Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-03-16
2001-07-03
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S663000, C420S106000, C420S109000
Reexamination Certificate
active
06254697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cast steel materials for the manufacture of casings and pressure vessels for use in steam turbine plants for thermal electric power generation, and a method of making a pressure vessel (or cast steel article) by using such a cast steel material.
2. Description of the Related Art
As casings and pressure vessels for use in steam turbine plants for thermal electric power generation, cast steel articles are frequently used in order to accommodate their complicated shapes. The properties required for such cast steel articles are such that they have excellent high-temperature strength and high creep rupture strength because they are used at high temperatures, and that they have excellent weldability because any defects in such cast steel materials need to be repaired by welding.
Generally known materials useful for this purpose include CrMoV cast steel, 2.25%CrMo cast steel and CrMo cast steel. Among them, 2.25%CrMo cast steel and CrMo cast steel have excellent impact resistance at ordinary temperatures and, as a result, good weldability. However, since they contain no strengthening element such as V, they have insufficient creep rupture strength and hence fail to meet the requirements of a material for the manufacture of casings of steam turbines having yearly rising operating temperatures.
On the other hand, the aforesaid CrMoV cast steel has high creep rupture strength and high mechanical strength owing to its high carbon content. However, it has poor impact resistance and, as a result., poor weldability. Accordingly, this material has the disadvantage that it cannot be easily repaired by welding in the process for the manufacture of casings and pressure vessels.
An object of the present invention is to provide a cast steel material for pressure vessels which has improved impact resistance (weldability) and toughness while maintaining its creep rupture strength at a level equal to or higher than the excellent creep rupture strength currently possessed by CrMoV cast steel, as well as a method of making a pressure vessel (or cast steel article) by using this cast steel material for pressure vessels which permits a pressure vessel to be made without requiring a material working step such as forging.
SUMMARY OF THE INVENTION
The present invention comprises the following embodiments.
[Embodiment 1]
A cast steel material for pressure vessels which contains, on a weight percentage basis, 0.04 to 0.1% C, 0.1 to 0.4% Si, greater than 0% and up to 0.2% Mn, 0.1 to 0.8% Ni, 3 to 4.5% Cr, 0.2 to less than 0.5% Mo, 0.2 to 0.4% V, 0.5 to 2% W, 0.01 to 0.06% Nb and/or Ta, 0.001 to 0.01% B, 0.005 to 0.045% Ti, 0.006 to 0.015% Al, greater than 0.005% and less than 0.01% N, 0 to 0.008% 0, 0 to 0.015% P as an impurity, and 0 to 0.007% S as an impurity, the balance being Fe and incidental impurities, provided that the aforesaid contents of Ti, Al, O and N satisfies the following relationship:
N−0.29{Ti−1.5(O−0.89Al)}≦0.0060%
[Embodiment 2]
A method of making a pressure vessel which comprises the steps of casting a cast steel material for pressure vessels in accordance with the above-described embodiment 1 to form a cast steel article in the form of a pressure vessel; normalizing the cast steel article by holding it at a temperature of 1,000 to 1,150° C. for 10 to 30 hours and cooling it to 200° C. or below; quenching the cast steel article by holding it at a temperature of 970 to 1,070° C. for 5 to 30 hours, cooling it at a cooling rate of 1 to 50° C. per minute until the temperature of various parts of the material reaches 600° C., and further cooling it to 200° C. or below; and tempering the cast steel article by holding it at a temperature of 680 to 740° C. for 5 to 20 hours.
The cast steel material for pressure vessels in accordance with the present invention is characterized in that the excellent high-temperature strength (in particular, creep rupture strength) possessed by a conventional cast steel material is further enhanced and, moreover, good ductility and toughness are exhibited. The outstanding feature thereof is that it has markedly improved weldability and can hence be more easily formed into pressure vessels than conventional cast steel materials.
Consequently, by using the cast steel material for pressure vessels in accordance with the present invention, it becomes possible to reduce the wall thickness of the product and decrease the number of welding steps, and thereby manufacture pressure vessels at a lower cost than in the case of conventional materials. Moreover, this cast steel material not only has excellent properties, but also can reduce the material cost by minimizing the addition of expensive alloying elements, thus producing remarkable effects from an industrial point of view.
Furthermore, the method of making a pressure vessel by using the cast steel material of the present invention can provide pressure vessels having a well-balanced combination of ductility, toughness and creep rupture strength.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The cast steel material for pressure vessels in accordance with the present invention and the method of making a pressure vessel by using the same will be more specifically described hereinbelow.
The reasons for content restrictions on various components contained in the cast steel material for pressure vessels (hereinafter referred to briefly as “cast steel materials) in accordance with the present invention are described below. In the following description, all percentages used to express contents are by weight unless otherwise stated.
C (carbon): C not only enhances the hardenability of the cast steel material, but also forms the carbides of Cr, Mo, Nb and V and thereby contributes to an improvement in creep rupture strength. If the content of C is less than 0.04%, no sufficient yield strength or creep rupture strength will be obtained. On the other hand, it is desirable that the content of C be as low as possible in order to secure weldability. That is, the content of C must be not greater than 0.1%. If the content of C is unduly high, it will be difficult to secure toughness. Moreover, carbonitrides will aggregate and coarsen during use to cause a reduction in strength upon long-term exposure to high temperatures. Accordingly, the content of C should be in the range of 0.04 to 0.1%. The preferred range is from 0.06 to 0.09%.
Si (silicon): Si is an element which is effective as a deoxidizer. Since castings are complicated in shape, the melt must be smoothly filled to all the corners of the mold. If not so, casting defects such as misrun and cold shut will occur and hence bring about a need for repair. Consequently, it is important to secure melt flowability, and Si is an element which is necessary for the securement of melt flowability. However, Si promotes segregation and thereby causes a reduction in the toughness of cast steel articles and also in the high-temperature strength thereof. If the content of Si is less than 0.1%, Si will not perform a proper function in acting as a deoxidizer and securing melt flowability. On the other hand, if Si is added in an amount of greater than 0.4%, the toughness and high-temperature strength of cast steel articles will be reduced. Accordingly, the content of Si should be in the range of 0.1 to 0.4%. The preferred range is from 0.2 to 0.35%.
Mn (manganese): Mn is an element which is useful in enhancing the hardenability of cast steel articles and is also effective in improving strength and toughness. However, the addition of an increased amount of Mn will tend to cause a reduction in the creep rupture strength of cast steel articles. Accordingly, the content of Mn should be greater than 0% (i.e., exclusive of 0%) and up to 0.2%. The preferred range is from 0.05 to 0.15%.
Ni (nickel): Ni enhances the hardenability of cast steel articles and is also effective in improving toughness. However, the addition of an unduly large amount of Ni will cause a reduction in th
Fujita Akitsugu
Kamada Masatomo
Tashiro Yasunori
Ueno Masakatsu
Mitsubishi Heavy Industries Ltd.
Myers Bigel Sibley & Sajovec P.A.
Yee Deborah
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