U.S. patent application number 10/066788 was filed with the patent office on 2002-11-07 for manufacturing process for a high strength work hardened product made of alznmgcu alloy.
Invention is credited to Warner, Timothy.
Application Number | 20020162609 10/066788 |
Document ID | / |
Family ID | 8859702 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020162609 |
Kind Code |
A1 |
Warner, Timothy |
November 7, 2002 |
Manufacturing process for a high strength work hardened product
made of AlZnMgCu alloy
Abstract
The purpose of the invention is a process for the manufacture of
a work-hardened product made of a high mechanical strength
Al--Zn--Mg--Cu aluminium alloy consisting of: casting an ingot made
of an alloy with composition (% by weight) Zn=9.0-11.0, Mg=1.8-3.0;
Cu=1.2-2.6 at least one of the elements Mn (0.05-0.4), Cr
(0.05-0.3), Zr (0.05-0.20), Hf (0.05-0.5), V (0.05-0.3), Ti
(0.01-0.2) and Sc (0.05-0.3), the remainder being made of aluminium
and inevitable impurities, possibly homogenisation of said ingot,
hot transformation of said ingot by rolling, extrusion or forging,
solution heat treatment and quenching of the product obtained,
possibly controlled stretching with a permanent set between 1 and
5%, annealing of the product at a temperature and with a duration
such that the product reaches the maximum compression yield stress
in the L direction. The invention is applicable particularly to
upper wing members of aircrafts. 1 Lgende des figures
Fran.cedilla.ais Anglais Seuil CSC CSC threshold Temprature
(.degree. C.) Temperature (.degree. C.) Dure de revenu (h)
Annealing duration (h) Mona-palier Single step Bi-palier Two-step
Temps quivalent de revenu Equivalent annealing time 120.degree. C.
(h) at 120.degree. C. (h) Temps quivalent a 120.degree. C.
Equivalent time at 120.degree. C. (h) (h) Tri-palier A Three-step A
Tri-palier B Three-step B Temps quivalent de revenu Equivalent
annealing time 120.degree. C. (h) at 120.degree. C. (h)
Inventors: |
Warner, Timothy;
(Ravenswood, WV) |
Correspondence
Address: |
Ira J. Schultz
DENNISON, SCHEINER & SCHULTZ
Suite 612
1745 Jefferson Davis Highway
Arlington
VA
22202
US
|
Family ID: |
8859702 |
Appl. No.: |
10/066788 |
Filed: |
February 6, 2002 |
Current U.S.
Class: |
148/439 ;
148/690; 148/697; 148/701 |
Current CPC
Class: |
C22F 1/053 20130101;
C22C 21/10 20130101 |
Class at
Publication: |
148/439 ;
148/690; 148/701; 148/697 |
International
Class: |
C22F 001/053 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2001 |
FR |
0101617 |
Claims
What I claim is:
1. Process for the manufacture of a work hardened product made of a
high mechanical strength Al--Zn--Mg--Cu aluminium alloy comprising:
casting an ingot made of an alloy with composition (% by weight)
Zn=7.0-11.0, Mg=1.8-3.0; Cu=1.2-2.6 at least one of the elements Mn
(0.05-0.4), Cr (0.05-0.3), Zr (0.05-0.20), Hf (0.05-0.5), V
(0.05-0.3), Ti (0.01-0.2) and Sc (0.05-0.3), the remainder being
made of aluminium and inevitable impurities, possibly
homogenisation of said ingot, hot transformation of said ingot by
rolling, extrusion or forging, solution heat treatment and
quenching of the product obtained, possibly controlled stretching
with a permanent set between 1 and 5%, annealing of the product at
a temperature and with a duration such that the product reaches the
maximum compression yield strength in the L direction.
2. Process according to claim 1, wherein the magnesium content of
the alloy is between 1.8 and 2.4%.
3. Process according to claim 1, wherein the copper content of the
alloy is between 1.6 and 2.2%.
4. Process according to claim 1, wherein the magnesium content of
the alloy is between 1.8 and 2.4%, and the copper content is
between 1.6 and 2.2%.
5. Process according to claim 1, wherein the alloy is 7349 or
7449.
6. Process according to claim 1, wherein the alloy is 7055.
7. Process for the manufacture of a work hardened product made of a
high mechanical strength Al--Zn--Mg--Cu aluminium alloy comprising:
casting an ingot made of an alloy with composition (% by weight)
Zn=7.0-11.0, Mg=1.8-3.0, Cu=1.2-2.6 at least one of the elements Mn
(0.05-0.4), Cr (0.05-0.3), Zr (0.05-0.20), Hf (0.05-0.5), V
(0.05-0.3), Ti (0.01-0.2) and Sc (0.05-0.3), the remainder being
made of aluminium and inevitable impurities, possibly
homogenisation of said ingot, hot transformation of said ingot by
rolling, extrusion or forging, dissolution and quenching of the
resulting product, possibly controlled stretching with a permanent
set between 1 and 5%, single step annealing at a temperature and
with a duration included within the parallelogram AEFG, in which
the vertices in the temperature-duration diagram have the following
coordinates: A: 120.degree. C.-100 h E: 145.degree. C.-5 h F:
150.degree. C.-40 h G: 120.degree. C.-700 h.
8. Process according to claim 7, wherein the annealing is a single
step annealing at a temperature and with a duration within the
parallelogram ABCD, in which the vertices in the
temperature-duration diagram have the following coordinates: A:
120.degree. C.-100 h B: 145.degree. C.-9 h C: 145.degree. C.-22 h
D: 120.degree. C.-230 h.
9. Process according to claim 1, wherein the equivalent annealing
time at 120.degree. C. is between 100 and 250 h.
10. Process according to claim 1, wherein the equivalent annealing
time at 120.degree. C. is 50 to 200 h longer than the time
corresponding to temper T651.
11. Process according to claim 1, wherein said annealing is a
two-step annealing comprising a first step at a temperature between
80.degree. C. and 120.degree. C., and a second step at a
temperature between 120.degree. C. and 160.degree. C., and wherein
said equivalent annealing time at 120.degree. C. is between 100 and
250 h.
12. Process according to claim 1, wherein said annealing is a
three-step annealing comprising a first step at a temperature
between 80.degree. C. and 120.degree. C., a second step at a
temperature between 120.degree. C. and 160.degree. C., and a third
step at a lower temperature than the second step and between
100.degree. C. and 140.degree. C., and wherein the equivalent
annealing time at 120.degree. C. is between 100 and 250 h.
13. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 1.
14. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 2.
15. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 3.
16. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 4.
17. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 5.
18. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 6.
19. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 7.
20. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 8.
21. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 9.
22. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 10.
23. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 11.
24. Structural element for mechanical construction and particularly
aeronautical construction, manufactured from at least one rolled,
extruded or forged product obtained by the process according to
claim 12.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to the manufacture of work-hardened
products by rolling, extrusion or forging made of a high mechanical
strength aluminium alloy of the AlZnMgCu type, particularly
products used for aeronautical construction and particularly for
upper wing members of aircrafts.
[0003] 2. Background Art
[0004] Al--Zn--Mg--Cu type alloys have been used for aircraft
construction for more than 50 years, and particularly for upper
wing members. Thus, 7075, 7178, 7050, 7150 alloys and more recently
7055 and 7449 alloys have been used. These alloys have usually been
used in the T6 temper, in other words annealing corresponding to
the maximum tensile yield strength, or an over annealed temper T76,
T79 or T77 to obtain better corrosion resistance. As an
illustration of this state of the art, these are patents EP 0020505
by Boeing related to the 7150 alloy, U.S. Pat. Nos. 4,477,292,
4,832,758, 4,863,528 and 5,108,520 by Alcoa on the T77 treatment,
Alcoa' patent EP 0377779 dealing with a manufacturing process for
the 7055 alloy, and the applicant's patent application EP 0670377
describing a process for the manufacture of plates made of 7449
alloy.
[0005] The properties of the 7449 alloy developed by the applicant
for plates intended for use on upper wing members have been studied
in the paper written by T. Warner et al. "Aluminium alloy
developments for affordable airframe structures", Conference on
Synthesis, Processing and Modelling of Advanced Materials, ASM
International, Paris, Jun. 25-27, 1977, pp 77-88. FIG. 2 in the
article reproduced as FIG. 1 in this application, shows typical
properties of plates from 15 to 40 mm thick made of this alloy,
specifically the ultimate strength and the tensile yield strength
in the L direction, the compression yield strength in the L
direction and the stress corrosion limit (in the ST direction), in
the T651 temper and in a T7.times.51 temper with improved corrosion
resistance. This temper was identified in later publications by the
same authors as T7951 (or T79511 for extruded products), for
example in the paper written by F. Heyms et al. "New aluminium
semi-products for airframe application", METEC congress,
Dusseldorf, June 1999 that uses the same figure. FIG. 1 attached to
this application shows that the compression yield strength in the
T79 temper is lower than the corresponding yield strength in the T6
temper. In the T7951 temper, plates made of 7449 have a 10% higher
compression yield strength, better resistance to exfoliation
corrosion and to stress corrosion and fatigue than plates made of
7150 in the T651 temper that are usually used for upper wing
members of commercial aircrafts, without any reduction in the
tolerance to damage.
[0006] In summary, the state of the art shows firstly that the
mechanical compression strength is an essential property for upper
wing members, and also that manufacturers of high strength alloys
offer products for this application eight in the T6 temper
corresponding to the maximum tensile yield strength, or an over
annealed T7 temper with better corrosion resistance but with lower
mechanical strength.
BRIEF DESCRIPTION OF THE INVENTION
[0007] The purpose of the invention is to further improve the
mechanical compression strength of products made of high strength
7000 alloys intended particularly for upper wing members of
aircrafts without losing any other usage properties.
[0008] The purpose of the invention is a process for manufacturing
a work-hardened product made of a high mechanical strength
Al--Zn--Mg--Cu aluminium alloy comprising:
[0009] casting an ingot made of an alloy with composition (% by
weight) Zn (7.0-11.0), Mg (1.8-3.0), Cu (1.2-2.6), at least one of
the elements Mn (0.05-0.4), Cr (0.05-0.30), Zr (0.05-0.20), Hf
(0.05-0.5), V (0.05-0.3), Ti (0.01-0.2) and Sc (0.05-0.3), the
remainder being made of aluminium and inevitable impurities,
[0010] possibly homogenisation of said ingot,
[0011] hot transformation of said ingot by rolling, extrusion or
forging,
[0012] solution heat treatment and quenching of the product
obtained,
[0013] possibly controlled stretching with a permanent set between
1 and 5%,
[0014] annealing of the product at a temperature and for a duration
such that the product reaches the peak compression yield strength
in the L direction.
[0015] Another purpose of the invention is the rolled, extruded or
forged product obtained by said process.
[0016] Another purpose of the invention is the structural element
for mechanical construction, and particularly for aeronautical
construction, made from at least one rolled, extruded or forged
product obtained according to said process.
BRIEF DESCRIPTION OF THE FIGURES
[0017] FIG. 1 shows typical properties, namely the tensile yield
strength (L direction), the ultimate tensile strength (L
direction), the compression yield strength (L direction) and the
stress corrosion threshold (ST direction) for plates between 15 and
40 mm thick made of the 7150-T651, 7449-T651 and 7449-T7951 alloys
according to prior art.
[0018] FIG. 2 shows the annealing time-temperature domain in the
process according to the invention.
[0019] FIG. 3 shows the ultimate strength and the tensile yield
strength of 38 mm thick plates made of 7449 alloy in example 1 as a
function of equivalent annealing time at 120.degree. C. for
different annealing temperatures.
[0020] FIG. 4 shows the ultimate tensile strength (L direction) and
the tensile and compression yield strength (L direction) of 38 mm
plates made of 7449 alloy in example 2 as a function of the
equivalent annealing time at 120.degree. C.
[0021] FIG. 5 shows the compression yield strength of plates made
of alloys A and B in example 3 as a function of the equivalent
annealing time at 120.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The invention is based on demonstrating a shift between the
peak mechanical tensile strength obtained with annealing,
corresponding to what is usually called the T6 temper, and the peak
mechanical compression strength. Although it has been known for a
long time that the upper wing members are stressed mainly in
compression, and that therefore the compression yield strength
controls the dimensions of structural elements of this part of the
wing, metallurgists have always used the tensile strength to define
the T6 temper of maximum strength achieved in annealing.
[0023] The inventors have found that there is a metallurgical state
between tempers T6 and T79 according to prior art, in which the
compression yield strength passes through a peak of between 20 and
25 MPa above the compression yield strength values of these two
tempers.
[0024] The process according to the invention is applicable to
Al--Zn--Mg--Cu type alloys with a high zinc content between 7 and
11%, with a magnesium content between 1.8 and 3%, and preferably
between 1.8 and 2.4%, and a copper content between 1.2 and 2.6% and
preferably between 1.6 and 2.2. The invention is not particularly
useful for a zinc content below 7% since these types of alloy are
not longer used in aeronautical construction for the fabrication of
structural elements stressed in compression. For zinc contents
higher than 11%, difficulties are encountered during industrial
casting of rolling ingots or billets large enough for the
production of plates, sections or forged parts suitable for the
manufacture of the said structural elements.
[0025] The process according to the invention is applicable
particularly for alloys used for manufacturing elements of the
upper wing members of aircrafts, for example 7055, 7349 and 7449
alloys in the form of work hardened products, in other words
rolled, extruded or forged products. This process comprises the
manufacture of an ingot, namely a rolling ingot for rolled
products, a billet or extrusion ingot for extruded products or a
forging ingot for forged products, in a known manner. This ingot is
preferably homogenised at a temperature close to the incipient
melting temperature of the alloy, as described in patent
application EP 0670377. It is then transformed by hot rolling,
extrusion or forging, to the required dimension. The product
obtained is solution heat treated also at a temperature fairly
close to the incipient melting temperature of the alloy, this
temperature being controlled by differential enthalpic analysis.
Solution heat treatment is followed by quenching, usually in cold
water. The quenched product is preferably subjected to controlled
stretching with a permanent set of between 1 and 5%.
[0026] The product is then annealed to obtain the peak compression
yield strength in the L direction. Annealing may be done in a
single step, in other words include a temperature rise gradient
that may or may not be linear as a function of time, a period of
time at a constant temperature within the limit of the temperature
tolerance of the furnace used, and cooling down to ambient
temperature. For single step annealing, the constant temperature is
between 120and 150.degree. C. with a duration within the
parallelogram AEFG in FIG. 2 and preferably between 120 and
145.degree. C. with a duration within the parallelogram ABCD in
FIG. 2. The latter annealing process is a particularly preferred
embodiment of this invention. For example, it can be used for
products made from the 7449 and 7439 alloys. Annealing may also be
done in two steps, with a first step at a temperature between 80
and 120.degree. C., and a second step at a higher temperature
between 120 and 160.degree. C. It may also be done in three steps,
with a first and a second step within the same limits as for the
two-step annealing, and a third step at a lower temperature than
the second step, between 100 and 140.degree. C. Considering the
time necessary for temperature rises in industrial furnaces, it is
difficult to envisage steps lasting less than 2 h, and preferably
less than 5 h.
[0027] In all cases, the two parameters (temperature and duration)
can be converted to a single parameter, the equivalent time at
120.degree. C. defined by the following formula: 1 T ( eq ) = exp (
- 16000 / T ) t exp ( - 16000 / T ref )
[0028] where T is the temperature of the annealing step in
.degree.K, t is the treatment duration in hours and T.sub.ref is
the reference temperature in this case assumed to be 120.degree.
C., namely 393.degree.K. The equivalent annealing time at
120.degree. C. is between 100 and 250 h, and 50 to 200 h more than
the equivalent annealing time for T651 annealing. The annealing
time necessary to reach the peak compression yield strength depends
on the composition of the alloy and particularly the Cu/Mg ratio,
the necessary duration increasing with this ratio.
[0029] The work hardened product, and particularly the rolled,
extruded or forged product obtained using the process according to
the invention, can advantageously be used for the manufacture of
structural elements, particularly for aeronautical construction.
Due to the increase in the compression yield strength resulting
from the process according to the invention, a structural element
manufactured from at least one extruded, rolled or forged product
according to the invention has a better resistance to compression
loads than a structural element with the same dimensions made from
work hardened, extruded or forged products according to prior art.
In one preferred embodiment of the invention, the structural
element is an upper wing member of an aircraft.
EXAMPLES
Example 1
[0030] 38 mm thick plates are made from 7449 alloy. The composition
of the alloy is (% by weight) Zn=8.11, Mg=2.19, Cu=1.94, Si=0.04,
Fe=0.07, Zr=0.09, Cr=0.06, Ti=0.025, the remainder aluminium and
impurities (<0.05 each).
[0031] Plates have been pre-widened to increase the plate width
from 1100 mm to 2500 mm, hot rolled to 38 mm with an exit
temperature of 378.degree. C., solution heat treatment at
475.degree. C., quenching with cold water and controlled stretching
to 2.8% permanent set after waiting for 1 h after quenching.
[0032] Samples taken from the mid-thickness of the plates were
subjected to 11 different single-step or two-step type annealings
as shown in table 7. The rise and fall gradients between steps
being 16.degree. C./h and 65.degree. C./h respectively,
corresponding to the rates observed for industrial heat treatment
furnaces. For each annealing, the equivalent time at 120.degree. C.
t.sub.eq is calculated using the following formula: 2 t ( eq ) =
exp ( - 16000 / T ) t exp ( - 16000 / T ref )
[0033] where T is the temperature of the annealing step in
.degree.K, t is the treatment duration in hours and T.sub.ref is
the reference temperature, in this case assumed to be 120.degree.
C., namely 393.degree.K.
[0034] The 11 tested annealings were between annealing T651 and
annealing T7951 according to prior art, and their parameters and
the corresponding equivalent times are shown in table 1.
[0035] In each case, the static tensile properties in the L
direction were measured (ultimate tensile strength R.sub.m, tensile
yield strength R.sub.0.2 and elongation A) on TOR 6 test pieces
taken from the central part of the plates. The results are the
average of at least two measurements and are shown in table 1 and
in FIG. 3.
2TABLE 1 Teq at Annealing Annealing 120.degree. C. R.sub.0.2(L)
R.sub.m(L) A 1.sup.st step 2.sup.nd step (h) (MPa) (MPa) (%) 24 h-
24 617 661 12 120.degree. C. 48 h- 48 623 661 12 120.degree. C. 96
h- 96 624 655 12 120.degree. C. 6 h-135.degree. C. 29 616 655 12 12
h- 55 619 651 11 135.degree. C. 24 h- 108 619 651 11 135.degree. C.
48 h- 215 611 642 11 135.degree. C. 24 h- 5 h-150.degree. C. 125
620 649 11 120.degree. C. 24 h- 9 h-150.degree. C. 196 613 642 11
120.degree. C. 24 h- 13 h- 265 607 636 10 120.degree. C.
150.degree. C. 24 h- 17 h- 336 595 627 10 120.degree. C.
150.degree. C.
[0036] It is found that close to the peak, the annealing processes
with the lowest temperature, in other words 120.degree. C., give
the highest values of R.sub.0.2 and R.sub.m. For two-step annealing
processes, this effect is controlled by the temperature of the
second step. Furthermore, the peaks for R.sub.0.2 and R.sub.m are
similar, but not at exactly the same location. The T651 peak
treatment can be defined as being the treatment that results in
values of R.sub.0.2 and R.sub.m within 5 MPa of the maximum
potential value, while remaining industrially acceptable. In this
case, it is a 48 h treatment at 120.degree. C.
Example 2
[0037] Samples taken from 38 mm thick plates made of 7449 alloy
with composition of Zn=8.38, Mg=2.15, Cu=1.96, Si=0.04, Fe=0.06,
Zr=0.11, the remainder aluminium and impurities (<0.5% each) are
made in exactly the same way as in example 1.
[0038] Eight different annealings were carried out on these
samples, between annealing T651 defined in example 1 and annealing
T7951. The temperatures and durations of these eight annealings and
the corresponding equivalent times at 120.degree. C. are shown in
table 2.
3TABLE 2 Annealing Parameters Equivalent time A (T651) 48
h-120.degree. C. 48 B 12 h-135.degree. C. 46 C 18 h-135.degree. C.
78 D 24 h-135.degree. C. 102 E 30 h-135.degree. C. 130 F 24
h-120.degree. C. + 5.5 h-150.degree. C. 130 G 24 h-120.degree. C. +
11 h-150.degree. C. 222 H (T7951) 24 h-120.degree. C. + 17
h-150.degree. C. 321
[0039] In addition to the mechanical tensile properties, the
compression yield strength was measured in the L direction on 13 mm
diameter and 25 mm long test pieces, and the electrical
conductivity was measured on samples taken from the surface. The
average results of the two measurements are shown in table 3 and in
FIG. 4, for R.sub.m and R.sub.0.2 in tension, and R.sub.0.2 in
compression.
4TABLE 3 R.sub.m ten R.sub.0.2 ten A R.sub.0.2 comp Conduct
Annealing (MPa) (MPa) (%) (MPa) (MS/m) A 633 676 12.4 596 18.4 B
639 673 11.8 599 18.7 C 637 668 12.0 611 19.1 D 634 663 11.0 614
19.7 E 633 663 10.5 615 20.0 F 635 662 11.2 613 20.1 G 619 648 10.5
608 21.2 H 597 621 10.7 590 21.9
[0040] It is seen that the annealing that gives the peak
compression yield strength (L direction) is at an equivalent time
of the order of 150 h, in other words at an equivalent time
intermediate between T651 annealing and T7951 annealing. The useful
range is between 100 and 250 h of equivalent time at 120.degree.
C., which is 50 to 200 h more than for a T651 annealing. This
annealing that results in the compression peak gives an improvement
of 19 MPa compared with a T651 annealing and 25 MPa compared with a
T7951 annealing.
Example 3
[0041] Plates made of two 7449 alloys with the thicknesses and
compositions indicated in table 4, were made in the same way as in
the previous examples as far as quenching.
5TABLE 4 e Alloy (mm) Si Fe Cu Mg Zn Zr Ti A 30 0.049 0.075 1.87
2.35 8.38 0.11 0.03 B 23 0.045 0.068 1.95 2.27 8.31 0.10
[0042] These plates were annealed as described in table 5, the
first 11 annealings being for alloy A and the last 7 for alloy B.
The compression yield strength R.sub.0.2 in the L direction, and
the modulus of elasticity in compression also in the L direction,
were measured on 13 mm diameter and 25 mm long test pieces taken
from the central part of the plates. The results are shown in table
5, and the yield strength results are shown in FIG. 5 as a function
of the equivalent annealing time at 120.degree. C.
6TABLE 5 Annealing Annealing Annealing R0.2 comp Modulus 1.sup.st
step 2.sup.nd step 3.sup.rd step (MPa) (MPa) 24 h-80.degree. C. 24
h-135.degree. C. 605 70281 24 h- 24 h-135.degree. C. 602 71200
100.degree. C. 24 h- 24 h-135.degree. C. 607 72335 120.degree. C.
24 h- 18 h-140.degree. C. 603 70598 100.degree. C. 24 h- 7
h-150.degree. C. 601 70618 100.degree. C. 24 h- 2.5 h- 607 72302
100.degree. C. 160.degree. C. 24 h- 30 h-140.degree. C. 600 72806
100.degree. C. 24 h- 18 h-140.degree. C. 24 h-120.degree. C. 616
71621 100.degree. C. 24 h- 7 h-150.degree. C. 24 h-120.degree. C.
615 70862 100.degree. C. 24 h- 2.5 h- 24 h-120.degree. C. 622 72569
100.degree. C. 160.degree. C. T7951 587 24 h-80.degree. C. 24
h-135.degree. C. 635 72910 24 h- 24 h-135.degree. C. 611 72222
120.degree. C. 24 h- 18 h-140.degree. C. 614 73244 100.degree. C.
24 h- 7 h-150.degree. C. 610 72349 100.degree. C. 24 h- 30
h-140.degree. C. 596 70181 100.degree. C. 24 h- 7 h-150.degree. C.
24 h-120.degree. C. 621 71303 100.degree. C. T7951 598
[0043] It is found that the peak compression yield strength occurs
for an equivalent annealing time at 120.degree. C. between 100 and
200 h, and that three-step annealing processes give higher values.
Furthermore, it is found that the compression yield strength is
about 15 MPa better than for the T7951 annealing for two-step
annealing processes, and about 25 MPa better for three-step
annealing processes.
* * * * *