Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/10—Sintering only
  • B22F3/1003—Use of special medium during sintering, e.g. sintering aid
  • B22F3/1007—Atmosphere
  • B22F3/101—Changing atmosphere
• • 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%
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2201/00—Treatment under specific atmosphere
  • B22F2201/01—Reducing atmosphere
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2201/00—Treatment under specific atmosphere
  • B22F2201/04—CO or CO2
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2201/00—Treatment under specific atmosphere
  • B22F2201/10—Inert gases

Definitions

• the invention is based on a method for producing a carbon-containing sintered body from steel powder, in which the steel powder is heated to sintering temperature in an atmosphere which at least temporarily contains carbon monoxide, is kept at sintering temperature for a predetermined period of time, and the sintered body formed in this way is subsequently cooled.
• the invention relates to a state of the art, as specified for example in Metals Handbook Ninth Edition Vol 7 Powder Metallurgy, pp. 360 and 361.
• a method for producing a sintered body is described in which steel powder is mixed with graphite powder and the resulting powder mixture is subsequently sintered.
• the graphite should do two things: on the one hand, it should be in the steel powder Reduce metal oxides, on the other hand, it should diffuse into the steel powder in order to bring the carbon content of the sintered body to a predetermined value. This is necessary, since otherwise the steel powder is considerably decarburized in the atmosphere which acts during sintering, which is often vacuum or contains an inert gas.
• the carbon in the steel powder escapes by reacting with oxygen from or on the grains of the steel powder or from the atmosphere to form carbon monoxide, which is generally flushed away or pumped away with the atmosphere.
• carbon monoxide which is generally flushed away or pumped away with the atmosphere.
• an extremely homogeneous and finely divided mixture of steel and graphite powder is required. This requires complex technology and can hardly be checked for the degree of distribution in a production process geared towards series production for the purposes of quality control.
• the invention is based on the object of specifying a method of the type mentioned at the beginning with which the carbon content of the sintered bodies produced thereafter can be adjusted in a simple manner suitable for series production.
• the method according to the invention is characterized in that, by means of comparatively simple technological measures, sintered bodies based on a steel powder with - for example, the starting steel powder corresponding - carbon content can be produced.
• the method according to the invention ensures a largely constant good quality of the sintered bodies produced thereafter and can be used particularly advantageously in the production of series products because of the cost advantages resulting therefrom and at the same time high reliability.
• an X20CrMoV121 steel powder is used as the starting material for producing sintered bodies.
• the chemical composition of this steel powder is: Percentage by weight component 0.21 C. 11.6 Cr 1.08 Mon 0.30 V 0.58 Mn 0.38 Ni 0.36 Si rest Fe
• the structure of this starting material is predominantly martensitic with smaller proportions of ⁇ -ferrite and austenite.
• the particle size of the powder grains determined by sieving is less than 50 ⁇ m.
• the oxygen content inside the powder is 55ppm and has both dissolved Oxygen as well as oxides. Added to this are oxides and adsorbed oxygen on the surface of the powder from 100-1000ppm.
• steel powders can also be used in the production of sintered bodies by the method according to the invention.
• 50g powder are each filled into cuboid shapes of approx. 10mm x 15mm x 10mm.
• the filled molds are placed in a sintering furnace provided with an aluminum oxide tube with a diameter of approx. 50mm.
• a sintering gas under atmospheric pressure is fed to the furnace at an inflow rate of approximately 0.5 1 / min.
• the loaded with the filled molds furnace is / are heated at a rate of about 10 o C min to a sintering temperature of about 1330 o C, left for about one hour to the sintering temperature, and thereafter at a rate of about 10 o C / min to Cooled to room temperature.
• an inert gas such as argon in particular, is first fed to the furnace as the sintering gas.
• a gas change takes place above a temperature of preferably approximately 1000 ° C.
• the sinter gas now supplied has at least carbon monoxide in addition to the inert gas.
• the steel powder is carburized at a temperature below about 1200 o C.
• Decarburization takes place above a temperature of approx. 1200 o C.
• the sintered bodies formed cool down at a temperature of approximately 1200 ° C. renewed change of the sintered gas. Below this temperature, only inert gas, such as argon, is again supplied. This avoids carburization of the sintered bodies following the decarburization previously carried out.
• inert gas such as argon
• the carbon content of the sintered bodies can be adjusted to a predetermined value which deviates from the carbon content of the steel powder by shifting one or both temperatures.
• the composition of the sintering gas can not only be changed step by step, as described above, but also continuously during the execution of the manufacturing process in order to adjust the carbon content to the predetermined value.
• the carbon content of the sintered body to be produced can be determined particularly precisely, since this then ensures compliance with the predetermined one Carbon content and the balance defined by the ratio of the partial pressures of carbon monoxide to carbon dioxide can be maintained by continuously changing the partial pressure of the carbon monoxide throughout the entire production process.
• the carbon monoxide content - as described above - is gradually changed, it is advisable to switch from the inert gas to a sintering gas containing carbon monoxide during heating at a temperature between 900 and 1200 oC.
• a switchover temperature of approx. 1000 o C and cooling of approx. 1200 o C has proven to be particularly favorable .
• the manufacturing process is carried out in a closed container.
• the sintering process is timed in such a way that carbon oxides formed in the steel powder at high temperatures are decomposed on cooling and carbon which is produced is reinstalled in the sintered body.
• the production method can also be carried out in a container in which the steel powder and thus also the sintered body to be produced are surrounded by elemental carbon, such as preferably graphite. It is necessary for the graphite to be in relatively close contact with the steel powder or the sintered body.
• the oxygen residue of the sintered gas then fetches the carbon required to build up a carbon monoxide-containing atmosphere localized around the sintered body and influences the carbon content of the steel powder or the sintered body only insignificantly.
• the steel powder be heat-treated in a reducing atmosphere before the sintering process. If such a heat treatment is carried out at temperatures of up to 1400 ° C., the oxygen present in the metal oxides of the steel powder, which are hardly avoidable or attached to the powder particles, is largely removed by reaction with the reducing component of the atmosphere. In the subsequent sintering process, this oxygen can no longer contribute to decarburization of the steel powder with the formation of carbon monoxide.
• a steel powder preheated in this way can be set to a predetermined carbon content much more easily in the subsequent sintering process than a steel powder which has not been heat-treated since Preheating one of the factors influencing the carbon content of the sintered body is switched off.
• the proportions of easily reducible oxides such as FeO and / or Cr2O3, are considerably reduced even at temperatures above 800 to 1000 o C.
• oxides that are difficult to reduce, such as MnO are reduced to a minimum by, for example, sulfur binding.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Powder Metallurgy (AREA)

Applications Claiming Priority (2)

Publications (2)

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ID=4194582

Family Applications (1)

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Cited By (3)

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• 1991
  • 1991-04-29 DE DE4113928A patent/DE4113928A1/de not_active Withdrawn
• 1992
  • 1992-02-20 EP EP19920102793 patent/EP0503326A3/de not_active Ceased
  • 1992-03-03 US US07/845,033 patent/US5162099A/en not_active Expired - Fee Related
  • 1992-03-05 JP JP4048914A patent/JPH0625710A/ja not_active Withdrawn

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