EP0081432A1 - 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 applications of these alloys to welded construction elements working 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% phosphorus, 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% of manganese and 0.2% to 0.5% 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 name "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 increasing 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 7,129,341 has proposed, for building elements in the cryogenic field, iron-nickel alloys loaded with manganese and having a limited sulfur content. These alloys contain by weight 36 to 36.5% of nickel, 0 to 0.25% 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, 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 are met. 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 present invention aims to provide alloys based on _iron for the building elements welded working under cryogenic conditions, not having a "ductility" hole marked nor any unacceptable tendency to "crack" solidification. These allasses have an average coefficient of expansion between -180 ° and 0 ° C lower or close to 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 carbon 35 to 39% of nickel, 0 to 20% of cobalt, 0 to 0.25% of silicon, 0 to 0 , 04% from / 0 to 0.004% sulfur, 0 to 0.008% phosphorus, 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° 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 limiting 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 score defined according to the Gueussier-Castro method, of the tendency to defect known as the "solidification crack", this tendency being all the more stronger than the note is higher.
• Figure 3 is a graph showing the average coefficient of expansion between -180 ° and 0 ° C of alloys according to the invention.
• Figure 4 shows 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 rupture curves of the alloys Ml and M2 (Figure 1) show that the "ductility hole" is erased in the alloys according to the invention containing more than 0.2% titanium.

The minimum titanium content is also critical from the point of view of weldability. Indeed, tests show that the alloys according to the in- _v ention do not exhibit cracks in welds crossings while alloys such as A in the present alloy and that occasionally free alloys titanium have systematically.

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 "grade" for which there are no difficulties in TIG welding.

The applications of the alloys according to the invention are those where these _a -bondings provide an average coefficient of expansion of less than 2.5 × 10 ^-6 ° C. under cryogenic conditions and a ductility hole sufficiently attenuated to allow welds in particular. weld crossings. 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. FIG. 4 shows a cryogenic duct 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 crossings.

It is understood that it is possible without departing from the scope of the invention to imagine variants and improvements of details and even to envisage the use of equivalent means.

Claims (3)

welded 1.- Iron-based alloys for building elements / working at cryogenic temperatures, containing by weight 35 to 39% of nickel, 0 to 20% of cobalt, 0 to 0.25% of silicon, 0 to 0, 0 4% carbon, 0 to 0.004% sulfur, 0 to 0.008% phosphorus, manganese, the rest being formed of iron and impurities, characterized by the fact that they contain 0.2 to 1.5% of manganese and 0.2 to 0.5% titanium. 2.- Alloys according to claim 1 characterized in that they contain 0.3 to 1% manganese. ₃ .- Applications of the alloys according to any one of the preceding claims, characterized in that these alloys are used in the manufacture of construction elements having weld crossings and requiring an average coefficient of expansion of less than 2.5 x 10 ^-6 / ° C.

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