JPS62170486A - Cast iron piston for internanl combustion engine and its production - Google Patents

Abstract

PURPOSE:To simply and easily produce an inexpensive piston having excellent resistance to thermal impact and thermal fatigue by disposing Al or Si to the prescribed section of the cast iron piston and irradiating high-density energy thereto, then subjecting said section to an adequate reheating treatment. CONSTITUTION:The Al and/or Si or the alloy thereof or the alloy with Fe is disposed on the surface of the section in the piston heat part of a rough shape material for the cast iron piston where the resistance to thermal impact and thermal fatigue is required. The high-density energy such as TIG arc is then irradiated thereon from above to form the alloy layer of the cast iron and the Al and/or Si by quick melting-quick resolidifying. The alloy layer is thereafter subjected to the reheating treatment for 1min-3hr at 800-1,100 deg.C. The alloyed cast iron layer in which the Al and/or Si is incorporated at 0.1-10wt% and fine block graphite is dispersed and crystallized is thereby forming to the above-mentioned section down to >=0.2mm depth from the surface and the cast iron piston having the partially improved resistance to thermal impact and thermal fatigue for an internal combustion engine is obtd.

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. This article relates to a cast iron piston whose surface layer is locally reinforced and a method for manufacturing the same.

Although conventional pistons for internal combustion engines made of cast iron are heavier than pistons made of aluminum alloy, they generally lack strength at high temperatures and are therefore used in large diesel engines and the like.

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 temperatures. 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, it has become desirable for the fuel chamber to be at a higher temperature of -1. There are increasing demands for improvement.

However, many conventional conventional cast iron pistons are made of ordinary cast iron, and it is difficult to fully meet the above requirements with such ordinary cast iron pistons.
The occurrence of cracks in the hot spot due to thermal stress, etc. cannot be avoided, and therefore the service life of the piston has to be shortened. Therefore, recently, thermal shock resistance,
It is also practiced to make screws from high-grade cast iron, such as alloyed cast iron, which is highly resistant to heat fatigue.

Problems to be Solved by the Invention As mentioned 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 area, and other parts require high-grade cast iron such as alloyed cast iron. is not required, and therefore, constructing the entire piston from expensive alloyed cast iron or the like will result in unnecessary loss. 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. In the case of rough alloy cast iron, machinability is poor and machining after casting may be difficult. In order to solve this problem, the piston is made of cast iron, which is inexpensive and has good castability and workability, such as ordinary cast iron, and parts of the piston, such as the hot spot part of the piston head, are strengthened. It is desirable to apply the method.

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-mentioned circumstances, and uses cast iron as a base material and partially strengthens the parts of the piston head part where thermal shock resistance and thermal 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 at least one of A1 and Sl in the piston head portion of the cast iron piston at a portion where thermal shock resistance and thermal fatigue resistance are required. An alloyed cast iron layer containing a total of 0.1 to 10% by weight of fine massive graphite or dispersed crystallization is formed over a depth of at least 0.2 m from the surface. be.

Second invention I&! How to make screws using cast iron as raw material
After casting the piston rough shape, one of Al and Si, an alloy thereof, or at least Al and Si is applied to the surface of the piston head of the piston rough shape where thermal shock resistance and thermal fatigue resistance are required. Cast iron and A1 and /
Alternatively, form an alloy layer with Sl, and then add 8 to this alloy layer.
Al2. An alloyed cast iron layer containing a total of 0.1 to 10% by weight of at least one of s and r and in which fine massive graphite is dispersed and crystallized is formed in the region to a depth of 0.2 m or more from the surface. That is.

The cast iron material used as the base material of the working piston is JIS Fe12. Fe12,F
Normal cast iron such as e12 is most preferred, but low alloy cast iron etc. can also be used.

In this invention, (a) only the parts of the head part of the piston made of ordinary cast iron such as small ones that require particular thermal shock resistance and thermal fatigue resistance, such as hot spot parts; A cast iron layer is formed.

To explain this hot spot part, Fig. 1 shows an example of a piston for a direct injection type diesel engine, and the piston body 1 is made of cast iron cast by sand casting or the like, and the piston head part is made of cast iron. A 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 of the nozzle 2 corresponds to a hot spot 4 that directly receives fuel injection and reaches the highest temperature. Further, FIG. 2 shows a hot spot portion 4 of a piston head portion 2 of another example of a piston for a direct injection type diesel engine. FIG. 3 roughly 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 particularly required, such as the above-mentioned hot spot area 4, at least one of Al and Si is contained in a total of 0.1 to 10% by weight, and fine lumpy graphite is uniformly distributed. A layer of dispersed crystallized alloyed cast iron is formed. Here, Ai', sr has a significantly strong tendency to form ferrite, and therefore the 71~ ricks of the alloyed cast iron layer becomes a ferrite 1~ or ferrite-based structure.

In the case of ordinary cast iron, which has traditionally been used for pistons, the base is usually pearlite structure, and in this case, as the piston's horatosbo part 1 to 1 becomes hot during use, cementite gradually turns into graphite and ferrite. It is believed that this decomposition caused volumetric expansion, which increased thermal stress in hot spots and the like, leading to cracking.

In addition, in conventional ordinary cast iron pistons, the crystallized graphite is flaky and large in size, making the flaky graphite easy to become the starting point and propagation point for cracks due to thermal stress, etc., which also causes cracks to occur. . However, the alloyed cast iron layer in the piston of the present invention has a stable ferrite or ferrite-based structure due to the alloying of Al and/or Si, and this ferrite phase has high toughness, so it may cause cracking. There is less risk, and the growth of cracks is also slow. Furthermore, since the shape of crystallized graphite is lumpy and fine, graphite is unlikely to become a starting point or a propagation part for cracks. Therefore, the hot spot areas and other areas where the alloyed cast iron layer is formed can be expected to have significantly improved thermal shock resistance and thermal fatigue resistance in terms of 81S.

Here, the reason for alloying Al and/or Sl is
As already mentioned, the base is made of ferrite or ferrite mainly by taking advantage of its large tendency to generate ferrite, thereby stabilizing the base structure, improving the toughness of the base, and preventing cracking. If the content is less than 0.1% by weight in total, these effects will not be fully maintained, while if it exceeds 10% by weight, it will become brittle, so the total content should be in the range of 0.11 to 10% by weight. Is it necessary to go inside?

Also, the alloyed cast iron layer (above) should be at least 0.2 mm from the surface to be drilled in hot spots, etc. A necessary hole is formed over a depth of Mi or more.

If the engine speed is less than 0.2mpt, the effects of (a) and (e) cannot be obtained sufficiently.

In addition, it is preferable that the average particle size of the fine lumpy graphite in the alloyed cast iron layer is 20 JJm or less.

By setting the thickness to 20 pm or less in this way, the effect of preventing crack generation can be more reliably 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 can be manufactured by casting using a cast iron material such as the above-mentioned ordinary cast iron as a raw material by a normal casting method such as sand casting.

The obtained piston rough shape is first subjected to alloying treatment with Al and/or Sl using a high-density energy source. In other words, A1 or Si or their alloys, or Fe-A1 alloys or Fe are applied to the surface of the piston head portion of the piston rough profile, in areas such as hot spots where thermal shock resistance and thermal fatigue resistance are particularly required. -Si
By placing an alloy or a Fe-A1-Si alloy and irradiating it with high-density energy such as a laser, electron beam, plasma arc, or TIG arc, Al and/or Si placed on the surface and its lower side can be removed. The cast iron is alloyed with Al and/or Sl by instantaneously and rapidly melting the surface layer of the cast iron base material, and then by moving the energy irradiation position or stopping the irradiation, the molten alloy layer is instantaneously and rapidly solidified. 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 the irradiation, the part melted due to heat transfer to the piston base material side. The alloy layer solidifies instantly and becomes an alloy layer having a fine structure.

Here, specific methods for disposing Al, Si, or their alloys, etc. in hot spots of the piston include, for example, placing or thermal spraying their powder, green compact, thin plate, etc., or spraying a slurry. It can be applied as a liquid, or it can be roughly processed into grooves in the necessary areas and filled in.

The alloy layer is then reheated to a temperature in the range of 800-1100°C for 1 minute to 3 hours, and then preferably heated to 50°C/
Cool at a cooling rate of 9 minutes or less, then cool naturally or slowly. By applying such reheating treatment, fine lumpy graphite crystallizes from the alloy layer, and the base structure becomes a ferrite phase or a ferrite-based structure. That is, as described above, an alloyed cast iron layer having a ferrite phase or a ferrite-based structure as a matrix and in which fine massive graphite is uniformly dispersed can be obtained.

Here, the heating temperature of the reheating treatment for the alloy layer is 800°C.
If it is below 1100°C, uniform and sufficient crystallization of graphite cannot be obtained, while if it exceeds 1100°C, there is a risk that partial eutectic melting may occur and graphite may be coarsely crystallized. Therefore, the heating temperature was within the range of 800 to 1iooC. Furthermore, if the heating time is less than 1 minute, graphite will not crystallize sufficiently even at high temperatures close to 1100°C, while even if heating is performed for a long time exceeding 3 hours, graphite crystallization will reach saturation, resulting in an increase in economic costs. Therefore, the heating time is 1
The duration was from 1 minute to 3 hours. Note that if the heating temperature is low, around ao o 'c, the heating time will be long, close to 3 hours, and
When the temperature is high, around 100° C., the heating time is preferably as short as about 1 minute. Also, within the above heating temperature and time ranges, it is particularly desirable to heat at 950 to 1050°C for 1 to 10 minutes. On the other hand, cooling after heating is
In order to make the base structure uniform ferrite, 50″C/
Although it is desirable that the cooling rate be less than 50°C/min, in this invention, Al and/or Sl, which have a strong tendency to form ferrite, are added, so even if the cooling rate exceeds 50°C/min, the base structure will be made mainly of ferrite. It is possible. In the above heat treatment, it is preferable to apply local heating in which only a portion of the alloy layer is heated, but depending on the case, the entire bislin may be heated. Specific heating means for local heating include high frequency induction heating and flame 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 an alloy layer of Al and/or 11' and cast iron is formed on the surface layer by alloying treatment using high-density energy, fine lumpy graphite is transformed into a ferrite phase or An alloyed cast iron layer uniformly dispersed and crystallized in a matrix consisting of a ferrite-based structure can be formed at desired locations such as hot spots.

Note that 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 m and a thickness of 50 m was created by sand casting.

This was machined into a disk shape with an outer diameter of 100 mm and a thickness of 5 m, and the center of the plate surface had a diameter of 20 mm. Within this range, Al was sprayed to a thickness of 0.2 m by plasma spraying, and then alloying treatment was performed thereon by TIG arc method using an AC TIG power source. The TIG arc current is 180A.
And so. Next, a reheating treatment was performed in which the material was heated at 1050'CX for 10 minutes using an electric furnace and then slowly cooled at a cooling rate of 10.degree. C./minute. Then, by machining, Fig. 4 (A>, (B))
A thermal shock test piece having a small hole (inner diameter 5 mm>5) in the center was fabricated as shown in FIG.

When we observed the metal structure within the center diameter of 20InIn after the above-mentioned reheating treatment, we found that Fig. 5 (400x magnification)
As shown in Figure 2, it was found that a layer of fine lumpy graphite with a ferrite matrix and an average grain size of 3 pm was uniformly formed into a layer of alloyed cast iron. In this case, the A18 degree of the alloyed cast iron layer was found to be 2.6% by weight as a result of analysis. For comparison, the structure of C25 cast iron is shown in FIG. 6 (400x magnification) without the above-mentioned TIG arc alloying and reheating treatment. In this case, it can be seen that the pearlite structure occupies a considerable portion of the base, and large flaky graphite crystals are precipitated.

A thermal shock test piece obtained as described above (present invention material) and a thermal shock test piece of the same shape made of FC25 cast iron (comparison material) that was not subjected to alloying and reheating treatment by TIG arc for comparison. A thermal shock test was conducted using repeated 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 expand roughly. . The test results are shown in FIG. As is clear from FIG. 7, compared to the comparative material of FC25 cast iron which was not treated according to the present invention, the material of the present invention has a much higher number of thermal shock cycles before cracking occurs, and therefore has a higher thermal shock resistance. It turns out that it is significantly superior.

Example 2 Alloying treatment and reheating treatment were performed under the same conditions as in Example 1 except that 81 was used instead of F10 in Example 1. The Si concentration of the alloyed cast iron layer in that case was 2.1% by weight. Further, the results of a thermal shock test similar to that in Example 1 are also shown in FIG. Also in this case, Example 1
It can be seen that it exhibits excellent thermal shock resistance comparable to or better than that of the A1 alloy.

Example 3 As a screw material, ordinary cast iron of JIS Fe12 to which 0.02% of cerium (Ce) was added as a deoxidizing agent was melted and directly molded into a shape as shown in Fig. 1 by sand casting. A rough profile of a screw for an injection type diesel engine was cast. The diameter of this piston rough shape is 120#, and the inner diameter of the nozzle part is 45#.

Next, 0.2% of Al was applied to a portion of the Holatos box 1 from the upper part of the inner surface of the nozzle part to the edge part of the rough piston material by plasma spraying. It was sprayed thickly and then alloyed by the TIG arc method with an average current of 210A. Then, using a high-frequency induction heating device, the alloyed part was heated at 1050°C for 10 minutes, cooled at a cooling rate of 10°C/min, and roughly machined to an outer diameter of 115 Irun1. Nozzle diameter 48.
Finished with 1 screw.

The diameter of the edge of the spout is 58. The inside of the nozzle part is filled with alloyed cast iron having a ferrite base as shown in FIG.
J), it was confirmed that a layer was formed. When the A18 degree of this alloyed cast iron layer was analyzed, it was found to be A11.8.
% by weight.

The piston obtained as described above was installed in a 3.51 direct injection diesel engine and subjected to a durability test by repeating the cycles of [350 rpm x full load x 20 minutes] and [idling x 10 minutes]. As a result, it was confirmed that no trouble occurred in the piston for up to 500 hours. On the other hand, for comparison, when the same durability test was conducted 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 at the mouth of the piston after 300 to 400 hours. . These results show 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 produced using Si in place of A1 in Example 3 and keeping the other conditions the same as in Example 3. In this case, the hot spot part 8
As a result of analysis, the i concentration was found to be 1.9% by weight. Further, when subjected to the same durability test as in Example 3, it was confirmed that it exhibited excellent durability almost the same as that of Al.

Effects of the Invention The cast iron piston of the present invention has a matrix formed by alloying Al and/or Si in areas of the piston head where thermal shock resistance and thermal fatigue resistance are required, such as hot spot areas. Since an alloyed cast iron layer consisting mainly of ferrite or ferrite with fine lumpy graphite uniformly dispersed is formed, the thermal shock resistance of that part is improved.
Thermal fatigue resistance has been significantly improved, so there is extremely little risk of cracks occurring in the -C hot spot due to thermal stress during piston use, making it possible to extend the life of the piston and prevent the high temperature of the combustor from forming. Therefore, it is possible to achieve higher output. In addition, the piston of this invention is reinforced only in the parts where thermal shock resistance and thermal fatigue resistance are particularly required, and inexpensive ordinary cast iron etc. can be used as the base material of the piston, so the raw material cost of the piston is reduced. In addition to being able to reduce the cost, it is also possible to obtain effects such as there is little risk of impairing workability or castability. Further, according to the manufacturing method of the present invention, a cast iron piston having the above-mentioned advantages can be simply and easily obtained.

[Brief explanation of drawings]

1 to 3 are longitudinal sectional views showing an example of a diesel engine piston to which the present invention is applied, and FIG.
Figures (A) and (B) are diagrams showing thermal shock test pieces in Examples, where (A) is a longitudinal cross-sectional view, (B) is a plan view, and Figure 5 is an alloying of uninvented materials in Examples. FIG. 6 is a photograph of the metal cross-sectional structure of the cast iron layer, FIG. 6 is a photograph of the metal cross-sectional structure of the comparative material, and FIG. 7 is a diagram without showing the thermal shock test results. 1... Piston, 2... Piston head part,
4...Hot spot portion (portion where alloyed cast iron layer is formed).

Claims (2)

[Claims] (1) Al and Si are added to the parts of the piston head of a cast iron piston that require thermal shock resistance and thermal fatigue resistance.
An alloyed cast iron layer containing a total of 0.1 to 10% by weight of at least one of these and in which fine massive graphite is dispersed and crystallized is formed over a depth of at least 0.2 mm from the surface. A cast iron piston for internal combustion engines. (2) After casting the piston rough shape using cast iron as raw material,
Al and S are applied to the surface of the piston head portion of the piston rough profile where thermal shock resistance and thermal fatigue resistance are required.
cast iron and Al and/or Si by placing one of i or an alloy thereof or an alloy of at least one of Al and Si with Fe and irradiating high-density energy from above to rapidly melt and resolidify the cast iron and Al and/or Si. This alloy layer is then subjected to reheating treatment at 800 to 1100°C for 1 minute to 3 hours to contain at least one of Al and Si in a total of 0.1 to 10% by weight. A method for manufacturing a cast iron piston for an internal combustion engine, characterized in that an alloyed cast iron layer in which fine lumpy graphite is dispersed and crystallized is formed in the region to a depth of 0.2 mm or more from the surface.