JPS6050115A - Method for working surface of cast steel - Google Patents

Abstract

PURPOSE:To vanish fine vacancies present on the surface of cast steel and to improve the corrosion resistance by subjecting only the surface layer of the cast steel to impactful compressive plastic deformation and by heating the cast steel to the diffusion temp. or above. CONSTITUTION:The surface of cast steel is peened to cause plastic deformation to an extent exceeding the size of fine vacancies present on the surface. The cast steel is then heat treated at >=1,000 deg.C diffusion temp. for >=1hr. By this surface working method, fine vacancies present on the surface of the cast steel and just under the surface are vanished, and the corrosion resistance is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a surface processing method for cast steel suitable for sealing micropores existing on and immediately below the surface of cast steel to impart corrosion resistance.

[Background of the invention]

In general, in cast steel products cast into product shapes, it is essentially inevitable that micro-defects, that is, micro-pores, will exist on the surface and just below the surface due to casting solidification shrinkage etc. due to the special nature of the casting method. That's true.

Therefore, conventional methods for completely removing such microscopic defects include (a) a method of remelting the cast steel LD coating, and (b) a method of remelting the cast steel LD coating.
High frequency transmission method (FC) is a method in which hydrostatic pressure is applied to the surface of the cast steel using a pressure medium such as an inert gas while heating the surface of the cast steel to a predetermined temperature using a laser beam irradiation method or soundly heating the entire cast steel. etc. are known.

However, the former method (a) cannot be applied to complex-shaped and thin-walled cast steel products because dimensional deformation occurs because the entire cast steel product is heated, and surface defects cannot be completely removed. The problem was that it could not be done.

On the other hand, in the latter method (b), heating by high frequency induction method or laser beam irradiation method cannot uniformly heat cast steel products with complex shapes, and cannot be applied to large cast steel products. In addition, in the method of compressing micropores using the hydrostatic pressure method, micropores in the surface layer F of cast steel can be compressed, but as a general rule, plastic deformation does not act on the surface, so the micropores are exposed on the surface. It is not possible to press the micro nitride that is present. Therefore, this method has a problem in that it is necessary to remove micropores on the trench surface that are not pressed after the treatment by mechanical processing such as surface grinding. Furthermore, since the hydrostatic pressure method rarely imparts plastic deformation to the surface, it has the problem that it rarely improves the metallic structure of the casting surface.

[Purpose of the invention]

This invention was made in order to solve these conventional problems as explained above, by taking a means to seal the micropores that exist on the surface of cast steel and just below the surface. Another object of the present invention is to provide a surface processing method for cast steel that can improve the corrosion resistance of cast steel and improve the metallographic structure of the surface layer of cast steel.

[Summary of the invention]

The method for improving the profile of cast steel according to the present invention applies impact compressive plastic deformation only to the surface layer of the cast steel, thereby crimping the micro-pores existing on the surface of the cast steel and just below the surface, and also compressing the crimped portion. It is characterized by diffusion bonding by heating to high temperatures.

The casting method can produce complex-shaped cast steel products all at once by press-fitting molten metal into a mold that has the same shape as the product and solidifying it. Furthermore, in recent years, with the advancement of precision casting technology, it has become possible to manufacture all final products as cast without performing finishing processes such as machining. In the case of cast steel products manufactured by such a casting method, it is currently unavoidable that micropores, which become microscopic defects, exist on the surface and just below the surface. These micropores not only deteriorate the corrosion resistance of cast steel, but also may serve as a starting point for stress concentration in stress corrosion cracking. To confirm this, we compared and investigated the corrosion resistance of Ni-containing 13% Cr cast steel and forged steel in high-temperature pure water.
Despite having the same chemical composition, 1. It is recognized that there is a large difference in corrosion resistance between /JA and forged steel.

Investigating the cause of this problem, we found that cast steel is a cast structure that inherently undergoes solidification shrinkage, and microscopic pores exist on the surface and just below the surface. (abbreviated as SCC). In more detail, after forging the as-cast four steels, quenching and tempering them, we conducted a low strain rate tensile test (hereinafter abbreviated as 88RT) to determine the SCC fracture ratio. When examining SCC susceptibility, cast steel had an SCC fracture rate of about 12%, but forged steel had an SCC fracture rate of 2.5%, which decreased to approximately 115%, significantly improving SCC resistance. I feel angry. This is because, during forging, the micropores that exist on the surface and just below the next bend are forged welded.

However, in reality, it is not possible to forge a separate item without dimensional deformation, and it is not possible to completely remove micropores from cast steel with a complex shape even using conventional techniques. Therefore, when we investigated the properties of the micropores that exist on the surface and just below the surface of #steel11, we found that most of the micropores have a diameter of several μm, ranging from 9.1 μm to a maximum of 200 μm.
The items up to this point are distributed. Most of them are close to spherical in shape, but there are also some irregularly shaped ones. On the other hand, the inner surface of the micropores is in an active state with a smooth metallic luster, so if the vicinity of the micropores is compressively deformed and the micropores are crimped, then the crimped part is heated to a high temperature. It is assumed that diffusion bonding is possible.

Therefore, the inventors adopted a known peening method as a method of imparting compressive plastic deformation only to the surface and just below the surface of cast steel, and also heated the crimped part by peening to a temperature higher than the diffusion temperature of the cast steel. We have found that it is possible to seal micropores that exist on and just below the surface of granite steel.

The surface processing method for cast steel according to the present invention is a method that completely removes microscopic defects on the surface of cast steel and just below the surface to improve the corrosion resistance of cast steel. The method is characterized in that after the microscopic holes are crimped, the crimped portion is diffusion bonded while being heated and maintained at a high temperature.

This peening treatment is a known method used to impart compressive residual stress to the surface of steel, which is commonly used. The impact load causes the temporary processing machine to reach its elastic limit, resulting in plastic deformation on the surface or a small area just below the surface. In this method, wires can be used in addition to grids. When wire is used, the surface hardening is greater and it is superior in terms of workability and plastic workability for flat workpieces.

The longer the peening process, the better from the perspective of plastically deforming the surface, but if the peening process continues for a long time, the plastically deformed surface layer will become too hard, and if peening is continued, the impact energy of the grid will cause the peening to become too hard. The surface layer begins to peel off. Therefore, it is desirable that the processing time for peening be less than a few minutes per unit surface area ICn1". Also, in order to compress micropores by peening, the depth of plastic deformation due to peening must be equal to or greater than the diameter of the pores. However, in actual peening, it is preferable to take into account large defects other than the desired pore diameter and give the depth of plastic deformation several times that value.

Note that the cast structure of cast steel cannot be made finer by mere heat treatment, but since the surface layer of the cast steel is plastically deformed by this peening, the cast structure is made finer by the subsequent diffusion treatment, quenching and tempering, etc., and its mechanical properties are improved. properties can be improved.

On the other hand, compressive plastic deformation due to peening is affected not only by the peening treatment time but also by the diameter of the grid. Even if the peening time is the same, the amount of plastic deformation increases when the diameter of the grid is increased. This is the material of the grid,
This is because if the air pressure at the time of firing is the same, a grid with a larger mass has a larger kinetic energy.

However, if the grid is too large, it will roughen the surface of the cast steel, so care must be taken when selecting the diameter.

In order to enhance the peening effect and prevent surface roughening, there is a method of using a large grid at the beginning of peening and then finishing with a small grid. If this method is repeated one after another, it is possible to prevent the surface from becoming rough, but in reality, this is not preferable as it complicates the number of work steps.

Next, peening the cast steel surface! l! After that, the cast steel is heated above the diffusion temperature and subjected to diffusion treatment. This diffusion treatment is to metallically diffusion bond the parts where the micropores have been crimped by the peening process, and it is to metallically diffusion bond the mechanical crimping defects that exist on the surface and just below the surface of the cast steel. The heating temperature is the temperature necessary for diffusion bonding with the micropores pressed together, and a high temperature is preferred. However, if the temperature is too high, the crystal grains of the cast steel become coarse and a burning phenomenon occurs in which the grain boundaries are preferentially oxidized, resulting in deterioration of the mechanical properties of the cast steel. Therefore, the heating temperature is higher than the diffusion temperature of cast steel, that is, 1000C or higher.
It is desirable to keep the temperature below 50C. As for the processing time, the longer the processing time, the more sufficient the diffusion bonding will be, but if it is 5 hours or more, the diffusion bonding will be saturated, although it depends on the processing temperature.

Therefore, in reality, about 5 hours is sufficient to achieve the purpose of the present invention.

As described above, the method for improving the surface of 'JjJ steel according to the present invention involves peening the surface of the cast steel to press the micro-pores, which are micro-defects, and heating the tE bonded area to perform diffusion bonding. Corrosion resistance can be significantly improved by completely removing microscopic defects.

Examples of the present invention will be described in detail below.

Example 1 First, 3.5NilaCr steel and 5.5NilaCr
After melting steel in an atmospheric atmosphere using a high-frequency melting furnace, the molten steel is poured into a mold and solidified to a thickness of 100 mm and a width of 100 mm.
A rectangular cast steel of 400+1111, length; 300 rtts was produced.

Next, after performing a wave number treatment of 1000'CX 5 hr, air cooling quenching of 950CX 2hr was performed.

Furthermore, 620CX5hr for 3.5Ni13Cr cast steel
On the other hand, the 5.5Ni13Cr cast steel was subjected to air-cooling tempering of 600 tr x 5 hr to prepare a test material. The chemical composition of each sample material is as shown in Table 1.

A 5SRT test piece 1 shown in FIG. 1 was taken from the sample material thus obtained and used in the practice of the present invention.

The peening treatment was carried out using a grid with a diameter of 0.7 mm.
The temperature of the cast steel was kept at room temperature, and the pressure for striking the grid was 7K.
9 f/α2, peening processing time 2° seconds,
The test was applied to the parallel portion 3 of the test piece including the R portion 2, as shown in FIG. 1, at various times of 40 seconds and 60 seconds. This treatment time refers to the time it takes to bombard the grid per unit surface area of the test piece (1 cm).
The specimens were subjected to a temperature range of 950C to 1150C and a diffusion time of l b r .

5hr, 10hr, 20hr and lft k change,,l
! I gave.

As described above, 5SRT tests were conducted on each of the horizontal pieces using the method of the present invention, and the corrosion resistance was compared. 5S here
In the RT test, each test piece is immersed in pure water with dissolved oxygen of about 8 p and an electrical conductivity of about 1 μS/crn while being maintained at 288 C, and the test piece is subjected to a strain rate of 8°3×1o-'/s. I went there. The corrosion resistance of each test piece was evaluated by the SCC fracture ratio obtained from the 5SRT test, and the smaller the SCC fracture ratio, the better the corrosion resistance. For comparison, a similar corrosion resistance test was also conducted on a material that was not subjected to the diffusion treatment, which is a component of the present invention.

The results are shown in Table 2.

It can be seen that the longer the peening treatment time, the greater the effect of improving corrosion resistance on the second surface treated specimens. on the other hand,
Looking at the same diffusion time, it seems that the effect of improving steel/furnace corrosion resistance at high temperatures is greater, but the SCC fracture ratio remains almost constant above 1050C, even if the manufacturing temperature is increased beyond that. It can be seen that the effect on corrosion resistance is small and reaches a saturated state. Further, when the diffusion temperature was as low as 950C, the effect of improving corrosion resistance was small.

Regarding the diffusion temperature, it was found that the longer the peening treatment was applied, the more the corrosion resistance was improved, but no significant effect could be expected when the diffusion temperature was longer than 5 hours.

In addition, 3.5%Ni13%Cr cast steel and 5.5%Ni13
%Cr cast steel, there was almost no difference in corrosion resistance depending on the steel type.

Although the same type of cast steel was subjected to appropriate heat treatment, the cast steel to which the present invention was not applied had a high SCC fracture ratio of about 12% and poor corrosion resistance. It can be seen that when the present invention is applied to such conventional products, the 8CC fracture surface ratio is reduced to about 3%, and the SCC susceptibility seen from the SCC fracture surface ratio is reduced to about 1/4. In other words, it has been revealed that the corrosion resistance of the cast steel according to the present invention is improved four times that of conventional products.

Embodiment 2 In this embodiment, as shown in FIG. 2, the parts of the internal pump 4, such as the impeller 5 and diffuser 6 on the high-temperature water side of the lower part of the pressure vessel, are cast with 3.5%Ni13%Cr steel. This figure shows the case where the method of the present invention is applied to this cast steel product after manufacturing it.

When such a cast steel impeller 2 and a cast diffuser are used in a nuclear reactor, if the corrosion resistance is poor, corrosion damage may occur, which may lead to a serious accident. Therefore, by applying the method of the present invention to these cast steel products, it was confirmed that the corrosion resistance of the cast steel products was improved.

First, the impeller and diffuser of the internal pump were cast using molten steel made by melting the 3.5% Ni1aCr steel used in Example 1 in a high-frequency melting furnace. It was subjected to a diffusion treatment and cooled in a furnace, then air-cooled and quenched for 950 x 5 hr, and further air-cooled and tempered for 620 tl:' x 10 br.

The impeller manufactured in this way was finished to product dimensions using a grinder, and then surface defects were examined using the Sumpp method. The Sumpu method involves selecting 10 arbitrary locations on the surface of an impeller cast steel product, polishing them so that minute defects are clearly visible, applying a cellulose acetate film to transfer the defects to the film, and applying Cr on top. The vapor-deposited material is observed using a scanning electron microscope. As in Example 1, most of the surface defects in this impeller cast steel range from 1 μm to several tens of μm. Considering the solidification process of casting, in which most defects of several μm T1 were distributed, the depth of the defects is as described above. In some cases, the dimensions are larger than those observed from the direction of the casting surface.

This is because dendrites grow inward from the casting surface during the solidification process of casting, and elongated micro defects are formed between the dendrites. Therefore, when performing peening treatment on impeller cast steel products, the plastic deformation depth should be set to 100 μm step by step, based on the above observation results.
Taking into account the importance of the product and the improvement of the metallographic structure, the plastic deformation depth was carried out to be 500 μm or more. The peening treatment was performed at a pressure of Kgf/cm2, and the Dalit diameter was reduced to 1.
Two types of grids are used: 2mm and 0.7rrrm, and 3mm grids with a diameter of 1.2m are used in areas where tensile stress is strongly applied.
Peening treatment was performed for 30 minutes using a grid with a diameter of 0.7fi, and peening was performed for 30 minutes using a grid with a diameter of 0.7fi.
diameter grid for 15 minutes, and 0.7 mm diameter grid for 15 minutes.
Peening treatment was performed for 5 minutes. Next, the peened impeller cast steel product was heated to 1050C in an inert gas and held for 5 hours, then cooled in a furnace and subjected to a diffusion treatment, and the Jan'4 product was further heated and held in an inert gas. rear,
The cast steel is quenched by blowing inert gas and cooling with south wind.
Furthermore, tempering of 320CXIQhr was performed.

After heat treatment in this manner, the surface was finished by honing and emery paper polishing to obtain an impeller cast steel product.

The impeller cast steel product obtained in this way has completely removed the microscopic defects that existed on the surface and just below the surface, and has significantly improved corrosion resistance, and mechanical tests have shown that it has good strength and toughness. Are better.

Example 3 In this example, a pump part for a cooling water purification system of a nuclear reactor was cast using 5.5% Ni tacr steel, and the method of the present invention was similarly applied to the cast steel product in the same manner as in Example 2. In this case as well, it was found that a vague effect could be obtained as in the example.

〔Effect of the invention〕

As explained above, according to the surface processing method for cast steel according to the present invention, after peening the micropores existing on the surface of the cast steel and immediately below the surface, the crimped portion is heated and held to perform diffusion bonding and form micropores. Since the pores are completely sealed, the corrosion resistance of the M product, especially the susceptibility to stress corrosion cracking, is significantly improved, and the blush structure of the cast steel product is refined to improve mechanical properties such as strength, toughness, and fatigue strength. child

[Brief explanation of drawings]

FIG. 1 is a plan view of a test piece used in Example 1 of this invention;
FIG. 2 is an overall schematic diagram of an internal pump showing an impeller and a diffuser that constitute an internal pump according to a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Test piece, 3... Parallel part, 4... Internal pump, 5... Impeller, 6... Gain user. Fraud 1 item 2r;? 'Il

Claims (1)

[Scope of Claims] 1. In a surface processing method for eliminating micropores existing on and immediately below the surface of cast steel, the surface of the cast steel is subjected to peening treatment, and then the cast steel is heated to a temperature higher than the diffusion temperature. Characteristic surface processing method for cast steel. 2. The method for surface processing of cast steel according to claim 1, wherein the peening treatment imparts plastic deformation larger than the size of micropores to at least the surface of the cast steel. 3. The cast steel surface construction method according to claim 1, wherein the diffusion temperature is 100OC or higher and the heating and retention time is 1 hour or longer.