EP0188995A1 - Hochchromhaltiger Gussstahl für ein Hochtemperaturdruckgefäss und Verfahren zu seiner thermischen Behandlung - Google Patents

Hochchromhaltiger Gussstahl für ein Hochtemperaturdruckgefäss und Verfahren zu seiner thermischen Behandlung Download PDF

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Publication number
EP0188995A1
EP0188995A1 EP85730139A EP85730139A EP0188995A1 EP 0188995 A1 EP0188995 A1 EP 0188995A1 EP 85730139 A EP85730139 A EP 85730139A EP 85730139 A EP85730139 A EP 85730139A EP 0188995 A1 EP0188995 A1 EP 0188995A1
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EP
European Patent Office
Prior art keywords
steel
temperature
steels
cast steel
chromium
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Application number
EP85730139A
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English (en)
French (fr)
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EP0188995B1 (de
Inventor
Makoto Nagasaki Technical Institute Nakamura
Yorimasa Nagasaki Technical Institute Takeda
Akitsugu Nagasaki Technical Institute Fujita
Yusaku Nagasaki Technical Institute Takano
Kazunari Japan Cast.& Forg.Corp. Takebayashi
Mitsuo Japan Cast.& Forg.Corp. Minami
Yasuhiro Japan Cast.& Forg.Corp. Tashiro
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Japan Casting and Forging Corp
Mitsubishi Heavy Industries Ltd
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Japan Casting and Forging Corp
Mitsubishi Heavy Industries Ltd
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Priority claimed from JP21632284A external-priority patent/JPS6196062A/ja
Priority claimed from JP59216323A external-priority patent/JPH0696741B2/ja
Application filed by Japan Casting and Forging Corp, Mitsubishi Heavy Industries Ltd filed Critical Japan Casting and Forging Corp
Publication of EP0188995A1 publication Critical patent/EP0188995A1/de
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium

Definitions

  • This invention relates to the art of steels and more particularly, to a high chromium cast steel which has good strength at high temperatures and high resistance to oxidation, good ductility and toughness at normal temperatures with good weldability, and high crack resistance, whereby the cast steel is particularly suitable for use as high-temperature, pressure containers such as, for example, a cylinder and a valve chest of steam turbines, which require high reliability.
  • the invention also relates to a method for the thermal treatment of the cast iron.
  • the cylinder or valve chest of steam turbines usually suffers high temperature and high pressure and should have high hot strengths such as, for example, creep rupture strength and load bearing capacity.
  • These parts have such a complicated, thick structure that when hot steam is flown therein at the time of starting, they are locally heated, where a compression strain takes place. When these portions are cooled, a great tensile stress may be left in the portions. In these portion, cracks are apt to initiate due to the fatigue by the heat. The crack initiation or the presence of the defects may lead to serious crackings by brittle failure.
  • the steel for these parts should have high ductility and toughness. If the parts are made by casting, the repair by welding is necessary. For the assembling and work, welding is necessary. In this sense, good weldability is essentially required, which, in turn, requires good ductility and toughness.
  • austenite steels which have relatively high hot strength and good oxidation resistance, are so-called austenite steels.
  • austenite steels Although austenite steels have high creep rupture strength at high temperatures, the load bearing capacity at low temperatures or normal temperatures is poor. In addition, there is the high tendency toward great thermal stress because of the small heat transfer efficiency.
  • the austenite steels are used to make cylinders or valve chests of steam turbines, they are liable to deform by the thermal stress at the time of starting or load variation. Thus, this will place an additional burden when the parts are designed. Since the austenite steels do not undergo the so-called transformation, it is not possible to control the structure by heat treatment. If these steels are made by casting, the cast structure remains as it is.
  • 12 chromium steels comprising about 8 - 15% of chromium.
  • the 12 chromium steels have such a high chromium content that they have better oxidation resistance than the afore-described low alloy steels, and have practical utility as a good hot strength steel as obtained by rolling or forging on a relatively small scale. This type of steel has been expected as having good hot strength even when obtained by casting.
  • a high chromium cast steel for a high-temperature, pressure container which comprises, on the weight basis, 0.08 - 0.12 of carbon, not larger than 0.7% of silicon, not larger than 0.8% of manganese, 0.4 - 0.7% of nickel, 9 - 11% of chromium, 0.65 - 1.00% of molybdenum, 0.13 - 0.20% of vanadium, 0.03 - 0.07% of niobium, 0.03 - 0.07% of nitrogen, and the balance of iron and inevitable impurities.
  • the cast steel may further comprise not larger than 0.7% of cobalt.
  • a method for thermally treating a cast steel of the type described above comprises heating the cast steel to a temperature of 1000 - 1100°C for normalization, cooling the steel down to a temperature not higher than 250°C, tempering the steel once in a temperature range of 550 - 750°C, cooling down to not higher than 150 0 C and tempering the resulting steel at 680 - 750°C.
  • the present invention is accomplished based on the following test results. We found that in order to obtain desirable properties of high chromium cast steels, the respective components contained in the steels had to be defined in specified ranges.
  • test steels used in the tests.
  • Each test steel was prepared by melting 50 kg of a steel material in a 50 kg vacuum high frequency blast furnace and casting the melt in a sand mold.
  • the resultant test steels were divided into two groups, which were each heated to 1030°C for 10 hours, cooled down to 300°C at a rate of 73°C/hr. After cooling in air, the respective groups were tempered at 650 and 700°C for 10 hours, respectively, and then subjected to tensile strength, impact strength, high-temperature tensile and creep rupture tests.
  • the reason why the test steels were kept at the respective temperatures for 10 hours and the cooling rate from 1030°C to 300°C was determined at 73°C/hour is that large-sized parts such as a steam turbine cylinder are assumed for the application of these steel materials.
  • Fig. 1 shows the drawing rate, which is considered to typically represent ductility, in relation to tensile strength, on the basis of the the results of tensile strength at normal temperatures.
  • Fig. 2 shows the results of an impact strength at normal temperatures, which typically represents toughness, in relation to tensile strength, in which the impact strength shown is a 2 mm V-notched Charpy impact strength.
  • Fig. 3 shows the results of a creep rupture strength, which is typical of high-temperature strength, in relation to tensile strength.
  • the steel Nos. 1 to 8 were tested in order to determine components of the cast steel of the present invention and careful attention was paid to the suppression of delta-ferrite and the hardenability. In other words, the test results were obtained under conditions where formation of delta-ferrite was suppressed and the hardenability was sufficiently ensured.
  • so-called 12 Cr cast steels show such a tendency as described above when small-sized articles were made at a large cooling rate.
  • various problems are presented due to the difference between the cases of the small large-sized articles. In order to avoid such problems, the test was conducted while taking the heat treatment into due consideration, from which the above fact was found.
  • Fig. 2 reveals that the impact strength also tends to decrease with an increase in content of carbon though not prominent as compared with the drawing rate.
  • the impact strengths of steel Nos. 8 and 12 are low. This is because of the formation of delta-ferrite.
  • Fig. 3 reveals that at the same tensile strength, the steels tested tend to have increasing creep rupture strengths with a decrease in content of carbon.
  • the carbon content should be low from the standpoint of the ductility, toughness and high-temperature strength and is determined to be in the range of 0.08 to 0.12% according to the above test results. Less amounts are unfavorable because delta-ferrite is liable to produce with the lack of hardenability, making it difficult to ensure toughness. This is why the lower limit is determined as 0.08%.
  • the upper limit of 0.12% is as follows: the test results reveal that larger amounts of carbon still keep good ductility and toughness, but are obtained merely from the test where relatively small-sized test materials, which are obtained under good ingotting conditions, arre used; with large-sized actual materials, there is the possibility that the lowerings of ductility and toughness with an increase of the carbon content become more pronounced; and when the carbon content is large at a tensile strength ranging from 70 to 80 kgf/mm 2 which is a practical range for the cast steel, there is the tendency of lowering the creep rupture strength.
  • the reason why the silicon content is determined to be not larger than 0.7% is as follows: when this type of steel is used as a cast steel, it is the usual to use a rather high content of silicon so as to ensure good forgeability and the silicon content is determined to be within the allowable range according to the usual practice. Higher silicon contents have effects of ensuring good fluidity of molten metal and killing molten metal and may be effective in preventing so-called cast defectives, but will tend to cause micro and macrosegregations, making it difficult to obtain steels of stable properties.
  • the upper limit of 0.7% is a range where the above problem does not appear pronouncedly.
  • manganese content is not larger than 0.8%.
  • manganese serves to mitigate the adverse influence of sulfur, prevents formation of delta-ferrite and improves the hardenability.
  • the content is allowed to such an extent.
  • higher contents may be used, there may be the fear that the properties of the cast steel may vary.
  • 0.8% is determined as the upper limit.
  • Nickel is used in a content of from 0.4 to 0.7%. With this type of cast steel, a smaller amount of nickel is preferred in order to improve the creep rupture strength. Too small amounts will tend to form delta-ferrite and precipitate pro-eutectoid ferrite, so that the toughness lowers to such an extent as not to be used as a cast steel material. Accordingly, the range of from 0.4 to 0.7% is determined around 0.5% which is ordinarily used for these purposes.
  • Chromium is determined in the range of 9 to 11%. This is because with this type of cast steel, higher contents of chromium result in more improved creep rupture strength. However, too higher contents tend to form delta-ferrite and precipitate pro-eutectoid ferrite, which makes it difficult to ensure high toughness. This is why the above range is used.
  • molybdenum content is determined in the range of from 0.65% to 1.00% is that molybdenum gives a well-balanced effect with regard to an improvement of the creep rupture strength when added in an amount of approximately 1%.
  • it too large contents are used, the resultant steel becomes embrittled on heating at high temperatures for a long time and there are tendencies toward the formation of delta-ferrite and precipitation of pro-eutectoid.
  • the content is determined in a slightly smaller range.
  • Vanadium is determined to range from 0.13% to 0.20%. In general, it is accepted that the creep rupture strength is improved when vanadium is added in an amount of about 0.25%. However, according to our experiment, it was confirmed that the creep rupture strength tended to be improved when the amount of vanadium was smaller similar to the case of carbon. Therefore, the content of vanadium was lowered within a range not impeding hardenability. Taking a controllable range into consideration, the above-defined range is determined. When-vanadium is added to the type of cast steel, to which the present invention is directed, in such an amount as recited above, it is considered necessary to determine the content as having a range by 0.06%.
  • Niobium presents the growth of crystal grains by interaction with nitrogen and serves to improve ductility and toughness. When used in combination with vanadium, it acts to improve the creep rupture strength. Similar to vanadium, too large amounts result in a lowering of the creep rupture strength with the case of large-sized materials. It was also experimentally confirmed that too large amounts caused carbon nitride to precipitate in segregated portions. The above range is determined while taking in view a controllable range about 0.055% at which its effect was experimentally confirmed.
  • Nitrogen is determined in a range of from 0.03 to 0.07%. As described before, the ductility and toughness lower at larger amounts of carbon when large-sized materials are made. Nitrogen has not such undesirable effects, but an effect of improving the creep rupture strength by increasing precipitation of carbon nitride similar in quality to the carbide when coexisting with carbon. In coexistence with vanadium and niobium, nitrogen has advantages of suppressing the growth of crystal grains and preventing the formation of delta-ferrite and the precipitation of pro-eutectoid ferrite. Nitrogen has also the effect of improving the hardenability with the attendant effect of imparting good ductility and toughness. However, too large a content of nitrogen has the tendency toward a lowering of the creep rupture strength similar to carbon. The above range is determined as a controllable range around 0.05% at which the effect nitrogen was confirmed.
  • cobalt may be added in order to prevent the formation of delta-ferrite and precipitation of pro-eutectoid and increase the hardenability.
  • Cobalt has similar effects as nickel but is different from nickel in that it gives little adverse influence on the creep rupture strength. If the nickel content is limited for some reasons by which there arise problems in the formation of delta-ferrite, the prevention of pro-eutectoid and the hardenability, it is favorable to add cobalt. However, if cobalt is added in too much an amount, the balance of the steel properties may be lost. The allowable range of cobalt is up to 0.7%, which is controllable range around 0.5% at which the effect was confirmed.
  • the steels having the above components correspond to steel Nos. 1, 2 and 3 in Table 2.
  • the results of Figs. 1 to 3 demonstrate that these steels are better in ductility, toughness and creep rupture strength than similar steel Nos. 4 to 7. This is mainly due to the effect of carbon.
  • steel No. 8 has a better creep rupture strength, ductility and toughness are not so high. This is attributed to the influence of nitrogen.
  • the high chromium .cast steel according to the invention has been made according to test results and the knowledge obtained therefrom and can solve the problems of lowering ductility and toughness involved in large-sized materials, which impedes practical utility of this type of cast steel.
  • the creep rupture strength is also improved.
  • the cast steel of the invention has high utility as a high chromium cast steel for high-temperature pressure containers such as a cylinder or valve chest of a steam turbine. Thus, it has a very high industrial value.
  • the cast steel of the type described above has to be used after proper thermal treatment in order to attain the purposes of the invention.
  • the present invention also provides a method for the thermal treatment of the cast steel in making a pressure container.
  • the ductility and toughness of the cast steel material of the invention tend to decrease with an increase of the tensile strength.
  • a certain limitation should be placed on the tensile strength.
  • the materials used in the test are of the small size and ingotted under well-balanced conditions. In practical applications as large-sized articles, there is the high possibility that the lowerings of the ductility and toughness by improving the tensi le strength becomes much more pronounced. Moreover, when the tensile strength is lowered to an extent, the creep rupture strength lowers. In view of these facts, the tensile strength was set in the range of from 70 kgf/mm to 80 kgf/mm. In the thermal treatment of the invention, consideration was first given to the tensile strength.
  • the steel of the invention has to be normalized in order to impart desired properties thereto.
  • the normalizing temperature should be 1000°C or higher. Too high temperatures may cause coarse crystal grains, presenting a problem with regard to ductility and toughness.
  • the upper limit of the temperature should be approximately 1100 C. Accordingly, the normalizing temperature is determined within the above range.
  • the steel After the normalization, the steel is cooled. When it is cooled down to normal temperatures, there is the fear of causing cracks as mentioned above. On the contrary, if the cooling is insufficient, residual austenite remains in large amounts. This austenite remains, as it is, even when the steel is subsequently tempered. After the tempering, hardened martensite is formed, with the tendency that cracking will occur as well. Accordingly, the steel should be cooled down to a temperature at which martensite transformation in adequate amounts takes place and non-transformed austenite is reduced in amount. With the steel of the invention, the steel is cooled to 250°C or below.
  • Subsequent tempering should be effected at temperatures not lower than 550°C, at which not cracks produce. However, if the temperature increases to high a level, the tensile strength cannot be maintained at 70 to 80 kgf/mm 2 . The upper limit for this is about 750°C. Accordingly, the temperature ranges from 550 to 750°C.
  • the cooling temperature after the first tempering is not low enough the austenite transformation, non-transformed austenite is left. This austenite remains as it is after the second tempering and is changed into hardened martensite upon subsequent cooling. .
  • the cooling should be carried to a satisfactory extent before the second tempering or after the first tempering. This cooling temperature should be 150°C or below in the practice of the invention.
  • the second tempering temperature should be determined to control the tensile strength in the range of 70 to 80 kgf/mm 2 .
  • the temperature should be in the range of from 680 to 750°C.
  • a one ton ingot material was made as shown in Fig. 4 and tested.
  • the dimension in the figure is expressed in terms of mm.
  • Table 5 shows the results of tensile strength at normal temperatures and an impact test using a 2 mm V-notched test specimen.
  • Figs. 5, 6 and 7 there are shown the results of high-temperature tensile strength, creep and creep rupture strengths, and high-temperature low cycle fatigue.
  • the method of the invention is effective in imparting good properties to the high chromium cast steel of the invention in view of the purposes of the steel as high-temperature pressure containers.
  • the present invention has industrially great merits.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
EP19850730139 1984-10-17 1985-10-11 Hochchromhaltiger Gussstahl für ein Hochtemperaturdruckgefäss und Verfahren zu seiner thermischen Behandlung Expired EP0188995B1 (de)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
JP21632284A JPS6196062A (ja) 1984-10-17 1984-10-17 高温圧力容器用高クロム鋳鋼
JP216323/84 1984-10-17
JP216322/84 1984-10-17
JP59216323A JPH0696741B2 (ja) 1984-10-17 1984-10-17 高温圧力容器用高クロム鋳鋼の熱処理方法

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EP0188995A1 true EP0188995A1 (de) 1986-07-30
EP0188995B1 EP0188995B1 (de) 1991-01-23

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2179674B (en) * 1985-07-25 1989-08-23 Nippon Kokan Kk 9% chromium heat-resistant steel excellent in toughness and having high cracking resistance and high creep strength in welded joint
WO1992000158A1 (en) * 1990-06-28 1992-01-09 Abb Powdermet Ab METHOD OF POWDERMETALLURGICALLY MANUFACTURING FULLY DENSE BODIES FROM HIGH TEMPERATURE MARTENSITIC Cr STEEL
EP0767250A2 (de) * 1995-08-25 1997-04-09 Hitachi, Ltd. Hochwarmfester Gusstahl, Dampfturbinengehäuse, Dampfturbinenkraftwerk und Dampfturbine
EP0860511A1 (de) * 1997-01-27 1998-08-26 Mitsubishi Heavy Industries, Ltd. Hochchromhaltiger, hitzebeständiger Gussstahl und daraus hergestellter Druckbehälter

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2848323A (en) * 1955-02-28 1958-08-19 Birmingham Small Arms Co Ltd Ferritic steel for high temperature use
GB1015043A (en) * 1962-05-31 1965-12-31 Gen Electric Improvements in steel alloys
GB1108687A (en) * 1966-03-29 1968-04-03 Hitichi Ltd Ferritic heat-resisting steel
BE855896A (fr) * 1977-06-20 1977-10-17 Centre Rech Metallurgique Ameliorations apportees aux aciers resistant au fluage et a l'oxydation a chaud
EP0073021A1 (de) * 1981-08-26 1983-03-02 Hitachi, Ltd. Martensitischer, hitzebeständiger Stahl
EP0083254A2 (de) * 1981-12-25 1983-07-06 Hitachi, Ltd. Hitzebeständiger Stahl

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2848323A (en) * 1955-02-28 1958-08-19 Birmingham Small Arms Co Ltd Ferritic steel for high temperature use
GB1015043A (en) * 1962-05-31 1965-12-31 Gen Electric Improvements in steel alloys
GB1108687A (en) * 1966-03-29 1968-04-03 Hitichi Ltd Ferritic heat-resisting steel
BE855896A (fr) * 1977-06-20 1977-10-17 Centre Rech Metallurgique Ameliorations apportees aux aciers resistant au fluage et a l'oxydation a chaud
EP0073021A1 (de) * 1981-08-26 1983-03-02 Hitachi, Ltd. Martensitischer, hitzebeständiger Stahl
EP0083254A2 (de) * 1981-12-25 1983-07-06 Hitachi, Ltd. Hitzebeständiger Stahl

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2179674B (en) * 1985-07-25 1989-08-23 Nippon Kokan Kk 9% chromium heat-resistant steel excellent in toughness and having high cracking resistance and high creep strength in welded joint
WO1992000158A1 (en) * 1990-06-28 1992-01-09 Abb Powdermet Ab METHOD OF POWDERMETALLURGICALLY MANUFACTURING FULLY DENSE BODIES FROM HIGH TEMPERATURE MARTENSITIC Cr STEEL
EP0767250A2 (de) * 1995-08-25 1997-04-09 Hitachi, Ltd. Hochwarmfester Gusstahl, Dampfturbinengehäuse, Dampfturbinenkraftwerk und Dampfturbine
EP0767250A3 (de) * 1995-08-25 1997-12-29 Hitachi, Ltd. Hochwarmfester Gusstahl, Dampfturbinengehäuse, Dampfturbinenkraftwerk und Dampfturbine
US5961284A (en) * 1995-08-25 1999-10-05 Hitachi, Ltd. High strength heat resisting cast steel, steam turbine casing, steam turbine power plant and steam turbine
EP0860511A1 (de) * 1997-01-27 1998-08-26 Mitsubishi Heavy Industries, Ltd. Hochchromhaltiger, hitzebeständiger Gussstahl und daraus hergestellter Druckbehälter
US6007767A (en) * 1997-01-27 1999-12-28 Mitsubishi Heavy Industries, Ltd. High chromium heat resistant cast steel material
US6123897A (en) * 1997-01-27 2000-09-26 Mitsubishi Heavy Industries, Ltd. High chromium heat resistant cast steel material and pressure vessel formed thereof

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DE3581527D1 (de) 1991-02-28
EP0188995B1 (de) 1991-01-23

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