US3669759A - Thermomechanical treatment for improving ductility of carbide-stabilized austenite stainless steel - Google Patents

Thermomechanical treatment for improving ductility of carbide-stabilized austenite stainless steel Download PDF

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US3669759A
US3669759A US860683A US3669759DA US3669759A US 3669759 A US3669759 A US 3669759A US 860683 A US860683 A US 860683A US 3669759D A US3669759D A US 3669759DA US 3669759 A US3669759 A US 3669759A
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steel
carbide
treatment
stainless steel
cold
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Tatsuo Kondo
Hajime Nakajima
Ryukichi Nagasaki
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Japan Atomic Energy Agency
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Japan Atomic Energy Research Institute
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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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys

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  • This invention relates to treatment of a class of austenite stainless steels of stabilized carbide type, that is, steels of AISI Type 321, 347, etc., of the AISI 300 series, in which a steel is first subjected to solution heat treatment, coldworked, and then subjected to a further heat treatment for precipitating carbides and making the matrix recrystallize.
  • the treatment diminishes brittleness, a defect in steels of this type, and provides new stainless steel materials which have high temperature tensile strength 20% or more higher than that of the conventional stainless steels, and elongation 30% or more greater than that of the conventional stainless steels.
  • the carbide-stabilized austenite stainless steel is, after solution treatment, subjected to the treatments to convert the dissolved carbon to carbide precipitates and further to coarsen the precipitated carbides. Because, in the austenite stainless steel in use for nuclear reactors, it has been believed that fine carbide precipitates tend to redissolve in the matrix by irradiation of neutrons, and it makes the matrix unstable; the aim has been to prevent redissolution of the carbide precipitates particles by coarsening them. However, in such a steel, reduction in ductility is remarkable because of the existence of the coarse carbides. Especially, this defect is fatal in steel containing titanium, and makes it less attractive as a reactor material.
  • the purpose of this invention is to provide a new class of carbide-stabilized austenite stainless steels having a tensile strength 20% or more higher than that of the prior art stainless steel, elongation 30% or more higher than that of the prior art stainless steel and structural stability equivalent to or superior to that of the prior art steel by preventing reduction in ductility by a specific combination of degree of working and time and temperature of heat treatment.
  • Brittleness that has been regarded as the most serious defect of the carbide-stabilized austenite stainless steels that is, steels of AISI Type 321, 347 etc. of the AISI 300 series, can now be diminished by a combination of cold working and heat treatment which comprises solution treatment, cold working, heat treatment for precipitation of carbides and recrystallization of the matrix.
  • This invention has relation to the steel-making industry, especially manufacture of stainless steel tubes, manufacture of nuclear reactor materials and manufacture of nuclear fuel elements.
  • the stainless steel of stabilized carbide type has been widely employed as the material usable at high temperatures in the chemical industry, since steel of this type has structural stability superior to that of the other stainless steels.
  • this class of stainless steels has a fatal defect in that its mechanical properties, especially ductility are impaired by the alloying elements (Ti, Nb, etc.) added for stabilizing the carbides.
  • carbide precipitates become more finely and more uniformly dispersed, and thus mechanical properties of the steel are remarkably improved.
  • dislocations are well removed in its microstructure, and it is in the so-called annealed state, residual strain caused by the cold working does not remain, and therefore, it is less susceptible to sigma brittleness than the prior art steel.
  • the carbide-stabilized austenite stainless steel in accordance with this invention is superior to the prior art steel of the same composition with respect to mechanical properties and micro-structural stability at high temperatures.
  • FIG. 1 (a) through (f) show the influence of cold working in the treatment of this invention.
  • FIG. 2 shows the relation between time and temperature in the heat treatment in accordance of this invention.
  • FIG. 3 shows the relation between conditions of the treatment and the obtained mechanical property (elongation FI G. 4 shows the relation between degree of working and time of heat treatment and the obtained mechanical properties.
  • FIG. 5 shows the post-irradiation mechanical properties of a stainless steel treated in accordance with thisinvention.
  • EXAMPLE 1 Specimens 95 x 15 x l2 mm. in dimension (thickness is selected by considering degree of cold working to undergo) are made of a heat of steel the analysis of which is as follows.
  • the specimens were subjected to solution heat treatment (heated at 1100 C. for minutes and quenched in water), and thereafter they were cold-rolled respectively by 10%, 30%, 50% and 70% reduction.
  • the coldworked specimens were then, for the purpose of stabilizing carbide (precipitation of the carbide) and recrystallization, heated at 790 C., 810 C., 820 C., 850 C., 900 C. and 950 C. for 1 minute to 192 hours, and were quenched in water.
  • the effects of the treatments were checked by means of electron microscope observation of carbide extraction replicas and thin foil of each specimen, X-ray difi'raction, electron diifraction, optical microscope observation and measurement of Vickers hardness (H FIG.
  • FIG. 2 The relation between time and temperature of the treatment for precipitation and recrystallization and microstructures that were identified by the above-mentioned tests is shown in FIG. 2 as an example.
  • the uppermost oblique (broken) line (1) represents the conditions under which recrystallization takes place
  • the next oblique (solid) line (2) represents the conditions under which the Vickers hardness reaches 230.
  • the third (broken) line (3) shows the conditions for completion of recrystallzation.
  • the area below the line of completion of recrystallization and right of the vertical straight broken line for 850 C. stands for an area in which carbide of chromium is stable.
  • the area left of the same vertical line is an area in which carbides of Ti and Nb are stable.
  • the area left of the vertical line for 950 C. is an area in which carbides dissolve.
  • the steel which exhibits excellent properties in accordance with this invention must be in the area in which carbides of Ti and Nb are stable, and that must be in the area on the upper side of the line (4) indicated as maximum period for heat treatment in the figure, since if the heat treatment is prolonged, excellency in the mechanical properties is lost.
  • the point at which the line of maximum period for heat treatment and the line for 850 C. intersect represents 4 hours.
  • FIG. 3 shows the relation between conditions of the heat treatment and elongation, one of the mechanical properties concerned in this invention.
  • Curve A tells that the steel which underwent 30% cold working and heat treatment at 850 C. shows remarkable change in its mechanical property (elongation) in the range 2-4 hours when heat-treated at 850 C.
  • Curves B and C show a similar relation with respect to 30% and 50% working respectively at 900 C. By comparing Curves A, B and C, it is understood that as degree of working and temperature of heat treatment increase, the mechanical properties improve.
  • FIG. 4 are shown the results of the high temperature tensile test at 650 C. of the specimens which was subjected to precipitation-recrystallization treatment at 850 C. after 10%, 30% and 50% working in comparison with the results with respect to the prior art treatment.
  • the prior art method no mechanical working is applied beforethe stabilization treatment, or if it is applied, the carbide stabilization treatment is carried out over a prolonged time (24 hours for instance), and therefore, strength and ductility decrease.
  • Example 3 The same specimens and a set of the specimens explained in the Example 3 which underwent solution heat treatment, 50% cold working, and precipitation-recrystallization treatment at 850 C. for 3 hours were irradiated with fast neutrons of 2 10 nvt. in an irradiation hole (VT-1) of the Japan Research Reactor No. 2 (JRR- 2) at the Tokai Establishment of Japan Atomic Energy Research Institute and then were subjected to tensile test at 750 C. The results are shown in FIG. 5.
  • (A) is a curve standing for a 16Cr-l0NilMo-Ti specimen (the above-mentioned steel) which underwent 50% cold working, and heat treatment at 850 C. for 3 hours;
  • (B) is a curve standing for a 16Cr-10NilMo-Nb specimen which underwent the same treatment;
  • (C) is a similar cunve for Steel A151 347.
  • Curve (D) stands forthe steel having the same composition as that of (A) which has been heat-treated at 850 C. for 24 hours without being subjected to cold working.
  • Curve (E) stands for A181 316 steel which simply underwent solution heat treatment. It is apparent that the steel materials of this invention have remarkably increased resistance to fracture.
  • EXAMPLE 2 The same steel as of Example 1 was subjected to solution heat treatment and was cold-rolled to 30% reduction. The specimen showed the following mechanical properties at 650 C.:
  • the method of this invention is applied to steel species of the so-called carbide-stabilized type such as AISI 321, 347 and 348, that is, a class of steels the composition of which is in the range shown in Table 1.
  • the steel in order to prevent formation of acicular precipitates, the steel must be worked to not less than 15% as explained hereinbefore. Though there is no upper limit for the degree of working, the degree is practically limited by working conditions. The preferred working degree is between 30% and 50% for all steels of the class.
  • Temperature and time of precipitation-recrystallimtion treatment are defined as '850-950 C. and less than 4 hours respectively as seen in FIG. 2. At temperatures in excess of 950 C., carbides tend to dissolve in the matrix, or if they precipitate, they grow coarse. Temperatures lower than 850 C. are not only impractical because of the prolonged treatment time required but also undesirable because precipitation of carbide of chromium takes place in preference to that of TiC, Nb'C, etc. Practical conditions being considered, the preferred precipitation-recrystallization treatment time is 900925 C. for all steels of the class.
  • Time required for precipitation-recrystallization treatment depends on treatment temperature, composition of steel, and degree of working. In any case, the satisfactory results are obtained when the heat treatment is finished within 4 hours.
  • the time required for the treatment is 25 minutes to 3 hours. If it is coldworked by about 30% and heat-treated at 925 C., the time required for the treatment is 15 minutes to 2 hours. If it is cold-worked by about 50% and heat-treated at 900 C., the time required for the treatment is 3 minutes to 25 minutes. If it is cold-worked by about 50% and heat-treated at 925 C., the time required for the treatment is 2 minutes to 25 minutes.
  • the time required for the treatment is 30 minutes to 3 /2 hours. If it is cold-worked by about 30% and heattreated at 925 C., the time required for the treatment is 20 minutes to 2 /2 hours. If it is cold-worked by about 50% and heat-treated at 900 C., the time required for the treatment is 4 minutes to 30 minutes. If it is coldworked by about 50% and heat-treated at 900 C., the time required for the treatment is 4 minutes to 30 minutes. If it is cold-worked by about 50% and heat-treated at 925 C., the time required for the treatment is 3 minutes to 25 minutes.
  • the annealing for removing the work hardening is carried out at a temperature between 850 C. and 950 C., preferably at the lowest temperature in this range.
  • a steel of AISI 321 is cold worked by about 30% and is annealed at 850 C., the time required for the annealing is not more than 7 minutes. If the steel is cold-worked by about 50% and annealed at 850 C., 3 minutes will suffice.
  • Ihe stainless steel material of this invention is primarily intended to be used as the nuclear reactor material such as cladding of fuel elements, but it is not restricted to such use only. If a high frequency electric heating, for instance, is utilized, stainless steel material even of considerable size can be rapidly heat-treated with ease as is well known to those who skilled in the art.
  • a process for manufacturing a carbide-stabilized austenite stainless steel which retains ductility at high temperature characterized in that carbide precipitates of the steel are stabilized in the form of fine particles comprising subjecting a carbide-stabilized austenite stainless steel to solution heat treatment, cold-working said steel not less than 15%, and heat-treating said steel at a temperaure between 850 C. and 950 C. for less than four hours.
  • a process for manufacturing a carbide-stabilized austenite stainless steel which retains ductility at high temperature characterized in that after a carbide-stabilized austenite stainless steel is subjected to solution treatment; said steel is cold-worked by not less than 15% of the degree of working, said worked steel is annealed at a ternperature between 850 C. and 950 C. until not more than 50% of the hardening by the working is removed, said cold working is repeated, again said steel is annealed at a temperature between 850 C. and 950 C. for less than 4 hours, and the 'above-mentioned working and annealing steps, are repeated at least once.
  • the treated steel is a steel of AISI Type 321, the steel is coldworked by about and annealed at about 850 C. for not more than 7 minutes.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
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US860683A 1968-09-27 1969-09-24 Thermomechanical treatment for improving ductility of carbide-stabilized austenite stainless steel Expired - Lifetime US3669759A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3740274A (en) * 1972-04-20 1973-06-19 Atomic Energy Commission High post-irradiation ductility process
US3917493A (en) * 1973-08-13 1975-11-04 Nippon Kokan Kk Austenitic heat resisting steel
US4336079A (en) * 1979-10-09 1982-06-22 Combustion Engineering, Inc. Stabilization of carbon in austenitic alloy tubing
CN113430344A (zh) * 2021-05-06 2021-09-24 上海大学 提高321不锈钢抗晶间腐蚀性能的晶界工程工艺方法

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SE452028B (sv) * 1982-04-30 1987-11-09 Skf Steel Eng Ab Anvendning av ror framstellda av kolstal eller laglegerat stal i sur, svavelvetehaltig miljo
SE451602B (sv) * 1982-08-18 1987-10-19 Skf Steel Eng Ab Anvendning av stang framstelld av kolstal eller laglegerat stal i sur, svavelvetehaltig miljo

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3740274A (en) * 1972-04-20 1973-06-19 Atomic Energy Commission High post-irradiation ductility process
US3917493A (en) * 1973-08-13 1975-11-04 Nippon Kokan Kk Austenitic heat resisting steel
US4336079A (en) * 1979-10-09 1982-06-22 Combustion Engineering, Inc. Stabilization of carbon in austenitic alloy tubing
CN113430344A (zh) * 2021-05-06 2021-09-24 上海大学 提高321不锈钢抗晶间腐蚀性能的晶界工程工艺方法

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