Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1991-12-19
1993-05-25
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148599, C21D 806
Patent
active
052136370
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to steel wires for the manufacture of flexible conduits which are resistant to corrosion in the presence of hydrogen sulfide (H.sub.2 S), and it also relates to the flexible conduits produced by means of these wires. Another object of the invention is the method of manufacturing these steel wires in order to make them resistant in an H.sub.2 S atmosphere.
It is known that in numerous applications, flexible conduits with metal reinforcements are used for the transporting of fluids and that in certain cases, particularly in the field of petroleum, these flexible conduits are subject to attack by sulfur-containing products.
In flexible conduits in which tightness is assured by one or more tubes or sheathings of polymeric material, such as thermoplastics or elastomers, the mechanical resistance to internal pressure and to the handling and operating stresses is assured by metal reinforcements made of helically wound steel wires.
These steel wires, which are generally shaped by hot or cold drawing, may have different profiles in the different layers of the metal reinforcements.
These wires may either be substantially flat wires having dimensions of about 2.times.5 mm to 4.times.10 mm, or wires of a hookable profile, for instance of Z, T or U-shape, permitting the hooking of a wire to the wire of the adjacent turn upon the winding, or else wires of circular cross section possibly assembled in the form of strands (cabled).
In the case of the production of flexible conduits intended to operate in the presence of H.sub.2 S, the grade of the steels serving for the manufacture of the reinforcement wires, as well as the mechanical and heat treatments carried out on these wires (in particular, strain-hardening upon shaping and then possibly annealing) must be selected in such a manner that these wires provide the necessary mechanical strength in operation, while at the same time they withstand corrosion in the presence of H.sub.2 S.
For several years, during which problems of the resistance of steel structures in an H.sub.2 S atmosphere have arisen, a large amount of research and experimentation has been carried out, both in the laboratory and in practical tests, to determine what characteristics the steels had to have in order to be suitable for use in an H.sub.2 S atmosphere.
This research has made it possible to determine that there was a correlation between the resistance to corrosion in the presence of H.sub.2 S and the hardness of the metal. More precisely, it was found that carbon steels and low-alloy steels having a hardness less than or equal to 22 HRc had, under stress, satisfactory resistance to H.sub.2 S corrosion and were therefore accepted as compatible with H.sub.2 S.
In the conclusions of this research, it was decided to characterize the metal by its HRc hardness, which permits simple non-destructive measurements. However, as is known, there is an equivalence, given in tables, between the hardness (HRc) and the rupture strength (Rm).
Thus, a hardness of 22 HRc corresponds to a rupture strength Rm of about 775-800 MPa.
In order to take this "H.sub.2 S compatibility/hardness" correlation into account, the manufacturers therefore have generally selected soft or semi-hard carbon steels (0.15% to 0.30% C) or low-alloy steels, which they subjected, after the strain-hardening resulting from the shaping (by drawing or rolling) to annealing suitable to bring the hardness to the accepted value, if necessary.
An accepted rule (NACE Standard 0175 National Association of Corrosion Engineers) has furthermore adopted the results of the research referred to above by stipulating that carbon steels, used in the field of petroleum, would be considered compatible with H.sub.2 S, without further tests, if they had a hardness of less than 22 HRc, the carbon content being stipulated as less than or equal to 0.38%.
This rule therefore leads, in the case of steel wires forming the reinforcements of flexible conduits, to relatively low mechanical properties, corresponding only to at most Rm=775 to 80
REFERENCES:
patent: 2527731 (1949-03-01), Ilacqua et al.
patent: 3264144 (1966-08-01), Frazier et al.
patent: 3591427 (1971-07-01), Hansen, Jr.
patent: 3668020 (1972-06-01), Lucht
patent: 3950190 (1976-04-01), Lake
patent: 4067754 (1978-01-01), Elias
patent: 4585062 (1986-04-01), Tiberg
Patent Abstracts of Japan, vol. 14, No. 47(C-682), Dec. 9, 1990 & JP-A-1 279 710 (Nippon Steel) Nov. 10, 1989.
Mallen Herrero Jose M.
Sugier Andre
Coflexip
Institut Francais du Pe'trole
Yee Deborah
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