JPH0450108B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a filler metal for welding a Ni-based heat-resistant alloy used in nuclear power plants that operate at high temperatures. [Conventional technology] For example, gas turbines, chemical plants, high-temperature gas-cooled nuclear power plants, etc. 600-1000℃
Heat-resistant alloys are used in the components of various devices that operate at high temperatures. Heat-resistant alloys are known to include Fe-based, Ni-based, and Co-based alloys, and heat-resistant alloys based on Ni are often used in the above-mentioned devices, such as those used in high-temperature gas-cooled nuclear power plants. The Ni-based heat-resistant alloy has the trade name Hastelloy X. The first example is the typical alloy composition.
Shown in the table.

[Table] When constructing a device using this Hastelloy X alloy, welding is usually involved. The welding method is tungsten electrode inert gas arc welding (abbreviated as
TIG) and alloy electrode inert gas arc welding (MIG) are used, and alloys are added to the welded part while melting during welding, and filler metal is required to maintain strength after welding. Regarding this filler metal, generally for welding Hastelloy X alloy,
Standard A5/14E R NiCrMo-2 defined by the American Welding Society (AWS) is widely used, and its chemical composition is shown in Table 2.

[Problem that the invention seeks to solve]

However, when equipment that operates at high temperatures of 600 to 1,000 degrees Celsius (as mentioned above) is used for long periods of time, the welds are susceptible to the well-known creep phenomenon in which the deformation of component parts due to small external forces progresses moment by moment, eventually leading to breakage. There is a problem that satisfactory strength cannot be obtained. In other words, the high-temperature creep strength of weld metals and welded joints welded by TIG or MIG using the previously mentioned Hastelloy High temperature strength is not reliable enough. For example, when performing a creep test at 900℃ and a stress of 4.5Kg/ ^mm2 , the creep rupture time is 100~
After 300 hours, only a low elongation at break of about 1 to 7% can be obtained. Particularly in the case of pipe joints, the weld metal is stretched by the well-known creep deformation of the base material and reaches its critical elongation at break in an extremely short period of time.
Creep rupture time is 1/2 compared to one without joints
It can be as short as ~1/10. The inventors investigated the cause of this problem and found that it was due to the behavior of unavoidable impurities in the filler metal, which are not specified in the above-mentioned AWS standards. This is clear from observation of the microscopic structure of weld metal, which shows that the weld metal has a structure in which a heat-resistant alloy has been locally melted and solidified, and is made up of columnar crystals that grow along the solidification direction. Impurities are concentrated and unevenly distributed in the grain boundaries, which become especially brittle at high temperatures, resulting in a significant decrease in not only creep strength but also elongation at creep rupture, reducing the reliability of welded joints. Therefore, in order to improve the high temperature creep strength of the weld zone, it is necessary to suppress the content of impurities mixed in the filler metal. The present invention has been made in view of the above points, and its purpose is to obtain weld metals and weld joints that are used for welding Ni-based heat-resistant alloys such as Hastelloy X and have excellent high-temperature creep strength and creep rupture ductility. Our goal is to provide filler metals that can be used. [Means for solving the problems] The present invention has C: 0.05 to 0.15% Si: 1.0% or less Mn: 1.0% or less Cr: 20.5 to 23.0% Co: 0.1% or less Fe: 17.0 to 20.0% Mo: 8.0 to 10.0% W: 0.2 to 1.0% Cu: 0.5% or less B: 0.003 to 0.01%, and further contains unavoidable impurities Al: 0.1% or less Ti: 0.005 to 0.05% Mg: 0.005 to 0.05% P: 0.04% or less S This is a filler metal for welding a Ni-based heat-resistant alloy having a composition of: 0.03% or less O: 0.01% or less N: 0.01% or less, and the balance is Ni. [Function] As mentioned above, the welding filler metal of the present invention is
Based on standard materials, we added elements that increase high-temperature creep strength and reduced the content of unavoidable impurities that remain or are mixed in during melting, which aggregates at the grain boundaries of the weld metal and causes intercrystalline formation at high temperatures. The behavior of these impurities, which act to weaken the bonding strength of the weld metal, is suppressed, and the high-temperature creep strength of the weld metal and weld joint is improved, making it possible to realize the original properties of a heat-resistant alloy. [Examples] The present invention will be described below based on Examples. In determining the composition of the filler metal alloy, the inventors first reviewed the alloy components in the AWS standard mentioned above, and found that among the elements considered to be the main components, C,
Since there were no problems with Si, Cr, Fe, Mo, W, and Cu, their explanations will be omitted without any changes; however, as an example, the study results regarding Mn will be described later. In addition, the main element Co
Only the content was changed, and B was added as an element that increases high-temperature creep strength. What is particularly important is determining the appropriate range of unavoidable impurities such as Al, Ti, Mg, and O that are not specified in AWS standards, but we also considered P and S at the same time. Therefore, in this example, from the perspective of the influence of filler metal constituent elements on weld metal on high-temperature creep strength, we will describe the results of experiments conducted mainly on elements that are considered important and elements that differ from AWS standards. . In this experiment, filler metals were produced by melting 20 lots of Ni-based alloys containing the above main components with different addition ranges of each element, and using these fillers, TIG welding of Hastelloy X alloy was performed. The obtained samples were subjected to a weld cracking test, a weld joint bending test, and a creep test. Here, the relationship between the content (wt%) of various elements contained in the filler metal and the creep rupture time is shown in Figures 1 to ² as diagrams obtained under the conditions of a temperature of 900℃ and a stress of 4.5Kg/mm2. It is shown in Figure 10. In each of these figures, a correlation coefficient determined on the assumption that the element content X and the creep rupture time Y have the relationship Y=AX+B is appended. The appropriate content of each element will be described below in the order of FIGS. 1 to 10. FIG. 1 is a diagram related to B, and B is effective as an element that increases high-temperature creep strength, and in JP-A-59-66994, which is a separate invention by the present inventors, the range is from 0.003 to
Although it is disclosed as 0.015%, in this case as well, by containing 0.003% or more, the creep rupture time reaches the base metal level of 500 hours or more.On the other hand, subsequent research has shown that 0.05Zr + 0.39B +
It was confirmed that if the relationship 0.11P+0.004S≦33ppm was satisfied, no cracking of the weld metal would occur.
In this formula, if Zr, P, and S are not contained at all, B can be allowed up to 0.008%, and considering that about 20% is lost due to oxidation during welding, it is preferable to set it at 0.01%. That is, the effective range of B is 0.003 to 0.01%. Figure 2 is a diagram related to Co, and in Figure 2
The higher the Co content, the shorter the creep rupture time, but since the correlation coefficient is only -0.0816, it can be said that it has almost no effect. However, when this alloy is used for the high-temperature gas-cooled nuclear reactor mentioned earlier, if it contains Co, which has a long half-life, the activated Co will circulate in the reactor system together with oxides, etc. It is better not to use Co, as it increases the radioactivity level in the working environment during periodic inspections.
Considering this point, the AWS standard sets the range of Co from 0.5 to
The content is set at 2.5%, but in the present invention it is set at 0.1% or less.
The reason for setting it below 0.1% is that Co is originally 2 to 3% in the Ni raw material.
About % is included, and by refining
This is because even if the Ni purity is increased, the Co concentration of the industrially obtained low CoNi raw material will be about 0.1%. Furthermore, it may be economically viable to use relatively pure Ni as the raw material for the filler metal. For high-temperature equipment other than nuclear reactors, the Co content is 0.1%.
It can be less or more. FIG. 3 is a diagram regarding Al. Since Al, which is highly oxidizing, is normally used as a deoxidizing agent when producing Hastelloy However, as shown in FIG. 3, the smaller the amount of Al, the better for creep characteristics. Particularly in Helium-cooled high-temperature gas furnaces with low oxidation potential, less Al is better since selective internal oxidation occurs due to Al. Al is a necessary element as a deoxidizing agent, but in order to keep the amount remaining in the alloy as low as possible, the creep rupture time is 500 hours or more at the base metal level from the relationship shown in Figure 3.
is the maximum allowable value. FIG. 4 is a diagram regarding Ti. Ti is also an element that cannot be avoided because it is used as a deoxidizing agent by utilizing its oxidizing power, but unlike Al, this element exhibits creep characteristics as the content increases, as shown in Figure 4. Therefore, it is not necessary to set the upper limit based only on creep characteristics. However, when the Ti content increases, there is a problem that it undergoes internal oxidation like Al, so in consideration of this, the creep rupture time is
The content should be in the range of 0.005% to 0.05%, depending on the balance between the two, so that internal oxidation does not occur significantly for more than 500 hours. Also, by including Ti to this extent, it contributes to reducing weld cracking and improving the bending ductility of welded joints. FIG. 5 is a diagram regarding Mn. Mn is one of the elements that is considered to be one of the main components, and the content is AWS
Although it can be left as is in the standard, since it is an element with a large influence, we have illustrated the relationship between content and creep rupture time as an example, as in the case of other elements, as a result obtained in the process of examining the main components. . As shown in FIG. 5, the higher the content, the lower the creep properties, the smaller the aperture of creep rupture, and the greater the correlation.
Mn is an element that contributes to oxidation resistance, but too much Mn deteriorates creep properties, so when emphasis is placed on creep properties, it is desirable to keep it at 0.8% or less. FIG. 6 is a diagram regarding Mg. Mg is an extremely effective element for reducing trace amounts of S, O, and N when added as a deoxidizing agent and desulfurizing agent, and is essential for melting filler metal. As the content increases, the creep properties tend to decrease. However, the correlation coefficient is −
Since it is small at 0.1477, it can be considered that there is almost no effect. Also, the more Mg there is, the less weld cracking will occur and the bending ductility of the welded joint will be better.However, if the content exceeds 0.05%, the flow of the metal during welding may deteriorate, so the maximum allowable amount for Mg should be set at 0.05%. %year,
The minimum allowable amount is 0.005%, which indicates that Mg has sufficiently deoxidized and desulfurized. FIG. 7 is a diagram regarding P, and FIG. 8 is a diagram regarding S.
P and S are normally regarded as harmful elements in metal materials, unless a special effect is expected, and the appropriate range is specified in the material standards, and they are also specified in the AWS standards, but in the present invention, In particular, since we focused on the influence of unavoidable impurities, Figures 7 and 8 were obtained for P and S. As a result, as shown in FIG. 7, the creep characteristics of P improve as the content increases, while the creep characteristics of S, on the contrary, tend to decrease as the content increases. but,
Considering that the inclusion of P increases the creep strength but deteriorates the bending ductility of welded joints, the maximum allowable amount of P is set at 0.04%, which is the same as the AWS standard, and the maximum allowable amount of S is also set as the same as the AWS standard. The same value is 0.03%, but
It is more advantageous for creep strength to reduce it as much as possible within this range. FIG. 9 is a diagram regarding O. O is an unavoidable impurity that enters from the atmosphere during melting of filler metal,
It gathers in the form of oxides at the grain boundaries of weld metal and weakens the high-temperature strength of the grain boundaries. Naturally, as shown in FIG. 9, the higher the content, the lower the creep characteristics. Therefore, it is important to determine the maximum allowable amount of O, which is not specified in the AWS standard.
To obtain the above, set it to 0.01%. FIG. 10 is a diagram regarding N. N is also an unavoidable impurity with the same meaning as O, and it is important to determine the limit value of its content. However, in order to increase the creep strength of N, there is a conventionally known technique in which, for example, in stainless steel, N is sometimes actively added as a solid solution strengthening element, but in the filler metal of the present invention, the As shown in Figure 10, the creep properties tend to decrease as the N content increases, and it was found that the lower the correlation coefficient, the better. This means that, like O, N also segregates at the grain boundaries of the weld metal and behaves so as to reduce the high-temperature strength. Therefore, the lower the N content, the better, but from the relationship shown in Figure 10, the creep rupture time
The maximum allowable amount of N for 500h or more should be 0.01%. Based on the above experimental results for determining the composition range of the filler metal of the present invention, we have found that raw materials not specified in the AWS standards contain unavoidable impurities that enter from the surrounding atmosphere during melting, and in particular, concentrate at grain boundaries. The most important feature of the filler metal of the present invention is that it has determined appropriate allowable amounts of O and N, which weaken the bond between crystals and reduce high-temperature creep strength. [Effect of the invention] Filler metals used for welding Ni-based heat-resistant alloys such as Hastelloy Since high-temperature strength is not taken into consideration, when applied to welding structural components of structures such as high-temperature gas-cooled nuclear reactors, the high-temperature creep properties are insufficient, making these devices unreliable for long-term use at high temperatures. In contrast, the filler metal of the present invention, as described in the examples,
Although the composition is based on the composition of AWS standard materials, we especially considered the remaining amount of unavoidable impurities that come in from raw materials and secondary materials during melting, and among these, Al,
By clarifying the appropriate range of various elements that are not specified in the AWS standard, such as determining the maximum allowable amount with emphasis on Ti, O, and N, and by improving or adding some of the constituent elements, the AWS standard Weld metal and welded joints at 900℃ and 4.5
Creep rupture time at Kg/mm ² is only 100 ~
In contrast, the filler metal of the present invention can obtain a creep rupture time of at least 500 hours under the same conditions, and has a large elongation at fracture, resulting in very good weldability. It has been possible to provide high reliability to welding of high-temperature drive devices using base heat-resistant alloys.

[Brief explanation of the drawing]

Figures 1 to 10 are diagrams showing the relationship between the amount of elements contained in the filler metal of the present invention and the creep rupture time of weld metal; Figure 1 is B, and Figure 2 is Co. , Figure 3 is Al, Figure 4 is Ti, Figure 5 is Mn,
FIG. 6 shows the relationship between the creep rupture time and the content of Mg, FIG. 7 shows the content of P, FIG. 8 shows the content of S, FIG. 9 shows the content of O, and FIG. 10 shows the relationship of the N content.

Claims (1)

[Claims] 1. C: 0.05 to 0.15 Si: 1.0 or less Mn: 1.0 or less Cr: 20.5 to 23.0 Co: 0.1 or less Fe: 17.0 to 20.0 Mo: 8.0 to 10.0 W: 0.2 to 1.0 Cu: 0.5 Contains the following B: 0.003 to 0.01, and further contains unavoidable impurities Al: 0.1 or less Ti: 0.005 to 0.05 Mg: 0.005 to 0.05 P: 0.04 or less S: 0.03 or less O: 0.01 or less N: 0.01 or less, and the balance is Ni A filler metal for welding a Ni-based heat-resistant alloy, characterized by having the following composition: