EP0081432B1 - Iron-based alloys for welded structures and uses of these alloys - Google Patents

Abstract

The present invention relates to iron-based alloys with a low coefficient of expansion and to the uses of these alloys for welded structural elements operating under cryogenic conditions. The alloys according to the invention contain by weight 35 to 39% of nickel, 0 to 20% of cobalt, 0 to 0.25% of silicon, 0 to 0.04% of carbon, 0 to 0.004% of sulfur, 0 to 0.008% of phosphorus, manganese, the remainder being formed by iron and by impurities. They are characterized by the fact that they contain 0.2% to 1.5% of manganese and 0.2 to 0.5% of titanium.

Description

The present invention relates to iron-based alloys with a low coefficient of expansion and which can be welded and to the applications of these alloys to welded construction elements working under cryogenic conditions, in particular to storage and transport tanks and transport conduits. liquefied gas.

The weldability of iron-nickel alloys with 35 to 50% nickel, including the alloy known under the brand "INVAR", is limited by two distinct phenomena: the tendency to "solidification crack" and the "ductility hole" ". The phenomenon known as “solidification crack” is due to the fact that interdendritic films are still liquid and therefore incapable of withstanding a tensile force at a temperature where the dendrites already formed constitute a continuous solid structure capable of transmitting the forces due to thermal contraction. The ductility hole corresponds to a minimum of ductility in the temperature range from 700 to 1000 ° C.

A filler metal is known for welding the above alloys in which manganese and titanium have been added to the base metal of the above type. A typical composition of this filler metal comprises 36% nickel, 0.1% silicon, 0.1% carbon, less than 0.01% sulfur, less than 0.01% phosphorus, 3% manganese and 1% titanium, iron forming the balance. The addition of manganese and titanium has the disadvantage of raising the coefficient of expansion of the alloy which cannot therefore be used as a base metal for the manufacture of construction elements in the cryogenic field. Furthermore, the use of this filler metal as a weld does not solve all the difficulties. In the case of crossed beads and if the stresses are high enough, cracking of the first bead occurs in the area affected by the second bead, not in the molten area but just at the limit of the latter in the metal of based.

French patent FR-A-2 148 954 has proposed iron-nickel alloys with manganese plating and having a limited sulfur content for building elements in the cryogenic field. These alloys contain by weight 36 to 36.5% of nickel, 0 to 25% of silicon, 0 to 0.04% of carbon, 0 to 0.012% of sulfur, 0 to 0.012% of phosphorus and 0.20 to 0, 40% manganese. Due to the limitation of the sulfur content and the presence of manganese, the construction elements made with these alloys can be welded without great difficulty. However, it is found that the metal of the molten zone of a weld bead produced with this alloy is unable to simultaneously resist a temperature of the order of 700 to 1000 ° C. and a tensile stress when these conditions meet. during a local recovery of a weld bead or a crossing of weld beads. This phenomenon is due to the drop in ductility observed in the temperature range given above.

The object of the present invention is to provide iron-based alloys intended for welded construction elements working under cryogenic conditions, not exhibiting a marked "ductility hole" or of an unacceptable tendency to "solidify". These alloys have an average expansion coefficient between -180 ° C and 0 ° C or lower neighbor 2.10- ⁶ / ° C and the present invention relates to applications requiring the above properties.

The iron-based alloys for building elements working at cryogenic temperatures in accordance with the invention contain by weight 35 to 39% of nickel, 0 to 20% of cobalt, 0 to 0.25% of silicon, 0 to 0, 04% carbon, 0 to 0.004% sulfur, 0 to 0.008% posphorus, manganese, the rest being formed by iron and by impurities and they are characterized by the fact that they contain 0.2 to 1, 5% manganese and 0.2% to 0.5% titanium.

According to one characteristic, the alloys contain 0.3 to 1% of manganese.

According to another characteristic of the invention, these alloys are used in the manufacture of construction elements having welded crossings.

• La figure 1 est un graphique donnant la striction à rupture mesurée par essai de traction rapide sur des échantillons forgés et traités une heure à 1100° C, en fonction de la température t.
• La figure 2 donne pour différentes teneurs en manganèse et en titane conformes à l'invention, une »note« définie selon la méthode Gueussier-Castro, de la tendance au défaut dit de la »crique« de solidification«, cette tendance étant d'autant plus forte que la note est plus élevée.
• La figure 3 est un graphique donnant le coefficient moyen de dilatation entre -180° C et 0"C d'alliages conformes à l'invention.
• La figure 4 représente un exemple d'élément de construction pour lequel les alliages conformes à l'invention sont spécialement adaptés.

The invention will now be described in more detail with reference to embodiments given by way of examples. This in no way limitative description refers to the appended drawings in which:

• FIG. 1 is a graph giving the necking at break measured by rapid tensile test on samples forged and treated for one hour at 1100 ° C., as a function of the temperature t.
• FIG. 2 gives, for different manganese and titanium contents in accordance with the invention, a "note" defined according to the Gueussier-Castro method, of the tendency to defect known as the "crack" of solidification ", this tendency being of as much stronger as the note is higher.
• Figure 3 is a graph showing the average coefficient of expansion between -180 ° C and 0 "C of alloys according to the invention.
• FIG. 4 represents an example of a building element for which the alloys according to the invention are specially adapted.

The alloys according to the invention are iron-based and contain 35 to 39% of nickel. They have an austenitic structure. They can contain 0 to 20% cobalt.

By way of example, the table gives two alloy compositions in accordance with the invention. These compositions are given by weight.

The alloys contain manganese and titanium. The combination of manganese addition and titanium addition is essential. Indeed the addition of manganese alone, even at the 3% level, has no effect on the "ductility hole". The manganese content is between 0.2 and 1.5%. Preferably it should not exceed 1% so that the average coefficient of expansion between -180 ° and 0 ° is low (Figure 3). Preferably the content is between 0.3% and 1%.

The minimum titanium content, equal to 0.2%, is critical with regard to the "ductility hole". Indeed, it is not reproducibly removed when the titanium content is below the limit mentioned. Thus the necking curve at break (Figure 1) of alloy A, the weight composition of which is given in Table II shows that the "ductility hole" exists when the titanium content is less than 0.2%.

On the contrary, the constriction at break curves of the alloys M1 and M2 (FIG. 1) show that the "ductility hole" is eliminated in the alloys according to the invention containing more than 0.2% of titanium.

The minimum titanium content is also critical from the point of view of weldability. In fact, tests show that the alloys according to the invention do not have cracks at the weld intersections, while alloys such as alloy A occasionally exhibit them and that titanium-free alloys systematically exhibit them.

The titanium content should not exceed 0.5% to avoid increasing the average coefficient of expansion and to avoid aggravating the tendency to crack solidification.

The sulfur content is between 0 and 0.004%. The graph in FIG. 2 shows that in the field of alloys according to the invention, the lowering of the sulfur content from 0.011% ("notes" circled) to 0.004% ("notes" underlined) causes the "note" to drop by 50 points to bring it well below 140 which is a "note" for which we do not encounter difficulties in TIG welding. The applications of the alloys according to the invention are those where these alloys provide an average coefficient of expansion of less than 2.5 x 10 _" ^s ° c under cryogenic conditions and a ductility hole sufficiently attenuated to allow welds, in particular crossings of The alloys according to the invention are suitable for welded construction elements working under cryogenic conditions and having weld crossings produced with metal fusion in the welding zones of said elements. Figure 4 shows a cryogenic conduit in which the annular bead 1 cuts the longitudinal cords 2 and 3. The alloys according to the invention are specially adapted to such parts having welded crossovers.

Claims (3)

1. Alloys based on iron for welded structural components which operate at refrigeration temperatures, the alloys containing, by weight, 35 to 39% of nickel, 0 to 20% of cobalt, 0 to 25% of silicon, 0 to 0.04% of carbon, 0 to 0.004% of sulphur, 0 to 0.008% of phosphorus and some manganese, the remainder consisting of iron and impurities, characterised in that they contain 0.2 to 1.5% of manganese and 0.2 to 0.5% of titanium.

2. Alloys according to Claim 1, characterised in that they contain 0.3 to 1% of manganese.

3. Use of the alloys according to any one of the preceding claims for the manufacture of structural elements which exhibit intersecting welds and require a mean coefficient of expansion of less than 2.5 × 10^-6/°C.

Images