EP1127952A1 - Process for making hollow bodies for pressure containers from AlZnMgCu alloy - Google Patents

Abstract

The subject of the invention is a method for manufacturing hollow bodies under pressure, in particular from compressed gas cylinders, comprising the following steps: a) casting a billet of alloy composition (% by weight): Zn: 6.25 - 8.0 Mg: 1.2 - 2.2 Cu: 1.7 - 2.8 Fe <0.20 Fe + Si <0.40 at least one of the elements of the group: Mn, Cr, Zr, V, Hf, Sc: 0.05 - 0.3 other elements <0.05 each and <0.15 in total, b) homogenization of this billet according to a temperature profile such that the temperature of the metal is at all times slightly lower than its starting melting temperature, c) softening annealing lasting 20 to 40 h between 200 and 400 ° C, with cooling of less than 50 ° C / h to a temperature below 100 ° C, in such a way that the hardness is <54 HB, d) cutting a piece, e) cold or warm spinning of a case, f) warping the case, g) dissolving at a slightly lower temperature at tempé Beginning of fusion, of a duration such that the specific energy associated with the AED signal is less (in absolute value) than 3 J / g (preferably <2 J / g). h) quenching in cold water, i) tempering between 100 and 200 ° C, lasting between 5 and 25 h. <IMAGE>

Description

Field of the invention

The invention relates to a method for manufacturing hollow bodies under pressure, in particular compressed gas cylinders of aluminum alloy AlZnMgCu, that is to say of the 7000 series according to the nomenclature of the Aluminum Association.

State of the art

The use of 7000 series aluminum alloys for the manufacture of bodies hollow pressure has been known for many years, these alloys having in the heat treated state a high mechanical resistance, which allows a lightening of the manufactured product. Manufacturing involves the casting of billets and their homogenization, the reverse spinning of a cylindrical case, the necking of the neck of the bottle and heat treatment by dissolving, quenching and tempering. Others properties sought in this application are formability, especially for the necking of the bottles, good corrosion resistance under tension and intercrystalline corrosion, and obtaining ductile behavior during bursting tests under internal hydraulic pressure.

Patent FR 2510231 by the applicant describes the use, for this application, of a 7475 type alloy of composition (% by weight):
Zn: 5.6 - 6.1 Mg: 2.0 - 2.4 Cu: 1.3 - 1.7 Cr: 0.15 - 0.25
Fe <0.10 Fe + Si <0.25. The reverse spinning operation can be done both hot and cold.

Patent EP 0081441 of the applicant describes a process for manufacturing high strength and high tenacity spun products made of alloy 7049A of composition:
Zn: 7.2 - 9.5 Mg: 2.1 - 3.5 Cu: 1.0 - 2.0 Cr: 0.07 - 0.17
Mn: 0.15 - 0.25 Fe <0.10 Si <0.08 Zr: 0.08 - 0.14.

The product is spun at a temperature of the order of 400 ° C.

The applicant's patent EP 0257167 provides for the use of an alloy 7060 of composition:
Zn: 6.25 - 8.0 Mg: 1.2 - 2.2 Cu: 1.7 - 2.8 Cr: 0.15 - 0.28
Fe <0.20 Fe + Si <0.40 Mn <0.20

Patent EP 0589807 is a variant of the previous one in which Cr is replaced by Zr (0.10 - 0.25%). 7060 bottles are produced industrially by hot spinning.

Patent application WO 94/24326 from Alcan International relates to a process for manufacturing a hollow body under pressure from an alloy of composition:
Zn: 5.0 - 7.0 Mg: 1.5 - 3.0 Cu: 1.0 - 2.7 Fe <0.30 Si <0.15
a recrystallization inhibitor (Cr or Zr in particular): 0.05 - 0.4, with a microstructure such that the volume fraction of phase S (CuMgAl ₂ ) is maintained below 1%, and preferably below 0, 2%. This microstructure is obtained, according to demand, by homogenizing the billet at approximately 475 ° C with a low rate of temperature rise when approaching this value. The spinning is preferably done, for cost reasons, cold or lukewarm. Tempering is an over-tempering leading to an elastic limit at around 20% below the peak, to improve toughness, resistance to fatigue and to crack propagation, as well as resistance to corrosion under stress. An alloy forming part of the claimed composition was subsequently registered with the Aluminum Association under the designation 7032.

Patent application EP 0670377 from Pechiney Recherche relates to alloys with high mechanical resistance of composition:
Zn: 7 - 13.5 Mg: 1.0 - 3.8 Cu: 0.6 - 2.7 Mn <0.5 Cr <0.4
Zr <0.2
possibly transformed by spinning to obtain hollow bodies. The homogenization and dissolution operations are carried out at less than 10 ° C., and preferably less than 5 ° C., from the starting melting point of the eutectics, under conditions such as in the state T6, l 'specific energy associated with the AED signal (differential thermal analysis) or, in absolute value, less than 3 J / g.

Problem

For some applications, it is desirable to use very high alloys resistance, so as to have bottles as light as possible, but also low manufacturing costs; this is the case, for example, with portable fire extinguishers.

One of the ways to lower the cost is actually to use cold spinning, that is to say with metal at room temperature at the start of spinning, or warm spinning, in which the metal is heated before spinning to a temperature below 300 ° C, significantly cheaper than hot spinning, for which the metal is heated between 350 and 450 ° C before spinning.

However, cold spinning of high strength alloys like 7060 leads to considerable spinning efforts, often incompatible with spinning presses normally used for this type of product, or in any case resulting in a period of shorter life of spinning tools. On the other hand, the application to alloy 7060 of the teaching of WO 94/24326 with regard to temperature homogenization of the billets (over 470 ° C) leads, in many cases, to reach the burn temperature of the alloy during homogenization.

The invention thus aims to develop a range of body manufacturing high pressure 7000 alloy pressure trough, such as 7060 alloy, using cold or lukewarm spinning under acceptable industrial conditions, so as to obtain a high mechanical resistance without prejudice to others properties required for this application.

Subject of the invention

a) coulée d'une billette en alliage de composition (% en poids) :
Zn : 6,25 - 8,0   Mg : 1,2 - 2,2   Cu : 1,7 - 2,8   Fe < 0,20   Fe + Si < 0,40
l'un au moins des éléments appartenant au groupe : Mn, Cr, Zr, V, Hf, Sc : 0,05 - 0,3 autres éléments < 0,05 chacun et < 0,15 au total,

b) homogénéisation de cette billette selon un profil de température tel que la température du métal soit à tout moment légèrement inférieure à sa température de fusion commençante,

c) recuit d'adoucissement d'une durée de 20 à 40 h entre 200 et 400°C, avec un refroidissement de moins de 50°C/h jusqu'à une température inférieure à 100°C, de telle manière que la dureté soit < 54 HB,

d) découpe d'un lopin,

e) filage à froid ou à tiède (température de début de filage < 300°C) d'un étui,

f) ogivage de l'étui,

g) mise en solution à une température légèrement inférieure à la température de fusion commençante, d'une durée telle que l'énergie spécifique associée au signal AED soit inférieure (en valeur absolue) à 3 J/g (de préférence < 2 J/g).

h) trempe à l'eau froide,

i) revenu entre 100 et 200°C, d'une durée comprise entre 5 et 25 h.

The subject of the invention is a process for manufacturing pressurized hollow bodies, in particular compressed gas cylinders, comprising the following steps:

a) casting of a billet of alloy composition (% by weight):
Zn: 6.25 - 8.0 Mg: 1.2 - 2.2 Cu: 1.7 - 2.8 Fe <0.20 Fe + Si <0.40
at least one of the elements belonging to the group: Mn, Cr, Zr, V, Hf, Sc: 0.05 - 0.3 other elements <0.05 each and <0.15 in total,

b) homogenization of this billet according to a temperature profile such that the temperature of the metal is at all times slightly lower than its starting melting temperature,

c) softening annealing lasting 20 to 40 h between 200 and 400 ° C, with cooling of less than 50 ° C / h to a temperature below 100 ° C, so that the hardness or <54 HB,

d) cutting a piece,

e) cold or lukewarm spinning (start of spinning temperature <300 ° C) of a case,

f) ogivage of the case,

g) dissolving at a temperature slightly below the starting melting temperature, of a duration such that the specific energy associated with the AED signal is less (in absolute value) than 3 J / g (preferably <2 J / g).

h) quenching in cold water,

i) tempering between 100 and 200 ° C, lasting between 5 and 25 h.

Description of the invention

The chemical composition of the alloy is within the limits defined in patents EP 0257167 (chromium alloy) or EP 0589807 (zirconium alloy). Chromium or zirconium can be replaced by vanadium, hafnium or scandium. Preferably, individually or in combination:
Zn> 6.75% Mg <1.95% Fe <0.12% Fe + Si <0.25% Mn <0.10%

The alloy is cast in billets in a manner known per se, for example by casting semi-continuous.

Homogenization takes place according to a temperature profile such that at all times the alloy temperature is a few degrees C lower than the temperature of starting alloy melting (burn temperature), which can vary from 470 to 485 ° C depending on the composition of the alloy. It is important that the homogenization is sufficient, otherwise there is a risk of seeing cracks due to coarse phase alignments with copper (e.g. AlCuZn), and cause the dissolution of local mergers, causing decohesions, burns or porosities. The quality of the homogenization can be assessed by analysis differential enthalpy. Insufficient homogenization translates into a beginning fusion with a significant endothermic peak, indicating the fusion metastable eutectic (αA1 + S, M, T). We can consider that this quality is good when, as indicated in patent EP 0670377, the AED thermogram indicates a specific energy, associated with the melting peak, of less than 3 J / g (in absolute value), and preferably at 2 J / g. We can also do this check only on the product put solution, and then judge the quality of the homogenization - solutionization pair.

To obtain good ductility, it is important that the burn temperature does not is not reached. To do this, it is preferable to homogenize in 2 insulating bearings of increasing temperature. The temperature of the first level also depends on the composition of the alloy. It is believed that when the composition is such that:% Mg <0.5% Cu + 0.15% Zn, the temperature of the first bearing must not exceed 465 ° C, and when Mg> 0.5Cu + 0.15Zn, it must not exceed 470 ° C.

The billets thus homogenized have high hardness which requires very significant efforts on the press during cold or lukewarm spinning, which results a reduction in the life of the tools. For this reason, it is essential to proceed to a softening annealing leading to an acceptable level of hardness, which can be situated at 54 HB, this Brinell hardness being measured with a ball of 2.5 mm in diameter and a load of 62.5 kg. This annealing preferably comprises several insulated bearings at decreasing temperatures between 400 and 200 ° C, total duration between 20 and 40 h, followed by a temperature drop fairly slow, less than 50 ° C / h, up to a temperature <100 ° C. The hardness obtained on softened billets no longer evolves by maturing at room temperature.

The softened billets are then cut into pieces corresponding to the quantity of metal necessary to obtain by cold or lukewarm spinning a blank bottle cylindrical case shape. We proceed to a so-called ogivage operation which consists of form the neck of the bottle by shrinking.

The part obtained is then dissolved in a temperature as close as possible of the starting melting temperature of the alloy, while avoiding the burn. The quality of the solution, which depends on both the quality of the prior homogenization, and of the conditions of the solution setting properly said, is also assessed by differential enthalpy analysis on samples in state T6. The specific energy (in absolute value) associated with the peak fusion of the AED thermogram must be less than 3 J / g, and preferably <2 J / g, regardless of the place of collection on the bottle. The result can indeed be different for the top and bottom of the bottle due to the variation in the speed of quench cooling. Indeed, if we immerse the bottle in the liquid of quench from the top, this part will undergo rapid cooling, while the bottom will cooled more slowly.

The tempering is carried out at a temperature between 100 and 180 ° C for a duration between 5 and 25 h. It is preferably an income comprising two insulating bearings at increasing temperature, the first at a temperature between 100 and 120 ° C lasting 4 to 8 hours, and the second at a temperature between 150 and 180 ° C and a duration between 5 and 20 h. This income must be adjusted for obtain a good compromise between the mechanical resistance, which decreases when the income is higher, and resistance to corrosion, especially corrosion under constraint, which increases with overrevenue. We get after income a structure recrystallized with fine grains leading to excellent ductility.

The method according to the invention makes it possible to obtain a remarkable set of properties, namely a breaking strength R _m > 490 MPa, a guaranteed yield strength R _0.2 > 460 MPa, an elongation at break A> 12%, an absence of intercrystalline corrosion, an absence of breakage at 30 days in stress corrosion under 350 MPa, while using, under acceptable industrial conditions, a cold or warm spinning technique more economical than hot spinning .

The process applies to the manufacture of high pressure cylinders intended including fire extinguishers, brewery gases, breathing apparatus, industrial gases. It is economically suitable for the production of used bottles unique, which simplifies distribution. It is also applicable to the manufacture of metallic liners for composite bottles wound with glass fibers, carbon or aramid.

Examples Example 1: influence of homogenization

Billet 7060 alloy of composition (% by weight) was poured:
Si = 0.02 Fe = 0.04 Cu = 2.07 Zn = 6.92 Mg = 1.76 Cr = 0.20

These billets were subjected to bi-level homogenization, with a first level at 460 or 465 ° C, and a second level at 470 ° C, by varying the durations of each of the levels according to a pre-established experience plan. For each homogenization treatment, a micrographic examination to assess the fragmentation and resorption of the copper phases was carried out 4 mm from the edge of the billet. The micrographs were classified according to a qualitative index from 1 (very good) to 7 (poor). Table 1 gives the different homogenization treatments and the corresponding qualitative index. Landmark Homogenization Total time Index 1 5h 465 ° + 25h 470 ° 30 hrs 1 2 19h 465 ° + 9h 470 ° 28 hrs 1 3 11h 460 ° + 13h 470 ° 24h 2 4 11h 460 ° + 19h 470 ° 30 hrs 3 5 11h 465 ° + 13h 470 ° 24h 3 6 5h 460 ° + 19h 470 ° 24h 4 7 5 p.m. 460 ° + 1 p.m. 470 ° 30 hrs 4 8 11h 460 ° + 7h 470 ° 6 p.m. 5 9 17h 460 ° + 7h 470 ° 24h 6 10 7h 465 ° + 9h 470 ° 4 p.m. 6 11 5h 460 ° + 13h 470 ° 6 p.m. 7 12 5h 460 ° + 25h 470 ° 30 hrs 7

The results were validated by image analysis and resulted in an area of recommendation represented in a triangular diagram, represented in the figure 1, having as coordinates the time of the first stage at 460 ° C, the time of the second plateau at 470 ° C and the total time. We note that a total duration of more than 26 h is necessary and sufficient for a good quality of homogenization. A deposit optimized for this treatment consists of a first stage of 1 p.m. at 460 ° C and a second stop at 2 p.m. at 470 ° C.

AED measurements confirm that the peak associated with fusion energy has practically disappeared, and the associated energy remains below - 0.20 J / g, wherever the withdrawal from the billet. In the absence of homogenization, there is a temperature on the order of 467 ° C, and a peak area on the order of - 15 J / g.

The volume fraction of phase S, which was 1.5% in the raw state of expansion, changes to 0.62% at the end of the first level at 460 ° C, and 0.17% at the end of the second level.

Example 2: influence of softening

Billet of the same alloy as in the previous example have been homogenized according to the set point from 1 p.m. to 460 ° C + 2 p.m. to 470 ° C. After returning to the at room temperature, they have a hardness greater than 70 HB. This hardness is not stable and grows over time. In order to soften the billet before spinning, we have carried out an annealing treatment comprising a 3 h stage at 400 ° C, a 6 h stage at 300 ° C, a 6 hour plateau at 230 ° C and cooling at a speed of 20 ° C / h until the metal drops below 100 ° C. After return to temperature ambient, the billet has a hardness of 52 HB which does not change over time.

This invariance of hardness over time indicates that the treatment softening is effective.

Example 3: influence of income

Billet diameter 153 mm in composition (% by weight) was poured:
Si = 0.02 Fe = 0.040 Cu = 2.06 Mg = 1.67 Zn = 7.14 Cr = 0.20

These billets were homogenized by a two-stage treatment of 13 h at 460 ° C and 14 h at 470 ° C. They were then softened by processing the previous example, then cut into 3.35 kg pieces to obtain a case by cold spinning, which after stretching and ogivage of the neck, is transformed into a cylinder body for compressed gases or liquefied with a capacity of 3 l, an external diameter of 117 mm, a length of 432 mm, intended after heat treatment to withstand a test pressure of 205 MPa.

These bottles were put in solution by a treatment of 2 h at 475 ° C. The quality of the solution of the entire bottle was assessed by differential enthalpy analysis using a Perkin-Elmer DSC7 device, with a temperature rise rate of 20 ° C / min. Samples were taken from the outside edge and the inside edge of the bottle, top, middle and bottom. The results are reported in Table 2: On-board sampling Sampling height Temp. Start of peak ° C Peak area (J / g) Outside High 452.0 - 0.13 Outside Middle 453.8 - 0.10 Outside Low 451.3 - 0.21 Interior High 449.5 - 0.19 Interior Middle 450.0 - 0.09 Interior low 449.5 - 0.25

Differential enthalpy analysis shows the good quality of the solution in all parts of the bottle. The peak areas are all less than 1 J / g (in absolute value), even if those corresponding to the bottom of the bottle have absolute values slightly higher than those corresponding to the middle or the top of the bottle.

After dissolving and maturing at room temperature for at least 72 hours, the bottles were immersed in a tank of cold water, then subjected to two-stage tempering, with a first stage of 6 hours at 105 ° C. and a second stage at 160, 165 or 170 ° C, with a duration of 10, 13.5 or 17 h. In the 9 cases, the tensile strength R _m (in MPa), the elastic limit at measurement, was measured from samples taken halfway up the body of the bottle, in the long direction and full thickness. 0.2% elongation R _0.2 (in MPa), elongation A (in%) and electrical conductivity (in MS / m). The results are shown in Table 3: ^2nd level income R _m MPa R _0.2 MPa AT % MS / m conductivity 10 h 160 ° 554.7 514.0 13.8 22.5 13.5 h 160 ° 542.0 498.3 16.4 23.0 5.16 p.m. 520.7 465.0 14.8 23.8 10:16 a.m. 519.3 463.3 14.4 23.8 13.5 h 165 ° 501.7 442.7 14.9 24.2 5:16 p.m. 485.7 419.0 16.3 24.5 10 h 170 ° 491.3 424.3 14.9 24.5 13.5 h 170 ° 486.0 414.7 12.5 24.8 17 h 170 ° 471.7 397.3 14.5 25.1

Light micrographs were performed on samples taken on the outside wall, mid-thickness and on the inside wall of the bottle, polished mechanically. They do not reveal any burn marks from the eutectics.

Whatever the income achieved, there is an absence of intercrystalline corrosion at the test according to European directive n ° 84/526 / CE (appendix 2). We measured also, according to the same standard, the corrosion behavior under stress on 3 test tubes requested at the same constraint for each type of income. We have not observed no breakage at 30 days under the constraints of 286, 316 and 353 MPa. Account given the EEC directive authorizing a guaranteed minimum elastic limit of 1.3 times the corrosion resistance stress under tension, we can guarantee an elastic limit of 460 MPa, which can be easily reached with the income of the first 4 lines of table 3. In particular, an income with a second stage of 10 h at 165 ° C allows an excellent compromise between mechanical strength and resistance to corrosion under stress.

Claims (13)

Method for manufacturing pressurized hollow bodies, in particular compressed gas cylinders, comprising the following steps: a) casting of a billet of alloy composition (% by weight):
Zn: 6.25 - 8.0 Mg: 1.2 - 2.2 Cu: 1.7 - 2.8 Fe <0.20 Fe + Si <0.40
at least one of the elements of the group: Mn, Cr, Zr, V, Hf, Sc: 0.05 - 0.3 other elements <0.05 each and <0.15 in total, b) homogenization of this billet according to a temperature profile such that the temperature of the metal is at all times slightly lower than its starting melting temperature, c) softening annealing lasting 20 to 40 h between 200 and 400 ° C, with cooling of less than 50 ° C / h to a temperature below 100 ° C, so that the hardness or <54 HB, d) cutting a piece, e) cold or lukewarm spinning (start of spinning temperature <300 ° C) of a case, f) ogivage of the case, g) dissolving at a temperature slightly below the starting melting temperature, of a duration such that the specific energy associated with the AED signal is less (in absolute value) than 3 J / g (preferably <2 J / g). h) quenching in cold water, i) tempering between 100 and 200 ° C, lasting between 5 and 25 h. Process according to claim 1, characterized in that Zn> 6.75%. Method according to one of claims 1 or 2, characterized in that Mg <1.95% Method according to one of claims 1 to 3, characterized in that Fe <0.12% and Fe + Si <0.25%. Method according to one of claims 1 to 4, characterized in that Mn <0.10%. Method according to one of claims 1 to 5, characterized in that the homogenization is such that the specific energy associated with the melting peak of the AED thermogram is <3 J / g. Method according to one of claims 1 to 6, characterized in that homogenization takes place in 2 isothermal steps at increasing temperature. Method according to claim 7, characterized in that Mg <(0.5Cu + 0.15Zn) and that the temperature of the first level is <465 ° C. Method according to claim 7, characterized in that Mg> (0.5Cu + 0.15Zn) and that the temperature of the first level is <470 ° C. Method according to one of claims 1 to 9, characterized in that the annealing softening is done by isothermal steps at decreasing temperature. Method according to one of claims 1 to 10, characterized in that the income is performed in 2 isothermal steps, the first at a temperature between 100 and 120 ° C and between 4 and 8 hours, and the second between 150 and 180 ° C duration between 5 and 20 h. Pressurized hollow body produced by a process according to one of Claims 1 to 11, characterized in that it has a breaking strength R _m > 490 MPa, an elastic limit R _0.2 > 460 Mpa, an elongation A > 12% and resistance to corrosion under stress such that there is no breakage in 30 days under a stress of 353 MPa. Hollow body according to claim 12, characterized in that it is reinforced externally by a winding of glass fibers, carbon or aramid.

Images