Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2001-03-02
2003-10-07
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C075S228000, C419S029000, C419S038000, C419S049000
Reexamination Certificate
active
06630102
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 of Austrian Patent Application No. 349/2000, filed on Mar. 3, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the powder metallurgical production of material having improved isotropy with a rectangular or flat elliptical cross section, so-called broad-flat material, in particular raw material for producing cutting or piercing tools, in which a powder of an alloy produced using gas, in particular pulverized using nitrogen, is placed in a capsule, compressed, and optionally sealed, after which a heating and isostatic pressing (HIP-ing) of the powder capsules occur and the hot-isostatically pressed slug is subjected to a forming by forging and/or rolling.
The invention further includes a material produced using powder metallurgy with a rectangular or flat elliptical cross section, so-called broad-flat material, with a width that is at least about 3.1 times the height and a degree of deformation of at least two times, in particular produced according to one of the processes named above.
2. Discussion of Background Information
In the solidification of alloys, demixing often occur that are impossible to compensate for or dissolve using diffusion in the case of ledeburite steels. Here, the size of the phases and/or grains deposited from the melt depends on the formation and/or setting time.
In conventional ledeburite tool steels produced by ingot casting, for example, coarse primary carbides and a carbide network may be present in the casting state. If these castings or blocks are subjected to a hot forming, the mechanical material properties are indeed improved, but the degree of improvement depends upon the stressing direction. Here, it is entirely possible for the results of a flexural impact test crosswise to the forming direction to amount to merely about 25 to 30% of the flexural impact values in comparison to those measured in the forming direction. This direction dependence of the material toughness can be explained by a distinctive carbide cell structure in conventionally produced material, which is also microscopically verifiable.
In order to achieve extensively isotropic mechanical material properties, processes were developed for the powder metallurgical production of work pieces. In this process, a separation of a fluid stream of metal into droplets occurs, in particular using gas streams with a high speed and energy, after which the droplets solidify in a short time. In the individual powder grains with a diameter, as a rule, of less than about 0.3 mm, the structural phases are homogeneously distributed and very fine because of the extremely short hardening time. The powder produced in that manner is then placed into a capsule, which is then closed and subsequently subjected to high temperature and high pressure from all sides, whereupon the powder grains connect metallically and/or the powder welds together or sinters. This process is called hot isostatic pressing (HIP-ing).
Such a material produced using powder metallurgy (PM material) can be used in an unformed state or be formed to improve its mechanical properties.
In the case of parts made of carbide-rich working steels, a fine homogenous microstructure is expected from the PM production, which is confirmed by structure images that show almost completely evenly distributed carbides of a uniformly small size and, due to this structure, no significant directional dependency of the mechanical properties in the formed material. While the differences in toughness in this working material in the direction of deformation and perpendicular to it have been reported, these differences amount to about 8 to 20% at the most and, up to now, have essentially been attributed to the not entirely preventable content of non-metal inclusions and a so-called fiber structure.
In practical use, cutting and piercing tools produced using powder metallurgy such as die plates, upper dies, and the like with a rectangular flat cross section form exhibited partially only a short lifespan; completely unexpected cases of damage occurred due to the breaking of tools. Extensive investigation of the mechanical properties, in particular the main stressing corresponding to the impact toughness of the material, was performed on so-called broad-flat bars. Samples were taken from the bar in the lengthwise, crosswise, and depth directions and each directionally oriented sample was tested with break-inducing impacts displaced from one another by 90°. The designation and position of the samples can be found in the table below and in FIG.
1
. They mean:
L-S Sample in the longitudinal direction, impact on the flat side in the direction of the depth
L-T Sample in the longitudinal direction, impact on the narrow side in the direction of the width
T-L Sample in the direction of the width, impact on the face side in the longitudinal direction
T-S Sample in the direction of the width, impact on the flat side in the direction of the thickness
S-L Sample in the direction of the thickness, impact on the face side in the longitudinal direction
S-T Sample in the direction of the thickness, impact on the narrow side in the direction of the width
Tests on broad-flat material (about 380×55 mm) made of high-speed steel (HS 6-5-3) had the following approximate results in % as compared to the impact strength in the L-S test.
L-S 100%
L-T 100%
T-S 80%
T-L 80%
S-T 25%
S-L 25%
The extremely low bending strength of powder metallurgically produced broad-flat material in the direction of the depth was completely unexpected and unknown in professional circles, but did explain the tool breakages mentioned above. In scientific experiments, a so-called fiber model was developed, whose effectiveness is derived from bonding faults and demixings on the boundary surface of the pulverized and formed particles. However, this is opposed by an absolute uniformity and purity of the raw material from the pulverization and HIP process, which does not lead to expectations of a fiber structure and, in the matrix for showing the carbide arrangement and carbide size, which is normally etched darkly, does not allow recognition of such a structure.
In further microscopic tests, structural regions were found with various etchings in comparison to the remaining regions of the material, which supported the fiber theory. However, a structure with coarse grains adapted to the forming process was not metallographically verifiable.
SUMMARY OF THE INVENTION
The present invention provides a process of the type mentioned at the outset by which an improved isotropy of the mechanical properties, in particular an increase in the impact strength and flexural breaking strength in the depth direction of broad-flat material of formed PM working pieces. Further, the present invention is directed to a material produced using powder metallurgy with a rectangular or flat elliptical cross section, i.e., a so-called broad-flat material with a width that is at least about 3.1 times the thickness and a degree of deformation of at least about 4 times.
The present invention includes a slug with such a rectangular or flat elliptical cross sectional shape is prepared and subjected to a shaping in such a way that the difference between the forming in the direction of the width and the forming in the direction of the depth of the cross section of the broad-flat material is at most two times, preferably about 1.5 times, the lower deformation value.
Moreover, when the hot isostatically pressed slug is subjected to a compressive shaping with a degree of compression of at least twofold, whereafter a stretch shaping of the compressed slug occurs while forming the broad-flat material.
Another aspect of the invention is for the hot isostatically pressed slug to be subjected to a diffusion annealing treatment with a maximum temperature of about 20° below the solidus temperature of the alloy and a
Böhler-Uddeholm Aktiengesellschaft
Greenblum & Bernstein P.L.C.
Mai Ngoclan
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