JPH0760481A - Welding material for high-cr and high-n austenitic steel - Google Patents

Abstract

PURPOSE:To provide stable strength at high temp. in the wide range of weld executing condition without enhancing sensibility of weld cracking at high temp. by specifying the contents of Mn, Cr, Ni, Nb and P, S as impurities in welding material. CONSTITUTION:The composition of a gas shielded arc welding material for a high-Cr and high-N austenitic steel consists of, by wt.%, 0.03-0.1% C, 0.1-0.5% Si, 1.2-8% Mn, 23-28% Cr, 18-30% Ni, 0.01-0.7% Nb, <=0.5% Al, 0.2-0.4% N, 0.5-1.5% Mo, <=0.01% B and <=0.01% P, <=0.02% (P+S) in P and S as impurities, further the balance Fe with inevitable impurities. Besides, 1-4% Cu is contained. By use of this material, adequate weldability and strength at high temp. are secured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding material used for gas shielded arc welding for austenitic steel of high Cr and high N, which is excellent in high temperature strength for power generation boilers and the like.

[0002]

2. Description of the Related Art Conventionally, 18-8 type austenitic stainless steel has been mainly used for power generation boilers, chemical reactors and the like used at high temperatures. However, with the recent severer operating conditions in boilers (higher temperatures and higher pressures), materials with higher high-temperature strength and higher corrosion resistance than the 18-8 series have been required. Austenitic steels characterized by high Cr and high N have been developed to meet such demands.
In both 72 and JP-A-59-64752, high Cr content,
An austenitic steel having improved high temperature strength and corrosion resistance by adding N, Nb and B is disclosed.

As a welding material used for welding such a high Cr high N austenitic steel, a base material is directly processed into a wire rod, or an arc welding material for high Ni alloys (for example, JIS welding rod DNiCr system). In addition, a welding material for high-Cr, high-N N-austenitic steels (see Japanese Patent Laid-Open No. 5-69187) has been proposed.

However, in the welding of high Cr and high N austenitic steel, while the base metal is melted, and then the structure is adjusted by rolling and heat treatment to secure the high temperature strength, whereas in most cases, the weld metal is Used in tissues during coagulation. Therefore, it is essentially not easy to increase the high temperature strength of the weld metal as compared with the base metal.

Further, the austenite structure has a high alloy composition, which increases the crack susceptibility and easily causes high-temperature welding cracking. Particularly, in the case of weld metal, solidification cracking that occurs in the weld metal during weld solidification poses a problem. Further, some of the components added to increase the high temperature strength of the austenitic steel material have a bad influence on the hot cracking of the weld, making its prevention more difficult.

Therefore, the welding material in which the base material is directly processed into a wire has the following problems.

For example, B which is effective in improving the high temperature strength of the base material
Has the effect of increasing the sensitivity of the weld metal to hot cracking. Further, in order to reduce the susceptibility to welding hot cracking, it is effective to reduce the amount of Ni and crystallize several% of δ ferrite.
This δ-ferrite transforms into a sigma phase during use at high temperature and causes embrittlement, so crystallization of δ-ferrite must be suppressed. On the other hand, if Nb which is effective in improving the high temperature strength is added under the condition that the crystallization of δ ferrite is suppressed, the crack susceptibility is further enhanced. Therefore, it has been difficult to provide the weld metal with excellent high-temperature strength and at the same time high resistance to high-temperature cracking by welding when the base metal is directly processed into a wire rod.

On the other hand, the strength of the welded portion of a power generation boiler or the like used at high temperature is easily softened by heating during welding, especially in an austenitic structure. Therefore, the conventional arc welding materials for high Ni alloys and the welding materials for eutectic high Cr high N austenitic steels previously proposed in Japanese Patent Laid-Open No. 5-69187 have been considered There is a problem that stable high temperature strength cannot always be obtained for a strong base material. In addition, arc welding materials for high Ni alloys are expensive,
It is not preferable in terms of economy.

[0009]

As described above, in the conventional welding materials, it was difficult to impart excellent high temperature strength and weld hot cracking resistance to the weld metal. The present invention has been made in view of such problems, and an object thereof is to provide a welding material that can obtain stable high-temperature strength in a wider welding execution condition range without increasing the susceptibility to high-temperature welding cracking of a welded portion.

[0010]

The present invention is based on the following welding materials.

(1) C: 0.03 to 0.1% by weight, Si:
0.1 to 0.5%, Mn: 1.2 to 8%, Cr: 23 to 28%, Ni: 18
~ 30%, Nb: 0.01 ~ 0.7%, Al: 0.5% or less, N: 0.2
.About.0.4%, Mo: 0.5 to 1.5%, and B: 0.01% or less, and P and S as impurities satisfy P: 0.01% or less and P + S: 0.02% or less, and the balance is Fe and inevitable impurities. Gas shielded arc welding material for high Cr high N austenitic steels.

This material may further contain 1 to 4 if necessary.
% Cu can be included.

(2) In% by weight, C: more than 0.1% and 0.24%
Up to Si: 0.1-0.5%, Mn: 8% or less, Cr: 23-28
%, Ni: 18 to 30%, Nb: 0.01 to 0.7%, Al: 0.5% or less, N: 0.15 to 0.35%, Mo: 0.5 to 1.5%, and B:
Contains 0.01% or less, and P and S as impurities P: 0.
01% or less and P + S: 0.02% or less, the balance is
A gas shielded arc welding material for high Cr high N austenitic steel, characterized by comprising Fe and inevitable impurities.

The material of (1) or (2) above may further contain Mg:
It is desirable to contain 0.02% or less or / and Ca: 0.02% or less.

[0015]

The present inventor has made a detailed study on a gas shielded arc welding material that can obtain stable high-temperature strength in a wider welding working condition range without increasing the susceptibility to high-temperature cracking of the welded part, and as a result, Obtained. The gas shielded arc welding referred to here is a method in which the arc is shielded by an inert gas such as Ar or He and welding is performed.

Since the high temperature strength can be improved by solid solution strengthening of N, increasing the N content of the welding material is effective for improving the high temperature strength of the weld metal.

However, there is a problem that N contained in the welding material does not remain in the weld metal during welding (no yield). This is particularly noticeable in high heat input welding, but because N scatters in the arc atmosphere, even if a large amount of N is contained in the welding material, effective N is retained in the weld metal. There is no situation. As a countermeasure against this, by setting the content of Mn to 1.2% or more, the activity of N in the weld metal can be reduced and the yield of N can be improved.
Here, the N activity is the solubility of N in the molten metal.

Since C also contributes to the improvement of high temperature strength as a solid solution strengthening element, increasing the C content of the welding material is effective for improving the high temperature strength of the weld metal. However, if C is excessively contained, C will be precipitated in a large amount as carbonitride, which will rather lower the high temperature strength. Therefore, in consideration of the improvement of the high temperature strength due to the synergistic effect with N, the content of C needs to be appropriately adjusted in relation to the content of N.

Cu is also a solid solution strengthening element, but since it has the effect of precipitation strengthening by fine precipitation of the Cu-rich compound phase together with solid solution strengthening, it is effective for high temperature strength of weld metal. Therefore, Cu may be added instead of increasing the C content, which causes the problem of carbonitride precipitation.

The present invention has been completed based on the above findings. Hereinafter, the reason for limiting the content of the components of the welding material in the present invention as described above will be described. Less than,
"%" Means "% by weight".

C: Contributes to improvement of high temperature tensile strength and creep strength of weld metal. However, if the content is excessive, as described above, precipitation occurs as a carbonitride, resulting in a decrease in high temperature strength. For example, precipitation strengthening of NbCrN and Cr ₂₃ C ₆ type carbides and solid solution strengthening of C matrix are effective for improving high temperature tensile strength and creep strength. However, when the C content is excessively contained, coarse Cr ₂₃ C ₆ is deposited and does not contribute to the improvement of strength. On the other hand, due to the precipitation of fine carbide
When Cr is consumed, N precipitates as Cr ₂ N, so that the solid solution N decreases and the strength of the weld metal can be prevented from lowering. Therefore, aiming at the synergistic effect of C and N, the amount of N is 0.2-
When the content is 0.4%, the C content is 0.03 to 0.1%, and the N content is 0.15 to 0.1%.
When the content was 0.35%, the C content was 0.1 to 0.24%.

Si: It is added as a deoxidizer, but its excessive content increases the sensitivity to hot cracking in the weld, so it was made 0.1 to 0.5%.

Mn: Although added as a deoxidizer, it fixes S, which adversely affects high temperature brittleness, and contributes to reduction of weld hot cracking susceptibility. Further, by reducing the activity of N in the weld metal, it is possible to suppress the scattering of N into the arc atmosphere during welding, increase the yield of N in the weld metal, and improve the high temperature tensile strength and creep strength. Contribute. Therefore, the amount of N is 0.
When the content is 2 to 0.4%, the lower limit of Mn content is 1.2%. However, the excessive content causes embrittlement of the weld metal, so the lower limit of the Mn content is 8%. 1.2 to 6% is preferable.

Cr: Necessary to secure high temperature strength, oxidation resistance and corrosion resistance, but excessive inclusion impairs hot workability. Therefore, the Cr amount is set to 23 to 28%.

Ni: essential for stabilizing the austenite structure and ensuring high temperature strength, but crystallizes δ ferrite. δ-ferrite reduces weld hot cracking susceptibility, but it also causes embrittlement during high temperature use. In the welding material of the present invention, while suppressing the crystallization of δ ferrite to prevent high temperature embrittlement, Ni is an expensive element, so considering that addition of a large amount impairs economic efficiency, the Ni amount is 18 to 30. %.

Fine precipitation of carbonitride as Nb: NbCrN is effective for improving high temperature strength. But,
The addition of Nb increases the hot cracking susceptibility of the weld. In particular, under the condition that δ ferrite is not crystallized, the crack susceptibility is remarkably increased. Therefore, the Nb content is 0.01-0.7%
And

Al: Although it is added as a deoxidizer, its effect is saturated even if it is added in a large amount, so the content is made 0.5% or less. Desirably 0.01 to 0.3%.

Ca, Mg: Effective for reducing the sensitivity to welding hot cracking and improving hot workability during wire processing. But,
Excessive inclusion is not preferable because it increases the inclusions in the weld metal. Therefore, even when these are added, the content is preferably 0.02% or less.

N: Indispensable for ensuring the high temperature strength of the weld metal. That is, N has a solidification solution in the matrix of the solidified structure to strengthen it, and partly precipitates as a nitride to have a precipitation strengthening action. However, excessive content causes a large amount of carbonitride to precipitate during use at high temperature, which causes embrittlement of the weld metal. Therefore, the amount of C is 0.03-0.
When the amount is 1%, the amount of N is 0.2 to 0.4%, and the amount of C is 0.1 to 0.
When it was 24%, the N content was 0.15 to 0.35%.

Mo: It can be solid-solved in the matrix of the solidified structure to enhance the high temperature strength of the weld metal. However, if the content is less than 0.5%, the effect is small, and
If it exceeds, not only the effect is saturated, but also the corrosion resistance is deteriorated. Therefore, the Mo content is set to 0.5 to 1.5%.

Cu: Fine precipitation as a Cu-rich phase effectively contributes to improvement of high temperature tensile strength and creep strength. Therefore, it may be desirable to add Cu instead of increasing the C content, which has the problem of carbonitride precipitation. When Cu is added, the content is 1% or more, and excessive addition causes a decrease in ductility, so the upper limit is 4%.

B: Segregation at the grain boundaries to increase the strength of the grain boundaries contributes to the improvement of creep strength. However, excessive addition promotes welding hot cracking, so the content is made 0.01% or less.

P, S: A eutectic having a low melting point is formed during solidification, which causes the occurrence of hot cracking in the weld. Therefore, P is 0.01% or less and P + S is 0.02% or less. The reason why the two components of P and S are simultaneously regulated in this way is to prevent the melting point of the eutectic crystal from decreasing due to the synergistic effect of P and S and increasing the cracking susceptibility. It is possible to suppress an increase in weld hot cracking susceptibility associated with the addition of B. Even with Nb-added steel, sufficient weldability is secured, and it becomes easy to secure high-temperature strength by adding Nb.

In the base material targeted for the welding material of the present invention, the main alloy components are Cr: 17-28%, Ni: 18-25%, and C:
It is a so-called high Cr high N austenitic steel containing 0.02 to 0.1%, N: 0.15 to 0.4%, Nb: 0.2 to 0.8% and B: 0.002 to 0.006%.

[0035]

EXAMPLES Examples of the present invention will be described in more detail below in comparison with comparative examples.

Using a high strength, high Cr, high N austenitic steel having the chemical composition shown in Table 1 as a base material, the performance comparison of gas shielded arc welding materials was carried out. This base material is a high Cr-based material containing N, Nb, and B components, and its creep strength (600
℃ × 100000 hours) is 18.8kgf / mm ^2, which is about 1.6 times higher than the creep strength of SUS 347H.

[0037]

[Table 1]

Twelve kinds of welding materials (six kinds for each of the present invention example and the comparative example) shown in Table 2 were prepared, and all were prepared by vacuum melting in a laboratory and then processing into a wire having an outer diameter of 2 mm. .

[0039]

[Table 2]

FIG. 1 is a view showing the shape of the groove used in the welding test, and the groove was set at 60 °. Further, although FIG. 2 is a diagram showing the shape of the joint produced in the welding test, the welding material evaluation test was performed using this joint.

In the welding test, first, as shown in FIG. 2, the base materials 1 of the pipes having an outer shape of 38 to 60 mm provided with the groove as shown in FIG. 1 were butted and restrained and welded onto the restraining rod 2 made of carbon steel. .

Next, multi-layer welding was performed on the groove with the test welding material to produce a joint for evaluation. The welded portions of the joint were the restraint welded portion 3 and the test welded portion 4 for the groove, and each welding was performed by the TIG method using Ar as a shielding gas. Here, restraint welding is performed by restraining both ends of the base metal so that cracks easily occur in the test welded portion 4 due to thermal stress generated during welding of the test welded portion 4 in the groove portion. This is because.

FIG. 3 is a view showing the shape of a side bending test piece taken from the joint. Further, FIG. 4 is a diagram showing the shape of a test piece used for a high temperature tensile test and a creep test taken from a joint.

After welding the joint, a side bending test piece shown in FIG. 3 was sampled by machining, and this was bent at a bending radius (8 to 14 mm) of twice the plate thickness and an angle of 180 degrees, and then opened. The presence or absence of hot cracking in the test weld 4 of the tip portion was examined. Further, a high temperature tensile test and a creep test were conducted using a test piece shown in FIG. 4 which was taken by machining.

The high temperature tensile test is conducted at 600 ° C., and the strength above the allowable stress value, which is the design standard of the base material, can be guaranteed.
Gf / mm ² was used as the criterion, and those exceeding this were regarded as acceptable.

In the creep test, the breaking time of the base metal was about 30.
A test was conducted under the conditions of a temperature of 600 ° C. for 00 hours and a load of 22 kgf / mm ² to examine the fracture time of the test welded portion 4. The criterion for evaluation was 80% of the fracture time of the base metal, and those that did not reach this were considered to have insufficient creep strength.

In the evaluation of the welding material, for welding high temperature cracks, ○ indicates no cracking, × indicates cracking, and high temperature tensile test indicates ○ exceeding the judgment standard of 47 kgf / mm ² , and × reached the judgment standard. Show what you don't. Further, regarding the creep strength, ◯ indicates that the breaking time exceeds 80% of the breaking time of the base material, and × indicates that it does not reach 80% of the breaking time of the base material. As for the welding conditions, the heat input (KJ / cm 2) and interlayer temperature (° C) along with the pipe dimensions of the base metal were changed. Where the welding heat input (KJ / cm
) Is {welding current (A) × welding voltage (V) / welding speed (c
m / min)} x 60, and the interlayer temperature (° C) is measured with a temperature-indicating paint that changes color in a specific temperature range. Table 3 shows the above evaluation results.

[0048]

[Table 3]

As is clear from Table 3, the welding materials Nos. A1 to A6, which are examples of the present invention, do not cause welding hot cracking even under severe conditions of large heat input and high interlayer temperature, and have high temperature tensile strength. Good results were also obtained in tests and creep tests. In particular, welding material No. A1 has a welding heat input of 9 to 13 KJ / cm 2 and an interlayer temperature in order to meet a wide range of construction conditions.
The evaluation was conducted under a wide range of conditions of 150 to 400 ° C. and pipe dimensions of 38 to 60 mm, and it was found that excellent hot crack resistance, high temperature tensile strength and creep strength were obtained under all the conditions.

On the other hand, in Comparative Example, welding material No. B1
~ B2 has high temperature cracking caused by welding. Welding material No. B3
B6 had insufficient strength in the high temperature tensile test and the creep test.

[0051]

The gas shielded arc welding material for high Cr high N austenitic steels of the present invention is a high Cr high N steel material, and is therefore less expensive than the welding material for high Ni alloys.

Further, under a wide range of working conditions, it is possible to give the weld metal excellent high temperature strength comparable to that of the base metal, further reduce the weld hot cracking susceptibility, and prevent weld cracking.

[Brief description of drawings]

FIG. 1 is a view showing a shape of a groove used in a welding test.

FIG. 2 is a diagram showing the shape of a joint produced in a welding test.

FIG. 3 is a view showing a shape of a side bending test piece taken from a joint.

FIG. 4 is a diagram showing the shapes of test pieces for high temperature tensile test and creep test taken from a joint.

[Explanation of symbols]

1 ... Base material, 2 ... Restraint bar, 3 ... Restrained weld, 4 ... Test weld

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takao Taka, 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Shigeru Matsumoto 1-17 Fuso-cho, Amagasaki City, Hyogo Prefecture No. Sumikin Welding Industry Co., Ltd. (72) Inventor Toshihiko Mizuta No. 17 Fuso-cho Amagasaki City, Hyogo Prefecture Sumikin Welding Industry Co., Ltd.

Claims (4)

[Claims] 1. C: 0.03-0.1%, Si: 0.1-
0.5%, Mn: 1.2-8%, Cr: 23-28%, Ni: 18-30
%, Nb: 0.01 to 0.7%, Al: 0.5% or less, N: 0.2 to 0.
4%, Mo: 0.5 to 1.5%, and B: 0.01% or less, and P and S as impurities satisfy P: 0.01% or less and P + S: 0.02% or less, and the balance is Fe and inevitable impurities. Gas shielded arc welding material for high Cr and high N austenitic steels. 2. In addition to the components according to claim 1, in a weight percentage, Cu: 1 to 4% is further contained, and P, S as impurities.
Satisfying P: 0.01% or less and P + S: 0.02% or less, and the balance consisting of Fe and unavoidable impurities, gas shield arc welding material for high Cr high N austenitic steel. 3. By weight%, C: more than 0.1% and up to 0.24%, Si: 0.1-0.5%, Mn: 8% or less, Cr: 23-28%,
Ni: 18-30%, Nb: 0.01-0.7%, Al: 0.5% or less,
N: 0.15 to 0.35%, Mo: 0.5 to 1.5%, and B: 0.01
% Or less, and P and S as impurities P: 0.01%
A gas shield arc welding material for high Cr high N austenitic steels, which satisfies the following and P + S: 0.02% or less, and the balance is Fe and inevitable impurities. 4. In addition to the component according to claim 1, claim 2 or claim 3, it contains Mg: 0.02% or less and / or Ca: 0.02% or less by weight%, and contains P as an impurity. , S satisfies P: 0.01% or less and P + S: 0.02% or less, and the balance is Fe and unavoidable impurities, which is a gas shield arc welding material for high Cr high N austenitic steel.