Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-07-21
2001-11-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000, C148S529000, C148S534000
Reexamination Certificate
active
06322642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a special steel and to the manufacture of pressure vessels. Preferably the pressure vessels are adapted to work under pressure under conditions in which there is a risk of H
2
S-induced stress cracking.
2. Discussion of the Related Art
In the petrochemical industry, pressure vessels are used for heating gases having high H
2
S contents. These vessels, which work under pressure and contain inflammable gases, pose significant safety problems which are solved by applying construction rules codified by various standards or construction codes, particularly the NACE MR 0175-97 standard and the codes of the ASME-code type. H
2
S, particularly in the presence of moisture, induces a risk of failure by stress corrosion, and the NACE standard defines H
2
S partial-pressure conditions for which particular construction rules have to be observed in order to guarantee safety of the plants. These construction rules are also defined by the standard and imposed on manufacturers.
In general, the NACE MR 0175-97 standard stipulates that the materials must give satisfactory results when they are subjected to cracking tests in the presence of hydrogen, defined by the NACE TM 0 177-90 standard, and indicates in a very general manner the materials and the operating conditions likely to give satisfaction. In the case of pressure vessels, it is theoretically possible to use carbon or low-alloy steels, both in the normalized state and in the quench-tempered state, as long as they contain less than 1% nickel and have a hardness of less than or equal to 22 HRC. If the vessels and their components were stress-relieved, the stress-relieving operation must have been carried out above 595° C. In addition, after the components have been joined together by welding, the vessels must be subjected to a postweld heat treatment at a temperature of greater than 620° C. so as to obtain a hardness of less than or equal to 22 HRC at any point.
In general, pressure vessels working under conditions in which there is a risk of H
2
S-induced stress cracking are manufactured using carbon and manganese steels in the normalized state, the guaranteed tensile strength R
m
of which does not exceed 485 MPa. As a result, the plants thus constructed have a large wall thickness and are therefore very heavy. The heavy weight is a problem, especially in the case of plants installed on offshore platforms.
In order to increase the guaranteed mechanical properties, it has been proposed to use carbon and manganese steels in the quench-tempered state. However, these steels do not allow a tensile strength of greater than 500 MPa to be guaranteed, nor a yield stress of greater than 400 MPa. Likewise, these properties can be guaranteed only in the case of thicknesses not exceeding approximately 80 mm.
It is also possible to use low-carbon steels microalloyed with vanadium or niobium and obtained by controlled rolling. These steels allow a guaranteed tensile strength level of approximately 550 MPa and a guaranteed yield stress level of approximately 450 MPa to be achieved. However, on the one hand these steels cannot be used to manufacture hot-formed components, and on the other hand they can only be used with thicknesses less than 40 mm.
Certainly, there are many low-alloy steels used in boilermaking in the quench-tempered state which allow higher design mechanical properties to be obtained, but these steels do not allow the conditions stipulated by the NACE standard to be met. In addition, they require welding precautions that it is not always easy to reliably comply with on work sites, especially when repair operations are being carried out. The use of these steels for the type of application envisaged here would run the risk of creating defects in the welds, and consequently the risk of serious incidents.
More specifically, in order to manufacture safe pressure vessels, suitable welding conditions must be chosen, these being characterized especially by a minimum preheat temperature and a minimum welding energy per unit length. These welding conditions may be combined in the form of a cooling time between 800° C. and 500° C. of the weld bead or of the zone affected by the welding heat (as defined in the NF: A 36-000 standard). To meet the maximum hardness criterion of 22 HRC, the inventors have found that this cooling time must be greater than a critical value which they call “800/500 cct” (which will be defined more fully later) and which depends on the steel used and on the constraints imposed by the construction codes. The welding is more difficult to carry out reliably the higher this value is. The quench-tempered steels used in boilermaking have an 800/500 cct (critical cooling time between 800° C. and 500° C.) of greater than 10 s, which is too long to allow these steels to be used tinder satisfactory conditions for manufacturing H
2
S-resistant pressure vessels.
SUMMARY OF THE INVENTION
One object of the present invention is to remedy these drawbacks by providing a way to manufacture pressure vessels working in an H
2
S medium, which are lighter than the known vessels, while being just as safe.
For this purpose, one subject of the invention is a process for manufacturing a pressure vessel intended and adapted to work under pressure between −40° C. and 200° C. under conditions in which there is a risk of H
2
S-induced stress cracking as defined by the NACE MR 0175-97 standard, incorporated herein by reference, especially section 1.3 thereof, most especially sections 1.3.1.1 and 1.3.1.2 thereof, in which:
components of the pressure vessel are manufactured from a steel whose chemical composition comprises iron and, by weight and based on total weight:
0.03%≦
C
≦0.15%
0%≦
Si
≦0.5%
0.4%≦
Mn
≦2.5%
0.5%≦
Ni
≦3%
0%≦
Cr
≦1%
0%≦
Mo
≦0.5%
0%≦
Al
≦0.07%
0%≦
Ti
≦0.04%
with, preferably Al+Ti≧0.01%
0%≦
B
≦0.004%
0%≦
V
≦0.02%
0%≦
Nb
≦0.05%
Cu
≦1%
S
≦0.015%
P
≦0.03%
and impurities resulting from the smelting operation, the chemical composition preferably being such that CET=C+(Mn+Mo)/10+(Cr+Cu)/20+Ni/40<0.35 and such that the 800/500 cct is less than 10 s, the components being quenched and tempered, after or before forming, so as to obtain a tampered martensitic or martensitic-bainitic structure containing less than 10% ferrite and preferably not containing any ferrite, the temper being carried out at a temperature T
T
preferably of less than 680° C.;
after the components have been formed, optionally a stress-relieving operation is carried out a temperature of greater than or equal to 595° C.;
the components of the pressure vessel are welded with a welding energy and preheat conditions such that the 800/500 ct (cooling time between 800° C. and 500° C.) of the heat-affected zone of the welding is greater than or equal to five seconds, and
a postweld heat treatment is carried out at a temperature T
PW
of greater than 595° C. and less than 680° C., preferably of less than 650° C., the steel then having a tensile strength of greater than or equal to 550 MPa, a yield stress of greater than or equal to 450 MPa, an elongation A % of greater than 17% and an impact strength K
CV
at −40° C. of greater than 40 joules, the hardness at any point of the surface of the vessel being less than 248 HV.
Preferably, the chemical composition of the steel is such that Nb+V≦0.02%; preferably too, it is such that:
0.04%≦
C
≦0.09%
Cr
≦0.6%
0.2%≦
Mo
0.5%.
The invention also relates to a pressure vessel intended to work under pressure between −40° C. and 200° C. under conditions in which there is a risk of H
2
S-induced stress cracking, as defined by the NACE MR 0175-97 standard, especially sections 1.3.1.1 and 1.3.1.2 thereof. This pressure vessel is made of a steel whose chemical composition comprises iron and, by weight based on total weight:
0.03%≦
C
≦0.15%
0%≦
Si
≦0.5%
0.4%≦
Mn
≦2.5%
0.5%≦
Ni
≦3%
0%≦
Cr
Bocquet Pierre
Bourges Philippe
Mabelly Philippe
Creusot Loire Industrie
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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