Powder metallurgy technology allows production of components which require high dimensional accuracy and have a complex structure in a shape markedly close to that of the finished product (in near net shape), thereby significantly decreasing the finishing cost. Therefore, many products produced by powder metallurgy are used as various components for machines and apparatuses in many fields.
In general, iron-based green compacts for powder metallurgy (green compacts) are produced as follows. First, an iron-based powder is mixed with alloying powder such as graphite powder and so forth, and lubricant powder such as stearic acid and lithium stearate to prepare an iron-based mixed powder. Then, the iron-based mixed powder is filled in a die, and is subjected to compacting, whereby the iron-based green compact is produced.
The iron-based powders are classified into iron powders (such as pure iron powder), alloy steel powder, and so forth, for example, based upon the components thereof. Also, the iron-based powders are classified into atomized iron powders, reduced iron powders, and so forth, for example, based upon the production method thereof.
In general, the iron-based green compacts are formed with a density of 6.6 to 7.1 Mg/M3. Furthermore, these iron-based green compacts are sintered to form sintered bodies. The sintered bodies are subjected to a sizing or a cutting process according to needs, whereby powder metallurgy products are produced. Furthermore, in some cases, the products are subjected to carburizing-quenching or bright-quenching after sintering for improving tensile strength or fatigue strength thereof.
Recently, iron-based powder metallurgy products with high strength or high fatigue strength are strongly desired due to the development of components with reduced size and weight.
In general, alloying elements (Ni, Cu, Mo, W, V, Co, Nb, Ti, and so forth) are added to the iron-based powders for improving the strength of the powder metallurgy products.
Note that examples of the methods for adding alloying elements include: a method for alloying the iron-based powder with a desired element (prealloying); a method for mixing an alloying powder (powder containing a desired alloying element) and the iron-based powder with or without binder; and a method for holding the mixture of the powder containing an alloying element and the iron-based powder at a high temperature so as to metallurgically combine these powders (diffusion bonding). Various properties of the alloy steel powder (or mixed powder), and various levels of uniformity and diffusion states of the alloying element after sintering are obtained depending on the method. Therefore, it is important to select the alloying element and the addition method for achieving the desired quality of the alloy steel powder (or mixed powder) or the desired quality of the sintered body.
For example, Japanese Examined Patent Application Publication No. 6-89365 discloses an alloy steel powder containing 1.5 to 20% by mass of Mo, which is a ferrite-stabilizing element, as a prealloy. According to the document in the sintering process of the aforementioned alloy steel powder, a single a phase is formed, leading to a high self-diffusion rate with respect to Fe. This accelerates sintering, resulting in a reduced size of the pores contained in the sintered body. Thus, pressure sintering of such an alloy steel powder provides a sintered body with improved densification. Furthermore, such an alloy steel powder contains no alloying element added by diffusion bonding, thereby providing a uniform and stable microstructure. However, the Mo content in the disclosure is relatively high, i.e., 1.8% by mass or more, leading to poor compressibility. This leads to the disadvantage that a green compact cannot be formed with high density (the density of the green compact). Accordingly, the sintered body obtained by performing a general sintering process (i.e., sintering in one step without pressurizing) has a low density, leading to insufficient strength and insufficient fatigue strength.
On the other hand, the pressure sintering method and the two-step sintering method including a repressing step have the disadvantage of high costs. Accordingly, a sintered body is preferably produced with high strength and high fatigue strength without involving such special sintering methods.
On the other hand, Japanese Examined Patent Application Publication No. 7-51721 discloses a steel powder which contains 0.2 to 1.5% by mass of Mo and 0.05 to 0.25% by mass of Mn as prealloyed elements, and which has a relatively high compressibility in compacting. However, it has been revealed by the present inventors that a single a phase is not formed using the aforementioned steel powder due to the Mo content of 1.5% by mass or less. Accordingly, the enhanced sintering between particles is not accelerated in a sintering step at a temperature (1120 to 1140° C.) of a mesh belt furnace generally used for powder metallurgy, leading to a problem of low strength of the sintering neck.
While the Japanese Examined Patent Application Publication No. 7-51721 discloses an iron powder as a comparative example, which contains Ni (3.8% by mass), Mo (0.5% by mass), and Cu (1.4% by mass) by diffusion bonding, the Patent document describes that the iron powder has poorer strength than that of the aforementioned alloy steel powder disclosed as an invention in the Patent document.
On the other hand, Japanese Examined Patent Application Publication No. 63-66362 discloses a technique in which Mo is added to an iron powder as a prealloyed element so long as compressibility is not impaired (Mo: 0.1 to 1.0% by mass), and Cu and Ni are bonded on the surfaces of the iron particles in the form of a powder by diffusion bonding. This technique provides both preferable compressibility during the compacting and high strength after sintering. However, the aforementioned technique has a limited ability to improve tensile strength and fatigue strength by adding Cu and Ni since the iron powder containing Mo as a prealloyed element cannot be sintered sufficiently, as with the technique disclosed in the Japanese Examined Patent Application Publication No. 7-51721.
On the other hand, Japanese Unexamined Patent Application Publication No. 8-49047 discloses an alloy steel powder limiting Mn content to 0.3% or less by mass as a prealloyed element as well as containing Mo of 0.1 to 6.0% by mass and V of 0.05 to 2.0% by mass (as prealloyed elements). The aforementioned alloy steel powder provides a sintered body with high strength after heat treatment while maintaining the compressibility thereof. Also, the patent document discloses that the alloy steel powder may contain one or more kinds of elements of Mo (4% by mass or less); Cu (4% by mass or less); Ni (10% by mass or less); Co (4% by mass or less); and W (4% by mass or less) in the form of powders by mixture or diffusion bonding.
On the other hand, Japanese Unexamined Patent Application Publication No. 7-233401 discloses an atomized iron powder (alloy steel powder) which contains Mn of 0.03 to 0.5% by mass and Cr of 0.03 to less than 0.1% by mass as prealloyed elements. The aforementioned atomized iron powder having excellent machinability of the sintered body, as well as providing superior dimensional-accuracy thereof. Also, the aforementioned Patent document discloses examples of strengthening elements that can be used as prealloyed elements, which include: Ni (4.0% by mass or less); Mo (4.0% by mass or less); Nb (0.05% by mass or less); and V (0.5% by mass or less). Furthermore, the Patent document discloses examples of strengthening elements (alloy powders) that can be added by diffusion bonding, which include: a Ni powder (5.0% by mass or less); a Mo powder (3.0% by mass or less); and a Cu powder (5.0% by mass or less).
However, according to the aforementioned techniques, such alloys are not designed from the perspective of the fatigue strength of components produced by sintering. This leads to difficulty in producing sintered metal components which satisfy the high fatigue strength desired in recent years, using a general sintering step.
For example, Japanese Unexamined Patent Application Publication No. 6-81001 and Japanese Unexamined Patent Application Publication No. 2003-147405 disclose alloy steel powders designed for improving the fatigue strength.
The Japanese Unexamined Patent Application Publication No. 2003-147405 discloses an alloy steel powder in which 0.5 to 1.5% by mass of Mo is bonded on the surfaces of a steel powder containing Ni of 0.5 to 2.5% by mass and Mo of 0.3 to 2.5% by mass as prealloyed elements by diffusion bonding. The aforementioned Patent document also discloses that a sintered body formed of the aforementioned alloy steel powder exhibits superior rolling contact fatigue strength after carburizing-quenching.
On the other hand, the Japanese Unexamined Patent Application Publication No. 6-81001 discloses an alloy steel powder in which Ni (0.5 to 5% by mass) and/or Cu (0.5 to 2.5% by mass) are bonded to an iron-based powder containing Mo of 0.05 to 2.5% by mass and at least one element of V, Ti, and Nb as prealloyed elements by diffusion bonding. The aforementioned Patent document discloses that the alloy steel powder provides a sintered body having superior rolling contact fatigue strength after carburizing-quenching, as well.