KR101551453B1 - Metallurgical powder composition and method of production - Google Patents

Abstract

The present invention relates to an annealed pre-alloyed water atomised iron-based powder suitable for the production of compressed and sintered parts having high abrasion resistance. The iron-based powder comprises less than 10 to 18 wt.% Cr, 0.5 to 5 wt.% Each of at least one Mo, W, V and Nb, and 0.5 to 2 wt.%, Preferably 0.7 to 2 wt. 1-2 wt.% C. The powder has a matrix containing less than 10% Cr by weight and a large M ₂₃ C ₆ - Type carbide and M ₇ C ₃ -type carbide. The present invention also relates to a process for the preparation of iron-based powders, as well as to methods of making compressed and sintered parts having high wear resistance and parts having high wear resistance.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a powder composition for metallurgy,

The present invention relates to iron-based powders. More particularly, the invention relates to wear resistant articles such as valve seat inserts (VSI) as well as powders suitable for the manufacture of parts made from powder.

Products with high abrasion resistance are used extensively and there is a constant need for less expensive products that have the same or better performance than existing products. Only valve seat inserts produce more than 1,000,000,000 parts per year.

The production of products with high abrasion resistance can be based on powders, such as iron or iron-based powders, containing carbon, for example in the form of carbides.

Carbides are very hard and have high melting points and high abrasion resistance properties in many applications. This abrasion resistance often makes carbide a desirable component in steel for high-speed steels (HSS), such as drills, lathe, valve seat inserts and the like, which require high wear resistance.

In a combustion engine, the VSI is a loop that is inserted where the valve contacts the cylinder head during operation. The VSI is used to prevent valve wear from the cylinder head. This is due to the materials used in the VSI, which can be more abrasive than the cylinder head material without wear on the valve. The materials for use in VSI are cast materials or more compacted and sintered PM materials.

The production of valve seat inserts into powder metallurgy provides a wide flexible and very cost effective product in the composition of VSI. The process for making PM valve seat inserts begins with the preparation of a mixture containing all the components required for the final part. The most common powder mixtures include iron or low alloyed powders that serve as matrices in the final part, basic alloy components C, Cu, Ni, Co, etc. which disperse more or less widely and increase strength and hardness with matrix materials . Moreover, materials of hard phases containing carbides and similar phases can be added to increase the wear resistance of the alloy. It is also common to have a machinability improving agent added to reduce tool wear during machining of the finished product, as well as a solid lubricant to aid lubrication during delivery to the engine. Furthermore, in all the compression ready mixtures, volatile lubricants are added to assist in the compression and injection of the compressed parts. The known VSI material produced by powder metallurgy is based on high speed steel powder as carbide containing matrix material. All commonly used powders have a particle size of less than 180 μm. The average particle size of the mixture is usually from 50 to 100 [mu] m so as to allow the mixture to flow and facilitate the production. In many cases, alloy and lubricant additives are finer in particle size compared to matrix powders, which improves the dispersion of components that are alloyed in the powder mixture and the final part.

The powder mixture is then fed into a tool cavity having the shape of a VSI loop. An axial pressure of 400-900 MPa was applied, resulting in a metallic VSI part similar to the original shape with a density of 6.4-7.3 g / cm ³ . In some instances, double compression is used to reduce the use of expensive alloy components. In double compression, two different powder mixtures are used. Higher cost with superior wear characteristics creates a wear surface of the VSI facing the valve, while relatively inexpensive provides the desired height of the part. After compression, individual grains are only loosely bonded through cold welding, and subsequent sintering processes require that individual particles be dispersed together and alloy components distributed. Sintering is generally carried out at a temperature of 112 ° C to 115 ° C under reduced pressure, which is usually based on nitrogen and hydrogen, but a temperature of up to 1300 ° C may also be used. During or after sintering, copper may penetrate into the pores of the component to increase hardness and strength, as well as improve thermal conductivity and wear characteristics. In many cases, subsequent heat treatment is performed to reach final properties. It is processed to the desired size to achieve the desired geometric accuracy of the VSI. In many cases, the final machining is performed after the VSI is installed on the cylinder head. The final machining is performed to provide a VSI and an inverted valve profile and to have a small dimensional change.

Examples of common iron-based powders with high abrasion resistance are described in U.S. Patent No. 6,679,932 for powder mixtures comprising tool steel powders with finely dispersed carbides and U.S. Patent No. 5,856,625 for stainless steel powders.

W, V, Mo, Ti, and Nb are strong carbide forming components and these components are particularly concerned with the manufacture of abrasion resistant products. Cr is another carbide forming component. However, most of these common carbide-forming metals are expensive and produce inappropriately high priced products. Thus, there is a need in the powder metallurgy industry for less expensive iron-based powders, or high-speed steels, which have sufficient abrasion resistance to be applied, for example, to valve seats and the like.

It may be desirable to use chromium as the predominant carbide-forming metal, since chromium is a common carbide-forming metal that is cheaper and easier to use than conventional metals used in general powders and hard phases with high abrasion resistance. In this way the powder and thus the compacted product can be made cheaper.

Carbides of ordinary high-speed steels are generally very small, but according to the present invention it is unexpectedly found that, for example, powders with equally favorable abrasion resistance for application to valve seats can be obtained with chromium as the predominant carbide- In the presence of a large positive carbide, it is supported by a small amount of finer and harder carbide.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a cheap iron-based powder for the production of a powder metallurgical product having high abrasion resistance.

Other objects which are evident from this discussion as well as from the discussion below are those which comprise less than 10 to 18% by weight of Cr according to the invention, 0.5 to 5% by weight of at least one Mo, W, V and Nb, 0.5 to 2% Based powder comprising an iron-base powder, wherein the iron-based powder comprises 10% by weight or more of an iron-based powder, Of Cr. ≪ / RTI > Moreover, the iron-based powder comprises a large chromium carbide and a finer and harder chromium carbide.

The amount of high Cr in the powder is a large type of carbide such as M ₂₃ C ₆ - Type, so that 18 wt% and more Cr will provide only very small amounts of fine, hard chromium carbide.

The novel powder according to the present invention for achieving the above object is characterized in that it comprises 10 to 18% by weight of Cr, 0.5 to 5% by weight of at least one of Mo, W, V and Nb, Based iron-based melt comprising 0.5 to 2 wt.%, Preferably 0.7 to 2 wt.%, And most preferably 1 to 2 wt.% Of C is subjected to water atomization and the desired carbide And then annealing the powder particles at a temperature and for a period of time sufficient to contain the iron-base powder.

In a preferred embodiment, a temperature in the range of 900-1100 [deg.] C and an annealing time in the range of 15-72 hours have been found to be sufficient to contain the desired carbide in the particles.

DETAILED DESCRIPTION OF THE INVENTION

The spray alloy powders of the present invention may comprise chromium, less than 10 to 18 wt%, at least one molybdenum, tungsten, vanadium and niobium, 0.5-5 wt% each, and carbon, 0.5-2 wt%, preferably 0.7-2 wt% Most preferably 1-2% by weight, with the remainder being iron, any other alloy component and inevitable impurities.

The spray alloy powder may optionally contain up to 2% by weight of other alloy components, such as silicon. Other alloy components or additives may also optionally be included.

It should be noted that very costly carbide-forming metal niobium and titanium are not required for the powders of the present invention.

The spray alloy powder preferably has an average particle size in the range of 40-100 mu m, preferably about 80 mu m.

In a preferred embodiment, the spray alloy powder comprises 12-17 wt% Cr, such as 15-17 wt% Cr, e.g., 16 wt% Cr.

In a preferred embodiment, the spray alloy powder comprises less than 12 to 18 wt% Cr, 1-3 wt% Mo, 1-3.5 wt% W, 0.5-1.5 wt% V, 0.2-1 wt% Si, -2 wt% C, and the balance Fe.

In a most preferred embodiment, the spray alloy powder comprises less than 14 to 18 wt% Cr, 1-2 wt% Mo, 1-2 wt% W, 0.5-1.5 wt% V, 0.2-1 wt% Si, 1-2 wt% C, and the balance Fe.

In another most preferred embodiment, the spray alloy powder comprises less than 12 to 15 weight percent Cr, 1-2 weight percent Mo, 2-3 weight percent W, 0.5-1.5 weight percent V, 0.2-1 weight percent Si , 1-2 wt% C, and the balance Fe.

In a preferred embodiment, the large chromium carbide is M ₂₃ C ₆ - (M = Cr, Fe, Mo, W), that is, at least one of Fe, Mo and W besides Cr may be present as a component for controlling carbide formation.

In a preferred embodiment, the finer and harder chromium carbide is an M ₇ C ₃ -type (M = Cr, Fe, V), ie there may be more than one Fe and V besides Cr as a component controlling carbide formation. Both types of carbides may also contain minor amounts of the other carbide forming components. The powder may further include other than the carbide type.

The large carbides of the powders of the present invention preferably have an average size in the range of 8-45 μm, more preferably 8-30 μm, a hardness of about 1100-1300 microbeakers, and preferably 10-30% Respectively.

The smaller carbides of the M ₇ C ₃ -type of the powder of the present invention are M ₂₃ C ₆ - It is smaller and harder than the larger type carbide. The smaller carbides of the M ₇ C ₃ -type of the inventive powder preferably have an average size of less than 8 μm, a hardness of about 1400-1600 microbeakers, and preferably occupy 3-10% by volume of the total powder.

Since carbide has a non-uniform shape, "size" is defined as the longest portion measured in a microscope.

To obtain these large carbides, the spray alloy powder is preferably annealed for a long time under vacuum. The annealing is preferably performed at a temperature in the range of 900-1100 DEG C, most preferably at about 1000 DEG C, which is the temperature at which chromium in the spray alloy powder reacts with carbon to form chromium carbide.

During annealing, new carbides form and grow and existing carbides continue to grow through reactions between chromium and carbon. The annealing preferably continues for 15-72 hours, more preferably over 48 hours, to obtain the desired size of carbide. The longer the annealing cycle, the greater the carbide grain grows. However, annealing consumes a lot of energy and can be a manufacturing bottleneck if it lasts for a long time. Thus, the average chromium carbide grain size of about 20-30 [mu] m large chromium carbide may be best, but depending on the priority, to finish the annealing more economically, the average chromium carbide grain size of the large chromium carbide is about 10 [ Can be more convenient.

Very slow cooling is applied, preferably in excess of 12 hours from the annealing temperature. Since larger amounts of carbide are thermodynamically stable at lower temperatures, slow cooling will allow additional growth of carbide. Slow cooling also ensures that the matrix becomes ferrite, which is important for the compressibility of the powder.

Annealing the powder also has other advantages in addition to the growth of carbide.

During annealing, the matrix grains also grow and the intrinsic pressure of the powder particles, obtained as a result of water atomization, is relaxed. These factors make the powder less rigid, for example giving greater compressibility to the molecule and making it easier to compress.

During annealing, the carbon and oxygen content of the powder can be controlled. It is generally desirable to keep the oxygen content low. During annealing, the carbon reacts with oxygen to form gaseous carbon oxides, which reduces the oxygen content of the powder. Additional carbon may be provided for annealing, in the form of graphite powder, if the carbon in the spray alloy powder itself to form carbides and reduce the oxygen content is not sufficient.

During annealing, much of the chromium in the spray alloy powder migrates from the matrix to the carbide, resulting in a matrix of annealed powders of less than 10 wt%, preferably less than 9 wt%, and most preferably less than 8 wt% Chromium content, and the powder is not stainless steel.

The matrix composition of the powder is designed such that the ferrite is transformed into austenite during sintering. As a result, the austenite can be transformed into martensite by cooling after sintering. The combination of large carbides and smaller and harder carbides in a mattingitic matrix will provide a compressed and sintered part of superior abrasion resistance.

The annealed powders of the present invention can be mixed with other powder components, such as other iron-based powders, graphite, volatile lubricants, solid lubricants, machinability enhancers and the like, before compression and sintering to produce products with high wear resistance. For example, the powder of the present invention may be mixed with pure iron powder and graphite powder or stainless steel powder. Solid lubricants, such as MnS, CaF ₂ , which may reduce friction during use of the sintered product and enhance its machinability, as well as lubricants that promote compression and evaporation during sintering, such as waxes, stearates, , MoS ₂ may be added. In addition to other common additives in the field of powder metallurgy, other machinability enhancers can be added.

The resulting mixture due to the excellent compression ratio is suitable for compacting into VSI parts close to the original shape with a chamfered reversing valve profile.

Figure 1 shows the microstructure of the OB1 substrate test material.
Figure 2 shows the microstructure of the M3 / 2 substrate test material.

Example  One

16.0 wt% Cr, 1.5 wt% Mo, 1.5 wt% W, 1 wt% V, 0.5 wt% Si, 1.5 wt% C and balance Fe were water atomized to form a spray alloy powder. The resulting powder was then vacuum annealed at 1000 ° C. for about 48 hours and the total annealing time was about 60 hours after which the powder particles were about 20 vol% M ₂₃ C _{6 with} an average grain size of about 10 μm in the ferrite matrix -Type carbide and about 5 vol% M ₇ C ₃ -type carbide with an average grain size of about 3 μm.

The resulting powder (hereinafter "OB1") was mixed with 0.5 wt% graphite and 0.75 wt% volatile lubricant. The mixture was compressed into test bars at a pressure of 700 MPa. The resulting samples were sintered at a temperature of 1120 ℃ under an atmosphere of 90N ₂ / 10H _2. After sintering, the sample was cryogenically cooled in liquid nitrogen and then tempered at 550 ° C.

A similar mixture based on the known HSS powder M3 / 2 was prepared and a test bar was prepared using the same process as described above.

The test bars were tested for hardness according to the Vickers method. The high temperature hardness was tested at three different temperatures (300/400/500 ° C). The results are summarized in the table below.

The microstructure of the OB1 test material (see FIG. 1) consists of a mixture of large and small carbides which is desired in a mattingitic matrix. The reference material (see FIG. 2) has a similar microstructure but a smaller carbide than the OB1 material.

The OB1 material has slightly higher porosity than the M3 / 2 material, which explains why the OB1 microhardness is higher than that of M3 / 2 but the OB1 hardness value (HV5) is lower than that of M3 / 2. In the manufacture of PM VSI parts, porosity is generally removed by copper infiltration during sintering and this effect can therefore be neglected. Taking this into account, the hardness value of the OB1 material can be compared to that of the control M3 / 2 material, giving a good indication that the material should have similar wear resistance. In particular, maintaining hardness at elevated temperatures is important for wear resistance in VSI applications. The high temperature hardness test results show that the OB1 material meets these requirements.

Example  2

The sprayed alloy powder was formed by water atomizing a melt of 14.5 wt% Cr, 1.5 wt% Mo, 2.5 wt% W, 1 wt% V, 0.5 wt% Si, 1.5 wt% C and balance Fe. The resulting powder was then vacuum annealed at 1000 ° C. for about 48 hours and the total annealing time was about 60 hours after which the powder particles were about 20 vol% M ₂₃ C _{6 with} an average grain size of about 10 μm in the ferrite matrix -Type carbide and about 5 vol% M ₇ C ₃ -type carbide with an average grain size of about 3 μm.

The powder was mixed with 0.5 wt% graphite and 0.75 wt% volatile lubricant to prepare a test bar in the same manner as in Example 1, and the resultant microstructure was very similar to that of Fig.

Claims (22)

An annealed pre-alloyed water atomised iron-based powder,
Less than 10 to 18% Cr;
0.5-5% by weight of one or more of Mo, W, V and Nb;
0-2 wt% Si;
0.5-2% by weight of C; And
The remaining Fe,
Wherein the iron-based powder has a matrix comprising less than 10% Cr by weight, wherein the iron-based powder comprises a first chromium carbide and a second chromium carbide having different average sizes and types, powder. The method according to claim 1,
Wherein said C is 0.7-2 wt%. The method according to claim 1,
Wherein said C is 1-2 wt%. The iron-based powder of claim 1 comprising a first chromium carbide having an average size of 8-45 μm and a second chromium carbide having an average size of less than 8 μm. The iron-based powder of claim 1 comprising a first chromium carbide having an average size of 8 to 30 μm and a second chromium carbide having an average size of less than 8 μm. 6. The iron-based powder of any one of claims 1 to 5, comprising from 10 to 30% by volume of the first chromium carbide and from 3 to 10% by volume of the second chromium carbide. 6. The iron-based powder of any one of claims 1 to 5, wherein the matrix is not stainless steel. delete 6. The iron-based powder of any one of claims 1 to 5 having an average particle size of from 40 to 100 [mu] m. 6. A composition according to any one of claims 1 to 5, comprising less than 12 to 18 wt% Cr, 1-3 wt% Mo, 1-3.5 wt% W, 0.5-1.5 wt% V, 0.2-1 By weight Si, 1-2% by weight C and the balance Fe. 6. A composition according to any one of claims 1 to 5, comprising less than 12 to 15% Cr, 1-2% Mo, 2-3% W, 0.5-1.5% V, 0.2-1 By weight Si, 1-2% by weight C and the balance Fe. 6. A composition according to any one of claims 1 to 5, which comprises less than 14 to 18% Cr, 1-2% Mo, 1-2% W, 0.5-1.5% V, 0.2-1 By weight Si, 1-2% by weight C and the balance Fe. The method of claim 1 wherein the first chromium carbide is selected from the group consisting of M ₂₃ C ₆ - Type, where M = Cr, Fe, Mo, W. The iron-based powder of claim 1, wherein the second chromium carbide is M ₇ C ₃ - type, wherein M = Cr, Fe, V. W, V and Nb, 0 to 2 wt.% Si, 0.5 to 2 wt.% C < RTI ID = 0.0 > And an iron-based melt comprising the balance Fe;
Annealing the powder particles in a period of time and at a temperature sufficient to obtain a first chromium carbide and a second chromium carbide having different average sizes and types within the particles,
A method of making an iron-based powder comprising a matrix having less than 10% Cr by weight. 16. The method of claim 15,
Wherein the C is from 0.7 to 2% by weight. 16. The method of claim 15,
Wherein the C is 1-2% by weight. Compressed and sintered parts made from at least powder according to claim 1. 19. A component according to claim 18, wherein a portion of the C-content is alloyed during sintering. 19. The method of claim 18, wherein the compacted and sintered component is compacted from a powder composition comprising the powder of claim 1 and at least one iron-based powder, graphite, volatile lubricant, solid lubricant, cutting enhancer, Sintered parts. 20. The part according to claim 18 or 19, wherein the pressed and sintered part is a valve seat insert which is pressed and sintered. 22. The compressed and sintered part of claim 21 comprising a chamfered mating surface having an inverted valve profile formed during compression.

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