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
  • 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
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • B22F2998/00—Supplementary information concerning processes or compositions relating to 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
  • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
  • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

• the present invention concerns a new stainless steel powder and stainless steel powder compositions including this new powder. Specifically the invention concerns stainless steel powder compositions for manufacturing sintered powder metallurgical parts having high densities .
• a primary goal in powder metallurgy is to achieve high density of compacted and sintered bodies.
• There are several methods of improving density one of those methods is warm compaction which improves the compressibility of the powder giving a green body with higher green density.
• the green density may also be increased.
• the use of high compaction pressures in combination with low amounts of lubricants also results in elevated green densities.
• Soft annealing of a stainless steel powder where the material is strain relieved and recrystallized, also improves the compressibility.
• After compaction the green body is subjected to a sintering operation in order to achieve a sintered body. High temperatures at sintering, i.e.
• Stainless steels have approximately above 10% chromium. Most often carbon is present in steels and will cause formation of chromium carbides. The formation of chromium carbides lowers the chromium content in the matrix, which in turn causes lower corrosion resistance. In order to avoid that the chromium content in the matrix is reduced, carbide forming stabilizers, such as niobium, are often used. In this way the formation of chromium carbides can be avoided and instead niobium carbides are formed, a result of which is that the corrosion resistance can be maintained.
• a problem with the use of niobium is that high sintering temperatures are necessary for obtaining high sintered densities and the energy consumption is considerable.
• the sintered parts manufactured by using the new powder are of particularly interest within the automotive industry where the demands on both costs and performance of the parts are high.
• the new powder can also be used for sintered parts in exhaust systems, and especially for flanges in exhaust systems.
• the present invention concerns stainless steel powder, stainless steel powder compositions as well as the compacted and sintered parts obtained thereof having high densities. Specifically the invention concerns stainless steel powder compositions for manufacturing powder metallurgical parts .
• vanadium as a stabiliser to a stainless steel powder
• the sintering temperature and accordingly the energy consumption can be reduced, while the sintered density is similar or even increased in comparison with the presently used niobium stabiliser.
• the vanadium should be present in an amount of at least 4 times the combined amounts of carbon and nitrogen, whereby the amount of nitrogen should be less than 0.07% by weight and the amount of carbon should be less than 0.1% by weight.
• the amount of vanadium should be in the range of O. ⁇ L-1% by weight.
• Stainless steel compositions including vanadium are disclosed in WO 03/106077 publication and in the US patent 5 856 625. In WO 03/106077 there is not disclosed any effect or any actual examples of powders including vanadium.
• the stainless steel powder preferably comprises 1.5-2.5% vanadium.
• This known stainless steel powder is intended for materials with high wear resistance and a high carbon content is necessary to achieve a proper amount of hard carbides in the matrix formed mainly from strong carbide forming elements such as Mo, V and W.
• the patent publication JP 59-47358 discloses a steel powder is comprising chromium, silicon, carbon and nitrogen. This powder may further contain nickel and/or copper and vanadium. The purpose of the the steel powder according to JP 59-47358 is to manufacture e.g. a sliding surface. DETAILED DESCRIPTION OF THE INVENTION
• the stainless steel powder according to the invention comprises 10-30% chromium, 0.1-1% vanadium, 0.5-1.5% silicon, less than 0.1% carbon and less than 0.07% nitrogen.
• the stainless steel powder comprises 10-20% chromium, 0.15-0.8% vanadium, 0.7-1.2% silicon, less than 0.05% carbon and less than 0.05% nitrogen.
• the vanadium content should be chosen so that vanadium carbides and nitrides are formed instead of chromium carbides and nitrides .
• the vanadium content will be chosen in relation to the actual carbon and nitrogen content in the sintered component to be able to form vanadium carbides and nitrides. It is believed that the vanadium carbides and nitrides formed are of type VC and NC and according to our present knowledge the vanadium content should preferably be minimum 4 times the carbon and nitrogen content of the powder.
• the actual carbon and nitrogen content in the sintered component may be higher than the content of the elements in the powder due to pick up during delubrication.
• the amount of silicon should be between 0.5% to 1.5%. Silicon is an important element as it creates a thin coherent oxide layer during atomisation of the stainless steel melt, i.e. the silicon content should be 0.5% by weight or above. The oxide layer prevents further oxidation. A too high silicon level will lead to a decrease in compressibility, hence the silicon content should be 1.5% by weight or lower.
• the amount of nitrogen should be as low as possible as nitrogen can have the same influence as carbon, i.e. sensitising the material through formation of chromium nitrides or chromium carbonitrides .
• Nitrogen has also a precipitation hardening effect which will decrease the compressibility. Therefore the nitrogen content should not exceed 0.07%, preferably not 0.05% by weight. In practice it is difficult to obtain nitrogen contents lower than 0.001%.
• alloying elements are added to enhance certain properties, such as strength, hardness etc.
• the alloying elements are selected from the group consisting of molybdenum, copper, manganese and nickel.
• ferritic stainless steels are preferred.
• Ferritic stainless steels are less expensive than austenitic stainless steels which are alloyed with nickel.
• a ferritic matrix has a lower coefficient of thermal expansion, which is beneficial for example in flanges in a stainless steel exhaust system. Therefore a preferred embodiment of the stainless steel according to the invention is essentially free from nickel.
• the ferritic stainless steel may comprise 10-20% by weight of chromium, 0-5% by weight of molybdenum, less than 1% by weight of nickel, less than 0.2% by weight of manganese.
• machinability improving agents such as calcium fluoride, manganese sulfide, boron nitride or combinations thereof.
• the stainless steel powder may be a gas or water atomised, pre-alloyed powder having an average particle size above about 20 ⁇ m, depending on the method of consolidation of the powder. Normally the average particle size is above about 50 ⁇ m. Most often a lubricant is added prior to compaction in order to enhance the compressibility of the powder and to facilitate the ejection of the green component. The amount of lubricant is typically between 0.1% and 2%, preferably between 0.3% and 1.5%.
• the lubricants may be chosen from the group consisting of metal sterates, such as zink or lithium stearate, Kenolube ® , amide polymers or amide oligomers, ethylene bisstearamide, fatty acid derivatives or other suitable substances with a lubricating effect. Die wall lubrication alone or in combination with internal lubricants may also be used.
• the stainless steel powder is mixed with lubricant and other optional additives.
• the powder mixture is compacted at 400-1200 MPa and sintered at 1150-1350 0 C for 5 minutes to 1 hour to obtain a density of at least 7.20 g/cm 3 .
• the powder according to the invention can be used for producing parts having lower sintered density in order to reduce processing costs.
• the compaction step could be performed as cold compaction or warm compaction.
• the high sintered density is obtained by increased shrinkage during the sintering and without being bound to any specific theory, it is believed that this shrinkage is a consequence of promoted volume diffusion. Vanadium carbides which are formed in presence of carbon will be dissolved at elevated temperatures, especially at sintering temperatures, but also at lower temperatures such as at annealing of the metal powder. Normally the sintering temperature for stainless steel powders is about 1150-1300 0 C.
• powder mixtures 4, 5 and 6 were compacted into tensile test samples according to ISO 2740 in a uniaxially compaction movement at ambient temperature at 600 MPa.
• the obtained green samples were sintered at 1200 0 C, 125O 0 C and 1300°C in an atmosphere of hydrogen for 20 minutes and 45 minutes, respectively.
• the example reveals a surprisingly great impact on the shrinkage during sintering of a green body produced from ferritic stainless steel powder according to the invention.
• Table 6 The example reveals a surprisingly great impact on the shrinkage during sintering of a green body produced from ferritic stainless steel powder according to the invention.

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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)
• Materials For Medical Uses (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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• 2004
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• 2005
  • 2005-07-01 EP EP05755291A patent/EP1768803B1/en not_active Not-in-force
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