JPS6330363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the performance of iron-based sintered parts such as gears that slide under high surface pressure.

The content of the present invention will be explained using a gear as an example. In gears with a large transmission torque, a phenomenon called pitting occurs in the surface layer, and this often determines the limit of its use. The cause of this pitting is that repeated shearing forces act on a portion slightly deeper than the sliding surface, resulting in fatigue failure.
It is known that the resulting cracks grow and lead to delamination of the surface layer.

Therefore, in order to increase surface pressure resistance, quenching is usually used.
A method has been adopted to increase the fatigue strength of the parts where cracks, which cause pitting, occur by performing treatments such as decharring and nitriding.

By the way, in reality, even if the meshing with the mating gear locally becomes point contact due to errors in the tooth profile of the gear or errors during assembly, the surface layer of the teeth usually undergoes plastic deformation at the beginning of operation. This increases the pressure-receiving area and creates a so-called conforming state, but this can actually be a problem for gears with hardened surface layers as described above, resulting in only localized contact without plastic deformation. . As a result, high stress was concentrated at the contact area, inducing fracture, and the material was unable to exhibit its inherent high surface pressure properties.

As a countermeasure against this problem, conventional methods have been to perform decarburization treatment when the member is a melt-molded material to form a soft ferrite phase on the surface of the hardened layer to improve conformability. However, if the member is a sintered alloy, this method is extremely difficult to apply because decarburization extends to the interior of the member through the continuous pores inherent in the sintered body.

For this reason, sintered parts have been used with their surfaces hardened by quenching, which is better than not quenching, but not the best.

The present invention is directed to compositions that cause martensitic transformation.
By heat-treating sintered steel parts to which elements that stabilize austenite such as Ni and Mn are added, the main idea is to create a martensite phase inside the part and an austenite phase from the surface to a depth of at least 0.02 mm. It is something.

That is, the above-mentioned crack occurrence area is made of a martensite phase with high fatigue resistance to prevent pitting, while the surface layer of the sliding surface is made of a relatively soft austenite phase to enable initial break-in. However, it is preferable that the thickness of the austenite phase be as thin as 0.02 mm, which is necessary to obtain an effective conforming effect. If it exceeds 0.3 mm, the austenitic phase will occur up to the above-mentioned crack occurrence site, and the load bearing capacity will decrease. Note that the thickness of the phase can be arbitrarily controlled by the amount of added components as described below. Moreover, as a remedy when the thickness is too high, it is possible to reduce the thickness by tempering.

Now, in the process of cooling steel containing 0.2 to 1.8% C from a high temperature, a change from austenite to martensite to C in austenite to cementite occurs at around 723℃, but the change in takes more time than the change in , so quenching etc. In the case of rapid cooling, the main change is the change in .

In this case, if Ni, Mn, etc., which have a strong ability to stabilize austenite, are present, the austenite phase, which is stable at high temperatures, will remain in the surface layer that cools quickly, while the interior will have a structure that changes to martensite. The thickness of the austenite phase in the surface layer at this time is approximately proportional to the amount of the added element. Figure 1 shows the experimental results for Ni, showing that the thickness of the austenite phase on the surface of the part can be controlled by the amount of Ni added;
By adding 3% or more of
It can be seen that an austenite phase of 0.02 mm is obtained and its lower limit is 3%.

The present invention will be described below with reference to experimental results including one example thereof.

The composition is Fe−2Ni−0.45Mo−0.15Mn and the particle size is
For commercially available atomized alloy powder of 100 mesh or less,
6% Ni powder of the same particle size was mixed and formed into a disk shape with a diameter of 60 mm and a thickness of 5 mm with a density of 7.0 g/cm ³ , and then sintered at 1250°C. Next, this sintered body was oil-quenched by holding it at 860° C. for 1 hour in a carburizing atmosphere containing a mixture of butane modified gas and butane gas, and then tempered at 180° C. to prepare Sample 1.
For comparison, as an example of a conventional material that does not generate a soft layer on the surface, the above atomized alloy powder (2% Ni) alone was molded, sintered and heat treated under the same conditions as Sample 1, and Sample 2 was created.

For both samples 1 and 2, the amount of carbon due to carburization is approximately 0.3%.
It is. Note that sintering in a state where carbon and Ni coexist causes significant dimensional changes, which is not preferable for parts with strict dimensional accuracy such as gears.
This is why, in this example, carbon was not added from the beginning and was diffused by carburizing from the atmosphere.

The graph in FIG. 2 shows the results of cutting these samples and measuring the hardness of the cross section, and the numbers in the figure indicate the sample numbers. As can be seen from the figure, sample 1 according to the present invention has a soft layer on its surface, whereas sample 2, which is a comparative example, does not have a soft layer.

Further, as a result of microscopic examination of the structure of the sample, it was found that the surface layer of Sample 1 was an austenite phase with an average thickness of 0.13 mm, and the inner layer was a martensite phase, while the entire sample 2 was a martensite phase.

Next, a durability test was conducted on both samples using a three-ball pitching tester, and the number of repetitions until failure was determined. The results are shown in FIG. The meanings of the numbers in the figure are the same as in FIG. The steel balls used in this test were SUJ2 material with a diameter of 9.53 mm, and the lubricating oil was
SAE #30, and the Hertzian surface pressure stress calculated by the usual method was used for the surface pressure.

As can be seen from the figure, sample 2 has no soft layer on the surface.
The critical surface pressure is approximately 60 kg, whereas the specimen 1 according to the present invention reaches 90 kg, making it possible to increase the critical surface pressure by 1.5 times.

As described in detail above, the surface layer of the sintered component according to the present invention is easy to conform to when sliding with a mating component, while the interior and the crack occurrence area that causes pitting have a structure with high fatigue resistance. Therefore, the service limit surface pressure can be much higher than before, and therefore, when applied to sintered gears, for example,
This has the effect of expanding its uses.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of nickel added to steel and the thickness of the austenite phase due to heat treatment.
Figure 2 is a graph comparing the hardness distribution of sample cross sections for Sample 1 according to the present invention and Sample 2 according to the conventional method. Figure 3 shows the results of a fatigue test using a three-ball pitting tester for both samples. This is a graph showing.

Claims (1)

[Claims] 1. An iron-based sintered part that can withstand high surface pressure, characterized in that the structure in a range of 0.02 to 0.3 mm from the surface to the inside is an austenitic phase, and the subsequent internal structure is a martensitic phase.