JPH0610284B2 - Sintered member manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a sintered member, which has excellent fatigue strength,
The present invention relates to a method for manufacturing an iron-based sintered member suitable as a mechanical material.

(Prior Art) A non-oxidizing heating / sintering furnace is used because the sintered body obtained by compression-molding iron-based metal powder and heating it to around 1000 ° C. to join the powder particles is porous. When it is taken out into the atmosphere, the air penetrates from the surface to a considerable inside through the pores of the molded body, so that the surface portion is oxidized or decarburized. When the forging is continuously performed, the surface portion of the sintered body is cooled by the forging die, the surface portion is less likely to be consolidated, and the holes are likely to remain. That is, the sintered member obtained through the conventional method for manufacturing a general sintered body,
The surface is a porous layer, and defects such as many voids are likely to occur even after forging. Therefore, the sintered member has drawbacks such as low mechanical properties such as fatigue strength and tensile strength.

Therefore, in the case of sintered members that require particularly fatigue strength, it has been common practice to re-compress and re-sinter the sintered body, and if necessary, further forge to densify the structure and strengthen the material. Met.

(Problems to be Solved by the Invention) However, in the above-mentioned conventional recompression / resintering method, the outermost surface is oxidized by the atmosphere or is rapidly cooled by contact with the recompression die during the process. As a result, defects such as insufficient bonding between powder particles and leaving fine cracks at part of the grain boundaries occur. There was a problem that it adversely affected.

As a solution, for example, by heating again after sintering forging, the oxidation of the surface layer is reduced and modified,
Various methods have been proposed, but there are problems that a large amount of energy is required, productivity is deteriorated, or strength is not improved so much, and a satisfactory method is still found. Not not.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a sintered member that exhibits high fatigue strength without causing an increase in manufacturing cost.

(Means for Solving Problems) Therefore, in the method for manufacturing a sintered member of the present invention, a step of compressing iron-based metal powder to obtain a green compact having a density ratio of 80 to 90%, A process of compressing a heated green compact obtained in an oxidizing atmosphere of 800 to 1000 ° C to obtain a repressed body having a density ratio of 95% or more, a step of sintering the repressed body to obtain a sintered body, a sintered body And a surface modification step in which only the surface layer within 1 mm is made to have a density ratio of 98% or more.

The above steps included in the method of the present invention need to be performed in the order of →→→, but during or before or after those steps, other steps such as coining after sintering, sizing, quenching, infiltration, etc. It may include steps normally employed when manufacturing a sintered member. It is particularly preferable to include a heat treatment step between the steps (1) and (2).

In the method for manufacturing a sintered member of the present invention, the repressed body before the sintering step has a density ratio of 95% or more, the surface thereof is compacted, and the pores of the surface portion are closed. Here, the density ratio means the ratio to the true density. Therefore, it is less likely that air will penetrate into the compression molded body during the sintering process. Therefore, it becomes difficult to form an oxide or decarburized layer on the surface of the sintered body. Therefore, it is possible to manufacture a sintered member having excellent fatigue strength characteristics and tensile strength characteristics.

The step (2) is a step of pressurizing the iron-based metal powder in a molding die for compression molding and consolidating the powder with a pressing force to obtain a compression-molded shaped body having a constant shape. The iron-based metal powder is not particularly limited, and a conventional iron-based sintered metal powder raw material used for ordinary sintered members can be used.
More specifically, a mixed powder of iron powder, copper powder, and graphite powder is often used as the iron-based metal powder raw material. In addition, low alloy steel powder can be used as a raw material after alloying, prealloying or premixing. For example, commercially available AI
SI4100 equivalent and AISI4600 equivalent powder are available. In this case, N
Alloying elements such as i, Mo, Cr, Mn, Co and C act effectively to improve strength. The proportion of copper powder is 0.5 to 10% by weight (hereinafter% means% by weight), the proportion of graphite powder is 0.3 to 10%, and the balance is generally iron powder. 0.5 to 1.0% of zinc stearate, which is a lubricant, is added to this.
It is mixed.

Here, the copper powder and the graphite powder usually serve as a solid solution in the iron powder in the sintering step, and play a role of improving the rigidity, strength and the like of the obtained sintered body.

The step of obtaining the green compact is usually performed at room temperature. When the density ratio of the green compact is less than 80%, the strength of the molded body is not sufficient, so that problems such as peeling of the surface portion and chipping of corners and corners are likely to occur. On the other hand, when the density ratio of the green compact exceeds 90%, the compression force during compression molding becomes extremely large. This is disadvantageous in terms of mold life.

In the process of, the green compact obtained as described above is
After heating to ~ 1000 ℃, re-compression molding, density ratio 95%
The above repressed body is formed. In this case, if the temperature is less than 800 ° C., the pressure applied when obtaining a repressed body having a density ratio of 95% or more becomes excessive. On the other hand, if the temperature exceeds 1000 ° C., the secondary compression molded body is easily oxidized, which is disadvantageous in terms of energy saving.
The process is preferably performed in a protective atmosphere, that is, a non-oxidizing atmosphere such as an inert gas atmosphere, a reducing atmosphere, or a nitrogen atmosphere, but may be performed in the air in some cases. This is because if the sintering process is performed in a reducing atmosphere, the repressed body can be reduced even if it is slightly oxidized.

The sintering step is a step of heating the repressed body in a non-oxidizing atmosphere to sinter the iron-based powder particles to integrate them. The conditions such as the sintering temperature and the sintering atmosphere can be arbitrarily selected depending on the type of iron-based metal powder used. As the atmospheric gas, an endothermic gas generally known as RX gas, decomposed ammonia gas commonly known as AX gas, and the like are preferable. Sintering temperature is 1100 ~
It is 1300 ° C. If the temperature is less than 1100 ° C, sintering will be insufficient.
Specifically, about 1150 ℃ is good, but it depends on the steel type, 1120 to 1150 ℃ for Fe-Cu-C system, 1150 to 1200 for 4600 system.
℃, 4100 series 1200 ~ 1250 ℃ is good. Sintering time is 20
Minutes are good.

The cooling step can be performed under the same conditions as conventional ones.

Shot peening (shot blasting) is a good means for forming a high density layer having a density ratio of 98% or more on the surface in the subsequent steps, and at that time, the thickness of the high density layer is set to a desired value. In order to do so, the hardness, particle size, material of shot particles, shot injection speed, injection angle and the like may be adjusted appropriately. However, if the high-density layer is formed to a thickness of 1 mm or more, a very large amount of energy will be added, and due to the residual stress, minute cracks will occur at the boundary between the high-density surface layer and the base, and On the contrary, the fatigue strength decreases due to the stress concentration, so the thickness of the high-density layer must be within 1 mm.

(Examples) In order to better understand the present invention, comparative examples and performance tests will be described together with examples.

Example 1 Based on iron powder (base powder), 2.5 wt% copper powder and 0.65 wt%
Graphite powder of 0.8 wt% and zinc stearate powder of 0.8 wt% were mixed, and this was uniformly mixed by a V-type mixer. All of these raw material powders are commercial products, iron powder is reduced iron powder, copper powder is electrolytic copper powder, and graphite is natural graphite powder.

The obtained homogeneous mixed powder was compression-molded with a mold to obtain a plate-like (thickness 7 mm) density ratio 83% as shown in FIG. 2 (front view, numerical values in the figure indicate mm dimensions). A green compact was obtained. This is 920 ° C × 2 in RX gas (butane metamorphic gas)
After heating and holding for 0 minutes, it is quickly transferred to a repressing die and compressed to form a repressed body with a density ratio of 97.1%, then quickly transferred to a heating furnace and heated and held in RX gas for 1130 ° C x 30 minutes (sintering) The cooling was performed from 900 ° C to 300 ° C at a rate of -40 ° C / min to obtain a sintered body having the shape shown in Fig. 3. Next, shot peening was applied to the sintered body, and the arc height amount was adjusted to increase the density ratio of the surface layer within 0.3 mm (layer from the surface to the depth of 0.3 mm) to 99%. I) was obtained.

Example 2 Example product II was obtained by the same method as in Example 1 except that the density ratio within 0.6 mm of the surface layer was changed to 99% by shot peening.

Example 3 Example product III was obtained in the same manner as in Example 1 except that the density ratio within 0.9 mm of the surface layer was changed to 99% by shot peening.

Example 4 Heat treatment was performed between the sintering step and shot peening fixing in Example 2, that is, after heating and holding at 900 ° C. for 30 minutes under vacuum (reduced pressure), oil quenching was performed, and then in a non-oxidizing atmosphere.
Example 2 except that an operation of tempering at 0 ° C. for 30 minutes was added.
In the same manner as described above, an example product IV having a density ratio within the surface layer of 6 mm of 99% was obtained.

Comparative Example 1 Comparative Example Product I was obtained in the same manner as in Example 1 except that the density ratio within the surface layer of 1.2 mm was set to 99% by shot peening.

Comparative Example 2 The same operation as in Example 1 was repeated to obtain the same green compact. It was, however, sintered without preheating. Sintering was carried out by heating and holding in RX gas at 1130 ° C. for 30 minutes, then the temperature was lowered to 920 ° C., immediately transferred to a repress mold and recompressed to a density ratio of 97%. Cooling from 900 ℃ to 300 ℃
Were cooled at a cooling rate of −40 ° C./min. Then shot peening is applied and 99% is within 0.6mm of the surface layer.
A comparative example product II having a density ratio of

Performance Test The tensile strength and fatigue strength (fatigue strength) of the example products I to IV and the comparative example products I and II obtained as described above were measured. The tensile test was carried out using a universal testing machine at a crosshead speed of 2 mm / min. The fatigue test was performed by a Schenk type plate bending fatigue tester, an SN diagram was prepared, and a fatigue limit (fatigue strength) was obtained. The results are shown in Fig. 1. As can be seen from the figure, there is no remarkable difference in tensile strength between each of the example products, while the fatigue strength of the example product is far superior to that of the comparative example product.

Example products I to III and comparative product I have a high density (99% density ratio) surface layer thickness of 0.3 to 1.2 mm by shot peening.
Between 0.3 and 0.9 mm,
The fatigue limit effectively increases with the layer thickness, but when the high-density layer reaches 1.2 mm (comparative example product I), the fatigue limit decreases conversely. This value is different from that of shot peening. The fatigue limit of the test piece was less than 26 Kgf / mm ^2, and it can be seen that there is a suitable range for providing a high-density layer.

The comparative example product II is equivalent to a product obtained by shot peening of a sintered body obtained by a method generally used conventionally, but the surface layer of the product has oxidation and a defective sintering portion. Therefore, compared to the fatigue limit of 19 Kgf / mm ² before adding shots, only an improvement effect of ² Kgf / mm ² can be measured. On the other hand, the fatigue limit of the example product II having the high-density layer (0.6 mm) having the same thickness as that of the comparative example product II is 34 Kgf / mm ² , which is lower than the fatigue limit of 26 Kgf / mm ² before the shot. 8 kgf
/ Mm ² has also improved.

That is, the method of the present invention can significantly improve the effect of shot peening after sintering as compared with the conventional method. In other words, it is possible to improve the fatigue limit equivalent to the conventional one with a shot amount which is extremely smaller than the conventional one. .

Further, comparison of Example product II and Example product IV reveals that good results can be obtained by performing quenching and tempering between the sintering process and the shot peening process.

(Effects of the Invention) As can be seen from the details described above, according to the method for manufacturing a sintered member of the present invention, it is possible to provide a sintered member having significantly improved mechanical strength, particularly fatigue strength. You can

By using such a member in, for example, an automobile engine, a power transmission unit, a suspension device, or the like, the service life thereof can be significantly extended.

INDUSTRIAL APPLICABILITY The method of the present invention can use the conventionally used sintering equipment as it is, can save the energy and the strength of the sintered member more than before, and can improve the strength of the sintered member. Since it can be used in place of a forged product or a cast product manufactured by a method with lower productivity, it also has a cost reduction effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing tensile strength and fatigue strength measurement test results of each sintered member obtained by an example of the method of the present invention and a comparative example, FIG. 2 and FIG. FIG. 3 is a front view showing a green compact and a sintered body obtained in the course of one example.

Claims (2)

[Claims] 1. A density ratio of 80-
Process to obtain 90% green compact, non-oxidizing green compact 800-100
A step of compressing the heated compact obtained by placing it in an atmosphere of 0 ° C. to obtain a repressed body having a density ratio of 95% or more, a step of sintering the repressed body to obtain a sintered body, a surface layer of the sintered body A method for producing a sintered member, which comprises sequentially including a surface modification step of making a density ratio of 98% or more only within 1 mm. 2. The method according to claim 1, further comprising a heat treatment step between the step of obtaining the sintered body and the surface modification step.