size of the atomised stainless steel powder having in this context a coarse particle size distribution which normally in the technical field is referred to as a "100 mesh powder" (particle size below 100 mesh, 150 μπι) or coarser. Such particle atomised stainless steel powder may have a mean particle size between 50-100μπι, 10-50 % of the particles being less than μπι and less than 5 % above 212 μπι. In an alternative embodiment the particle size distribution may be coarser such that less than 10 % being below 45 μπι and at least 40, preferably at least 60 % by weight being above 106 μπι or 212 μπι.

Alloying elements in the stainless steel powder

Chromium, Cr

The amount of Cr is between 12-30% of the stainless steel powder. Cr contributes to corrosion resistance in various atmospheres such as a sulphur containing atmosphere at elevated temperatures. Cr also contributes to high temperature creep- and rupture strength. Cr may also form carbides or nitrides which may be beneficial for wear resistance. A minimum of 12 % Cr is needed in order to obtain sufficient corrosion resistance, above 30 % Cr the compressibility of the stainless steel powder will be negatively influenced.

Nickel, Ni

A minimum of 5 % Ni is needed in order to obtain sufficient heat resistance. Ni contributes to the stability of austenite which enhance the strength of the component at high temperatures. Ni improves also the toughness and ductility and increases the resistance of the component against oxidisation, carburization, nitriding, thermal fatigue and strong acids. The upper limit for Ni is 25% in order not to negatively influence the compressibility of the powder.

Molybdenum, Mo

Mo contributes to improved pitting resistance and enhances high temperature mechanical properties. Mo may also act as a carbide forming element. The content of Mo is between 0-5%. Contents above 5% are not regarded as cost effective. Vanandium, V, Tungsten, W and Nibium, Nb

One or more of these elements may be present at a content of 0-5%. These elements are carbide and nitride forming elements thus contributing to wear resistance. As in the case of Mo, content above 5% is not regarded as being cost effective.

Carbon, C

The content of C in the stainless steel powder shall be less than 0.1% in order not to deteriorate the compressibility as the presence of C has a great impact on the hardness of the powder.

Silicon, Si

Si may be present up to 3%. Si may decrease the melting temperature of the melt prior to atomisation thus facilitating the atomisation process. Si may also act as desoxidising agent in the melt and during atomisation. A content above 3 % will negatively influence the compressibility.

Manganese, Mn

Mn may contribute to enhanced strength of the sintered component and may be present at a content up to 2 %. As Mn is easily oxidized a content above 2 % will give to high content of oxide inclusions in the steel powder. Mn may also be present in the form of MnS which provides machinability enhancing properties to the sintered component.

Sulphur, S

In the case that it is desired to form MnS during the course of production of the stainless steel powder or at sintering of the component a certain amount of S is needed.

The stainless steel powder further comprises inevitable impurities and being balanced with iron.

Steel powder composition

Graphite

The steel powder composition is obtained by mixing the steel powder with graphite in an amount of 0.1-3%. In order to for sufficient amount of carbides during sintering the minimum amount of graphite is 0.1%. A content of graphite above 3% is relevant in relation to the amount of carbide forming elements in the stainless steel powder. Lubricant

One or more conventional lubricants may be added to the stainless steel powder composition at an amount of up to 1.5%by weight of the composition.

Copper powder, Cu

A copper or copper containing powder may be added to the stainless steel powder composition at a content of 2%. Cu will during sintering form a liquid phase thus facilitate sintering. Additions above 2% Cu will however have a negative influence of the dimensional stability of the component.

Phosphorous containing powder, P

A phosphorous containing powder, such as Fe ₃ P may be added up to 2%. P has the same effect as Cu.

Machinability enhancing agents

One or more conventional machinability enhancing agents may be added in an amount of up to 1%. Above a content of 1% the compressibility of the composition will be negatively influenced. The machinability enhancing agent may be chosen from the group of MnS, CaF ₂ , MoS ₂ , hexagonal BN, bentonite or mica such as muscovite. Alternatively, during the coarse of the manufacture of the stainless steel powder or during sintering, MnS may be formed from in the melt present Mn and S.

Production of the component

The process for producing the heat resistant stainless sintered steel part includes the following steps; - Providing a steel powder composition comprising a stainless steel powder having a mean particle size between 50-100 μιτι, the stainless steel powder comprising;

12-30% by weight of Cr,

5-25 % by weight of Ni,

0-5 % by weight of Mo,

0-5 % by weight of V,

0-5 % by weight of W, 0-5 % by weight of Nb,

0-3 % by weight of Si,

0-2 % by weight of Mn,

0-0.3 % by weight of S,

less than 0.1 % by weight of C,

the balance being inevitable impurities and Fe,

the powder composition further includes,

graphite in an amount of 0.1-3 % by weight of the composition, preferably 0.1-2 % by weight of the composition,

optionally Cu powder up to 2 % of the composition,

optionally a phosphorous containing powder up to 2 % by weight of the composition, optionally machinability enhancing agent up to 1 % by weigh of the composition, optionally lubricant up to 1.5 % by weight of the composition, - transfer the stainless steel powder composition to a compaction die.

- uniaxially compacting the composition to a density of at least 6.6 g/cm3, preferably 6.7 g/cm3, even more preferably 6.8 g/cm3 and ejecting the compacted component from the die,

- subjecting the ejected component to presintering at a temperature between 700- 1000°C in an inert atmosphere, a reducing atmosphere or in vacuum,

- transfer the the presintered component to a die and subjecting the component to unixial recompaction to a density above 7.0 g/cm ³ , preferably above 7.1 g/cm ³ , more preferably above 7.2 g/cm ³ ,

- sinter the recompacted component at a sintering temperature between 1100-1350°C in an inert atmosphere, a reducing atmosphere or in vacuum, the density of the sintered component being above sintered density of the component being above 7.0 g/cm ³ , preferably above 7.1 g/cm ³ , more preferably above 7.2 g/cm ³ , even more preferably above 7.25 g/cm ³ , or even more preferably above 7.3 g/cm ³ .

Although the compaction and recompaction may be performed in a conventional unaxial powder compaction equipment the recompaction may be performed in a preferred embodiment according to high velocity compaction (HVC) with the aid of a compaction machine, example of such machine is for example described in US patent 6,202,757. In order to reach sufficient density of the recompacted body at the recompaction step when high velocity compaction is used, sufficient amount of energy has to be transferred from the ram to the body to be recompacted. This minimum energy corresponds to a minimum of about 800-1 000 MPa calculated from the peak force registered during the high velocity compaction and from the surface of the component hit by the ram.

It is also possible to perform the first compaction step according to HVC. Sintered component

Embodiments of the present invention disclosed herein provide compacted and sintered components produced from the above mentioned iron-based powder composition. Such components may be a unison or nozzle ring or a sliding nozzle to be used in a turbocharger. Components such as these are used in environments that require good resistance to wear and corrosion at elevated temperature, the microstructure of the component should therefore be mainly austenitic. The content of machinability enhancing agents, such as MnS, in the sintered component is preferably 0.1-1, more preferably 0.1-0.6 % by weight. When using MnS as a machinability enhancing agent the mean value of the size of the MnS phase is preferably below 4 μπι. The size of a MnS phase being determined by measuring its longest extension. The component may further include carbides and/or nitrides in order to enhance mechanical properties such as strength, hardness wear and corrosion resistance. Carbon and nitrogen can be provided to the composition, and/or through the atmosphere during sintering and/or heat treatment. Preferably, the size of the carbides and/or nitrides have a mean value below 3 μπι. The size of a carbide/nitride grain being determined by measuring the longest extension of the grain.

The upper limit for the density of the sintered component is believed to be about 7.7 g/cm ³ . The sintered component is also characterised by its pore structure manifested in the presence of coalesced rounded small pores along the former particle boundaries and no presence of elongated pores. The mean value of the size of the pores being less than 3 μπι, preferably less than 2 μπι. The size of the pores measured as its longest extension. The pore structure, i.e. small spherical pores will enhance mechanical properties such as tensile and fatigue strength. The chemical composition of the sintered component is essentially the same as the chemical composition of austenitic stainless steel powder used by having a higher C content as added graphite to the powder composition will diffusion into the matrix during the presintering enabling carbide formation.

Thus, an austenitic stainless steel sintered component according to the present invention may be characterised by having a content of;

12-30% by weight of Cr,

5-25 % by weight of Ni,

0-5 % by weight of Mo,

0-5 % by weight of V,

0-5 % by weight of W,

0-5 % by weight of Nb,

0-3 % by weight of Si,

0-2 % by weight of Mn,

0-0,3 % by weight of S,

0.1-3 % by weight of C, preferably 0, 1-2 % by weight,

optionally up to 3 % by weight, preferably up to 2% by weight,

optionally up to 2 by weight % of Cu,

optionally up to 2 % by weight of P,

optionally machinability enhancing agent up to 1 % by weight,

the balance being inevitable impurities and Fe and,

a density above 7.0 g/cm ³ , preferably above 7.1 g/cm ³ , more preferably above 7.2 g/cm ³ , even more preferably above 7.25 g/cm ³ and,

a pore structure wherein the majority of the numbers of pores being small rounded pores having a size less than 3 μιτι, preferably less than 2 μιη.

Example 1

The following non- limiting example serves to illustrate the present invention, pre- alloyed stainless steel powder having the chemical content according to table 1 was used. Table 2 shows the particle size distribution of the powder. Element [% by weight]

Cr 20.6

Ni 13.0

Si 2.5

Mn 0.11

Mo 0.03

Cu 0.02

W 0.09

Co 0.05

V 0.15

C 0.01

s 0.01

P 0.02

0 0.26

N 0.042

Fe balance

Table 1; composition of stainless steel powder used in example 1

Table 2; particle size distribution of stainless steel powder used in example 1 The stainless steel powder was mixed with 0.5% A- wax and 0.55 graphite UF-4. The obtained powder composition was compacted according to HVC in a HYP 35- 5062 high velocity compaction machine available from Hydropulsor AB, Sweden, at 90 mm stroke length into rings having dimensions 050/30* 10 mm to a density of 6,89 g/cm ³ ..

The obtained rings were thereafter soft annealed at 750°C in an atmosphere of DA (dissociated ammonia) having a dew point of -40°C for 20 minutes.

After soft annealing the components were subjected HVC compaction (90 mm stroke length). Prior to compaction the die was painted with DWL Zn- stearate suspended in acetone. The obtained density was 7.24g/cm ³ .

The obtained components were further subjected to sintering in 90/10 at a temperature of 1280°C for 30 minutes. The density obtained was determined to 7.33 g/cm ³ . As the component has increased its density from 7,24 g/cm ³ to 7.33 g/cm ³ it is shown that so called active sintering has taken place. This is also confirmed by the microstructure which shows no presence of elongated pores but instead coalesced spherical small pores along the former particle boundaries. Testing;

Micrographic examinations were performed by light optical microscopy. Figure 1 shows the presences of small round pores along the particle boundaries, the size of the pores being less than 3 μπι.

Figure 2 shows the shape of present carbides, the size being substantially less than 4 μπι.

Example 2

Manganese and sulphur present in the pre- alloyed stainless steel melt to be atomized, thus MnS is formed during the coarse of production of the stainless steel powder.

A pre- alloyed stainless steel powder having chemical composition according to table 3 below was used. Table 4 shows the particle size distribution of the powder. Element [% by

weight]

Cr 20.7

Ni 13.1

Si 2.5

Mn 0.98

Mo 0.03

Cu 0.02

W 0.09

Co 0.06

V 0.14

C 0.10

s 0.23

P 0.02

0 0.28

N 0.042

Fe balance

Table 3, chemical composition of stainless steel powder used in example 2

Table 2; particle size distribution of stainless steel powder used in example 1 The stainless steel powder was mixed with 0.5% A- wax and 0.55 graphite UF-4. The obtained powder composition was compacted according to example 1 into rings having dimensions 050/30* 10 mm to a density of 6,58g/cm ³ .

The obtained rings were thereafter soft annealed at 750°C in an atmosphere of DA having a dew point of -40°C for 20 minutes.

After soft annealing the components were subjected HVC compaction according to example 1. Prior to compaction the die was painted with DWL Zn- stearate suspended in acetone. The obtained density was 6.88g/cm ³ . The obtained components were further subjected to sintering in 90/10 at a temperature of 1280°C for 30 minutes. The density was 7.17 gem ³ . As the component has increased its density from 6.88 g/cm ³ to 7.17g/cm ³ it is shown that so called active sintering has taken place. This is also confirmed by the microstructure which shows no presence of elongated pores but instead coalesced spherical small pores along the former particle boundaries. However, the obtained finished density was lower compared to example 1.

Testing;

Micrographic examinations were performed by light optical microscopy. Figure 3 shows the presences of small round pores along the particle boundaries, the size of the pores being less than 3 μπι.

Figure 4 shows the shape of present carbides, the size being substantially less than 4 μπι.

Example 3 (MnS added to the stainless steel powder composition)

In example 3 the same stainless steel powder as used in example 1 was used.

The stainless steel powder was mixed with 0.5% A- wax and 0.55 graphite UF-4. and 0.5% of MnS having a mean particle size of less 5μπι. The obtained powder composition was compacted according to example 1 into rings having dimensions 050/30* 10 mm to a density of 6.84 g/cm ³ .

The obtained rings were thereafter soft annealed at 750°C in an atmosphere of DA having a dew point of -40°C for 20 minutes.

After soft annealing the components were subjected HVC compaction in a die. Prior to compaction the die was painted with DWL Zn- stearate suspended in acetone. The obtained density was 7.23 g/cm ³ .

The obtained components were further subjected to sintering in 90/10 at a temperature of 1280°C for 30 minutes. The obtained sintered density was 7.33 g/cm ³ . As the component has increased its density from 7.23 g/cm ³ to 7.33 g/cm ³ it is shown that so called active sintering has taken place. This is also confirmed by the microstructure which shows no presence of elongated pores but instead coalesced spherical small pores along the former particle boundaries.

Testing;

Micrographic examinations were performed by light optical microscopy. Figure 5 shows the presences of small round pores along the particle boundaries, the size of the pores being less than 3 μπι.

Machinability testing Machinablity was tested on components made according to the three examples, as reference material a similar full dense cast material was used.

Testing was performed by drilling holes, without lubricating material, to a depth of 7 mm. The drills used were Drill Dornier 03.5 mm, A002. The feed rate 0.06

mm/revolution. The number of holes drilled before the drills were worn out at various cutting speeds were noted as a measurement of the machinabilty.

The following table 5 shows the results; Cutting speed Component Component Component Reference,

[meter/minute] according to according to according cast

Ex 1 [no of Ex 2 [ no of to Ex 3 [no material holes] holes] of holes] [no of

holes]

61 31

10

20

210 121 15

30 24 8 15 12

40 7 7

Table 5, results from machinabilty testing

Table 5 shows that sintered parts made according to the invention exhibit improved machinability properties even if no machinability agent is added, example 1, as compared to the reference material. Presence of machinability enhancing agent further improves the machinability, example 2 and 3. Best result is obtained when the machniablity agent, MnS, is present as small inclusions in the matrix, the size of the MnS phase being less than 4 μιτι, example 2.