JP7595312B2 - Reprocessed parts and manufacturing method for reprocessed parts - Google Patents

Description

The present invention relates to reprocessed parts and methods of manufacturing reprocessed parts.
This application claims priority based on Japanese Patent Application No. 2021-076702, filed on April 28, 2021, the contents of which are incorporated herein by reference.

Gears and bearings that make up reducers are used in large numbers and are expensive. For this reason, there is a demand for them to be reused. Repair methods using build-up technology have been proposed for damaged parts, but due to their high cost and low quality, they are rarely used. Parts that are not damaged are also reused, but some of them end up breaking down before their expected lifespan.

When it comes to reusing gears and bearings that are not damaged, their remaining lifespan has been measured and they have only been reused within that range, but there have been no attempts to restore functionality or extend their lifespan. This is because it is commonly believed that it is difficult to restore functionality or extend their lifespan using simple, inexpensive methods.

On the other hand, regarding the technology of reusing missing parts, methods of repairing missing parts by various techniques are known. For example, Patent Document 1 describes a repair method by welding heat-resistant metal alloys used in gas turbine engines. Patent Document 2 describes a technology of irradiating a crack generated in the steel material of an existing steel structure with laser light to melt the crack and repair it. Patent Document 3 describes a method of evaluating the remaining life of high-temperature components, and further describes a highly reliable high-temperature equipment maintained by this evaluation method and a reheat treatment method (repair method) for damage detected by this evaluation method. Patent Document 4 describes a gas turbine blade that regenerates the structure after use to the same form as the structure of unused material and has the same strength as unused material, and a reheat treatment method thereof.

JP 2004-216457 A JP 2013-86163 A Japanese Patent Application Publication No. 8-271501 JP 2000-80455 A

However, the above technologies are costly, require precise processes and are primarily relevant for aircraft and power generation components.

The present invention has been made in consideration of the above problems, and aims to provide reprocessed parts and a manufacturing method for reprocessed parts in which the functionality of carburized parts is restored and their service life is extended by a simple and inexpensive method using only heat treatment.

To solve the above problems, the present invention provides the following means.

The reprocessed part according to the first aspect of the present invention has a carburized layer on its surface, and within the carburized layer, three layers are arranged adjacent to each other in the depth direction from the surface: a first hardened layer, a second hardened layer, and a third hardened layer, all of which have a higher hardness than the core, and the first hardened layer has the highest hardness of the three layers, and the second hardened layer has the lowest hardness of the three layers.

In the reprocessed part relating to the above aspect, the minimum hardness value of the second hardened layer may be higher than the hardness of the core portion.

In the reprocessed part relating to the above aspect, the thickness of the first hardened layer may be 50 μm or more and 300 μm or less.

In the reprocessed part relating to the above aspect, the prior gamma grain size of the first hardened layer may be 5 μm or more and 15 μm or less.

In the reprocessed part relating to the above aspect, the volume fraction of the retained austenite in the first hardened layer may be 10% or more and 35% or less.

The method for manufacturing a reprocessed part according to the second aspect of the present invention is a method for manufacturing a reprocessed part according to the above aspect, and includes the steps of preparing a carburized part having a carburized layer on its surface, and flash-hardening the carburized part to form the three layers within the carburized layer.

The manufacturing method for reprocessed parts according to the above aspect may include a step of performing the flash hardening by a laser, determining a range of laser output when performing the flash hardening such that no cracks are generated in the first hardened layer and the thickness of the first hardened layer is a predetermined thickness, and a step of performing the flash hardening by laser within the determined output range.

According to the present invention, it is possible to provide reprocessed parts whose service life has been extended by a simple and inexpensive method using only heat treatment for carburized parts.

FIG. 2 is a schematic cross-sectional view showing a reprocessed part according to the present embodiment. 1 is a graph showing a hardness distribution in a typical cross-sectional depth direction of a reprocessed part according to an embodiment of the present invention. FIG. 1A is a schematic diagram of an example of an apparatus configuration for performing a laser hardening process, and FIG. 1B is a concept for explaining instantaneous cooling in an evaluation body after the laser hardening process. FIG. 4A is a flow diagram of a sliding fatigue test, and FIG. 4B is a conceptual diagram for explaining each step. 1A and 1B are schematic diagrams illustrating an outline of a sliding fatigue test, in which FIG. 1A is a schematic plan view and FIG. 1 is a graph showing the results of a sliding fatigue test performed on each evaluation body, and measuring the number of cycles at which pitting occurred. 1 is a graph showing the relationship between an increase in the residual γ rate due to laser hardening and an improvement in the sliding fatigue life. FIG. 13 is a diagram showing the hardness distribution in the depth direction of the evaluation body at step 1 and after step 3. 13A is an SEM image of the evaluation object at step 1, and FIG. 13B is an SEM image of the evaluation object after step 3. (a) is an SEM image at step 1, (b) an SEM image after step 2, (c) an SEM image after step 3, and (d) an SEM image after step 4. 1A is an SEM image of the evaluation object at step 1, and FIG. 1B is an SEM image of the evaluation object after step 3. 1 shows the results of observation of the internal structure of the first hardened layer by STEM (scanning transmission electron microscope). 1 shows the results of observation of the internal structure of the first hardened layer by a TEM (transmission electron microscope).

The present embodiment will now be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show enlarged characteristic parts for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the effects.

[Reprocessed parts]
FIG. 1 is a schematic cross-sectional view showing a reprocessed part according to the present embodiment.
As shown in FIG. 1, the reprocessed part 100 of this embodiment has a carburized layer 10 on its surface. Within the carburized layer 10, there are three adjacent layers from the surface to the depth direction, namely, a first hardened layer 1, a second hardened layer 2, and a third hardened layer 3, all of which have a harderness than that of the core 20. The first hardened layer 1 has the highest hardness of the three layers, and the second hardened layer 2 has the lowest hardness of the three layers.
Here, "the first hardened layer has the highest hardness among the three layers" means that the first hardened layer has a region with the highest hardness among the three layers. Also, "the second hardened layer has the lowest hardness among the three layers" means that the second hardened layer has a region with the lowest hardness among the three layers. The relationship of the average hardness of each of the first hardness layer, the second hardness layer, and the third hardness layer is first hardness layer>third hardness layer>second hardness layer. In this specification, the average hardness of a layer refers to the average hardness of the maximum hardness and the minimum hardness in the layer, obtained by using a graph of the hardness distribution in the thickness direction based on the hardness obtained at two or more measurement points in the thickness direction (depth direction) of the layer.

The reprocessed part 100 can be obtained by subjecting a "carburized part" to a flash hardening treatment, which will be described later, after it has been used for a certain period of time, or when the part is unused. In this specification, among the reprocessed parts, those obtained by subjecting a used "carburized part" to a flash hardening treatment are referred to as "reused parts".
In this specification, the term "carburized part" refers to a part in which a surface layer (carburized layer) with a high carbon content is formed by diffusing and penetrating carbon from the surface of a steel part, and then the carburized layer is hardened by quenching. The term "carburized part" is not limited to carburized parts as long as it is a part in which a carburized layer is formed on the surface layer, and includes parts in which other elements are diffused and penetrated together with carbon, for example, nitriding parts in which nitrogen is diffused and penetrated. The term "carburized part" is produced by subjecting a steel part to surface heat treatment classified as surface hardening and thermal diffusion treatment, and this surface hardening includes known surface hardening such as induction hardening, and thermal diffusion treatment includes known thermal diffusion treatment such as carburizing. Such parts are not particularly limited as long as they are steel parts, but typically include mechanical parts such as gears and bearings. Examples of such parts include, but are not limited to, cutlery, springs, wheel spindles, piston ring grooves, connecting rods, shafts, pulleys, etc. If a steel part that has been carburized and has a carburized layer on the surface is quenched, the carburized layer will have a higher martensite structure, which will be harder than the core and improve wear resistance. On the other hand, it is possible to leave some austenite structure (retained austenite) without transforming everything during quenching. Since the austenite structure is soft and highly ductile, the retained austenite portion acts as a shock absorber and improves toughness.
In addition, a carburized part has a carburized layer and a core. When the carburized part consists of a carburized layer and a core, the carburized layer refers to the part from the surface of the carburized part to a depth where the hardness is indistinguishable from that of the core, and the core refers to the part that is located deeper than the carburized layer.

<Structural features>
The reprocessed part 100 has a common feature with the carburized part in that it has a carburized layer 10 and a core 20 .
The reprocessed part 100 includes a first hardened layer 1 having the highest hardness, and a second hardened layer 2 having the lowest hardness (in other words, relatively softer than the first hardened layer 1 and the third hardened layer). The reprocessed part 100 also includes a second hardened layer 2 having a relatively softer hardness between the first and third hardened layers having high hardness. The hardened layer and the softened layer are so-called lamellar-shaped, which provides composite reinforcement and is advantageous in terms of toughness. In other words, a hard layer alone would be brittle, but the presence of a soft layer within the hard layer increases the toughness as a whole.
Such structural features can be obtained by subjecting conventional carburized parts (i.e., parts having a carburized layer on their surface) to a "flash hardening" treatment, which will be described later, after a certain period of use or when the carburized part is still unused.

<Carburized layer>
The carburized layer 10 is already present at a stage before the "flash hardening" treatment described later is performed. The carburized layer 10 has a higher hardness than the core 20.
The thickness of the carburized layer 10 is not limited, but is usually about several mm, typically about 2 to 5 mm.

To extend the service life of gears, bearings, etc., carburizing and induction hardening are generally performed, and the surface area is hardened from the outermost surface to a depth of several mm to form a carburized layer 10. The carburized layer, which is located on the surface and has a harder hardness than the core, is subjected to load over time during operation, and its structure and mechanical properties change. An example of a structural change in the surface area is processing-induced martensitic transformation of the retained austenite (γ) phase, and an example of a change in mechanical properties is a change in residual stress and half-value width (reflecting dislocation density).

During operation, the surface layer of a carburized part that is subjected to load over time decreases the residual γ phase and increases the ratio of the processing-induced martensite phase, but no damage occurs in the part at this point. For example, in the case of a reducer, at the first or second maintenance, when regular maintenance is observed, the residual γ phase decreases in many gears and bearings, but no damage occurs.
The inventors have succeeded in significantly improving the lifespan of carburized parts without defects (including not only those that have been used for a certain period of time, but also those that are unused) by controlling only the surface structure using a simple and inexpensive method of heat treatment alone. The inventors have succeeded in significantly improving the lifespan of carburized parts by applying flash quenching (instantaneous heating and cooling) in the remanufacturing (recycling) of the carburized parts.

2 shows a graph of the hardness distribution in the depth direction of a typical cross section of the reprocessed part according to this embodiment, where the horizontal axis is the depth (mm) from the surface, and the vertical axis is the Vickers hardness (Hv).
In FIG. 2, (1) shows the hardness distribution in the depth direction of the carburized part before use, (2) shows the hardness distribution in the depth direction of the carburized part after use, and (3) shows the hardness distribution in the depth direction of the reprocessed part according to this embodiment, in which the carburized part after use has been flash-quenched (reheat treated).

In the hardness distribution in the depth direction of the reprocessed part shown in Figure 2, the region indicated by the symbol A in the depth direction indicates the first hardened layer, the region indicated by the symbol B in the depth direction indicates the second hardened layer, the region indicated by the symbol C in the depth direction indicates the third hardened layer, and the region indicated by the symbol D in the depth direction indicates the carburized layer.

In this specification, the first hardened layer, the second hardened layer and the third hardened layer are defined as follows, using the typical cross-sectional depth-wise hardness distribution shown in FIG.
The second hardened layer is a range including the minimum hardness (symbol b). The position where the minimum hardness (symbol b) and the maximum hardness (symbol a) in the first hardened layer are 1/2 (intermediate) is defined as the boundary position between the first hardened layer and the second hardened layer (the boundary position between the region of symbol A and the region of symbol B). The position where the minimum hardness (symbol b) of the second hardened layer and the maximum hardness (symbol c1) of the third hardened layer are 1/2 (intermediate) is defined as the boundary position between the region of symbol B and the region of symbol C. The position of the inner boundary of the third hardened layer is a position (symbol c2) that is deeper inside than the position showing the maximum hardness (symbol c1) of the third hardened layer and 1/2 (intermediate) is defined as the hardness (symbol P) of the minimum hardness (symbol b) of the second hardened layer and the maximum hardness (symbol c1) of the third hardened layer.
Between the third hardened layer and the core there is a layer 4 (see FIG. 1) whose hardness is lower than that of the third hardened layer and higher than that of the core.

<First hardened layer>
The first hardened layer is the layer disposed closest to the surface of the three layers, and has the highest hardness region among the three layers. The first hardened layer also has the highest average hardness among the three layers.
The hardness of the first hardened layer is typically in the range of Vickers hardness (Hv) of Hv 700 to Hv 900. Hv 700 corresponds to the hardening hardness of the surface layer, and Hv 900 corresponds to the hardness of the laser hardened hardened layer region.
In the example shown in Fig. 2, the maximum hardness of the first hardened layer is about Hv 900 in Vickers hardness (Hv). Before the laser hardening (reheat treatment), the hardness in the vicinity of the first hardened layer was about Hv 700, but the hardness is now higher, and the wear resistance of the first hardened layer is improved compared to before the laser hardening (reheat treatment).

The thickness of the first hardened layer is preferably 50 μm or more and 300 μm or less.
By appropriately controlling the thickness of the first hardened layer, macroscopic composite reinforcement with the second hardened layer (relatively soft phase) located inside the first hardened layer is realized, which contributes to toughness.
In the example shown in FIG. 2, the thickness of the first hardened layer is about 200 μm.

By subjecting the first hardened layer to a quenching treatment that involves instantaneous heating and cooling, the inventors have succeeded in retaining the high carbon concentration inherited from the carburizing treatment and in revealing the residual gamma phase at a frequency equal to or higher than that of new material (unused material). In addition, the size of each residual gamma phase is refined to the same or smaller size as that of new material, and is uniformly dispersed. Incidentally, instantaneous heating and cooling do not require atmospheric control and can be performed in air.

In addition, by instantaneous heating and cooling, the grain size of prior austenite (γ) in the first hardened layer is made finer than that of a new carburized part.
In the examples, the prior γ grain size of the carburized steel material was about 10 μm, but was refined to about 5 μm by quenching treatment using instantaneous heating and instantaneous cooling.
Although the first hardened layer produced by instantaneous heating and cooling hardens to a Vickers hardness (Hv) of Hv900, it maintains high toughness due to the effect of refining the crystal grains and the combined strengthening effect of the residual gamma phase (soft phase) and martensite phase (hard phase). Metal materials are generally prone to fracture from grain boundaries. When the grain size is reduced and there are many grain boundaries, forces are not concentrated on one grain boundary. Therefore, the more grain boundaries there are, the less likely the material is to break and the less likely grain boundary fracture will occur. As a result, the fatigue life is extended.

In addition, due to the instantaneous heating and cooling, the volume fraction of the residual γ phase in the first hardened layer becomes larger than that of a new carburized part.
In the examples, the volume fraction of the residual γ phase in the first hardened layer was about 20% before sliding fatigue, but the volume fraction of the residual γ phase decreased to about 10% due to sliding fatigue, and the volume fraction of the residual γ phase increased to about 30% by instantaneous heating and cooling. In addition, no coarse residual γ (e.g., 2 μm or more) was formed.

<Second hardened layer>
The second hardened layer is adjacent to the first hardened layer and is located deeper than the first hardened layer, and includes a region with the lowest hardness among the three layers. The second hardened layer also has the lowest average hardness among the three layers. The second hardened layer 2 is a relatively soft layer located between the first hardened layer 1 and the third hardened layer 3, which have high hardness, and improves the overall toughness among the hard layers.
It is preferable that the minimum hardness of the second hardened layer is higher than the hardness of the core, because in this case, the second hardened layer can also contribute to wear resistance.
The hardness of the second hardened layer is preferably, in Vickers hardness (Hv), Hv 500 or more and less than Hv 700. If it is Hv 500 or more, it is harder than the core, and if it is less than Hv 700, it is relatively softer than the first hardened layer (a difference of about Hv 300 from the maximum hardness is preferable for improving toughness), and the lower limit of the first hardened layer can be set as the upper limit of the second hardened layer.
In the example shown in Fig. 2, the minimum hardness of the second hardened layer is about Hv 530 in Vickers hardness (Hv). Before the laser hardening (reheat treatment), the hardness in the vicinity of the first hardened layer was slightly less than Hv 700, but the hardness is now lower, which contributes to toughness.

The thickness of the second hardened layer depends on the hardening conditions, but is typically approximately 200 μm or more and 300 μm or less.

<Third hardened layer>
The third hardened layer is a layer disposed adjacent to the second hardened layer at a deeper position than the second hardened layer, and has a lower hardness than the first hardened layer and a higher hardness than the second hardened layer.
The hardness of the third hardened layer is typically, in Vickers hardness (Hv), Hv 550 or more and Hv 600 or less. Hv 550 is typically about Hv 50 higher than the lower limit of the second hardened layer, and Hv 600 is a state in which the hardness is lower than the lower limit of the first hardened layer and higher than the hardness of the second hardened layer and the hardness at a depth of 1.5 mm.

(Manufacturing method for reprocessed parts)
The method for producing a reprocessed part according to the present invention includes the steps of preparing a carburized part having a carburized layer on its surface, and flash-quenching the carburized part to form the three layers in the carburized layer (flash-quenching step). The flash-quenching step can be performed in air without controlling the atmosphere.

In this specification, "instantaneous hardening" refers to hardening in which the surface of a part is irradiated with particle beams such as laser light, electron beams, or neutron beams (hereinafter collectively referred to as "laser light, etc.") and then quenched and hardened by instantaneous heating and instantaneous cooling. The principle is that the surface layer reaches the heat treatment temperature locally and in a very short time by irradiation with laser light, etc. ("instantaneous heating"), and when the irradiation with laser light, etc. is stopped, the heat is quickly conducted to the inside of the part, causing the surface layer to cool rapidly (natural cooling) (i.e., "instantaneous cooling"), and the part is hardened.

In the method for producing reprocessed parts of the present invention, the flash hardening is preferably performed by a laser from the viewpoints of simplicity and cost.
In this specification, instantaneous hardening by laser may be simply referred to as "laser hardening", and the instantaneous hardening process may be referred to as the laser hardening process.

When the flash hardening is performed by a laser, it is preferable to have a step of determining the range of laser output when performing the flash hardening so that no cracks are generated in the first hardened layer and the thickness of the first hardened layer is a predetermined thickness. By having such a step, it is possible to perform the flash hardening step after confirming the laser output range required to obtain the reprocessed parts according to the present invention for each device, etc., and to avoid problems depending on the device and environment.
This predetermined thickness is a thickness that provides a characteristic hardness distribution in the depth direction in the carburized layer as described above. If the first hardened layer is too thick, it may become too hard and crack, whereas if it is too thin, the effect of hardening cannot be obtained. The thickness is, for example, 100 μm or more and 300 μm or less.

The output of such lasers typically ranges from 750 W to 1150 W. In this case, if the output exceeds 1150 W, the output is too high and may cause cracks, and if the output is less than 750 W, the hardening effect cannot be achieved.

Fig. 3(a) shows a schematic diagram of an example of an apparatus configuration for performing the laser hardening process, and Fig. 3(b) is a conceptual diagram for explaining instantaneous cooling of the evaluation body after the laser hardening process.
Laser light from a laser oscillator 101 is conveyed through an optical fiber 102 and converged by a condenser lens 103, and the converged laser light L is irradiated onto an evaluation object S. Instantaneous heating occurs locally on the surface layer of the evaluation object irradiated with the laser light L, and then instantaneous cooling occurs when the laser irradiation is interrupted, thereby hardening the surface layer.
As the laser device for performing the laser hardening, a known device can be used.

[Example]
Reprocessed parts according to the present invention were produced, and a sliding fatigue test was carried out to evaluate the fatigue life.

Fig. 4(a) shows a flow diagram of the sliding fatigue test carried out, and Fig. 4(b) is a conceptual diagram explaining each step.
The sliding fatigue test consists of the following four steps:
Step 1: preparing an evaluation object (carburized part).
Step 2: Making a sliding contact.
Step 3: After step 2, laser hardening is performed.
Step 4: After step 3, a step of performing sliding contact again (restart step).

The evaluation specimen prepared in step 1 corresponds to a carburized part.
The sliding contact in step 2 simulates the operation of a carburized part. In the example, the width of the sliding surface between the evaluation body and the roller (see FIG. 5) was about 2 mm.
Carrying out step 3 corresponds to performing instantaneous hardening using a laser on the carburized part after operation for a certain period of time, forming a three-layer structure of a first hardened layer, a second hardened layer and a third hardened layer in the carburized layer, thereby producing a reprocessed part according to the present invention. In the embodiment, the output of the laser light during laser hardening was 1050 W (peak output), and the moving speed of the laser light on the evaluation body was 1500 mm/min.
Step 4 is a step of performing sliding contact again on the reprocessed part produced in step 3.

<Fatigue life evaluation method>
For the fatigue life evaluation, a "sliding (sliding) fatigue test" was performed using the evaluation body (test piece) 200 and the roller R. An outline of the test is shown in Fig. 5. Fig. 5(a) is a schematic plan view, and (b) is a schematic side view.

As the evaluation body 200, a steel material of JIS standard SCM420 was used.
Specifically, the following steel composition was used:
C: 0.18-0.23%, Mn: 0.60-0.85%, Si: 0.15-0.35%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, P≦0.030%, Si≦0.030%, Ni≦0.25%, Cu≦0.30%; the above percentages are mass%.

After carburizing, the evaluation body 200 was ground (sharpened) to prepare the surface (sliding surface) 200a. The shape and dimensions of the evaluation body 200 used are shown in FIG. 5(b). The carburizing process was carried out so that a carburized layer of about 4 mm to 5 mm was formed on the surface (sliding surface) 200a of the evaluation body 200.

Figure 5(b) also shows the shape and dimensions of the roller R used in combination with the evaluation body 200 during the sliding fatigue test. This roller R is made of steel material of JIS standard SCM420. Note that all dimensional numbers in Figure 1 are in millimeters [mm].

The sliding fatigue test was performed by contacting the outer diameter surface of roller R with the sliding surface of evaluation body 200 (width of sliding surface is approximately 2 mm (see Figure 4)) with a surface pressure load of 3.4 [GPa], and rotating evaluation body 200 and roller R around their respective axes so that the contact surfaces of both rotate at speeds of 1890 rpm and 2700 rpm, respectively, to create contact (sliding contact) accompanied by sliding (sliding). Note that in this test, lubricating oil (GL490 (manufactured by ENEOS Holdings Corporation)) with a temperature of 80±5 [°C] was supplied to this sliding contact area at a flow rate of 300-450 [mm/min] while the test was performed.

The test conditions for the sliding fatigue test are as follows:
Load: 3.4 [GPa]
Slip rate: 30% (= 100 x (2700 rpm - 1890 rpm) / 2700 rpm)
Lubricant: GL490 (manufactured by ENEOS Holdings Corporation)
・Lubricating oil temperature: 80±5 [℃]
Flow rate: 300-450 [mm/min]

A sliding fatigue test was performed on the evaluation body 200 with the above configuration and conditions, and the test was stopped when pitting (peeling marks) with a diameter of 1 mm or more occurred on the sliding surface 200a, and the total number of rotations at that point was defined as the number of cycles until pitting occurred (sliding fatigue life). Here, the "diameter" of 1 mm or more refers to the maximum diameter from one end to the other end of the planar shape of the pitting (peeling marks).

6 is a graph showing the results of measuring the number of cycles to pitting (sliding fatigue life) at which pitting (peeling marks) occurred when a sliding fatigue test was performed on evaluation specimens 1 to 6. The numbers on the horizontal axis are the numbers of the evaluation specimens, and the vertical axis shows the number of cycles to pitting.
The evaluation sample 1 corresponds to a conventional carburized part, and corresponds to the "carburized product" in the legend in FIG.
Evaluation specimens 2 to 6 correspond to the reprocessed parts of the present invention. Of these, evaluation specimens 2 to 5 correspond to used carburized parts that were subjected to flash quenching, and correspond to the "reprocessed part (interrupted test material)" in the legend in Fig. 6. Evaluation specimens 2 to 5 also correspond to the above-mentioned "reused parts." Evaluation specimen 6 corresponds to an unused carburized part that was subjected to flash quenching, and corresponds to the "reprocessed part (untested material)" in the legend in Fig. 6.

For evaluation sample 1, the number of cycles at which pitting occurred was measured at step 2 without proceeding to step 3 (laser hardening (reheat treatment)).
For evaluation specimens 2 to 5, the fatigue cycle of evaluation specimen 1 was used as the standard, and step 2 (operation step) was carried out up to 15%, 50%, 75%, and 85% of the standard cycle, respectively, and then step 3 (laser hardening (reheat treatment)) was carried out, and further step 4 (re-operation step) was carried out to measure the number of cycles until pitting occurred.
For evaluation sample 6, step 2 (operation step) was not carried out, but step 3 (laser hardening (reheat treatment)) was carried out, and then step 4 (restart step) was carried out to measure the number of cycles at which pitting occurred.

The sliding fatigue lives of evaluation bodies 2 to 5 showed a life improvement effect of reheat treatment by laser hardening of 3.8 times, 3.2 times, 2.7 times, and 1.3 times, respectively, compared to the sliding fatigue life of evaluation body 1. From this result, it was found that the life of carburized parts that did not suffer any damage even after operating for a certain period of time can be improved by performing flash re-hardening treatment.
The sliding fatigue life of evaluation body 6 was improved by 8.2 times compared with the sliding fatigue life of evaluation body 1.
The results of evaluation samples 2 to 6 show that by applying flash quenching to carburized parts, whether before or after use, the carburized parts are reborn as parts with a longer life. The effect of flash quenching on the extension of life is greatest before use, and the shorter the time of use after use, the greater the effect.

Figure 7 is a graph showing the relationship between the proportion of residual gamma phase due to laser hardening and sliding fatigue life. The horizontal axis is the ratio of the proportion of residual gamma phase after step 3 (laser hardening) of evaluation bodies 2 to 6 to the proportion of residual gamma phase in evaluation body 1 (equivalent to a conventionally carburized part (equivalent to the state of evaluation bodies 2 to 6 before step 2 (before operation))), and the vertical axis is the ratio of the sliding fatigue life of evaluation bodies 2 to 6 to the sliding fatigue life of evaluation body 1. The proportion of residual gamma phase was determined by X-ray diffraction (XRD).

The evaluation samples 1 to 6 are as described above.
In FIG. 7, the sliding fatigue life ratios of 1.3, 2.7, 3.2, and 3.8 are the results for evaluation samples 5, 4, 3, and 2, respectively, and the sliding fatigue life ratio of 8.2 is the result for evaluation sample 6.
From FIG. 7, it can be seen that, up to a certain range of the ratio of the residual γ phase, the sliding fatigue life improves with an increase in the ratio of the residual γ phase, and a correlation between the residual γ phase and the life can be seen.

Figure 8 shows the hardness distribution in the depth direction of the evaluation body at step 1 (corresponding to the hardness distribution in the depth direction of a conventional carburized part) and the hardness distribution in the depth direction of the evaluation body after step 3 (laser hardening).

It can be seen that the hardness distribution in the depth direction at step 1 of the evaluation body before laser hardening gradually decreases with increasing depth from the surface, and gradually approaches the hardness of the core.
In contrast, the hardness distribution in the depth direction of the evaluation body (corresponding to the recycled product of the present invention) that had been laser hardened (reheat treated) showed that there was a first hardened layer with a hardness of 700 Hv to 900 Hv on the outermost surface, a second hardened layer exhibiting extremely low hardness closer to the core than the first hardened layer, and a third hardened layer with a harderness than the second hardened layer closer to the core than the second hardened layer.

FIG. 9(a) is a scanning electron microscope (SEM) image of a cross section of the evaluation body at step 1, and (b) is a SEM image of a cross section of the evaluation body that has been laser hardened (reheat treated).
In the SEM image of Fig. 9(b), there is a dark area from the surface to a depth of about 200 μm, which corresponds to the difference in structure due to laser hardening. This layer corresponds to the first hardness layer. On the other hand, there is no such dark area in the SEM image of Fig. 9(a).

10(a) to (d) show cross-sectional structures of evaluation samples for steps 1 to 4 observed by SEM.
The arrows in the SEM images indicate areas of residual γ phase.
Based on the SEM images, it can be seen that the residual γ phase, which was present at a high ratio in step 1, decreased after step 2 (sliding contact (operation)), but recovered to a high ratio again after step 3 (laser hardening).

In addition, the results of evaluating the microstructural changes in the surface layer of the specimens after each step (ratio of residual γ phase, residual stress, FWHM) using X-ray diffraction (XRD) are shown in Table 1;

XRD analysis revealed that the residual gamma phase, which was present at about 18% in step 1, decreased to about 10% after step 2 (sliding contact (operation)), but increased to about 28% after step 3 (laser hardening), higher than before the sliding contact.

11A shows an SEM image of the evaluation body at step 1, and (b) shows an SEM image of the evaluation body after step 3. The areas surrounded in each SEM image are crystal grains.
From Fig. 11(a), it can be seen that the prior γ grain size (in the first hardened layer) was about 10 μm before sliding contact (operation), and from Fig. 11(b), it can be seen that the prior γ grain size was refined to about 5 μm by laser hardening (reheat treatment).
Although the Vickers hardness of the first hardened layer is hardened to 700Hv to 900Hv by flash quenching, high toughness can be maintained due to the effect of refining crystal grains and the combined strengthening effect of the residual γ phase (soft phase) and the martensite phase (hard phase).

Figure 12 shows the results of an observation of the internal structure of the first hardened layer at a depth of about 10 μm from the surface layer using a STEM (scanning transmission electron microscope). In the low-magnification image of step 1, the lath martensite is arranged in parallel in a band shape with the same contrast, as indicated by the yellow arrow. These are aligned in the same crystal orientation, and the presence of several packets can be seen depending on the orientation of the arrangement. On the other hand, in the high-magnification image, the contrast in the lath martensite is also visible. This indicates a high dislocation density. In step 2, which involves sliding fatigue, as indicated by the red arrows in areas A and B, it can be seen that the twin deformation is observed along the grain boundaries with slip lines. These slip lines can be seen in approximately 10 lines on a scale of 200 nm. Therefore, the interval between the slip lines is assumed to be about 10 nm, which is considered to correspond to the crack growth rate per cycle. On the other hand, there are regions like C where slip lines are not visible, and this view reflects that slip lines have not been introduced and are localized. In step 3, some slip lines remain in some packet regions near the red arrow, but when viewed overall, it can be clearly seen that the slip lines are gradually decreasing over the entire area, and the intervals between them are also widening, and the state has been restored to that of step 1. This can be said to be one of the characteristic effects of laser hardening. Finally, in step 4, the influence of plastic flow is large with the load over time, and the slip lines become unclear, and subgrain formation is progressing. It is thought that these are formed by elongation into an isotropic shape as sliding fatigue progresses, and then subgrain formation occurs due to rearrangement. In addition, slip lines are visible in one direction throughout the entire view.

Figure 13 shows the results of observation of the internal structure of the first hardened layer using a TEM (transmission electron microscope). In step 1, lath martensite (yellow arrow) is apparent before testing, as in the structure image in Figure 12. This is followed by subgrain formation (orange arrow) along with sliding fatigue in step 2. However, in step 3, where laser hardening was performed, the subgrains decrease, but the lath martensite recovers and the contrast within the lath becomes uniform. In step 4, which is the stage at which pitting finally occurs, the TEM structure image confirmed that excessive plastic flow causes subgrain formation to progress so significantly that even the lath martensite becomes unclear.

The evaluation specimens simulating the reprocessed parts of the present invention showed that the toughness and wear resistance of the carburized parts were improved by a simple and inexpensive method of heat treatment alone, and that functionality was restored and the lifespan was extended.

Reference Signs List 1: First hardened layer 2: Second hardened layer 3: Third hardened layer 10: Carburized layer 20: Core 100: Reprocessed part

Claims (8)

With a carburized layer on the surface,
the carburized layer has three adjacent layers in order from the surface to the depth direction, the first hardened layer, the second hardened layer and the third hardened layer, each of which has a hardness higher than that of the core;
The first hardened layer has the highest hardness among the three layers, and the second hardened layer has the lowest hardness among the three layers;
A reprocessed component, wherein the prior gamma grain size of the first hardened layer is 5 μm or more and 15 μm or less, and the volume fraction of retained austenite in the first hardened layer is 10% or more and 35% or less. (delete) The reprocessed part described in claim 1, wherein the thickness of the first hardened layer is 50 μm or more and 300 μm or less. (delete) (delete) A method for manufacturing a reworked part according to claim 1 or claim 3, comprising the steps of:
preparing a carburized component having a carburized layer on a surface thereof;
and flash quenching the carburized component to form the three layers within the carburized layer. The instantaneous hardening is performed by a laser,
determining a range of laser output when performing the instantaneous hardening such that no cracks are generated in the first hardened layer and the thickness of the first hardened layer is a predetermined thickness;
and performing a laser flash hardening within the determined power range. 4. The reprocessed part according to claim 1, wherein the reprocessed part is a carburized part that has been subjected to load over time due to use for a certain period of time.

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