KR100264673B1 - Surface hardening of gray cast iron for automotive cylinder block using high energy electron beam irradiation - Google Patents

Abstract

PURPOSE: A method for hardening the surface of gray cast iron material used for engine block material by projecting electron beam with a specified amount of projection energy density is provided, which can conduct operations in atmospheres, improves productivity and prevents oxidation of material surface. CONSTITUTION: A high voltage accelerated electron beam is projected with an amount of projection energy density of 1.2 to 2.1KW/cm¬2 to the surface of gray cast iron material used for engine block material comprising at least one component selected from the group consisting of 2.0 to 4.0% by weight of C, 1.0 to 3.0% by weight of Si, 1.0% or less by weight of Mn, Cr, Cu, P and S; and the balance of Fe to form a surface hardening layer having a Vickers hardness of 600 to 850 VHN and comprising martensite.

Description

Surface hardening method of gray cast iron material for engine block using high voltage accelerated electron beam

The present invention relates to a method for hardening the surface of gray cast iron material for an engine block, and more particularly, to a method for hardening the surface of a gray cast iron material for an engine block of a vehicle using a high voltage accelerated electron beam.

Gray cast iron typically contains 2.0 to 4.0% by weight of C and 1.0 to 3.0% by weight of Si, and other small amounts of elements such as Mn, S, and P, and the balance is Fe. Is ferrite and / or pearlite, and in particular the texture state of the gray cast iron material used for the engine block of the vehicle is primarily pearlite. Gray cast iron is not only easy to manufacture inexpensive and complicated castings, but also has excellent hardness and wear resistance due to the large amount of flake graphite inside the tissue, but also has excellent cutting ability, adaptability to lubricants, and vibration damping ability. It is excellent in compressive strength, dimensional stability, and so on, and is widely used in automobile parts such as engine blocks of vehicles.

Conventionally, the flame hardening method, the high frequency induction hardening method, etc. are mentioned as a conventional surface hardening method of such steel materials. Despite the advantages that these surface hardening methods are relatively economical and have a relatively good technical accumulation, they are low in energy efficiency, easy to form an oxide layer on the surface, and may be accompanied by deformation of the material itself due to surface distortion. In the case where the shape is complicated or only local surface hardening is required, the application of these methods is difficult and there are serious problems such as the necessity of using a refrigerant for cooling the material.

As a conventional method for solving the above problems, a surface hardening method using a laser has recently been proposed, but this method has a problem of inferior productivity due to low thermal efficiency of less than 40% and low processing speed, thus resulting in large volume production. The application to priority industrial sites has a difficult problem.

In recent years, when high-output focusing energy obtained by accelerating an electron beam is directly projected onto the surface of a material, high kinetic energy of electrons strikes a material lattice to form phonons, which are instantly converted into thermal energy, and thus can be used as a powerful heat source. The surface of the alloy can be hardened by projecting an electron beam having high energy to it, and the degree of hardening and the depth of hardening vary depending on the composition of the alloy, in particular, the amount of carbon, the process conditions during the electron beam projection, and the amount of heat input during the projection. The general fact that it can be changed is reported (see Korean Journal of Metals, vol.31, pp.1993). However, studies on the surface hardening of metal alloy materials using high voltage accelerated electron beams are currently insufficient. In particular, proper process parameters during acceleration electron beam projection to achieve optimum surface hardening in gray cast iron materials have been established. There is no.

The present inventors have studied to develop a surface treatment method using an accelerated electron beam of a steel alloy material that can be processed in a relatively short amount of time with a low cost, and as a result, a specific amount of the gray cast iron material used as a material for engine block of a vehicle By hardening the surface by projecting an electron beam at the input energy density, the surface hardness is increased by 2 to 3 times by the hardening mechanism known as martensite transformation, which increases the wear resistance by 3 to 4 times. From the fact that the durability of engine parts can be dramatically improved, the present invention has been completed to establish the optimum process parameters in the acceleration electron beam projection for the gray cast iron material described above.

Accordingly, an object of the present invention is to effectively martite the microstructure of the projection surface layer of gray cast iron material by establishing an optimal process parameter in the acceleration electron beam projection for the gray cast iron material used as the engine block material of the vehicle. The present invention provides a method for hardening the surface of a gray cast iron material for an engine block using a high voltage accelerated electron beam, which can significantly improve the surface hardness value and the wear resistance thereof.

According to a preferred aspect for achieving the above object of the present invention, the surface of a gray cast iron material for automotive engine blocks comprising 2.0 to 4.0 wt% carbon, 1.0 to 3.0 wt% silicon and trace amounts of other alloying elements A high voltage acceleration electron beam in the range of 1.0 to 2.5 McV at a projected energy density of 1.2 to 2.1 kW / cm 2, preferably 1.5 to 2.0 kW / cm 2, particularly preferably 1.6 to 1.95 kW / cm 2. A method of surface hardening of gray cast iron material for an engine block using an electron beam is provided.

1 (a), (b), (c), and (d) are enlarged cross-sectional photographs by an optical microscope showing a changed state of microstructure with depth after projecting a high energy electron beam.

2 (a), (b) and (c) are enlarged photographs by an optical microscope showing the changed microstructure state after projecting a high energy electron beam.

3 is a graph showing the change in hardness with depth from the material surface in the preferred embodiments of the present invention.

4 is a graph showing the improvement of wear resistance in preferred embodiments of the present invention.

First, the preferred composition of the gray cast iron material in the method of the present invention is C: 2.0 to 4.0% by weight, Si: 1.0 to 3.0% by weight, the balance is Fe, in addition to Mn, S, P, Cr, It may contain 1 weight% or less of 1 type or more components chosen from the group which consists of Cu. However, such an alloy component composition to which the present invention is applied is not limited, and may be used as the gray cast iron material for an engine block of a vehicle in general.

In the gray cast iron material to which the present invention is applied, the most important essential alloy components for determining the mechanical properties thereof are C and Si, and since gray is present in the form of flake graphite in gray cast iron, it is abrasion resistance and resistance to sliding friction. It has excellent adhesion and damping ability. Si is a strong graphite stabilizer that precipitates free carbon (graphite) in iron or promotes graphite formation by graphitization which decomposes cementite (Fe ₃ C) into free carbon and iron.

C is preferably 2.0 to 4.0% by weight, and when the content of C is less than 2.0% by weight, iron carbide, i.e., cementite structure, which is a hard and fragile compound, is increased, so it is not preferable. It is also undesirable because of the coarsening and the increase of the network structure, which lowers the strength, weakens the base, and causes internal notches or internal cracks.

Si, which is a component that promotes graphitization alone in gray cast iron, is preferably in the range of 1.0 to 3.0% by weight, and when it is less than 1.0% by weight, the hard and fragile cementite structure is increased. It is also not preferable because the strength decreases due to the coarsening of the flake graphite and weakens the base.

Mn, S, P, Cr, and Cu as optional components are conventionally included in gray cast iron materials, either alone or as a combination of two or more components, and the composition ratio is usually 1.0 wt. Less than%, but this is not limiting.

The high voltage accelerating electron beam used in the present invention is generated using an electron accelerating device, and conventional conventional accelerating electron beam devices have energy of several tens to hundreds of kV, so that the operation in the vacuum device becomes essential, and therefore it is not suitable for the present invention. In addition, the use of an accelerated electron beam apparatus having a high energy of 1.0 to 2.5 MeV enables electron beam transmission in the atmosphere, so that work can be performed in the atmosphere, and therefore it is preferable because it can be easily applied to industrial sites. In the case of using a device having an energy of less than 1.0 MeV, it is difficult to work in the air or it takes a relatively long time to harden gray cast iron material, which may lead to a decrease in working efficiency and an insufficient hardening effect. It is not preferable because it exceeds 2.5MeV, since the surface layer may instantly melt during accelerated electron beam projection to change into a structure similar to white cast iron.

The process conditions and heat input during the accelerated electron beam projection can be expressed as the energy density projected onto the material surface per unit area, that is, the input energy density, and the input energy density is the electron beam power (electron beam energy × beam current), electron beam movement speed, and specimen. It is determined by the size of. In the present invention, the input energy density of the gray cast iron material for the engine block of the vehicle is 1.2 to 2.1 kW / cm 2, preferably 1.5 to 2.0 kW / cm 2, particularly preferably 1.6 to 1.95 kW / cm 2. When the input energy density of less than 1.2 kW / cm 2 is used, the microstructure changes are insufficient, and the surface hardening effect is insignificant. When the input energy density of more than 2.1 kW / cm 2 is used, melting occurs on the material surface. It is also undesirable because additional processing is required.

Typically the structure of the gray cast iron material for the engine block is primarily a ferrite, which is a eutectic of ferrite and cementite.

In order to explain the possible changes in the microstructure of the high-voltage accelerated electron beam projection on such a pearlite matrix, the projection surface with the input energy density of 2.19 kW / cm 2 slightly exceeding the scope of the present invention will be described. From (a) melt zone, (b) transformation zone, (c) heat affected zone, and (d) original zone.

The molten region of (a) has a temperature rise above the melting point by the projection of a high-voltage accelerated electron beam to melt flake graphite and a new dedrite solidified structure and γ + Fe ₃ C of complex form of known structure. ledeburite), which shows a structure similar to typical white cast iron during melting and quenching, but its matrix is mainly composed of martensite with high hardness due to its fast cooling rate. In the present invention, the presence of such a melted zone is not preferable because the surface of the engine block is roughened due to melting and solidification and requires a separate processing treatment.

The transformation region of (b) is a region in which only the known Perlite structure is phase-transformed to martensite having a high hardness value and flake graphite remains almost intact while electron beam projection heat is transferred from the surface to the inside, and the molten region of (a) As long as the temperature does not rise, local melting of the material does not occur, but after forming austenite, which is a high temperature stable phase, it is quenched and transformed into a plate-like martensite, and (a) it enters the interior at the interface of the melting region / (b) transformation region. Increasingly, the plate-like martensite becomes smaller and denser and the amount of retained austenite tends to decrease, leading to higher hardness values. The lower part of the transformation region (b) maintains its original tissue state, and only partially martensite transformation occurs, so this region may be referred to as the partial transformation region (b1). Generation of such regions (b) and (b1) is a preferred region in the present invention.

The heat affected zone of (c) has little difference from the original zone, and the interface of the molten zone of (a) / the transformation zone of (b) is clearly distinguished, but the transformation zone of (b) / the heat of (c) The interface of the affected area is unclear.

The original area of (d) is an area which is not affected by heat, and is an area that is completely different from the tissue in the initial state.

The most influential factor on the wear resistance of gray cast iron is the hardness value, and the fine hardness value for only the matrix structure except the flaky graphite will be examined.

The Vickers hardness value (VHN) of the molten region and the transformation region is increased by two to three times compared to 300 VHN, which is a known hardness before projection.

The Vickers hardness value (VHN) in the melting zone is constant at around 800 VHN, where it rapidly decreases to around 600 VHN at the beginning of the transformation zone, then gradually increases to almost constant and reaches about 850 VHN, then to the heat affected zone where the matrix appears. It tends to decrease sharply again. The three regions exhibiting such a change in hardness value coincide with (a) molten region, (b) transformation region, and (b1) partial transformation region, respectively.

The low hardness at the beginning of the metamorphic region is due to the formation of many soft tissues such as retained austenite around the coarse platen martensite, but even in this case twice as much as the base due to the presence of martensite. High hardness value is shown. In addition, in the transformation region, the hardness value gradually increases as it enters the inside, because the amount of retained austenite decreases and finer and densified plate-like martensite enters the interior of the transformation region. In the partial transformation region, the martensite transformation occurs inside the partially unresolved cementite lamellae, and the variation in hardness value is relatively severe depending on the position. The heat affected zone has a hardness value similar to that of the matrix because the projected heat passes and is transferred to the heat transfer zone, and thus does not involve a large change in the microstructure.

The thickness of the surface hardened layer showing a high hardness value relative to the known has a direct correlation with the beam input energy density.

As the beam injection energy density increases, the hardness of the projection surface hardened layer is increased, and as the thickness thereof is increased, wear resistance is greatly improved, and abrasion resistance is improved by 3 to 4 times by high voltage accelerated electron beam surface treatment.

As described above, by improving the microstructure of the gray cast iron projection surface layer for the engine block of the vehicle by the high energy accelerated electron beam treatment, the surface hardness can be greatly increased, and the thickness of the hardened layer can be adjusted. It can greatly contribute to improving wear resistance of engine parts.

In the case of using a relatively high input energy density of about 2.1 to 2.2 kW / cm 2 which slightly exceeds the range in the present invention, the surface is slightly melted to change into a white cast iron structure having a high hardness. When the hardened layer by this is included, the surface hardened layer of about 2 mm can be obtained. However, in this case, it is not preferable because additional processing on the surface layer is required due to the surface non-uniformity by melting of the surface.

When using a lower input energy density of about 1.5 to 2.0 kW / cm 2, which is a preferable range in the present invention, a surface hardened layer of about 0.5 to 1.5 mm can be obtained by martensitic transformation without melting the surface. In addition, since a uniform surface can be obtained without oxidation, direct use of the final product without further processing is preferable.

In addition, martensiticization of the pearlite matrix hardens the surface of gray cast iron, and also improves fatigue, fracture, and high temperature deformation characteristics mainly caused by flake graphite.

EXAMPLE

Hereinafter, the present invention will be described in more detail with reference to examples, and it is necessary to understand that these examples are not intended to limit the present invention but are merely examples of the best embodiments.

[Preparation of Psalms]

The gray cast iron material (FCH2D) specimen for the engine block of the vehicle used in the following Examples and Comparative Examples was cut to 30 mm x 30 mm by cutting the material to about 19 mm, which is the maximum thickness that can be collected from the finished product made through the casting process. After processing to a size of × 19mm was used as a specimen, the chemical composition was shown in Table 1 below.

TABLE 1

[Projection Conditions of Electron Accelerator and Accelerated Electron Beam]

The high energy accelerated electron beam projection test was conducted using an electron accelerator (model name ELV-6) of the Budker Institute of Nuclear Physics (BINP) of Novosibirsk, Russia.

The energy range of the electron accelerator is 0.5 to 1.5 McV, the maximum electron beam current is 70 mA, and the maximum power is about 100 kW. Table 2 shows the test conditions for the acceleration electron beam projection. Other parameters except the beam current were fixed constantly and only the beam current was changed to change the energy input to the sample surface per unit area, that is, the input energy density, and the width of the projection beam was adjusted by changing the electromagnetic field in the scanning device. The projection width was fixed to 3.2 cm, slightly larger than the specimen so that the entire surface could be projected sufficiently.

TABLE 2

[Test Methods]

(1) microstructure observation

The central part of the specimen to which the electron beam was projected was cut perpendicular to the projection direction, and the cutting of the specimen was minimized by material loss and deformation by using an electro-discharge machine (EDM). The microstructures were observed with an optical microscope and a scanning electron microscope (SEM).

(2) Measurement of phase percentage

Phase fractions of residual austenite, Fe ₃ C carbide, martensite, ferrite, etc. were quantitatively measured using a Mossbauer analyzer. As a source, Co ⁵⁷ gamma rays and thin disk-shaped specimens having a thickness of about 100 μm and a diameter of 10 mm taken from the projection surface layer were used for analysis.

(3) hardness change measurement

The hardness change before and after the accelerated electron beam treatment was measured with the hardness from the surface using a Vickers microhardness tester.

(4) wear resistance evaluation

The abrasion test was carried out in a ring-on-disc manner using a friction wear tester (Model EFM-III-EN / F, Orientec Co., Japan). The upper ring was fabricated by processing a SUS 420 J2 stainless steel with an outer diameter of 25.6 mm and an inner diameter of 20 mm, and the lower disk was processed by 30 mm x 30 mm x 5 mm from the surface of the specimen to which the accelerated electron beam was projected. The accelerated electron beam projection specimen is worn out by mutual friction with the counterpart while the lower rotating shaft rotates while being pressed from the top of the wear tester body.

Wear conditions are 50kgf. The rotation speed was 67 rpm and the wear distance was 1500 m for about 5 hours. No lubricant was used. The wear amount of the accelerated electron beam projection specimen was measured to 0.1mg unit after the test was completed, and averaged by performing the abrasion test three times for each specimen.

[Examples 1-3 and Comparative Examples 1,2]

The specimens having the composition shown in Table 1 above were projected with the input energy density changed by changing the beam current (Examples 1 to 3 and Comparative Examples 1 and 2), and the projection layer in each case. Table 3 shows the depth and the maximum hardness values of.

TABLE 3

*: Each thickness of the melt zone, transformation zone and partial transformation zone is 900/850/300.

**: same as original known hardness value

(1) Example 1

In the case of Example 1, it was projected at an input energy density of 1.93 W (kW / cm 2), and as can be seen from the optical micrograph shown in FIG. 1 (a), the melting region was not visible and only the transformation region was weak. It was formed to a thickness of 700 μm, and the thickness of the partial transformation region was about 150 μm.

As can be seen from the optical micrographs shown in FIGS. 2 (a) and 2 (b), it can be seen that flake graphite remains as it is and only the matrix structure is changed from pearlite to martensite. The size tends to be smaller and denser and the amount of retained austenite tends to decrease.

Quantitative phase measurement of residual austenite, Fe ₃ C carbide, martensite, ferrite, etc. using the Mossbauer analyzer showed that the transformation region was 79.5% martensite, 11% austenite, and 7.3% Fe _3- xC. ₁ + x (reported to precipitate in twin-boundry of martensite as an orthorohmbic structure, such as Fe ₃ C), unknown presence of 2.3%.

The hardness measurement results according to the depth from the surface using the micro hardness tester are shown in FIG. 3, and the surface layer increased to about 600 VHN or suddenly to about 800 VHN, and then showed a relatively constant value up to 600 µm. The thickness of the microstructure change layer in FIG.

As a result of the abrasion test as described above, the amount of abrasion was very good as 26 mg and the results are shown in FIG.

(2) Examples 2 and 3

In the case of Examples 2 and 3, the projection was performed at an input energy density of 1.67 W (kW / cm 2), and as can be seen from the optical micrographs shown in FIGS. 1 (b) and 1 (c), the melting was performed. The area is not visible and only the transformation area is formed to have a thickness of about 500 μm and about 200 μm, respectively. Similarly to the second (a) and (b), flake graphite is intact and only the matrix structure is changed from pearlite to martensite. It can be seen that the phase percentage was almost the same as in Example 1.

The hardness measurement results according to the depth from the surface using the micro hardness tester are shown in FIG. 3, and the overall change pattern is similar to that of Example 1, and the curing depth was decreased to about 500 μm and 200 μm, respectively. The maximum hardness value was about 800 VHN, which was almost the same as in Example 1.

The amount of wear by the abrasion test was 30 mg and 44 mg, respectively, which was very good in Example 2, but relatively good in Example 3. The results are shown in FIG.

(3) Comparative Example 1

In the case of Comparative Example 1, the projection surface was projected with an input energy density of 2.19 W (kW / cm 2) slightly exceeding the scope of the present invention, and as can be seen from the optical micrograph shown in FIG. From the depth of about 900㎛, the temperature rises above the melting point, there is a melting region where the flake graphite is melted and the matrix structure is converted into a solidified structure of complex form, and the transformation region is formed at a depth of about 900-2000㎛. The thickness of the transformation region was about 150 μm.

As can be seen from the optical micrograph shown in FIG. 2 (c), it can be seen that the microstructure of the molten region is composed of a dendrite-shaped solidification structure and is changed into a structure similar to white cast iron.

Quantitative phase measurement of residual austenite, Fe ₃ C carbide, martensite, ferrite, etc. using the Mossbauer analyzer showed that the melting region was 41.3% Fe ₃ C, 44.8% martensite, 2.8% austenite, 4.7% Fe _3- xC ₁ + x, 6.4% unknown phase. The region-specific phase percentages are shown in Table 4 below.

TABLE 4

Although the hardness measurement results according to the depth from the surface using the micro hardness tester are not shown, the hardness value from the surface to the depth of about 900 μm is constant at about 800 VHN and then rapidly decreases to 600 VHN at the beginning of the transformation region, to 1800 μm. After increasing slowly to reach 850VHN, it showed a tendency to decrease again to 2100 mu m where the matrix appeared.

The surface of the specimen of Comparative Example 1 has a rough surface due to the presence of the molten region and thus requires a separate surface treatment treatment, and therefore is not preferable in the present invention.

(4) Comparative Example 2

In the case of the comparative example 2, it projected by the input energy density 1.14W (kW / cm <2>) which falls short of the scope of this invention, and the change of the microstructure by electron beam projection was hardly recognized.

As a result of the measurement of the phase fraction by using the Mossbauer analyzer, it was found that it consists of 82.1% of ferrite and 17.9% of Fe ₃ C, which is consistent with the fact that the base structure of gray cast iron is composed of pearlite. It means that it was hardly affected by the fighter.

As a result of measuring the hardness value using the fine hardness tester, it was about 300 VHN of the hardness value of the original matrix structure.

This surface hardening method of the present invention is capable of working in the air and can harden a surface of about 72 m2 per hour locally or extensively, so that the productivity is very high and it is required for industrial sites requiring a continuous process. It is easy to apply, uniform heating and cooling is performed without any refrigerant or subsequent process, almost no pores or cracks are formed, no deformation or twisting occurs on the surface of the material after surface treatment, and the time that the material is heated is very short. This has the advantage of preventing oxidation of the surface of the material.

Moreover, using the surface hardening method by the accelerated electron beam projection of the present invention, the surface hardness value is improved by about 2.5 times, thereby improving the wear resistance by about 3 to 4 times.

Claims (4)

2.0 to 4.0 wt% C: 1.0 to 3.0 wt% Si; At least one component selected from the group consisting of Mn, Cr, Cu, P, and S in a total amount of less than 1.0 wt%; And the Vickers hardness value (Vickers) is 600 to 850 VHN by projecting a high voltage accelerated electron beam at an input energy density in the range of 1.2 to 2.1 kW / cm 2 to the gray cast iron material for the engine block of the vehicle composed of Fe. The surface hardening method of the gray cast iron material for engine blocks using the high voltage accelerated electron beam which forms most the surface hardened layer which consists of plate-like martensite. The method of claim 1, wherein the energy of the high voltage accelerated electron beam is in the range of 1.0 to 2.5 MeV permeable in the atmosphere. The surface curing method of claim 2, wherein the energy input density of the high voltage accelerated electron beam is in a range of 1.5 to 2.0 kW / cm 2. 2. The method according to claim 1, wherein by projecting the high-voltage accelerated electron beam in the range of input energy density of 1.6 to 1.95 kW / cm &lt; 2 &gt;, flake graphite is melted and the matrix structure is converted into dendrite coagulation structure and redeburite. Surfaces characterized by substantially no melting zones showing a similar structure to the white cast iron being changed, the thickness of the surface hardening layer being 0.5 to 1.5 mm and abrasion resistance 3 to 4 times higher than that of the matrix. Curing method.