Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Patent
1994-05-02
1995-10-17
Kastler, Scott
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
75246, 75255, B22F 100
Patent
active
054586704
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
Iron powder used for powder metallurgy is roughly divided into two types pure iron powder and alloying steel powder.
This invention relates to iron powder and mixed powder for powder metallurgy belonging to the above former pure iron powder as well as a method of producing such iron powder.
BACKGROUND ART
Iron powder for powder metallurgy is used in the production of a sintered part having usually a density of 5.0-7.2 g/cm.sup.3. The part is made by adding and mixing iron powder with Cu powder, graphite powder and the like, shaping into a green compact in a mold, sintering and, if necessary, sizing a sintered body for dimensional correction.
However, the sintered body produced by adding Cu powder, graphite powder or the like to the iron powder is high in the strength, so that it has a drawback that the dimensional correction can not be conducted to a satisfactory extent due to spring-back of the sintered body even if the sizing for dimensional correction is conducted.
As a method of ensuring a desired dimensional accuracy without sizing, therefore, JP-B-56-12304 proposes a technique of enhancing the accuracy of dimensional change by improving particle size distribution of the starting powder, and JP-A-3-142342 proposes a technique of controlling a given size by predicting the dimensional change during sintering from the shape of powder.
However, the iron powder for powder metallurgy is added with Cu powder, graphite powder, lubricant and the like, or mixed for the uniformization of properties in the steps from powder formation to the shaping, or further transferred for replacement with a new vessel, so that the properties such as particle size distribution, shape and the like are apt to be changed at these steps. Also the position change of ingredients due to segregation of Cu powder or graphite powder added to the iron powder occurs and consequently the dimensional accuracy can not necessarily be obtained to a satisfactory extent.
DISCLOSURE OF INVENTION
The invention advantageously solves the above problems and provides iron powder and mixed powder for powder metallurgy capable of providing a dense sintered body with a high accuracy by enhancing the accuracy of dimensional change in the sintering (concretely green density: about 6.90 g/cm.sup.3, scattering width of dimensional change: within 0.10%, preferably 0.06%) without impairing compressibility as well as a method of advantageously producing such iron powder.
The inventors have made various studies with respect to the composition of iron powder and the compounding ratio of additives in order to achieve the above object and found the following:
(1) The dimensional change in the sintered body is strongly correlated to the amount and particle size of graphite added to iron powder;
(2) Even when the amount and particle size of graphite changes, if an oxide of a particular element is existent on the surface of the iron powder at a constant quantity or more, the scattering width of dimensional change or the fluctuating width of dimensional change reduces; and
(3) As the scattering width of the oxide quantity becomes small, the fluctuating width of dimensional change is small.
The invention is based on the above knowledges.
That is, the essential points and construction of the invention are as follows.
1. Iron powder for powder metallurgy consisting of 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/1 mol of O.sub.2, not more than 0.30 wt % of oxygen and the reminder being Fe and inevitable impurities, in which not less than 20% of the above element forms an oxide.
2. Iron powder for powder metallurgy consisting of 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/1 mol of O.sub.2, not more than 0.30 wt % of oxygen and the reminder being Fe and inevitable impurities, in which not less than 20% of the above element forms an ox
REFERENCES:
patent: 4518427 (1985-05-01), Tengzelius et al.
Ishikawa Hiroyuki
Maeda Yoshiaki
Nitta Minoru
Ogura Kuniaki
Ohtsubo Hiroshi
Kastler Scott
Kawasaki Steel Corporation
Miller Austin R.
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