JPH01215489A - Welding material for high cr ferrite steel - Google Patents

Abstract

PURPOSE:To improve the stiffness and high temp. creep property of a welding metal by regulating the chemical component including Ni, Mo, W, V, Nb, N, O, etc., in a material and related parameter value in specified range respectively. CONSTITUTION:In weight %, 0.05-0.17% C, 0.01-1.5% Si, 0.01-2% Mn, <=0.025% P, <=0.015% S, 8-12% Cr, <=0.8% Ni, 0.5-3% Mo, 0.5-3% W, 0.1-0.5% V, 0.01-0.2% Nb, <=0.04% Al, 0.03-0.05% N, <=0.01% O and 0.0005-0.01% Ca are contained and the parameter Creq and Qc shown by equations I, II are regulated <=13% and <=0.2%. The stiffness and high temp. creep property of the welding metal to be formed are improved because the regulation of the chemical component and parameter is performed both.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a welding material for use in high-chromium ferrite steel for high temperature use, which can be used in a high-temperature environment with a metal temperature of 600° C. or higher.

[Conventional technology]

High-temperature materials used for boiler superheater tubes, reheater tubes, heat exchangers for the chemical industry, atomic crab industry, etc. are often required to have heat resistance of 600° C. or higher.

Conventionally well-known high-temperature materials include austenitic stainless steel and 2+ACr-1Moli.
There are three types of ferrite steels: low alloy steel such as 9Cr-IMo steel, and high Cr ferritic steel represented by 9Cr-IMo steel. Among these, high Cr ferritic steel is less prone to high-temperature corrosion and stress corrosion cracking, is inexpensive, and has a high Cr content and excellent heat resistance, making it difficult to use at temperatures above 600°C. It is beginning to be widely used to replace low-alloy steel.

In high-temperature materials such as high-Cr ferritic steel, high-temperature long-term creep strength and structural stability are especially important in heat resistance. In high-temperature high Cr ferritic steel, the m-weave is tempered to form martensite, and in order to stabilize creep strength over long periods of time at high temperatures, tempering is performed at least 150°C higher than the service temperature and 30 to 50°C lower than the Ac+ point. It is customary to receive. In this tempering, 60
Assuming a working temperature of 0°C or higher, the tempering temperature should be 750°C or higher, preferably 800°C.
The above is required. However, in the case of 12Cr steel, AC
, point is around 800°C, and tempering above 800°C cannot be adopted.

For this reason, it is necessary to design an alloy to increase the Ac, point. For example, in JP-A No. 62-89842, for 8-13Cr ferrite steel, St, Mn
, Mo, W, V, Nb, Ni.

A:0. A high Cr ferritic steel has been proposed, which has a higher A C+ point through adjustment of NIL, etc., which enables high-temperature tempering and ensures excellent high-temperature long-term creep strength.

On the other hand, when high temperature materials such as high Cr ferritic steel are used in boilers,
When used as heat exchanger parts, welding is often required. When welding high Cr ferritic steel, welding materials include austenitic stainless steel, Ni-based alloys, and high Cr ferrite. The use of steel is considered.

[Problem to be solved by the invention]

However, in the case of austenitic materials, there is a large difference in thermal expansion between the base metal and the weld zone, which causes cracking due to the generation of thermal stress.Additionally, during use, carbon migrates from the base metal to the weld zone, forming a decarburized layer. cracking is also induced. In the case of N+ base material, there are problems in terms of cracking due to the difference in thermal expansion and material cost. Furthermore, in the case of co-metallic thread materials, there is a fatal flaw in that toughness cannot be ensured.

Judging from the above, at present, austenitic or Ni3-based welding materials are being used, although they still have various problems.

In view of the current situation, it is an object of the present invention to provide a welding material for alloy-based high Cr ferritic steel that can prevent a decrease in the toughness of the weld metal.

[Means to solve the problem]

When welding high-chromium ferritic steel for high temperature use, the welding material is basically a metal thread material that is similar in composition and structure to the base metal. If a matching welding material can be used, cracks due to differences in thermal expansion and cracks due to the formation of decarburized layers can be prevented, and the material cost can be significantly reduced compared to Ni-based materials.

However, when using a matching thread material, it becomes difficult to ensure the toughness of the weld metal as described above.

The present inventors believed that an ideal welding material could be obtained if the reduction in toughness when using a matching thread material could be prevented, and conducted research into the cause of this. As a result, we obtained the following knowledge.

○ If a welding material with the same composition as the base metal is used for high-temperature high Cr ferritic steel, the weld metal will have a two-phase structure of ferrite and martensite with the same composition as the base metal. However, the amount of ferrite is not the same as the base metal, that is, the amount of ferrite is greater than the base metal, which reduces the toughness of the weld metal.

○ After welding, the weld metal undergoes only so-called post-weld heat treatment, whereas the base metal undergoes hot working, normalizing, and tempering before welding. The difference in thermal history between the two is also a major factor in reducing toughness.

Based on the above knowledge, we investigated various measures to prevent a decrease in toughness in co-metal thread materials, and as a result, we found that a combination of the following three measures is effective.

○ Optimize the amount of ferrite in the weld metal by controlling the Cr of the welding material.

0 In order to sufficiently recover the toughness of the weld metal only by post-weld heat treatment, we introduced the Qc value defined by the following formula, and calculated this Qc4t! for the welding material. Ingredients are regulated by

Qc=C+Mn/20+Si/30 (%) ○ By regulating the zero content in the welding material, the matrix itself becomes tougher.

The present invention was made based on this knowledge, and in terms of weight percent, C: 0.05 to 0.17% and Si: 0.01 to 0.5.
%, Mn+0.01-2%, P: 0.025% or less, S
: 0.015% or less, Cr: 8-12%, Ni: 0.8
% or less, Mo: 0.5-3%, W: 0.5-3%,
V: 0.1-0°5%, Nb: o, o 1-0.
2%, Al: 0.04% or less, N: 0.63~o, os
9/6.0: 0.01% or less, Ca: 0.0005~
Contains 0.01%, and further CrIIQ: 13% or less, provided that Cr = Cr+63i+4M.

+: 0.5W + 11V + 5Nb + 12Al-40C-3ON -4Ni-2Mn (%) Qc: 0.2% or less However, Q c = C + M n / 20 + S
i / 30 [%], and the balance consists of Fe and unavoidable impurities.
This article focuses on welding materials for r-ferritic steel.

[For production]

The reasons for limiting each requirement in the welding material of the present invention will be explained below.

C: Combines with Cr, M O% W, V% Nb to form carbides, increasing the high temperature long-term creep strength of weld metal. If C is less than 0.05%, this effect will not be sufficient, and if it exceeds 0.17%, the weld metal will harden.
Decrease toughness. Therefore, C is 0605~0.1
7%.

31, Mn: Effective as a deoxidizing agent, but if it is less than 0.01%, the cost will increase economically;
If each exceeds 0%, toughness decreases.

Therefore, SL is 0.01~:0.5%, Mn is 0.0
1 to 2.0%.

P, S: Both elements are impurities that are harmful to weldability and high-temperature long-term creep strength. From this point of view, P is 0.
S is limited to 0.025% or less, and S is limited to 0.015% or less, and it is preferable that both of them be smaller.

Cr: One of the basic components of high-Cr ferritic steel for high temperatures, and requires 8% or more in order to ensure the oxidation resistance of the weld metal. However, when it exceeds 13%, the amount of ferrite in the weld metal increases and the toughness decreases. Therefore, Cr is set at 8 to 13%.

Ni stabilizes martensite as a niostenite-forming element, but if it exceeds 0.8%, it impairs high-temperature long-term creep strength, so Ni should be kept at 0.8% or less.

Mo, W: Both increase high temperature long-term creep strength and
．． If it is less than 5%, the effect is small, but if it exceeds 3%, intermetallic compounds will precipitate, reducing the toughness of the weld metal. Therefore, Mo and W are both set at 0.5 to 3%.

It combines with VjC and N, precipitates fine carbonitrides, and contributes to improving high-temperature long-term creep strength. If it is less than 0.1%, this effect will be small, and if it exceeds O or S, carbonitrides will become coarser, which will actually reduce the high temperature long-term creep strength.

Like NbjV, the fine precipitation of carbonitrides improves the high-temperature long-term creep strength. Furthermore, the toughness is improved by precipitating NbC and refining the structure. 0.01
If it is less than 0.2%, this effect is small, and if it exceeds 0.2%, Nb
The large amount of C precipitated reduces solid solution CN and lowers the strength of the weld metal. Therefore, Nb is 0.01~0゜2%
shall be.

Ajl: Al is added as a deoxidizing agent, but if it exceeds 0.04%, it will impair the high temperature long-term creep strength of the weld metal, so Al should be 0.04% or less.

N: Combines with V and Nb to precipitate carbonitrides, contributing to improvement in high-temperature long-term creep strength. If it is less than 0.003%, this effect is small, and if it exceeds O,OS%, it becomes difficult to provide sufficient toughness to the weld metal by post-weld heat treatment at 800°C or less. Therefore, N is 0.003-0.05%
shall be.

0: Since it weakens the bonds of iron atoms in the weld metal and makes the matrix brittle, it significantly reduces the toughness of the weld metal, so it is limited to 0.01% or less, and the lower the better.

Ca: Added as a deoxidizer, contributes to lowering the zero content of weld metal and improving toughness. If it is less than 0.005%, the effect will be small, and if it exceeds 0.01%, it will remain as inclusions in the weld metal, which will actually reduce the toughness. Therefore, Ca
is o, o o s ~ 0.01%, and in addition to the above components, M
g: o, oi% or less, Ce: 0°02% or less, La: 0
．． 02% or less of one or more types may be added.
Like Ca, these contribute to lowering the zero content of the weld metal and improve its toughness.

Creq: When high-Cr ferritic steel for high temperature use is welded with a co-metallic material, the weld metal has a ferrite-martensitic two-phase mixed structure similar to the base metal. In weld metals with a two-phase mixed structure of ferrite and martensite,
The larger Creq expressed by the following formula, the larger the amount of ferrite. Furthermore, an increase in the amount of ferrite reduces the toughness of the weld metal.According to the investigation by the present inventors, Creq
exceeds 13, the amount of ferrite in the weld metal is 60%
As a result, practically sufficient toughness (Charpy impact value of 5 pupils f-m/as" or more at 0° C.) cannot be ensured.

Therefore, Cr is regulated to 13% or less.

Cr,,=Cr+6S l+4Mo+:0.5W+11
V+5Nb+12Al-40C-3ON-4Ni-2M
n (%) QC: According to the research conducted by the present inventors, the decrease in the toughness of the weld metal when high-Cr ferritic steel for high temperature use is welded with co-metal thread material is due to the increase in the amount of ferrite as well as the thermal history of the weld metal. It was found that a major cause was that the thermal history of the base metal differed from that of the base metal.In other words, after hot forming, the base metal underwent normalization and tempering treatments unique to high-chromium ferritic steel for high temperature use before welding. In contrast, the weld metal only undergoes so-called post-weld heat treatment after welding.In order to compensate for the lack of toughness of the weld metal due to this difference in thermal history, the present inventors A parameter Qc expressed by the following formula was introduced.

Qc=C+Mn/20+Si/30 (%) According to the research conducted by the present inventors, the higher Qc becomes, the harder the martensitic phase in the weld metal becomes, reducing the toughness after post-weld heat treatment.Particularly when Qc is 0. If it exceeds 2%, 800℃
The following post-weld heat treatment does not ensure practically sufficient toughness (Charpy impact value 5 mm f-m 7 cm or more at 0° C.).

On the other hand, to make Qc less than 0.05%, C is 0.05%.
Qc must be less than 0.05% to 0.2% because it is greater than 0 and reduces the high temperature long-term creep strength.
shall be. That is, Mn and 31 are regulated based on their relationship with the C content so that Qc is 0.05 to 0.2%.

〔Example〕

Next, the results of using the welding material of the present invention will be explained in comparison with comparative materials.

Steels having the chemical compositions shown in Table 1 as A1 to A9 and B1 to B15 are melted in a 50-pupil vacuum melting furnace, further forged and rolled to have an outer diameter of 2 fins and a length of 1100 mm (1). Materials and comparative welding materials were produced.

On the other hand, as a base material, 50k steel with the chemical composition shown in Table 2 was used.
g Melted in a vacuum melting furnace and forged at 1150-950℃,
A plate having a thickness of 10 fi was produced by rolling.

The manufactured base material was subjected to AC normalization at 1050°C x 1 hr and AC tempering at 830°C x 0.5 hr. This base material is a high Cr ferritic steel for high temperature use proposed in JP-A No. 62-89842, and has an increased F01 point of 800.
The composition is designed to allow tempering at temperatures above ℃.

Then, the groove shown in FIG. 1 was provided in this base material, and five layers of TIG welding, shown in FIG. 1, were performed using the various welding materials described above under the conditions shown in Table 3. After welding, the welded parts were subjected to post-weld heat treatment of AC at 760℃ x 0.3hr,
Thereafter, a Charpy impact test piece with a notch in the center of the weld metal shown in FIG. 2 and a creep test piece shown in FIG. 3 were cut out from the welded parts, and each test was conducted.

The Charpy impact test was conducted at 0°C, and the creep test was conducted at a temperature of 650°C and a load stress of 9 kg r/fi, and the time until rupture was investigated.

The test results are shown in Table 1 in correspondence with the welding materials.

As is clear from Table 1, when using the Al-A9 welding material whose chemical composition, Crll@ and Qc are within the range of the present invention, a creep rupture time of over 5000 h'r can be obtained. This result is equivalent to the rupture time of the base material (5000 hr or more). Also, regarding toughness, 5 kg f
-m/d (5kg f/a”)l] (7) Charpy impact value is ensured.

On the other hand, Bl welding material has too much Cr, B2 has too much C, B4 has too much O and insufficient Ca, and B6
In this case, there is too much Mo, in BS there is too much W, and in B9 there is too much Ca.
too much, too much N in Bll, Cr in B16,...
Because there are too many, the Charpy impact values of all of them are lower than those of A1 to A9.

On the other hand, regarding creep strength, B1 lacks N,
B3 and 5 lack C, B4 has too much Ni, and B5
There is a shortage of Mo in B7, a shortage of W in B7, a shortage of V in B9, too much V in BIO, a shortage of Nb in B11, too much Nb in B12, and too much Al in B13. However, compared to the rupture time of the material (more than 5000 hr), the results were quite low.

Furthermore, if the Qc value exceeds 0.2% as seen in B13, or Cr and q exceeds 13% as seen in 816, sufficient toughness (5k, r-/cd) cannot be obtained. It disappears.

In addition, Figure 4 shows the relationship between Charpy impact value and zero amount for Al,
2,5.7 and B4,14.15. As is clear from the figure, the toughness of the weld metal is greatly affected by the amount of zero in the welding material, and the smaller the amount of zero, the more the toughness is improved.

〔Effect of the invention〕

As is clear from the above explanation, the welding material for high Cr ferritic steel of the present invention requires regulation of chemical components (particularly 0It),
By regulating the Cr value and Qc value, it is possible to give the weld metal excellent toughness even though it is a cometal thread material, and to give it the same or better performance than the base metal in terms of high-temperature long-term creep strength. Furthermore, since it is a cometal thread material, it prevents cracking induced due to the difference in thermal expansion with the base material and the formation of a decarburized layer, and is significantly superior in terms of economy compared to Ni-based base materials.

Therefore, the welding material of the present invention is, for example, Ac.

It is applied to high-temperature high-Cr ferritic steel materials with increased points, and is high-performance and economical by taking advantage of the advantages of the base material.

This technology makes it possible to perform flexible welding, and is highly effective in improving the characteristics and economic efficiency of welded joints.

[Brief explanation of the drawings] Figure 1 is a cross-sectional view showing the groove shape and lamination method of the welded part in the welding test, Figures 2 and 3 are side views of the Charpy impact test piece and the creep test piece, and Figure 4 is a side view of the Charpy impact test piece and creep test piece. The figure is a chart showing the relationship between the Charpy impact value of weld metal and the amount of O. In the figure, 18 weld beads, 2: welded member. Figure 1 Figure 2 Figure 4 Figure 3

Claims (1)

[Claims] 1. C: 0.05 to 0.17% by weight, Si: 0.0
1-1.5%, Mn: 0.01-2%, P; 0.025
% or less, S: 0.015% or less, Cr: 8-12%, N
i: 0.8% or less, Mo: 0.5~3%, W: 0.5~
3%, V: 0.1-0.5%, Nb: 0.01-0.2
%, Al: 0.04% or less, N: 0.03 to 0.05%
, O: 0.01% or less, Ca: 0.0005 to 0.01
%, and further Cr_e_q: 13% or less However, Cr_e_q=Cr+6Si+4Mo+1.5W
+11V+5Nb +12Al-40C-30N -4Ni-2Mn [%] Qc: 0.2% or less However, Qc=C+Mn/20+Si/30 [%] is satisfied, and the balance is Fe and unavoidable impurities. Welding material for ferritic steel.