DE2137761A1 - High-alloy steel powder - Google Patents

Description

The invention relates to powder metallurgical methods of extraction high-alloy, homogeneous steels, e.g. B. of high-speed steels, with good tempering resistance and high Eotglut hardness and a density that approximates to about 100 $ of the theoretical density. In particular is concerned with the invention with steel powders, the salient one Properties related to their carbon and oxygen content have, and with a process for the production of blanks from these materials, from which then high-alloy steels according to known methods can be produced.

High-alloy steel with a high carbon content, such as high-speed steel ■ and tool steel, is metallographic by a two-phase structure characterized by carbides embedded in a steel structure consists. The distribution and the size of these carbides influence extremely important properties of the material, such as Hardenability, millability and mechanical strength. These Properties can be improved if the staiil is very fine and Evenly distributed carbide particles in a finely grained structure contains.

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An advantageous uniform distribution of the carbides cannot be achieved if one assumes a traJibloek in steelmaking. When solidifying, carbides of a eutectic type precipitate and form a network in the block. The graininess of this carbide network depends on the cooling speed of the block and therefore indirectly on the size of the shape. It is certainly possible to create a large part of the network by subsequent forging or rolling to break up, provided that the angry Verimlerung the Sufficient cross-sectional area. However, the dimensions are possible of the semi-finished product after the end of the deformation limited, and once through an upper limit, which is in front of the block size, to the another of a lower limit, that of the prevention of the area depends. With this method you can get a completely even distribution the carbide is not retained in the final product.

If, on the other hand, a steel material is produced in the form of powder by atomizing a molten steel, then each powder particle solidifies almost instantly and one obtains an extremely fine-grained structure with the desired arbitrary and smooth distribution of the carbides. Methods for heat compression of capsules filled with such powders to densities that approximate a theoretical density of about 100 % are already known. Lim? The first prerequisite for achieving such high density values, apart from the use of high pressure and high temperature, is that the surface of a single powder particle has a composition which does not differ significantly from that in its center, i.e. ii. that the surface of the powder is practically free of metal oxides. The limit value for the maximum permissible oxygen content is below ΰ.05% (the percentage is based on percent by weight). Since the above-mentioned types of materials alloy elements with a high affinity for oxygen, such as. ;}. Chromium and vanadium, and the surface exposed to the atmosphere is very large. it is very difficult to avoid oxidation of the powder particles during atomization. If the powder is atomized and cooled in an inert gas, such as nitrogen or argon, the oxygen content can be reduced more than with one

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= 3 -

Atomization in water or steam. The aforementioned high of $ 0.05 is retained even at einsr sputtering in an inert gas is very difficult to sustain 211 ,, & s svar luekanat and technically feasible, high-alloy material has the _s strong © xydationseapfindliehe Legierunyseleraente, with oxygen levels below 0.05 ^ / έ to be generated by atomization and cooling of the powder in argon gas (international Journal of Powder Metallurgy ₉ K (3) »1969 _S page 7 - 17), but this technique is associated with very high manufacturing costs ₉ which are based on the heavy consumption of Inert gas and the high investment capital "

Methods for hot pressing of pressure cans filled with alloy steel powder to densities close to 100 % of the theoretical density are well known, e.g. B ₀ from the US-PSS 2,725,288 and 2,235,958, British Patent Specification 842,226 _a Casiadisehen the patent specification 616 393 ^{a nd} the Swedish patent application (utläggningskrift) 315 085. ^ ie usual methods consist in the following steps:

1. Filling the capsule sai-t steel powder *;

2. Pre-compression of the powder capsule at liaumteraperatur;

3. Preheating the powder capsule;

4. Hot pressing at 900 - 1200 C by means of the so-called isostatic compression, by extrusion or by pressing the capsule in a water-cooled Shape.

The pre-compression is used to improve the contact between the Powder — particles, and that in turn makes it easier to heat them up Compression temperature.

It is also known that by coating or mixing the powder with a carbonaceous material, e.g. B. lamp soot, a high proportion of these surface oxides can be reduced if the pressure capsules during heating to compression - temperature evacuated (US Pat. No. 5,341,325). For this purpose, the powder capsule is used

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connected to a vacuum pump. The problem with this method lies in that all the carbon reacts with the oxides and none elemental carbon remains after degassing.

The previously known methods for the production of high-alloy steels, by hot pressing of powder material filled in a pressure capsule had one thing in common: that the composition of the pre-alloyed steel powder is completely identical to the desired analysis of the End product was.

The invention relates to a powder material for the production of high-alloy steels with good tempering resistance and high hot hardness, characterized in that the powder material is a high-alloy steel powder, the alloy elements at least one Element with high affinity for oxygen contains in which

^C tot ^. 1.1.,
stubborn

° dead

+ k O _dead

where C,, is the carbon content of the powder in wt, jt, to %

C .... the carbon content in wt. ^, the stoichiometric

would be required for the formation of carbides with the alloying elements,

0,, the total: content of oxygen of the powder in wt. Ii toi

mean, and

k and k consi

and can accept 1.0 »

k and k represent constants that have values between 0.7

The disadvantages and difficulties of the previously known methods of pulverine metallurgical production of tool steel are at least in

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its most important aspects are overcome or eliminated by the invention. The process is based on a pulverized steel material, which can contain the usual alloying substances such as molybdenum, tungsten and cobalt, and also oxidation-sensitive alloying elements such as titanium, chromium and vanadium. The powder material is a high alloy steel powder preferably containing 10 Ji or more of a carbide-forming additive. The carbide-forming elements are selected in the following proportions: chromium: 0-30 $, molybdenum: 0-20 {6, tungsten: 0-20% and vanadium: 0-20 #; other carbide-forming elements, such as titanium, tantalum, niobium, zirconium and hafnium, can also be used, in each case in the range from 0-10 ¹ Jo. In addition to the carbide-forming additive, 0 - 30 ¹ Jo cobalt and 0 - 10 % aluminum can be added. The Gesanit content of alloy elements should not exceed 60 ^.

Characteristic of the new powder material used here is that its carbon content is higher than it is to compensate for those present Alloying elements in the stoichiometric ratio corresponding to the carbides formed is required. The carbon content is further determined by the total oxygen content of the powder material in accordance with the equation given above. The carbon content is usually between 0.6 and 5 ^. the Powder particles should preferably, as far as possible, be irregularly shaped and not spherical.

As will be described further below, the invention also encompasses a method for the subsequent treatment of the powder material with the ultimate goal of producing high-alloy steel with a density that is equal to or practically equal to the theoretical density.

The best properties are achieved in a tool steel when the carbon content in a certain proportion to the amount of Carbide-forming elements present in the steel, e.g. B. chromium, tungsten, Molybdenum and vanadium.

The ratio of nasty carbon content to the content of the alloy

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elements is determined by the stoichiometric composition of the carbides that are formed during the steel's temperature, e.g. B. of Cr ^, (J / -, W ₂ C, Mo ₂ C and V, C,. 3 The required amount of carbon (C ...,) for the stoichiometric equilibrium with some alloy elements is shown in Table 1.

Table 1

Emptying elements Carbide C .... 1 ί alloy roll element each

stocn.

Vanadium Tungsten Molybdenum Ψ Chromium

V ₃ W ₂ C

Mo ₂ C

0.2
0.03 0.06 0.06

The following values (Table 2) for the ratio of carbon content: C ..., were obtained for some standard high-speed steels: s to ch

Table 2

AISI

Composition in

C content

G W. Mon Cr V 00 Co C. stubborn M2 0.85 6.00 5.00 4.00 2, 00 0 , 7 M2 higher
C content 1.10 6.00 5.00 4.00 O 00 1 , 0 M3 (2) 1.20 6.00 5.00 4.00 i » 00 - 0 , 9 M41 1.10 6.75 3.75 4.25 2, 15th 5.00 1 M42 1.10 1.50 9.50 3.75 1, 00 8.00 1 M43 1.25 1.75 ö, 75 3.75 2, 00 8.25 1 Tl 0.70 18.00 - 4.00 1, 00 - 0 , 7 T15 1.50 12.00 - 4.00 5, J, 00 0

As can be seen from Table 2, does not have a £> _c hnellstuhl a ratio which is greater than unity. The reason for that is

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the proportion of retained austenite after hardening increases sharply with increasing carbon content. As an example, it may be mentioned that in the case of high-speed steel AISI M2 an increase in the carbon content of 0.85 to 1.1 $ (ratio of carbon content: C ..., = 1) an increase in the amount of retained austenite of 25 55-55 $ (Trans, of the ASM, 57 (1964), pp. 925-948). This proportion of retained austenite can be removed by repeated tempering and a high secondary hardness can be achieved. With a carbon content of 1.3 $ (carbon content: C .... = I, l) there is 70 $ of remaining austenite. This high content of retained austenite makes successful heat treatment of the steel impossible. Therefore, the carbon content of 1.1j6 represents the upper practical limit in a cutting steel of the AISI M2 type.

According to the invention, a powder with a total carbon content, C,., Which is considerably above that of dead

which is required for the formation of the alloy carbides, ie ^c tot ^{: c} stöch> ¹ » ¹ r

The carbon content of the powder material is determined by the composition of the steel to be produced as the end product and the total oxygen content, 0,., of the powder material after atomization. Experimental results based on a wide variety of studies are summarized in the following equation that the carbon content to the content of alloy elements and relates the total oxygen content of the material:

C., = k C .... + k ¹ 0,, dead stabbed dead

where Ic and Ic_ can assume values between 0.7 and 1.0. That means, that the total oxygen content, 0 ..., of the material is so high

must be that} the ratio C.,: C ..., exceeds 3.1. In the 'stabbed dead

Practice has shown that the oxygen content must not fall below 0.15 % .

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A suitable composition of the powder material is:

Chromium: 3.5 to 4.5 / 6, molybdenum: 3.5 to 5 #, tungsten: 5 to 7 #, vanadium: 2 to 4 #, cobalt: 5 to 10 %, carbon: 1.15 to 2, 0 j6, Best iron and impurities, including oxygen.

The powder is made by atomizing with water or steam, each powder material being subjected to almost instantaneous cooling. This process gives a very fine grain and uniform carbide structure in each powder particle. Because of the with a high carbon content, the temperature M, the temperature at which the formation of martensite begins, down to below 100 C. As a result, despite the rapid cooling, the austenite becomes so stable that that the amount of retained austenite exceeds $ 70. This high austenite content leads to a strong prevention of hardness for each individual powder particle after atomization, as compared with the conventional powders that are made with conventional carbon held. Due to this decrease in hardness the compressibility of the powder is improved.

During the atomization, the surface of the particles is oxidized. As a result, the oxygen content of the powder exceeds 0.15%. The combination of a high carbon content with a high oxygen content gives the powder unique properties that make it very suitable as a drilling material for the powder metallurgical production of tool steel Surface oxides, provided that a sufficiently low pressure, i . E. less than 10 * "Torr. Since the carbon is evenly distributed in each individual powder particle, the transport route of the carbon or the carbon-containing gases is also the shortest conceivable in practice achieve low oxygen levels, as illustrated in the following examples.

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example 1

With a water-atomized high-speed steel powder AISI M2 (analysis: C = 0.78 Ji, Cr = 4.45 Ji, Mo = 5.10 J6, V = 1.84 Ji ₁ W = 5.28 Ji), up to one Sieve fineness of 40 meshes, with a total oxygen content of 0.28 %, graphite is mixed in amounts between 0.1 and 0.3 %. The various powder mixtures are then poured into cylindrical pressure capsules, 60 mm in diameter χ 100 mm, and compressed in these at room temperature to a density of 62% of the theoretical density. Thereafter, the pressure capsules are ^{heated up to 115O 0} C, and at the same time the interior of the - ^ ruckkapsel is constantly kept under vacuum. This temperature is kept for over 4 hours. At the end of the treatment, the pressure in the pressure capsules is about 10 Torr. The following results were obtained after the end of the heat treatment and the hot isostatic pressing at 1150 ° C:

Graphite mixture Compressed powder

Carbon content ^ total oxygen $ of theo-

Content % ret. Density

0.1 0.74

0.2 0.82

0.3 0.90

During the metallographic examination of the compacted and heat-treated Samples was the presence of Ilest graphite particles and unreduced oxides detected in all samples listed above.

After emptying from the pressure capsule, the material with the highest density after hot pressing, i.e. the material with 0.2 % added graphite, was hot forged at 1150 C; the volume of the sample decreased by 56 fi. During the metallographic examination of the forged Itohlings it could be clearly seen that the porosities, which originated from the ßest-graphite and the unreduced oxide,

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0.19 82 0.12 93 0.13 86

were preserved even after forging. So it wasn't possible to close a full light after the second treatment obtain.

Example 2

A water-atomized high-speed steel powder AISI M2 (analysis:

Cr = 4.47 #, Mo = 5.09 £, V = 1.72 £, W = 5.92 %), up to 30 meshes, with a carbon content of 1.35 # (C: C .. = 1, 24), was filled in the same way as in Example 1 into a pressurized can, 60 min diameter χ 100 mm, and compressed to a density of 62 $> the theoretical density. The pressurized powder can was then heated up to 1150 C with simultaneous evacuation.

This temperature was held for 1 hour. After this time has elapsed the pressure in the pressurized can was 10 torr. After completion of the heat treatment, the pressure can was closed and then under isostatically hot-pressed at a pressure of 1.5 k bar. As in the previous example, the temperature was 1150 C. The following results were received:

Powder after hot isostatic pressing

Carbon content # 1.35 1.02

Ge saint-sauer-

Fabric content $ 0.36 0.01

$ the theor.

Density 99

As in Example 1, this blank was subjected to a second warm-machining process. subjected to processing by forging. The Schraiede temperature was ll60 C and the degree of diminution 34 Ji, at the subsequent me ^ - tallographic examination revealed no Itest porosity at all found; the material was completely tight.

Example 1 shows that a decrease in the oxygen content in the atomized, high-alloy steel was made possible by reduction with added carbon under vacuum and at high temperature.

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The decrease, however, was not sufficient on the whole, and that after the Samples obtained from heat treatment could not be processed into a usable tool steel.

In example 2, the liquid melt became carbon before atomization added in such an amount that stabilization of the austenite was effected while rapidly cooling the powder particles. This resulted in a somewhat increased compressibility and a higher one Density after compaction as in example 1. The reduction of the oxides in the vacuum heat treatment of the pre-compacted powder body is caused by an excess of carbon, which is responsible for the solution reactions in each powder particle is required, further greatly improved, deviating from Example 1, where the carbon is through First introduced.

Atomization with a gas leads to powder — particles of spherical shape Shape. With this powder one obtains a density which appears to be relatively high, but because of the coherence, it is high Map and the spherical shape of the particles can make this powder not to compacts with such mechanical strength, the so-called "liriin hardness", that the pressed material is pressed can be handled without disintegration. Hence it is necessary to use the powder to be enclosed in a pressurized can prior to vacuum treatment. Used if water is used as the atomizing agent, irregular powder particles are obtained. These can also be made into compacts when cold be pressed with sufficient Lirün hardness to allow handling of the compressed material without disintegration.

When using a powder according to the invention, the green card can further increase, because uiese powders have a higher density because of their high carbon content and consequently lower hardness can be priced.

Isostatic hot pressing is used for the final compaction on, then an enclosure in the pressure cell is necessary to enable compression. If a powder according to the invention is used

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det, then it can be advantageous to enclose the material in the pressurized can only after the pre-compression and the reduction treatment have been carried out. It has thus been found that evacuation of the pre-compacted powder can be made more effective if it is not enclosed in a sheet steel can. This fact is due to the minimal resistance to the flow of the escaping reaction products, which is achieved when the (rase can escape over the entire surface of the compact instead of just over part of it, which then causes the P {^ iΓi3 · ■ ^ » ^Γ : ίίP ⁿ ^; ■ "d3 ^ έ ^ i ^ ® & l ^: ^ · uφ - d; i ^ β- · ^ί · P ^ rXv-er · fi ■ - into the · -pressure cell in front of the vacuum ^ Treatment as described above is carried out ¹ , This is shown in the following example .;.

Example 3 -.

The same high-speed steel powder with a high carbon content as in Example 2 is compacted by isostatic pressing (using a rubber mold which was subsequently removed) at a pressure of 3 k bar into a compact with dimensions of 75 × 100 mm. The density of the compact was 5-33 g / cm, ie 64 f> the theoretical density. The compact was then heat-treated at a temperature of 1150 ° C. in vacuo. During the treatment at 1150 ° C., the pressure in the furnace was 10 torr. After the end of the heat treatment, the furnace was filled with argon and cooled to room temperature. The powder body was then inserted into a sheet steel can and isostatically hot-pressed at 1.5 kbar at 1150.degree. The pressurized can containing the powder body was evacuated before heating. The density after hot pressing was 99-100 % of the theoretical density and the oxygen content was 0.003 %. . . _ ■ ..

This is how the porous body, which after cold pressing and Reduction treatment is obtained, a suitable material for further densification by isostatic hot pressing, by, ^! Χ-ν trusion or compression molding. About hot isostatic pressing to allow and the material when heated to a suitable. ..--. *.

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To protect the treatment temperature against oxidation, it must be enclosed in a pressure-resistant box made of sheet steel. When the can is not evacuated during the closing itself, it has to go with it an evacuation line should be provided to allow the removal of drafts Air before or during heating to the appropriate hot pressing temperature becomes possible. An inert gas, e.g. 3. Argon to be introduced into the can to keep it during to protect against oxidation over a longer period of time.

In some cases it has been found beneficial to use the warm Body or plank in a pressure-resistant container or a pruck dose to give as soon as a sufficiently low oxygen content had been discontinued during the induction treatment, whereby the Pressure has been maintained or decreased. This grants the advantage that the enclosed material can be transferred to a Preö apparatus while it is still warm, which an additional, repeated Saves heating.

If a powder according to the invention is cold-pressed, then powder compacts can be formed, the geometrical shape of which corresponds to that of the end product, as in the usual powder metallurgical processes. If the structure is complicated, the powder is difficult to encapsulate and hot isostatic pressing is not suitable. However, it has been found that the theoretical density can be obtained by forging a powder material having a complicated shape when prepared from a powder according to the invention which has been prepared by hot pressing and a reduction treatment, y ± e described above , provided that the body has been forged in an inert atmosphere on | c.? ^c "-, n '33 ·? ' .isr- iTc '/'ftr;« α.? οϊ :. % ;; U -;: \ cZ.sn λά tiaer ivavt & n Λίμα-

Sphere or by means of a vacuum or by covering the material with a chemical protective layer is shielded against oxidation. This is explained in the following example,

Oi ^ h liird explained in the x ^ Igümisn Tieiapiei ^. . .

example

A ring-shaped body with an outside diameter of 25 mm and

A r-ingioi "i22. ^ Sr norpsr with; an outer diameter of 23 msi una

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A thickness of 9 mm was produced by cold pressing in a press mold. After a heduction treatment in a vacuum oven at 1150 ° C., as in Example 3, and subsequent cooling to room temperature, the body was coated with a chemical protective layer, the main component of which consisted of water glass. The body was then induction-heated to 1150 ° C in an argon atomic sphere. After heating, the body was quickly placed in a frontal mold and forged, the time of transfer from the furnace to the mold being less than 3 seconds. After forging the density was more than 99 fi of the theoretical density,

To suit the needs of the season

The high-alloy steel powder contains one or more elements with strong affinity for oxygen, such as titanium, Ghrpm and yanauin.

The oxygen content exceeds Q '! 5% _t ^un ^ ^ ^he Kphlenstpffgehalt is based on the oxygen content and the theoretical carbon content, which would be stoichiometrically required lor the formation of carbides with the Legierungseiementen. Porous objects are made by heating the powder to 9Q0 | jis 1.25P ⁰ P iJ? Vacuum created to reduce the oxygen content to 0.05 % .

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Claims (10)

...; ..- ■ - claims 1. Powder material for the production of high-alloy steel with good quality Resistance to tempering and high Warali hardness, characterized by that the powder material is a high-alloy steel powder, that of alloying elements at least one element with high affinity to oxygen, in which ■ ^ ~> v «.. ^C tot ρ
stubborn ° tot> ⁰ · ¹⁵ and C., = k C .... + k 0.. is, dead stuck dead ' where C., the carbon content of the Palver in weight, "tot C ... - the carbon content in percent by weight, which is stoichiometric would be required for the formation of carbides with the alloying elements, 0., the total content of oxygen in the powder in parts by weight * fco x> mean, and k and k consi I ₁ O can accept. k and k represent constants, the fourth between 0.7 and 2. Powder material according to claim 1, characterized in that the powder material is a high-alloy steel powder, at least 10 Gev.jf of a carbide-forming additive, which consists of at least one carbide-forming element of the group: chromium with 0 to 30 Gew.ji, molybdenum 0 to 20 wt. £ _f tungsten with 0 to 20 Gew.ji, vanadium with 0 to 20 wt. £, titanium with 0 to 10 wt. #, tantalum, 0 to 10 wt ^ ji, niobium with 0 to 10% by weight, zirconium with 0 to 10% by weight and hafnium with 0 to 10% by weight, and 0 to ΪΚ>% ORIGINAL BATHROOM 30 wt. ^ Cobalt, 0 to 10 wt. Fi aluminum and 0.6 to 5 wt. F> carbon, wherein the total content of 'alloying elements 00 wt. ^ Does not exceed, and reads iron and impurities, oxygen included , represent· 3. Powder material according to claim 1, characterized in that the carbide-forming additive consists of at least one carbide-forming element of the group: chromium with 0 to 30 wt. % _T molybdenum with 0 to 20 wt. #, Tungsten with 0 to 20% by weight and vanadium with 0 to 20% by weight, and 0 to 30% by weight of cobalt and 0.6 to 5% by weight of carbon. 4. Powder material according to .Anspruch 1, characterized in that the The shape of the particles is essentially irregular " 5. Powder material according to claim 1, characterized in that it consists of 3 »5 to 4.5 wt. % Chromium, 3.5 to 5 wt.% Molybdenum, 5 to 7 wt.% Tungsten, 2 to 4 wt . £ vanadium, 5 to 10 wt. $ cobalt, 1.15 to 2.0 Gew.j6 carbon and balance iron and impurities, oxygen included is. 6. A method of making porous blanks for use in the Production of high-alloy steel according to Claims 1 to 5, characterized in that that the powder in a vacuum to a temperature of 900 to 1250 C under a pressure that is at the beginning and at the end the treatment is below 10 ~ Torr, heated for a period of time sufficient to reduce the total content of oxygen after heating below 0.05 wt. 7. The method according to claim 6, characterized in that the heating is carried out in a gas-tight furnace. 8. The method according to claim 7 »characterized in that the Bring warm material into a gas-tight container when a sufficiently low oxygen content has been reached, and the Maintains pressure in the container largely at the same level as in the furnace, 209810/1121 BATH ORIGINAL 9. The method according to claim 7 »characterized in that the Fills the furnace with an inert gas when the material has a sufficiently low oxygen content » 10. The method according to claim 6, characterized in that one before Bring the powder into a gas-tight pressurized can after heating » BATH ORIGINAL 209810/1121