JPH02299767A - Method for joining spheroidal graphite cast iron - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for joining spheroidal graphite cast iron used for mutually joining spheroidal graphite cast iron or joining spheroidal graphite cast iron and mild steel.

[Conventional technology]

The castability of cast iron has been improved by adding large amounts of C and Sk to Fe, but due to this, spheroidal graphite cast iron has significantly inferior mechanical properties, especially ductility and toughness, compared to steel. In order to improve the ductility and toughness, precipitated graphite is spheroidized by heat treatment. However, when this spheroidal graphite cast iron is subjected to a welding heat cycle, graphite dissolves in the matrix during heating and becomes austenite, and during cooling it undergoes martensitic transformation and hardening, resulting in cracking.

In addition, when welding spheroidal graphite cast iron, Ni-based welding materials are often used to improve the ductility of the weld metal, but in such welding, the melting point of the weld metal exceeds the melting point of the base metal. A part of the weld heat affected zone called HAZ may melt, and cracks may occur in the HAZ due to thermal strain.

To solve these problems, a welding method that is an improvement on gas welding and a welding method that uses plasma keyhole welding has been proposed for joining spheroidal graphite cast iron (Japanese Patent Laid-Open No. 56-36384). Publication, Japanese Patent Application Laid-open No. 59785367). These welding methods suppress hardening, embrittlement, and cracking caused by structural changes caused by welding heat cycles, but they all have major problems in workability.

[Problems to be solved by the invention] In other words, the gas welding method requires a high degree of technical skill, has poor workability, and, moreover, the reliability of the joint depends on the skill of the constructor ( On the other hand, the plasma keyhole welding method is suitable for downward position welding because it uses a plasma arc to make a keyhole in the molten pool and holds it while forming a back wave. Therefore, when melting one side of a pipe, it is necessary to fix the welding position and perform welding while rotating the pipe.However, when working on piping inside a plant or large equipment, it may be necessary to perform circumferential welding while the pipe is fixed. In that case, extremely advanced technology is required to form the Uranami, and even if welding is performed using advanced technology, it is difficult to form a good Uranami over the entire circumference.In addition, the reproducibility of the weld bead shape is , there are also problems that depend on individual skill.

The present invention solves these construction problems, and its purpose is to join spheroidal graphite cast iron easily and efficiently, and to stably obtain a sound joint. The purpose is to provide a method.

[Means for Solving the Problems] Cracking, which is a problem in welding spheroidal graphite cast iron, is caused by melting of the base material. Diffusion bonding using insert metal is well known as a bonding method that does not melt the base metal. The present inventors have determined that this diffusion bonding method is suitable for bonding spheroidal graphite cast iron, and have investigated its practical use. As a result of repeated various experimental studies toward this goal, we obtained the following knowledge.

In order to melt only the insert metal, it is necessary to add a melting point lowering element to the insert metal and melt the insert metal at a temperature below that at which the base metal melts.

As the melting point lowering element, select an element that easily diffuses into the base material.

By doing so, the elements that lower the melting point are removed from the bonding layer, and embrittlement due to residual elements is prevented. As a result, not only can the joint be made sound, but the conditions of the joining atmosphere and the roughness of the surfaces to be joined can be relaxed, and workability is improved. In addition, by appropriately controlling the pressing force, deformation such as blistering can be prevented.

After the insert metal is melted, the cooling rate from the high-carbon austenite phase is reduced to suppress the formation of the martensite phase, thereby preventing hardening of the joint and improving ductility.

By combining the above means, spheroidal graphite cast iron can be easily and efficiently joined, and a sound joint can be stably obtained.

The present invention was made based on the above-mentioned findings, and includes an Fe5 or Ni group containing at least one of Si: 8% or less, 81% or less, P: 11% or less, and has a melting point of 11% or less.
Insert metal of 00'C or less is sandwiched between the joints, and the joint is pressurized at a pressure of 0.5 to 1.0 kg r / 2 and heated to 1100C or less and above the melting point of the insert metal for 60 seconds or more. Oz: 1.5℃ up to at least 500″C after being maintained in an atmosphere of 200ρpm or less
The gist of this invention is a method for joining spheroidal graphite cast iron, which is characterized by cooling at a rate of less than 1/2 seconds.

In the insert metal used in the joining method of the present invention, one or two of Cr: 25% or less and Co: 25% or less can be added.

After joining, heating may be performed at 500 to 700''C for 2 hours or more. This heating further promotes prevention of hardening of the joint and improvement of ductility.

The joining method of the present invention can be used not only for joining spheroidal graphite cast iron to each other, but also for joining spheroidal graphite cast iron and mild steel.

[Function] As the spheroidal graphite cast iron to be joined by the joining method of the present invention, for example, ASTM Au grade A339-55, A39
Examples include B-58.

It is difficult to stably obtain sound joints with these cast irons by fusion welding.

The insert metal used in the joining method of the present invention includes Si,
The reason for adding at least one of B and P is to lower the melting point of the insert metal. Furthermore, these elements easily diffuse into the base material during bonding, and their diffusion rate is also high. Therefore, embrittlement of the bonding layer due to element addition is prevented. In order to sufficiently lower the melting point of the insert metal, it is preferable to contain these elements in an amount of 2% by weight or more.

However, if the amount added is too large, it becomes difficult to diffuse and remains in the bonding layer.
Each is limited to 511% by weight.

At least one or more of St, B, and P may be contained within the above range.

Cr and Co are effective in refining graphite and contribute to improving the ductility of the joint. However, when added in excess, the bonding layer becomes brittle. Therefore, Cr, C.

When adding, the upper limit is 25% for each element.
shall be.

The melting point of the insert metal must be 1100°C or lower to prevent local melting of the base metal, and even if the insert metal satisfies the above composition, the melting point is 1100°C or lower.
Temperatures exceeding 00°C are unsuitable. Although the lower limit of the melting point is not particularly required, it is realistically about 700°C.

The thickness of the insert metal is 10-100μm.
1 Preferably 20 to 50 μm. If it is extremely thin, there will be insufficient insert metal and the desired effect will not be obtained.
If it is too thick, the bonding time will be long and it is not economical.

Regarding the bonding atmosphere, O1 is used to prevent oxidation of the bonding layer.
Limit the amount to 200 ppm or less. -Finally, in the diffusion bonding method, it is important to control the bonding atmosphere (O amount) to prevent the bonding layer from becoming brittle, which leads to deterioration in workability. The melting point lowering element added to the metal turns the bonding layer into a liquid phase, thereby preventing the occurrence of defects due to residual oxides. As a result, the allowable range of the bonding atmosphere is wider than that of general diffusion bonding methods.
Construction becomes easier. Further, the conditions regarding the pressurizing force and the roughness of the surfaces to be joined, which will be described later, are also relaxed.

The pressurizing force during welding is 0.5 to ensure that the surfaces to be joined are in close contact with each other.
kg r/mm" or more is required, but l kg
f/m”, the deformation of the joint will increase, so
The amount should be 0.5 to 1 kgr/kaku2.

The roughness of the surfaces to be joined should be as smooth as possible, but in the joining method of the present invention, the roughness is 60 μm.
The desired effect can be obtained even with Rmax.

The joining temperature was 1100″C to prevent local melting of the base metal.
below and above the melting point of the insert metal. However, even if the melting point of the insert metal is 1000° C. or lower, the bonding temperature is preferably 1000° C. or higher in order to shorten the bonding time.

The bonding temperature holding time is required to be 60 seconds or more in order to sufficiently diffuse the melting point lowering element into the base material, but it is desirable to keep it as short as possible within this condition in order to ensure economic efficiency.

After being heated and maintained, it is cooled to at least 500° C. at a rate of 1.5° C./second or less in order to prevent hardening of the bonded portion. That is, at the time of joining, the base material structure near the joint becomes high-carbon austenite, and if martensitic transformation occurs during cooling after heating and holding, hardening progresses and becomes brittle. In order to prevent this phenomenon, it is necessary to reduce the cooling rate sufficiently (up to 500℃),
By setting the speed to less than /second, the base material near the joint becomes a mixed structure of bainite, pearlite, and ferrite, and hardening is suppressed. Although a lower limit of the cooling rate is not particularly required, from the viewpoint of bonding efficiency, a cooling rate of 0.5° C./sec or more is desired. 5
Cooling below 00°C does not require limiting the cooling rate.

After joining, 500~
Heat treatment at 750'C for 2 hours or more is performed as necessary. If the heat treatment temperature is less than 500℃, the tempering effect will be small, and if it exceeds 750℃, part of the structure will become austenitic.
Since it becomes martensite when cooled, the heat treatment temperature is 50℃.
The temperature shall be 0 to 750°C.

The holding time is required to be 2 hours or more to ensure the tempering effect, and from the economic point of view, the shorter the time the better.

(Example) Outer diameter 216.3+mg, wall thickness 111II! l, length 50
A 0 m spheroidal graphite cast iron pipe and a mild steel pipe of the same size were cut at right angles to the pipe axis, and the end faces were finished to 50 μmRmax as test materials. The composition of the sample material is shown in Table 1.

As the insert metal, an amorphous ribbon having a thickness of 20 to 50 μm produced by a rapid solidification method was used. The chemical composition of the insert metal used is shown in Table 2.

For the test, as shown in Fig.
An annular heating head 2 was used, which was fixed by butting b.

The heating head 2 has a half-split shape, and has restraining parts 3a and 3b for the test materials 1a and lb at both ends, and an inductor 4 between them. Then, using this heating head 2, the - set of specimens 1a and lb are fixed with insert metal sandwiched between their end faces, and the inner surfaces of specimens 1a and lb and the heating head 2 are fixed.
The abutting portions of the test materials 1a and 1b were heated by an inductor 4 while flowing N2 gas inside. By this heating, the insert metal between the end faces is melted, and the test materials 1a and 1
b extends in the axial direction, and pressurizing force is applied between the joint surfaces. As a result, the end faces are diffusion bonded. The pressurizing force is the restraint part 3
It was adjusted by changing the rigidity of a and 3b to alleviate the reaction force due to thermal expansion of the test materials 1a and lb. Further, the bonding temperature, heating holding time, and cooling rate were adjusted by operating the high frequency power source 5 of the inductor 4.

After the test materials 1a and 1b were joined, a test piece shown in FIG. 2 was taken from the joint, and the test piece was subjected to a tensile test. The results and joining conditions are shown in Table 3.

Table 2 Tests A1 to A10 outside the conditions of the present invention are examples in which spheroidal graphite cast iron pipes were joined together. Since the bonding conditions are controlled within the scope of the present invention,
Except for AIO, which was heat treated after bonding, the bonding strength of all of them exceeds 60 kgf/kaku2. In addition, All is an example of joining a spheroidal graphite cast iron pipe and a mild steel pipe, and it was confirmed that a break occurred in the mild steel base material and a sound joint was made i).

On the other hand, B1 has a low bonding strength because the bonding temperature does not reach the melting point of the insert metal. B2 had low bonding strength due to insufficient heating holding time, and B3 had sagging at the bonded portion due to excessive pressure. B4,
In No. 5, the cooling rate and bonding temperature were excessive, and cracks occurred in the bonded portion. B6 has a low bonding strength because the amount of 0 in the sea urchin atmosphere is large. B7 and 9.10 have low bonding strength due to excessive melting point depressing elements in the insert metal, and B8
Because the melting point lowering element is insufficient, the insert metal does not melt and the bonding strength is low. .

〔Effect of the invention〕

The joining method of the present invention imparts excellent joint strength to the joint of spheroidal graphite cast iron, and prevents cracking and deformation of the joint. Furthermore, the joint strength is controlled by the joining conditions and is not affected by the skill of the worker, making construction easier and the joint quality stable. Furthermore, since the joining conditions are relaxed, workability is significantly superior.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the joining method adopted in the embodiment of the present invention,
FIG. 2 is an explanatory diagram of the shape of the test piece. In the figure, la, lb: sample material, 2: heating head.

Claims (1)

[Claims] 1. Si: 8% or less, B: 4% or less, P: 11 in weight%
An insert metal containing at least one type of Fe group or Ni group with a melting point of 1100°C or less is sandwiched between the joint parts, and the joint part is heated at a rate of 0.5 to 1.0 kgf/mm^2
Under a pressure of 1100℃ or less, above the melting point of the insert metal for 60 seconds or more O_2: 200pp
1. A method for joining spheroidal graphite cast iron, which comprises maintaining the spheroidal graphite cast iron in an atmosphere of 1.5° C. or less and then cooling it to at least 500° C. at a rate of 1.5° C./sec or less. 2. Insert metal weight% Cr: 25% or less, C
The method for joining spheroidal graphite cast iron according to claim 1, characterized in that it contains one or two types of o: 25% or less. 3. The method for joining spheroidized graphite cast iron according to claim 1 or 2, wherein heating is performed at 500 to 700°C for 2 hours or more after joining. 4. The method for joining spheroidal graphite cast iron according to any one of claims 1 to 3, characterized in that spheroidal graphite cast iron and mild steel are joined.