Steel and steel tube for high- temperature use

Abstract

The invention concerns steel for high temperature use containing by weight: 0.06 to 0.20% of C, 0.10 to 1.00% of Si, 0.10 to 1.00% of Mn, not more than 0.010% of S, 10.00 to 13.00% of Cr, not more than 1.00% of Ni, 1.00 to 1.80% of W, Mo such that (W/2+Mo) is not more than 1.50%, 0.50 to 2.00% of Co, 0.15 to 0.35% of V, 0.040 to 0.150% of Nb, 0.030 to 0.12% of N, 0.0010 to 0.0100% of B and optionally up to 0.0100% of Ca, the rest of the chemical composition consisting of iron and impurities or residues resulting from or required for preparation processes or steel casting. The chemical constituent contents preferably verify a relationship such that the steel after normalizing heat treatment between 1050 and 1080° C. and tempering has a tempered martensite structure free or practically free of delta ferrite.

Description

[0001] The invention relates to steels for use under stress at high temperatures of about 600.degree. C. to 650.degree. C., more particularly steels known as ferritic steels with a high chromium content with a tempered martensitic structure both at ambient temperature and at service temperatures.[0002] The invention is applicable to tubular metal products such as superheater tubes, reheater tubes, headers or pipings for superheated or reheated steam for boilers, or tubes for furnaces for chemistry or petrochemistry.BACKGROUND TECHNIQUE[0003] Such products are usually seamless tubes obtained after a severe hot plastic deformation operation carried out on solid bars of highly specialized steel.[0004] Apart from ferritic steels with 2.25% Cr-1% Mo according to ASTM A213 type T22, austenitic stainless steel tubes according to ASTM A213 (ASTM=American Society for Testing and Materials) type TP321H, TP347H have long been known, containing about 0.05% C, 18% Cr, 11% Ni and stabilized with ...

AI Technical Summary

Problems solved by technology

In contrast, they suffer from major drawbacks due to their austenitic structure, which renders them incompatible with steels with a ferritic or martensitic structure, which are of necessity used in other parts of the boiler that are less exposed to high temperatures; hence, the search for materials with a ferritic or martensitic structure is of great importance.

Said steels T91, P91, T92, P92 contain 9% Cr and some of their users believe that such a Cr content is insufficient to resist hot oxidation and/or corrosion by steam beyond 600.degree. C., in particular at 650.degree. C. because of the metal temperature envisaged for the tubes of the superheaters in future power stations.

Certainly, the presence of an oxide layer on the inner surface of the tubes of superheaters, which layer derives from corrosion of the steel by the steam moving in the tubes, creates a thermal resistance which increases with the thickness of said layer and, at constant thermal flux, entrains an increase in the mean temperature of the tubes and thus a large reduction in their service life.

Further, flaking of said layer when it is too large may lead to accumulation of debris in the bends in the superheaters, impeding the movement of steam with a supplemental risk of overheating the tubes.

Flaking can also result in debris being entrained into the turbine and can thus damage its blades.

That steel is claimed to be more resistant to hot oxidation than T91 or T92 because of its Cr content, but it is far less resistant to creep rupture than T91/P91 and it is difficult to weld, in particular when very thick.

Adding a large amount of Ni, though, has the disadvantage of greatly reducing the Ac1 point and thus limiting the maximum tempering temperature of the tubes; it also appears to be deleterious to the creep rupture strength.

However, adding Cu causes problems as regards forgeability when fabricating tubes for superheaters by hot rolling.

Such a steel is expensive to produce, however.

European patent application EP-A-0 892 079 also proposes adding Co in amounts of 0.2% to 5% but in a steel containing less than 10% Cr, which does not solve the problem described above.

However, we described the unacceptable disadvantages of such additions above.

Adding Ti or Zr, however, suffers from the majo

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