JPH0825055B2 - Welding material for high Cr ferritic steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a welding material used for welding high Cr ferritic steel for high temperature.

[Conventional technology]

Boilers, heat exchanger tubes for the chemical industry or nuclear industry, high-temperature materials used for heat-resistant pressure piping, austenitic stainless steel, low alloy steel such as 2 1 / 4Cr-1Mo steel,
Three types of high Cr ferritic steels represented by 9Cr-1Mo steel are well known. Among them, high Cr ferritic steels are beginning to be widely used because they are resistant to high temperature corrosion and stress corrosion cracking and are inexpensive.

This high Cr ferritic steel is usually used by adjusting its structure and mechanical properties by heat treatment. For example, the high Cr ferritic steel disclosed in JP-A-62-89842 is based on 8 to 13 Cr ferritic steel and contains Mo, W, V, Nb,
The amount of Ni, Al, etc. was adjusted to ensure excellent high-temperature long-term creep strength.
It is supposed to be tempered at 700-800 degrees.

On the other hand, when welding to such a high Cr ferritic steel, the welding material is austenitic stainless steel,
Three types of Ni-based alloys and gold alloys are applied. Among these, the common metal welding material is difficult to secure the toughness of the weld metal, but cracks due to thermal expansion difference and cracks due to decarburization layer formation can be prevented, and the material cost is low. It is often used when the ambient temperature is relatively low, assuming that the toughness of the steel is improved. Therefore, high
The common metal welding materials for Cr ferritic steel are designed on the premise of post-weld heat treatment.

[Problems to be Solved by the Invention]

However, the post-weld heat treatment is a factor that increases the welding construction cost and also causes deformation. Therefore, if the toughness of the weld metal is ensured, it is desirable that the post-weld heat treatment be omitted.

The present invention, in order to meet such a demand, imparts particularly excellent corrosion resistance to the weld metal, and can impart excellent toughness to the weld metal even without post-weld heat treatment. The purpose is to provide a welding material.

[Means for solving the problem]

When a welding material having the same composition as the base material is used for high temperature Cr ferritic steel, the weld metal has a ferrite / martensite two-phase structure like the base material. However, the ratio differs from that in the base metal. According to the research conducted by the present inventors, the toughness of the weld metal when the high Cr ferritic steel is welded with the co-metal type welding material is determined by the ratio of ferrite / martensite in the weld metal, particularly this ratio in the base metal. It turns out that the differences are strongly dominated. Also,
It was clarified that the hardness and O content of the weld metal also strongly affect the toughness.

Based on such a factual relationship, the inventors of the present invention have studied various toughness reduction preventive measures in a co-metal material, and as a result, suppress the O content in the welding material, increase the martensite content, and further increase the hardness. High Cr by reducing
It was found that the toughness of the weld metal can be sufficiently increased without post-weld heat treatment even when higher corrosion resistance is secured.
Further, the weld metal is usually subjected to post-weld heat treatment, and if the post-weld heat treatment is omitted, it is subjected only to the heat cycle by welding. On the other hand, the base material undergoes hot working, normalizing, and tempering before welding, and is also heated during welding. It was also found that the difference in thermal history between the two causes a decrease in toughness.

The present invention has been made on the basis of such findings, and in weight%, C: 0.01 to 0.1%, Si: 0.01 to 1%, Mn: 0.01 to 2%, C
r: more than 10.5% and 12% or less, Ni: 2-5%, Mo: 0.1-3%, W: 0.
3 to 3%, V: 0.1 to 0.5%, Nb: 0.01 to 0.2%, Al: 0.04% or less, N: 0.003 to 0.08%, O: 0.01% or less, Ca: 0.0005 to 0.01
%, And Cr _eq : 12% or less, but Cr _eq = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb + 12Al-40C-30N -4Ni-2Mn [%] Qc: 0.15% or less, but Qc = C + Mn / 20 + Si / 30 [%] is satisfied. , A balance of Fe and unavoidable impurities, and a welding material for high Cr ferritic steel.

The welding material of the present invention has an important point that excellent corrosion resistance is ensured by increasing Cr, and that the ferrite / martensite ratio is improved by increasing Ni in order to prevent the toughness deterioration that becomes a problem under high Cr. There is. That is, it is effective to reduce the amount of ferrite in order to improve the toughness of the weld metal, and for that purpose, it is necessary to increase the amounts of C, N and Ni. However, since C and N are elements accompanied by an increase in hardness, the ratio of ferrite / martensite was mainly improved by increasing the amount of Ni. Ni increases the amount of martensite without increasing the hardness and contributes to the improvement of the ferrite / martensite ratio, but on the other hand, it also has the effect of lowering the creep strength. However, it was also clarified that creep strength of 600 ° C or less can be secured by adjusting other components.

[Action]

The reasons for limiting each requirement in the welding material of the present invention will be described below. In addition,% represents% by weight unless otherwise specified.

C: Combines with Cr, Mo, W, V, and Nb to form a carbide, which increases the creep strength of the weld metal. If C is less than 0.01%, this effect is not sufficient, while if it exceeds 0.1%, the weld metal is hardened by the welding heat cycle, and sufficient toughness cannot be secured without post-weld heat treatment. Therefore, C is 0.01 to 0.1
%.

Si, Mn: Effective as a deoxidizer, but if it is less than 0.01%, the cost is economically increased, and if it exceeds 1% or 2%, the toughness decreases. Therefore, Si is 0.01-1% and Mn is
0.01 to 2%.

Cr: One of the basic components of high Cr ferritic steel for high temperature,
From the viewpoint of ensuring the oxidation resistance of the weld metal, it is necessary to add more than 10.5%. Fig. 4 shows the relationship between the scale layer thickness and the Cr content of the welding material when steam-oxidizing the weld metal.
Particularly good oxidation resistance is obtained in the region where the amount is larger than about 10%. However, if it exceeds 12%, the amount of ferrite in the weld metal increases and the toughness decreases. Therefore Cr is 10.5
% To 12% or less.

Ni: An austenite forming element, which contributes to the optimization of the ferrite / martensite ratio in the weld metal by increasing the amount of martensite. It also contributes to improving the toughness of the matrix. For that purpose, it is necessary to add 2% or more. Fig. 5 shows the relationship between the Charpy impact value of the weld metal and the Ni content of the welding material. Under the condition of Cr> 10.5% and no heat treatment after welding, the impact value increases sharply at Ni = about 2%. However, excessive addition increases the effect of lowering the creep strength and impairs the economical efficiency, so the content is made 5% or less.

Mo and W: Both increase the high temperature long-term creep strength, and if Mo is less than 0.1% and W is less than 0.3%, the effect is small. However, if it exceeds 3%, an intermetallic compound is precipitated and the toughness of the weld metal is reduced. Therefore Mo is 0.
1 to 3% and W to 0.3 to 3%.

It combines with V: C, N and precipitates fine carbonitrides, contributing to the improvement of high-temperature long-term creep strength. If it is less than 0.1%, this effect is small, and if it exceeds 0.5%, carbonitrides become coarse,
On the contrary, it lowers the creep strength at high temperature for a long time. Therefore, V is 0.1 to 0.5%.

Similar to Nb: V, it improves the high temperature long-term creep strength by fine precipitation of carbonitride. Furthermore, NbC is precipitated to improve the toughness by refining the structure. If it is less than 0.01%, this effect is small, and if it exceeds 0.2%, a large amount of NbC is precipitated to reduce the amount of solid solution C, and the strength of the weld metal is lowered.
Therefore, Nb is 0.01 to 2%.

Al: It is added as a deoxidizer, but if it exceeds 0.04%, the high temperature long-term creep strength of the weld metal is impaired. Therefore Al
Is 0.04% or less.

It combines with N: V and Nb to precipitate carbonitrides and contributes to the improvement of high-temperature long-term creep strength. If it is less than 0.003%, this effect is small, and if it exceeds 0.08%, it becomes difficult to give sufficient toughness to the weld metal without post-weld heat treatment. Therefore N is
Set to 0.003 to 0.008%.

O: Weakly bonds the iron atoms of the weld metal and weakens the matrix, resulting in a significant decrease in the toughness of the weld metal. Therefore, O is limited to 0.01% or less, and the smaller the content, the better.

Ca: added as a deoxidizer, contributes to lowering the O content of the weld metal and improves toughness. If it is less than 0.0005%, its effect is small, and if it exceeds 0.01%, it remains as an inclusion in the weld metal and rather reduces the toughness. Therefore, Ca is 0.0005 to 0.
01%

Other than the above components, Mg: 0.01% or less, Ce: 0.02% or less, La:
One or more than 0.02% may be added. Like Ca, they contribute to lowering the O content of the weld metal and improve their toughness.

Both the P and S impurity elements are harmful impurities for weldability and high temperature long-time creep strength. From this viewpoint, P is preferably set to 0.025% or less and S is set to 0.015% or less.

Cr _eq : When high-temperature high-Cr ferritic steel is welded with a co-metallic material, the weld metal has a ferrite-martensite two-phase mixed structure, similar to the base metal. In a weld metal having a two-phase mixed structure of ferrite and martensite, the larger the Cr _eq expressed by the following formula, the larger the amount of ferrite.
Then, an increase in the amount of ferrite reduces the toughness of the weld metal. According to the investigation by the present inventors, when Cr _eq exceeds 12, the amount of ferrite in the weld metal becomes 60% or more, and the toughness is practically sufficient (Charpy impact value 5 kgf · m / cm at 0 ° C.).
² or more) cannot be secured. Therefore, Cr _eq is regulated to 12% or less.

Cr _eq = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb + 12Al-40C -30N-4Ni-2Mn [%] Qc: According to the study of the present inventors, the toughness of the weld metal when high temperature high Cr ferritic steel is welded with a common metal type material It was found that the decrease is largely due to the increase in the amount of ferrite and that the heat history of the weld metal is different from that of the base metal. That is, the base material is high-Cr for high temperature after hot forming.
While the ferritic steel is subjected to normalizing and tempering treatments and then used for welding, the weld metal is only subjected to a thermal cycle by welding if it is not subjected to post-weld heat treatment.
In order to compensate for the insufficient toughness of the weld metal due to the difference in the thermal history by the component design of the welding material, the present inventors introduced a parameter Qc represented by the following formula.

Qc = C + Mn / 20 + Si / 30 [%] According to the investigation by the present inventors, the higher the Qc, the harder the martensite phase in the weld metal and the lower the toughness. Especially when Qc exceeds 0.15%, practically sufficient toughness without heat treatment after welding (Charpy impact value at 0 ° C of 5k
gf ・ m / cm ² or more) is not secured. Therefore, Qc is 0.1
5% or less. To reduce Qc to less than 0.01%, C
Must be less than 0.01%, resulting in a decrease in high temperature long-term creep strength. Since C is secured at 0.01% or more, Qc becomes 0.01% or more as the amount of C is secured.

Although the welding material of the present invention is premised on the omission of the post-weld heat treatment, the post-weld heat treatment does not adversely affect the quality of the weld metal, so that the post-weld heat treatment should be performed for residual stress, toughness assurance, and other reasons. It doesn't matter. In this case, the temperature is preferably 760 ° C. or lower in order to prevent the martensite from austenizing and the creep strength from lowering.

〔Example〕

Next, the characteristics of the welding material of the present invention will be described in comparison with the comparative material.

50 steels with the chemical composition shown in Table 1 as A1-A6 and B1-B6
kg melted in a vacuum melting furnace, further forged and rolled to an outer diameter of 2 m
The welding material of the present invention and the comparative welding material having a length of m and a length of 1000 mm were produced. On the other hand, as a base material, steel having the chemical composition shown in Table 2 was melted in a 50 kg vacuum melting furnace, forged at 1150 to 950 ° C., and rolled to manufacture a plate having a thickness of 10 mm. The manufactured base metal was subjected to 1050 ° C. × 1 hr, AC normalizing and 830 ° C. × 0.5 hr, AC tempering. This base material is a high Cr ferritic steel for high temperature proposed in JP-A-62-89842.

Then, the base material was provided with the groove shown in FIG. 1, and 5 layers of TIG welding shown in FIG. 1 were performed under the conditions shown in Table 3 using the various welding materials described above. After welding, some welded joints are post-weld heat treated at 715 ℃ × 0.3hr, AC, and others are not heat treated after welded, and a notch is provided in the center of the weld metal shown in Fig. 2 from the welded joints. The Charpy impact test piece and the creep test piece shown in FIG. 3 were cut out and each test was performed. Further, a test piece of 2 × 5 × 25 mm was taken from the weld metal and heated in steam at 700 ° C. × 1000 h, and the scale thickness was rated as less than 100 μm and rated as 100 μm or more.

The Charpy impact test was carried out at 0 ° C., and the creep test was conducted under the conditions of a temperature of 600 ° C. and a load stress of 14 kgfm / mm ² to investigate the time until breakage.

 The test results are shown in Table 4 in correspondence with the welding materials.

As is clear from Table 4, the welded joints using the welding materials A1 to A6 all exhibited a toughness of more than 7 kgfm / cm ² and a creep rupture time of more than 3500 hours, even though there was no heat treatment after welding. There is. However, other than A4 and A5, the corrosion resistance is poor because the Cr content is small. That is, even though only A4 and A5 have high Cr, they have high toughness and high-temperature strength.

The welded joint using the welding material A6 shows a toughness of 9.2 kgfm / cm ² even without post-weld heat treatment, but the toughness is improved to 12.8 kgfm / cm ² by the post-weld heat treatment, and creep rupture occurs. Time is virtually unchanged.

On the other hand, the welded joint using the welding material B1 has good toughness, but the creep rupture time does not reach 1000 hours. Welded joints using other welding materials (B _{4 to} B ₆ )
Although it has excellent corrosion resistance due to Cr, the creep rupture test was omitted under such high Cr because the toughness of the weld metal was poor without post-weld heat treatment.

〔The invention's effect〕

The welding material for high Cr ferritic steels of the present invention ensures high corrosion resistance and, at the same time, imparts sufficient toughness to the weld metal without heat treatment after welding. Therefore, post-weld heat treatment can be omitted,
It is possible to significantly reduce the welding construction cost and prevent deformation due to heat treatment after welding. Moreover, since the welding material of the present invention is a co-metal material, it is possible to prevent induction cracking due to the difference in thermal expansion from the base material and the formation of the decarburized phase, and the material cost is significantly lower than that of the Ni-based material.

[Brief description of drawings]

Fig. 1 is a cross-sectional view showing the groove shape of the welded portion and the laminating method in the welding test, Figs. 2 and 3 are side views of the Charpy impact test piece and the creep test piece, and Fig. 4 is the Cr content of the welding material. Is a graph showing the relationship between the corrosion resistance of the weld metal and FIG. 5 is a graph showing the relationship between the Ni content of the welding material and the toughness of the weld metal. In the figure, 1: weld metal, 2: base metal.

Claims (1)

[Claims] 1. C: 0.01-0.1%, Si: 0.01-1% by weight%,
Mn: 0.01 to 2%, Cr: more than 10.5% and 12% or less, Ni: 2 to 5%, M
o: 0.1-3%, W: 0.3-3%, V: 0.1-0.5%, Nb: 0.01-
0.2%, Al: 0.04% or less, N: 0.003 to 0.08%, O: 0.01% or less, Ca: 0.0005 to 0.01%, and Cr _eq : 12% or less, but Cr _eq = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb + 12Al-40C- 30N-4Ni-2Mn [%] Qc: 0.15% or less However, Qc = C + Mn / 20 + Si / 30 [%] is satisfied, and the balance is Fe and inevitable impurities.