US20210262050A1

US20210262050A1 - Modified high speed steel particle, powder metallurgy method using the same, and sintered part obtained therefrom

Info

Publication number: US20210262050A1
Application number: US17/269,164
Authority: US
Inventors: Denis Oshchepkov; Christophe Szabo
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 2018-08-31
Filing date: 2018-08-27
Publication date: 2021-08-26
Also published as: WO2020043718A1; JP2021535282A; CN112639150A; KR20210038691A; EP3844315A1; BR112021002876A2

Abstract

Particles made from a High Speed Steel (HSS) that is modified to contain dispersed precipitations of manganese sulfide (MHSS), a Powder Metallurgy (PM) method using the same, and a part produced by the PM process using the modified HSS particles. By forming a melt of a HSS and 1) Mn or a Mn-containing compound and 2) S or an S-containing compound, followed by an atomization process, modified HSS particle can be obtained containing dispersed sulfide precipitations containing mainly manganese sulfide. The amount of Mn and S are chosen such that the weight ratio of Mn to S (Mn/S), in wt-% of the total weight of the particle, is in the range of 8.0-1.0. An article obtained by a PM manufacturing method using the particles has improved machinability as compared to a an article prepared from a corresponding non-modified HSS.

Description

FIELD OF THE INVENTION

The present invention relates to particles made from a High Speed Steel (HSS) that is modified to contain dispersed precipitations of manganese sulfide (MHSS), and to a Powder Metallurgy (PM) method using the same. It also relates to a part produced by the PM process using the modified HSS particles.

BACKGROUND OF THE INVENTION

High Speed Steel is a highly alloyed steel and is conventionally used in a variety of applications due to its high hardness at high temperature, flexural strength and durability. A typical application of HSS alloys is cutting equipment for machining apparatuses or valve seat inserts (VSI) for combustion engines.
Due to its high hardness, machining or cutting of HSS itself is a costly and wear-intensive operation. Yet, such operations are typically required even for articles manufactured by powder metallurgy in order to obtain an article having exactly the required dimensions, such as for a VSI.
It is further known to add additives that improve machinability to alloys other than HSS. For instance, WO 93/19875 describes a method for the manufacture of a sintered ferrous-based material that comprises the steps of making a mixture of a ferrous-based powder, the mixture including a compound containing at least one metal from the group comprising manganese and the alkaline-earth series of metals; at least one sulphur donating material; pressing the powder mixture and sintering the pressed mixture so as to cause the formation by reaction during sintering of at least one stable metal sulfide within the sintered material. Materials and articles made by the method are also described.
Another approach is described in an article by L. G. Roy et al in “Prealloyed Mn/S powders for improved machinability in P/M parts”, Progress in Powder Metallurgy A. 1987, vol. 43, pp. 489-499. The authors describe the pre-alloyed Mn/S iron powder MP 37S and its predecessor MP 36S, from which it differs with respect to its higher Mn/S ratio. In these materials, Mn and S were added to form manganese sulphide (MnS) inclusions in order to improve the machinability of iron powders. These inclusions are deformed in the shear plane in the flow zone adjacent to a tool surface, they contribute to higher cutting speeds, longer tool life, good surface finish of machined parts, lower tool forces. However, the document also emphasizes that for powders used for PM processes, a variety of additional requirements need to be met, such as growth characteristics of the powder during sintering, as well as the mechanical properties of the sintered part. The authors thus investigated the effect of increasing the Mn/S ratio on the machinability, dimensional change and strength of the sintered part. From a chemical analysis, it was also found that the inclusions in MP 36S and MP37 not only contain MnS, but also MnFeO, SiO2, S, and/or FeS.
For High Speed Steels, the high hardness and strength needs to be maintained. It is thus believed that adding additives that improve the machinability, such as MoS₂or MnS, may possibly improve machinability, but will at the same time be accompanied by an impairment of other properties, such as a reduction in hardness and strength.

Objects of the Invention

It is an object of the present invention to provide a means for improving the machinability of High Speed Steel materials, and to provide a High Speed Steel based material—having improved machinability.
It is a further object of the invention to provide a means for improving the machinability of High Speed Steel materials without significantly impairing the physical properties of the High Speed Steel. Preferably, the physical properties of the High Speed Steel, in particular strength, are not impaired at all as compared to a High Speed Steel not including the means of the present invention.
It is yet another object of the present invention to provide a material that is suitable for a Powder Metallurgy (PM) Manufacturing Process and a PM method using the same. The part obtained from the PM should be suitable for applications that require final machining prior to use yet should possess properties that withstand harsh conditions of use, such as present in a combustion engine.
Manganese sulfide powder which is commonly used in powder metallurgy mixes for machinability improvement has mean particle size D50=4-6 μm, for example MnS-E commercially produced by Höganäs AB. This makes the powder and mixes containing it very prone to dusting. MnS is known to cause serious eye irritation, skin irritation for the human exposed to it (http://echa.europa.eu/registration-dossier/-/registered-dossier/10223/2/1. It is hence a further object of the present invention to provide a material that has improved machinability with a reduced health risk.

SUMMARY OF THE INVENTION

The present inventors found that forming a melt of a HSS and 1) Mn or a Mn-containing compound and 2) S or an S-containing compound, followed by an atomization process to form particles, is able to modify an HSS particle to contain dispersed sulfide precipitations containing mainly manganese sulfide. The amount of Mn and S are chosen such that the weight ratio of Mn to S (Mn/S), in wt-% of the total weight of the particle, is in the range of 8.0-1.0, preferably 5.5 to 3.0, such as from 5.5 to 3.5 or from 5.5 to 4.1. An article obtained by a PM manufacturing method using the particles has improved machinability as compared to a an article prepared from a corresponding non-modified HSS to which 1) Mn or a Mn-containing compound and 2) S or an S-containing compound were not added. Surprisingly, the physical properties of the article obtained from the modified HSS particles are not or not significantly impaired as compared to an article that is prepared from corresponding non-modified HSS particles.
The material of present invention does not contain free MnS particles therefore MnS dusting and abovementioned hazards are dramatically reduced.
The present invention includes the following aspects. Further aspects and features of the invention will become more apparent in view of the following detailed description.

- 1. A High Speed Steel (HSS) particle that is modified to contain dispersed precipitations of manganese sulfide, wherein the weight ratio of Mn to S (Mn/S), in wt-% of the total weight of the particle, is in the range of 8.0-1.0, preferably 5.5 to 3.0, such as from 5.5 to 3.5 or from 5.5 to 4.1.
- 2. High Speed Steel particle that is modified to contain dispersed precipitations of manganese sulfide according to item 1, having a composition consisting of, in weight %,
  - C: 0.75-1.40
  - Mn: 0.41-2.00
  - Si: 0.10-0.45
  - Cr: 3.75-5.00
  - Ni: up to 0.20
  - Mo: 4.50-6,50
  - W: 5.50-7.50
  - V: 1.75-4.50
  - Cu: up to 0.20
  - S: 0.050-0.300
  - the balance being Fe and unavoidable impurities in an amount of up to 0.5% by weight, preferably 0.2% by weight.
- 3. HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to item 1, wherein the longest axis of the manganese sulfide precipitations is from 1 to 10 μm, as determined by SEM image analysis, preferably 1 to 8 μm, more preferably 1 to 5 μm.
- 4. HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to any one of items 1 to 3, wherein the total amount of Mn and S (Mn+S) is from 0.10-3.80% by weight, preferably from 0.10-3.00% by weight, further preferably from 0.20-2.50% by weight, such as from 0.30-2.00% by weight.
- 5. HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to any one of items 1 to 4, wherein the amount of Mn in the overall particle is 3.00% by weight or less, preferably 2.00% by weight or less.
- 6. HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to any one of items 1 to 5, wherein the particle contains 4.00% by weight or more of at least one element selected from Mo, W, V and Cr.
- 7. HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to any one of items 1 to 6, wherein the amount of S is 0.10% by weight or more, and the amount of Mn is 0.45% by weight or more.
- 8. A multitude of HSS particles that are modified to contain dispersed precipitations of manganese sulfide as defined in any one of items 1 to 7, wherein 98% by weight or more of the particles have an particle size of 0 to 300 μm and wherein the a mount of particles bigger than 300 μm is 2% by weight or less, as determined by sieve analysis according to ISO4497:1983; and wherein preferably 98% by weight or more of the particles have a particle size of 0 to 200 μm and the amount of particles bigger than 200 μm is 2% by weight or less.
- 9. Method for forming a powder metallurgy product, which comprises the steps of:
  - a. Providing a multitude of particles as defined in any one of items 1 to 8;
  - b. Compacting a composition comprising the particles to form a green part;
  - c. Sintering the green part, and
  - d. optionally heat-treating the part obtained from step c; and
  - e. optionally machining the sintered part obtained from step c. or from step d.
- 10. A method for forming a powder metallurgy product according to item 9, wherein the step a. of providing the particles includes the steps of:
  - a.1-1 adding Mn or an Mn-containing compound and S or an S-containing compound to HSS material, and melting the obtained mixture; or
  - a.1-2 adding Mn or an Mn-containing compound and S or an S-containing compound to a melt of HSS;
  - a.2. forming particles from the melt, preferably by water atomization or gas atomization.
  - a.3. optionally annealing the particles in vacuum, inert or reducing atmosphere.
- 11. Sintered part, obtainable from the particles as defined by in any one of items 1 to 9 or by the process as defined in any one of items 9 and 10.
- 12. Sintered part according to item 11, which is a part for a combustion engine, preferably a valve seat insert.
- 13. Use of a combination of Mn or a Mn-containing compound and S or an S-containing compound for improving the machinability of products made from HSS, preferably powder metallurgy products, the use including the formation of dispersed precipitations of manganese sulfide in a HSS.
- 14. Use according to item 13, wherein the formation of dispersed precipitations of manganese sulfide in a HSS is achieved by forming a melt of HSS and Mn or a Mn-containing compound and S or an S-containing compound.

Definitions

The following terms and definitions will be used and apply in the following

DETAILED DESCRIPTION

Any given range referred to by a lower and upper limit, such as for example “2 to 5” or “between 2 and 5”, includes the lower and the upper value, as any value in between. Values greater than the lower limit or lower than the upper limit are explicitly included. The term is thus to be understood as abbreviation for the expression “[lower limit] or greater, but [upper limit] or lower”.
Whenever reference is made to ranges and more preferred ranges, the lower and upper limits can be freely combined. As one example, the phrase “5 to 10, preferably 6 to 8” also includes the ranges of 5 to 8 and 6 to 10.
In the present invention, all physical parameters are measured at room temperature (20° C.) and at atmospheric pressure (10⁵Pa), unless indicated differently or prescribed differently by a standard such as ISO or ASTM. In case there should be a discrepancy between a standard method and the methods described and referred to in the following description, the present description prevails.
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, and the phrase “about 60” denotes a range of 60±3.
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”. The term “consisting essentially of” allows 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, which other materials may also be completely absent. In the latter case, the composition “consists of” the recited components.
In the present invention, the term High Speed Steel (HSS) denotes an alloy as defined by its composition in Table 1 of ASTM 600-92a(2010), not including the Intermediate High Speed Steels M50 and M52, or an alloy having a composition as disclosed in WO2009/040369, which is hereby incorporated in its entirety be reference. This alloy may be in any form, e.g. in the form of a pre-alloyed water atomized iron-based powder, and it has a composition comprising 10 to less than 18% by weight of Cr, 0.5-5% by weight of at least one of Mo, W, V, and Nb, 0.5-2% by weight, preferably 0.7-2% by weight, more preferably 1-2% by weight of C, optionally 0-2% by weight of Si, the remainder being Fe. The alloy according to this definition is not limited in structure, yet it may comprise a matrix containing less than 10% by weight of Cr, as well as large chromium carbides having an average size of 8-45 μm, preferably 8-30 μm, and smaller and harder chromium carbides having an average size of less than 8 μm. The large chromium carbides are more preferably present in an amount of 10-30% by volume, and the smaller and harder chromium carbides are preferably present in an amount of 3-10% by volume.
A Modified High Speed Steel (MHSS) refers to an alloy that satisfies the compositions as defined in Table 1 of ASTM 600-92a(2010) or defined above with reference to WO2009/040369 except for the amounts of manganese (Mn) and sulfur (S), which are higher for a modified High Speed Steel as compared to the corresponding High Speed Steel.

DETAILED DESCRIPTION OF THE INVENTION

1. Modified High Speed Steel Particle

In one aspect, the present invention relates to a High Speed Steel (HSS) particle that is modified to contain dispersed precipitations of manganese sulfide. Such a particle is also referred to a s Modified High Speed Steel (MHSS) particle. Such a particle can be obtained by the method of the present invention that will be described later, and is generally obtainable by adding Mn or an Mn-containing compound and S or an S-containing compound to a melt of HSS as defined in ASTM 600-92a(2010) or an alloy as disclosed above with reference to WO2009/040369, or by melting HSS together with Mn or an Mn-containing compound and S or an S-containing compound, followed by atomization in order to form particles. Of course, the Mn-containing compound and the S-containing compound can be replaced by a compound containing both Mn and S, such as MnS. It is however necessary to form a homogeneous melt of the HSS and the material(s) that are added in order to increase the Mn and S content as compared to the HSS on which the MHSS is based, as adding MnS to HSS particles is unable to form the structure of the particles of the present invention containing dispersed precipitations of manganese sulfide.
The HSS on which the MHSS is based is not particular limited, but is in one embodiment preferably a M-type HSS, such as M2 (regular or high C), M3 (class 1 or class 2) or M35. Other HSS on which the MHSS may be bases include 061, available from Höganäs AB, Sweden, and T15.
In one embodiment, the MHSS consists of, in weight %,

- C: 0.75-1.40, preferably 1.00-1.25
- Mn: 0.41-2.00
- Si 0.10-0.45
- Cr 3.75-5.00
- Ni: up to 0.20
- Mo: 4.50-6.50, preferably 5.00-6.50
- W: 5.50-7.50
- V: 1.75-4.50, preferably 3.00-3.80
- Cu: up to 0.20
- S: 0.050-0.300

the balance being Fe and unavoidable impurities in an amount 0.5% by weight or less, preferably 0.2% by weight or less. Herein, the unavoidable impurities include any element not listed above, such as P or Si. The lower limit of Mn can also be 0.50, and/or the lower limit of S can also be 0.100.
The MHSS particle of the present invention has precipitations of manganese sulfide. This, however, does not mean that elements other than Mn and S are completely absent in these precipitations. If a precipitate is analyzed by e.g. SEM/EDS mapping on 50 or more precipitations (point analysis). In such an SEM/EDS mapping, the amount of Mn is 40 at. % or more, in a normalized connotation wherein the amounts of the elements Si, S, V Cr, Mn, Fe, Mo and W (insofar present) form 100 at. %. Herein, Fe typically forms 15 to 25 at. %, and S typically forms 25-35 at. %.
The size of the precipitations is not particularly limited, but is typically not too large in order not to disturb the structure of the HSS too much and to provide dispersed regions that are believed to improve machinability. The longest axis of all precipitates, as observed by SEM on the surface of the particles, is typically 10 μm or less, such as 1 to 8 μm and typically from 1 to 5 or from 2 to 5 μm.
The total amount of Mn and S (Mn+S) is generally from 0.10-3.80% by weight, preferably from 0.10-3.00% by weight, further preferably from 0.20-2.50% by weight, such as from 0.30-2.00% by weight, relative to the weight of the MHSS particle. The lower limit in each of these ranges can however also be 0.50, 0.60, 0.70, 0.80 or 0.90% by weight or more, such as 0.95% by weight or more.
It has been found that the weight ratio of Mn to S (i.e. weight of Mn/weight of S, in wt-% of the total weight of the MHSS particle) is of relevance for at least the following two reasons.
The first reason is that this ratio is required to obtain a proper structure. If the relative amount of Mn (i.e. the weight ratio of Mn to S) is too high, excess Mn may lead to distortions. If the ratio is too low, excess S may form sulfide layers at grain boundaries, which may potentially lead to an impairment of physical properties, such as rupture strength of an article obtained by a PM process using the MHSS particles.
Secondly, the present inventors have surprisingly found that adjusting the ratio of Mn and S allows avoiding a substantial loss of sulfur from the melt, as would generally be expected due to the relatively high vapor pressure of sulfur at elevated temperatures. Without wishing to be bound by theory, it is believed that when there is an sufficiently enough manganese in relation to sulfur, the sulfur immediately reacts with (or associates with) the manganese. Thereby the amount of elementary sulfur that could get lost due to its high vapor pressure is thus minimized. This is turn avoids or minimizes the need for maintenance work for removing sublimed sulfur, lowers the environmental burden and also makes the MHSS cheaper and easier to produce.
For the above at least two reasons, the weight ratio of Mn to S (Mn/S), in wt-% of the total weight of the particle, is in the range of 8.0-1.0, more preferably 5.5 to 3.0, such as from 5.5 to 3.5 or from 5.5 to 4.1.
The Mn content in HSS is mostly 0.15-0.40% by weight or less, depending on the type of HSS as listed in Table 1 of ASTM A600-92a (2010). In the MHSS particle of the present invention, the Mn content is higher than that, and is in one embodiment 3.00% by weight or less, preferably 2.00% by weight or less, but 0.41% by weight or more, such as 0.45% by weight or more, or even 0.50% by weight or more or 0.75% by weight or more.
In the MHSS particle, the amount of S is also higher as in corresponding HSS, wherein it is limited to 0.03% by weight. In one embodiment of the present invention, the amount of S is 0.05% by weight or higher, such as 0.10% by weight or higher or 0.15% by weight or higher. The upper limit is not particularly limited, but may be 0.50% by weight or lower, such as 0.30% by weight or lower or 0.25% by weight or lower.
HSS alloys contain more than one of Mo, W, V and Cr in different amounts, depending on the type of HSS. In one embodiment of the present invention, the MHSS particle contains 4.00% by weight or more of at least one element selected from Mo, W, V and Cr, and in one embodiment contains 4.00% by weight or more of two or more of these elements. For instance, the MHSS particle may contain 4.00% by weight or more of each of Mo, W and Cr.
The MHSS particle of the present invention can be used in a PM manufacturing process, and to this end, a multitude of MHSS particles are needed. In one embodiment of the multitude of MHSS particles of the present invention, 98% by weight or more of the particles have an particle size of 0 to 300 μm, wherein the amount of particles bigger than 300 μm is 2% by weight or less, as determined by sieve analysis according to ISO4497:1983. The amount of particles bigger than 300 μm can also be 1% by weight or less, or such particles can be completely absent.
In one embodiment, 98% by weight or more of the particles have a particle size of 0 to 200 μm and the amount of particles bigger than 200 μm is 2% by weight or less. The amount of particles bigger than 200 μm can also be 1′)/0 by weight or less, or such particles can be completely absent.
The MHSS particles of the present invention can be formed by providing a melt of the raw components, typically a commercially available HSS, S and Mn (or compounds comprising S and Mn), followed by atomization from the melt by a conventional technique. The raw components may be HSS scrap, but could also involve virgin raw materials such as Fe, FeCr, FeV etc. The atomization can be effected by e.g. gas atomization or water atomization, with water atomization being preferred.
The MHSS particles can be annealed in vacuum or protective atmosphere in order to reduce hardness and improve compressibility. Such an annealing can be conducted under conditions that are generally known, such as at 900-1100° C. for 15-72 hours.
The MHSS particles are suitable for use in PM manufacturing process. They possess suitable powder compressibility and are able to provide a low dimensional change during sintering.

2. Method for Forming a Powder Metallurgy Product

The method for forming a powder metallurgy product of the present invention comprises the steps:

- a. Providing a multitude of MHSS particles as described above;
- b. Compacting a composition comprising the particles to form a green part;
- c. Sintering the green part, and
- d. optionally heat-treating the part obtained from step c; and
- e. optionally machining the sintered part obtained from step c. or from step d.

Other than the MHSS particles described above, the steps are conventional in the field of PM manufacturing methods. Any suitable conventional method can be employed in the present invention.
The composition referred to in step b. may consist of the MHSS particles only, but may optionally also contain other metal or alloy components, such us graphite powder (up to 1%), iron powder, low alloyed iron powder, Cu powder, hard additives, such as Tribaloy, and may also contain other additives such as a glidant (e.g. amide wax) in an amount of e.g. 1% by weight. The MHSS particles of the present invention typically form 90% by weight or more, such as 95% by weight or more or 99% by weight or more of the composition of step b.
Sintering step “c” can optionally be carried out with the presence of an Cu based alloy. In this case Cu based alloy is placed in contact with the green part in the sintering furnace. At sintering temperature the infiltrating alloy is melted. The liquid Cu alloy will penetrate into the pores by capillary forces and form a nearly fully dense composite material. Infiltrating alloys typically contain at least 95% Cu and optionally further elements for improving infiltration behavior, as is well known in the art. Cu infiltration increases the hardness and strength of the final PM component and improves the thermal conductivity.

3. Sintered Part

The sintered part of the present invention is a part that is obtainable by using a composition comprising or consisting of the MHSS particles as described above in a PM manufacturing process, e.g. the process described above.
The sintered part of the present invention obtainable by using the composition comprising or consisting of the MHSS particles as described above has improved machinability as compared to a sintered part made from the corresponding HSS under the same conditions. Surprisingly, the sintered part of the present invention has still very satisfactory properties. In particular, the sintered part may still have the same hardness and strength.
Due to these beneficial properties, the present invention is particularly suitable for providing sintered parts (parts produced by a PM manufacturing method) for use under harsh conditions. The sintered part of the present invention is thus in one embodiment a part for a combustion engine, such as a valve seat insert (VSI) or valve guide.

EXAMPLES

Examples 1 and 2

MHSS particles with the following compositions were prepared (balance: Fe and unavoidable impurities):

TABLE 1

Chemical Composition of MHSS particles of Examples 1 and 2

Ex	C	S	Mo	Ni	Cu	W	V	Cr	Si	Mn

1	1.44	0.14	1.62	—	—	1.60	1.25	15.90	0.47	0.42
2	1.10	0.19	5.60	0.11	0.08	6.40	3.30	4.39	0.21	0.93

Melts of the materials were prepared by adding Mn and S to virgin materials, including Fe and alloy thereof such as ferrochromium, ferrovanadium, ferrotungsten and ferromolybdenum and melting the composition, followed by atomization and sieving using a −212 μm screen. Reference materials, in the following referred to as Ref1 and Ref2, were prepared in the same manner, except for no addition of Mn and S. The powders were vacuum annealed. The particles had the following particle size distribution:

TABLE 2

Particle Size distribution of MHSS particles (weight-%)

Particle Size (μm)	Example 1	Example 2

+250	0	0
+18-250	0	0
+150-180	0.1	1.43
+106-150	6.35	11.78
+75-106	13.47	18.09
+45-75	32.43	32.8
−45	47.65	36.9

The vacuum annealed powders were investigated by SEM. It was observed that precipitates of manganese sulfide having a longest axis of 2-5 μm had been formed.
An elemental mapping (point chemical analysis) using EDS was performed on more than 200 precipitates. The following results were obtained:

TABLE 3

Composition of precipitations in Examples 1 and 2 (at.-%, normalized)

Element	Example 1	Example 2

Si	0.47	0.68
S	27.65	29.02
V	0.91	0.83
Cr	6.95	1.30
Mn	45.78	46.27
Fe	16.92	20.20
Mo	1.25	1.69
W	0.06	—

25 kg of the powders of Examples 1 and 2 and of the non-modified HSS powders were admixed with 1% amide wax as a lubricant and used in a PM manufacturing process. The compressibility of modified and non-modified HSS particles was identical, compacted parts showing similar porosities after compaction at 800 MPa and 50° C.
The compacted parts (green parts) were then sintered at 1120° C. for 20 minutes in nitrogen with 10 vol. % H₂. Thereafter, the particles were sub-zero cooled dipping in liquid nitrogen for 20 minutes in order to transform retained austenite. Thereafter, a final tempering at 560° C. in N₂was performed for 2 hours.
The dimensional change and the hardness of the sintered materials obtained from the MHSS particles (Examples 1 and 2) and the corresponding HSS particles (Ref1 and Ref2) are shown in Table 4 below.

TABLE 4

Properties of sintered parts

	Dimensional change
Composition	from die size, %	Hardness, HV10

Ex. 1	0.497	336
Ref1	0.433	345
Ex. 2	0.127	349
Ref2	0.157	344

The hardness values given in Table 4 overlap in their margin of error (±15). It is thus concluded that the modification of the present invention does not impair important properties of the sintered part, including hardness. Further, all sintered parts obtained from the MHSS of Examples 1 and 2 showed similar porosity as compared to the sintered part obtained from the respective reference particles made from HSS without addition of Mn or S.
The machinability of the sintered parts was evaluated by using ring shaped samples of 55*45*15 mm under the following cutting parameters:
Cutting Tool:
Sandvik CNGA120404T010208B, material—CBN, radius of cutting tip: 0.4 mm
Cutting Depth: 0.4 mm
Cutting Speed: 120 m/minute
Radial Feed Rate: 0.07 mm/rev.
The wear (mm) of the tool tip wear or breakage was used as a criterion for machinability. The test was repeated two times for each material. The results are given in Table 5.

TABLE 5

Results of machinability evaluation.

Composition	Cutting distance, m	Wear (mm)

Ex. 1	269	0.055
	806	0.069
	2688	0.114
	3763	0.117
	6720	0.141
Ref.1	269	0.083
	538	Break
Ex. 2	269	0.069
	806	0.093
	2688	0.133
	3763	0,162
	6720	0.179
Ref2	269	0.048
	806	0.079
	2688	0.185
	2834	0.448(break)

The material of Ref2 achieved cutting distance 2834 m until the tool tip was broken. The MHSS material of Example 2 could be machined up to 6720 m. Tool tip wear was about 0.18 mm.
The material of Ref1 could not be machined given that the tool tip was broken shortly after beginning of the test. A variation in the cutting conditions to 400 m/m in and a reduction of the radial feed rate could slightly improve the situation, but still the tip broke after 48 cuts (538 m, data shown in Table 5).

Comparative Example 1

In order to evaluate whether the addition of MnS to particles of HSS has the same effect, the HSS material of Ref2 was admixed with MnS powder (D50=5 μm) to prepare the powder of Comp. Ex. 1. Then, sintered parts were prepared in the same manner as described above. The following results were obtained:

TABLE 6

Properties of materials after sintering and heat treatment

	Density,	Porosity,	TRS,	Hardness,
Compositionn	g/cc	%	MPa	HV10

Ref2	6.69	17.6	873	344
Ex. 2	6.70	17.5	871	349
Comp Ex 1	6.74	16.6	829	370

The machinability was evaluated in the same manner as in Examples 1 and 2. The machinability was at the same level for Comp. Ex 2 as for Example 2. Example 2 and Comp. Ex. 1 provided a tool life of at least 3 times as Ref 2. Yet, admixing of MnS in Comparative Example 1 resulted in a considerable reduction of material strength, which was not observed for the material of Example 2.

Claims

1. A High Speed Steel (HSS) particle that is modified to contain dispersed precipitations of manganese sulfide, wherein the weight ratio of Mn to S (Mn/S), in wt-% of the total weight of the particle, is in the range of 8.0-1.0.

2. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, having a composition consisting of, in weight %,

C: 0.75-1.40

Mn: 0.41-2.00

Si: 0.10-0.45

Cr: 3.75-5.00

Ni: up to 0.20

Mo: 4.50-6.50

W: 5.50-7.50

V: 1.75-4.50

Cu: up to 0.20

S: 0.050-0.300

the balance being Fe and unavoidable impurities in an amount of up to 0.5% by weight.

3. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, wherein the longest axis of the manganese sulfide precipitations is from 1 to 10 μm, as determined by SEM image analysis.

4. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, wherein the total amount of Mn and S (Mn+S) is from 0.10-3.80% by weight.

5. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, wherein the amount of Mn in the overall particle is 3.00% by weight or less.

6. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, wherein the particle contains 4.00% by weight or more of at least one element selected from Mo, W, V and Cr.

7. The HSS particle that is modified to contain dispersed precipitations of manganese sulfide according to claim 1, wherein the amount of S is 0.10% by weight or more, and the amount of Mn is 0.45% by weight or more.

8. A multitude of HSS particles that are modified to contain dispersed precipitations of manganese sulfide as defined in claim 1, wherein 98% by weight or more of the particles have an particle size of 0 to 300 μm and wherein the a mount of particles bigger than 300 μm is 2% by weight or less, as determined by sieve analysis according to ISO4497:1983.

9. A method for forming a powder metallurgy product, which comprises the steps of:

providing a multitude of particles as defined in claim 1;

compacting a composition comprising the particles to form a green part;

sintering the green part, and

optionally heat-treating the part obtained from step c; and

optionally machining the sintered part obtained from step c. or from step d.

10. A method for forming a powder metallurgy product according to claim 9, wherein the step a. of providing the particles includes the steps of:

a.1-1 adding Mn or an Mn-containing compound and S or an S-containing compound to HSS material, and melting the obtained mixture; or

a.1-2 adding Mn or an Mn-containing compound and S or an S-containing compound to a melt of HSS;

a.2. forming particles from the melt, preferably by water atomization or gas atomization.

a.3. optionally annealing the particles in vacuum, inert or reducing atmosphere.

11. A sintered part, obtainable from the particles as defined by in claim 1.

12. The sintered part according to claim 11, which is a part for a combustion engine.

13. (canceled)

14. (canceled)