Sinter-hardening powder and their sintered compacts

Abstract

A sinter-hardening powder can yield a sintered compact with high strength, high hardness, and high density. A raw powder for sintering includes Fe as its primary component and also comprising 0.1-0.8 wt % C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, and 0.1-2.0 wt % Mo, wherein the mean particle size of the raw powder for sintering is 20 μm or less. The sintered and tempered compact, without any quenching treatment, has high hardness, high strength, high density, and good ductility.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is a continuation-in-part of a prior application Ser. No. 11 / 308,824, filed on May 11, 2006. The prior application is a continuation-in-part application of application Ser. No. 10 / 907,155, filed on Mar. 23, 2005, now abandoned, which claims the priority benefit of Taiwan application serial no. 93116634, filed on Jun. 10, 2004 and Taiwan application serial no. 93126297, filed on Sep. 1, 2004. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention generally relates to sinter-hardening powders, in particular, to compositions useful for yielding high hardness, high strength, and high density in sintered parts by the metal injection molding process or the press-and-sinter process.[0004]2. Description of Related Art[0005]To attain high hardness and high st...

AI Technical Summary

Benefits of technology

[0019]This invention has solved the above-mentioned problems as a result of the carefully selected combination of the base powder and the optimum amounts and types of the alloying elements. The powder mixture or granulated powder uses elemental iron powder, such as atomized, reduced, or carbonyl iron powder, as the base powder. The mean particle size is 20 μm or less. The alloying elements includes 0.1-0.8 wt % of C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, 0.1-2.0 wt % of Mo. The above composition can further contain at least one other minor strengthening element in the amount of 5.0 wt % or less. The strengthening elements can be selected from the group consisting of Cu, Mn, Si, Ti, Al, and P. The carbon can be provided by adding graphite or carbon black, or using carbon-containing carbonyl iron powders. The mixed powder is intended for the production of MIM compacts. The granulated powder is intended for the production of press-and-sinter compacts. Furthermore, the present invention provides a sintered compact by using the powder mixture or the granulated powder. The compact can be sinter-hardened at a normal furnace cooling rate of 3-30° C. / minute in the sintering furnace without the need for fast cooling, which is required for other sinter-hardening powders. The sintered compact does not require any quenching treatment. Only low temperature tempering is needed to obtain optimum mechanical properties. The sintered body with the above-mentioned sinter-hardening powder has unprecedented hardness, tensile strength, and good ductility. Since no quenching process is required, the production cost is lower. A higher production yield is also attained due to the elimination of defects, such as cracks and distortions, which occur during quenching.

Problems solved by technology

However, when performing quenching, several problems such as deformation, size inconsistency, or cracking may occur due to the volume expansion when the part transforms from austenite to martensite, or due to the thermal stress caused by the fast cooling of the quenching treatment.

In addition, performing heat treatment on the components incurs additional costs.

However, the strength and other properties, in particular the ductility and toughness, of these sinter-hardening powders are still unsatisfactory.

Although these press-and-sinter alloys are of the sinter-hardening type, the mechanical properties are not satisfactory, and the required cooling rate is still very fast, at a minimum of 30° C. / min.

In addition, these high cooling rates, while slower than those of quenching in oil or water, are still fast enough to cause problems such as deformation, inconsistency of the dimensions, and quenching cracks.

Although mechanical properties of the metal injection molded products can be obtained by heat treatment after sintering, the costs of the heat treatment and the yield loss due to quenching increase the total production cost.

However, according to the Metal Powder Industries Federation Standards, no sinter-hardening alloys are listed for the metal injection molded products.

Moreover, few patents have been disclosed to date on sinter-hardening powder that can attain strength, hardness, and density similar to those of the quenched-and-tempered compacts.

However, the production cost of fine atomized powders is expensive because only a small portion of the as-atomized powders can be used.

Another disadvantage is the lack of flexibility in making alloys with special compositions.

To produce parts with such customer-designed compositions, special orders for the prealloyed powder must be placed to a powder supplier, which usually means a long waiting time that delays the delivery of the final sintered products.

Since the amounts of the alloying elements added are small, the cost of stocking these alloying powders is low.

Furthermore, although prealloyed powder provides homogeneous alloying, such an effect causes poor compressibility and serious wear on the tooling when the compaction method is used in making the conventional press-and-sinter products.

In the MIM process, the wear on the kneaders, molding machines, and molds are also more severe due to the higher hardness of the prealloyed powder.

The addition of Mn and Si is, however, undesirable in powder mixtures because both Mn and Si are prone to oxidation during sintering unless the dew point is carefully controlled to a very low level.

The reason is that complete homogenization cannot be attained during sintering in practice.

Due to these disadvantages of using prealloyed powders, only stainless steels, tool steels, and some other special alloys that require completely homogeneous alloying are usually produced with prealloyed powders.

However, the prealloyed powder has many disadvantages as described above, such as low sintered density, low strength, and low hardness.

To attain high sintered densities, prealloyed powder must be sintered at high temperatures, which is expensive due to the high energy cost and the high capital investment of high temperature sintering furnace.

This has become a major disadvantage, particularly where energy consumption is a great concern.

Furthermore, high temperature sintering causes coarser grains, which impair the mechanical properties.

Such a combination is not easy to design or select even by people who are familiar with the skill and practice of the press-and-sinter and metal injection molding process.

However, application of fine powders in the traditional press-and-sinter process is difficult because of the poor flowability of the powder, which in turn makes it difficult to fill the powders into the die cavity, and thus automated pressing cannot be used.

Images