JPH0559994B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a cast iron piston used in internal combustion engines such as large diesel engines and a method for manufacturing the same, and particularly relates to a cast iron piston used in internal combustion engines such as large diesel engines and a method for manufacturing the same. This invention relates to a cast iron piston whose surface layer is locally reinforced, and a method for manufacturing the same.

Conventional technology Although cast iron pistons for internal combustion engines are heavier than aluminum alloy pistons,
Generally, it has excellent high-temperature strength, so it is used in large diesel engines, etc.

By the way, in a diesel engine, particularly a direct injection type diesel engine, the head portion of the piston receives fuel injection and is exposed to high temperature. In particular, the hot spot portion of the nozzle portion of the piston head portion where the injected fuel is directly sprayed becomes extremely hot, so it is required to have excellent thermal shock resistance and thermal fatigue resistance. With the recent increase in the output of diesel engines, there is a demand for higher temperatures in the fuel chamber, which has led to improvements in the thermal shock resistance and thermal fatigue resistance of the piston head, especially the hot spot, as described above. The demand for this is becoming stronger and stronger.

However, conventional cast iron pistons are often made of ordinary cast iron, and it is difficult for such ordinary cast iron pistons to fully meet the above requirements, and hot spots may occur due to thermal stress etc. It is unavoidable that cracks will occur in the
Therefore, the useful life of the piston has to be shortened. Therefore, recently, pistons have been made of high-grade cast iron, such as alloyed cast iron, which has excellent thermal shock resistance and thermal fatigue resistance.

Problems to be Solved by the Invention As described above, when a piston is made using high-grade cast iron such as alloyed cast iron, there is a problem in that the cost of raw materials for the piston unnecessarily increases. In other words, excellent heat resistance is required of the entire piston, especially the piston head, especially the hot spot, and other parts do not have such heat resistance that high-grade cast iron such as alloyed cast iron is required. Therefore, constructing the entire piston from expensive alloy cast iron or the like will lead to unnecessary cost increases. Furthermore, castability of high-grade cast iron such as alloyed cast iron is often inferior to that of ordinary cast iron, resulting in a problem of lower yield. Furthermore, alloy cast iron has poor machinability and may be difficult to machine after casting. In order to solve these problems, a method is to make the piston using cast iron, which is inexpensive and has good castability and workability, such as ordinary cast iron, and to partially strengthen parts of the piston, such as the hot spot part of the piston head. It is desirable to apply However, the reality is that such a method has not been established in the past.

This invention was made against the background of the above circumstances, and uses cast iron as a base material and partially strengthens the parts of the piston head part where thermal shock resistance and heat fatigue resistance are particularly required, such as the hot spot part. The object of the present invention is to provide a cast iron piston for an internal combustion engine that has improved thermal shock resistance and thermal fatigue resistance at that part, and a method for manufacturing the same.

Means for Solving the Problems The cast iron piston for an internal combustion engine of the first invention has Cr and
An alloyed cast iron layer containing a total of 0.3 to 2.0% by weight of at least one of Mo and in which massive graphite is dispersed and crystallized in a pearlite base or a pearlite-based base extends to a depth of at least 0.2 mm from the surface. It is characterized by being formed as follows.

In the manufacturing method of the second invention, after casting a piston rough shape using cast iron as a raw material, Cr and Mo are added to the surface of the piston head portion of the piston rough shape where thermal shock resistance and thermal fatigue resistance are required. at least one or an alloy thereof or Cr;
An alloy of at least one of Mo and Fe is placed, and high-density energy is irradiated from above to cause rapid melting and rapid resolidification to form an alloy layer of cast iron and Cr and/or Mo, and then this alloy The layer is reheated at 800 to 1050°C for 1 to 10 minutes to produce an alloyed cast iron layer containing a total of 0.3 to 2.0% by weight of at least one of Cr and Mo and in which massive graphite is dispersed and crystallized. 0.2 from the surface to the above part
It is characterized by being formed over a depth of mm or more.

Function As the cast iron material that becomes the base material of the piston, JIS FC20, FC25,
Normal cast iron such as FC30 is most preferred, but low alloy cast iron and the like can also be used.

In this invention, the head portion of the piston made of ordinary cast iron as described above has particularly high thermal shock resistance.
An alloyed cast iron layer as described later is formed only in areas where thermal fatigue resistance is required, for example, hot spots.

To explain this hot spot part, FIG. 1 shows an example of a piston for a direct injection type diesel engine.The piston body 1 is made of cast iron cast by sand casting or the like, and the piston head part 2 The cross-hatched area from the upper part of the inner wall surface to the edge of the concave nozzle 3 formed on the top surface corresponds to the hot spot 4 which directly receives the fuel injection and reaches the highest temperature. FIG. 2 also shows a hot spot portion 4 of a piston head portion 2 of another example of a direct injection type diesel engine piston. Furthermore, FIG. 3 shows a hot spot portion 4 of a piston head portion 2 of a conventional piston for a diesel engine.

In areas where thermal shock resistance and thermal fatigue resistance are especially required, such as the above-mentioned hot spot portion 4, the matrix structure is pearlite or pearlite-based and massive graphite containing at least one of Cr and Mo in a total of 0.3 to 2.0% by weight. An alloyed cast iron layer is formed in which the particles are uniformly dispersed and crystallized. Here, Cr and Mo are strong carbide-forming elements, and therefore the matrix of the alloyed cast iron layer becomes pearlite reinforced with cementite or a pearlite-based structure.

In the case of ordinary cast iron, which has traditionally been used for pistons, the matrix is usually pearlite, and in this case, as hot spots of the piston become hot during use, the cementite gradually decomposes into graphite and ferrite. At this time, it is thought that volumetric expansion occurred and this expansion increased thermal stress in hot spots and the like, leading to cracking.
Furthermore, since the crystallized graphite in conventional ordinary cast iron pistons is flaky, the flaky graphite tends to become the starting point or propagation part of cracks due to thermal stress, etc., and this has also been a cause of crack occurrence. However, the alloyed cast iron layer in the piston of the present invention is particularly composed of Cr and/or
Alternatively, the cementite in the pearlite structure of the base is strengthened and stabilized by alloying with Mo, so there is less risk of cracks occurring due to cementite decomposition and volume expansion, and the growth of cracks is also slow. be. Furthermore, since the crystallized graphite is in the form of a lump, the graphite is less likely to become a starting point or a propagation part of a crack. Therefore, the thermal shock resistance and thermal fatigue resistance of the areas such as hot spots where the alloyed cast iron layer is formed are significantly improved.

Here, the reason for alloying Cr and/or Mo is to use their strong tendency to form carbides to strengthen and stabilize the cementite in the pearlite structure of the base, thereby preventing cracking. However, if the total content is less than 0.3% by weight, the effect of stabilizing cementite and preventing cracking will not be sufficiently achieved, while if it exceeds 2.0% by weight, free cementite will remain. The total content should be within the range of 0.3 to 2.0% by weight, since there is a strong tendency that a pearlite structure cannot be obtained, leading to a decrease in toughness.

Further, the alloyed cast iron layer as described above needs to be formed at a depth of at least 0.2 mm from the surface at hot spots and the like.

If the depth is less than 0.2 mm, the above-mentioned effects cannot be sufficiently obtained.

Next, a method for manufacturing a piston having an alloyed cast iron layer as described above, that is, a second invention will be described.

First, the piston rough shape may be manufactured by using a cast iron material such as the above-mentioned ordinary cast iron as a raw material and casting by a normal casting method such as sand mold casting.

The obtained piston rough shape is first subjected to alloying treatment with Cr and/or Mo using a high-density energy source. That is, in the piston head portion of the piston rough shape material, Cr, Mo, or an alloy thereof,
Or Fe-Cr alloy, Fe-Mo alloy or Fe
- By placing a Cr-Mo alloy and irradiating it with high-density energy such as a laser, electron beam, plasma arc, or TIG arc, the Cr and/or Mo alloys placed on the surface and their underlying The cast iron is alloyed with Cr and/or Mo by instantaneously and rapidly melting the surface layer of the cast iron base material on the side, and then the molten alloy layer is instantaneously and rapidly solidified by moving the energy irradiation position or stopping the irradiation. . Here, the mass of the part melted by high-density energy irradiation is much smaller than the mass of the entire piston, so by moving the high-density energy irradiation position or stopping irradiation, heat transfer to the piston base material side The molten alloy layer solidifies instantly,
This results in a chilled alloy layer containing free cementite.

In addition, as a specific method for disposing Cr, Mo, or their alloys, etc. in the hot spot portion of the piston, for example, their powder, green compact,
It may be done by placing a thin plate or the like, by thermal spraying, or by applying it as a slurry, or by cutting a groove in a necessary area and filling it therein.

The chilled alloy layer is then heated to 800~1050
After reheating to a temperature within the range of 1 to 10 minutes, the sample is air cooled, left to cool, or slowly cooled. By applying such reheating treatment, massive graphite crystallizes from the chilled alloy layer, and the base structure becomes a pearlite phase or a pearlite-based structure. That is, as described above, an alloyed cast iron layer having a pearlite phase or a pearlite-based structure as a matrix and in which massive graphite is dispersed is obtained.

Here, if the heating temperature of the reheating treatment for the chilled alloy layer is less than 800℃, uniform and sufficient crystallization of graphite will not be obtained, while if it exceeds 1050℃, graphitization will proceed excessively and pearlite will be formed in the matrix. There is a risk that you will not be able to obtain it. Therefore, the heating temperature is 800 to 1050
The temperature was within the range of ℃. Furthermore, if the heating time is less than 1 minute, graphite will not crystallize sufficiently even at high temperatures close to 1050℃.
On the other hand, if heating is carried out for a long time exceeding 10 minutes, graphitization will proceed excessively and pearlite will not be obtained.
Therefore, the heating time was set to 1 minute to 10 minutes. Note that when the heating time is low, around 800°C, the heating time is preferably a long time of about 10 minutes, and when it is high, around 1050°C, the heating time is preferably short, about 1 minute. Also, even within the above heating temperature range, especially
Heating at 950-1050°C is more desirable.

In the above heat treatment, it is preferable to apply local heating in which only a portion of the chilled alloy layer is heated, but in some cases, the entire piston may be heated. As a specific heating means for local heating, high frequency induction heating, fire heating (burner heating), etc. can be used, and in the case of heating the entire piston, furnace heating can be used.

As described above, after forming a chilled alloy layer of Cr and/or Mo and cast iron on the surface layer through alloying and chilling treatment using high-density energy, reheating treatment is performed to form cementite. It is possible to form an alloyed cast iron layer in which massive graphite is dispersed and crystallized in a matrix consisting of a pearlite phase or a pearlite-based structure, which is strengthened and stabilized, at desired locations such as hot spots.

After the above-mentioned reheating treatment, mechanical processing such as grinding or polishing may be performed as appropriate to finally form the product piston shape.

Examples Example 1 JIS FC25 cast iron was melted and an experimental cast iron disk having an outer diameter of 120 mm and a thickness of 50 mm was created by sand casting.
This was machined into a disk shape with an outer diameter of 100 mm and a thickness of 5 mm, and Cr was sprayed to a thickness of 0.1 mm using plasma spraying within a 20 mm diameter area at the center of the plate surface. Alloying and chilling treatments were performed from above using the TIG arc method using an AC TIG power source. In addition
The TIG arc current was 180A. Next, a high-frequency induction heating coil was placed near the surface of the chilled alloy layer, and a reheating treatment was performed in which the chilled alloy layer was heated to 1000° C. for 2 minutes and then allowed to cool. Thereafter, it was machined into a thermal shock test piece having a small hole (inner diameter 5 mm) 5 in the center as shown in FIGS. 4A and 4B.

After the above-mentioned reheating treatment, we observed the metallographic structure within a 20 mm diameter area as shown in Figure 5 (400 magnification).
As shown in Fig. 2), it was found that an alloyed cast iron layer was obtained in which the matrix was made of pearlite reinforced with cementite and in which massive graphite crystallized. The analysis revealed that the Cr concentration in the alloyed cast iron layer in this case was 0.8% by weight. For comparison, alloying, chilling and reheating using TIG arc as described above were not performed.
The structure of FC25 cast iron is shown in Figure 6 (400x magnification).

Thermal shock test specimen (invention material 1) obtained as described above and FC25 without alloying/chilling by TIG arc and reheating treatment for comparison.
A thermal shock test piece of cast iron of the same shape (comparison material) was subjected to a thermal shock test by repeating heating-cooling cycles. The heating in this test was performed using a propane-oxygen burner at a heating rate of 11.5°C/sec.
The heating temperature was 450°C, and cooling was performed using cooling water with a water temperature of 18°C.

In this thermal shock test, if the thermal shock resistance is low, a crack will occur from the small hole 5 in the center due to the repeated thermal stress of expansion and contraction in the circumferential direction due to heating and cooling, and the crack will further expand. do. The test results are shown in FIG. As is clear from FIG. 7, compared to the comparison material of FC25 cast iron that was not treated according to the present invention, the number of thermal shocks that were repeated before cracking occurred in the material 1 of the present invention was significantly higher. It can be seen that the thermal shock resistance is extremely excellent.

Example 2 Alloying/chilling treatment and reheating treatment were performed under the same conditions as in Example 1 except that Mo was used instead of Cr in Example 1. The Mo concentration in the alloyed cast iron layer in that case was 1.1% by weight. The results of a thermal shock test similar to that of Example 1 (inventive material 2) are also shown in FIG. It can be seen that this case also exhibits excellent thermal shock resistance comparable to or better than the case of Cr alloying in Example 1.

Example 3 As a piston material, ordinary cast iron of JIS FC25 with 0.02% cerium (Ce) added as a deoxidizing agent was melted and made by sand casting method to create a piston with the shape shown in Figure 1 for a direct injection diesel engine. The rough shape of the piston was cast. The diameter of this piston rough shape is
120mm, and the inner diameter of the nozzle part is 45mm.

Next, Cr was sprayed to a thickness of 0.1 mm by plasma spraying on the hot spots from the upper part of the inner surface of the nozzle part to the edges of the rough piston material, and then alloyed and chilled using the TIG arc method with an average current of 210A. A chemical treatment was applied. After that, using a high-frequency induction heating device, the alloyed/chilled part was heated to 1000℃ for 2 minutes and then allowed to cool. The piston has a nozzle diameter of 48mm. This piston has an alloyed cast iron layer based on pearlite, as shown in FIG. 5 of Example 1, formed on the edge of the nozzle up to 58 mm in diameter and within the nozzle up to a depth of 8 mm from the top surface. It was confirmed that An analysis of the Cr concentration in this alloyed cast iron layer revealed that it was 0.7% by weight.

The piston obtained as above was installed in a 3.5 direct injection diesel engine, and
It was subjected to a durability test by repeating the cycles of [× full load × 20 minutes] and [idling × 10 minutes]. As a result, it was confirmed that no trouble occurred with the piston for up to 500 hours. On the other hand, for comparison, when we conducted the same durability test on an FC25 cast iron piston of the same size without the above-mentioned treatment, many thermal cracks were observed in the hot spot area of the nozzle after 300 to 400 hours. . From these results,
It can be seen that the piston according to the present invention has excellent thermal shock resistance and thermal fatigue resistance.

Example 4 A piston for a direct injection type diesel engine was prepared using Mo instead of Cr in Example 3, and under the same conditions as in Example 3 except for the following conditions. Analysis showed that the Mo concentration in the alloyed cast iron layer at the hot spot in this case was 0.8% by weight. Furthermore, when subjected to the same durability test as in Example 3, it was confirmed that it exhibited excellent durability almost the same as in the case of Cr alloying.

Effects of the Invention The cast iron piston of the present invention strengthens the cementite by alloying Cr and/or Mo in the piston head, especially in the hot spot area where thermal shock resistance and thermal fatigue resistance are required.・Since an alloyed cast iron layer with stabilized pearlite or pearlite-based matrix structure and crystallized massive graphite is formed, the thermal shock resistance and thermal fatigue resistance of that part are significantly improved. Therefore, there is extremely little risk of cracks occurring in the hot spot portion due to thermal stress during use of the piston, making it possible to prolong the life of the piston, and also to increase the temperature of the combustion chamber and thus increase the output. In addition, the piston of the present invention is reinforced only in the parts where thermal shock resistance and thermal fatigue resistance are particularly required, and inexpensive ordinary cast iron or the like can be used as the base material of the piston. The cost of raw materials can be reduced, and there is also less risk of impairing workability or castability. Further, according to the manufacturing method of the present invention, a cast iron piston having the excellent advantages as described above can be obtained simply and easily.

[Brief explanation of the drawing]

1 to 3 are longitudinal cross-sectional views showing an example of a diesel engine piston to which the present invention is applied, and FIGS. 4A and 4B are views showing thermal shock test pieces in the example. Longitudinal cross-sectional view, B is a plan view, Figure 5 is a photograph of the metal cross-sectional structure of the alloyed cast iron layer of the present invention material in the example, Figure 6 is a photograph of the metal cross-sectional structure of the comparative material, and Figure 7 is the thermal shock test result. FIG. 1... Piston, 2... Piston head section, 4
...Hot spot part (part where alloyed cast iron layer is formed).

Claims (1)

[Scope of Claims] 1. Cr is added to the piston head of a cast iron piston in a region where thermal shock resistance and thermal fatigue resistance are required.
and Mo in total from 0.3 to
The alloyed cast iron layer containing 2.0% by weight and in which massive graphite is dispersed and crystallized in a pearlite base or pearlite-based base is at least 0.2% by weight from the surface.
A cast iron piston for internal combustion engines, characterized by being formed over a depth of mm or more. 2. After casting a piston rough shape using cast iron as a raw material, at least one of Cr and Mo, or an alloy thereof or At least one of Cr and Mo
An alloy layer of cast iron and Cr and/or Mo is formed by placing an alloy with Fe and irradiating it with high-density energy to cause rapid melting and rapid resolidification. Cr, Mo
A cast iron product, characterized in that an alloyed cast iron layer containing a total of 0.3 to 2.0% by weight of at least one of the above and in which massive graphite is dispersed and crystallized is formed in the above region to a depth of 0.2 mm or more from the surface. A method for manufacturing pistons for internal combustion engines.