Metal treatment – Stock – Ferrous
Patent
1991-12-16
1993-06-22
Yee, Deborah
Metal treatment
Stock
Ferrous
148662, 148663, 148538, C22C 3810, C21D 128
Patent
active
052213749
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an agent for improving the hydrogen cracking resistance of low-alloy boiler forging and/or structural steels which can be employed especially in hydrogen-containing media such as H.sub.2 S-enriched gases and also for improving the weldability of these steels and their suitability for gas cutting or for cutting with a plasma torch.
It is well known that steel is sensitive to the presence of hydrogen, which can give rise to unacceptable cracking. This hydrogen can result from the manufacture of steel or can be introduced by a mechanism of the corrosion type, in particular when the steel is employed in a medium containing H.sub.2 S, or during welding or cutting operations using conventional gas or plasma torch cutting.
This sensitivity of the steel is accentuated by the presence of segregated regions containing more carbon and alloying elements than the average composition.
These segregated regions are all the more important when the steel is employed in the form of bulky products, for example sheets whose thickness ranges from 5 mm to several hundred millimetres.
To reduce this sensitivity of the steel to hydrogen cracking, a person skilled in the art knows that it is necessary, in combination or separately, to reduce nonmetallic inclusions as much as possible, to reduce the content of alloying elements to the minimum contents needed to obtain the desired mechanical characteristics and, preferably, to perform a heat treatment of quenching and tempering to obtain a tempered martensitic structure.
However, the pressure vessel construction codes, for example the ASME code, do not always permit the use of tempered quenched steels.
The fact of reducing the contents of alloying elements to a minimum often makes it difficult to obtain the mechanical characteristics imposed by the same codes.
European Patent EP-0,021,349 proposes a steel with a high elastic limit, containing cobalt to improve the resistance to hydrogen cracking induced by the presence of H.sub.2 S.
However, this patent requires that the steel should be quenched and tempered.
Moreover, it clearly indicates that the quenching operation must be performed after a very fast austenitisation, at a rate of the order of 2.degree. C./s, and this limits this steel to applications employing thin products which are therefore free from major segregated regions. In fact, this patent relates to steels for thin-walled tubes (not exceeding a few millimetres).
This patent indicates that cobalt would appear to act by forming a cobalt-enriched layer at the surface of the steel, slowing down the entry of hydrogen due to corrosion by H.sub.2 S.
This barrier is formed all the better the faster the austenitisation before quenching.
This patent also explains that the effect of cobalt is very weak when the structure is ferrite pearlite obtained either by normalisation or by controlled rolling.
Finally, it says nothing about the influence of cobalt on welding and on the suitability for cutting using gas cutting or plasma.
The problem raised above thus remains in its entirety.
The objective of the invention is therefore to find a means making it possible to improve the hydrogen cracking resistance of steels employed especially at great thickness and consequently containing segregated regions, while making it possible in particular to satisfy the codes of construction of pressure boiler forged equipment, whether the hydrogen enters the metal by corrosion in an acidic medium, such as wet H.sub.2 S, or during thermal soldering or cutting operations; consequently, the objective of the invention is also to improve the weldability and the suitability for cutting.
The subject of the invention is an agent intended to improve the hydrogen cracking resistance of steels employed especially in hydrogen-containing media and intended especially to improve the weldability and the suitability of these steels for cutting by thermal means, when they are intended for the production of thick pieces.
This agent is cobalt added in weight contents of between 0.05% and
REFERENCES:
patent: 3155500 (1964-11-01), Anthony, III et al.
patent: 3615370 (1971-10-01), Ridal
patent: 4407681 (1983-10-01), Ina et al.
Beguinot Jean
Blondeau Regis
Bourges Philippe
Charles Jacques
Coudreuse Lionel
Creusot Loire Industrie
Yee Deborah
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