Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
  • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S75/00—Specialized metallurgical processes, compositions for use therein, consolidated metal powder compositions, and loose metal particulate mixtures
  • Y10S75/95—Consolidated metal powder compositions of >95% theoretical density, e.g. wrought
  • Y10S75/951—Oxide containing, e.g. dispersion strengthened

Definitions

• the oxygen content exceeds 0.15%, and the carbon content is related to the oxygen content and to the theoretical carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements.
• Porous billets are produced by heating the powder in a vacuum to 900-1250 C. to reduce the oxygen content to 0.05%.
• the present invention relates to powder metallurgical processes for obtaining high-alloyed homogeneous steels, e.g., high speed steels, with good temper resistance and high red hardness, as well as with a density close to about 100% of the theoretical density. More particularly the invention concerns steel powders with special properties with regard to the carbon and oxygen content, as well as a method of producing from these materials porous billets from which high-alloy steels can be manufactured by means of known methods.
• High alloy steel with high carbon content such as high speed steel and tool steel
• the distribution and size of these carbides strongly affect such important properties of the material as hardenability, grindability, and mechanical strength. These properties are improved if the steel contains very fine and uniformly distributed carbide particles in a fine-grained matrix.
• the aforesaid types of material contain such alloying elements as e.g. chromium and vanadium with a high aflinity to oxygen, and since the surface exposed to the atmosphere is very large, it is very difiicult to avoid oxidation of the powder particles during the atomization.
• an inert gas such as nitrogen or argon
• the maximum value of 0.05% referred to above is, however, very difiicult to maintain even when atomizing in an inert gas.
• Hot compression at 900-1200 C. by so-called isostatic compression, by extrusion, or by compacting the capsule in a water-cooled die.
• Pre-compaction serves to improve the contact between the powder particles, which facilitates heating up to the compaction temperature.
• the present invention provides a powder material for the production of high alloy steel with good temper resistance and high hot hardness, the powder material being a high alloy steel powder containing alloying elements including at least one element with a strong afiinity to oxygen, in which:
• C is the carbon content of the powder in weight percent
• Csmch is the carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements, in weight percent
• O is the total oxygen content of the powder in weight percent
• k and k are constants which can assume values between 0.7 and 1.0.
• the disadvantages and difliculties of the previously known methods of powder-metallurgical production of tool steel are overcome or eliminated at least in their most important aspects by the present invention.
• the process starts with a powdered steel material, which may contain the conventional alloying substances, such as molybdenum, tungsten, and cobalt, as well as such oxidationsensitive alloying elements as titanium, chromium, and vanadium.
• the powder material is a high alloy steel powder which preferably contains 10% or more of a carbide-forming addition.
• the carbide-forming elements may be chosen within the following proportional ranges: Cr 30%, M0 0-20%, W 020%, and V 020%; other carbide-forming elements such as Ti, Ta, Nb, Zr, and Hf, each in the range 0 10%, may also be used.
• carbide-forming addition Co 030% and Al 0-l0% may be included.
• the total content of alloying elements should not exceed 60%.
• the characteristic of the new powder material used here is that its carbon content is higher than that which is necessary to balance the constituent alloying elements in the proportion corresponding to the stoichiometry of the formed carbines.
• 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 will usually be from 0.6 to 5%.
• the powder particles preferably should, in so far as possible, be of irregular shape and not spherical.
• the invention also includes a method for further treatment of the powder material with the intention, of finally obtaining high alloy steel having a density that is equal or substantially equal to the theoretical density.
• the optimal properties of a tool steel are achieved if the carbon content bears a certain relation to the quantity of carbide-forming elements, e.g. chromium, tungsten, molybdenum, and vanadium, contained in the steel.
• carbide-forming elements e.g. chromium, tungsten, molybdenum, and vanadium
• the ratio of the carbon content to the contents of the alloying elements is determined by the stoichiometric composition of the carbides which are formed when the steel is tempered, for example Cr C W C, Mo C and V C
• the required quantity of carbon (C for the stoichiometric equilibrium with some alloying elements will be found in Table 1.
• a powder is produced with a total carbon content, C which is considerably above that required for forming the alloy carbides, i.e. Ct t/C t 1 1-1.
• the carbon content of the powder material is determined by the composition of the steel envisaged as the end product and by the total oxygen content, O of the powder material after atomization.
• Experimental data based on a large number of tests has been condensed in the following equation which relates the carbon content to the content of alloyiing elements and the total oxygen content of the material:
• One suitable composition of the powder material is Cr 3.5 to 4.5%, M0 3.5 to 5%, W 5 to 7%, V 2 to 4%, Co 5 to 10%, carbon 1.15 to 2.0%, and balance Fe and impurities including oxygen.
• Powder is produced by atomizing with water or steam whereby every powder particle is subjected to almost instantaneous cooling. This procedure yields a very finegrained and uniform carbide structure in every powder particle. Owing to the high carbon content, the M temperature, the temperature at which martensite begins to form, will be lowered to below C. As a consequence, in spite of the rapid cooling, the austenite becomes so stabilized that the amount of retained austenite exceeds 70%. This high austenite content leads to a great reduction in hardness for every individual powder particle after atomizing as compared with conventional powders produced with the usual carbon contents. Owing to this decrease in hardness the compressibility of the powder is improved.
• Example 1 As in Example 1 this billet was subjected to a second hot working by forging.
• the forging temperature was 1160 C. and the degree of reduction 34%. No residual porosity at all was found in subsequent metallographic investigation and the material was completely dense.
• Example 1 shows that a decrease in the oxygen content of the atomized high alloy steel has been made possible by reduction with admixed carbon under vacuum and at an elevated temperature. The decrease, however, has been altogether insufiicient and the specimen obtained after the heat treatment could not be worked into an acceptable tool steel.
• Example 2 carbon has been added to the steel melt before atomization in such an amount as to obtain the stabilization of the austenite during the rapid cooling of the powder particles. This results in a somewhat enhanced compressibility and a higher density after the compacting than in Example 1.
• the reduction of the oxides in the vacuum heat treatment of the pre-compacted powder body is further greatly increased by having the excess carbon required for the reactions in solution in every powder particle, unlike Example 1 where the carbon has been introduced by admixture.
• Atomizing with a gas results in powder particles of spherical shape.
• a relatively high apparent density is certainly obtained with such a powder, but owing to the combination of the great hardness with the spherical particle shape, this powder cannot be pressed into compacts of such mechanical strength, so-called green strength, that the pressed materials can be handled without disintegrating. It is, therefore, necessary to encapsulate the powder before the vacuum treatment. If water is used instead as the atomizing medium, irregular powder particles are obtained. These can be pressed even in the cold state into compacts of suflicient green strength to enable the pressed material to be handled without falling apart.
• the green strength is further increased, because these powders can, owing to their high carbon content, and hence lower hardness, be compressed to a higher density.
• EXAMPLE 3 The same high speed steel powder with high carbon content as in Example 2 was pressed by isostatic compaction (using a rubber mould which was subsequently removed) under a pressure of 3 kbar into a compact having the dimension 75 x mm.
• the density of the compact was 5.33 g./cm. i.e. 64% of the theoretical.
• the compact was heat treated in vacuum at a temperature of 1150 C.
• the pressure in the furnace was 10* torr throughout the treatment period at 1150 C.
• the furnace was filled with argon and cooled to room temperature.
• the powder body was then put into a sheet steel capsule and hot isostatically compacted with 1.5 kbar at 1150 C.
• the capsule containing the powder body was evacuated before being heated up.
• the density after hot compaction was 99100% of the theoretical and the oxygen content was 0.003%.
• the porous body which is obtained after the cold compacting and reduction treatment forms a suitable material for further consolidation by hot isostatic compacting, by extrusion, or by pressing in a die.
• a pressuretight capsule made of sheet steel.
• the capsule is evacuated during the encapsulation itself, it must be provided with an evacuation tube in order to make possible the evacuation of entrapped air before or during the heating up to a temperature suitable for hot compacting.
• an inert gas e.g. argon
• powder compacts When a powder according to the invention is cold compacted, powder compacts can be formed whose geometrical shape corresponds to that of the final product, as in the case of the conventional powder-metallurgical methods. If this shape is complicated the powder compact is difficult to encapsulate, so that isostatic hot compaction is unsuitable. It has proved, however, that the theoretical density can be reached by forging a powder material of complicated shape, starting from a powder according to the present invention and produced by cold compaction and a reducing treatment as above, provided that the body is shielded against oxidation either by carrying out the forging in an inert atmosphere, or vacuum, or by protecting the material by a chemical protective coating. This is elucidated in the following example:
• EXAMPLE 4 An annular body 25 mm. outside diameter and 9 mm. thick was produced by cold pressing in a compacting die. After reduction treatment in a vacuum furnace at 1150 C., as in Example 3, and subsequent cooling down to room temperature, the body was coated with a chemical protective layer, the main constituent of which was water glass. Thereafter the body was induction-heated to 1150 C. in an argon atmosphere. After heating, the body was quickly transferred into a pre-heated forging die, the forging then being carried out, the time of transfer from the furnace to the die being less than 3 seconds. The density after forging exceeded 99% of the theoretical density.
• a powder material for the production of high alloy steel with good temper resistance and high hot hardness by hot compaction the powder material being a high alloy steel powder consisting, in weight percent, of Co -30, Al 0-10, carbon 0.6-5, and at least 10% of a carbide-forming addition consisting of at least one carbideforming element selected from the group consisting of Cr 0-30, Mo 0-20, W 0-20, V 0-20, Ti 0-10, Ta 0-10, Nb 0-10 Zr 0-10, and Hf 0-10; the total content of alloying elements not exceeding 60% by weight; the balance being Fe and impurities including oxygen; and in which:
• O is the total oxygen content of the powder in weight percent
• k and k are constants which can assume values between 0.7 and 1.0.
• a powder material for the production of high alloy steel with good temper resistance and high hot hardness by hot compaction the powder material being a high alloy steel powder consisting, in weight percent, of Co 11-30, carbon 0.6-5, and at least 10% of a carbide-forming addition consisting of at least one carbide-forming element selected from the group consisting of Cr 0-30, Mo 0-20, W 0-20, and V O-20; the total content of alloying elements not exceeding by weight; the balance being Fe and impurities including oxygen; and in which:
• 0 is the carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements, in weight percent
• O is the total oxygen content of the powder in Weight percent
• k and k are constants which can assume values between 0.7 and 1.0.
• a powder material as claimed in claim 1 consisting of Cr 3.5 to 4.5%, M0 3.5 to 5%, W 5 to 7%, V 2 to 4%, Co 5 to 10%, carbon 1.15 to 2.0%, and balance Fe and impurities including oxygen.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)

Applications Claiming Priority (1)

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Family Applications (1)

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Cited By (13)

Families Citing this family (5)

• 1970
  • 1970-08-28 SE SE7011689A patent/SE344968C/xx unknown
• 1971
  • 1971-02-23 US US118186A patent/US3704115A/en not_active Expired - Lifetime
  • 1971-07-28 DE DE19712137761 patent/DE2137761A1/de active Pending
  • 1971-08-12 AT AT704471A patent/AT337743B/de not_active IP Right Cessation
  • 1971-08-13 ES ES394219A patent/ES394219A1/es not_active Expired
  • 1971-08-25 FR FR7130874A patent/FR2106034A5/fr not_active Expired
  • 1971-08-27 JP JP46065198A patent/JPS5036807B1/ja active Pending
  • 1971-08-27 BE BE771878A patent/BE771878A/fr unknown

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