EP0589807B1 - Use of an aluminium alloy for making bottles for compressed gas - Google Patents

Abstract

Alloy of the 7000 series and a specific heat treatment with a view to the manufacture of hollow bodies under pressure and, in particular, of metal bottles for compressed gases. The alloy contains, in weight %: 6.25 </= Zn </= 8.0 Mn </= 0.20 1.2 </= Mg </= 2.2 Ti </= 0.05 1.7 </= Cu </= 2.8 0.10 </= Zr </= 0.25 others, each </= 0.05 Fe </= 0.20 '' total </= 0.15 Si+Fe </= 0.40 remainder: Al Cr </= 0.05. The final annealing is preferably performed in 3 stages: - 1st stage between 105 and 120 DEG C for 6 to 12 h - 2nd stage: between 170 and 190 DEG C for 0.5 to 20 h - 3rd stage: between 105 and 120 DEG C for 12 to 36 h.

Description

The invention relates to an AI alloy which can be used for the manufacture of bottles. of metal for compressed gases.

In its application EP-A-0257167, the applicant claimed an alloy type 7000 particularly suitable for the use considered above.

However, the latter noted that in certain cases, the modification of the chemical composition on the one hand and the final heat treatment on the other improve the burst characteristics (facies of the tear) while maintaining the level of mechanical characteristics and resistance to corrosion under stress required.

Furthermore, the article by P.K. BALASUBRAMANIAN "Extrusion of Al Zn Mg Cu Zr Alloys "Materials Science and Technology, vol.1, n ° 6, 1985, pages 470-474, describes the extrusion of tubes of various Al Zn Mg Cu Zr alloys that can resist pressure tests up to 550 MPa.

The alloys used according to the invention have the following weight composition (in%):
6.25 ≤ Zn ≤ 8.0
1.2 ≤ Mg ≤ 2.2
1.7 ≤ Cu ≤ 2.8
0.10 ≤ Zr ≤ 0.25
Cr ≤ 0.05
Fe ≤ 0.20
Fe + Si ≤ 0.40
Mn ≤ 0.20
Ti ≤ 0.05
Others each ≤ 0.05
total ≤ 0.15
Rest: Al

The Mg content is preferably kept below 2%, and even 1.95%, and the Zr content is preferably between 0.10 and 0.18%, the contents in Fe + Si being ≤ 0.25% with Fe ≤ 0.12%, an Mn content ≤ 0.10% and / or the Zn content ≥ 6.75.

If the Zr content is greater than 0.25%, we observe the presence of large precipitates which cause serious difficulties during casting and structure is not recrystallized. For Zr contents ≤ 0.10%, the structure is recrystallized, but with large grains.

The manufacturing and control process are similar to those described in EP-A-0257167, but preferably the treatment of final income type T73 is replaced by an income in 3 stages, the 1st stage being carried out between 105 and 120 °. C for 6 to 12 h, the 2nd step being carried out between 170 and 190 ° C for 0.5 to 20 h and the 3rd step being carried out between 105 and 120 ° C, for 12 to 36 h.
These steps can be carried out continuously or discontinuously (return to ambient temperature between each of them or some of them).
The durations and temperatures actually used are chosen by a person skilled in the art so as to obtain both a high electrical conductivity (corresponding to good resistance to corrosion under tension) and a high elastic limit.
The improvement in the cracking characteristics is probably due, but this is an assumption, to the fact that the structure is better recrystallized (Zr being an anti-recrystallizing element less powerful than Cr), the relative loss of resistance to corrosion under tension being compensated by the triple final income.

The invention will be better understood using the following examples:

Example 1 - Replacement of Cr by Zr for two-phase income type T73.

Two alloys, one in accordance with the request EP-A-0257167- alloy 1, the other similar apart from the fact that chromium has been replaced by zirconium -alloy 2- have been prepared and transformed into bottles of 6 liters according to the following production range:
165 mm diameter billet casting
Sawing into plots
Reheating plots
Hot reverse spinning of cases
Hot drawing
Cold drawing
Bottom machining
Cut to length
Hot icing
Bottom drilling and machining
Pickling
Dissolution
Quenching
Income 6 hours at 105 ° C + 3 hours at 170 ° C.

The weight composition (in%) of these 2 alloys is given in the following table: Alloy Cu Mg Zn Fe Yes Cr Zr 1 2.4 1.9 7.0 0.08 0.07 0.20 2 2.3 1.9 6.9 0.08 0.08 0.02 0.11

The characteristics obtained on the corresponding bottles are as follows: Alloy R 0.2 (MPa) Rm (MPa) AT (%) CSC at 280 MPa Ruptures / non ruptures. (NR) Burst crack length (mm) 1 404 470 15.6 3 NR at 60 days 512 - 498 - 480 2 392 459 15.2 1 NR at 60d, 55d, 52d 446 - 423 - 421

In conditions for which the burst characteristics are correct (longitudinal crack for the most part, not branched, limited to a sector of angle ± 90 ° around the crack main, limited towards the bottom and towards the neck to areas of which the thickness is less than 1.5 times the thickness of the body), the length developed from the crack was noted as a good indication of suitability bursting: the longer the crack, the closer we get to conditions for which the burst would be bad.

The results presented above show that the replacement of chromium with zirconium makes it possible to very significantly improve the quality of bursting, but at the expense of corrosion resistance under stress and, weakly, mechanical strength. However, both series of bottles are suitable for use.

For a given type of income, in this case two-tier income, the mechanical strength and resistance to stress corrosion are unequivocally linked, which makes it better to speak of a loss on the compromise between mechanical strength and corrosion resistance under constraint. In other words, the replacement of chromium by zirconium negatively affects corrosion resistance or resistance mechanical, depending on how long you maintain the second level you choose.

Example 2 - Using income in 3 steps.

The following example shows the gain that can be obtained by performing a returned in three stages to the bottles manufactured in alloy 2 according to the range of the previous example.

Electrical conductivity is taken as an indicator of resistance to stress corrosion, in accordance with standard practice. All values in the table below are averages of 3 individual values. Rep Returned R0.2 (MPa) Rm (MPa) AT (%) Conductivity (MS / m) Burst (mm) AT 6h 105 ° C + 15h 170 ° C (ref.) 392 459 15.2 24.6 430 B 6h105 ° C + 15h170 ° C + 36h110 ° C 417 485 14.9 24.7 461 VS 6h105 ° C + 20h170 ° C + 36h110 ° C 397 464 15.51 25.6 430 D 6h105 ° C + 3h190 ° C + 36h110 ° C 400 457 15.7 24.4 429

• le revenu à 3 étapes permet d'améliorer le compromis résistance à la corrosion sous contrainte/résistance mécanique. Entre le revenu A et le revenu C, la conductivité augmente de manière importante, avec un léger gain de résistance mécanique. L'augmentation de conductivité se traduit bien par une augmentation de la résistance à la corrosion sous contrainte puisqu'on n'a pas de rupture à 60 j sous 280 MPa.
• le revenu tri-paliers avec un deuxième palier à 190°C conduit à un compromis résistance à la corrosion sous contrainte/résistance mécanique à peine meilleur que celui obtenu avec un revenu bipalier. Le domaine de température intéressant pour le deuxième palier est donc limité vers le haut à 190°C.
• grâce au revenu tri-paliers, le compromis résistance à la corrosion sous contrainte/résistance mécanique obtenu avec l'alliage 2 est équivalent à celui obtenu avec l'alliage 1 traité avec un revenu bi-palier. On bénéficie alors pleinement de l'influence du zirconium sur la qualité de l'éclatement, puisque la longueur moyenne des fissures est passée de 497 mm avec l'alliage au chrome à 430 mm avec l'alliage au zirconium.

Using this table, we can establish the following points:

• 3-stage tempering improves the compromise between stress corrosion resistance and mechanical resistance. Between income A and income C, the conductivity increases significantly, with a slight gain in mechanical strength. The increase in conductivity translates well into an increase in resistance to corrosion under stress since there is no rupture at 60 d at 280 MPa.
• the tri-level income with a second level at 190 ° C leads to a compromise in resistance to stress corrosion / mechanical resistance hardly better than that obtained with two-bearing income. The temperature range of interest for the second level is therefore limited upwards to 190 ° C.
• thanks to tri-level tempering, the compromise between stress corrosion resistance and mechanical strength obtained with alloy 2 is equivalent to that obtained with alloy 1 treated with bi-level tempering. We then fully benefit from the influence of zirconium on the quality of bursting, since the average length of the cracks went from 497 mm with the chrome alloy to 430 mm with the zirconium alloy.

Claims (8)

1. Use of an alloy containing, in % by weight
      6.25 ≤ Zn 8.0    Mn ≤ 0.20
      1.2 ≤ Mg ≤ 2.2    Ti ≤ 0.05
      1.7 ≤ Cu ≤ 2.8
      0.10 ≤ Zr ≤ 0.25    Others in each case ≤ 0.05
      Fe ≤ 0.20    "  in total ≤ 0.15
      Si+Fe ≤ 0.40    Balance Al
      Cr ≤ 0.05
   for the production of bottles for compressed gas.
2. Use of an Al alloy according to Claim 1, having a content of Mg of ≤ 2.0.
3. Use of an Al alloy according to either of Claims 1 or 2, having a content of Mg of ≤ 1.95%.
4. Use of an Al alloy according to either of Claims 1 or 3, having a content of Zn of ≥ 6.75.
5. Use of an alloy according to one of Claims 1 to 4, having contents of Fe and Si such that Fe is ≤ 0.12% and Fe+Si is ≤ 0.25%.
6. Use of an alloy according to one of Claims 1 to 5, having a content of Mn ≤ 0.10%.
7. Use of an alloy according to one of Claims 1 to 6, having a content of Zr of between 0.10 and 0.18.
8. Process for producing bottles for compressed gas according to one of Claims 1 to 7, characterized in that the final ageing is carried out in 3 stages:

   1st stage: between 105 and 120°C for 6 to 12 h

   2nd stage: between 170 and 190°C for 0.5 to 20 h

   3rd stage: between 105 and 120°C for 12 to 36 h.