Metal treatment – Stock – Nickel base
Reexamination Certificate
2002-07-11
2004-03-09
Sheehan, John (Department: 1742)
Metal treatment
Stock
Nickel base
C420S443000, C420S445000, C420S446000, C420S447000, C420S448000, C420S449000, C420S450000
Reexamination Certificate
active
06702906
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a Ni-base heat resistant alloy excellent in hot workability, weldability and carburization resistance and in strength at elevated temperatures, and relates to a welded joint thereof. In particular, it relates to a Ni-base heat resistant alloy to serve as a material for manufacturing tubes, plates and other members with a view to giving a welded structure to be used in a cracking furnace or a reformer furnace in an ethylene plant, and to a welded joint thereof. The cracking furnace or reformer furnace in an ethylene plant is a furnace in which a hydrocarbon raw material, such as naphtha, propane, ethane or a gas oil, is cracked or reformed at elevated temperatures, not lower than 800° C., to produce fundamental petrochemical products such as ethylene and propylene.
BACKGROUND ART
The temperature at which the cracking furnace and reformer furnace in an ethylene plant are used tends to become higher and higher in order to increase the yield of ethylene and the like.
The tube material, for use in such a cracking furnace and a reformer furnace, is required to have good heat resistance, inclusive of it's strength at elevated temperatures resistance to carburization, since the inner surface thereof is exposed to a carburizing atmosphere. On the other hand, the so-called coking, namely the phenomenon of the deposition of the carbon on the tube inner surface, occurs during operation and, as the amount of the deposit increases, the tube's internal pressure increases, the heating efficiency decreases and other operational troubles occur.
Therefore, in actual operation, the so-called decoking work for removing carbon deposit by means of air or steam is performed at regular intervals. The shutdown during decoking and the number of working steps are major problems. The severity of such coking and related problems increase as the tube becomes a smaller diameter tube, which is favorable for yield improvement.
A prior art technology for preventing coking, as shown in JP Kokai H02-8336, causes an alloy to contain not less than 28 mass % of Cr and thus causes a formation of a firm and stable Cr
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3
film on the alloy surface. The film prevents Fe and Ni, which are catalyst elements, promoting carbon deposition, from the exposure on the surface, and thereby suppress coking.
On the other hand, in order to improve the carburization resistance, it is known to be effective to increase the Si content in the alloy, as described in JP Kokai S57-23050, for instance.
However, these prior art technologies have problems, as mentioned below.
In applying a high Cr alloy, as proposed in JP Kokai H02-8336, as an elevated temperature strength material from the viewpoint of coking prevention, it is necessary to render the metallographic structure austenitic by raising the Ni content in the alloy. However, when merely an austenitic structure is obtained, the strength at elevated temperatures is low compared with conventional alloys, hence it is difficult to apply the material singly as an elevated temperature strength material. JP Kokai H02-8336 indicates the use of the material in combination with other elevated temperature strength material in the form of a double-layers tube. However, the double-layers tube has problems from the viewpoint of production cost and reliability.
When the Si content in the alloy is increased, as taught by JP Kokai S57-23050, the susceptibility to weld crack increases, hence there arises a problem that it cannot be practically used in the form of a welded structure.
On the contrary, alloys caused to form a firm and dense Al
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3
film on the metal surface by increasing the Al content therein, as shown in JP Kokai H04-358037, JP Kokai H05-239577, JP Kokai H05-33092 and JP Kokai H06-207235, show a markedly improved resistance to carburization and to coking as compared with conventional alloys. When the Ni content is increased in such high Al alloys, &ggr;′ phase precipitates finely in the matrix during use at elevated temperatures and the creep rupture strength is also markedly improved. Thus, the alloys described in the above-cited publications are characterized by having good resistance to carburization, and to coking at elevated temperatures, and high creep strength; therefore are suited for use as tubes of cracking furnace and reformer furnace in an ethylene plant.
However, for the alloys described in the above-cited publications, no due consideration was taken for the weldability, in particular weld crack resistance, in composition designing. No sufficient consideration was given, either, in designing the composition of the weld metal itself for constituting welded joints. Ni-base alloys, having a high Al content, are susceptible to cracking in the heat affected zone (hereinafter also referred to as “HAZ”) affected in the step of welding, as well as in the weld metal and, in addition, the weld metal tends to become lower in creep strength at elevated temperatures as compared with the base metal.
Since the weld metal is used in a solidified structure, unlike the base metal that is in a hot-worked and heat-treated condition, the creep strength at elevated temperatures of the weld metal tends to be low. Therefore, for obtaining materials useful in practical use, it is important, in designing the composition of the base metal and of the weld metal, to take into consideration measures for reducing the susceptibility to cracking in the step of welding and also for preventing the creep strength of welded joints from decreasing.
DISCLOSURE OF INVENTION
It is an objective of the present invention to provide a Ni-base heat resistant alloy and a welded joint made thereof, which are excellent in resistance to carburization and to coking in the environment, in which tubes of cracking furnace and reformer furnace in an ethylene plant are placed, namely in the environment in which carburization, oxidation and repeated temperature changes occur, and which have good weldability and strength at elevated temperatures.
The gist of the present invention consists in (1) a Ni-base heat resistant alloy as mentioned below, and (2) a welded joint made thereof as mentioned below. In the following, “%” for each constituent content is “% by mass”.
(1) A Ni-base heat resistant alloy consisting of C: not more than 0.1%, Si: not more than 2%, Mn: not more than 2%, P: not more than 0.025%, S: not more than 0.005%, N: not more than 0.04%, Cr: 10 to 30%, Al: 2.1 to less than 4.5%, and Mo: 2.5 to 15% or W: 2.5 to 9% or Mo and W: 2.5 to 15% in total, Ti: 0 to 3%, Nb: 0 to 1%, V: 0 to 1%, Ta: 0 to 2%, Zr: 0 to 0.2%, Hf: 0 to 0.8%, B: 0 to 0.03, Mg: 0 to 0.01%, Ca: 0 to 0.01%, Fe: 0 to 10%, La: 0 to 0.1%, Ce: 0 to 0.1%, Nd: 0 to 0.1%, Y: 0 to 0.1%, Cu: 0 to 5%, and Co: 0 to 10%, and the balance being substantially Ni, and satisfying the relation (1) given below.
(104Si+1980P+1980S+9Al+15Ti+11Nb+1.8W+11600B)≦{1.1(240−20000S−1900P−30Al−10Ti−9W+17000B) (1)
wherein the symbols for elements in the above relation (1) are the contents (% by mass) of the respective elements contained in the alloy.
Among the above constituents, the content of Ti is desirably not more than 1.5%.
(2) A welded joint in which each of the base metal and weld metal is made of a Ni-base heat resistant alloy consisting of C: not more than 0.1%, Si: not more than 2%, Mn: not more than 2%, P: not more than 0.025%, S: not more than 0.005%, N: not more than 0.04%, Cr: 10 to 30%, Al: 2.1 to less than 4.5%, and Mo: 2.5 to 15% or W: 2.5 to 9% or Mo and W: 2.5 to 15% in total, Ti: 0 to 3%, Nb: 0 to 1%, V: 0 to 1%, Ta: 0 to 2%, Zr: 0 to 0.2%, Hf: 0 to 0.8%, B: 0 to 0.03%, Mg: 0 to 0.01%, Ca: 0 to 0.01%, Fe: 0 to 10%, La: 0 to 0.1%, Ce: 0 to 0.1%, Nd: 0 to 0.1%, Y: 0 to 0.1%, Cu: 0 to 5% and Co: 0 to 10%, and the balance substantially being Ni, and satisfying the relation (1) given below, and further satisfying the condition that the ST value of the weld metal is larger than the ST value of the base metal,
Kitamura Kazuyuki
Nishiyama Yoshitaka
Ogawa Kazuhiro
Okada Hirokazu
Sheehan John
Sumitomo Metal Industries Ltd.
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