JPH08246093A - Weldable spheroidal graphite cast iron material and its welding joint product - Google Patents

Abstract

PURPOSE: To produce a spheroidal graphite cast iron material capable of welding free from the generation of cracks by reducing the content of carbon in the surface part in the welding part of a spheroidal graphite cast iron material. CONSTITUTION: The whole body of the base material of a spheroidal graphite cast iron material (such as FCD450) or only the welding part and its vicinity are buried in a decarburizing material (such as iron sand and mill scale). The temp. of the base material is increased to about 950 to 1100 deg.C in a heating furnace, and it is heated to the same temp. range for about 40 to 70hr. After that, it is cooled to about 200 to 300 deg.C for about 5 to 10hr and is discharged from a furnace. Thus, the content of carbon in the range from the surface of the base material to a depth of about 0.5 to 2mm can be reduced to about <=1.0wt.%, by which a weldable spheroidal graphite cast iron material can be obtd. This iron material is joined with an inexpensive steel by welding, so that an inexpensive structure having strength and toughness can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weldable spheroidal graphite cast iron material and a welded article thereof.

[0002]

2. Description of the Related Art Spheroidal graphite cast iron is toughness among various cast irons.
Due to its high ductility, it is widely used in various castings such as iron pipes, valves and vehicle parts.

However, spheroidal graphite cast iron is said to be difficult to weld, and structural welding has not been carried out. The reason is as follows.

That is, when the base material of spheroidal graphite cast iron is rapidly heated by welding heat during welding, the peripheral portion (mainly ferrite structure) of the spheroidal graphite is selectively remelted because of its low melting point and crystallized graphite. Is dissolved in the melt, the chemical composition of the melt is close to the chemical composition at the time of casting of the base material, but the cooling rate of the welded portion is significantly higher than the cooling rate at the time of casting. When the liquid solidifies, white pig iron (cementite F
This is because the formation of e ₃ C) easily occurs and the risk of cracking is great.

[0005]

In view of the above-mentioned prior art,
An object of the present invention is to provide a weldable spheroidal graphite cast iron material.

Another object of the present invention is to provide a welded joint product of a spheroidal graphite cast iron material and a steel material.

[0007]

The present invention relates to (1) a weldable spheroidal graphite cast iron material characterized in that the carbon content in the surface layer portion of the welded portion of the spheroidal graphite cast iron material is reduced.

Further, the present invention is (2) in the invention described in the above item (1), wherein the carbon content is reduced so that the surface layer portion has a substantially ferrite structure. Spheroidal graphite cast iron material.

Further, the present invention relates to (3) a welded article characterized in that the weldable spheroidal graphite cast iron material described in the above (1) or (2) is welded to a steel material.

[0010]

[Operations and Examples] As described above, when the spheroidal graphite cast iron material is to be welded as it is, the white pig iron caused by the rapid heating and quenching at the time of welding makes the welding impossible. If the carbon content in the melt is reduced to prevent the formation of cementite, welding without cracks should be possible.

The melting depth of the base material at the welded part is not deep, and the rate of diffusion of carbon from the spherical graphite of the base material is smaller than that of combined carbon, so that the thickness of the base metal at a certain thickness from the surface of the welded part If the carbon content of the base metal in the range is reduced and the carbon content necessary for cementite formation is not supplied, the carbon content in the melt at the welding site will be low and crack-free welding will be possible. .

Based on the above findings, the present invention enables welding of spheroidal graphite cast iron material by reducing the carbon content in the surface layer portion of the welded portion of spheroidal graphite cast iron material.

In the present invention, the reduction of the carbon content in the surface layer portion which becomes the fusion zone of the welded portion of the base material spheroidal graphite cast iron material can be preferably carried out by decarburizing by heat treatment.

A decarburization method by heat treatment is known as a method for removing carbon from a cast iron material, which is a basic technique in the production of white-core malleable cast iron. Such a decarburization method is applied to the surface layer of a spheroidal graphite cast iron material. The present invention is the first to be applied to decarburization of a part.

In the present invention, the decarburization method by heat treatment is applied to the spheroidal graphite cast iron material to reduce the carbon content of the surface layer, so that the surface layer having a thickness of about 0.5 to 2 mm from the surface is formed. It is possible to obtain a cast iron product having a ferrite type spheroidal graphite cast iron structure in which the entire structure is substantially ferrite and the central portion has spheroidal graphite in the ferrite.

Since the cast iron product thus obtained has a low carbon content in the surface layer, it can be easily welded to a steel material and the like, and the central part has a spheroidal graphite cast iron composition with high toughness,
An inexpensive structure having strength and toughness can be obtained by producing a complicated-shaped portion by casting and welding and joining it with an inexpensive steel material.

In comparison with the case where the decarburization method by heat treatment is applied to the white-centered malleable cast iron to reduce the carbon content in the surface layer, in the case of spheroidal graphite cast iron, the carbon content diffuses from the spheroidal graphite. Since the speed is smaller than that of cementite, it is easy to reduce the carbon content of only the surface layer portion.

Next, the present invention will be described in detail.

The spheroidal graphite cast iron material used as the base material in the present invention is appropriately selected in consideration of the mechanical strength required for the final product. For example, FCD45
0, FCD370, FCD400, FCD500 and the like, and their equivalents can be used.

The matrix structure of the spheroidal graphite cast iron material is preferably a mixed structure of ferrite and pearlite from the viewpoint of mechanical strength,
From the viewpoint of easily obtaining a cast product having such a structure, the chemical composition is C3.5 to 3.9 wt%, Si2.5 to 2.9 wt%, Mn 0.4 wt% or less, and S0.015 wt% or less. ,
0.025 to 0.050% by weight of Mg, the balance being substantially F
The composition of e is preferable.

In the present invention, the base material is subjected to decarburization treatment to reduce the carbon content at least in the surface layer portion (melted portion) of the welded portion.

From the viewpoint of weldability, it is preferable that the spheroidal graphite existing in the surface layer portion is eliminated so that the entire structure becomes a substantially ferrite structure. From this point of view, the carbon content in the range from the surface of the base material to the depth of 0.5 to 2 mm is preferably 1.5% by weight or less, particularly 1.0% by weight or less.
When the carbon content exceeds the above range, decarburization is insufficient and weldability becomes poor. Although the lower limit of the carbon content is not particularly limited, it is about 0.1% by weight under practical decarburization treatment conditions.

The decarburization treatment is usually carried out by burying the entire base material in the decarburized material and heating it in a heating furnace such as a continuous tunnel furnace. In addition, you may make it decarburize only a welding site and its vicinity.

The decarburizing material is usually iron oxide (mainly Fe ₂
O. ₃ ) is used, and specifically, iron sand, mill scale, hematite powder, etc. can be used.

The heating conditions in the decarburizing treatment may be appropriately determined depending on the type and size of the base material, the depth of the decarburizing treatment, etc., but the surface layer portion is substantially composed of a ferrite structure,
The temperature of the base metal is usually 950 to 11 from the viewpoint that a treated product in which the matrix of the central portion is substantially composed of a ferrite structure is easily obtained.
After raising the temperature to 00 ° C, the temperature within this range (holding temperature)
Heating for 40-70 hours (holding step), then 5-10
After cooling to 200 to 300 ° C. over a period of time (gradual cooling step), it is preferable to discharge from the furnace. If a processed product in which the base in the central portion is rich in pearlite is desired, the slow cooling step may be stopped and the product may be rapidly cooled.

The step of raising the temperature to the holding temperature (950 to 1100 ° C.) (temperature raising step) is usually about 8 to 10 hours, although it depends on the size of the base material.

The processed product thus obtained is optionally annealed, normalized, etc., and if necessary, the black skin is removed, and then welded to a steel material.

The partner steel material may be appropriately selected depending on the use of the final product. For example, rolled steel for general structure (JIS
G 3101), rolled steel for welded structure (JIS G 3
106), carbon steel pipe for general structure (JIS G 344
4) and their equivalents can be used.

The welding method is also not particularly limited, and arc welding, covered arc welding or the like can be mainly used. As the arc welding, any of inert gas arc welding (MIG welding, TIG welding), active gas arc welding (mag welding, etc.) can be adopted.

Although any appropriate welding rods, welding wires, etc. may be used, the present invention has the advantage that ordinary inexpensive welding rods and wires can be used.

In the present invention, the weldable spheroidal graphite cast iron materials of the present invention may be welded to each other, and a product having a complicated shape can be produced by split casting and then weld welding, which is advantageous. is there.

A specific example of the welded joint product of the present invention will be described by taking an erection pipe used for assembling a scaffold at a construction site as an example.

FIG. 1 is a front view showing an embodiment of this erected pipe, FIG. 2 is a side view thereof, and FIG. 3 is an enlarged sectional view of an essential part showing its weld joint shape.

In the drawings, reference numeral 1 is a steel pipe, and joint parts 2 made of the spheroidal graphite cast iron material of the present invention are welded and joined to both ends thereof at welded portions 3.

The joint component 2 is composed of a tubular main body 10,
From the surface of the main body 10, a plurality (four in this embodiment) of wing-shaped protrusions 11 extending in the longitudinal direction extend in the radial direction.
The wing-shaped protrusion 11 is provided with a hole 12. Body 10
A flange portion 13 is provided on the side opposite to the welding side end portion of
Bolt holes 14 are provided in the flange portion 13.

As shown in FIG. 4, the erected pipes can be connected by abutting two of them at a flange portion 13 and fastening and fixing them with bolts and nuts. By connecting the erection pipes sequentially in this manner, the pipes can be extended to a desired length. The erected pipes extended to a predetermined length are fixed to each other by arms or braces. The holes 12 of the wing-shaped projections 11 are for fixing members such as braces.

Next, the present invention will be described in detail with reference to examples.

Example 1 FCD450 spheroidal graphite cast iron base metal was spheroidized with a Fe-Si-Mg alloy, and then the melt inoculated with the Fe-Si alloy was poured into a mold to form a round bar with a diameter of 20 mm. Cast. The composition of the molten metal is C 3.79 wt%, Si2.5
4% by weight, Mn 0.18% by weight, S 0.009% by weight,
Mg was 0.036% by weight, P was 0.043% by weight, and the balance was substantially Fe.

The round bar was embedded in sand iron placed in a cast iron pot, placed in a continuous tunnel furnace, decarburized under the following conditions, and then discharged from the furnace.

Temperature raising step Time: 8 hours Holding step Temperature: 1070 ° C. Time: 72 hours Slow cooling step Temperature: 1070 ° C. → 200 ° C. Time: 12 hours The obtained test piece was cut at a right angle to the central axis and its cross section was taken. An optical micrograph was taken, and the graphite area ratio (%) at each site along the radial direction from the surface toward the center was determined. The results are shown in Table 1 and FIG. Table 1 shows the low carbonization rate (ratio when the graphite area ratio of the central part is 100%).
And the carbon content (the value obtained by calculation from the graphite area ratio and the carbon content of the base material (3.79% by weight)) are also shown.

[0041]

[Table 1]

Further, FIG. 6 is an optical micrograph (magnification: 10) of a cross section of the base material (as-cast product) of the test piece before decarburization treatment.
8 (A) and 8 (B) show optical microscope photographs (magnification 400 times) of the cross section of the decarburized test piece. Here, FIG. 8A is a cross-sectional photograph of the surface layer portion, and FIG.
(B) is a cross-sectional photograph of the central portion. FIG. 7 is an explanatory diagram of the photograph of FIG.

As is apparent from FIG. 6, the structure of the base material before the decarburization treatment is the spherical graphite particles 20, the island-shaped ferrite structure 21 around the spherical graphite particles 20, and the pearlite structure between the island-shaped ferrite structures 21. It consists of 22 and.

From the results shown in Table 1 and FIGS.
It can be seen that in the test piece obtained by subjecting the base material having the structure shown in FIG. 6 to the decarburization treatment, the carbon content is greatly reduced from the surface to a depth of about 2 mm or less. Further, it can be seen from FIG. 8 that in the surface layer portion, the spheroidal graphite is miniaturized and the structure becomes ferrite as a whole, and the matrix inside the surface layer portion also disappears with the pearlite structure and becomes ferrite.

Example 2 A test plate of a spheroidal graphite cast iron material of the present invention and a test plate of steel material were welded and joined.

The size of both test plates is length 300 mm × width 1
It was 50 mm × thickness 4.5 mm, and one end face in the width direction of each test plate was inclined by 30 ° so that the weld abutting portions of both test plates had a V-shaped groove of 60 °.

A test plate of the spheroidal graphite cast iron material of the present invention (hereinafter,
The FCD test plate) was produced by casting using a molten metal having the same composition as in Example 1 and subjecting the obtained cast product to decarburization treatment under the same conditions as in Example 1.

The steel material test plate is SS400 material (JIS G
3101 general structural rolled steel) was cut out and produced (hereinafter, this test plate is referred to as SS400 test plate).

Both test plates were butted against each other so as to form a V-shaped groove of 60 ° and welded on one side under the following conditions.

Welding conditions Welding machine: MAG welding machine 350 (using carbon dioxide gas) manufactured by Matsushita Electric Industrial Co., Ltd. Welding wire: MG50T (diameter 1.2 mm) [Kobe Steel Co., Ltd., JISZ 3312 (mild steel and high tensile strength) YGW12 specified for MAG welding solid wire for steel) Welding current: 140-150A Welding voltage: 19V Gas flow rate: 20 liters / min CO ₂ Number of passes: 1 pass From the welded product obtained, JIS Z 3121 (butt) The No. 1 test piece specified in the tensile test method for welded joints) was cut out and the tensile strength was measured. Table 2 shows the results. Table 2
Shows the values for two test pieces A and B.

Example 3 A welded joint was prepared in the same manner as in Example 2 except that the FCD test plate was used in place of the SS400 test plate and butt welding was performed between the FCD test plates. The test piece was cut out and the tensile strength was measured. Table 2 shows the results. Table 2 shows the values for two test pieces C and D.

[0052]

[Table 2]

From Table 2, it can be seen that in the welded joint product of the spheroidal graphite cast iron material and the steel material of the present invention and the welded joint product of the spheroidal graphite cast iron materials of the present invention, the strength of the welded portion is high.

Example 4 The erected pipe shown in FIGS. 1 to 4 was produced by welding the joint part 2 made of the spheroidal graphite cast iron material of the present invention and the steel material pipe 1.

The following was used as the steel pipe 1.

Material: STK steel pipe specified in JIS G 3444 (carbon steel pipe for general structure) Dimensions: length 250 mm, outer diameter 76.3 mm, inner diameter 68 m
The dimensions of the joint component 2 were as follows.

Length of the main body 10: 125 mm Outer diameter of the main body 10: 80 mm Inner diameter of the main body 10: 64 mm Thickness of the main body 10: 8 mm Flange size: 145 mm × 145 mm The joint shape of the steel pipe 1 and the joint part 2 is The shape shown in FIG. 3 was used. The groove was a 60 ° V-shaped groove.

The joint part 2 was produced by casting a molten metal having the same composition as in Example 1 and subjecting the obtained cast product to decarburization treatment under the same conditions as in Example 1.

The joint parts 2 are attached to both ends of the steel pipe 1 as shown in FIG.
Butt and welded as shown in FIG. Welding was performed under the following two conditions.

Welding condition A Welding machine: MAG welding machine Daihen 350 manufactured by Osaka Transformer Co., Ltd.
(Use carbon dioxide gas) Welding wire: SCO (diameter 1.2 mm) [YGW12 specified by Sumitomo Metal Industries, Ltd., JISZ 3312 (Magnetic welding solid wire for mild steel and high strength steel)] Welding current: 210 A Welding voltage : 27V Gas flow rate: 20 liters / min CO ₂ Number of passes: 1 pass Welding condition B Welding machine: Daihen 350 Welding wire: MG50T (diameter 1.2 mm) Welding current: 200 A Welding voltage: 30 V Gas flow rate: 20 liters / min CO Number of ₂ passes: 1 pass The cross section of the joint part 2 after the decarburization treatment and the welded joint between the joint part 2 and the steel pipe 1 were observed with an optical microscope. The results are shown in Figure 9.
~ 11. FIG. 12 is an explanatory diagram of the photograph of FIG.

FIG. 9: Cross-sectional photograph of the surface layer of the decarburized joint part 2 (magnification 100 times) FIG. 10: Cross-sectional photograph of the center of the decarburized joint part 2 (magnification 100 times) 11: Cross-sectional photograph of the welded joint portion of the joint component 2 and the steel pipe 1 (magnification 100 times) As shown in FIG. 9, the surface layer portion of the welded butted portion of the decarburized joint component 2 was pulverized into spherical graphite, It can be seen that has a ferrite structure. In FIG. 9, the upper right corner is the surface and the black skin is formed.

As shown in FIG. 10, it can be seen that the matrix structure of the decarburized joint part 2 has a ferrite spheroidal graphite cast iron structure.

In FIG. 12, 30 is a joint portion of the joint component 2, 31 is a surface (black skin) portion thereof, 32 is a weld overlay portion, 3
3 indicates a welded joint.

As is clear from FIG. 11, no abnormalities were observed in the structure of the welded portion 30 of the joint component 2 and the weld overlay 32, and the welded state was good.

Further, a tensile test was conducted on the erected pipe. In addition, in this tensile test, the one in which the joint component 2 was welded to only one end of the steel pipe 1 was used. Two such erection pipes are shown in FIG.
The butt joints in 3 and the bolts and nuts tightened and fixed were subjected to a tensile test with a 30 ton universal tensile tester manufactured by Shimadzu Corporation. The results are shown in Table 3.

[0066]

[Table 3]

[0067]

EFFECTS OF THE INVENTION The spheroidal graphite cast iron material of the present invention has a low carbon content in the surface layer, so that it can be easily welded to steel and the like, and the central portion has a spheroidal graphite cast iron structure with high toughness, so that the shape is complicated. It is possible to obtain an inexpensive structure having strength and toughness by forming a large portion by casting and welding and joining it to an inexpensive steel material.

[Brief description of drawings]

FIG. 1 is a front view showing an embodiment of a erected pipe which is a welded joint product of a steel pipe and a joint component made of the spheroidal graphite cast iron material of the present invention.

FIG. 2 is a side view showing the erection pipe.

FIG. 3 is an enlarged sectional view of an essential part showing a welded joint shape of the erected pipe.

FIG. 4 is a partial front view showing a connection state between the erected pipes.

5 is a graph showing the relationship between the distance from the surface and the graphite area ratio in the decarburization-treated test piece obtained in Example 1. FIG.

FIG. 6 is a micrograph (magnification: 100 times) showing the metal structure of a test piece before decarburization treatment.

7 is an explanatory diagram of the photograph of FIG. 6. FIG.

FIG. 8 is a micrograph (magnification: 400 times) showing the metal structure of the test piece after the decarburization treatment, (A) is a cross-sectional photograph of the surface layer portion, and (B) is a cross-sectional photograph of the central portion.

FIG. 9 is a photomicrograph (magnification: 100 times) showing the metal structure of the surface layer portion of the decarburized joint component 2 obtained in Example 4.

FIG. 10 is a photomicrograph (magnification 100 times) showing the metal structure of the central portion of the decarburized joint component 2.

FIG. 11 is a micrograph (magnification: 100 times) showing a metal structure of a welded joint between the joint part 2 and the steel pipe 1.

FIG. 12 is an explanatory diagram of the photograph of FIG. 11.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel pipe 2 Joint parts 3 Welding part 20 Spheroidal graphite particles 21 Ferrite structure 22 Perlite structure

Claims (3)

[Claims] 1. A weldable spheroidal graphite cast iron material having a reduced carbon content in the surface layer portion at the welded portion of the spheroidal graphite cast iron material. 2. A weldable spheroidal graphite cast iron material having a reduced carbon content such that the surface layer portion has a substantially ferrite structure. 3. A welded article, characterized in that the weldable spheroidal graphite cast iron material according to claim 1 or 2 is welded to a steel material.