EP2248918A1 - Austenitischer edelstahl und wasserstoffentfernungsverfahren dafür - Google Patents

Austenitischer edelstahl und wasserstoffentfernungsverfahren dafür Download PDF

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EP2248918A1
EP2248918A1 EP09714474A EP09714474A EP2248918A1 EP 2248918 A1 EP2248918 A1 EP 2248918A1 EP 09714474 A EP09714474 A EP 09714474A EP 09714474 A EP09714474 A EP 09714474A EP 2248918 A1 EP2248918 A1 EP 2248918A1
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Prior art keywords
hydrogen
stainless steel
austenitic stainless
diffusible
ppm
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French (fr)
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EP2248918A4 (de
Inventor
Yukitaka Murakami
Saburo Matsuoka
Yoji Mine
Toshihiko Kanezaki
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National Institute of Advanced Industrial Science and Technology AIST
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National Institute of Advanced Industrial Science and Technology AIST
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    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • 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
    • 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
    • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
    • C21D3/02—Extraction of non-metals
    • C21D3/06—Extraction of hydrogen
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/28—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/30—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • 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 stainless steel having reduced hydrogen embrittlement and to a method for removing hydrogen therefrom. More specifically, the present invention relates to an austenitic stainless steel wherein there is a reduced influence of hydrogen present therein on the growth of fatigue cracks that occur in the austenitic stainless steel, and to a method for removing hydrogen therefrom.
  • the components of a typical austenitic stainless steel are set forth in Table 1.
  • Table 1 lists the names of stainless steels and heat-resistant steels as defined in JIS (Japanese Industrial Standards).
  • the last column of Table 1 shows the Vickers hardness of the stainless steel (hereinafter, HV).
  • Other columns correspond to the chemical compositions of the stainless steel, with amounts of the components expressed in wt %.
  • the content of hydrogen (H) is expressed as wt ppm (parts per million by weight).
  • Non-patent Documents 1 and 2 It is known that hydrogen penetrates into metallic materials and decreases the static strength and fatigue strength thereof.
  • Patent Documents 1 and 2 Various methods for removing such hydrogen, and methods for predicting the effect of hydrogen, have been proposed.
  • Patent Document 2 for example, an austenitic stainless steel is thermally treated after a plating process by being kept at a temperature of 270 to 400°C for 10 minutes or more, to remove the hydrogen and prevent hydrogen embrittlement.
  • Patent document 3 discloses a method wherein the extent of hydrogen embrittlement of an austenitic stainless steel is predicted and judged based on the chemical composition thereof.
  • Patent Documents 4 and 5 disclose austenitic stainless steel wires that are subjected to a dehydrogenation treatment.
  • Patent Document 4 (Table 2 and paragraphs [0015] to [0016]) discloses a high-strength austenitic stainless steel wire having 1.5 ppm or less of hydrogen, as a result of a dehydrogenation treatment (300°C for 24 hours).
  • the stainless steel wire does not exhibit drawing longitudinal cracking in a tensile test, at tensile strengths from 1900 N/mm 2 to about 2200 N/mm 2 .
  • the wire has a content of mechanically-induced martensite of 30 to 75% after drawing.
  • Patent Document 5 (Table 2, paragraph [0042]) discloses a high-strength austenitic stainless steel wire in which the hydrogen content is reduced to 1.5 ppm through a dehydrogenation treatment (low-temperature aging at 400°C for 30 minutes).
  • the stainless steel wire does not break in a tensile test, at tensile strengths from 2000 N/mm 2 to 2800 N/mm 2 .
  • the wire has a content of mechanically-induced martensite of 25 to 75% after drawing.
  • the inventions disclosed in Patent Documents 4 and 5, however, are inventions of austenitic stainless steel wire, and disclose nothing as regards fatigue testing under slow cycling frequency.
  • Non-patent Document 1 discloses fatigue test results for austenitic stainless steels according to SUS304, SUS316, and SUS316L. The fatigue tests are conducted by comparing these austenitic stainless steels with their hydrogen-charged counterparts. The fatigue crack growth rate of hydrogen-charged SUS304 and SUS316 is faster than in the corresponding uncharged steels. However, no clear difference is seen with SUS316L.
  • Non-patent Document 1 discloses fatigue test results for JIS SUS304 and SUS316L austenitic stainless steels after a test piece is prestrained and a microhole of about 100 ⁇ m is formed therein.
  • the fatigue crack growth rate is accelerated ten-fold in hydrogen-charged SUS304 compared with the uncharged counterpart.
  • the fatigue crack growth rate is accelerated two-fold in SUS316L.
  • Non-patent Document 1 discloses results that overturn this common belief. This finding is the more significant in that the results were obtained by applying cyclic loading at a low frequency of 5 Hz or less.
  • Non-patent Document 2 points out the following: "(3) The martensitic phase resulting from transformation in the austenitic stainless steel becomes a pathway for hydrogen diffusion throughout the material, and the diffusion coefficient of hydrogen is increased thereby" (see page 130).
  • Low-frequency cyclic loading occurs also due to, for instance, temperature variations in the outside air temperature.
  • a conceivable example of cyclic loading due to variations in the outside air temperature is, for instance, thermal stress resulting from compression and expansion of the stainless steel itself, and of the parts connected to stainless steel components, as a result of temperature differences between day and night.
  • the frequency of the cycle the temperature difference between day and night can range from only a few degrees to ten degrees centigrade or more, and one cycle is 24 hours long.
  • high pressure hydrogen tanks in fuel cell vehicle-related facilities, high-pressure hydrogen tanks, facilities for supplying fuel for fuel cells, and the like have a cycle measured in single day units as noted above, and the hydrogen fill time is long.
  • a fuel cell-powered vehicle is dependent on the environment in which it operates, and experiences hence temperature differences of several °C to several tens of °C, and cycles ranging from sub-seconds to several hours.
  • An object of the present invention is to provide an austenitic stainless steel for reducing the influence of hydrogen on the growth rate of fatigue cracks that occur in the austenitic stainless steel, and to provide a method for removing the hydrogen therefrom.
  • a further object of the present invention is to provide an austenitic stainless steel in which diffusible hydrogen and non-diffusible hydrogen are removed therefrom, and to provide a method for removing the hydrogen therefrom, by focusing on diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement in the austenitic stainless steel.
  • a further object of the present invention is to provide an austenitic stainless steel in which diffusible hydrogen and non-diffusible hydrogen are removed therefrom, and to provide a method for removing the hydrogen therefrom, by focusing on diffusible hydrogen and non-diffusible hydrogen that become problematic under cyclic loading with a long cycle time.
  • a further object of the present invention is to provide an austenitic stainless steel wherein diffusible hydrogen and non-diffusible hydrogen that are present in the austenitic stainless steel are removed therefrom during a manufacturing step of the austenitic stainless steel, and to provide a method for removing the hydrogen therefrom.
  • a further object of the present invention is to provide an austenitic stainless steel wherein diffusible hydrogen and non-diffusible hydrogen that are present in the austenitic stainless steel are removed therefrom during a manufacturing step of the austenitic stainless steel, in particular during a thermal treatment in an air atmosphere, and to provide a method for removing the hydrogen from the austenitic stainless steel.
  • a further object of the present invention is to provide an austenitic stainless steel that allows slowing down the growth rate of fatigue cracks during repeated low-frequency loading, and to provide a method for removing the hydrogen from the austenitic stainless steel.
  • Hydrogen charging means causing hydrogen to penetrate into a material.
  • Hydrogen charging method refers to a method in which a material is exposed in a high pressure hydrogen chamber, a method in which cathodic charging is performed, or a method in which the material is immersed in a chemical solution and the like.
  • Fatigue crack growth refers to the enlargement of defects and cracks that occur in a material , as a result of cyclic loading, during the manufacturing process, or cracks from bores, holes and the like that are artificially opened into the material.
  • Fatigue crack growth rate denotes the rate at which a fatigue crack progresses.
  • An austenitic stainless steel refers to Cr-Ni steel wherein Cr and Ni are added to Fe to produce a stainless steel having an austenitic phase and exhibiting increased corrosion resistance in corrosive environments and the like. Table 1 gives a list of such stainless steels.
  • An austenitic phase denotes a phase of iron, at a temperature range of 911 to 1392°C, in 100% pure iron (Fe), having a face-centered cubic lattice structure (hereinafter, FCC lattice structure).
  • Fig. 11A illustrates a face-centered cubic lattice.
  • the austenitic phase can also exist at room temperature when alloying elements such as Cr and Ni are added to Fe.
  • a martensitic phase is a structure obtained by quenching steel from a high-temperature stable austenitic phase.
  • the martensitic phase has a body-centered cubic lattice structure (hereinafter, BCC lattice structure).
  • Fig. 11B illustrates a body-centered cubic lattice.
  • the martensitic phase may arise through the action of stress, such as cold-working and the like, on austenitic-phase stainless steel at ordinary temperatures.
  • Diffusible hydrogen refers to hydrogen that is present in the material and escapes from the material over time at room temperature. Diffusible hydrogen causes hydrogen embrittlement in the material. Hydrogen that cannot escape from the material over time even at temperatures from room temperature to about 200°C, is called non-diffusible hydrogen.
  • the present invention achieves the above objects on the basis of the following means.
  • the inventors of the present invention found that non-diffusible hydrogen in an austenitic stainless steel is related to fatigue crack growth, and on the basis of this finding, the inventors invented an austenitic stainless steel and a method for removing hydrogen therefrom.
  • the present invention relates to an austenitic stainless steel having an austenitic phase whose crystalline structure is a face-centered cubic lattice structure, and to a method for removing hydrogen from the austenitic stainless steel.
  • the austenitic stainless steel of the present invention has an austenitic phase where a crystalline structure is a face-centered cubic lattice structure, and the austenitic stainless steel is subjected to a thermal treatment in an air atmosphere, at a heating temperature ranging from 200°C to 1100°C, to remove thereby diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement in the austenitic stainless steel, and remove a content of hydrogen (H) in the austenitic stainless steel to 0.0001 wt% (1.0 wt ppm) or less.
  • H hydrogen
  • a method for removing hydrogen from an austenitic stainless steel of the present invention comprises a step of heating the austenitic stainless steel, having an austenitic phase where a crystalline structure is a face-centered cubic lattice structure, in an air atmosphere, at a heating temperature ranging from 200°C to 1100°C, to remove thereby diffusible hydrogen and non-diffusible hydrogen in the austenitic stainless steel, to a content of 0.0001 wt% (1.0 wt ppm) or less
  • the diffusible hydrogen and the non-diffusible hydrogen that are removed in accordance with the method for removing hydrogen from an austenitic stainless steel of the present invention diffuse via a mechanically-induced martensitic phase, brought about by cyclic loading at low-frequency, accumulate in cracks that are under stress concentration, increase thereby the growth rate of fatigue cracks, and cause hydrogen embrittlement in the austenitic stainless steel.
  • the austenitic stainless steel of the present invention preferably, the diffusible hydrogen and the non-diffusible hydrogen in the austenitic stainless steel are removed so that the hydrogen (H) in the austenitic stainless steel is 0.00007 wt% (0.7 wt ppm) or less.
  • the diffusible hydrogen and the non-diffusible hydrogen in the austenitic stainless steel are removed so that the hydrogen (H) in the austenitic stainless steel is 0.00002 wt% (0.2 wt ppm) or less.
  • the austenitic stainless steel is subjected to a thermal treatment at a heating temperature of 200°C or higher in an air atmosphere.
  • the upper limit of the heating temperature is 1100°C, and in particular, the temperature of the thermal treatment is lower than the sensitization temperature, which is the temperature at which carbides of chromium (Cr) in the austenitic stainless steel precipitate due to heating.
  • the duration of the thermal treatment ranges from 2 hours to 500 hours.
  • the diffusible hydrogen and the non-diffusible hydrogen diffuse via a mechanically-induced martensitic phase brought about by cyclic loading at low-frequency, accumulate in cracks that are under stress concentration, increase thereby the growth rate of fatigue cracks, and cause hydrogen embrittlement in the austenitic stainless steel.
  • Removal of diffusible hydrogen and non-diffusible hydrogen from the austenitic stainless steel of the present invention may be carried out in a dedicated step for removal.
  • removal is performed not in a separate step but in the manufacturing process of the austenitic stainless steel, in the form of a thermal treatment for a predetermined duration to remove hydrogen to a hydrogen (H) content of 0.00007 wt% (0.7 wt ppm) or less.
  • the manufacturing process of the austenitic stainless steel can be streamlined as a result, since there is included no special process for removing diffusible hydrogen and non-diffusible hydrogen.
  • the temperature in the thermal treatment is 200°C or higher but lower than the melting point temperature of the stainless steel.
  • the duration of heating in the manufacturing process varies depending on the volume of the material, but ranges in practice from 2 hours to several tens of hours.
  • the manufacturing process takes place preferably in an inert gas flow atmosphere.
  • the manufacturing process of an austenitic stainless steel includes herein a solution thermal treatment and an aging treatment that are used in the manufacture of stainless steel.
  • the temperature in the thermal treatment is most preferably of 920°C or higher. In the case of an aging treatment, the temperature in the thermal treatment is most preferably 700°C or higher.
  • the stainless steel of the present invention is preferably an austenitic stainless steel or a heat-resistant austenitic steel.
  • an appropriate heating temperature range is found to be 200 to 1100°C.
  • the rationale for this range is as follows. As shown in Fig. 14A and Fig. 14B in the below-described Additional experimental example 3, hydrogen is released at a temperature of 200°C or higher. This indicates that the stainless steel must be heated at a temperature equal to or higher than a lowest temperature of 200°C, i.e. indicates that there is a lower limit to the heating temperature.
  • the upper limit of the heating temperature is a temperature below the melting point of the stainless steel.
  • Austenitic stainless steels contain ordinarily more hydrogen than 1 ppm in conventional steelmaking methods.
  • the amount of hydrogen depends on the size of the material upon shipping.
  • Fig. 15 is a graph illustrating the relationship between material size and the amount of hydrogen in the material of Table 1. The graph plots the results of measurements performed by the inventors of the present invention on various materials purchased from material manufacturers.
  • the horizontal axis in the graph represents the size of the material.
  • the size denotes herein the smallest value of the material dimensions upon shipping.
  • the size corresponds to the diameter of 20 mm
  • the size corresponds to the plate thickness of 10 mm
  • the size corresponds to the length of 10 mm.
  • the above heating times denote the time required for hydrogen to diffuse out of a sample. This time depends on the size of the material. The heating time can be estimated through calculations based on the diffusion coefficient of hydrogen in the material at the heating temperature. Samples of a smaller size can be thermally treated in a shorter time, of several hours.
  • Samples of a larger size need a fairly prolonged treatment period, of 2 weeks or more.
  • a heating time longer than the time required for hydrogen to diffuse out of the sample results arguably in no change in the amount of hydrogen, no matter how long the heating time. Therefore, although the effective range of aging treatment time depends on the heating temperature and sample size, it is found that 500 hours or less is a practical range.
  • the aging treatment time is not limited thereto. As Fig. 15 shows, a greater amount of hydrogen is present in a case where there is used a large material, for instance structural materials for power plants and the like, owing to the large material size.
  • an austenitic stainless steel is thermally treated at a temperature of 200°C or higher, in an air atmosphere, to remove non-diffusible hydrogen and diffusible hydrogen that are present in the austenitic stainless steel, thereby making it possible to provide an austenitic stainless steel that is highly resistant to fatigue crack growth.
  • the materials used were the SUS304, SUS316, and SUS316L(A) (hereinafter, simply SUS316L) austenitic stainless steels shown in Table 1.
  • a solution thermal treatment was performed on the SUS304, SUS316, and SUS316L steels used.
  • the shape of the fatigue test piece is shown in Fig. 1A .
  • the surface of the test piece was finished by buffing after polishing with # 2000 emery paper.
  • an artificial microhole 100 ⁇ m in diameter and 100 ⁇ m deep was opened in the center of the fatigue test piece in the lengthwise direction with a drill having a radial tip angle of 120°, to facilitate observation of fatigue crack growth.
  • the artificial microhole was drilled in the center of the test area of the test piece.
  • the test area was a cylindrical portion, approximately 20 mm long, at the center of the test piece.
  • the top and bottom faces of the cylinder were parallel to each other and lay perpendicular to the lengthwise axis of the test piece.
  • Fig. 2 illustrates an outline of the test area and the shape of the drilled artificial microhole. In the case of a hydrogen-charged fatigue test piece, the piece was buffed again immediately after hydrogen charging was over, and the artificial microhole was drilled.
  • the amount of martensite in the test area of the fatigue test piece of an austenitic stainless steel was measured by X-ray diffraction.
  • X-ray diffraction was performed using a miniature X-ray stress measurement apparatus PSPC-RSF/KM manufactured by Rigaku Corporation (Akishima city, Tokyo, Japan).
  • Quantitative analysis was determined from the integrated intensity ratio of the diffraction peaks of the austenitic phase ⁇ 220 ⁇ plane and the martensitic phase ⁇ 211 ⁇ plane, using CrK ⁇ rays.
  • SUS304, SUS316, and SUS316L the content of martensite in the test area before fatigue testing was about 3%.
  • the content of martensite in the hydrogen-charged test areas was also about 3%.
  • the content of martensite was measured in two places before drilling of the artificial microhole.
  • the first measurement region was a circular region 1 mm in diameter centered on the spot at which the artificial microhole was to be drilled.
  • the second measurement region was a region 1 mm in diameter centered on a spot defined by rotating the lengthwise axis of the test piece 180° from the spot where the artificial microhole was to be drilled. In other words, the second measurement region was located on the opposite side of the cylinder from the first measurement region.
  • Hydrogen charging was performed using a cathodic charging method.
  • Hydrogen charging was performed for 672 hours (4 weeks) at a solution temperature of 50°C (323 K), and 336 hours (2 weeks) at a solution temperature of 80°C (353 K).
  • the sulfuric acid solution was replaced once a week to minimize changes in the sulfuric acid concentration resulting from evaporation.
  • the test piece was polished, and then the amount of martensite in the test area after prestraining was measured by X-ray diffraction. The martensite content was 65 to 69% by specific volume in SUS304, and 26 to 28% by specific volume in SUS316L. The amount of martensite was measured at two places before the artificial microhole was drilled. The measurement regions were 1 mm in diameter centered on the spot where the artificial microhole was to be drilled, and a spot defined by rotating the lengthwise axis of the test piece 180° from the spot where the artificial microhole was to be drilled.
  • the cycling frequency was adjusted so that the surface temperature of the test area did not exceed 60°C during the fatigue test. Fatigue cracks were observed using the replica method. The length of the fatigue cracks was measured.
  • replica film acetyl cellulose film
  • the replica film was peel off once it had dried, 2 or 3 minutes after affixing.
  • Gold was vapor- deposited on the recovered replica film, and the fatigue cracks in the test area were observed with a metallurgical microscope.
  • a sample 7 mm in diameter and 0.8 mm thick was cut out from the test area immediately after the end of fatigue testing, was placed in a vacuum chamber, and was heated at a constant heating rate.
  • the vacuum chamber internal pressure was 1 ⁇ 10 -7 to 3 ⁇ 10 -7 Pa before the sample was heated.
  • the temperature was raised up to 800°C at a heating rate of 0.5°C/sec.
  • TDS quadrupole mass analyzer-type thermal desorption spectrometer
  • Fig. 4 is a photograph of fatigue cracks that developed from the artificial microhole drilled in hydrogen-uncharged SUS304 after fatigue testing. The photograph shows that fatigue cracks spread from the artificial microhole. These fatigue cracks develop bilaterally from the artificial microhole, and grow in a roughly symmetrical manner.
  • Figs. 5A-5C show results of X-ray examination of the austenitic phase and martensitic phase in the test area surface before fatigue testing and the fatigue-cracked surface after fatigue testing.
  • the dotted lines in Figs. 5A-5C denote results upon measuring of the surface of the test area before fatigue testing.
  • the solid line denotes results upon measuring fatigue-cracked surface after fatigue testing.
  • Fig. 5A shows the measurement results for SUS304. The measurement indicates that, after fatigue testing, the austenitic phase has decreased and the martensitic phase has increased, as compared with before fatigue testing.
  • Fig. 5B shows the measurement results for SUS316. The measurement indicates that, after fatigue testing, the austenitic phase has decreased a little and the martensitic phase has increased, as compared with before fatigue testing.
  • Fig. 5C shows the measurement results for SUS316L. The measurement indicates that, after fatigue testing, the martensitic phase has increased, as compared with before fatigue testing, although virtually no change in the austenitic phase is seen for SUS316L.
  • Figs. 6A-6C are graphs showing the relationship between the length of the cracks caused by fatigue testing and number of cycles.
  • Fig. 6A shows the results for SUS304, Fig. 6B for SUS316, and Fig. 6C for SUS316L.
  • the measurement results are shown for hydrogen-charged pieces and uncharged pieces for each material (SUS304, SUS316, and SUS316L).
  • the cycling frequency is 1.2 Hz for SUS304 and SUS316, and 5 Hz for SUS316L.
  • the graph indicates that the crack growth rate is accelerated in hydrogen-charged SUS304 and SUS316, as compared with the uncharged material. For example, the number of cycles N until crack length 2a reaches 400 ⁇ m is lower in a hydrogen-charged material than in the uncharged material. In these cases, the fatigue crack growth rate is approximately twice as fast in the hydrogen-charged pieces.
  • the fatigue crack growth rate for SUS316L is slightly higher in the hydrogen-charged material than in the uncharged material, but no significant difference is appreciated.
  • Fig. 7 shows photographs of fatigue cracks in SUS304, SUS316, and SUS316L observed by the replica method.
  • the fatigue cracks grow essentially symmetrically, and therefore the photographs in Fig. 7 depict only one half of the photograph.
  • the photographs reveal that the fatigue cracks in hydrogen-charged material tend to grow more linearly than in uncharged material.
  • the hydrogen-charged material the slip bands occur over a broad region, whereas in the uncharged material the slip bands are localized near the fatigue cracks.
  • Fig. 8 is a graph showing the results of fatigue testing of SUS316L.
  • Fig. 8 shows the fatigue test results of two materials with a hydrogen content of 0.4 wt ppm and 2.6 wt ppm when uncharged, and results after the material with a hydrogen content of 2.6 wt ppm was charged with hydrogen to raise the content thereof to 3.9 wt ppm.
  • the cycling frequency until the fatigue cracks reached a length of 200 ⁇ m was 1.5 Hz. Once the length of the fatigue cracks reached 200 ⁇ m, the cycling frequency was changed from 1.5 Hz to 0.0015 Hz.
  • the fatigue cracks grew in a material with a hydrogen content of 2.6 wt ppm and 3.9 wt ppm.
  • FIG. 9 is a graph showing the results of fatigue testing of SUS316L. This figure shows the fatigue test results of two materials with a hydrogen content of 0.4 wt ppm and 2.6 wt ppm when uncharged, and results after the material with a hydrogen content of 2.6 wt ppm was charged with hydrogen to raise the content thereof to 3.9 wt ppm and 5.1 wt ppm. Two cycling frequencies were used, namely 1.5 Hz and 0.0015 Hz.
  • the graph indicates that fatigue cracks grow in the material having a hydrogen content of 2.6 wt ppm and in the same material charged with hydrogen to a content of 3.9 wt ppm and 5.1 wt ppm.
  • the cycling frequency is a low 0.0015 Hz
  • the fatigue crack growth rate is faster than at a cycling frequency of 1.5 Hz.
  • the graph shows that in the material with a hydrogen content of 0.4 wt ppm the fatigue crack growth rate is slower at both cycling frequencies of 0.0015 Hz and 1.5 Hz. This indicates that fatigue cracks do not grow much when the hydrogen content in the material is 0.4 wt ppm or less.
  • Fig. 10 is a conceptual diagram illustrating a situation wherein diffusible hydrogen and non-diffusible hydrogen diffuse through a transformed martensitic phase.
  • the tip of the fatigue crack in the figure undergoes martensitic transformation, and the diffusible hydrogen and the non-diffusible hydrogen diffuse via the martensitic phase. That is, hydrogen migrates through a route made up of the martensitic phase, which has a high hydrogen diffusion rate, and accumulates at the tip of the fatigue crack.
  • This phenomenon relates to hydrogen diffusion and migration time.
  • the rate of diffusion of the hydrogen in the austenitic phase (FCC) is four orders of magnitude slower than the rate of diffusion in the martensitic phase (BCC).
  • the fatigue crack periphery undergoes martensitic transformation, and the surrounding hydrogen diffuses through this martensitic phase and gathers at the tip of the fatigue crack.
  • the austenitic stainless steel is also called Cr-Ni stainless steel, and is obtained through the addition of Cr and Ni to Fe.
  • the principal components of the austenitic stainless steel are Fe (iron), Cr (chromium), and Ni (nickel), with various additives given in Table 2 below.
  • Table 2 below shows preferred examples of the austenitic stainless steel of the present invention, but the way in which the present invention is embodied is by no means limited thereto.
  • Table 2 Component Composition 1 Composition 2 (weight ratio) (weight ratio) C 0.030 or less 0.08 or less Si 1.00 or less 1.50 or less Mn 2.00 or less 2.00 or less Ni 12.00 to 15.00 8.00 to 27.00 Cr 16.00 to 18.00 13.50 to 26.00 Mo 2.00 to 3.00 or less 3.00 or less Al - 0.35 or less N - 0.50 or less Ti - 2.35 or less V - 0.50 or less B - 0.010 or less H 0.00007 (0.7 ppm) or less 0.00007 (0.7ppm) or less Other Balance Fe and unavoidable impurities Balance Fe and unavoidable impurities Balance Fe and unavoidable impurities
  • Ni is added to Fe to improve corrosion resistance.
  • Ni is added to Fe in combination with Cr to increase corrosion resistance.
  • Ni and Mn are elements for securing nonmagnetic properties after cold rolling.
  • the Ni content must be 10.0 wt% or more to secure nonmagnetic properties after cold rolling.
  • the content of Ni must the adjusted in accordance with the content of Si (silicon) and Mn in such a manner so as to preclude generation of a mechanically-induced martensitic phase of 1 vol% or greater.
  • Mn also has the effect of improving the solid solubility of N (nitrogen).
  • C is an element used for forming strong austenite.
  • C is an effective element for enhancing the strength of stainless steel.
  • Si is added for deacidification and strengthening of the solid solution. Adding only a small amount thereof is preferred since generation of the martensitic phase during cold-working is promoted by the Si content. Nitrogen brings about solution hardening.
  • Mo mobdenum
  • Mo mobdenum
  • Ti titanium
  • B boron
  • A1 aluminum
  • A1 is an element added for deacidification during steelmaking and is effective in precipitation hardening, in a similar manner to Ti.
  • the present invention can also be embodied by adding elements such as Nb, Cu or the like, as needed, in addition to the elements described in Table 2 above.
  • Nb can serve as a substitute for titanium.
  • the austenitic stainless steel wherein the austenitic phase is essentially 100% of the total volume is preferred.
  • the austenitic stainless steel having no martensitic phase is preferred.
  • the average grain size is preferably about 50 ⁇ m or less. In modern materials the average grain size is about 50 ⁇ m, but a smaller the average grain size is preferred.
  • Removal of the diffusible hydrogen and the non-diffusible hydrogen involves performing a thermal treatment on the austenitic stainless steel at a heating temperature of 200°C or higher.
  • the thermal treatment is performed in a vacuum environment.
  • the vacuum environment is 0.2 Pa or less.
  • the austenitic stainless steel is kept under vacuum at the heating temperature for 460 hours or less.
  • the temperature of the thermal treatment is lower than the sensitization temperature, which is the temperature at which carbides of chromium (Cr) in the austenitic stainless steel precipitate due to heating.
  • the upper limit of the heating temperature is 500°C. It becomes possible to remove as a result the non-diffusible hydrogen and the diffusible hydrogen (which are present in the austenitic stainless steel, diffuse via the mechanically-induced martensitic phase brought about by cyclic loading, and build up at crack sites that are under concentrated stress, causing thereby hydrogen embrittlement).
  • a thermal treatment such as the above makes it possible to remove, from the austenitic stainless steel, diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement in the austenitic stainless steel, to lower thereby the hydrogen (H) content in the austenitic stainless steel down to 0.0001 wt% (1.0 wt ppm) or less.
  • the preferred content of hydrogen (H) in the austenitic stainless steel after this thermal treatment is 0.00007 wt% (0.7 wt ppm) or less.
  • a more preferred content of hydrogen (H) in the austenitic stainless steel after this thermal treatment is 0.00002 wt% (0.2 wt ppm) or less, and more preferably 0.000007 wt% (0.07 wt ppm) or less.
  • the experiment was performed on a thermally-treated test piece of SUS316.
  • the test piece was a round bar 7 mm in diameter.
  • a disc 7 mm in diameter and 0.8 mm thick was cut from the round bar.
  • the test piece was thermally-treated at 800°C for 20 minutes.
  • the atmospheres during the experiment were an air atmosphere, a vacuum environment (approximately 0.006 Pa), and an Ar gas atmosphere.
  • the thermal treatment was performed while supplying Ar gas thereto.
  • the heating rate was 0.5°C/second up to 700°C.
  • the escaped hydrogen was measured for heating up to 700°C.
  • Fig. 12 shows the measurement results.
  • the horizontal axis represents the measurement temperature
  • the vertical axis represents the hydrogen release intensity.
  • the hydrogen concentration of the test piece that had not been thermally-treated was 1.5 wt ppm.
  • the hydrogen concentration of the test piece became 0.7 wt ppm when the thermal treatment was performed in air.
  • the hydrogen concentration of the test piece became 0.4 wt ppm when the thermal treatment was performed in a vacuum.
  • the experiment was performed on a thermally-treated test piece of SUH660.
  • the test piece was a round bar 7 mm in diameter.
  • a disc 7 mm in diameter and 0.8 mm thick was cut from the round bar.
  • the test piece was thermally-treated at 720°C for 16 hours.
  • the experimental atmosphere was an air atmosphere and a vacuum environment (approximately 0.006 Pa).
  • the hydrogen concentration of the test piece before the aging treatment was 1.3 wt ppm.
  • the hydrogen concentration of the test piece after the aging treatment was 0.6 wt ppm.
  • the experiment was performed on thermally-treated test pieces of SUS304 and SUS316L.
  • the test pieces were disc-shaped samples having a diameter of 7 mm and a thickness of 0.4 mm.
  • the experiment atmosphere in this thermal treatment was an air atmosphere at approximately 0.1013 MPa.
  • the test pieces were aged by being placed in an air atmosphere at a temperature of 300°C and 450°C, where a thermal treatment was carried out for 2 hours.
  • Figs. 14A and 14B show the measurement results.
  • the horizontal axis represents the measurement temperature
  • the vertical axis represents the hydrogen release intensity.
  • the heating rate for the TDS measurement was 0.5°C/second up to 700°C.
  • the escaped hydrogen was measured for heating up to 600°C.
  • Fig. 14A illustrates the measurement results of a test piece of SUS304.
  • the hydrogen concentration of the test piece before the aging treatment was 2.3 ppm.
  • the hydrogen concentration of the test piece after the aging treatment was 0.19 ppm, in a case where the thermal treatment was performed at a temperature of 300°C, and of 0.07 ppm, in a case where the thermal treatment was performed at a temperature of 450°C.
  • Fig. 14B illustrates the measurement results of a test piece of SUS316L.
  • the hydrogen concentration of the test piece before the aging treatment was 2.6 ppm.
  • the hydrogen concentration of the test piece after the aging treatment was 0.07 ppm, in a case where the thermal treatment was performed at a temperature of 300°C, and of 0.03 ppm, in a case where the thermal treatment was performed at a temperature of 450°C.
  • the present invention is good for use in corrosion resistance and in fields that employ high-pressure hydrogen. More specifically, the present invention is good for use in products that have a concern of hydrogen embrittlement and delayed fracture due to hydrogen penetration, such as metal gaskets, various types of valves used in automobiles, springs, steel belts, cutting blades, fuel cells, as well as materials for valves, springs and the like ancillary to fuel cell systems.
  • the present invention can also be used in building structures, machinery, plant and equipment.

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  • Crystallography & Structural Chemistry (AREA)
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  • Heat Treatment Of Sheet Steel (AREA)
EP09714474.5A 2008-02-29 2009-02-09 Austenitischer edelstahl und wasserstoffentfernungsverfahren dafür Withdrawn EP2248918A4 (de)

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DE102010053385A1 (de) * 2010-12-03 2012-06-21 Bayerische Motoren Werke Aktiengesellschaft Austenitischer Stahl für die Wasserstofftechnik
EP3017072A4 (de) * 2013-07-05 2017-08-02 Outokumpu Oyj Rostfreier stahl mit beständigkeit gegenüber verzögerter rissbildung und verfahren zu dessen herstellung

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CN107741449B (zh) * 2017-09-14 2019-12-13 浙江大学 奥氏体不锈钢中马氏体体积分数的测试装置
JP2019196918A (ja) * 2018-05-07 2019-11-14 日本電信電話株式会社 鋼材破断起点推定方法、鋼材破断起点推定装置及び鋼材破断起点推定プログラム
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KR102202390B1 (ko) * 2018-12-13 2021-01-13 한국표준과학연구원 가스 처리로부터 개질된 내수소취화 스테인레스 분말결합체 및 이를 위한 스테인레스 분말
KR102597735B1 (ko) 2019-03-26 2023-11-02 제이에프이 스틸 가부시키가이샤 페라이트계 스테인리스 강판 및 그 제조 방법
KR102290782B1 (ko) 2019-09-26 2021-08-18 주식회사 포스코 내구성이 우수한 고강도 코팅강판의 제조방법
CN112666066B (zh) * 2020-12-15 2022-11-11 中国石油大学(华东) 基于氢扩散动力学的管道氢脆温度阈值预测方法和应用
WO2024043080A1 (ja) * 2022-08-24 2024-02-29 日鉄ステンレス株式会社 オーステナイト系ステンレス鋼

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EP3017072A4 (de) * 2013-07-05 2017-08-02 Outokumpu Oyj Rostfreier stahl mit beständigkeit gegenüber verzögerter rissbildung und verfahren zu dessen herstellung
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