JPS6261082B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention is applicable to a valve seat for an internal combustion engine or a ring for a spinning machine, which has a thickness (length) relative to its diameter.
This article relates to a method of sintering parts that are small in size and have strict requirements for roundness and flatness using powder metallurgy, while maintaining their roundness and flatness with high precision. .

The role of the sintered body in these parts is equivalent to the raw material for the product, and is often completed by performing machining such as cutting or grinding on both the upper and lower end surfaces and the inner and outer diameter surfaces of the sintered body.

Therefore, since the dimensions such as the overall length and inner and outer diameters of the sintered body include necessary machining allowances in advance, the accuracy of these dimensions themselves may be rough to some extent.

However, distortions in roundness and flatness that occur during sintering are extremely difficult to correct in post-processing due to the influence of spring back when removed from the processing machine, and the processing cost is also large, so it is difficult to correct them from the beginning. It is necessary to sinter it so that no distortion occurs.

In the past, the distortion that occurred during sintering was either caused by stacking powder compacts in several stages and packing them into a sintering case, and the cores of the stacked powder compacts were misaligned from the beginning (as shown in Figure 1). (Reference) Distortion will occur if the powders shift during movement in the sintering furnace, so either sinter them in just one stage without stacking them, or if they are stacked, insert a core that slides into the inner diameter of each powder compact, for example. It has been found that no distortion occurs if sintering is performed in a manner that prevents misalignment (see Figure 2).

However, sintering in one layer is extremely uneconomical in terms of furnace efficiency, and
Other means of using a jig are unavoidable because it takes time and effort to carefully insert the fragile green compact into the jig, and the heat loss of the furnace cannot be ignored because the jig itself has a large heat capacity. is not the preferred method.

In this invention, in order to give the powder compact itself a function to prevent misalignment during sintering, a convex portion 4 is provided on one opposing end surface of the powder compacts stacked with their cores aligned, and a convex portion 4 is provided on the other end surface. A recessed portion 5 is provided in the recessed portion 5, and the fitting between the recessed portion and the recessed portion prevents misalignment.

An example in which the present invention is applied to a valve seat material will be described below. The standard dimensions of this product are the outer diameter
31.2mm, inner diameter 27.0mm, thickness (length) 7.0mm, roundness
The flatness is within 0.15 and the flatness is within 0.1.

First, as shown in FIG. 3, a compact is shaped such that it has an annular protrusion 4 with a height of 0.2 mm on the outer edge of the upper end surface and an annular recess 5 with a depth of 0.3 mm on the outer edge of the lower end surface. The powder was compacted to a powder density of 6.75 g/cm ³ using Cr-Mo alloy steel powder as the raw material powder, then stacked in 10 layers as shown in Figure 4, packed in a sintering case, and placed in a reducing atmosphere. Sintered in a furnace at a temperature of 1250°C for 30 minutes.

Next, as a comparative example, a compact was molded that differed from the above only in that no concave and convex parts were visible on the outer edge.
The cores were stacked in 10 layers and sintered under the same conditions as above, without using a jig of that size. When the distortion of the thus obtained sintered body was measured, the distortion variation in the comparative example was 0.22 in roundness and 0.14 in flatness, requiring post-processing or sorting, whereas in the present invention, the roundness was 0.08. Good results were obtained, with the flatness remaining at 0.05 and satisfying the standards with a margin as sintered. In addition, in the above example, 0.2 mm on the top surface,
Although a 0.3mm unevenness remains on the bottom surface, this is within the processing allowance added to the material, so it does not bring any new negative factors to the manufacturing process or cost.

Next, as an embodiment other than the example shown in FIG. 3, the concave portion and the convex portion may be provided on the inner edge portion or in the middle as shown in FIG. 5, or they may be provided radially as shown in FIG. 6. In either case, it is preferable that the depth of the recess is slightly larger than the height of the convex part, and
If stacked with the protrusions facing down, the load will be concentrated at the bottom, so it is appropriate to stack them with the protrusions facing up.

[Brief explanation of the drawing]

FIGS. 1 and 2 are drawings explaining the problems of the conventional sintering method, FIGS. 3 and 4 are drawings explaining one embodiment of the present invention, and FIGS. 5 and 6 are drawings explaining the problems of the conventional sintering method. It is a drawing showing an embodiment of. 1... Green compact, 2... Sintering case, 3... Core, 4... Convex portion, 5... Concave portion.

Claims (1)

[Claims] 1 A plurality of green compacts to be stacked with their cores aligned are provided with a convex portion 4 on one opposing end surface and a recessed portion 5 on the other end surface, and are stacked in a state in which the centers cannot shift due to the fit of the concave and convex portions. A method for manufacturing thin-walled cylindrical sintered parts with little sintering distortion, characterized by sintering.