Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-03-28
2003-02-04
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000
Reexamination Certificate
active
06514359
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a heat resistant steel having a Cr content of not more than 8% by mass and suited for such uses as heat exchangers, steel pipes for piping, heat resistant valves and members or parts required to be welded in the fields of boilers, chemical industries and nuclear energy utilization, among others, in particular to a heat resistant steel having a Cr content of not more than 8% by mass and excellent in creep strength at elevated temperatures not lower than 400° C. and in toughness. In the description which follows, a Cr steel having a Cr content of not more than 8% by mass is referred to as “low/medium Cr steel”.
BACKGROUND OF THE INVENTION
So far, in high temperature environments not lower than 400° C., austenitic stainless steels, Cr steels with a Cr content of 9 to 12% by mass (hereinafter referred to as “high Cr steels”), low/medium Cr steels and carbon steels have been used selectively in respective matched fields taking into consideration both the environment (e.g. temperature, pressure) and the economical feature.
Among the various heat resistant steels mentioned above, low/medium Cr steels contain Cr and therefore are superior to carbon steels in oxidation resistance, high temperature corrosion resistance, strength at elevated temperatures and creep strength. Furthermore, although low/medium Cr steels are inferior to austenitic stainless steels in strength at elevated temperatures or creep strength, they have smaller thermal expansion coefficient and, in addition, are much more inexpensive. Comparing with the high Cr steels as well, low/medium Cr steels are more inexpensive and are characterized in that they have superior in toughness, weldability and heat conductivity.
Therefore, the so-called “Cr—Mo steels”, namely the low/medium Cr heat resistant steels have been used in many instances, for example the steels STBA 20, STBA 22, STBA 23, STBA 24 and STBA 25 as defined in JIS G 3462, also known as 0.5 Cr-0.5 Mo steel, 1.0 Cr-0.5 Mo steel, 1.25 Cr-0.5 Mo steel, 2.25 Cr-1.0 Mo steel and 5.0 Cr-0.5 Mo steel, respectively, based on the Cr and Mo contents on the % by mass basis.
Meanwhile, improvements in strength at elevated temperatures and creep strength of low/medium Cr heat resistant steels have so far been achieved by addition of V, Nb, Ti, Ta and the like, which are precipitation strengthening elements. Well known as such precipitation-strengthened low/medium Cr heat resistant steels are, for instance, 1% Cr-1% Mo-0.25% V steel, which is a material for turbines, and 2.25% Cr-1% Mo—Nb steel, which is a material of construction of fast breeder reactors, so called based on the contents on the % by mass basis.
Furthermore, low/medium Cr ferritic steels of the precipitation strengthening type have been disclosed in patent specifications, for example in J P Kokai S63-18038, J P Kokai H01-316441, J P Kokai H02-217439, J P Kokai H06-220532, J P Kokai H08-134585 and WO 96/14445.
SUMMARY OF THE INVENTION
Generally, the strength at elevated temperatures and creep strength of heat resistant steels are very important in designing pressure members or parts, and are desired to have high strength regardless of the temperature the steel is to be used. In particular, in the case of heat-resistant pressure steel pipes used in boilers, chemical industries, nuclear energy utilization and like fields, steels having high strength at elevated temperatures and creep strength are required, and the wall thicknesses of the steel pipes are determined based on the strength at elevated temperatures and creep strength of the materials. Therefore, improvements in strength at elevated temperatures and creep strength of low/medium Cr steels have so far been achieved by solid-solution strengthening and precipitation strengthening. However, the strength at elevated temperatures and the creep strength after a long period of use are not always compatible with each other.
The improvements in strength at elevated temperatures of low/medium Cr heat resistant steels have been generally achieved by increasing the contents of C, Cr, Mo and W. However, in the case of steels having increased strength at elevated temperatures as a result of containing these alloying elements beyond their solubility limit, carbides and/or intermetallic compounds, which comprise C, Cr, Mo and W as main components, may precipitate after a long period of use at elevated temperatures, leading to decreases in creep strength on the higher temperature after a long period of use. Thus, even the conventional “Cr—Mo steels” cannot avoid this problem.
On the other hand, when the strength, in particular strength at elevated temperatures, of low/medium Cr steels is increased by precipitation strengthening, no adequate metallographic control leads to the following problems.
(a) Although unused materials or materials used for only short period of time exhibit high strength at elevated temperatures and high creep strength, materials used at elevated temperatures for 10,000 hours or longer reduce effects of precipitation, so that they may not have stable strength at elevated temperatures and creep strength any longer. This is because while carbides, nitrides and intermetallic compounds contribute to precipitation strengthening in unused materials or materials used for only short period of time, the aging occurring during a long period of time at elevated temperatures results in coarsening of these precipitates, whereby the precipitation strengthening effect may be lost.
(b) In precipitation hardening steels, the grain inside has been strengthened, so that the strength of grain boundaries is relatively weak, and this may lead to deteriorations in toughness and corrosion resistance.
(c) When the microstructure of a steel is a dual-phase consisted of bainite and ferrite or martensite and ferrite, fine precipitates are precipitated inside bainite or martensite, whereby the strength at elevated temperatures and creep strength increase whereas, in ferrite, the precipitates easily become coarsened and the precipitation strengthening effect reduces. Thus, the each phase forming the above dual phase exhibits different deformabilities (e.g. strength at elevated temperatures and ductility) and the toughness and/or creep strength may deteriorate. Further, during use at elevated temperatures, the precipitates may become coarsened at the boundary between bainite and ferrite or at the boundary between martensite and ferrite, leading to deterioration in toughness and/or fatigue property.
Therefore, 1% Cr-1% Mo-0.25% V steel, 2.25% Cr-1% Mo—Nb steel and the precipitation strengthening type low/medium Cr steels proposed in the above-cited patent specifications have the following problems, respectively.
In the case of 1% Cr-1% Mo-0.25% V steel, the amount of V carbonitride precipitates becomes excessive and, in addition, the precipitates readily become coarsened and, therefore, the toughness and/or creep strength may deteriorate.
In the case of 2.25% Cr-1% Mo—Nb steel, grain boundary precipitates such as M
6
C carbides readily become coarsened and the amount of Mo in solid solution in the matrix rather decreases, so that the toughness and creep strength may deteriorate.
In the case of the 3% Cr-1% Mo—W—V steel proposed in J P Kokai S63-18038, M
6
C carbides are easy to precipitate and the amounts of Mo and W in solid solution in the matrix rather decrease, leading to deterioration in creep strength, in particular creep strength after a long period of use where the time to rupture exceeds 6,000 hours, as the case may be.
The “heat resistant steel excellent in toughness” proposed in J P Kokai H01-316441 is a heat resistant steel based on Cr—Mo steel and containing V. However, it is necessary that the metallography should be of the dual phase comprising ferrite and bainite or ferrite and pearlite. Furthermore, as described in the example section, the ferrite phase content is not less than 70%. Therefore, it is poor in strength at elevated temperatures in some instances.
The “high strength low alloy steel excellent in corrosi
Armstrong Westerman & Hattori, LLP
Sumitomo Metal Industries Ltd.
Yee Deborah
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