Electric heating – Metal heating – Of cylinders
Patent
1997-08-07
1998-11-17
Shaw, Clifford C.
Electric heating
Metal heating
Of cylinders
219 73, B23K 918
Patent
active
058379568
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of fabricating a high strength and high toughness large-diameter welded steel pipe used as a line pipe and, in particular, relates to improvement in strength, toughness, and cracking characteristics of welded joint of a large-diameter welded steel pipe.
BACKGROUND ART
It is well-known that pipelines are used as means for large-scale transport of crude oil or natural gas mined as crude fuel, liquids or gases obtained by refining the crude fuel, or other kinds of liquids, gases, or slurries. These pipelines are extremely effective as means for large-scale transport of industrial crude fuel, and line-pipes durable to withstand more severe environment are required.
Large-diameter UOE steel pipes are used as line-pipes. Recently, the above energies are supplied to various places such as cold districts and the sea floor, therefore higher characteristics are in demand for line-pipes.
In particular, line-pipes placed in cold districts are required to have both strength and excellent low-temperature toughness. Furthermore, in the case of gas pipelines, transport pressure inside pipes tends to increase to improve transport efficacy and, correspondingly, higher strength is in demand for line-pipes. To meet the above requirements, development of steel plates and welding materials have continued so that the strength of UOE steel pipes has gradually improved from the X 50 grade and, at present, steel pipes of the X 70 grade are being put to practical use.
Appropriate quantities of Ti, B, and Mo are added to weld metal for UOE steel pipes of up to the X 70 grade to improve strength and toughness of the weld metal. The micro-structure of the weld metal thereby essentially consists of acicular ferrite. Since acicular-ferrite has fine grains, the weld metal can attain high strength and high toughness.
For example, Japanese Examined Patent Publication No. 57-17637 proposes a submerged arc welding method in which a high toughness weld metal is obtained by addition of Ti, B, and Mo to a weld wire to adjust the composition of weld metal so as to achieve a refined micro-structure in the weld metal.
Furthermore, Japanese Unexamined Patent Publication No. 63-2588 discloses a welded steel pipe excellent in field weldability. In this steel pipe, the chemical composition (P.sub.cm) of steel-pipe material is set to a low value and the chemical composition of seam weld metal, in particular the contents of Ti, B, and Mo, are controlled so that low hardness is achieved in a weld metal with relatively low weling heat input in the field and in portions heat-affected the field welding, thereby providing a pipe showing both corrosion resistance and high toughness.
Moreover, Japanese Unexamined Patent Publication No. 3-285770 proposes a method of fabricating a large-diameter steel pipe which reduces the hardness of a girth weld metal and its heat affected zone by, in particular, containing Ti in the chemical composition of steel-pipe material and weld metal while balancing against Al, O, and N to achieve excellent sour gas resistance. In this method, the composition of weld metal is controlled such that the weld metal forms acicular ferrite.
According to each of the above methods, large-diameter steel pipes of the X 60 to 70 grades can be manufactured while lowering the hardness of weld metal by controlling the chemical composition of the weld metal. However, demand for higher strength line-pipes is increasing and high strength line-pipes up to the X 100 grade have been investigated.
The largest problem that has to be overcome for achieving higher strength steel pipes is the prevention of cracks occurring in welded joints of steel pipes. Cracks in welded joints include hot cracking and cold cracking. Hot cracking occurs at a temperature range of above approximately 1,000.degree. C. during cooling process of welding and is grain-boundary cracking due to the formation of compounds having low melting points, such as P, S, and Ni, in the grain boundaries. Cold cracking occurs during or
REFERENCES:
patent: 4020312 (1977-04-01), Araki et al.
Okabe Takatoshi
Yasuda Koichi
Kawasaki Steel Corporation
Miller Austin R.
Shaw Clifford C.
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