JPH0613155B2 - Flash welding method for spheroidal graphite cast iron. - Google Patents

Description

TECHNICAL FIELD The present invention relates to a flash butt welding method for spheroidal graphite cast iron.

[Prior Art] The difficulty of welding cast iron is well known, and the case of spheroidal graphite cast iron is no exception. The reason is that rapid cooling of the weld metal causes carbides, martensite, etc. to precipitate in the weld, shrinkage during cooling, and rapid strain such as expansion due to precipitation of martensite. Precipitated carbides have poor plastic deformability and therefore often generate cracks. Furthermore, carbon contained in cast iron in a large amount oxidizes during welding to generate CO ₂ gas, which causes blowholes in the weld metal. It depends on how things are born.

For this reason, the reliability of the strength of the weld is low,
The present situation is that welding of cast iron is used only for repairing defects in cast iron and joint welding is not performed at present. On the other hand, joint welding is being actively attempted. This is because it can be expected that the application will be significantly expanded if the joint welding can be established, the proportion of spheroidal graphite cast iron in cast iron is high, e.g. lengthening of the ductile cast iron pipe with one side flange by centrifugal casting, Another reason is that it greatly contributes to shortening of the construction period and cost reduction, such as the production of ductile cast iron pipes with flanges on both sides, which cannot be centrifugally cast.

Therefore, an example of the flash butt welding method, which has been conventionally tried as a welding method for spheroidal graphite cast iron, will be given. In this welding method, a large current is passed while lightly contacting the butt end surfaces of the material to be welded, the contact surface is heated and melted by Joule heat and arc heat generated at this time, and then a sudden pressure called upset is applied to the contact surface. In addition, it is a method of pressure welding. Therefore, when the flash butt welding method is applied to spheroidal graphite cast iron, unlike conventional arc welding, TIG welding, etc., since no filler metal is used, there is no risk of dissimilar metals existing in the welded portion, and upset CO ₂ gas Since the weld metal that tends to remain is extruded from the joint, there is an advantage that clean welding can be obtained and brittleness and strength reduction of the joint can be prevented. However, on the other hand, there is a problem that the quality of the material deteriorates due to the rapid cooling of the heat affected zone.

[Problems to be Solved by the Invention] This is 30,000 in the conventional flash butt welding method.
Since the welding condition is to butt-weld a large current of up to 40,000 A for an extremely short time of 6 to 10 seconds, the heat distribution area in the direction orthogonal to the welding line is very narrow, and therefore cooling can be performed quickly. There was a problem that the elongation was small and the standard value could not be satisfied even in the flatness test because of the fact that it was difficult to avoid the precipitation of cementite, martensite, etc. in the heat affected zone.

Therefore, as a result of striving to improve the conventional method, the present invention provides practical spheroidal graphite by means of welding conditions such as slowing down the cooling rate of the welded portion and its vicinity and predetermined post-heat treatment conditions to be given thereafter. A flash butt welding method for cast iron has been established, and an object thereof is to provide such a welding method.

[Means for Solving the Problems] In the flash butt welding method for spheroidal graphite cast iron according to the present invention, the welding current is set to a small region and the flash time is set to a large region as welding conditions, and a predetermined post heat treatment is performed after welding. More specifically, the welding current is set to ² to 6 A / mm ² per welding unit cross-sectional area, the flash time is set to 20 to 100 seconds, the post heat treatment is held at 700 to 750 ° C., and then air cooled. .

[Operation] In the present invention, first, a welding current is set to be small as a welding condition.
Flash butt welding of spheroidal graphite cast iron is performed by setting the flash time within a large area. Specifically, the welding current is set to ² to 6 A / mm ² per unit welding area, and the flash time is set to 20 to 100 seconds.

FIG. 1 conceptually shows a region to which the welding conditions of the present invention are applied. However, in the normal flash butt welding method, the welding conditions are large welding current and short flash time.
Welding is performed in the area A. On the other hand, in the present invention, the welding current is small and the flash time is long, and the welding is performed in the region B. This is because, when welding is performed in the area A, the heat distribution area in the direction orthogonal to the welding line is very narrow, and therefore cooling is performed quickly, while in the area B, the heat distribution area is widened, resulting in cooling. This is because the speed becomes slow. For this reason, the cooling rate of the heat-affected zone can be made slower than before, and the precipitation of martensite and cementite can be prevented to some extent.Therefore, a joint with high tensile strength and large elongation can be obtained even in as weld. To be The regions C and D in FIG. 1 are regions where welding is not appropriate. In region C, welding is not possible due to insufficient heating, and in region D, conversely, excessive heating causes defective joints.

When welding is performed in the region B as described above, the cooling rate of the heat-affected zone becomes slower, so that the quality of the joint is improved. However, this alone cannot completely prevent the precipitation of martensite and cementite in the joint. Therefore, next, a predetermined post heat treatment is performed after welding to decompose these generated carbides. The conditions for the post heat treatment include
After heating and holding at 700 to 750 ° C., air cooling is performed.
This makes it possible to reduce the hardness of the welded portion and satisfy a predetermined flatness amount while maintaining the strength and elongation of the joint at or above the required standard values. Further, it has been confirmed that the welded structure and the structure in the vicinity thereof are sufficiently a pearlite + ferrite structure.

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

Generally, the standard values of mechanical properties required for welded joints in the case of ductile cast iron pipe are as shown in Table 1.

Next, the sample used for the experiment is manufactured by flash butt welding two ductile cast pipes cast by the centrifugal casting method. In this case, the dimensions, chemical composition and tensile properties of the sample are shown in Table 2.

The welding conditions are shown in Table 3.

Next, the experimental results are shown in Table 4 in relation to the heat treatment temperature.

However, at each heat treatment temperature, it was held in the furnace for 30 minutes and then air cooled. The size of each test piece is 4.0φm
m and GL = 20 mm.

From the results in Table 4, the most preferable heat treatment temperature is 700 to
It can be seen that the temperature is 750 ° C. This is because all the fracture positions are the base metal positions, which indicates that the strength of the joint is equal to or higher than that of the base metal. Also, as welded ones satisfy the conditions of tensile strength and elongation, but cannot satisfy the hardness and flatness amount described later.

When the heat treatment temperature exceeds 750 ° C., the tensile strength increases, but the elongation remarkably decreases, making it difficult to use. This is because the A3 transformation temperature of iron is 738 ° C., and when heated to 780 ° C. or higher, the raw material is completely austenized and the base metal portion is hardened.
On the other hand, if the temperature is 750 ° C. or lower, the material is hardly transformed and only the hardness of the welded portion is selectively reduced.

2 and 3 are graphs showing the experimental results shown in Table 4.

Next, FIG. 4 shows the hardness (H _v10 ) distribution of the joint in relation to the heat treatment temperature. The hardness measurement range of the base metal is a range of 10 to 30 mm to the left and right of the welding line (W.L). 2 each
It is measured at 0 points.

The hardness of the weld decreases as the heat treatment temperature increases, but at 780 ° C or higher, the hardness of the base metal increases and the elongation decreases, as can be predicted from the results in Table 4. In the case of as weld, the hardness of the weld was the highest, and cracks occurred in the weld even in the flatness test. 70 to this
At 0 to 750 ° C., the standard value of Table 1 is satisfied, and the flatness also increases to about 100 mm at 750 ° C. as shown in FIG. The dimensions of the sample used for the flatness test are 200mm in outer diameter, 7.5mm in thickness, and 2 lengths with the welding line in the center.
It was set to 50 mm.

[Effects of the Invention] As described above, according to the present invention, flash butt welding of spheroidal graphite cast iron can be realized by the above-mentioned welding conditions and post heat treatment conditions, and the versatility of this type of cast iron material can be expanded. Greatly contribute.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a region to which welding conditions according to the present invention are applied, FIGS. 2 and 3 are views showing experimental results of heat treatment temperature and tensile strength and elongation, and FIG. 4 is heat treatment temperature. And the hardness distribution of the joint, and FIG. 5 is a diagram showing the relationship between the heat treatment temperature and the amount of flattened cracks.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hisashi Kobayashi 3-16-9, Uchikanda, Chiyoda-ku, Tokyo Inside Nihon Cast Iron Pipe Co., Ltd. (72) 3-16-16, Uchikanda, Chiyoda-ku, Tokyo No. 9 Nihon Cast Iron Pipe Co., Ltd. (72) Inventor Toshiyuki Yonezu 3-16-9 Kanda, Uchi, Chiyoda-ku, Tokyo Nihon Cast Iron Pipe Co., Ltd.

Claims (1)

[Claims] 1. In flash butt welding of spheroidal graphite cast iron material, the welding current is set to ² to 6 A / mm ² per welding unit cross-sectional area and the flash time is set so that the cooling rate in the welded portion and its vicinity is slow. 20-10
A flash butt welding method for spheroidal graphite cast iron, characterized in that the post heat treatment condition after welding is maintained at 700 to 750 ° C. and air cooling is performed after 0 second.