Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Patent
1999-04-12
2000-01-11
Jenkins, Daniel J.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
419 38, 419 54, B22F 316, B22F 324
Patent
active
060132256
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to powder metallurgy, and in particular to a method of making a surface densified and hardened powder metal part.
2. Discussion of the Prior Art
Many components of machines are required to function in rolling and rubbing contact at high surface pressures. This action causes wear by a variety of well known mechanisms including: abrasion, scuffing (galling) and surface pitting (rolling contact fatigue). An economical and well-known method of manufacture of machine components is by powder metallurgy (P/M) which involves compaction of a blend of fine powders of appropriate composition in a set of tools that result in a precise shaped preform. The preform is then subjected to heat in a process called"sintering" which bonds the powder particles together and alloys the blend ingredients to form the desired microstructure. The sintered product can then be heat treated conventionally to harden the surface or whole part to increase wear resistance.
It has been found by experiment that wear resistance against heavy rolling contact requires a surface layer or skin of high integrity to withstand the subsurface micro scale cracks. These cracks eventually propagate and join together until a small fragment of surface material breaks away leaving a small pit. This process spreads to form larger areas of pitting. Eventually the machine operating noise becomes a problem or in extreme cases, the surface fails leading to mechanical breakdown of the machine. In order to raise the surface integrity of a P/M mechanical component such as a roller or gear or sprocket, the surface density must be increased to an appropriate level. This can be accomplished in several ways including raising the density by repressing the whole sintered product in the cold or heated condition. Alternatively the surface can be densified locally by a rolling action. In the case of a gear or sprocket, the latter involves rolling and meshing against a master gear or sprocket at higher pressure. This process requires an expensive precision master former which has limited life due to wear, and depth of densification is limited.
SUMMARY OF THE INVENTION
The invention provides a method of making a surface densified powder metal part in which, after initial compressing and sintering, the cooled part is surface heated to a surface heated depth so as to produce a hot skin which is at a temperature above the core temperature of the part. The part is then repressed in a second die set. This compresses the hot skin between the die and the cooler, less malleable core of the part, to densify the surface of the part.
Following surface densification in this manner, the part may be resintered and/or heat treated or hardened.
Preferably, the surface heating is done to a temperature which is just below the critical temperature, which is the temperature at which the steel material of the part transforms from a ferritic to an austenitic microstructure.
In addition, the second die set, in which the part is repressed, is advantageously smaller in at least one dimension than the surface heated part by approximately 10% of the surface heated depth, to provide a desirable degree of surface densification.
These and other objects and advantages of the invention will be apparent from the detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention teaches an alternative less costly and potentially deeper skin approach to surface densification which involves localized surface heating of the P/M component being treated, followed by compression of the hot surface by pressing the component into a shaped die which is slightly smaller (e.g., in diameter) than the product. This causes compressive forces in the hot surface. The unheated core acts as a restraint against which the hot surface is compressed. This is in contrast to conventional hot repressing in which the whole body is pre-heated and the hot core also compresses, thereby preventing effective surface densification.
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Cadle Terry M.
Geiman Timothy E.
Gray Michael S.
Mandel Joel H.
Jenkins Daniel J.
Zenith Sintered Products, Inc.
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