JPH04371376A - Electron beam welding method for cast material - Google Patents

Abstract

PURPOSE:To obtain a good welded zone having low residual stress without occurrence of a microcrack on the welded zone in joining cast material. CONSTITUTION:In welding where at least one member is cast material, a chill crystal 4 is formed on the weld zone of the cast material and joining is performed by electron beam welding. Since the chill crystal 4 is formed on the weld zone of the cast material, the crystal. grain diameter of the welded zone becomes finer and inclusion of contained gas components such as impurities, hydrogen, oxygen and nitrogen is extremely limited on the grain boundary of this fine crystal grain. Accordingly, the passage of an electron beam is improved. Consequently, electron beam welding with low welding heat input can be applied to this and sound welding of the cast material can be performed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for electron beam welding of welded joints in which at least one member is a cast material.

0002

2. Description of the Related Art In general, large and thick casting materials have a slow solidification rate and therefore have coarse crystal grains after casting. It is also known that even small casting materials, such as austenitic stainless steel, exhibit this tendency markedly depending on their chemical composition. Many impurities and gas components containing hydrogen, oxygen, nitrogen, etc. are present in the grain boundaries of these coarse grains. Therefore, when electron beam welding is performed to join such casting materials, these impurities present in the grain boundaries and gas components containing hydrogen, oxygen, nitrogen, etc. gasify and expand, causing them to segregate in the welded area. Blowholes occur. As a result, the beam passes poorly and the welded part becomes unsound. Therefore, electron beam welding is not suitable for casting materials with coarse grain size, and currently MIG welding or TIG welding is used. Furthermore, the application of electron beam welding to iron-based materials has been limited to, for example, rolled materials and forged materials with small crystal grain sizes.

[0003] As an application example of MIG welding of casting materials with coarse grain size, there are hearth rolls used in various heating furnaces, soaking furnaces, preheating furnaces, etc. for heat treating steel plates. Conventionally, the following methods are known as methods for manufacturing hearth rolls of this type. First, the parts that make up the hearth roll are:
It consists of a shaft, axle, and barrel, all of which are made of heat-resistant castings that are heat-resistant and oxidation-resistant so that they can withstand the atmosphere during use. Typical manufacturing methods and materials for these are: the shaft is made of SUS310 material by forging, and the axle is made of SCH19 material by sand casting or die casting.
For equipment with relatively low requirements for heat resistance and oxidation resistance, such as preheating furnaces and slow cooling furnaces, the barrel is YU, which has a plate-wound welded structure.
S material is also used as a high alloy material by centrifugal casting for equipment such as heating furnaces and soaking furnaces that require high heat resistance and oxidation resistance.
CH22 material etc. are used respectively.

[0004] Furthermore, the above-mentioned parts are assembled by welding as described below. First, welding between the shaft and axle will be described. The shaft is made of a forged material, and the axle is made by sand casting or die casting, and after V-grooving the joints of both members, MIG welding and TIG welding are performed.

Next, welding between the axle and the barrel will be described. The barrel is manufactured by centrifugal casting as described above. At this time, a metal mold is used as a mold for centrifugal casting,
In order to prevent the molten metal from sticking to the mold during casting, the inner surface of the mold is coated. As a material for the coating mold, a water-soluble coating material that is widely used in centrifugal casting rolls is generally used.

[0006] Here, the procedure for applying the water-soluble mold coating material to the inner surface of the mold is as follows. First, a mold is heated, and then a water-soluble mold coating material is applied to the inner surface of the heated mold. In this way, on the inside of the mold,
After the mold coating material is applied and dried, molten metal is poured into a mold to manufacture the barrel. After the coating material is applied to the inner surface of the mold and dried, until the molten metal begins to be poured into the mold, it takes time due to installation of the mold into the centrifugal casting equipment, waiting for the composition of the molten metal to be adjusted, etc. The coating material that has been coated and dried in advance absorbs moisture. The gas components to be absorbed and their amounts are, for example, 300 to 500 PPM of nitrogen, 3 to 500 PPM of hydrogen,
4PPM, oxygen 300PPM, etc. Therefore, if the thickness of the coating material applied to the mold is too thin and the molten metal is poured into the mold, the gas absorbed by the coating material will enter the molten metal, causing air bubbles to form in the product. defects occur. Therefore, the water-soluble coating material is applied to a thickness of approximately 3 mm in order to ensure that the gas absorbed by the coating material does not lose a place to escape and to facilitate ease of application. On the other hand, if the coating is made too thin, it is difficult to coat the mold uniformly and the coating material may partially peel off from the mold, which is not preferable.

[0007] As described above, when molten metal is poured into a mold coated with a coating material of about 3 mm, the cast molten metal is gradually cooled due to the heat insulating effect of the coating material, and the structure after cooling has a coarse crystal grain size. It becomes an austenite structure. Therefore, if electron beam welding is used to weld the axle and the cast barrel, which has an austenitic structure with a coarse grain size, the beam will not pass through and the welded part will become unhealthy, so welding of cast materials with a coarse grain size. Construction is carried out using commonly used MIG welding and TIG welding.

[0008]

[Problems to be Solved by the Invention] As mentioned above, MIG is used as a welding method for casting materials with coarse grain size.
Welding and TIG welding had the following problems.

In MIG welding and TIG welding, the welding heat input during welding is high, so components with relatively low melting points such as NbO2 contained in the base metal melt and precipitate, causing microcracks at the weld line boundary. arise. In order to prevent this, if the welding heat input during welding is lowered, poor fusion will occur between the base metal and the weld metal, making it impossible to obtain a sound weld and also requiring a long welding time. Furthermore, welding requires a welding groove such as a V-groove, which requires a large amount of filler metal. Therefore, residual stress after welding was high and heat treatment was required after welding.

0010

[Means for Solving the Problems] In order to solve the above-mentioned problems, the welding method of the present invention is such that when at least one member is a cast material, chill crystals are formed in the welded part of the cast material and an electron beam is applied to the welding method. It is characterized by joining by welding.

[0011]

[Operation and Examples] When at least one member is a cast material, chill crystals are formed in the welded part of the cast material, so the crystal grain size of the welded part becomes fine, and the grain boundaries of these fine grains The presence of impurities and gas components such as hydrogen, oxygen, and nitrogen is extremely reduced. Therefore, even when electron beam welding is applied, impurities present in the grain boundaries and gas components such as hydrogen, oxygen, and nitrogen will not gasify and expand, segregate in the welded area, and impair the beam passage, resulting in a healthy welding process. Welding is possible. Furthermore, by applying electron beam welding to welding cast materials, the welding heat input during welding is low and no microcracks occur in the welded part. Furthermore, since the volume of molten metal in the joint is extremely small, residual stress after welding is low, and heat treatment is not required.

As an example of the present invention, a hearth roll having a barrel diameter of 800 mm as shown in FIG. 1 was welded and manufactured. In FIG. 1, the hearth roll is composed of a shaft 1, an axle 2, and a barrel 3. The shaft 1 is forged from SUS310 material, the axle 2 is made by die casting with SCH19, and the barrel 3 is centrifuged with SCH22 using a thermal spray coating material with a coating thickness of about 1 mm. Manufactured by casting.

[0013] The method of spraying the mold onto the mold was as follows. That is, after preheating the mold, powder raw materials for the coating mold were charged into a CO2-acetylene gas burner and applied by spraying. Therefore, no moisture is contained in the coating mold. Barrel 3 was cast using the above mold by centrifugal casting. FIG. 2 shows how chill crystals 4 are generated in the barrel 3 in this case. As is clear from the figure, chill crystals 4 with a depth of about 28 mm were formed over the entire thick section of the joint weld of the barrel 3 due to the cooling acceleration effect of the mold and the thin spray coating mold.

Axle 2 and barrel 3 manufactured as described above
were welded and constructed in vacuum by electron beam welding, which is the method of the present invention. Figure 3 shows the state of welding in the cross section of the weld. As is clear from the figure, the weld is limited to an extremely narrow area and a healthy structure is obtained, with no welding defects such as microcracks or blowholes. Ta.

As described above, according to the method of the present invention, it is possible to perform electron beam welding of casting materials with coarse grain sizes, and high-quality joints can be manufactured at low cost. In addition, the following effects were found by using a thermal spray coating mold compared to the case of using a conventional water-soluble coating mold. Since the adhesion to the mold after thermal spraying is strong, peeling does not occur after coating, and the thickness of the coating can be reduced. In addition, there was no moisture absorption after the mold coating, suppressing blowhole defects during casting, and the cooling of the molten metal after casting was facilitated because it had higher thermal conductivity than conventional water-soluble mold coating materials.

Next, in order to confirm the action and effect of the present invention,
As a comparative example, barrel 3 was cast using a conventional water-soluble coating mold with a coating thickness of approximately 3 mm, and axle 2 and barrel 3 were welded using conventional MIG welding. . FIG. 4 shows the state of formation of chill crystals 4 in the barrel 3 in this comparative example. As is clear from the figure, the depth of the chill crystals on the outer surface is as shallow as about 5 mm.

FIG. 5 shows the state of welding in a cross section of a welded part in a comparative example. This comparative example is an example in which the welding heat input was low in order to prevent microcracks during welding.
As a result, poor integration 7 into the base metal and poor fusion 8 occur within the welding beam.

FIG. 6 shows the examples of the present invention and comparative examples.
The measurement results of residual stress before and after heat treatment of the weld are shown. As is clear from the figure, the residual stress of the example of the present invention was 10 to 1 in both the circumferential direction and the axial direction before and after heat treatment.
It is as low as 5kgf/mm2 and poses no problem in use. Therefore, it is possible to omit heat treatment after electron beam welding. In the comparative example, the residual stress before heat treatment was approximately 30 kgf/mm2 in the circumferential direction and approximately 50 kgf/mm2 in the axial direction.
This requires high heat treatment, and even after heat treatment, about 30 kgf/mm2 remains in the axial direction.

FIG. 7 shows the examples of the present invention and comparative examples.
The hardness test results of welded parts are shown. As is clear from the figure,
The hardness of the weld heat affected zone in the embodiment of the present invention is slightly higher than the hardness of both members, and its range is limited to a narrow range. Further, the hardness of the welded portion is the hardness of both members. In the comparative example, the hardness of the weld heat affected zone is considerably higher than the hardness of both members, and its range is wide. Further, the hardness of the welded portion is lower than that of both members.

Next, the diameter of the barrel 3 is 650, 800, 1
A 200 mm hearth roll was welded and manufactured by electron beam welding after forming chill crystals on the welded part in the same manner as in the above example, and was actually used in an actual furnace as a hearth roll for a steel sheet heat treatment furnace. did. As a result, in all cases, the welded parts after manufacture had no defects, were sound, and had excellent durability during use.

[0021] In this embodiment, when forming the chill crystal 4 at the welded part of the barrel 3, the coating material and coating thickness are selected as means for controlling the cooling rate of the molten metal after casting.
That is, although the inner surface of the mold was thermally sprayed to a thickness of 1 mm, the method for forming the chill crystal 4 is not limited to this. For example, after casting with a conventional water-soluble coating mold, the entire barrel 3 or only the joint welded portion may be heated and rapidly cooled to form the chilled crystal 4.

The above description has been about the barrel 3, but regarding the formation of chilled crystals 4 in other parts, for other sand casting materials, chilling is performed by using chilled metal or a material with a large cooling effect for the aggregate of the sand mold. Crystal 4 may also be formed. As for the range of chill crystal 4 formation, the entire casting material may be chilled as in this embodiment, or it may be partially chilled.

[0023]

[Effects of the Invention] Conventionally, electron beam welding could not be applied to large or thick casting materials due to the large crystal grain size after casting. It is now possible to apply. As a result, the following effects are achieved.

[0024] By applying electron beam welding, welding heat input is low, microcracks do not occur in the welded part, and it has become possible to manufacture high quality joints of cast materials. ■Welding residual stress is low, and no heat treatment is required to remove residual stress after welding, resulting in lower manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 shows a cross section of a hearth roll according to an embodiment of the present invention.

FIG. 2 shows the state of formation of chill crystals in the barrel of the example of the present invention.

FIG. 3 shows the state of welding in a cross section of a welded part in an example of the present invention.

FIG. 4 shows the generation of chill crystals in a barrel in a comparative example, which is a conventional method.

FIG. 5 shows the state of welding in a cross section of a welded part in a comparative example, which is a conventional method.

FIG. 6 shows measurement results of residual stress before and after heat treatment in an example of the present invention and a comparative example using a conventional method.

FIG. 7 shows the hardness test results of welded parts of an example of the present invention and a comparative example using a conventional method.

[Explanation of symbols]

1 axis 2 Axle 3 Barrel 4 Chill crystal 5. Granular crystals 6 Segregation layer 7. Poor penetration into the base material 8. Poor fusion within the weld bead

Claims (1)

[Claims] 1. A method for electron beam welding of cast materials, characterized in that at least one of the members is a cast material, and the welding is performed by forming chill crystals in the welded portion of the casting material and performing the joining by electron beam welding.