JP3165902B2 - High Cr steel welding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for welding high Cr steel. More specifically, for example, line pipes used for the transportation of oil and natural gas, or containers used for storage,
Alternatively, the present invention relates to a welding method excellent in corrosion resistance, strength and toughness, which is suitable for welding high Cr steel used in applications requiring further strength and corrosion resistance.

[0002]

2. Description of the Related Art In recent years, an increasing number of petroleum and natural gas containing wet carbon dioxide and hydrogen sulfide have been increasing. It is a well-known fact that carbon steel and low alloy steel are significantly corroded in such an environment. As a countermeasure against corrosion of steel pipes during transportation of such corrosive oil and natural gas, addition of a corrosion inhibitor has conventionally been common. However, in a marine oil well, the cost required for the addition / recovery treatment 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 a corrosion-resistant material that does not require the addition of a corrosion inhibitor 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 heat treatment to ensure strength and toughness. In general, the structure is tempered martensite. For example, JP-A-4-99154 and JP-A-4-99155 disclose high Cr steels for line pipes which reduce C and N and which are added with a substitutional austenite stabilizing element and have excellent weldability. ing.

By the way, line pipes and pressure vessels are connected or manufactured by welding, but there has been no welding material or welding method suitable for high Cr steel having excellent weldability as described above. "NKK Technical Report", 1989
Issue 129, pp. 15-22, AISI4
An example is reported in which ten steels are manufactured as UOE steel pipes, and TIG welded joints (equivalent to local circumferential welding of line pipes) are made using a co-metallic material to which Ni is added. But,
As can be seen from this report, the hardness of the weld metal becomes extremely hard even when the high-Cr steel co-metallic material contains a large amount of Ni. In this case, in a use environment, there is a problem that if a small amount of hydrogen sulfide is mixed, stress corrosion cracking may occur in the weld metal. Although it is possible to reduce the hardness of the weld metal by performing a tempering heat treatment after welding, in the case of a high Cr steel, the heat treatment temperature and time are extremely high, such as 620 ° C. × 1 hour. Long heat treatment is required. 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, since TIG (tungsten inert gas) welding has a low welding speed and a small amount of weld metal in one pass, there is a problem in that the application cost is extremely high when applied to welding of a structure. Therefore, it is difficult to weld a high Cr steel using a common metal or martensitic stainless steel welding material.

On the other hand, if low alloy steel is used as a welding material, welding is easy, and post-weld heat treatment of the weld metal is unnecessary.
However, low-alloy steel has poor corrosion resistance in an environment containing carbon dioxide, and if a low-alloy steel weld metal with poor corrosion resistance contacts a high-Cr steel base metal with excellent corrosion resistance, the weld is selectively corroded. In addition, the safety of the structure is extremely dangerous,
Can not be adopted.

Further, if high Ni austenitic stainless steel or Ni-base superalloy having excellent corrosion resistance is used as a welding material, selective corrosion of a weld does not occur, the hardness of the weld metal is low, and the toughness of the weld metal is low. Can be secured. But,
Austenitic stainless steel and Ni-based superalloys have a problem in that their crystal structures have low strength. Generally, the yield strength of these materials is 300-400 N / mm
^There are only about ^two . When a high Cr steel base material having a yield strength of 551 N / mm ² or more is welded with a very low strength weld metal, the weld metal is intensively deformed when external stress is applied, which may lead to fracture. Yes (called undermatching). Therefore, austenitic stainless steel and high N
There was also a great difficulty in welding high Cr steels using the i alloy as a welding material.

As described above, in a welded structure, undermatching in which the strength of the weld metal is lower than that of the base metal is often disliked, and it is often preferable that the strength of the weld metal is higher than that of the base metal. Assuming that the strength of the base metal is constant, it is necessary to increase the strength of the weld metal. In this case, the reinforcing means should not significantly reduce other properties such as corrosion resistance and toughness.

[0008]

SUMMARY OF THE INVENTION In view of these circumstances, the present invention provides a method for welding high Cr steel which has excellent corrosion resistance in an environment containing carbon dioxide gas and has excellent toughness and strength of a welded portion. It is an object.

[0009]

Means for Solving the Problems The present invention is as follows.

In the first invention, Cr: 7.5% by weight.
-18.0%, and the microstructure is substantially a martensite single phase or a martensite-based welding method for a high Cr steel partially containing ferrite.
C: more than 0.03% to 0.12% or less, 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 + 0.5W: 1.0 to 5.0%, Al: 0.005 to 0.5%, P is 0.03% or less, and S is 0.01%. Less than,
N is reduced to 0.03% or less, arc welding is performed using a welding material consisting of the balance of Fe and unavoidable impurities, and the microstructure of the weld metal is a two-phase structure of austenite and ferrite, and the austenite fraction is 30 to 70%. % Of the high-Cr steel.

A second invention is based on the first invention,
This is a method for welding high Cr steel in which the welding material further contains Cu: 3% or less by weight as an additional component.

[0012] The third invention is the first invention or the second invention.
In the present invention, a method for welding a high Cr steel in which a welding material further contains, as an additional component, 1.0% or less by weight of one or more of Nb, V, Ti, Zr, and Ta in total. It is.

[0013]

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

C: C is added as an element that greatly increases the strength of the weld metal and as an austenite-forming element. If the addition amount of C is 0.03% or less, the strength of the weld metal is insufficient, and it is easy to cause undermatching. C is C
r is an element that reduces carbides by forming carbides. However, if the C content is 0.12% or less, the decrease in corrosion resistance due to the addition of C is not so large, and is lower than that of the high Cr steel as the base material. There is no. However, C content is 0.12%
If it exceeds, the corrosion resistance and toughness of the weld metal decrease, so the upper limit is made 0.12%.

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

Mn: Mn is required 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 must be contained at 0.02% or more. However, even if the content exceeds 3.0%, the effect is no longer saturated, and it is difficult to excessively contain Mn during the production of the material, so the upper limit content is 3.0%. I do.

Cr: Cr must be contained at 18.0% or more in order to secure the corrosion resistance and strength of the weld metal. However, if the Cr content exceeds 28.0%, the structure of the weld metal is adjusted. To do so, a large amount of alloying element must be added as an austenite-forming element, which only increases the cost unnecessarily. Therefore, the content of Cr 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. When the content is the same, the effect of W is 1/2 of the effect of Mo, and the effect of adding both is represented by Mo + 0.5W. And when Mo + 0.5W is less than 1.0%,
If the corrosion resistance and strength of the weld metal are not sufficient, and if it exceeds 5.0%, it will be difficult to maintain a proper fraction of ferrite and austenite in the weld metal, or to make the austenite fraction 30-70%. Austenitic forming elements must be added excessively. Therefore, Mo + 0.
The value of 5W is set to 1.0 to 5.0%. In addition, even if either one of Mo and W is added alone or both are added in combination, the amount represented by Mo + 0.5W is 1.0 to 5.0.
What is necessary is just to be in the range of 0%.

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

P: Since the presence of a large amount of P lowers the toughness, it must be reduced to 0.03% or less.
The smaller the better, the better.

S: Hot workability, if S is present in a large amount,
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.

N: N is an element that enhances the strength of the weld metal, but if it is contained in a large amount, problems such as formation of blowholes in the weld metal and bond portions occur. Therefore, N
The amount is limited to 0.03% or less.

The above are the basic components of the welding material used in the method of the present invention. If necessary, the following elements may be further added to use a welding material having further improved properties.

Cu: Cu has a remarkable effect on enhancing 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. Therefore, the upper limit content is set to 3.0%.

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 added in excess, these effects are saturated, but the toughness is reduced. Therefore, the total content of one or more of Nb, V, Ti, Zr, and Ta is 1.0%. Shall not exceed

The welding material used in the method of the present invention may contain, in addition to the above-mentioned components, for the purpose of adjusting the productivity, toughness, corrosion resistance and the like, or as an impurity accompanying the added alloy element.
Sn, Sb, B and the like can be contained. In order to improve the manufacturability, rare earth elements (REM), Ca,
Mg and the like can be contained. Here, the rare earth element means an atomic number of 57 to 71 or 89 to 1
Indicates element 03 and Y. In the present invention, the content of oxygen is not particularly limited. However, since oxygen is an impurity which is a source of generating oxide-based nonmetallic inclusions, the smaller the oxygen content, the better.

Next, in the present invention, the microstructure of the weld metal is limited. That is, the microstructure of the weld metal needs to have a two-phase structure of austenite + ferrite in order to simultaneously satisfy a plurality of required characteristics such as strength, impact toughness, hardness, and corrosion resistance. In a ferrite single phase or ferrite-based structure, impact toughness is poor,
In the case of an austenitic single phase or austenite-based structure, the strength of the weld metal is insufficient. Further, if the structure is mainly composed of martensite single phase or martensite, the hardness is high and the impact toughness is poor. With the two-phase structure of austenite + ferrite, the strength of the weld metal is increased, while preventing the hardness from excessively increasing. Therefore, welding cracks do not occur even without preheating or post heat treatment at the time of welding. Further excellent impact toughness is obtained.

The austenite fraction in the weld metal is 30%
If it is less than 10, the ferrite fraction becomes excessively large and the impact toughness of the weld metal decreases, and the ferrite hardness becomes too high. On the other hand, if the austenite fraction exceeds 70%, it becomes difficult to secure the strength of the weld metal, no matter how the components are adjusted. Therefore, the austenite fraction of the weld metal must be in the range of 30 to 70%.

The method of the present invention employs arc welding. It is not particularly limited if it is ordinary arc welding,
Covered arc welding is used for welding line pipes and pressure vessels, etc.
MIG (metal inert gas) welding, MAG (metal active gas) welding, TIG (tungsten inert gas)
Welding and the like are preferred. The welding may be automatic, semi-automatic, or manual, and is not particularly limited. In the case of a MIG, a mag, or a TIG, the components of the wire may be within the scope of the present invention.

The high Cr steel of the present invention has a Cr content of 7.5 to 18.0%, and has a microstructure substantially composed of a single martensite phase or a martensite-based partly 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 483 N / mm ² or more, and the strength of the base material is 551 N / mm ^2.
/ Mm ² or more is more effective.

[0032] Even if it is a high Cr steel, when the structure is composed of a single phase of ferrite or substantially composed of ferrite,
Since the strength of the steel itself is not always high, it is often unnecessary to employ the method of the present invention. This is because such steel is often used mainly as a thin plate 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. Of course, there is no problem if the method of the present invention is applied to a high Cr steel having a ferrite single phase or substantially ferrite.

[0033] 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. For example, to improve weldability, C and N
To obtain a martensitic structure
i, Cu, Mn, and Mo and W for the purpose of further improving corrosion resistance, and Nb, V, Ti, Zr, and T for the purpose of improving impact toughness and heat-affected zone hardness.
It is also possible to add at least one of rare earth elements (REM) and Ca for the purpose of adding a and the like, and for improving corrosion resistance and hot workability. Generally, impurities P, S, and O are reduced, and Si and Al are added for the purpose of deoxidation, and B may be added for the purpose of strength.

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

[0035]

Embodiments of the present invention will be described below.

The high Cr steel pipe (outer diameter 27
(3.1 mm, wall thickness 11.1 mm) was used as a base material, and welding was performed using the welding materials shown in Table 2 under the welding methods and conditions shown in Table 3 to produce welded joints. In addition, the steel pipes shown in Table 1 are steel pipes that have been subjected to quenching-tempering heat treatment after production as steel pipes and have a yield strength of 551 N / mm ² or more. No preheating was performed 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 the cross section of each welded joint was etched to reveal the structure of the weld metal.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

Next, a JIS No. 4 impact test piece (full size) was sampled from each of the welded joints so that the notch was located in the weld metal, and an impact test was performed. The maximum hardness of the weld metal was measured as Vickers hardness under a load of 1 kg. Further, 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. As a corrosion test condition in a wet carbon dioxide gas environment, the steel sheet was immersed in a 5% aqueous NaCl solution at a carbon dioxide gas pressure of 40 atm for 30 days in an autoclave at a test temperature of 120 ° C., and the corrosion rate was calculated from the weight change before and after the test. Although the unit of the corrosion rate is mm / y, the corrosion rate of a certain material in a certain environment is generally 0.1 mm / y.
If less than y, the material is considered to be sufficiently corrosion resistant and usable.

The test results are shown in Table 4. In the impact test results in Table 4, ○ indicates that the fracture surface transition temperature is −30 ° C. or less, × indicates that the fracture surface transition temperature exceeds −30 ° C. and 0 ° C. or less, and XX indicates that the fracture surface transition temperature exceeds −0 ° C. It represents that there was. In the results of the tensile test, ○ indicates that the fracture occurred in the base metal portion but did not occur in the weld metal portion, and X indicates that the fracture occurred in the weld metal portion. The corrosion rate was shown as the corrosion test result.

[0042]

[Table 4]

As is clear from Table 4, No. 1 of the present invention was used. In the case of 1 to 5, the required properties such as excellent impact toughness of the weld metal, high strength of the weld metal and no breakage of the weld metal, low maximum hardness of the weld metal, and excellent corrosion resistance of the weld metal are simultaneously satisfied. It turns out that we were satisfied. In addition,
As described above, these characteristics were obtained as they were without preheating or post-heating during welding.

On the other hand, in Comparative Example No. In No. 6, the corrosion resistance and impact toughness of the weld metal were extremely poor because the welding material was based on a low alloy steel. Comparative Example No. In No. 7, the welding material was a 13% Cr-4Ni co-metal system, had a martensite structure as quenched as it was welded, and the hardness of the weld metal was very hard. Also, in Comparative Example No. In No. 8, an austenitic welding material was used, and although corrosion resistance and impact toughness were good, the strength of the weld metal was extremely low and was completely insufficient. Comparative Example No. In No. 9, although the microstructure of the weld metal was a ferrite + austenite two-phase mixed structure,
The impact toughness of the weld metal was significantly poor due to the inadequate composition and the inappropriate austenite fraction of the weld metal.

[0045]

The present invention is 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-A-8-57683 (JP, A) JP-B-28-3307 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 9/23 B23K 35 / 30 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A method of welding 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. %, C: more than 0.03% and 0.12% or less, 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 + 0.5W: 1.0 to 5.0%, Al: 0.005 to 0.5%, P is 0.03% or less, and S is 0.01% or less, N is reduced to 0.03% or less, arc welding is performed using a welding material consisting of the balance of Fe and unavoidable impurities, and the microstructure of the weld metal is changed to a two-phase structure of austenite and ferrite. Characterized in that the fraction is 30-70% High C
r Steel welding method. 2. The method for welding a high Cr steel according to claim 1, wherein the welding material further contains Cu: 3% or less by weight as an additional component. 3. The welding material according to claim 1, further comprising, as an additional component, 1.0% or less by weight of one or more of Nb, V, Ti, Zr, and Ta. 3. The method for welding a high Cr steel according to item 2.