US20180216215A1 - Austenitic heat-resistant alloy and welded structure - Google Patents

Austenitic heat-resistant alloy and welded structure Download PDF

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US20180216215A1
US20180216215A1 US15/736,395 US201615736395A US2018216215A1 US 20180216215 A1 US20180216215 A1 US 20180216215A1 US 201615736395 A US201615736395 A US 201615736395A US 2018216215 A1 US2018216215 A1 US 2018216215A1
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austenitic heat
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Hiroyuki Hirata
Hiroyuki Semba
Kana JOTOKU
Atsuro Iseda
Toshihide Ono
Katsuki TANAKA
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • 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
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • the present invention relates to an austenitic heat-resistant alloy and a welded structure including this alloy.
  • JP 2004-250783 A proposes an austenitic stainless steel with improved high-temperature strength and corrosion resistance, where the N content is 0.1 to 0.35% and the Cr content is higher than 22% and lower than 30%, and a metallic microstructure is specified.
  • JP 2009-084606 A proposes an austenitic stainless steel with improved high-temperature strength and corrosion resistance, where the N content is 0.1 to 0.35% and the Cr content is higher than 22% and lower than 30%, and impurity elements are specified.
  • JP 2012-1749 A discloses an austenitic heat-resistant steel with improved high-temperature strength and hot workability containing 0.09 to 0.30% N and having large amounts of Mo and W in composite addition.
  • WO 2009/044796 A1 discloses a high-strength austenitic stainless steel containing 0.03 to 0.35% N and one or more of Nb, V and Ti.
  • WO 2009/044796 A1 discussed above states that limiting the amounts of the elements that cause embrittlement of the grain boundaries and the elements that strengthen the grain interiors to certain ranges prevents cracking that would occur during use for a prolonged period of time. Indeed, these materials prevent cracking under certain conditions.
  • austenitic heat-resistant alloys with large amounts of W, Mo etc. added thereto to further improve properties such as high-temperature strength has become widespread.
  • structure shapes and sizes, for example, these austenitic heat-resistant alloys may not prevent cracking in a stable manner. More specifically, they may not prevent cracking in a stable manner for high welding heat inputs, heavy plate thicknesses or high use temperatures such as above 650° C.
  • An object of the present invention is to provide an austenitic heat-resistant alloy that provides good crack resistance and high-temperature strength in a stable manner.
  • An austenitic heat-resistant alloy has a chemical composition of, in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up to 0.03% P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr; 1 to 5% W; 0.1 to 0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B; 0.001 to 0.02% Sn; up to 0.03% Al; up to 0.02% O; 0 to 0.5% Ti; 0 to 2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0 to 0.2% REM; and the balance being Fe and impurities, the alloy having a microstructure with a grain size represented by a grain size number in accordance with ASTM E112 of 2.0 or more and less than 7.0.
  • the present invention provides an austenitic heat-resistant alloy that provides good crack resistance and high-temperature strength in a stable manner.
  • FIG. 1 is a cross-sectional view of a bevel produced for the Examples, showing the shape of the groove thereof.
  • the present inventors conducted a detailed investigation to solve the above-discussed problems, and revealed the following findings.
  • the inventors thoroughly investigated SIPH cracks occurring, during use, in welded joints using austenitic heat-resistant alloys with high N contents. They found that (1) cracks developed along grain boundaries in weld-heat-affected zones with coarse grains near the fusion lines, and (2) clear concentrating of S was detected on the fractured surfaces of cracks. They further found that (3) large amounts of nitrides and carbonitrides had precipitated within grains near the cracks. This was particularly significant for high Nb contents. In addition, they found that (4) the larger the initial grain size of the used austenitic heat-resistant alloy, the larger the grain size in weld-heat-affected zones became and the more likely cracking occurred.
  • the inventors revealed that, in order to prevent SIPH cracking in an austenitic heat-resistant alloy containing 0.04 to 0.14% C, 0.05 to 1% Si, 0.5 to 2.5% Mn, up to 0.03% P, 23 to 32% Ni, 20 to 25% Cr, 1 to 5% W, 0.1 to 0.3% N, 0.0005 to 0.01% B, up to 0.03% Al, and up to 0.02% O, it is effective to exactly control the Nb and S contents to be in the range of 0.1 to 0.6% and below 0.001%, respectively, and to have an initial grain size of the base material represented by a grain size number as defined by the American Society for Testing and Material (ASTM) of 2.0 or more.
  • ASTM American Society for Testing and Material
  • the inventors found that the grain size as represented by grain size number needs to be less than 7.0.
  • V which has a lower precipitation strengthening property than Nb, in a content of 0.1 to 0.6% is necessary to achieve a predetermined creep strength without impairing SIPH crack resistance.
  • austenitic heat-resistant alloys are generally welded for assembly. When they are welded, a filler material is usually used. However, for small parts with thin wall thicknesses, or even for components with heavy wall thickness for root running or tack welding, gas shield-arc welding may be performed without using a filler material. If the penetration depth is insufficient at this time, unwelded abutting surfaces remain as weld defects, and the strength required of a welded joint cannot be obtained. While S reduces SIPH crack resistance, S has the effect of increasing the penetration depth. Thus, the inventors found that the problem of insufficient penetration depth tends to be apparent if the S content is exactly controlled to be below 0.001% to address the issue of SIPH crack resistance.
  • welding heat input may be simply increased.
  • increasing welding heat input brings about grains coarsening in weld-heat-affected zones, and the inventors failed to prevent SIPH cracking even when the initial grain size of the base material had a grain size number of 2.0 or more.
  • the austenitic heat-resistant alloy according to the present embodiment has the chemical composition described below.
  • “%” in the content of an element means mass percent.
  • Carbon (C) stabilizes the austenite microstructure and forms fine carbide particles to improve creep strength during use at high temperatures. 0.04% or more C needs to be contained in order that these effects are sufficiently present. However, if an excess amount of C is contained, large amounts of carbides precipitate, which reduces SIPH crack resistance. In view of this, the upper limit should be 0.14%.
  • the lower limit of C content is preferably 0.05%, and more preferably 0.06%.
  • the upper limit of C content is preferably 0.13%, and more preferably 0.12%.
  • Silicon (Si) has a deoxidizing effect, and is effective in improving the corrosion resistance and oxidation resistance at high temperatures. 0.05% or more Si needs to be contained in order that these effects are sufficiently present. However, if an excess amount of Si is contained, the stability of the microstructure decreases, which decreases toughness and creep strength. In view of this, the upper limit should be 1%.
  • the lower limit of Si content is preferably 0.08%, and more preferably 0.1%.
  • the upper limit of Si content is preferably 0.6%, and more preferably 0.5%.
  • Mn manganese
  • Mn has a deoxidizing effect. Mn also contributes to the stabilization of austenite microstructure. 0.5% or more Mn needs to be contained in order that these effects are sufficiently present. However, if an excess amount of Mn is contained, this causes embrittlement of the alloy, and creep ductility decreases. In view of this, the upper limit should be 2.5%.
  • the lower limit of Mn content is preferably 0.6%, and more preferably 0.7%.
  • the upper limit of Mn content is preferably 2%, and more preferably 1.5%.
  • Phosphorus (P) is contained in the alloy in the form of an impurity, and, during welding, segregates on grain boundaries in weld-heat-affected zones, thereby increasing liquation cracking susceptibility. P also decreases creep ductility after use for a prolonged period of time.
  • an upper limit should be set for P content, which should be 0.03% or lower.
  • the upper limit of P content is preferably 0.028%, and more preferably 0.025%. It is preferable to minimize P content; however, reducing it excessively causes increased steel-manufacturing cost.
  • the lower limit of P content is preferably 0.0005%, and more preferably 0.0008%.
  • sulfur (S) is contained in the alloy in the form of an impurity, and, during welding, segregates on grain boundaries in weld-heat-affected zones, thereby increasing liquation cracking susceptibility. S also segregates on grain boundaries during use for a prolonged period of time and causes embrittlement, which significantly reduces SIPH crack resistance.
  • the S content needs to be less than 0.001%.
  • the upper limit of S content is preferably 0.0008%, and more preferably 0.0005%. It is preferable to minimize S content; however, reducing it excessively causes increased steel-manufacturing cost.
  • the lower limit of S content is preferably 0.0001%, and more preferably 0.0002%.
  • Nickel (Ni) is an element indispensable for providing sufficient stability of the austenite phase during use for a prolonged period of time. 23% or more Ni needs to be contained in order that this effect is sufficiently present within the limits of Cr and W contents of the present embodiment. However, Ni is an expensive element, and large amounts of Ni contained mean increased costs. In view of this, the upper limit should be 32%.
  • the lower limit of Ni content is preferably 25%, and more preferably 25.5%.
  • the upper limit of Ni content is preferably 31.5%, and more preferably 31%.
  • Chromium (Cr) is an element indispensable for providing sufficient oxidation resistance and corrosion resistance at high temperatures. Cr also forms fine carbide particles to contribute to the provision of sufficient creep strength, too. 20% or more Cr needs to be contained in order that these effects are sufficiently present within the limits of Ni content of the present embodiment. However, if an excessive amount of Cr is contained, the microstructure stability of the austenite phase at high temperatures deteriorates, which decreases creep strength. In view of this, the upper limit should be 25%.
  • the lower limit of Cr content is preferably 20.5%, and more preferably 21%.
  • the upper limit of Cr content is preferably 24.5%, and more preferably 24%.
  • Tungsten (W) dissolves in the matrix, or forms fine intermetallic compounds to significantly contribute to the improvement of creep strength and tensile strength at high temperatures. 1% or more W needs to be contained in order that these effects are sufficiently present. However, if an excess amount of W is contained, the deformation resistance with grains becomes high and SIPH crack resistance reduces, and creep strength may decrease. Further, W is an expensive element, and large amounts of W contained mean increased costs. In view of this, the upper limit should be 5%.
  • the lower limit of W content is preferably 1.2%, and more preferably 1.5%.
  • the upper limit of W content is preferably 4.5%, and more preferably 4%.
  • Niobium (Nb) precipitates in the form of fine MX carbonitride particles, and, in addition, precipitates in the form of Z phase (CrNbN) within grains to significantly contribute to the improvement of creep strength and tensile strength at high temperatures.
  • 0.1% or more Nb needs to be contained in order that these effects are sufficiently present. However, if an excess amount of Nb is contained, the strengthening property of these precipitates is too high, which reduces SIPH crack resistance and causes a decrease in creep ductility and toughness.
  • the upper limit should be 0.6%.
  • the lower limit of Nb content is preferably 0.12%, and more preferably 0.15%.
  • the upper limit of Nb content is preferably 0.55%, and more preferably 0.5%.
  • Vanadium (V) precipitates in the form of fine MX carbonitride particles within the grains to contribute to the improvement of creep strength and tensile strength at high temperatures. 0.1% or more V needs to be contained in order that these effects are sufficiently present. However, if an excess amount of V is contained, large amounts of carbonitrides precipitate, which reduces SIPH crack resistance and causes a decrease in creep ductility and toughness. In view of this, the upper limit should be 0.6%.
  • the lower limit of V content is preferably 0.12%, and more preferably 0.15%.
  • the upper limit of V content is preferably 0.55%, and more preferably 0.5%.
  • N Nitrogen
  • 0.1% or more N needs to be contained in order that these effects are sufficiently present. However, if an excessive amount of N is contained, it dissolves during use for a short period of time, or large amounts of fine nitride particles precipitate within grains during use for a prolonged period of time, thereby increasing the deformation resistance within grains, which reduces SIPH crack resistance. Further, creep ductility and toughness decrease.
  • the upper limit should be 0.3%.
  • the lower limit of N content is preferably 0.12%, and more preferably 0.14%.
  • the upper limit of N content is preferably 0.28%, and more preferably 0.26%.
  • B Boron
  • B provides fine dispersed grain-boundary carbide particles to improve creep strength, and segregates on grain boundaries to strengthen grain boundaries. 0.0005% or more B needs to be contained in order that these effects are sufficiently present. However, if an excess amount of B is contained, the weld thermal cycle during welding causes a large amount of B to segregate in weld heat affected zones near melt boundaries to decrease the melting point of grain boundaries, thereby increasing liquation cracking susceptibility.
  • the upper limit should be 0.01%.
  • the lower limit of B content is preferably 0.0008%, and more preferably 0.001%.
  • the upper limit of B content is preferably 0.008%, and more preferably 0.006%.
  • Tin (Sn) has the effect of increasing the penetration depth during welding by evaporating from the molten pool to increase the current density of the arc. 0.001% or more Sn needs to be contained in order that these effects are sufficiently present. However, if an excess amount of Sn is contained, the liquation cracking susceptibility in weld-heat-affected zones during welding and the SIPH crack susceptibility during use become high. In view of this, the upper limit should be 0.02%.
  • the lower limit of Sn content is preferably 0.0016%, and more preferably 0.002%.
  • the upper limit of Sn content is preferably 0.018%, and more preferably 0.015%.
  • Aluminum (Al) has a deoxidizing effect. However, if an excess amount of Al is contained, the cleanliness of the alloy deteriorates, which decreases hot workability. In view of this, the upper limit should be 0.03%.
  • the upper limit of Al content is preferably 0.025%, and more preferably 0.02%. No lower limit needs to be set; still, it should be noted that decreasing Al excessively causes an increase in steel-manufacturing cost. In view of this, the lower limit of Al content is preferably 0.0005%, and more preferably 0.001%.
  • Al as used herein means acid-soluble Al (sol. Al).
  • Oxygen (O) is contained in the alloy in the form of an impurity, and has the effect of increasing the penetration depth during welding. However, if an excess amount of O is contained, hot workability decreases and toughness and ductility deteriorate.
  • the upper limit should be 0.02%.
  • the upper limit of O content is preferably 0.018%, and more preferably 0.015%.
  • No lower limit needs to be set; still, it should be noted that decreasing O excessively causes an increase in steel-manufacturing cost.
  • the lower limit of O content is preferably 0.0005%, and more preferably 0.0008%.
  • the balance of the chemical composition of the austenitic heat-resistant alloy in the present embodiment is Fe and impurities.
  • Impurity as used herein means an element originating from ore or scrap used as raw material for the heat-resistant alloy being manufactured on an industrial basis or an element that has entered from the environment or the like during the manufacturing process.
  • some of the Fe may be replaced by one or more elements selected from one of the first to third groups provided below. All of the elements listed below are optional elements. That is, none of the elements listed below may be contained in the austenitic heat-resistant alloy of the present embodiment. Or, only one or some of them may be contained.
  • only one group may be selected from among the first to third groups and one or more elements may be selected from this group. In this case, it is not necessary to select all the elements belonging to the selected group.
  • a plurality of groups may be selected from among the first to third groups and one or more elements may be selected from each of these groups. Again, it is not necessary to select all the elements belonging to the selected groups.
  • the element belonging to the first group is Ti.
  • Ti improves the creep strength of the alloy through precipitation strengthening.
  • Titanium (Ti) combines with carbon or nitrogen to form fine carbide or carbonitride particles, thereby contributing to the improvement of creep strength. These effects are present if a small amount of Ti is contained. On the other hand, if an excess amount of Ti is contained, large amounts of precipitates are produced, which reduces SIPH resistance and creep ductility. In view of this, the upper limit should be 0.5%.
  • the lower limit of Ti content is preferably 0.01%, and more preferably 0.03%.
  • the upper limit of Ti content is preferably 0.45%, and more preferably 0.4%.
  • the elements belonging to the second group are Co, Cu, and Mo. These elements improve the creep strength of the alloy.
  • cobalt (Co) is an austenite-forming element, and increases the stability of the austenite microstructure to contribute to the improvement of creep strength. These effects are present if a small amount of Co is contained. However, Co is a very expensive element, and large amounts of Co contained mean increased costs. In view of this, the upper limit should be 2%.
  • the lower limit of Co content is preferably 0.01%, and more preferably 0.03%.
  • the upper limit of Co content is preferably 1.8%, and more preferably 1.5%.
  • Cu copper
  • the upper limit should be 4%.
  • the lower limit of Cu content is preferably 0.01%, and more preferably 0.03%.
  • the upper limit of Cu content is preferably 3.8%, and more preferably 3.5%.
  • Mo molybdenum
  • the lower limit of Mo content is preferably 0.01%, and more preferably 0.03%.
  • the upper limit of Mo content is preferably 3.8%, and more preferably 3.5%.
  • the elements belonging to the third group are Ca, Mg and REM. These elements improve hot workability of the alloy.
  • the lower limit of Ca content is preferably 0.0005%, and more preferably 0.001%.
  • the upper limit of Ca content is preferably 0.01%, and more preferably 0.005%.
  • magnesium (Mg) improves hot workability during manufacture. This effect is present if a small amount of Mg is contained. On the other hand, if an excess amount of Mg is contained, it combines with oxygen to significantly decrease the cleanliness of the alloy, which decreases hot workability.
  • the upper limit is 0.02%.
  • the lower limit of Mg content is preferably 0.0005%, and more preferably 0.001%.
  • the upper limit of Mg content is preferably 0.01%, and more preferably 0.005%.
  • rare-earth metals improve hot workability during manufacture. This effect is present if a small amount of REM is contained. On the other hand, if an excessive amount of REM is contained, it combines with oxygen to significantly decrease the cleanliness of the alloy, which decreases hot workability.
  • the upper limit should be 0.2%.
  • the lower limit of REM content is preferably 0.0005%, and more preferably 0.001%.
  • the upper limit of REM content is preferably 0.15%, and more preferably 0.1%.
  • REM is a collective term for a total of 17 elements, i.e. Sc, Y and the lanthanoids, and “REM content” means the total content of one or more REM elements.
  • REMs are usually contained in mischmetal. Thus, for example, mischmetal may be added to the alloy such that the REM content is in the above-indicated range.
  • Nd has a strong affinity for S and P, and has the effect of reducing weld liquation cracking susceptibility by forming sulfides or phosphides, and thus it is more preferable to utilize Nd.
  • Grain Size Number 2.0 or More and Less than 7.0
  • the austenitic heat-resistant alloy according to the present embodiment has a microstructure having a grain size represented by a grain size number in accordance with ASTM E112 of 2.0 or more and less than 7.0.
  • the grains of the microstructure before welding need to be fine grains, i.e. their size as represented by grain size number in accordance with ASTM E112 needs to be 2.0 or more, in order to prevent the grains in the weld-heat-affected zones from becoming excessively coarse even after being affected by the heat cycle from the welding.
  • the grain size number should be 2.0 or more and less than 7.0.
  • the microstructure having the above-specified grain size can be provided by performing a heat treatment on the alloy with the above-specified chemical composition under appropriate conditions.
  • This microstructure may be achieved by, for example, shaping the alloy of the above-specified chemical composition into a predetermined shape by hot working or cold working before performing a solution heat treatment in which it is held at temperatures of 900 to 1250° C. for 3 to 60 minutes before water cooling.
  • the solution heat treatment involves holding the alloy at temperatures of 1120 to 1220° C. for 3 to 45 minutes before water cooling, and yet more preferably holding the alloy at temperatures of 1140 to 1210° C. for 3 to 30 minutes before water cooling.
  • the austenitic heat-resistant alloy according to an embodiment of the present invention has been described.
  • the present embodiment provides an austenitic heat-resistant alloy providing good crack resistance and high-temperature strength in a stable manner.
  • the materials labeled A to J having the chemical compositions shown in Table 1 were melted in a laboratory and ingots were cast, which were subjected to hot forging and hot rolling in the temperature range of 1000 to 1150° C. to provide plates with a thickness of 20 mm. These plates were further subjected to cold rolling to the thickness of 16 mm.
  • the plates were subjected to a solution heat treatment in which they were held at 1200° C. for a predetermined period of time before water cooling. After the solution heat treatment, they were machined to plates with a thickness of 14 mm, a width of 50 mm and a length of 100 mm.
  • the groove shown in FIG. 1 was provided along the longitudinal direction of each plate produced as described above. With grooved plates abutting each other, two joints for each mark were subjected to butt welding using gas-tungsten arc welding to produce welded joints. The welding did not use filler material, and the amount of heat input was 5 kJ/cm.
  • Table 2 also shows the grain size number of the austenitic heat-resistant alloy for each mark.
  • Each of the welded joints using the austenitic heat-resistant alloys with Marks A-1 to A-4, B to D and I as the base material had an appropriate chemical composition, where the initial grain size of the base material had a grain size of 2.0 or more and less than 7.0.
  • Each of these welded joints had a back bead across the entire length after root running, and had good weldability in fabrication. Further, though the thickness of the base material was 14 mm, which is relatively large, no cracks were produced in weld-heat-affected zones even after aging, meaning good crack resistance. Further, the creep-rupture strength at high temperatures was sufficient.
  • the welded joint using the austenitic heat-resistant alloy with Mark A-6 as the base material had good crack resistance, but the creep-rupture time was below the target. This is presumably because the grain size of the austenitic heat-resistant alloy with Mark A-6 was too small.
  • the welded joint using the austenitic heat-resistant alloy with Mark F as the base material contained no Sn but a large amount of S such that a sufficient back bead was produced. However, cracks that are believed to be SIPH cracks were produced after aging.
  • the present invention can be suitably used as an austenitic heat-resistant alloy used as a high-temperature part such as a main steam tube or high-temperature reheating steam tube in a thermal power boiler.

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EP4023776A4 (en) * 2019-08-29 2022-08-31 Nippon Steel Corporation HEAT RESISTANT AUSTENITIC STEEL
CN116529396A (zh) * 2021-04-14 2023-08-01 日铁不锈钢株式会社 耐焊接高温开裂性优异的高Ni合金
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