Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Patent
1996-09-13
1998-07-21
Jenkins, Daniel J.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
419 14, 419 15, 419 36, 419 38, 419 53, 419 58, 75236, 75246, 75950, B22F 310
Patent
active
057846812
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of making a sintered article particularly, though not exclusively, a valve seat insert for heavy-duty applications in internal combustion engines.
BACKGROUND OF THE INVENTION
It is well known to produce valve seat inserts for applications in gasoline engines by a powder metallurgy route. Frequently, a ferrous powder mixture comprising a prealloyed ferrous powder and various additives is pressed uniaxially to about 80% of its theoretical density, sintered and often infiltrated to fill the residual porosity with copper. The materials used, usually medium-alloy steels or high speed steels, may be diluted with relatively high proportions of iron powder to assist compressibility and to lessen stresses on the press tools. Where such dilution is used, densities of up to 85% of theoretical may be achieved. The valve seat inserts so produced are generally used in low to medium-duty applications.
Conventionally, valve seat inserts for heavy-duty applications such as in large diesel engines such as are used in truck, marine, industrial and generator set applications, for example, have been produced by casting. The alloys used have been cobalt-, nickel- or iron -based materials. In premium heavy duty applications cobalt-rich alloys such as Stellite (trade mark) have been favoured. However, cobalt is both an expensive and a strategic material and its cost has been rising rapidly over recent years to levels which restrict its economic viability for valve seat insert applications.
Cast valve seat inserts rely for their performance in respect of wear and abrasion resistance on the formation during solidification of hard carbide phases. However, such hard phases tend to be gross in nature and relatively inhomogeneous in distribution when produced by the cast route. This is a disadvantage in what is essentially a component of low cross-sectional area, leading to a lack of fracture toughness. A further disadvantage of such materials having gross hard carbides is their poor machinability and poor compatibility with valve materials.
GB-A-2 187 757 describes a sintered iron-based material which, amongst other uses is stated as being suitable for valve seats.
The material described utilises a relatively high boron content to promote liquid phase sintering to thereby encourage some degree of densification during sintering. However, the use of boron as an aid to densification makes the material extremely critical to sintering temperature in respect of uniformity of shrinkage and distortion during sintering. It is common practice with such boron containing materials to sinter them under vacuum conditions in suitable furnaces, thus increasing the cost of production. Furthermore, although reference is made to the use of the materials described as valve seats, the entire thrust of the reference is concerned with the wear resistance of parts in sliding contact with each other rather than by impact contact as is the case with valve seat inserts and co-operating valves. It is also stated in this reference that the sintered alloys described preferably have a theoretical density ratio of more than 90%.
Phosphorus is also similarly employed to promote liquid phase sintering and also suffers from similar disadvantages.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of the present invention to produce a sintered, high-density valve seat insert suitable for heavy-duty applications and a method for the production of such a valve seat insert.
It is a further object of the present invention to enable the valve seat insert to be produced, and the method for its production to be carried out, on conventional production plant equipment in order for the production of such a valve seat insert to be cost effective. Such plant may include conventional uniaxial compacting presses and compacting tools operating at pressures up to about 770 MPa; conventional conveyor, walking beam furnaces, or batch furnaces employing protective gas atmospheres to maintain a reducing atmosphere or
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Farthing Leslie John
Holme David
Purnell Charles Grant
Brico Engineering Limited
Jenkins Daniel J.
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