Metal treatment – Stock – Ferrous
Patent
1997-11-11
1999-08-31
Yee, Deborah
Metal treatment
Stock
Ferrous
148608, C22C 3844, C21D 800
Patent
active
059449217
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention refers to a supermartensitic stainless steel having high mechanical strength and corrosion resistance deriving from its composition and from the thermal treatment to which its manufactured articles are submitted during their working phases.
The invention also concerns the production process of the manufactured articles, in particular pipes for drilling, production and pipage in the hydrocarbons field.
PRIOR ART
A particular application field of the stainless steels is the pipes for drilling, production and pipage in the hydrocarbons field one, where it is well known to resort to compositions of steels guaranteeing the necessary mechanical strength and corrosion resistance. However, the more and more frequent exploitation of deposits of hydrocarbons of the so called acid, or sour kind, owing to a high content in sulphidric acids and/or of carbon dioxide, often even in presence of high contents in chlorides and at high temperatures, makes more and more difficult the choice of suitable materials.
In this connection, the various oil companies and the planning societies in the attempt to prevent to the outmost the accidents deriving from possible breakings in the working cycle of the pipes, which would cause great production losses and damages to the environment, fixed very rigorous limits of mechanical strength and corrosion resistance of the materials to employ.
At present stainless steels, either of biphasic or austenoferritic kind, where are simultaneously present the two ferritic and austenitic phases in such ratios among them to give interesting properties to the steel from the point of view of the mechanical or corrosion resistance either austenitic kind homogeneous steels, which however are even more expensive owing to the high amount of components which must be added to the basic alloy, are on the market and used.
The Applicant too, operating since many years in the production of pipes for drilling and for the production and pipage of hydrocarbons, has described and claimed in the Patent Application EP 93106675.7. a super-duplex kind steel to be used for manufactures employable in acid environments. An important market section in the drilling pipes and in the production and pipage of hydrocarbons, is constituted by that one using pipes in martensitic steels. Said pipes are substantially employed in "sweet" environments, that is characterized by high contents in carbon dioxide and chlorides with absence or presence in traces only of hydrogen sulfide.
The martensitic steels have a much lower cost than other stainless steels and their use finds an increasing favour; unfortunately their use is limited from the cracking sensitivity in presence of hydrogen sulfide.
In order to overcome this limitation of the martensitic steels, either the alligation with chromium amounts higher than 13%, or combinations of chromium and molybdenum have been experimented obtaining encouraging results. Sometimes nickel in such amounts to guarantee the total austenitization, necessary for the complete transformation in martensite has been also added.
In the present text, steels having a chromium content higher than 14% by weight, or a combination Cr/Mo in which Cr is >12% and Mo is >1%, if necessary with the addition of nickel in amounts higher than 0.5% are pointed out with the adjective "supermartensitic", the presence of said metallic elements however being such as to guarantee a complete transformation of the steel in martensitic one, after the cooling subsequent to the austenization treatment.
In the technical-patent literature informations are also found relative to the influence of other elements on the behavior of the martensitic stainless steels.
By way of example, we remember the Patent Application JP-A-3120337/91 of May 22, 1991 where the influence of the amounts of Mo, Mn and S in a Ni/Cr austenitic steel in order to increase its corrosion resistance capacity is discussed. In particular a maximum limit equal to 0.5% of Mn has been pointed out, as the presence of
Barteri Massimo
Cumino Giuseppe
Dalmine S.p.A.
Yee Deborah
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