GB2228013A - An austenitic steel with improved resistance to neutron-induced swelling and helium embrittlement - Google Patents

Description

An austenitic steel with improved resistance to neutron-induced swelling

and helium embrittlement The invention relates to an austenitic steel with improved resistance to neutron-induced swelling and helium embrittlement.

The structural core materials of atomic reactors, more especially of highspeed breeder reactors, are inclined to swell because of the influence of neutron radiation. In addition, helium is incorporated in the material, and such diffuses to the grain boundaries and leads to the embrittlement of the material.

German Offenlegungsschrift 3011432 discloses an alloy of iron, nickel and chromium with an improved resistance to swelling, its composition being shown in Table 1.

Table I

Composition of the Alloy according to German Offenlegungsschrift 3011432 Percentage range Preferred percentage 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 Aluminium 0.2 - 0.6 0.3 Carbon 0.02 - 0.1 0.03 Boron 0.002- 0.015 0.005 Manganese 0.05 - 0.4 0.2 Iron Remainder Remainder 1 Gamma-dash-W) or Gamma-double dash-(J11) phases are contained in this alloy. It has been shown that these phases are not stable under irradiation. The ductility of these materials is insufficient after the irradiation.

The publication of a conference report by K.Ehrlich and K. Anderko IAEASM-284/17, Lyons (France), 22-26 July 1985 discloses an alloy under the name B801, which has the composition given in Table II.

Table 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 has a favourable swelling behaviour but no mention is made of its ductility after irradiation.

1 1 The object of the invention is to provide an austenitic steel which has a high swelling resistance as well as a high-ductility after irradiation. The steel is to be particularly suitable for component parts in reactor cores of high-speed breeder reactors and should have a high long-term stability.

According to the invention, the object is achieved by an austenitic steel having the composition given in Table III.

Table III

Composition of the steel according to the invention Percentage range Preferred percentage 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 Niobium 0.0 - 0.4 0.2 - 0.3 Titanium 0.3 - 0.6 0.3 - 0.4 Aluminium 0.1 - 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 0.01 0.05 Boron 0.004 0.008 0.006 0.007 Iron Remainder Remainder The following are essential for the good properties of the steel according to the invention: The relatively high nickel content; a chromium content which is smaller, by a factor of 2.3 to 2.7, than the nickel content; a finely-dispersive separation of stable carbo-nitrides; and a grain boundary strengthening as a result of borides with the addition of zirconium.

The steel according to the invention is not inclined to swell under neutron- irradiation. At the same time, its ductility after irradiation is not substantially reduced.

The high ductility is achieved because helium, which is produced from & radiation in the material, is retained at the place of its origin and cannot diffuse to the grain boundaries. The finely-dispersive separation of nitride, carbide and/or carbo-nitride phases is responsible for this effect, whereby a plurality of lattice imperfections are produced where helium is fixed.

The elements boron and zirconium are added in order to stabilise the grain boundary separations. They form a second safety means for counteracting the embrittling effect of helium in the material.

During its processingg the steel according to the invention has to be subjected to at least one final coldforming step.

The processing is preferably concluded by a final cold-forming process of 13 to 16%. The resistance to swelling is further increased hereby.

1 - R- It has been shown that the above-mentioned, finelydispersive carbonitride phases are achieved, more especially by the admixture of titanium and vanadium and silicon.

This effect is further incr-eased by the admixture of niobium and, more especially, aluminium and zirconium. because of the simultaneous admixture of zirconium and boron, the grain boundary separations do not agglomerate during the processing of the material.

The invention is explained more fully hereinafter with reference to examples.

Example 1

A charge of 20 kg was smelted in a vacuum-induction furnace.

The individual elements were mixed in elementary form; nitrogen and carbon were added as iron nitrides and carbides.

The charge was subsequently re-smelted in a vacuumarc furnace and then initially rough-forged at approx. 11500C and then forged-out at 10000C. The material was homogenised for 1 hour at 11000C. The steps of turning and smoothing as well as a test for cracks were then effected.

1 An analysis produced the following composition:

Fe Mo Nb Mn c 55.1 % Ni 29.5 Cr 11.2 1.9 % v 0.63 Si 0.54 0.34 % Ti 0.31 % AI 0.16 0.1 % Zr 0.041 % N 0.025 0.022 % p 0.014 % B 0.006 Example 2

A combustion rod encasing tube having an external diameter of 6 mm and a wall thickness of 0.38 mm was produced from the material according to Example 1 by the following method steps:

cold-forming of approx. 50% with subsequent recrystallization at 10750C/3 min. in an approx. 10-times alternating cycle up to the preliminary dimension; a penultimate cold-forming operation of 50%; a final recrystallization at 9500 C/30 min.; a final cold-forming operation of 13 to 16%.

Example 3

Small discs having a diameter of 3 mm and a thickness of 0.18 mm were produced according to Example 2 and irradiated in a 'Wariable Energy Cyclotron" (VEC). Initially, approx. 17.5 appm helium were pre-implanted, in order to simulate the He transmutation during a neutronirradiation process. Subsequently, irradiation was carried out at 5750C with 66 MeVNi6+ ions up to a dose of approx.

1 1 dpa + 10% (=64 dpa NRT). Since the layer of maximum damage is located at a depth of approx. 3.5 jum during the technique which is used, the layer located thereabove was removed by vibration-polishing. For the investigations in the transmission electron microscope, the samples were thinned by the "back- thinning" process. The photos from the regions with pores were evaluated according to pore diameter and concentration and the volume swelling was calculated in %. The maximum value for the alloy according to example 1 was 0.2%.

Example 4

Flat tensile samples with a dimension of the measuring length of 25 x 4 x 0.5 were produced according to Example 2 and irradiated in a sodiumfilled irradiation capsule at a temperature of T = 6500C up to a neutron dose of approx. 1022 D/CM2, whereby approx. 68 appm He were accumulated. After the irradiation process, the tensile samples were examined in hot cells in tensile and creep tests. The breaking tension, measured at a testing temperature of 7000C, was 16.8% in the tensile test A and 11% in the creep test A (for a service life of approx. 2000 hours).

1

Claims (3)

1. An austenitic neutron-induced swelling 26 - 33 11 - 14 1.7 - 2.1 0.4 0.8 0.4 - 0.8 0 - 0.4 0.3 - 0.6 0.1 - 0.2 maximum 0.5 0.03 0.02 0.02 0.01 0.004 0.05 % 0.08 % 0.06 % 0.06 % 0.008% steel with imDroved resistance to comprising Nickel; Chromium; Molybdenum; Vanadium; Silicon; Niobium; Titanium; Aluminium; Manganese; Zirconium; Nitrogen; Carbon; Phosphorus; Boron; and remainder: Iron with incidental impurities.

2. An comprising 29.5 11.0 1.8 0.6 0.5 0.2 0.3 austenitic steel as claimed 30.5 % Nickel; 12.0 % Chromium; 2.0 % Molybdenum; 0.7 % Vanadium; 0.7 % Silicon; 0.3 % Niobium; 0.4 % Titanium; in claim 1 1; 1 0 remainder:

maximum 0.2 0.3 % Aluminium; % Manganese; 0.03 0.05 % Zirconium; 0.02 0.08 % Nitrogen; 0.02 - 0.04 % Carbon; 0.01 - 0.05 % Phosphorus; 0.006 0.007% Boron; and Iron with incidental impurities

3. An austenitic steel with improved resistance to neutron-induced swelling, substantially as hereinbefore described and exemplified.

Publ shed 1990a.ThE;Pa:tentOffice- State House.66 71 High Holborn. London WC1R4TP Further copies may be obtained from The Patent Office Sales Branrh. St Mary Cray. Orpington. Kent BR5 3RD Printed by Multiplex techniques ltd. St Mary Cray. Kent, Con. 1'87