FR2642437A1 - AUSTENITIC STEEL HAVING IMPROVED RESISTANCE TO NEUTRON-INDUCED SWELLING AND HELIUM FRAGILIZATION - Google Patents

Abstract

The aim of the invention is to provide an austenitic steel which simultaneously exhibits high resistance to swelling and high ductility after irradiation. This steel must be suitable in particular for the manufacture of constituent elements of reactors for fast breeder reactors and must have great stability over time. This problem is solved by an austenitic steel with improved resistance to neutron-induced swelling, which consists of: 26 - 33% Nickel 11 - 14% Chromium 1.7 - 2.1% Molybdenum 0.4 - 0.8% Vanadium 0.4 - 0.8% Silicon 0 - 0.4% Niobium 0.3 - 0.6% Titanium 0.1 - 0.2% Aluminum maximum 0.5% Manganese 0.03 - 0.05% Zirconium 0.02 - 0.08% Nitrogen 0.02 - 0.06% Carbon 0.01 - 0.06% Phosphorus 0.004 - 0.008% Boreet the remainder being iron with possible impurities.

Description

"Austenitic steel with improved strength against induced swelling

  by neutrons and embrittlement by helium "

Description

  The invention relates to austenitic steel with improved resistance to induced swelling

  by neutrons and embrittlement by helium.

  The nuclear structure materials of nuclear reactors, particularly fast breeders, tend to swell under the action of neutron radiation. In addition, helium is inserted into the material, diffuses at the grain boundaries

  and leads to embrittlement of the material.

  German Offenlegungsschrift No. 30 11 432 discloses an iron-nickel-chromium alloy with improved resistance to swelling, the composition of which is given in US Pat.

Table I.

T A B L E A U I

  Composition of the alloy according to German Offenlegungsschrift No. 30 11 432.

  Range of preferred composition content in% in% Nickel 33 - 39.4 37 Chromium 7.5 - 16 12 Niobium 1.5 - 4 2.9 Silicon 0.1 - 0.7 0.2 Zirconium 0.01 - 0, 2 0.05 Titanium 1 - 3 1.75 Aluminum 0.2 - 0.6 0.3 Carbon 0.02 - 0.1, 0.03 Bore 0.002 - 0.015 0.005 Manganese 0.05 - 0.4 0.2 Iron Rest Rest This alloy has gamma-prime (t ') or gamma-second (y ") phases, but it has been found that these phases are not stable to radiation, the ductility of these materials after irradiation is not By a publication of a meeting report by K. Ehrlich and K. Anderko, IAEASM-284/17, Lyon, France, 22-26 July 1985, an alloy known as B 801 is known to have been the composition

given in Table II.

T A B L E A U II

  Composition of the alloy according to IAEA-SM-284/17.

  Carbon 0.01% Chromium 11.1% Nickel 30.5% Molybdenum 2.0% Vanadium 0.7% Silicon 0.6% Manganese 0.39% Boron 0.0055% Nitrogen 0.11%

  This alloy exhibits a favorable behavior

  swelling, but there is no information on

its ductility after irradiation.

  The object of the invention is to provide an austenitic steel which exhibits, together with a high resistance to swelling, high ductility after irradiation. This steel must in particular be suitable for the manufacture of constituent elements of the reactor core of fast breeders and present a

long lasting stability.

  This problem is solved according to the invention thanks to an austenitic steel which has the composition

shown in Table III.

T A B L E A U III

  Composition of the steel of the invention

  Range Preferred composition content in% in% Nickel 26 - 33 29.5 30.5 Chromium 11 - 14 11 - 12 Molybdenum 1.7 - 2.1 1.8 - 2.0 Vanadium 0.4 0.8 0, 6 - 0.7 Silicon 0.4 - 0.8 0.5 - 0.7 Niobiumr 0.0 - 0.4 0.2 - 0.3 Titanium 0.3 - 0.6 0.3 - 0.4 Aluminum 0 ,. - 0.2 0.1 - 0.2 Manganese maximum 0.5 maximum 0.3 Zirconium 0.03 - 0.05 0.03 - 0.05 Nitrogen 0.02 - 0.08 0.02 - 0.08 Carbon 0,02 - 0,06 0,02 - 0,04 Phosphorus 0,01 - 0,06 0,0_ - 0,05 Boron 0,004 - 0,008 0,006 - 0,007 Iron Remainder - Rest To obtain the advantageous properties of

  the steel according to the invention, it is essential that

  It has a relatively high nickel content and a chromium content 2.3 to 2.7 lower than the nickel content, and exhibits finely dispersed precipitation of stable carbonitrides and reinforced grain boundaries by borures with

addition of zirconium.

  The steel according to the invention does not tend to

  swell under the effect of neutron irradiation.

  Simultaneously, its ductility after irradiation is

  not significantly reduced.

  The high ductility value is obtained in that the helium formed in the material from the α-radiation is maintained at the place of its formation and can not diffuse towards the grain boundaries. This effect is a consequence of the finely dispersed precipitation of nitride, carbide and / or carbonitride phases, thus creating a large number of

  defects on which helium is attached.

  Boron and zirconium elements are added to stabilize precipitation at grain boundaries. This addition corresponds to a second security measure with respect to the weakening effect of

helium in the material.

  The steel according to the invention must be subjected, during its processing, to at least one

  final stage of cold deformation.

  Preferably, we finish the transformation

  by a last cold deformation of 13 to 16%.

  This further improves the swelling resistance. It has been found that the above-mentioned finely dispersed carbonitride phases are obtained primarily by the addition of titanium,

vanadium and silicon.

  The addition of niobium and especially aluminum

  and zirconium can further enhance this effect.

  The simultaneous addition of zirconium and boron makes it possible to prevent the agglomeration of precipitation at

  grain boundaries when machining the material.

  The invention will be described in more detail with the aid of the following precipitation examples:

Example i

  A 20 kg load was melted in a vacuum induction furnace. Each element was added in elementary form; we added nitrogen and carbon

  in the form of nitride and iron carbide.

  The charge was then recast in a vacuum arc furnace, and then pre-forged at about 1150 ° C and then forged at 1000 ° C. The material was homogenized for 1 hour at 1100C. Then, machining and finishing machining processes were carried out, as well as

that a crack controlE.

  The following composition was determined by analysis: Fe 55.1% Ni 29.5% Cr 11.2% Mo 1.9% V 0.63% Si 0.54% Nb C, 34% Ti 0.31% Al 0.16% Mn 0, 1% Zr 0.041% N 0.025%

C 0.022% P 0.014% B 0.006%

Example 2

  Using the material of Example I, a fuel rod with an outside diameter of 6 mm and a wall thickness of 0.38 mm was produced by carrying out the following process steps: cold deformation about 50% followed by recrystallization at 1075 C / 3min in about 10 cycles to roughing dimensions, - before last cold deformation of 50%, - last recrystallization at 950 ° C / 30 min,

  - last cold deformation of 13 to 60%.

Example 3

  Analogously to the method of Example 2, small discs with a diameter of 3 mm and a thickness of 0.18 mm were made and irradiated in a "Variable Energy Cyclotron" (VEG). Probably about 17.5 pm of helium were first incorporated, so as to simulate the transmutation of helium during neutron irradiation. Then, a bombardment at 575 ° C. with Ni6 + ions of 66 MeV was carried out until a dose of approximately 70 dpa 10% (= 64 dpa NRT). As in the technique used, the most damaged layer is at a depth of about 3.5 μm, the upper layer was removed by vibrating polishing. To study the

  electron microscope samples by trans-

  mission, they were thinned using the "back thinning" process. The pore diameter and pore concentration of the pore area photos were evaluated and the% volume swell was calculated. The value for the alloy of Example 1 was

maximum of 0.2%.

Example 4

  Example 2 stamped specimens were prepared with the following dimensions: x 4 x 0.5, and bombarded after placing them in a sodium-filled irradiation capsule at a temperature of T = 650 ° C. , up to a neutron dose of about 1022n / cm2 which allowed to accumulate about 68 helium app. After irradiation, the specimens were tested in hot cells from the point of view of traction and creep. The elongation at break measured at a test temperature of 700 C was A = 16.8% in the tensile test and A = 11% in the creep test (for a duration of 2000 hours test). 8-

Claims (2)

R E V E N D I C AT IO N-S   1) Austenitic steel with improved resistance to neutron induced swelling, characterized in that it consists of: 26 - 33% nickel 11 - 14% chromium 1.7 - 2.1% Molybdenum 0.4 - 0.8% Vanadium 0.4 - - 0.8% Silicon 0 - 0.4% Niobium 0.3 - 0.6% Titanium 0.1 - 0.2% Aluminum at most 0.5% Manganese 0.03 - 0.05% Zirconium 0.02 - 0.08% Nitrogen 0.02 - 0.06% Carbon 0.01 0.06% Phosphorus 0.004 - 0.008% boron   and the rest being iron with possible impurities tual.   2 ") Austenitic steel according to the   cation 1, characterized in that it consists of: 29.5 - 30.5% of nickel 11.0 - 12.0% of chromium 1.8 - 2.0% of molybdenum 0.6 - 0.7 % Vanadium 0.5 - 0.7% Silicon 0.2 - 0.3% Niobium 0.3 - 0.4% Titanium 0.1 - 0.2% Aluminum at most 0.3% of Manganese 0.03 - 0.05% of Zirconium