JPH09295185A - Welding method of high chromium steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding method suitable for welding the high Cr steel to be used in a line pipe for petroleum and natural gas, and excellent in high temperature cracking resistance, low temperature cracking resistance, toughness, strength, and corrosion resistance of the weld metal. SOLUTION: The gas shielded metal arc welding is performed using a stainless steel wire having a composition consisting of 0.005-0.12% C, 0.01-1% Si, 0.02-2% Mn, 13-18% Cr, 3-5% Ni, 0.05-0.12% N, and with the ratio of the Cr equivalent to the Ni equivalent is >=1.6 to <=2.0 in the welding of the high Cr steel containing 7.5-12% Cr, the micro structure of the weld metal is of three phase structure of austenite, ferrite and martensite.

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, to a line pipe used for transportation of oil and natural gas, a container used for storage, or strength, toughness and corrosion resistance. Suitable for welding high Cr steel used in applications where
The present invention relates to a welding method which does not require preheating and post heat treatment, has good crack resistance, and has excellent strength, toughness, and corrosion resistance.

[0002]

2. Description of the Related Art In recent years, petroleum and natural gas produced have increased in amount containing moist carbon dioxide and hydrogen sulfide.
It is a well-known fact that carbon steel and low alloy steel are significantly corroded in such an environment. Therefore, in transporting such corrosive oil and natural gas, addition of a corrosion inhibitor has heretofore been generally used as an anticorrosion measure for steel pipes. However, the corrosion inhibitors are becoming difficult to use due to the enormous cost required for addition and recovery processing of the corrosion inhibitors in the ocean oil well and due to the problem of marine pollution. Therefore, the need for corrosion resistant materials that do not require the addition of corrosion inhibitors has recently increased.

[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
In the publication, a high Cr steel for line pipes is proposed in which C and N are reduced and a substitutional austenite stabilizing element is added and which has excellent weldability.

By the way, it is well known that line pipes and pressure vessels are connected or manufactured by welding, but a welding material or welding method suitable for high Cr steel having excellent weldability as described above. There has never been. 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 the high-Cr steel co-metal material, the hardness of the weld metal becomes very hard even if a large amount of Ni is contained. Also in this case,
Pre-heat treatment or post-heat treatment is essential because the weld metal is highly susceptible to cold cracking. Therefore, it is difficult to weld a high Cr steel using a conventional co-metal or martensitic stainless steel welding material.

On the other hand, when high Ni austenitic stainless steel or Ni-base superalloy having excellent corrosion resistance is used as the welding material, selective corrosion of the welded portion does not occur, the hardness of the welded metal is low, and The toughness can be secured. However, austenitic stainless steels and Ni-base superalloys
Due to its crystal structure, there is a problem that strength is low. When welding is performed with a weld metal having a very low strength, when an external stress is applied, the weld metal is intensively deformed and may be broken (referred to as undermatching). Therefore,
Welding high Cr steel using austenitic stainless steel or high Ni alloy as a welding material has also been very difficult. Further, in recent years, a duplex stainless steel welding material has also been used, but the strength of the weld metal is low and often undermatches.

[0006]

In view of these circumstances, the present invention does not require preheating and post heat treatment when welding high Cr steel, and does not require high temperature crack resistance, low temperature crack resistance and toughness of the welded portion. It is an object of the present invention to provide a welding method having excellent strength and excellent corrosion resistance in a carbon dioxide gas-containing environment and the like.

[0007]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems, and in% by weight (hereinafter the same), Cr: 7.
In the welding method of high Cr steel containing 5 to 12.0% and having a microstructure of martensite single phase, or 50% or more of martensite and the balance ferrite, C: 0.
005 to 0.12%, Si: 0.01 to 1.0%, M
n: 0.02 to 2.0%, Cr: 13.0 to 18.0
%, Ni: 3.0 to 5.0%, N: 0.05 to 0.12
%, And if necessary Mo: 0.1-2.0
% Cu: 0.1 to 2.0% of one or two kinds are contained,
Furthermore, P is limited to 0.03% or less and S is limited to 0.01% or less, and the balance is Fe and inevitable impurities, and Cr
Equivalent weight is Cr + Mo + 1.5Si, Ni equivalent weight is Ni + 0.
When 5Mn + 30C + 30N, the Cr equivalent / Ni equivalent ratio is 1.6 or more and 2.0 or less, gas shield arc welding is performed using a stainless steel wire, and the microstructure of the weld metal is austenite + ferrite + martensite It is a welding method for high Cr steel characterized by having a structure.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the range of each component in the present invention will be described below. First, the reasons for limiting the components of the stainless steel wire will be described. C: C is added as an element that greatly increases the strength of the weld metal and as an austenite forming element in an amount of 0.005% or more. Further, although C is an element that forms Cr carbides and lowers the corrosion resistance, if the C content is 0.12% or less, the deterioration of the corrosion resistance due to the addition of C is not so large,
It does not fall below that of the high Cr steel that is the base material. However, if the C content exceeds 0.12%, the corrosion resistance and toughness of the weld metal deteriorate, so the upper limit is made 0.12%.

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

Mn: Mn is necessary as a deoxidizing agent for the weld metal, and is also important as an austenite forming element for adjusting the structure of the weld metal, and it is necessary to contain 0.02% or more. However, even if the content exceeds 2.0%, not only the effect is already saturated, but excessive addition of Mn causes difficulty in manufacturing the material, so the upper limit content is 2.0%. To do.

Cr: Cr is required to be contained in an amount of 13.0% or more in order to secure the corrosion resistance and strength of the weld metal, but when it is contained in excess of 18.0%, the strength of the weld metal is secured. Therefore, it becomes difficult to generate a martensite structure. Therefore, the Cr content is 13.0 to 18.
0%.

Ni: Ni is important as an element that stably forms austenite in the structure of the weld metal and secures toughness and corrosion resistance. If the content is less than 3.0%, the impact toughness is insufficient. If the Ni content exceeds 5.0%, the austenite fraction may become excessive and the strength of the weld metal may decrease, while the effect of improving impact toughness is saturated. Therefore, the content of Ni is set to 3.0 to 5.0%.

N: N is added as 0.05% or more as an austenite forming element for ensuring toughness, but 0.12%
If it is contained in excess of 0.1%, problems such as formation of blowholes in the weld metal occur, so the upper limit content is made 0.12%.

P: If a large amount of P is present, the high temperature crack resistance and toughness of the weld metal are deteriorated. Therefore, it is desirable that the amount be small, and it is necessary to reduce it to 0.03% or less.

If a large amount of S: S is present, the weld hot cracking resistance, hot workability, ductility and corrosion resistance are deteriorated. Therefore, a smaller amount is desirable, and it is necessary to reduce it to 0.01% or less. In order to further improve the manufacturability 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.

Cr equivalent / Ni equivalent: When the Cr equivalent / Ni equivalent is less than 1.6, the weld metal becomes austenite single phase solidification and the hot cracking susceptibility increases. On the other hand, if it exceeds 2.0, the ferrite content increases and the toughness decreases. Therefore, the Cr equivalent / Ni equivalent ratio is limited to 1.6 to 2.0. The Cr equivalent is Cr + Mo + 1.5Si, including Mo added as required later, and the Ni equivalent is Ni.
+ 0.5Mn + 30C + 30N.

The above are the basic components of the stainless steel wire used in the method of the present invention. In the present invention, however, the following elements are further added, if necessary, to further improve the properties of the welding material. There is.

Mo: Mo is added to secure the corrosion resistance and high strength of the weld metal. If the Mo content is less than 0.1%, the corrosion resistance and strength of the weld metal will not be sufficient, and if it exceeds 2.0%, an intermetallic compound will be generated in the weld metal and the toughness will decrease. Therefore, the content of Mo is set to 0.1 to 2.0%.

Cu: Cu has a remarkable effect in enhancing the strength and corrosion resistance of the weld metal, and is added as an austenite forming element for ensuring toughness in an amount of 0.1% or more, but 2.0%. If added in excess, the effect will no longer be saturated, but the manufacturability of the welding material will be reduced, so the upper limit content is made 2.0%.

Secondly, the reason why the microstructure of the weld metal is limited in the present invention will be explained. The microstructure of the weld metal must be austenite + ferrite + martensite in order to simultaneously satisfy multiple required properties such as low temperature crack resistance, high temperature crack resistance, strength, impact toughness and corrosion resistance. Is. A ferrite single phase has poor impact toughness and insufficient strength. The austenitic single phase has a high risk of hot cracking and has a significantly lower strength. Martensite single phase is poor in impact toughness and there is a risk of cold cracking. In the two phases of ferrite and austenite, crack resistance and toughness are good, but strength is insufficient. The two phases of ferrite and martensite have high cold cracking susceptibility and poor impact toughness. The two phases of austenite and martensite are highly susceptible to hot cracking.

By forming the three-phase structure of austenite + ferrite + martensite, the toughness and cold crack resistance are secured by austenite, the strength is martensite, and the hot crack resistance is secured by ferrite. Strength and toughness are enhanced and hot cracking is prevented. Furthermore, the occurrence of low temperature cracking is suppressed even without performing preheating or post heat treatment.

The high Cr steel targeted by the present invention has a Cr content of 7.5 to 12.0% and has a microstructure of martensite single phase or contains martensite of 50% or more and the balance ferrite. A steel that requires high strength. Here, if the martensite content is less than 50%, the strength is insufficient, and if the Cr content exceeds 12%, 50% or more of martensite cannot be secured. Also, the Cr content is 7.5%
If it is less than this, the corrosion resistance becomes insufficient. The present invention is particularly effective when the yield strength of the base material is 700 N / mm ² or more.

Even if it is a high Cr steel, if the structure of the steel is a ferrite single phase or the ferrite is more than 50%, the strength of the steel itself is not necessarily high, so that the strength required for the welded portion is high. In many cases it is not very expensive and it is not necessary to apply the method of the invention. Of course, the structure is a single ferrite phase, or high C with more than 50% ferrite.
There is no problem in applying the method of the present invention to r steel. In the high Cr steels targeted by the present invention, other components are not particularly limited as long as the Cr content is in the above-mentioned range, and any of them can be applied.

[0024]

Embodiments of the present invention will be described below. A groove having a groove angle of 60 ° and a root face of 1 mm was prepared using a high Cr steel having the components shown in Table 1 (sheet thickness: 14.5 mm) as a base material. The steel sheets in Table 1 are subjected to quenching-tempering heat treatment and have a yield strength of 710 N / mm ² or more. Further, based on the above findings, steel having the chemical composition shown in Table 2 was melted by vacuum melting and then drawn by a usual method to obtain a welding wire. Using these welding wires for the base metal in Table 1, 2
MIG welding was performed under the conditions of 00A-24V-40 cm / min. In addition, in any case, no preheating was applied at the time of welding and no heat treatment was performed after welding.

[0025]

[Table 1]

[0026]

[Table 2]

The microstructure of the weld metal was subjected to etching on the cross section of each welded joint, and was distinguished from the exposed structure. The results are shown in Table 3. Next, a JIS No. 4 impact test piece (full size) was sampled from each welded joint so that the notch was located in the weld metal, and then an impact test was performed. In addition, in the direction orthogonal to the welding line, JIS5 should include the weld metal, the weld heat affected zone, and the base metal in the parallel portion.
The No. 10 tensile test piece was sampled and the tensile test was performed at room temperature.

[0028]

[Table 3]

On the other hand, a test piece was taken from the weld metal of each welded joint and a corrosion test was performed in a wet carbon dioxide gas environment. Corrosion test conditions in a wet carbon dioxide environment include:
40 carbon dioxide in an autoclave at a test temperature of 120 ° C
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. Although the unit of the corrosion rate is expressed in mm / y, in general, when the corrosion rate of a material in an environment is less than 0.1 mm / y, the material is considered to be sufficiently corrosion resistant and usable. Furthermore, JIS Z31 is used for the hot crack test.
The FISCO test described in 55 was adopted, and the U-type weld crack test described in JIS Z3157 was adopted for the low temperature crack test.

The results of each test are shown in Table 3. In the results of the hot cracking test and cold cracking test in Table 3, ◯ means that no crack was observed, and x means that cracking occurred. Further, in the impact test results, ◯ indicates that the fracture surface transition temperature is −30 ° C. or lower, × indicates that the fracture surface transition temperature exceeds −30 ° C. and is 0 ° C. or less, and XX indicates that the fracture surface transition temperature exceeds 0 ° C. It means that In the tensile test results, ○ was broken in the base metal part, not broken in the weld metal part,
X represents a fracture at the weld metal part. The corrosion test results showed the corrosion rate.

As is apparent from Table 3, No. 1 which is an example of the present invention. In Nos. 1 to 6, the weld metal has a three-phase structure of austenite + ferrite + martensite, and the high temperature cracking resistance and the low temperature cracking resistance are good even without preheating or post heat treatment during welding. It can be seen that a large number of required properties such as excellent impact toughness of the weld metal, high strength of the weld metal (which does not break in the weld metal), and excellent corrosion resistance of the weld metal can be satisfied at the same time.

On the other hand, No. In No. 7, since the weld metal is a single ferrite phase, the impact toughness is extremely poor and the strength is low. Comparative Example No. 8 and No. 12
Indicates that the weld metal is austenite single-phase solidified and therefore weld hot cracking occurs. In addition, No. No. 8 has low strength, and No. No. 12 has low toughness. Comparative Example No. 9 and No. In No. 11, the weld metal has a martensite single phase or a martensite + ferrite structure, low temperature cracking occurs, and the impact toughness is remarkably lowered. Comparative Example No. In No. 10, the weld metal has a two-phase structure of ferrite + austenite and is excellent in crack resistance, toughness and corrosion resistance, but the weld metal fractures due to insufficient strength.

[0033]

As described above, the present invention does not require preheating and post-heat treatment, and does not require hot crack resistance, cold crack resistance,
It is possible to provide a welding method for high Cr steel excellent in toughness, strength and corrosion resistance, and it is extremely important to contribute to the development of industry.

Claims (2)

[Claims] 1. Cr: 7.5% by weight (hereinafter the same)
In the welding method for high Cr steel containing 12.0% and having a microstructure of martensite single phase, or martensite 50% or more and the balance ferrite, C: 0.005 to 0.12%, Si: 0. 01-1.0%, Mn: 0.02-2.0%, Cr: 13.0-18.0%, Ni: 3.0-5.0%, N: 0.05-0.12% In addition, P is limited to 0.03% or less, S is limited to 0.01% or less, the balance is Fe and inevitable impurities, and the Cr equivalent is Cr + 1.5Si and the Ni equivalent is Ni + 0.5.
When Mn + 30C + 30N, gas shield arc welding is performed using a stainless steel wire having a Cr equivalent / Ni equivalent ratio of 1.6 or more and 2.0 or less, and the microstructure of the weld metal is austenite + ferrite + martensite A method for welding high Cr steel characterized by having a structure. 2. A stainless steel wire further contains one or two of Mo: 0.1 to 2.0% Cu: 0.1 to 2.0%, and Cr + as a Cr equivalent.
The method for welding high Cr steel according to claim 1, wherein Mo + 1.5Si is used.