WO2020069795A1

WO2020069795A1 - Composition comprising high melting iron alloy powder and modified high speed steel powder, sintered part and manufacturing method thereof, use of the high speed steel powder as additive for sintering

Info

Publication number: WO2020069795A1
Application number: PCT/EP2019/072031
Authority: WO
Inventors: Denis Oshchepkov
Original assignee: Höganäs Ab (Publ)
Priority date: 2018-08-20
Filing date: 2019-08-16
Publication date: 2020-04-09

Abstract

The present invention relates to a composition comprising certain iron alloy particles (A2) in combination with other high melting alloy particles (A1), in particular high speed steels and cobalt-based alloys, the iron alloy particles (A2) having a lower melting point than the high melting alloy particles (A1). The use of the particles (A2) in a sintering manufacturing allows broadening the process window and to obtain sintered parts having a high density, high hardness and high strength under various process conditions. This is particular useful for the PM manufacture of high melting alloys such as tool steel (including high speed steel (HSS)), which are otherwise difficult to process.

Description

COMPOSITION COMPRISING HIGH MELTING IRON ALLOY POWDER AND MODIFIED HIGH SPEED STEEL POWDER, SINTERED PART AND MANUFACTURING METHOD THEREOF, USE OF THE HIGH SPEED STEEL POWDER AS ADDITIVE FOR SINTERING

Background

Parts made from hard and high melting alloys are of significant use in industry due to their high hardness and strength. Examples include parts made from tool steels, such as high speed steel as defined for instance in ASTM A600-92a (Reapproved 2010).

Parts made from metals or alloys can be obtained via a wide variety of ways. In many instances, parts are made from cast ingots by forging or machining, thereby changing the shape or removing excess material to obtain the desired part. Such processes are however inferior in that a lot of waste material is generated and/or that significant wear to the production machinery is caused. This applies in particular to hard materials, which are generally more difficult to process in these ways.

It is known that these drawbacks can to some extent be avoided by using Powder Metallurgy (PM), such as PM press and sinter processes. Herein a metal or alloy powder is prepared, compacted in a die and subsequently sintered. Yet, this technology has its own challenges, and may be considered less suitable to form materials of high strength, high hardness and high density, as oftentimes voids remain in the sintered parts, reducing the strength and density of the sintered part.

These problems can to some extent be addressed by using additives that form a liquid phase at lower temperatures, such as boron. The liquid phase present during the sintering process allows reducing the amount of pores remaining in the sintered part and thereby increases density. However, the material forming the liquid phase during sintering remains in the sintered part and forms defects in the structure that are often the origin of cracks and a loss of structural integrity of the sintered part during later use.

The strength and hardness of sintered parts obtained by blending such low-melting additives (e.g. boron) into the powder mixture is hence insufficient. If materials of high density and simultaneously high strength and hardness are desired, the use of such low melting additives generally needs to be avoided. The high strength/high hardness metal or alloy is then typically sintered without such additives in order to avoid structural defects in the sintered part.

The sintering of high melting alloy powders, such as high speed steel powders, to full density or near full density (supersolidus sintering) is a known process. However, such a process has a very narrow temperature process window (i.e. a temperature slightly exceeding the solidus temperature of the high melting alloy powder by about 2 - 3 degrees), and is therefore not widely used industrially due to the delicate process conditions. Such parts then often show less-than-optimum density and/or- inferior strength if the process conditions are not exactly controlled. Accordingly, the rate of failure of such processes is high, making them

economically unattractive.

In addition to the narrow temperature process window that prevents a wide spread application in industry, a sintered part obtained from a sintering process of high melting alloy powders needs to satisfy further requirements. This include in particular high density (relative to the bulk density of the high melting alloy powder), as a lower relative density is indicative of a porous structure, leading to a weakness in the structural integrity of the obtained part. This leads to lower hardness and lower resistance to external forces. Also the strength of the material can be affected in case the density is not high enough.

Sintering at temperature which is lower than optimal typically leads to excessive remaining porosity and therefore low strength, hardness and wear resistance. From the other hand, exceeding the optimal sintering temperature leads to excessive formation of a liquid phase which increases shape distortion and reduces strength by formation of a brittle grainboundary eutectic network.

Problems to be solved by the invention

The present invention in one aspect aims at providing a composition that allows broadening the temperature process window in a manufacturing process using high melting alloy powders, such as tool steels, including high speed steel powders.

It is another object of the present invention to provide a composition that allows obtaining a sintered part made essentially from a high melting alloy powder, the sintered part having a high density, high hardness and high strength. The composition is preferably able to provide such a sintered part under a variety of process conditions. The present invention further aims at providing an industrially useful process for the PM manufacture of parts essentially made from a high melting alloy, which process is able to produce parts having high density, high strength and high hardness. In a further aspect, the process is less sensitive to variations in process conditions.

It is a further object of the present invention to provide the use of a material as additive in a sintering manufacturing process of high melting alloy powders, in particular tool steel powders (e.g. high speed steel powders), allowing to broaden the process window and to obtain a high theoretical density of the sintered part.

Summary of the Invention

The present inventors have studied the problems associated with the prior art, and unexpectedly found that the use of certain iron alloy particles (in the following referred to as "iron alloy particles A2") allows solving the above-mentioned problems when used in combination with other high melting alloy particles, in particular high speed steels and cobalt- based alloys (in the following referred to as "alloy particles Al"). The use of the particles A2 in particular allows broadening the process window and to obtain sintered parts having a high density, high hardness and high strength under various process conditions. This is particular useful for the PM manufacture of high melting alloys such as tool steel (including high speed steel (HSS)), which are otherwise difficult to process.

The present invention thus includes the following aspects:

1. Composition comprising

A two or more alloy particles, the two or more alloy particles

comprising first alloy particles Al made from an alloy having a melting point of 1220 °C or higher and second iron alloy particles A2 made from an iron alloy having a melting point that is lower than the melting point of the alloy from which the particles Al are made; and

optionally B one, two or more additives; wherein the first alloy particles A1 are selected from the group consisting of high speed steel particles and particles made of alloy M35, wherein the alloy M35 consists of, in weight-%: 0.80-0.90 C, 4.50-5.50 Co, 3.75-4.50 Cr, up to 0.40 Cu, up to 0.50 Mn, 4.75-5.50 Mo, up to 0.45 Ni, up to 0.1 O, up to 0.040 P, up to 0.040 S, up to 0.040 Si, 1.75-2.25 V and 6.00 to 6.75 W, the remainder Fe and unavoidable impurities; and the iron alloy forming the second iron alloy particles A2 consisting of

carbon in an amount of 0.70 to 1.20 % by weight;

tungsten in an amount of 7.0 to 11.0 % by weight;

manganese in an amount of 0.90 to 1.80 % by weight;

chromium in an amount of 2.5 to 4.5 % by weight;

silicon in an amount of 0.60 to 1.80 % by weight;

boron in an amount of 0.50 to 1.40 % by weight;

vanadium in an amount of 1.40 - 2.00 % by weight;

titanium in an amount of 0.10 to 0.80 % by weight;

niobium in an amount of 1.10 to 1.90 % by weight;

optionally one or more of cobalt, copper, nickel, molybdenum, oxygen, nitrogen, phosphorous, and sulfur in a total amount of 2.50 % by weight or less, preferably 1.50 % by weight or less, and more preferably 1.00 % by weight or less;

the balance being iron and optionally unavoidable impurities, the unavoidable impurities forming 0.50 % by weight or less, preferably 0.20 % by weight or less, of the alloy; wherein the two or more particles A form 95% to 100 % by weight of the total weight of the composition. Composition comprising

A two or more alloy particles, the two or more alloy particles

comprising first alloy particles A1 made from an alloy having a melting point of 1220 °C or higher and second iron alloy particles A2 made from an iron alloy having a melting point that is lower than the melting point of the alloy from which the particles A1 are made; and

carbon in an amount of 0.70 to 1.20 % by weight;

tungsten in an amount of 7.0 to 11.0 % by weight;

manganese in an amount of 0.90 to 1.80 % by weight;

chromium in an amount of 2.5 to 4.5 % by weight;

silicon in an amount of 0.60 to 1.80 % by weight;

boron in an amount of 0.50 to 1.40 % by weight;

vanadium in an amount of 1.40 - 2.00 % by weight;

titanium in an amount of 0.10 to 0.80 % by weight;

niobium in an amount of 1.10 to 1.90 % by weight;

the balance being iron and optionally unavoidable impurities, the unavoidable impurities forming 0.50 % by weight or less, preferably 0.20 % by weight or less, of the alloy; wherein the two or more alloy particles A form 95% to 100 % by weight of the total weight of the composition; wherein the two or more alloy particles A are formed to 95% by weight or more by the alloy particles A1 and A2, or wherein the alloy particles A consist of the particles A1 and A2; and

wherein the amount of the particles A1 is 90.0% by weight or more, preferably 95.0% by weight or more, but 99.5% by weight or less, preferably 99.0% by weight or less, and the amount of the particles A2 is 10.0% by weight or less, preferably 5.0% by weight or less, but 0.5% or more, preferably 1.0% by weight or more, relative to the total weight of the particles A1 and A2. Composition according to item 1 or item 2, wherein the alloys forming the particles A2 have the following composition, in weight% of the respective alloy:

and/or wherein the particles A1 made from an alloy selected from the group consisting of M2, regular and high, M3 Class 1 and Class 2, M35 and T15, wherein M2, regular and high, M3 Class 1 and Class 2, and T15 are alloys as defined in ASTM A600-92a and M35 is an alloy as defined in item 1. Composition according to any one of items 1 to 3, wherein one, two or more additives B is/are present, and/or the additive B is non-metallic. Composition according to any one of items 1 to 4, wherein the optional one, two or more additives B is selected from the group consisting of hydrocarbons having a molecular weight of 130 g/mol or more, fatty acids, fatty acid amides, graphite, polyolefin or polyamide waxes, and machinability enhancing agents such as MnS. Composition according to any one of items 1 and 3 to 5, wherein the two or more alloy particles A are formed to 95% by weight or more by the alloy particles A1 and A2, or wherein the alloy particles A consist of the particles A1 and A2. Composition according to any one of items 1 and 3 to 6, wherein the amount of the particles A1 is 90.0% by weight or more, preferably 95.0% by weight or more, but 99.5% by weight or less, preferably 99.0% by weight or less, and the amount of the particles A2 is 10.0% by weight or less, preferably 5.0% by weight or less, but 0.5% or more, preferably 1.0% by weight or more, relative to the total weight of the particles A1 and A2. Composition according to any one of items 1 to 7, wherein the alloy particles A have a particle size, expressed as D_v50 and determined by a laser light scattering method, of 200 pm or less, preferably 150 pm or less, but 10 pm or more, preferably 25 pm or more. Method for producing a sintered part, comprising the following steps in this order: a. providing a composition as defined in any one of items 1 to 8; b. compacting the composition to form a green body; and c. sintering the green body to form a sintered part under vacuum, a reducing atmosphere or an inert atmosphere.

10. Method for producing a sintered part according to item 9, wherein the method additionally comprises a step d., performed after steps a., b., and c., have been performed, of tempering the sintered part at a temperature of 450 to 700 "C.

11. Method for producing a sintered part according to any one of items 9 to 10, wherein the sintered part obtained after step c. or, if conducted, after step d. has a sintered density of 85% or more, preferably 90% or more, more preferably 95 % or more, or between 97 and 100% of the density of the alloy Al.

12. Method for producing a sintered part according to any one of items 9 to 11, wherein the sintered part obtained after step c. or, if conducted, after step d. has a Hardness HRC of 30 or more, preferably 40 or more, more preferably 45 or more or 50 or more, such as 55 or more or 60 or more.

13. Sintered part made from an iron alloy, the sintered part having a density of of 85% or more, preferably 90% or more, more preferably 95 % or more, or between 97 and 100% of the density of the alloy forming the particles Al, and/or a Hardness HRC of 30 or more, preferably 40 or more, more preferably 45 or more, or 50 or more, which is obtainable by using the composition as defined in any one of items 1 to 8 or by the method according to any one of items 9 to 12.

14. Use of particles A2 as defined in any one of items 1 or 3 in a sintering

manufacturing process of powders comprising or consisting of particles as defined for particles Al in item 1 or item 2.

Further and preferred aspects of the present application will become apparent in view of the following detailed description. The following detailed description as well as the examples provided in the following are however not intended to limit the scope of the present invention in any way, and are given mainly for illustrative purposes.

Definitions

In the present invention, the following definitions apply to the terms used herein:

In the present invention, all parameters and product properties relate to those measured under standard conditions (25 °C, lO^ Pa) unless stated otherwise or prescribed by a certain standard or test protocol.

As used herein, the indefinite article "a" indicates one as well as more than one and does not necessarily limit its reference noun to the singular.

The term "about" means that the amount or value in question may be the specific value designated or some other value in its neighborhood, generally within a range of ±5% of the indicated value. As such, for instance the phrase "about 100" denotes a range of 100 ±5.

The term and/or means that either all or only one of the elements indicated is present. For instance, "a and/or b" denotes "only a", or "only b", or "a and b together". In the case of "only a" the term also covers the possibility that b is absent, i.e. "only a, but not b".

The term "comprising" as used herein is intended to be non-exclusive and open-ended. A composition comprising certain components thus may comprise other components besides the ones listed. However, the term also includes the more restrictive meanings "consisting of' and "consisting essentially of', which are used synonymous with "made of' and "made essentially of", respectively. The terms "consisting essentially of' and "made essentially of' allow for the presence of up to and including 10 weight%, preferably up to and including 5% of materials other than those listed for the respective composition or material, which other materials may also be completely absent.

Whenever a range is expressed as "from x to y", or the synonymous expression "x - y", the end points of the range (i.e. the value x and the value y) are included. The range is thus synonymous with the expression "x or higher, but y or lower".

The term "tool steel" and "high speed steel" used in the present invention have their common meaning in the art, unless specified differently. Suitable definitions are well known to a skilled person, and are listed e.g. in ASTM A600-92a (Reapproved 2010) for high speed steel or ASTM A681 - 08(2015) for tool steel.

Brief description of the drawings

Fig. 1 shows the porosity of the sintered part obtained by sintering at 1235 °C the compositions of Comparative Example 1, Example 1 and Example 2;

Fig.2 shows the microstructure of the sintered part obtained by sintering at 1235 °C the compositions of Comparative Example 1, Example 1 and Example 2;

Fig. 3 shows the evolution of the porosity of the material obtained by sintering the composition of Example 1 depending on the sintering temperature;

Fig. 4 shows the effect on the microstructure of different sintering temperatures for the composition of Example 1; and

Fig. 5 shows the influence of the sintering temperature on the microstructure of the sintered part obtained from the composition of Example 2.

Detailed Description of the Invention

In the following, a detailed description of the composition, the method and the use according to the present invention will be given. Also, each of the components will be described in detail.

Composition

The composition of the present invention comprises A two or more alloy particles, the two or more alloy particles comprising first alloy particles Al, and iron alloy particles A2. The particles Al may subsequently also be referred to as "high melting" alloy particles, and the alloy particles A2 may subsequently be referred to as "low melting" alloy particles. Hereby it is expressed that the "low melting" iron alloy particles A2 made from an iron alloy having a melting point that is lower than the melting point of the alloy from which the particles Al are made. The composition of the present invention may contain additional metal or metal alloy particles A beyond the particles A1 and A2, yet typically the alloy particles A consist of or essentially consist of the high melting alloy particles A1 and the low melting alloy particles A2. As such, the alloy particles A1 and A2 form 95% by weight or more of all alloy particles A, and preferably 97% by weight or more of the alloy particles A. In a preferred embodiment, the particles A consist of the particles A1 and A2. In the latter embodiment, the composition of the present invention does not contain any alloy particles (and also no other metal or alloy particles) other than the particles A1 and A2.

Since the composition of the present invention has been developed with the aim of providing a composition that is suitable for obtaining a sintered particle that essentially consists of high speed steel, the amount of the particles A1 is 90.0 % by weight or more, preferably 95.0% by weight or more, yet typically 99.5 % by weight or less, preferably 99.0% by weight or less of the total of the particles A1 and A2. Further, the amount of the particles A1 is 99.0 % by weight or more, preferably 95.0 % by weight or more, but 99.5 % by weight or less, preferably 99.0% by weight or less, of the entirety of the alloy particles A. Incidentally, these definitions are identical in case the alloy particles A consist of the alloy particles A1 and A2.

The iron alloy particles A2 are made of a modified high speed steel alloy, and basically serve as an additive to the high melting alloy particles A1 in a PM manufacturing process.

Accordingly, the amount of the iron alloy particles A2 is 10.0 % by weight or less, preferably 5.0% by weight or less, but typically 0.5% by weight or more, preferably 1.0 % by weight or more, such as 1.5% by weight or more or 2.0 % by weight or more, of the total of the alloy particles A1 and A2. The same range applies to the total of the alloy particles A, and incidentally again the definition is identical in case the iron alloy particles A consist of the iron alloy particles A1 and A2.

It follows that the alloy particles A preferably consist of 90.0 % by weight or more of the high melting alloy particles A1 and of 10.0% by weight or less of the low melting alloy particles A2. Herein, additional alloy particles may be completely absent, or may be present in an amount of up to 5.0% by weight, relative to all alloy particles A. However, such additional alloy particles are preferably absent, and the alloy particle A is preferably formed by only the alloy particles A1 and A2.

In a sieve analysis (ROTAP), the particle size distribution of both the alloy particles A is typically such that 95% by weight or more is passing a sieve with openings of 300 pm.

Additionally, the alloy particles A have typically a median particle size, expressed as Dv50 and determined by a laser light scattering method, of 200 micrometer or less, preferably 150 micrometer or less, yet 10 micrometer or more, preferably 25 micrometer or more. The amount of particles having a size (longest axis) of 300 pm or more is hence 5% or less, and may in some embodiments be 2% or less, 1% or less, or zero.

The particles A1 are not particularly limited with regard to their physical appearance and their method of manufacture. However, it is preferable to use water-atomized particles, as such particles are readily available. Further, water-atomized powders are more irregular in shape and are hence more suitable for compaction, since their shape facilitates interlocking of the particles during compaction. This contributes to the green strength of the part obtained after compaction, which facilitates further handling.

The particles A1 are preferably water-atomized particles, more preferably water-atomized and vacuum annealed particles. Since water-atomized particles that have not been subjected to vacuum annealing are typically covered by a thicker oxide layer as compared to gas- atomized powders, it is optionally possible to pre-treat the particles in a reducing atmosphere to remove possible oxides, yet this step is entirely optional and the particles can be used as is without any pre-treatment.

The particles A2 can be produced by any suitable method, as the chemistry/composition is believed to be more important than their specific shape. The particles A2 can be gas- atomized, water-atomized, or be prepared by any other suitable method.

In addition to the particles A, the composition further optionally comprises one or two more additives 8, which will be described in more detail below. The amount of the optional additives 8 is generally 10% or less, preferably 5% or less, more preferably 2.5% or less and even more preferably 2.0% by weight or less, based on the entire composition comprising the alloy particles A and the optional additive B.

While the composition is defined in claim 1 as comprising the iron alloy particles A, and optionally the one, two or more additives B, additional components may be present as long as they do not interfere with the success of the present invention. Yet, typically the composition consists of the iron alloy particles A and the optional one or two or more additives B, in which case any component that is not an alloy particle A is encompassed by the one, two or more additives B. The above-references ranges for the amounts of the optional additives B also apply in this case. In consequence, the two or more iron alloy particles A form 90% by weight or more of the composition, preferably 95% by weight or more of the composition, and still further preferably 97% by weight or more of the entire composition. In specific embodiments, the iron alloy particles form 98% or more of the entire composition, such as 98.5% by weight or more of the entire composition.

High melting alloy particles A1

The high melting alloy particle A1 is selected from the group consisting of A1 high speed steel particles and M35 particles. These are made of an alloy that has a melting point of 1220 °C or higher, such as from 1220 to 1300 °C, and preferably 1230 - 1280 °C.

The high melting high speed steel particles A1 are made of (i.e. consist of) high speed steel, as defined in ASTM A600-92a (Reapproved 2010), or a made from the alloy M35.

The alloy M35 consists of (all in weight-%): 0.80-0.90 C, 4.50-5.50 Co, 3.75-4.50 Cr, up to 0.40 Cu, up to 0.50 Mn, 4.75-5.50 Mo, up to 0.45 Mi, up to 0.1 O, up to 0.040 P, up to 0.040 S, up to 0.040 Si, 1.75-2.25 V and 6.00 to 6.75 W, the remainder Fe and unavoidable impurities.

Since high speed steels are well defined in the art, the term has a well-established meaning to a skilled person. The high speed steel used in the present invention can be selected from high speed steels known in the art, such as Tl, T2, T4, T5, T6, T8, T15, Ml, M2 (regular and high C), M3 (Class 1 and Class 2), M4, M5, M6, M7, M10 (regular and high C), M30, M33, M34, M36, M41, M42, M43, M44, M46, M47, M48, M50, M52 and M62, as defined in ASTM A600-92a (2010).

Preferred Examples of the alloy forming the particles A1 are M2 (regular and high), M3 (Class 1 and Class 2), M35 and T15.

Low Melting Iron alloy particles A2

The iron alloy particles A2 form the component that allows increasing the temperature process window for sintering process in the presence of a liquid phase, due to its low melting point.

The iron alloy particles A2 are formed from an alloy that has a lower melting point as compared to the particles Al, and the melting point of the alloy forming the iron alloy particles A2 is typically 1000 °C or higher, preferable 1100 °C or higher, such as 1150 °C or higher, but less than 1220“C, such as 1200 °C or lower. Without wishing to be bound by theory, it is believed that this component facilitates the creation of a liquid phase at an earlier stage of the sintering, thereby widening the temperature process window and facilitating the densification/sintering of the alloy particles, thereby allowing obtaining a sintered part having a high density.

The iron alloy particles A2 are generally formed from an alloy consisting of: carbon in an amount of 0.70 to 1.20 % by weight;

tungsten in an amount of 7.0 to 11.0 % by weight;

manganese in an amount of 0.90 to 1.80 % by weight;

chromium in an amount of 2.5 to 4.5 % by weight;

silicon in an amount of 0.60 to 1.80 % by weight;

boron in an amount of 0.50 to 1.40 % by weight;

vanadium in an amount of 1.40 - 2.00 % by weight;

titanium in an amount of 0.10 to 0.80 % by weight;

niobium in an amount of 1.10 to 1.90 % by weight;

optionally one or more of cobalt, copper, nickel, molybdenum, oxygen, nitrogen,

phosphorous, and sulfur in a total amount of 2.50 % by weight or less, preferably 1.50 % by weight or less, and more preferably 1.00 % by weight or less;

the balance being iron and optionally unavoidable impurities, the unavoidable impurities forming 0.50 % by weight or less, preferably 0.20 % by weight or less, of the alloy. An unavoidable impurity in the sense of the present definition is any element that is not cited above, in this case any element (i.e. one or more elements) other than Fe, C, W, Mn, Cr, Si, B, V, Ti, Nb, Co, Cu, Ni, Mo, O, N, P and S. The amounts indicated above refer to the total amount of such impurities, i.e. the other one or more elements, which may however also be completely absent.

In a preferred embodiment, the alloy forming the particles A2 consists of the following:

Here, the "other elements" denote unavoidable impurities, as explained above, and include any (i.e. one or more) element that is not otherwise listed. The amount of 0.00 - 0.50 refers to the total amount of these one or more other elements., and again such other elements may also be completely absent.

With the above general and preferred compositions, the required low melting point can be provided by combination of B, Si and C with the metallic alloying elements. Another feature of the particles A2 is that, after sintering and solidification, it provides a microstructure which is very similar to the particles Al-1, and thus provides good mechanical properties, in particular hardness and strength to the obtained sintered part. Without wishing to be bound by theory, the particles A2 are believed to diffuse into the particles A1 and thereby provide for a strong bonding and a more homogeneous microstructure as compared to conventional additives that have been used to form a liquid phase, such as boron. The present invention thus also encompasses a composition wherein the particles A1 and A2 are diffusion-bonded.

Additive B

The additive B is optional, and hence may be completely absent. It is however preferably present and forms at the most 5% by weight of the total weight of the composition. In this regard, the composition may consist of the alloy particles A and optionally the additive B, and the alloy particles A form 95-100% by weight of the total weight of the composition.

If the optional additive B is present, its amount is greater than 0% by weight, and is preferable 0.1% by weight or more, or preferably 0.2% by weight or more. The upper limit is 5% by weight of the total of the composition, yet is preferably 4% by weight or less, more preferably 3% by weight or less. A typical amount of the additive B is in the range of 0.2-1.5% by weight of the total composition.

The additive B is in one embodiment non-metallic, and considering that the additive B covers any component that is not an alloy particle A, the composition is in this case free of any metal element that is not contained in the iron alloy particles A.

The additive B may be a component that reduces the friction amongst the alloy particles A at the time of compaction, and is therefore preferable selected from group consisting of hydrocarbons having a molecular weight of 130 g/mole or more, fatty acids, fatty acid amides, fatty acids esters, graphite and polyolefin- or polyamide-waxes. In this regard, a "wax" is defined as a component having a melting point of 35 °C or higher, such as 40 °C or higher. The melting point of the wax can be as high as 80 °C or higher, or even 100 °C or higher.

The additive B is preferably a compound containing one or more carbon atoms. Without wishing to be bound by theory, it is believed that the presence of carbon at the time of sintering facilitates the formation of carbides , which may then increase strength and hardness.

The additive can be composed of one component only, or the additive B can be composed of two or more compounds. For instance, the additive B can be formed by a combination of a polyolefin or polyamide wax and graphite or other types of carbon, such as carbon black. A preferred additive is graphite, which is further preferably combined with a wax such as polyamide wax or polyolefin wax. Further additives that may be used are machinability enhancing additives, such as MnS.

Method for producing a sintered part

The method of the present invention includes steps of a. providing a composition containing the alloy particles A and optional the additive B, as defined above or in the claims,

b. compacting the composition to form a green body, and

c. sintering the green body at a form a sintered part under vacuum, a reducing atmosphere or an inert atmosphere.

These steps are optionally followed by a tempering treatment that is performed at 450- 700 °C, which can be conducted once, twice, three times or even more times. In between the different tempering treatments, the part is cooled to 150 °C, and typically to room temperature.

Hence, the optional step d. of tempering the particles can be a single, double or triple tempering, which is typically performed at a temperature of 440-600 °C. Alternatively, a subzero cooling (i.e. a cooling to less than 0 °C) and a single tempering at 450-700 °C (preferably 540-600 °C) can be conducted.

The steps of the method are described in some more detail below:

Step a. of providing a composition

For providing the composition comprising the two or more alloy particles A and the optional additive B, the respective components are thoroughly mixed. In this respect, any equipment typically used for the mixing of particles can be used, such as a double cone mixer.

Step b of compacting the composition to form j green body

In this step, the composition provided in the previous step is pressed, typically at a compaction pressure of 400 MPa or more, such as 800 MPa or more. The step of compacting the composition is not particularly limited, and any method typically used in the art can be employed, such as compacting in a cold die. However, instead of a cold die compaction, also warm compaction, warm die compaction, or another known other method can be used.

Step c. of sintering the green body

Following the formation of the green body by compaction in step b., the green body is sintered, preferably at a temperature that falls between the solidus temperature of the alloy forming the particles A1 and the liquidus temperature of the alloy forming the particles A2. If the particles A1 and/or A2 are each formed by two different alloys (e.g. particles A1 comprise a mixture of particles of different high speed steels), the sintering is preferably performed between the lowest solidus temperature of all alloys forming the particles A1 and the highest iiquidus temperature of all alloys forming the particles A2.

While thus the preferred temperature window for the sintering depends on the exact materials used, this temperature window is typically located between 1180 and 1300 °C, and the sintering may be performed for any suitable time, such as from 10 minutes to 2 hours.

If the temperature is too low, no liquid may be formed at the boundaries of the particles, so that the reduction/elimination of voids or pores is believed to be difficult. A higher temperature may thus facilitate the removal of pores, as long as it does not exceed the solidus temperature of the alloy forming the particles Al. Conversely, higher temperatures can lead to an increase in the size of carbides that are formed in case carbon-containing materials are included in the composition, and hence the temperature must be adjusted accordingly. Given that the Iiquidus and solidus temperature of an alloy is well-known to a skilled person or can easily be determined, the suitable sintering temperature can be determined by a skilled person based on common general knowledge and few orienting tests, as demonstrated in the following examples.

During the sintering, vacuum (i.e. a pressure of 10² Pa or less), a reducing atmosphere (i.e. consisting of reducing species such as hydrogen or methane, and optionally inert gases such as nitrogen or argon) or an inert atmosphere (e.g. consisting of nitrogen, argon, helium, or any mixture thereof) can be used. Using such an atmosphere avoids oxidation of the alloy particles, which may otherwise lead to inferior properties of the sintered part.

Sintered part

The sintered part obtained by the method of the present invention including the steps a., b., c. and optionally d., is characterized by a high density of 85% or more, preferable 90% or more, more preferably 95%, or between 97 and 100% of the density of the alloy forming the particles Al. Accordingly, the sintered part obtained by the method of the present invention and/or by using the composition of the present invention is a part having high strength and high hardness, as voids are essentially absent. As such, the hardness, expressed as HRC, is preferably 30 or more, more preferably 40 or more, still further preferably 45 or more or 50 or more, such as 55 or more or 60 or more, determined according to ISO 4498:2010.

The density of the sintered part can be determined by an ordinary method, such as by weighing the sintered part and determining its volume by immersion into water and observing the volume of water that is displaced, e.g. by observing the rise of water level in a container of known dimensions and therefrom calculating the volume of water displaced by the sintered part. While the conditions are generally not critical, the sintered part can be immersed for 1 hour in distilled water at 20 degrees Celsius before the amount of water displaced is determined. Thereby, weight and volume of the sintered part are obtained, from which the density can be calculated. The density of the alloy can be determined similarly on a test piece formed from an alloy by melting the alloy and casting it to form a dense test piece, weighing the test piece and immersing it in water under the above condition in order to determine the volume of the test piece.

Examples

The present invention will now be described by way of Examples and Comparative Examples. The present invention is however not limited to these Examples, which are given merely for illustrative purposes.

Examples 1 and 2 and Comparative Example 1 Preparation of Compositions

Compositions comprising the components listed below in Table 3 were mixed thoroughly in a windmill laboratory mixer. For each composition, 500 g in total were prepared.

The compositions comprised, as HSS particles Al, a commercial water atomized and vacuum annealed powder obtainable from Hoganas (Great Britain), Ltd, under the designation PX- M2-04. The composition of the alloy was not determined, but falls within the ranges given by the manufacturer as listed below in Table 1:

Table 1: Compositional window of High Speed Steel (HSS) particles Al used in the Examples 1 and 2 and Comparative Example

As particles A2, iron alloy particles available from Hoganas AB, Sweden were used. According to the manufacturer, the composition of the alloy forming these particles falls within the compositional window given in Table 2 below:

Table 2: Compositional window of iron alloy particles A2 used in the Examples

The particle size of A1 was less than 150 pm. The particle size of A2 was less than 53 pm

The following compositions were prepared with the above-described iron alloy particles A1 and A2:

Table 3: Compositions according to Examples 1, 2, and Comparative Example 1 (in weight % of the entire composition)

Formation of Green Part and Production of Sintered Parts

TRS bars were pressed at compaction pressure 800MPa in a cold die. For each composition, four TRS bars were prepared, which were then respectively sintered under the following conditions in a BU92 belt furnace:

1. Sintering at 1150°C for 20 min, Atmosphere 90/10 N₂/H₂+

2. Vacuum sintering at 1235°C, 30 min, High vacuum (less th

3. Vacuum sintering 1225°C, 30 min, High vacuum (less than

4. Vacuum sintering 1215°C, 30 min, High vacuum (less than

It is to be noted that the sintering conditions 1 are not accordance with the present invention, which requires a sintering at 1180“C to 1300 °C

The samples after vacuum sintering were subjected to double tempering at 560°C for lhour in N2- Sintered density, hardness, TRS and SEM investigations were performed.

Results

The following results were obtained under the sintering conditions 1 (not encompassed by the method of the present invention):

The results show that the addition of particles A2 does not have a beneficial effect under these sintering conditions. The dimensional change after sintering is relatively small, and sintered density and in particular the strength of the sintered part are inferior.

The following Table 5 provides the results obtained after vacuum sintering at different sintering temperatures under vacuum:

Table 5: Results after vacuum sintering at 1215, 1225 and 1235“C

From the example results provided above in Table 5, it can be seen that for all materials a higher sintering temperature leads to an increase in sintered density and transverse rapture strength. However, the results obtained for comparative example 1 (not containing any particles A2) show that the temperature is still insufficient for obtaining a high sintered density close to that of the alloy. This shows that the material still contains many voids, which is also reflected in a poor hardness and a poor strength. Conversely, the compositions according to example 1 and 2 show a much higher dimensional change (which indicates the reduction of voids), and consequently a higher density. The sintered parts obtained therefrom also exhibit a much higher strength and hardness, which shows that also the microstructure is homogeneous and that the different components are strongly bound to each other. This shows that the use of the alloy particles A2 in an amount of 3% is able to facilitate the elimination of voids in the material and exhibits a structure that provides compatibility between the materials on a microscopic level.

This is independently confirmed by the observations shown in Figure 1. Here, the porosity of the materials (sintered parts) obtained at 1235 °C are shown for the parts obtained from Comparative Example 1 (containing no particles A2) and Examples 1 and 2. As is apparent from Figure 1, the sintered part obtained from the composition according to Comparative Example 1 still shows many voids, while these are essentially absent in the sintered parts obtained from the compositions of Example 1 and 2.

Figure 2 shows the microstructure of the materials sintered at 1235 °C. Here, also the result provided in Figure 1 discussed above is confirmed. Further, it becomes apparent that the maximum carbide size in the sintered parts obtained from the compositions of Examples 1 and 2 is slightly higher than in the sintered part obtained from the composition of

Comparative Example 1, but still does not exceed 5-10 micrometer. This shows that this processing temperature does not lead to an increase in the presence of large carbide areas.

At higher magnifications (not shown) increase of very fine submicron carbides can be seen in the sintered part obtained from the compositions according to Examples 1 and 2, and without wishing to be bound by theory, it is believed that these contribute to the much higher hardness of the materials.

Accordingly, the compositions in accordance with Examples 1 and 2 allow increasing the hardness vis-a-vis a sintered part that is obtained from the particles A1 only (Comparative Example 1).

Figure 3 shows the evolution of the porosity of the sintered part obtained from the composition according to Example 1, depending on the temperature. As shown here, samples sintered at temperatures of 1235 °C have still a considerable amount of pores, while already giving satisfactory results with regard to strength and hardness. Increasing the sintered temperature did not influence the maximum carbide size, however more fine carbides were found in samples sintered at 1225 °C and higher. This is also shown in Figure 4, which shows the evolution of carbides of the sintered part obtained from the composition of Example 1 depending on the temperature.

Even though it is believed that the sintering of the compositions of Examples 1 and 2 led to the presence of a liquid phase, it was not possible to identify any traces of liquid in the microstructure. It was however found that the areas where the particles A2 were located might be areas with increased concentration of strengthening/hardening carbides.

The material of Example 2 differed from the material of Example 1 only in the addition of 0.2% graphite. As derivable from Figure 5, and in particular in comparison with Figure 4 and also taking into account the results provided in Table 5, it is believed that the addition of graphite reduced the solidus temperature of the material A1 and therefore helped to obtain fully dense material in all used sintering temperatures. Addition of graphite can be used to optimise sintering temperature widow in component production (note that the full density, i.e. the theoretical maximum, is 8.16 g/cm-3 for the alloy forming the particles A1 in the compositions of Examples 1 and 2/Comparative Example 1). The maximum size of carbides in the sintered part obtained from the composition of Example 2 was less than 10 micrometer for all sintering temperatures, yet it seemed that the size of carbides increased from a temperature of 1225 °C onwards.

To summarize, the Examples above show that it is possible to achieve a full density sintered part by adding 3% of the particles A2. The addition of small amounts of graphite reduced the temperature that was necessary for obtaining full density sintering. Further, the sintering process window was at least 20 °C, which is four times wider than what could previously be achieved.

In consequence, the compositions of the present invention allow performing the process of the present invention in a manner that is industrially more favourable, and which in particular does not require a delicate control of the sintering temperature. Put differently, the compositions of the present invention are able to provide sintered parts with near full or full density and with increased hardness and strength, and are less demanding from a production point of view.

Example 3

In order to verify the results of Examples 1 and 2 for diffusion bonded powders wherein the particles A1 and A2 are diffusion bonded, unannealed particles A1 as used in Examples 1 and 2 was mixed with 3% particles A2, also as used in Examples 1 and 2, in wind mill mixer. The mix was then vacuum annealed.

The diffusion bonded powder was then mixed with an amide wax in an amount of 1% by weight of the entire composition and graphite powder in an amount of 0.2 % by weight of the entire composition, so that the amount of the diffusion bonded powder was 98,8 % by weight. This was compared with a reference powder wherein the diffusion bonding had not been conducted. By subjecting the materials to vacuum sintering at temperature 1225°C, 30 min in fine vacuum, cooling in liquid nitrogen and tempering at 550°C for 1 hour in nitrogen. The following properties were determined:

Table 6: Compositions for diffusion bonding trials

The results show that the excellent of the Examples 1 and 2 can also be obtained if a diffusion-bonded alloy is used as precursor.

Examples 4 and 5 and Comparative Example 2

In a procedure similar to Examples 1 and 2, compositions comprising a high speed steel T15 (Example 4) and a high speed steel M35 (Example 5) were prepared. For comparison, a composition using a tool steel H13 were also prepared. The compositions are shown below (all values in weight % of the entire composition); the particles A2 are the same as used in Examples 1 and 2.

T15 used in Example 4 is a tungsten-type High Speed Steel, and M35 is a molybdenum-type High Speed Steel. H13 is a Tool Steel, but not a High Speed Steel, and is hence not encompassed by the definition of the particles Al.

The mixes were prepared in windmill type mixer. Compaction of TRS bars was performed at 800MPa in cold die. Sintering was done at 1240°C for 30 min applying fine vacuum. The samples were sub-zero cooled in liquid nitrogen directly after sintering and tempered 560°C for 2 hours.

The following results were obtained:

The results above show that full density or near-full density could be achieved with the High Speed Steels T15 and M35, but that this result is not obtained for the Tool Steel H13 (bulk density 7.75 g/cm³), achieving only 89% density. This is reflected by the porosity of 11.0 %. In consequence, also Hardness and TRS values are inferior.

In an attempt to improve the properties of the sintered part obtained in Comparative Example 2, an additional sintering at 1300°C was performed for 30 minute under vacuum. Yet, still only a density of 7.02 g/cm³ and a porosity of 9.4% was obtained.

Claims

1. Composition comprising

A two or more alloy particles, the two or more alloy particles

carbon in an amount of 0.70 to 1.20 % by weight;

tungsten in an amount of 7.0 to 11.0 % by weight;

manganese in an amount of 0.90 to 1.80 % by weight;

chromium in an amount of 2.5 to 4.5 % by weight;

silicon in an amount of 0.60 to 1.80 % by weight;

boron in an amount of 0.50 to 1.40 % by weight;

vanadium in an amount of 1.40 - 2.00 % by weight;

titanium in an amount of 0.10 to 0.80 % by weight;

niobium in an amount of 1.10 to 1.90 % by weight;

optionally one or more of cobalt, copper, nickel, molybdenum, oxygen, nitrogen, phosphorous, and sulfur in a total amount of 2.50 % by weight or less, preferably 1.50 % by weight or less, and more preferably 1.00 % by weight or less; the balance being iron and optionally unavoidable impurities, the unavoidable impurities forming 0.50 % by weight or less, preferably 0.20 % by weight or less, of the alloy; wherein the two or more particles A form 95% to 100 % by weight of the total weight of the composition.

2. Composition comprising

A two or more alloy particles, the two or more alloy particles

carbon in an amount of 0.70 to 1.20 % by weight;

tungsten in an amount of 7.0 to 11.0 % by weight;

manganese in an amount of 0.90 to 1.80 % by weight;

chromium in an amount of 2.5 to 4.5 % by weight;

silicon in an amount of 0.60 to 1.80 % by weight;

boron in an amount of 0.50 to 1.40 % by weight;

vanadium in an amount of 1.40 - 2.00 % by weight;

titanium in an amount of 0.10 to 0.80 % by weight; niobium in an amount of 1.10 to 1.90 % by weight;

the balance being iron and optionally unavoidable impurities, the unavoidable impurities forming 0.50 % by weight or less, preferably 0.20 % by weight or less, of the alloy; wherein the two or more alloy particles A form 95% to 100 % by weight of the total weight of the composition;

wherein the two or more alloy particles A are formed to 95% by weight or more by the alloy particles A1 and A2, or wherein the alloy particles A consist of the particles A1 and A2; and

wherein the amount of the particles A1 is 90.0% by weight or more, preferably 95.0% by weight or more, but 99.5% by weight or less, preferably 99.0% by weight or less, and the amount of the particles A2 is 10.0% by weight or less, preferably 5.0% by weight or less, but 0.5% or more, preferably 1.0% by weight or more, relative to the total weight of the particles A1 and A2.

3. Composition according to claim 1 or claim 2, wherein the alloys forming the

particles A2 have the following composition, in weight% of the respective alloy:

and/or wherein the particles A1 made from an alloy selected from the group consisting of M2, regular and high, M3 Class 1 and Class 2, M35 and T15, wherein M2, regular and high, M3 Class 1 and Class 2, and T15 are alloys as defined in ASTM A600-92a and M35 is an alloy as defined in claim 1.

4. Composition according to any one of claims 1 to 3, wherein one, two or more additives B is/are present, and/or the additive B is non-metallic.

5. Composition according to any one of claims 1 to 4, wherein the optional one, two or more additives B is selected from the group consisting of hydrocarbons having a molecular weight of 130 g/mol or more, fatty acids, fatty acid amides, graphite, polyolefin or polyamide waxes, and machinability enhancing agents such as MnS.

6. Composition according to any one of claims 1 and 3 to 5, wherein the two or more alloy particles A are formed to 95% by weight or more by the alloy particles A1 and A2, or wherein the alloy particles A consist of the particles A1 and A2.

7. Composition according to any one of claims 1 and 3 to 6, wherein the amount of the particles A1 is 90.0% by weight or more, preferably 95.0% by weight or more, but 99.5% by weight or less, preferably 99.0% by weight or less, and the amount of the particles A2 is 10.0% by weight or less, preferably 5.0% by weight or less, but 0.5% or more, preferably 1.0% by weight or more, relative to the total weight of the particles A1 and A2.

8. Composition according to any one of claims 1 to 7, wherein the alloy particles A have a particle size, expressed as D_v50 and determined by a laser light scattering method, of 200 pm or less, preferably 150 pm or less, but 10 pm or more, preferably 25 pm or more.

9. Method for producing a sintered part, comprising the following steps in this order: a. providing a composition as defined in any one of claims 1 to 8;

b. compacting the composition to form a green body; and

c. sintering the green body to form a sintered part under vacuum, a reducing atmosphere or an inert atmosphere.

10. Method for producing a sintered part according to claim 9, wherein the method additionally comprises a step d., performed after steps a., b., and c., have been performed, of tempering the sintered part at a temperature of 450 to 700 °C.

11. Method for producing a sintered part according to claim 9 or 10, wherein the

sintering in step c. is performed at a temperature between the solidus

temperature of the alloy forming the particles A1 and liquidus temperature of the alloy forming the particles A2.

12. Method for producing a sintered part according to any one of claims 9 to 11,

wherein the sintered part obtained after step c. or, if conducted, after step d. has a sintered density of 85% or more, preferably 90% or more, more preferably 95 % or more, or between 97 and 100% of the density of the alloy Al.

13. Method for producing a sintered part according to any one of claims 9 to 11,

wherein the sintered part obtained after step c. or, if conducted, after step d. has a Hardness HRC of 30 or more, preferably 40 or more, more preferably 45 or more or 50 or more, such as 55 or more or 60 or more.

14. Sintered part made from an iron alloy, the sintered part having a density of of 85% or more, preferably 90% or more, more preferably 95 % or more, or between 97 and 100% of the density of the alloy forming the particles Al, and/or a Hardness HRC of 30 or more, preferably 40 or more, more preferably 45 or more, or 50 or more determined according to ISO 4498:2010, which is obtainable by using the composition as defined in any one of claims 1 to 8 or by the method according to any one of claims 9 to 12.

15. Use of particles A2 as defined in any one of claims 1 or 3 in a sintering

manufacturing process of powders comprising or consisting of particles as defined for particles Al in claim 1 or claim 2.