RU2458172C2 - Metallurgical powdered composition and method for its obtaining - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: ferrum-based powder contains 15-30 wt % of Cr, 0.5 - 5 wt % of at least one of Mo, W and V and 0.5 - 2 wt %, preferably 0.7 - 2 wt % and more preferably 1-2 wt % of C, and unpreventable impurities. The powder has matrix that contains less than 10 wt % of Cr and includes chromium carbides with average size 8-45 microns. Method of powder obtaining consists in water spraying of ferrum-based liquid alloy that contains 15-30 wt % of Cr, 0.5 - 5 wt % each of at least one of Mo, W and V and 0.5 - 2 wt %, preferably 0.7 - 2 wt % and more preferably 1 - 2 wt % of C, ferrum the rest, optionally silicium up to 2 wt % and unpreventable impurities to obtain powder particles on the base of ferrum and powder particles annealing at the temperature of 900 -1100°C for 15-72 hours with the use of slow cooling preferably more 12 hours.

EFFECT: obtaining ferrum-based powder for manufacturing powder metallurgy parts with high wear-resisting properties.

18 cl, 2 dwg, 1 tbl, 4 ex

Description

FIELD OF THE INVENTION

The present invention relates to an iron-based powder. In particular, the invention relates to a powder suitable for the production of wear-resistant products.

State of the art

Products with high wear resistance are very widely used, and there is an ongoing need for cheaper products that have the same or better characteristics than existing products.

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

Carbides are usually very hard and have high melting points, and it is these qualities that give them high wear resistance in many applications. This wear resistance often makes carbides desirable as components of steels, for example high speed steels (HSS), which require high wear resistance, such as steels for drills, lathes, valve seats and the like.

Examples of conventional iron-based powders with high wear resistance are described, for example, in US Pat. No. 6,679,932 relating to a powder mixture comprising powder of tool steel with finely dispersed carbides and US Pat. No. 5,856,625 relating to stainless steel powder.

W, V, Mo, Ti and Nb are strong carbide forming elements, which makes them especially important for the production of wear-resistant products. Another carbide forming element is Cr. However, most of these traditional carbide-forming metals are expensive, and this leads to an excessively high price of the product. Thus, in powder metallurgy, there is a need for less expensive iron-based powders or high-speed steels that provide sufficient wear resistance to pressed and sintered products, such as valve seats or the like.

Since chromium is a much cheaper and more readily available carbide-forming metal than other such metals used in traditional powders and solid phases with high wear resistance, it would be desirable to be able to use chromium as the main carbide-forming metal. Thanks to this, the powder and, therefore, the pressed product would be cheaper to manufacture.

Carbides in standard high-speed steels are usually quite small, but in accordance with the present invention it was unexpectedly shown that powders having the same preferred wear resistance, for example, in the area of valve seats, can be obtained with chromium as the main carbide-forming metal, provided that the carbides are sufficient large.

The essence of the invention

Thus, it is an object of the present invention to provide an inexpensive iron-based powder for the production of powder metallurgy products having high wear resistance.

This goal, as well as other goals that emerge from the following discussion, are achieved according to the present invention by annealed, pre-alloyed, water-sprayed iron-based powder containing 15-30 wt.% Cr, 0.5-5 wt.% Each of at least one of Mo, W, and V; and 0.5-2%, preferably 0.7-2%, and most preferably 1-2 wt.% C, the iron-based powder has a matrix containing less than 10 wt.% Cr and wherein the iron-based powder contains large chromium carbides.

Although it was found that a Cr content in the range of 15-30 wt.% Leads to a sufficient amount of carbides of the appropriate type, size and hardness, it was found that a Cr content of 18 wt.% Or higher enhances this effect and leads to a particularly high amount of carbides suitable type, size and hardness. Accordingly, in some embodiments, the annealed, pre-alloyed, water-sprayed iron-based powder contains 18-30 wt.% Cr.

In some embodiments, the annealed, pre-alloyed, water-sprayed iron-based powder contains 15-30 wt.% Cr, 0.5-5 wt.% Mo and 1-2 wt.% C.

According to the present invention, this new powder, which achieves the above objectives, can be obtained by a method for producing an iron-based powder, comprising spraying water with an iron-based melt containing 15-30 wt.% Cr, 0.5-5 wt.% At least one of Mo, W and V, and 0.5-2%, preferably 0.7-2%, and most preferably 1-2 wt.% C, to obtain iron-based powder particles and annealing the powder particles at and for time sufficient to produce large carbides inside the particles.

In preferred embodiments, it has been found that temperatures in the range of 900-1100 ° C. and annealing times in the range of 15-72 hours are sufficient to produce the desired carbides in the particles.

In some embodiments, the iron-based melt contains 18-30 wt.% Cr.

In some embodiments, the iron-based melt contains 15-30 wt.% Cr, 0.5-5 wt.% Mo and 1-2 wt.% C.

Brief Description of the Drawings

Figure 1 shows the microstructure of a test material based on A3.

Figure 2 shows the microstructure of the test material based on M3 / 2.

Detailed Description of Preferred Embodiments

The pre-alloyed powder according to the invention contains chromium (15-30%, preferably 18-25% by weight), at least one of molybdenum, tungsten and vanadium (0.5-5 wt.% Each) and 0.5-2% preferably 0.7-2% and most preferably 1-2% by weight of carbon, the remainder being iron, optionally other alloying elements and unavoidable impurities.

Pre-alloyed powder may optionally include other alloying elements, such as tungsten up to 3 wt.%, Vanadium up to 3 wt.% And silicon up to 2 wt.%. Optionally, other alloying elements or additives may also be contained. In one embodiment, the doped powder comprises up to 2% by weight of silicon.

It should be noted that very expensive carbide-forming metals: niobium and titanium are not necessary in the powder of the present invention.

The pre-alloyed powder preferably has an average particle size in the range of 40-100 μm, preferably about 80 μm.

In preferred embodiments, the pre-alloyed powder consists of 20-25 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, the rest Fe, or from 20-25 wt.% Cr, 2-4 wt.% Mo, 1-2 wt.% C, the rest Fe.

In other preferred embodiments, the pre-alloyed powder consists of 19-23 wt.% Cr, 1-2 wt.% Mo, 1.5-3.5 wt.% W, 0.5-1.5 wt.% V, 0.2-1 wt.% Si, 1-2 wt.% C, the rest Fe, or from 20-25 wt.% Cr, 2-4 wt.% Mo, 1-2 wt.% C, the rest Fe.

The carbides of the powder according to the invention preferably have an average size in the range of 8-45 microns, more preferably in the range of 8-30 microns, and preferably comprise up to 20-40% of the total volume of the powder.

Since carbides are irregular in shape, “size” refers to the largest measurement measured under a microscope.

Although other types of large carbides are suitable, in some embodiments, the large powder carbides of the invention are carbides of type M ₂₃ C ₆ (M = Cr, Fe, Mo, W), i.e. in addition to Cr as the dominant carbide forming element, one or more of Fe, Mo, and W may be present. Large carbides may also contain small amounts of others other than the carbide forming elements mentioned above.

To obtain these large carbides, the doped powder is subjected to continuous annealing, preferably in vacuum. Annealing is preferably carried out in the range of 900-1100 ° C., most preferably at about 1000 ° C. — the temperature at which chromium from the doped powder reacts with carbon to form chromium carbides.

During annealing, new carbides are formed and increase in size, and existing carbides continue to grow as a result of the reaction between chromium and carbon. Annealing preferably lasts 15-72 hours, more preferably over 48 hours, to obtain carbides of the desired size. The longer the annealing time, the more carbide grains grow. However, annealing consumes a lot of energy, and if it continues for a long time, problems may arise in the production scheme. Thus, although the average carbide grain size of about 20-30 microns may be optimal, from an economic point of view it is more convenient, depending on the priorities, to finish annealing earlier when the average carbide grain size is about 10 microns.

Very slow cooling is applied, preferably over 12 hours, from the annealing temperature. Slow cooling allows further carbide growth, since at lower temperatures a greater amount of carbides is thermodynamically stable. Slow cooling will also ensure that the matrix becomes ferritic, which is important for the compressibility of the powder.

Powder annealing also has other advantages besides carbide growth.

During annealing, matrix grains also grow, and the internal stresses of the powder particles resulting from spraying with water are weakened. These factors make the powder less hard and more easily compacted, that is, they provide a powder with higher compressibility.

During annealing, the carbon and oxygen content of the powder can be controlled. It is usually desirable to keep the oxygen content low. During annealing, carbon reacts with oxygen to form gaseous carbon monoxide, which reduces the oxygen content in the powder. If carbon in the alloyed powder itself is not enough for both carbide formation and oxygen reduction, additional carbon in the form of graphite powder can be provided for annealing.

Since, upon annealing, a lot of chromium from the alloyed powder migrates from the matrix to carbides, the matrix of the obtained annealed powder has a dissolved chromium content of less than 10% by weight of the matrix, preferably less than 9 wt.% And most preferably less than 8 wt.%, Therefore, the powder is not stable to corrosion.

The composition of the powder matrix is designed so that during sintering, ferrite turns into austenite. Thus, after cooling following sintering, austenite can turn into martensite. Large carbides in the martensitic matrix will give good wear resistance to the pressed and sintered parts.

Although the bulk of the carbides in the powder of the invention are chromium carbides, some carbides can also be formed by other carbide-forming compounds in the alloyed powder, such as the aforementioned molybdenum, tungsten and vanadium.

The annealed powder according to the invention can be mixed with other powder components, such as other iron-based powders, graphite, evaporative lubricants, solid lubricants, machinability additives and the like, before pressing and sintering, to obtain a product with high wear resistance. You can, for example, mix the powder according to the invention with pure iron powder and graphite powder or with stainless steel powder. Grease, such as wax, stearate, metal soap or the like, which facilitate pressing and then evaporate during sintering, as well as solid grease, such as MnS, CaF ₂ , MoS ₂ , which reduce friction when using a sintered product and which also increase its workability. You can also add other machinability improving additives, as well as other additives traditional in the field of powder metallurgy.

Example 1

Melt 21.5 wt.% Cr, 1.5 wt.% Mo, 1.5 wt.% W, 1 wt.% V, 0.5 wt.% Si, 1.5 wt.% C and the rest Fe was sprayed water, getting doped powder. Then, the obtained powder was annealed in vacuum at 1000 ° C for approximately 48 hours, the total annealing time being approximately 60 hours, after which the powder particles contained approximately 30 vol% chromium carbides with an average grain size of approximately 10 μm in a ferrite matrix.

Example 2

The melt was 21.5 wt.% Cr, 3 wt.% Mo, 1.5 wt.% C and the rest Fe was sprayed with water to obtain a doped powder. Then, the obtained powder was annealed in vacuum at 1000 ° C for approximately 48 hours, the total annealing time being approximately 60 hours, after which the powder particles contained approximately 30 vol% chromium carbides with an average grain size of approximately 10 μm in a ferrite matrix.

Example 3

Melt 21.0 wt.% Cr, 1.5 wt.% Mo, 2.5 wt.% W, 1 wt.% V, 0.5 wt.% Si, 1.6 wt.% C and the rest Fe was sprayed water, getting doped powder. Then, the obtained powder was annealed in vacuum at 1000 ° C for approximately 48 hours, the total annealing time being approximately 60 hours, after which the powder particles contained approximately 30 vol% chromium carbides with an average grain size of approximately 10 μm in a ferrite matrix.

The resulting powder (hereinafter referred to as A3) was mixed with 0.5 wt.% Graphite and 0.75 wt.% Of the evaporating lubricant. The mixture was compacted into a bar sample for testing at a pressure of 700 MPa. The resulting samples were sintered in an atmosphere of 90N ₂ / 10H ₂ at a temperature of 1120 ° C. After sintering, the samples were subjected to cryogenic cooling in liquid nitrogen, followed by tempering at 550 ° C.

Using the same method as described above, a similar mixture was prepared based on the known HSS powder M3 / 2, and bar samples were prepared for testing. Rod samples were tested for hardness according to the Vickers method. High temperature hardness was determined at three different temperatures (300/400/500 ° C). The results are summarized in the table below.

Powder mix Porosity(%) HV _0.025 Hv ₅ High temperature hardness (NM5) 300 ° C 400 ° C 500 ° C A3 23 825 356 286 256 268 M3 / 2 17 836 415 363 326 267

The microstructure of test material A3 (see FIG. 1) consists of many large carbides in a martensitic matrix, while the reference material has a microstructure (see FIG. 2) with significantly smaller carbides in a martensitic matrix.

Material A3 has a slightly higher porosity than material M3 / 2, which explains why the hardness values (HB ₅ ) for A3 are lower than for M3 / 2, although the microhardness values (HV _0.025 ) are almost the same for both materials. In the production of components of the PM VSI powder components, porosity is usually eliminated by impregnation with copper during sintering, and therefore these effects can be neglected. In light of this, the hardness values of material A3 are comparable to those for the reference material M3 / 2, which gives good indications that these materials will have comparable wear resistance. In particular, maintaining hardness at elevated temperatures is important for wear resistance in VSI applications. High temperature hardness test results show that A3 meets these requirements.

Example 4

The melt was 21.5 wt.% Cr, 3 wt.% Mo, 1.5 wt.% C and the rest Fe was sprayed with water to obtain a doped powder. Then, the obtained powder was annealed in vacuum at 1000 ° C for approximately 48 hours, the total annealing time being approximately 60 hours, after which the powder particles contained approximately 30 vol% chromium carbides with an average grain size of approximately 10 μm in a ferrite matrix.

Processing this powder mixed with 0.5 wt.% Graphite and 0.75 wt.% Evaporated lubricant to obtain a bar sample for testing in the same way as in example 3, led to a microstructure very similar to the microstructure of figure 1 .

Claims (18)

1. An iron-based powder containing:
15-30 wt.% Cr,
0.5-5 wt.% At least one of Mo, W and V and
0.5-2 wt.%, Preferably 0.7 to 2 wt.% And most preferably 1-2 wt.%, C,
the rest being iron, optionally silicon up to 2 wt.%, and inevitable impurities,
moreover, the iron-based powder has a matrix containing less than 10 wt.% Cr, and contains chromium carbides with an average size of 8-45 microns. 2. The powder according to claim 1, containing 18-25 wt.% Cr. 3. The powder according to claim 1, containing 15-30 wt.% Cr, 0.5-5 wt.% Mo and 1-2 wt.% C. 4. The powder according to claim 1, consisting of 20-25 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 rest Fe. 5. The powder according to claim 1, consisting of 19-23 wt.% Cr, 1-2 wt.% Mo, 1.5-3.5 wt.% W, 0.5-1.5 wt.% V, 0.2-1 wt.% Si, 1-2 wt.% C and the rest Fe. 6. The powder according to claim 1, consisting of 20-25 wt.% Cr, 2-4 wt.% Mo, 1-2 wt.% C and the rest Fe. 7. A powder according to any one of claims 1 to 6, containing carbides having an average size of 8-30 microns. 8. The powder according to any one of claims 1 to 6, containing 20-40 vol.% Carbides. 9. The powder according to any one of claims 1 to 6, in which the matrix is not corrosion resistant. 10. The powder according to claim 7, containing 20-40 vol.% Carbides. 11. The powder according to claim 7, in which the matrix is not corrosion resistant. 12. A powder according to any one of claims 1 to 6, having an average particle size of 40-100 microns. 13. The powder according to claim 7, having an average particle size of 40-100 microns. 14. The powder of claim 8, having an average particle size of 40-100 microns. 15. The powder according to claim 9, having an average particle size of 40-100 microns. 16. A method of producing iron-based powder, including:
spraying water with an iron-based melt containing
15-30 wt.% Cr,
0.5-5 wt.% Each of at least one of Mo, W and V,
0.5-2%, preferably 0.7-2% and most preferably 1-2 wt.%
C, the rest is iron, optionally silicon is up to 2 wt.%, And unavoidable impurities to obtain iron-based powder particles, and annealing the powder particles at a temperature of 900-1100 ° C for 15-72 hours, using slow cooling, preferably above 12 h, to obtain a powder having a matrix containing less than 10 wt.% Cr, and containing chromium carbides with an average size of 8-45 microns. 17. The method according to clause 16, and the iron-based melt contains 18-25 wt.% Cr. 18. The method according to clause 16, wherein the iron-based melt contains 15-30 wt.% Cr, 0.5-5 wt.% Mo and 1-2 wt.% C.

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