Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-04-02
2002-04-30
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C138S171000, C138S177000, C148S909000, C420S034000, C420S104000, C428S682000, C428S686000
Reexamination Certificate
active
06379821
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a martensitic stainless steel welded pipe excellent in corrosion resistance, in particular in stress corrosion cracking resistance. More particularly, it relates to a martensitic stainless steel large-diameter and thick-wall welded pipe exceeding 20 inches in outside diameter and exceeding 0.5 inch in wall thickness, which is used in a pipeline, in particular a trunk line, for conveying a fluid such as oil or natural gas readily corroding metals.
BACKGROUND OF THE INVENTION
Large-diameter, thick-wall stainless steel pipes exceeding 20 inches in outside diameter and 0.5 inch in wall thickness are widely used in pipelines, in particular trunk lines, for conveying oils or natural gases which readily cause corrosion of metals.
Such large-diameter, thick-wall pipes are generally produced by a process which comprises forming a thick plate or hot strip into an open pipe or spiral form by bending and then welding the joining portions together. The steel pipes produced by such a process are called UO pipes or spiral pipes.
UO pipes are produced by a forming process comprising the steps referred to by the alphabetic letters (U and O) indicative of the name given thereto. In this process, a thick plate is formed into an open pipe form bent by using a U press, then the side edges are joined to each other to form a pipe shape by means of an O press and the joining portions are welded together.
Spiral pipes are produced by forming a hot strip into a pipe shape by spirally bending the hot strip in succession and then welding the joining portions together, side edge to side edge of the strip.
Processes other than those mentioned above are also available for the production of large-diameter, thick-wall pipes. For example, there is a process which comprises forming a thick plate into a pipe-like shape using a 3-roll type forming machine called roll bender and then seam-welding the side edge to side edge in order to join portions of the thick plate together.
In producing these large-diameter, thick-wall pipes, the submerged arc welding method (hereinafter referred to as “SAW method”) is widely used. In producing large-diameter, thick-wall pipes by welding, one-layer welding is generally carried out from each of inside and outside of the pipe. Further, when the wall of the material is thick, multi pass (at least three) layer welding, in which two or more bead layers are formed from one or both sides, may be conducted in some instances.
Heretofore, in producing large-diameter, thick-wall welded pipes to be used in conveying fluids, such as oils and natural gases, readily causing corrosion of metals, steel plates made of carbon steel or low alloy steel containing at most about 1% by mass of Cr have been used as the base metals together with welding materials. The reason why large-diameter, thick-wall welded pipes made of carbon steel, which is inferior in corrosion resistance, have been used is that carbon steel is economically more advantageous. However, carbon steel is poor in corrosion resistance. Therefore, for pipelines constituted of welded pipes made of carbon steel, it has been a common practice to subject the crude oil or natural ngas obtained from an oil well to dehydration to thereby reduce the corrosiveness of the fluid.
However, the cost of construction of the dehydration equipment and the platform for the installation thereof is high. Therefore, the use of a more corrosion-resistant material has been begun for the production of pipes for pipelines while omitting the dehydration equipment. The material having higher corrosion resistance includes stainless steel.
In this case, neither dehydration equipment nor platform therefor is required at exploration locations and this fact is very advantageous to the exploration of small-scale oil or gas wells, for example horizontal wells, which cannot have been drilled. Specifically, it is an advantage that crude oils can be conveyed through such pipelines to an existing platform and collectively treated there for dehydration.
In high-latitude districts at north latitude 70° or higher where future exploration is expected, for example oil wells in the North Sea, the platform construction itself is difficult from the viewpoint of waves on the sea. In that case, it is necessary to transport crude oils through pipelines without dehydration treatment.
With such background, large-diameter, thick-wall welded pipes enabling the omission of dehydration treatment are more and more desired for conveying fluids readily corrosive against metals.
Some stainless steels which are highly corrosion-resistant and suitable for conveying such fluids as mentioned above, and seamless pipes or electric resistance welded pipes or laser welded pipes made thereof with a relatively small diameter and a relatively thin wall, have been proposed. As for large-diameter, thick-wall welded pipes made of stainless steel, welded pipes and base metals therefor are disclosed in JP Kokai H10-60599 and JP Kokai H12-8144, for instance.
For the above application, martensitic stainless steel containing 9-13% by mass of Cr are used from the economical viewpoint. This is because martensitic stainless steel has, in addition to the economical feature, sufficient corrosion resistance under such circumstances as mentioned above, and is excellent in hot workability and, therefore, can readily be made into thick plates or hot-rolled plates, which are materials for the production of welded pipes.
It has been considered that these martensitic stainless steels used for base metals and the weld metals of seam weld portions are excellent in stress corrosion cracking resistance (hereinafter referred to as “SCC resistance”), carbon dioxide gas corrosion resistance (hereinafter referred to as “CO
2
resistance”) and sulfide stress corrosion resistance (hereinafter referred to as “SSC resistance” or “sour gas resistance”).
However, it has been revealed that when welded pipes made of martensitic stainless steel are used in a pipeline for conveying a corrosive fluid without dehydration, stress corrosion cracking (hereinafter referred to as “SCC”) tends to occur at the weld portion of the pipe inside surface. In particular, with large-diameter, thick-wall welded pipes produced by the SAW method without cutting off the weld bead on the pipe inside and outside surfaces, the tendency toward occurrence of SCC is significant. Furthermore, it has become apparent that even if the pipes have SCC resistance, the base metal and weld metal may be poor in sour gas resistance and the weld metal may be high in weld hot cracking susceptibility in some instances.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a large-diameter, thick-wall martensitic stainless steel welded pipe excellent in corrosion resistance, in particular stress corrosion cracking resistance (SCC resistance), at the base metal portion and the seam weld portion of pipe inside surface and, further, excellent in sulfide stress corrosion resistance (sour gas resistance) and carbon dioxide gas corrosion resistance (CO
2
resistance).
The stainless steel welded pipe of the invention is composed of a base metal which is a stainless steel containing not more than 0.05% by mass of C and 9-20% by mass of Cr and having metallurgical microstructures comprising a full martensite phase or a martensite phase as the main constituent with a ferrite phase contained therein, and a seam weld metal which is a stainless steel containing not more than 0.1% by mass of C and 7-20% by mass of Cr and having metallurgical microstructures comprising a martensite phase as the main constituent with an austenite phase contained therein. Further, the seam weld bead on the inside surface of the welded pipe of the invention satisfies the following relation (1):
L≦
0.2
×W
(1)
where
L: the length of the portions of the seam weld bead showing a value of h which exceeds 1.25 as calculated by the expression (2) shown below:
h={
1+(2
×H/W
)}×(
YS
B100
/YS
w100
)&e
Hamada Masahiko
Kondo Kunio
Kushida Takahiro
Ogawa Kazuhiro
Omura Tomohiko
Clark & Brody
Koehler Robert R.
Sumitomo Metal Industries Ltd.
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