JP3164978B2 - High Cr steel welding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for welding high Cr steel, and more particularly, for example, a line pipe used for transporting oil and natural gas, a container used for storage, or a material that requires further strength and corrosion resistance. The present invention relates to a welding method excellent in corrosion resistance, strength and toughness suitable for welding a high Cr steel used in a used application.

[0002]

2. Description of the Related Art In recent years, the production of petroleum and natural gas containing moist carbon dioxide and hydrogen sulfide has been increasing.
It is a well-known fact that carbon steel and low alloy steel are significantly corroded in such an environment. Therefore, when transporting such corrosive oil and natural gas, as a corrosion prevention measure for steel pipes,
The addition of corrosion inhibitors has been common in the past. But,
In a marine oil well, the cost required for the addition and recovery of the corrosion inhibitor is enormous, and the use of the corrosion inhibitor is becoming difficult due to the problem of marine pollution. Therefore, the need for corrosion-resistant materials that do not require the addition of corrosion inhibitors,
Recently it is getting bigger.

[0003] For this purpose, many steels or steel pipes having excellent corrosion resistance in an environment containing carbon dioxide gas and the like and excellent weldability have been proposed. These contain about 11 to 15% of Cr in order to obtain corrosion resistance in an environment containing carbon dioxide, reduce C for the purpose of improving weldability, and quench-temper to secure strength and toughness. Generally, the structure is tempered martensite by heat treatment. For example, JP-A-4-99154 and JP-A-4-99155
Japanese Patent Application Laid-Open Publication No. H11-216, proposes a high Cr steel for line pipes which has reduced C and N and which has a substituted austenite stabilizing element and has excellent weldability.

It is well known that line pipes and pressure vessels are connected or manufactured by welding, but a welding material or a welding method suitable for high Cr steel having excellent weldability as described above. There was no conventional. N
KK Technical Report, 1989, No. 129, No. 15-22
On the page, AISI410 steel is manufactured as UOE steel pipe,
There has been reported an example in which a TIG welded joint (corresponding to on-site circumferential welding of a line pipe) was formed using a co-metallic material to which Ni was added. However, as seen in the NKK technical report,
In a high Cr co-metal material, the hardness of the weld metal becomes very hard even if a large amount of Ni is contained. In this case, there is a problem that in a use environment, a small amount of hydrogen sulfide may cause stress corrosion cracking in the weld metal.

Further, it is possible to reduce the hardness of the weld metal by performing a tempering heat treatment after welding. However, in the case of a high Cr steel, a very high temperature and a long time heat treatment such as "620 ° C. × 1 hour" is required as the heat treatment temperature and time. Actually, it is extremely difficult to apply such a heat treatment at a line pipe laying site, or to apply such a heat treatment to a welded portion or a whole of a huge pressure vessel, and it requires enormous cost and time. In addition, TIG (tungsten inert gas) welding has a disadvantage that the welding cost is extremely high when applied to the welding of a structure because the welding speed is low and the amount of weld metal in one pass is small. Therefore, it is difficult to weld a high Cr steel using a common metal or martensitic stainless steel welding material.

On the other hand, when low alloy steel is used as a welding material, welding is easy, and post-weld heat treatment is not required for the weld metal. However, low alloy steel has poor corrosion resistance in a carbon dioxide gas-containing environment. If a low-alloy steel weld metal with poor corrosion resistance contacts a high-Cr steel base material with excellent corrosion resistance, the weld is selectively corroded, which is extremely dangerous for the safety of the structure. , Can not be applied.

Further, when high Ni austenitic stainless steel or Ni-base superalloy having excellent corrosion resistance is used as a welding material, selective corrosion of the weld does not occur, the hardness of the weld metal is low, and the weld metal is low. Toughness can be secured. However, austenitic stainless steel and Ni-based superalloy have a problem that their strength is low due to their crystal structure. Generally, the yield strength of these materials is between 300 and 4
00N / mm ² about only. Yield strength is 560N / m
When a high Cr steel base material having a size of m ² or more is welded with a weld metal having a very low strength, the weld metal may be intensively deformed when external stress is applied, leading to breakage. Therefore, there was also a great difficulty in welding high Cr steels using austenitic stainless steel or high Ni alloy as a welding material.

[0008]

In view of these circumstances, the present invention has excellent corrosion resistance when welding high Cr steel in a carbon dioxide gas-containing environment and the like, and also has excellent toughness, strength, and the like in a welded portion. It is intended to provide a welding method.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has a Cr content of 7.5 to 18.0% by weight.
The microstructure is substantially a martensitic single phase,
Alternatively, in welding of high Cr steel mainly containing martensite and partially containing ferrite, Si: 0.0
1 to 1.0%, Mn: 0.02 to 3.0%, Cr: 1
8.0-28.0%, Ni: 5.0-10.0%, Mo
+ 0.5W: 1.0 to 5.0%, Al: 0.005 to
0.5%, C: 0.03% or less, P: 0.03
%, S: regulated to 0.01% or less, and arc welding using a welding material consisting of the balance Fe and inevitable impurities,
In addition, the microstructure of the weld metal is a two-phase structure of austenite and ferrite, and the austenite fraction is 30 to 70%.
It is a method for welding high Cr steel. Further, if necessary, the content of the welding material is% by weight, and Cu: 3%.
Hereinafter, N: contains one or two kinds of 0.2% or less, and contains one or more kinds of Nb, V, Ti, Zr, and Ta in a total content of 1.0% or less by weight%. It is also a feature.

[0010]

The reasons for limiting the range of each component in the present invention will be described below. In the present invention,% means% by weight unless otherwise specified.

Si: Si is effective as a deoxidizing agent and a strengthening element for a weld metal. However, if the content is less than 0.01%, the deoxidizing effect is not sufficient, and the content is more than 1.0%. However, the effect is not only saturated but also lowers the impact toughness, so the content range of Si is limited to 0.01 to 1.0%.

Mn: Mn is required as a deoxidizing agent for a weld metal, and is also important as an austenite-forming element for adjusting the structure of a weld metal, and must be contained at 0.02% or more. However, even if the content exceeds 3.0%, the effect is no longer saturated, and the excessive addition of Mn causes difficulty in the production of the material, so the upper limit content is 3.0%. I do.

[0013] Cr: Cr must be contained at 18.0% or more in order to ensure the corrosion resistance and strength of the weld metal. However, if the Cr content exceeds 28.0%, the structure of the weld metal is reduced. In order to adjust it, a large amount of alloying element must be added as an austenite forming element, which only increases the cost unnecessarily. Therefore, C
The content of r is set to 18.0 to 28.0%.

Ni: Ni is important as an element that stably forms austenite in the structure of the weld metal and ensures toughness and corrosion resistance. If the content is less than 5.0%, impact toughness is insufficient, and it becomes difficult to secure an austenite fraction. If the Ni content exceeds 10.0%, the austenite fraction may become excessive and the strength of the weld metal may decrease, but the effect of improving the impact toughness is no longer saturated. Therefore, the content of Ni is set to 5.0 to 10.0%.

Mo + 0.5W: Mo and W are added to secure the corrosion resistance and high strength of the weld metal. here,
When the content is the same, the effect of W is one of the effects of Mo.
/ 2, the total effect of both is Mo + 0.5W
Is represented by If Mo + 0.5W is less than 1.0%, the corrosion resistance and strength of the weld metal are not sufficient, and 5.0%
If the ratio exceeds 3, it becomes difficult to keep the ferrite and austenite fractions of the weld metal properly, or in order to make the austenite fraction 30 to 70%, the austenite forming element must be excessively added. Therefore, Mo
The value of +0.5 W is set to 2.0 to 5.0%. Where M
Any of o and W may be added alone, or both may be added in combination. The important thing is Mo + 0.5
That is, the amount represented by W is in the range of 2.0 to 5.0%.

Al: Al is 0.005% as a deoxidizing agent
The above addition is necessary. However, if added in excess of 0.5%, coarse oxide-based inclusions are formed to impair the corrosion resistance and impact toughness of the weld metal, so the upper limit content is 0.5%.
And

C: Since C forms carbides with Cr to lower the corrosion resistance and greatly increase the hardness of the weld metal, the content of C is limited to 0.03% or less. Alternatively, it is more remarkable that the reduction to 0.02% or less improves the corrosion resistance and toughness, which is more preferable.

P: Since the presence of a large amount of P lowers the toughness, it is desirable to reduce the P, and it is necessary to reduce it to 0.03% or less, and the smaller the P, the better.

S: When S is present in a large amount, hot workability,
Since the ductility and the corrosion resistance are reduced, a smaller amount is desirable, and it is necessary to reduce the content to 0.01% or less. In order to further improve the productivity as a welding material and further improve the corrosion resistance of the weld metal, it is more preferable to reduce S to 0.005% or less.

The above are the basic components of the welding material used in the method of the present invention. In the present invention, the following elements are further added as necessary to further improve the properties of the welding material.

Cu: Cu has a remarkable effect on increasing the strength and corrosion resistance of the weld metal, and is also a useful element for adjusting the austenite fraction to a predetermined range. However, if the addition exceeds 3.0%, the effect is no longer saturated, but the productivity of the welding material is reduced due to hot cracking or the like. Therefore, the upper limit content is set to 3.0%.

N: N is useful as an austenite-forming element for adjusting the austenite fraction in the weld metal to a predetermined range and as an element for increasing the strength of the weld metal. However, since it is difficult to contain more than 0.2%, and unnecessarily increases the cost,
The upper limit content is set to 0.2%.

Nb, V, Ti, Zr, Ta: Nb, V,
Ti, Zr, and Ta have the effect of reducing the hardness of the weld metal and improving the corrosion resistance. However, even if it is added excessively, these effects are saturated but the toughness is reduced, so that the total content of one or more of Nb, V, Ti, Zr, and Ta is 1.0%. Shall not exceed.

In the welding material used in the method of the present invention, in addition to the above-mentioned components, Sn, Sb, B, etc., for the purpose of adjusting the productivity, toughness, corrosion resistance and the like, or as impurities accompanying the added alloy element. Can be contained.
In order to improve manufacturability, rare earth elements (RE
M), Ca, Mg and the like can be contained. Here, the rare earth element means an element having an atomic number of 57 to 71 or 89 to 103 and Y. In the present invention, the content of oxygen is not particularly limited. However, since oxygen is an impurity serving as a source for generating oxide-based nonmetallic inclusions, it is natural that the smaller the oxygen content, the better.

Next, the reason why the microstructure is limited in the present invention will be described. The microstructure of the weld metal must have a two-phase structure of austenite + ferrite in order to simultaneously satisfy a plurality of required properties such as strength, impact toughness, hardness, and corrosion resistance. A ferrite single phase or a structure mainly composed of ferrite has poor impact toughness. On the other hand, if the structure is mainly composed of a single phase of austenite or austenite, the strength of the weld metal is insufficient. If the structure of the weld metal is a martensite single phase or a structure mainly composed of martensite, the hardness is large and the impact toughness is poor. The two-phase structure of austenite + ferrite increases the strength of the weld metal while preventing the hardness from excessively increasing. Therefore, welding cracks do not occur even if preheating or post heat treatment is not performed during welding. And further excellent impact toughness.

Here, when the austenite fraction in the weld metal is less than 30%, the ferrite fraction becomes excessively large, so that the impact toughness of the weld metal decreases and the ferrite hardness becomes too high. On the other hand, the austenite fraction is 70%
If the ratio exceeds the limit, it becomes difficult to secure the strength of the weld metal, no matter how the components are adjusted. Therefore, it is important and necessary that the austenite fraction of the weld metal be in the range of 30 to 70%.

In the method of the present invention, the welding method is intended for arc welding. There is no particular limitation on ordinary arc welding, but welding of line pipes, pressure vessels, and the like includes covered arc welding, MIG (metal inert gas) welding, MAG (metal active gas) welding, and TIG (tungsten inert gas). ) Welding is commonly used. The welding may be automatic, semi-automatic, or manual welding, and is not particularly limited.

The high Cr steel of the present invention has a Cr content of 7.5 to 18.0% and has a microstructure of substantially a single martensite phase, or mainly martensite and partially containing ferrite. It is a steel that requires high strength. The present invention is particularly effective when the yield strength of the base material is 448 N / mm ² or more, and the strength of the base material is 517 N / mm ^2.
/ Mm ² or more is more effective.

Even if it is a high Cr steel, if the structure of the steel is composed of a ferrite single phase or substantially ferrite, the strength of the steel itself is not necessarily high, so that it is not necessary to apply the method of the present invention. Often. The reason is that such steels are often used mainly as thin plates requiring workability, and there is no requirement for the impact toughness of the base material or the welded portion, or the requirement level is low. Further, the strength required for the welded portion is not so high.

In the high Cr steel to which the present invention is applied,
The other components are not particularly limited as long as the Cr content is in the above range, and any of them can be applied. The following is an example of the concept of typical components just in case. (1) C and N are reduced for the purpose of improving weldability. (2) Ni, Cu, M to obtain martensite structure
Add n. (3) Alternatively, for the purpose of improving corrosion resistance, Mo,
Add W. (4) Alternatively, Nb, V, Ti, Zr, Ta or the like is added for the purpose of improving impact toughness and heat-affected zone hardness. (5) At least one of rare earth elements (REM) and Ca is added for the purpose of improving corrosion resistance and hot workability. In addition, P, S, and O, which are impurities, are generally reduced. Si and Al are added for the purpose of deoxidation. B may be added for the purpose of strength.

The present invention can of course be applied to circumferential welding of steel pipes such as line pipes, and of course can be applied to welding of steel plates used in pressure vessels and structures.

[0032]

Embodiments of the present invention will be described below. A high-Cr steel pipe (wall thickness: 12.7 mm) whose components are shown in Table 1 was used as a base material, and welding was performed with a welding material shown in Table 2 to form a welded joint. The steel pipes shown in Table 1 were manufactured as steel pipes, and then subjected to quenching-tempering heat treatment to yield a yield strength of 551 N / mm ² or more. Table 3 shows the welding conditions. The numbers of the steel pipes to which each welding material was applied are shown. In addition, no preheating was applied at the time of welding, and no heat treatment was performed after welding. Table 3 also shows the results of measuring the austenite fraction by the point count method after etching to reveal the structure of the cross section of each welded joint.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Next, from each of the welded joints, a JIS No. 4 impact test piece (full size) was placed so that the notch was located in the weld metal.
After sampling, an impact test was performed. The maximum hardness of the weld metal was measured as Vickers hardness at a load of 1 kg.
In a direction perpendicular to the welding line, a JIS No. 5 tensile test piece was sampled so as to include a weld metal, a weld heat affected zone, and a base material in a parallel portion, and a tensile test was performed at room temperature.

On the other hand, test specimens were taken from the weld metal of each welded joint and subjected to a corrosion test in a wet carbon dioxide gas environment. Corrosion test conditions in a wet carbon dioxide gas environment include:
In an autoclave at a test temperature of 120 ° C., carbon dioxide gas 40
It was immersed in a 5% NaCl aqueous solution for 30 days under atmospheric pressure, and the corrosion rate was calculated from the weight change before and after the test. The unit of the corrosion rate is expressed in mm / y. Generally, when the corrosion rate of a certain material in an environment is less than 0.1 mm / y, it is considered that the material has sufficient corrosion resistance and can be used.

The test results are shown in Table 3. In the impact test results in Table 3, ○ indicates that the fracture surface transition temperature is −30 ° C. or less,
Indicates that the fracture surface transition temperature exceeded −30 ° C. and was 0 ° C. or less, and XX indicates that the fracture surface transition temperature exceeded −0 ° C., respectively. In the results of the tensile test, ○ was broken at the base material,
The ones that did not break at the weld metal part and the ones that failed at the weld metal part represent crosses. Corrosion test results include:
The corrosion rate was shown.

As is evident from Table 3, No. 1 of the present invention was used. Nos. 1 to 5 have many required properties such as excellent impact toughness of weld metal, high strength of weld metal (not broken by weld metal), low hardness of weld metal, and excellent corrosion resistance of weld metal. It turns out that it can be satisfied simultaneously.
In addition, these characteristics are obtained as-welded without performing preheating or post-heating during welding.

On the other hand, in Comparative Example No. Since No. 6 is based on a low alloy steel, the corrosion resistance and impact toughness of the weld metal are extremely poor. Comparative Example No. No. 7 is a 13% Cr-4Ni co-gold system and has a martensitic structure, and the hardness of the weld metal is very large as it is. Also, in Comparative Example No. 8
Is an austenitic welding material, but the corrosion resistance and impact toughness are good, but the strength of the weld metal is very low, which is completely insufficient. Comparative Example No. No. 9 is a ferrite + austenite two-phase mixed structure, but the components are not appropriate and the austenite fraction of the weld metal is inappropriate, so that the impact toughness of the weld metal is extremely poor.

[0041]

As described above, according to the present invention, Cr and Ni are contained in a well-balanced manner as a welding material to form a weld metal into a two-phase structure of austenite and ferrite.
This, together with further defining other components, makes it possible to provide a method for welding high Cr steel having excellent corrosion resistance, strength, and toughness, and greatly contributes to industrial development.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-123497 (JP, A) JP-A-53-40656 (JP, A) JP-A-50-158546 (JP, A) 40345 (JP, A) JP 28-3307 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 9/23 B23K 35/30

Claims (3)

(57) [Claims] 1. A welding method for a high Cr steel containing 7.5 to 18.0% by weight of Cr and having a microstructure substantially of martensite single phase or martensite and partially containing ferrite. Si: 0.01 to 1.0%, Mn: 0.02 to 3.0%, Cr: 18.0 to 28.0%, Ni: 5.0 to 10.0%, Mo + 0 0.5 W: 1.0 to 5.0%, Al: 0.005 to 0.5%, C: 0.03% or less, P: 0.03% or less, S: 0.01% or less And arc welding using a welding material consisting of the balance Fe and unavoidable impurities, and changing the microstructure of the weld metal to austenite and ferrite.
A method for welding a high Cr steel, comprising a phase structure and an austenite fraction of 30 to 70%. 2. The high Cr material according to claim 1, wherein the welding material further contains one or two of Cu: 3% or less and N: 0.2% or less by weight. Steel welding method. 3. The welding material further comprises Nb, V, Ti, Z.
The method for welding a high Cr steel according to claim 1 or 2, wherein one or more of r and Ta are contained in a total content of 1.0% or less by weight%.