Metal treatment – Stock – Ferrous
Patent
1985-12-20
1987-04-14
Rutledge, L. Dewayne
Metal treatment
Stock
Ferrous
148337, C22C 3842, C22C 3858
Patent
active
046576061
DESCRIPTION:
BRIEF SUMMARY
This invention relates broadly to high chromium corrosion resistant duplex steels of the type containing in excess of 23%, and typically 25% by weight of chromium and in excess of 4%, and typically 5% by weight of nickel together with additions of molybdenum and silicon; with less than 0.1% carbon and importantly, with controlled additions of copper, nitrogen and manganese, and with the balance of the composition consisting of iron.
Hitherto, such high chromium duplex steels are always produced such that after melting, casting and in some cases, hot working, and in other cases, welding, the alloy is subjected to solution heat treatment in the temperature range 1050.degree. C. to 1150.degree. C. This, in the case of steels as described in British Patent specification No. 1,158,614, is often carried out for at least one hour per inch of cross-section, and sufficient to produce a product which is substantially free of precipitated particles and of which 40% to 60% (by weight) is a ferritic matrix, the matrix having austenite grains embedded therein.
Such heat treatment is followed by rapid quenching in oil or water. Resistance to corrosion, in combination with good strength and ductility, can only be maximized in such duplex steels, if, after casting, hot working or welding, they are subjected to this solution heat treatment followed by rapid cooling whereby deleterious precipitates are taken into solution, and retained in solution, during cooling to room temperature.
Typically, in the solution heat treated condition, an alloy as described for example, in British Patent specification No. 1, 158 614, will achieve mechanical properties as follows:
______________________________________ 0.2% Proof Stress - tons/sq. in
28
Ultimate tensile strength - tons/sq. in.
48
Elongation - % 25
Impact Value - Joules 50
______________________________________
These mechanical properties are achieved, together with good resistance to corrosion, in both acidic and salt water conditions.
A well known measure of resistance to corrosion is the standard test for resistance to pitting corrosion in the 3% sodium chloride solution at 30.degree. C., using a saturated calomel electrode (S C E), where the voltage is measured, at which breakdown occurs of the passive oxide film protecting such steels; and where higher breakdown potentials indicate better pitting corrosion resistance.
In a typical duplex steel according to G.B. Patent specification No. 1,158 614, in the solution heat treated condition, such breakdown does not occur until a potential in excess of 800 millivolts is reached. However in the case of an identical alloy, which has not been solution heat treated, a breakdown in the passive film may occur at potentials as low as 100 millivolts.
Such a non solution heat treated duplex steel will have mechanical properties, typically as follows:
______________________________________ 0.2% Proof Stress - tons/sq. in.
30
Ultimate Tensile Strength - tons/sq. in.
50
Elongation - % 17
Impact values - Joules 20
______________________________________
It can therefore be seen that if the known high chromium duplex steels are not so solution heat treated and then rapidly cooled, they fail to achieve the required degree of ductility, and in important combination therewith, resistance to corrosion.
However, the need for such solution heat treatment followed by rapid cooling incurs a number of disadvantages, for example as follows:
(1) Such heat treatment often has to be carried out after welding and this causes difficulty in the case where the steel is located at the site of final use; for example after welding the alloy at an offshore installation it is impractical to carry out solution heat treatment.
(2) Moreover, when defects, discovered during machining, are rectified by a weld, it would be clearly disadvantageous if a further heat treatment had to be carried out on a component in the finished machined condition.
(3) The solution heat treatment followed by rapid cooling involves the risk of exce
REFERENCES:
patent: 3082082 (1963-03-01), Bidwell et al.
patent: 3567434 (1968-03-01), Richardson et al.
patent: 4032367 (1977-06-01), Richardson et al.
patent: 4591644 (1986-05-01), Leerschool et al.
Bonar Langley Alloys Limited
Rutledge L. Dewayne
Yee Deborah
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