IE47383B1 - Nickel-based alloy for nuclear power station - Google Patents

Abstract

The alloy consists by weight, of 1.4 to 2.5% of carbon, 0 to 2% of silicon, 25 to 33% of chromium and 6 to 15% of molybdenum, the balance consisting of nickel and unavoidable impurities, the total amount of cobalt, tungsten and boron in the unavoidable impurities being preferably less than 0.2%, based on the weight of the alloy. The alloy is suitable for components such as stopcocks, valves etc. in nuclear power stations. The alloy has comparable mechanical properties to the cobalt-based alloys conventional for such purposes, but is less susceptible to activation by fission or fusion products.

Description

The invention relates to a nickel-based alloy containing chromium and molybdenum, which is suitable for the manufacture of moulded components and for the hardsurfacing and coating of components, in particular of . components used in the construction of nuclear reactors.

The increasing requirements in respect of the security of personnel working in power stations which utilise nuclear fission or fusion phenomena have led the constructors of the power stations to emoloy materials 1θ· which contain increasingly less of elements which can readily be activated by the fission of fusion products.

Such power stations comprise numerous components (such as stopcocks, valves and the like) which are subjected to friction phenomena and require, in addition to low . coefficients of friction, good resistance to corrosion, abrasion and scoring. Such good friction properties are generally achieved by means of a surface coating of a hard metal or hard alloy which is deposited as a continuous layer and then machined.

, Usually, a cobalt-based alloy is used for such a surface coating because cobalt, in the pure state or as a component of metal phases of alloys, assists in achieving a low coefficient of friction and good resistance to wear if the metal phases are of sufficient hardness.

. The most commonly used cobalt-based alloy consists of 1% of carbon, 1% of silicon, 4.5% of tungsten and 28.5% of chromium, the remainder consisting of cobalt and unavoidable impurities, but other cobalt-based alloys of similar composition are known for this purpose.

. Typical such alloys are those marketed under the trade mark Haynes Stellite Alloy Nos. 6, 12 and 21.

Such cobalt-based alloys contain high proportions of elements, such as tungsten, boron and, of course, cobalt which can be activated by products of primary . fission or fusion reactions, in particular by neutrons.

This may be harmful to the safety of personnel working in the power stations, either because the alloy coatings become contaminated or because some of the activated products thereof become detached from the coatings as . a result of abrasive wear or corrosion, and entrained by fluids in contact with the coated components.

Various attempts have been made to find replacements for cobalt-based alloys which are less easy to activate in a nuclear reactor environment. In particular, an alloy . containing substantially 60% of nickel, 15% of cobalt, 15% of chromium and 10% of boron has been proposed (see, for example, Welding Research Supplement, October 1976 by E. Lugscheider, 0. Knotek and K.H.Battenfeld), which alloy has a hardness-temperature curve and . mechanical properties which are comparable to those of the alloys of the stellite No. 6 type.

There have also been proposed (Metallurgical Reports of the CRM, No. 49 of December 1966) alloys in which part of the cobalt is replaced by molybdenum, which alloys have . better resistance to abrasion in an aqueous medium whilst still having sufficient hardness. If such alloys are used for the coating of components subjected to friction in nuclear reactors, their capacity to become activated is of course less than those containing more cobalt.

. However, it is desirable to develop alloys of less susceptibility to radioactive activation, which would enable use thereof in nuclear power stations less hazardous than the use of conventional alloys.

We have now developed a nickel-based alloy which has a 5. structure, and mechanical properties, in the body of the material and at the surface, which are close to those of conventional cobalt-based alloys, but which is less susceptible to activation by fission or fusion products.

According to the invention, there is provided a nickel10. based alloy which consists, by weight, of 1.5 to 2.5% of carbon, 0 to 2% of silicon, 25 to 33% of chromium and 6 to 15% of molybdenum, the balance consisting of nickel and unavoidable impurities.

The total amount of cobalt, tungsten and boron in the 15. unavoidable impurities in the alloy according to the invention is preferably less than 0.2% by weight, based on the weight of the alloy.

The alloys according to the invention can be produced easily, for example, in a forced-draught induction . furnace. They can be used as indicated above, as components of nuclear power plant, in applications where cobalt-based alloys of the Haynes-Stellite type, as mentioned above, have hitherto been used.

The alloys according to the invention may be used to form 5. coatings, the coatings being applied using the alloy in powdered form or in the form of bare, coated or cored wire. The alloy may also be used to form a welding rod. Coatings formed from the alloys according to the invention may have good homogeneity and a good metallurgical bond . to the substrate to which they are applied.

In order that the present invention may be more fully understood, the following Example, in which reference will be made to the accompanying drawings, are given by way of illustration only. In the Example, all parts and . percentages are by weight.

EXAMPLE A melt was produced in a forced-draught induction furnace from the constituent elements, so as to give an alloy having the following approximate composition: 1.70% of carbon, 1% of silicon, 28% of chromium and 7.5% of molybdenum, remainder nickel, iron less than 0.5%, with an oxygen content of less than 200 ppm.

A precise analysis of the melt was carried out, giving 5. the following result: 1.69% of carbon, 0.96% of silicon, 27.93% of chromium, 7.24% of molybdenum, 0.12% of iron, 0.010% of oxygen, the remainder consisting almost solely of nickel.

The molten alloy was converted to a powder by spraying a jet 10. of the metal, during tapping off, by means of an inert gas.

Coatings of 3 to 8 mm thick were subsequently produced on steel components by remelting the powder in a plasma torch. A steel plate having a 4 mm coating thus . obtained was used for various tests.

The results of these tests are shown in the accompanying drawings, in which:Figure 1 shows the micro-structure of 200 x magnification of the coating after exposure to Grosbeck reagent; and Figure 2 shows the hardness curves of the coating as a function of temperature and, by way of comparison, the corresponding hardness curve of the alloy Haynes Stellite No. 6.

Referring firstly to Fig. 1, the structure of the alloy coating was homogeneous, close to the eutectic. . an X-ray diffraction analysis of the precipitated phases was also carried out and two carbide phases with hexagonal structure were identified, one of these phases having a composition of the M7C3 type and the other of these phases having an MgC structure, where M is a metal which - can be chromium. The matrix had an entirely austenitic structure.

High temperature hardness measurements were also carried out on the coatings produced and on similar coatings obtained using the alloy Haynes Stellite No. 6.

. Referring to Fig. 2, it can be seen that the high temperature hardness properties of the alloy according to the invention is entirely comparable with that of Haynes Stellite No. 6, the hardness of the alloy according to the invention being even greater than that of Haynes Stellite No, 6 in the . 400—600°C temperature range.

The coefficient of expansion of the alloy produced as described above was also measured, over various temperature ranges. The results of these expansion measurements and, by way of comparision, the results . of similar measurements carried out on a sample of Haynes Stellite No. 6 are shown in Table 1.

TABLE 1 Expansi on coeffi cient Temperature Alloy according Alloy of the range to the invention Stellite No. 6 type. 20-100°C 11.5 x 106 11.35 χ IO6 20-200°C 11.95 13.0 20-300°C 12.3 13.6 20-400°C 12.7 13.9 20-500°C 13.1 14.2 20-600°C 13.6 14.5 20-700°C 14.0 14.7 20-800°C 14.5 15.0

Claims (5)

1. CLAIMS :1. A nickel-based alloy which consists, by weight, of 1.5 to 2.5% or carbon, 0 to 2% of silicon, 25 to 33% of chromium and 6 to 15% of molybdenum, the balance 5. consisting of nickel and unavoidable impurities.

2. An alloy according to claim 1, in which the total amount of cobalt, tungsten and boron in the unavoidable impurities is less than 0.2%, based on the weight of the alloy.

3. A nickel-based alloy substantially as herein described 10. in the Example.

4. A coating powder obtained by spraying a melt of an alloy according to any of claims 1 to 3.

5. A welding rod or a bare, coated or cored wire, said rod or wire being formed of an alloy according 15. to any of claims 1 to 3.