Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1994-11-23
1997-05-20
Jordan, Charles T.
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75231, 75246, 75252, 419 38, C22C 3800
Patent
active
056314310
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a machinability-improving powder to be added to an iron-based powder composition for use in powder-metallurgical manufacture of components.
BACKGROUND OF THE INVENTION
Powder-metallurgical manufacture of components often involves the following process steps. A base powder, generally an iron or steel powder, is admixed with alloying elements, such as nickel, copper, molybdenum and carbon, in the form of a powder, and a lubricant. The powder mixture is thereafter compacted in a press tool yielding what is known as a green body of almost final geometry. After compacting, the compact is sintered so as to obtain its final strength, hardness, elongation etc.
One of the major advantages of powder-metallurgical manufacture of components is that it becomes possible, by compacting and sintering, to produce blanks of final or very close to final shape. There are however instances where subsequent machining is required. For example, this may be necessary because of high tolerance demands or because the final component has such a shape that it cannot be pressed directly but requires machining after sintering. More specifically, geometries such as holes transverse to the compacting direction, undercuts and threads, call for subsequent machining.
By continuously developing new sintered steels of higher strength and thus also higher hardness, machining has become one of the major problems in powder-metallurgical manufacture of components. It is often a limiting factor when assessing whether powder-metallurgical manufacture is the most cost-effective method for manufacturing a component. Hence, there is a great need for new and more effective additives to improve the machinability of sintered steels. It then is important that this additive does not appreciably affect the mechanical properties, such as tensile strength and elongation, of the sintered material.
Today, there are a number of known substances which are added to iron-based powder mixtures to facilitate the machining of components after sintering.
The commonest powder additive is MnS, which is mentioned e.g. in EP 0 183 666, describing how the machinability of a sintered steel is improved by the admixture of such powder. Materials which are difficult to machine, in this context materials having a hardness above about 180 HV, cannot however be machined properly by adding MnS. Moreover, the addition of MnS often entails an unacceptable reduction of the strength of the material after sintering.
U.S. Pat. No. 4,927,461 describes the addition of hexagonal BN (boron nitride) to iron-based powder mixtures to improve machinability after sintering. By agglomerating very fine BN powder (0.05-1.0 .mu.m), it is possible to achieve a similar improvement of the machinability of iron-based powder mixtures after sintering as by the addition of MnS. However, the sintered strength is affected to a lesser extent if a correct amount of BN powder is added, than when adding MnS. As in the case of MnS, BN additions do however not make it possible in industrial production to machine materials having a hardness above 200 HV.
WO 91/14526 describes how small amounts of Te and/or Se together with MnS are used to improve the machinability about twice in powder-metallurgical materials that are difficult to machine. The addition of Te and/or Se is already conflicting with environmetal considerations, in that the hygienic limit values for these additives are very low and there is a tendency towards ever more stringent environmental regulations.
SUMMARY OF THE INVENTION
The invention provides an iron-based powder composition essentially free from hard phase material for compacting and sintering into products having enhanced machinability. The composition contains, in addition to iron, at least one of the alloying elements C, P, Cr, Mn, Cu, Ni and Mo. The iron-based powder contains 0.1-0.6% by weight of CaF.sub.2 powder particles as agent for improving the machinability and such particles are unattached to graphite particles. In addit
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Carroll Chrisman D.
Hoganas AB
Jordan Charles T.
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