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
  • C22C33/0207—Using a mixture of prealloyed powders or a master alloy

Definitions

• a sintering alloy powder for use in forming products by powder metallurgical techniques contains from 2 percent to 13 percent chromium, optionally up to 5 percent copper and up to 2.5 percent carbon and the remainder except for impurities iron.
• the alloy powder is preferably made by annealing an iron-chromium alloy powder having a particle size less than 150 pm for 2 hours at a temperature of between 850 and 950C and subsequently mixing the annealed poiser with iron powder with a particle size less than 400 um to adjust the chromium content to the desired value.
• iron-chromium alloy powder with a particle size less than 150 pm is mixed with a fine iron powder with a particle size less than 40 pm, the mixture is annealed with exclusion of air for 2 hours at a temperature of from 850 to 950C and after this the annealed powder is reduced in particle size and is mixed with iron powder to give the desired chromium content.
• This invention relates to sintering alloys containing a major proportion of iron, together with chromium and also, if desired, copper. These alloys are highly suitable for manufacturing metal products by powder metallurgical processes.
• One of the most important fields of application for powder metallurgy is in the manufacture of precision parts from metallurgical powders containing mainly iron.
• a variety of methods are used, the simplest of which may be called the single pressing process. This involves essentially four process stages. In the first stage the metal powder is prepared to form a freely flowing and easily pressable powder. In the second stage the powder is given a preliminary compaction by a mechanical or hydraulic press which is usually an automatic press, using shaped tools, to form a stable blank. In the third stage of the process the resulting blank is sintered in an atmosphere of inert gas in a sintering furnace at temperatures between l000 and I300C for between 30 and 240 minutes.
• the sintered product has considerable mechanical strength.
• the density ratio in the product, as well as its physical properties, depends on the sintering temperature and sintering time. During the sintering the dimensions of the blank are somewhat mofidied.
• the sintered product therefore usually has its dimensions corrected in a fourth stage by a calibration. This gives the product the desired precise dimensions but hardly changes its physical properties.
• a sintered product made of iron powder by the process described above that is to say by the single pressing process, has a tensile strength of between 22 and 24 kp/mm and an elongation at rupture of between 18 percent and 20 percent.
• the inital powder is usually a mixture of iron powder and copper powder.
• the two powders are mixed together in specified proportions.
• the mixture of powders is compacted under a pressure which does not exceed 6 Mplcm to produce a blank which is subsequently sintered in an atmosphere of inert gas at a temperature of between 1000 and l300C.
• the resulting sintered product has a tensile strength of between 36 and 38 kp/mm, depending on the copper content, and an elongation at rupture of between 2 and percent.
• the object of the present invention is to provide a sintering alloy capable of satisfying the requirements in regard to these two main physical properties, and which therefore has a wider field of application.
• products made from alloys in accordance with the invention have higher tensile strengths compared with products made from conventional ironcopper sintering alloys, without any less elongation at rupture than that of products made from unalloyed sintering iron powder.
• a sintering alloy powder contains a major proportion of iron, and from 2 to 13 percent chromium together with impurities.
• the particle size of the powder is preferably less than um.
• the sintering alloy powder may also contain up to 5 percent of copper, there then being preferably at least 0.5 percent of copper present.
• the products made from the binary iron-chromium sintering alloy powder in accordance with the invention not only have higher tensile strengths than products made from unalloyed sintering iron powder but also have a smaller reduction in elongation at rupture than products made from conventional iron-copper sintering alloys.
• Products made from the alloy powders in accordance with the invention which also contain copper have the further advantage that the tensile strength is increased, compared with those made from conventional ironcopper alloys.
• Binary sintering alloys of the ironchromium system with chromium contents within the range covered by the present invention, and ternary sintering alloys of the iron-chromium-copper system with compositions within the range covered by the invention have not hitherto been used in powder metallurgical processes and neither has their use been suggested been described in the literature. Apparently these alloys have not aroused technical interest in this field, because it has not hitherto been possible to make sintering powders with high chromium contents, having sufficiently good pressing and sintering properties.
• the invention provides sintering alloy powders from which sintered products can be made which have the physical properties mentioned above, the powders themselves having excellent pressing and sintering properties.
• the sintering alloy powders are, in accordance with another aspect of the invention, preferably made by either one of the following processes:-
• an iron-chromium. alloy powder with a particle size less than 150 ,u.m is annealed for 2 hours at a temperature of between 850 and 950C, after which the annealed powder is mixed with iron powder with a particle size less than 400 am. to adjust the chromium content to the desired value.
• an iron-chromium alloy powder with a particle size less than 150 pm is mixed with a fine iron powder with a particle size less than 40 ,um, the mixture is annealed with exclusion of air for 2 hours at a temperature of from 850 to 950C, whereupon the annealed powder is reduced in particle size and is mixed with iron powder to give the desired chromium content.
• the iron chromium alloy powder is a ferro-chrome with a chromium content of between 35 and 50 percent by weight. If desired the annealed powder may also be mixed with copper powder with a particle size less than 150 um.
• the iron powder with which the annealed powder is mixed can consist of pure iron, for example electrolytic iron, or it can be an iron containing small quantities of other elements, for example a pig iron powder.
• the small particle size of the powder can be obtained by means of the usual size reducing processes, for example by spraying or by reduction from iron compounds. Both pig iron sprayed powder and reduced powder is suitable for the process according to the invention.
• either a mixture of ferrochrome powder with iron powder, the mixture having the finally desired 2 to 13 percent chromium content may be used, or alternatively a powder mixture containing more chromium, for example between 18 and 30 percent chromium, can be annealed, the chromium concentration being subsequently brought down to between 2 and 13 percent by adding further iron powder.
• the powdered alloy is pressed to make a shaped blank and the blank is then sintered at a temperature of between 1000 and l300C preferably between l200 and l300C.
• the resulting sintered products have chromium contents between 2 and 13 percent, depending on the initial powder mixture, and if desired copper contents between 0.5 and percent.
• the sintering powder prepared in accordance with the invention has very good pressing properties, corresponding approximately to those of pig iron sprayed powders and reduction powders, as is shown in the following Table 1.
• Table 2 shows the density ratios obtained when the powder contains 9 percent of chromium, the Table showing the values for four different mixtures.
• Powder No. 1 is a mixture of annealed ferrochrome powder and electrolytic iron powder.
• Powder No. 2 consists of ferrochrome powder and iron powder annealed together.
• Powder No. 3 is a mixture of ferrochrome powder and iron reduction powder.
• Powder No. 4 is a mixture of ferrochrome powder and pig iron sprayed powder together with an addition of 1 percent Zn stearate as a lubricant.
• blanks are made by compressing the alloy powder in a press under a pressure of from 4 to 7 Mplcm giving a blank with a density of between 6.4 and 6.9 g/cm.
• the blank is sintered at a temperture of 1000 to 1300C and the resulting sintered product can if desired be calibrated to give it precise dimensions.
• Table 3 were obtained for tensile strength, elongation at rupture and Brinell hardness, the values depending on the chromium content and the sintering temperature. All the values shown in the Table relate to a density of 6.7 g/cm in the blank. Sintering was continued in all cases for 2 hours in an atmosphere of hydrogen. A higher blank density gives somewhat higher values, and a lower blank density somewhat lower values.
• An iron-copper sintering material containing 3 percent chromium, using electrolytic iron powder, compacted to a blank density of 6.7 g/cm was sintered at l200C.
• the sintered product showed a tensile strength of 28 kp/mm and an elongation at rupture of 6 percent.
• an iron-chromium sintering, material containing 4 percent chromium, using electrolytic iron powder, compacted to a blank density of 6.7 g/cm was sintered at l200C. In this case the sintered product showed a tensile strength of 29 kp/mm and an elongation at rupture of 12 percent.
• alloys containing 4.5 percent copper which is. usually the highest copper content
• alloys containing 4 percent of chromium and, thirdly, alloys containing 6 percent chromium, operating conditions being otherwise the same in all cases:
• ternary sintering alloys in accordance with the invention prepared as described above and containing from 0.5 to 5 percent of copper, between 0.5 and 1.2 percent of a lubricant, for example zinc stearate can with advantage be added to the powder.
• the resulting mixed powder can be pro-pressed at a pressure of between 4 and 6 Mp/cm to give a blank, which is then sintered at a temperature of from 1000 to I300C.
• the sintered product is if necessary calibrated to provide it with precise dimensions.
• Table 4 shows tensile strengths, elongations at rupture and Brinell hardness of sintered products made in accordance wtih the invention from ternary ironchromium copper sintering alloys with various copper contents.
• Electrolytic iron powder l-lVA-Star was used for the mixture, and a fine annealed ferrochrome powder. The powder mixture was compressed to a density of 6.7 g/cm in the blank, which was sintered for 2 hours in an atmosphere of hydrogen. The mechanical properties are shown in dependence on the sintering temperature, the chromium content and the copper content.
• Table 4 shows, to begin with, that even using comparatively low sintering temperatures quite good tensile strengths are obtained.
• both tensile strength and elongation at rupture increases sharply at sintering temperatures from l200C upwards.
• the sintering temperature is preferably from l200 to 1300C.
• a sintering temperature between 1050 and I 150C is sufficient.
• the powder mixture was pre-pressed under a pressure of 6 Mp/cm to form a blank.
• the blank had a density of 6.8 g/cm.
• the blank was sintered for 2 hours in an atmosphere of cracked ammonia gas in a closed box sealed against the entry of water vapour and oxygen by means of a getter substance.
• the sintered product had a tensile strength of 51 Kp/mm and an elongation at rupture of 9 percent.
• EXAMPLE 3 A mixture containing 4 percent of chromium in the form of finely ground ferrochrome powder with a chromium concentration of 42 percent, 4 percent of electrolytic copper powder, 1 percent of zinc stearate and the remainder iron powder in the form of a reduction powder with a maximum particle size of um was pre-pressed under a pressure of 4 Mp/cm to give a blank which had a density of 6.4 g/cm.
• the blank was sintered in a closed box, in which the blank was protected by a getter substance against penetration by the oxygen and moisture of the sintering atmosphere, in a travelling belt furnace at 1100C. After cooling the product had a tensile strength of 33 Kp/mm and an elongation at rupture of 2.5 percent.
• a sintering alloy powder consisting essentially of about 2 to 13 percent of chromium and about 0.5 to 5 percent of copper, with the balance iron and impurities.
• a sintering alloy consisting essentially of 2 to 13 percent of chromium

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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)

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Citations (3)

• 0
  • BE BE759464D patent/BE759464A/xx unknown
• 1970
  • 1970-11-30 GB GB5680570A patent/GB1331090A/en not_active Expired
  • 1970-11-30 FR FR707042904A patent/FR2077565B1/fr not_active Expired
  • 1970-12-14 SE SE7016925A patent/SE373879C/xx unknown
• 1971
  • 1971-12-16 US US00098965*[A patent/US3853537A/en not_active Expired - Lifetime

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