DK155838B - PROCEDURE FOR REDUCING LOSS OF REVERSIBILITY BETWEEN THE MARTENSITIC AND AUSTENITIC CONDITIONS IN A METAL MATERIAL THAT CAN BE TRANSMITTED BETWEEN THESE CONDITIONS - Google Patents

Description

DK 155838 B

The present invention relates to a method of reducing loss of reversibility between the mercenitic and austenitic state of a metal material which can be converted between these states and which has shape-recall properties, which loss would otherwise occur when storing the material at room temperature.

Metal materials, e.g. alloys capable of undergoing a reversible conversion from austenitic to martensitic state are known, and some of these may be heat-recoverable articles. Such alloys are e.g. those disclosed in U.S. Patent Nos. 3,021,882, 3,174,851, 3,351,463, 3,567,523, 3,753,700 and 3,759,552, as well as Belgian Patent No. 703,649 and British Patent Nos. 1,315. 652, 15 1,315,653, 1,346,046 and 1,346,047, the four of which are in the name of the Fulmer Research Institute and will henceforth be referred to as the "Fulmer Patents".

Such alloys are also described in NASA Publication SP110, "55-Nitinol - the alloy with of memory, 20 etc." (U.S. Government Printing Office, Washington, D.C., 1972), as well as N. Nakanishi et al., Scripta Metallurgica, 5, 433-440 (Pergamon Press 1971)).

One characteristic of these and other alloys is their ability to undergo shear conversion by cooling from a high temperature (austenitic) to a low temperature (martensitic) state. Thus, if an object of such an alloy is deformed when in its martensitic state, it will remain deformed. If it is reheated to a temperature at which it is austenitic, it will tend to return to its undeformed state. The transition from one state to another, in each direction, occurs over a temperature range.

The temperature at which martensite is formed upon cooling is referred to as Ms, while the temperature at which this process is completed is referred to as Mf, each of these temperatures being those obtained at high 2

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speeds (eg 100 ° C / min) for changing the temperature of the sample, ie. Similarly, the temperatures for beginning and ending the conversion to austenite are referred to as As 5 and Af, respectively. Generally, Mf is a lower temperature than As, and Ms is a lower temperature than Ms may be equal to or lower than or higher than As, depending on the alloy composition as well as the thermochemical history of the alloy The conversion from one form to another can be followed by measuring one of several physical properties of the material except for the one described above. the deformation inverse, such as its specific electrical resistance, which exhibits an anomaly as the transformations take place. If curves of specific resistance or the influence 15 versus temperature are plotted, a line connecting points Ms, Mf, As and Af will go back to Ms, forming a loop, referred to as the hysteresis loop, for many materials Ms and As are at approximately the same temperature.

A particularly useful alloy that possesses heat recoverability and shape memory is the intermetallic compound TiNi disclosed in U.S. Patent No. 3,174,851. The temperature at which deformed articles of the alloys return to their original shape depends on the alloy composition, such as e.g. are disclosed in British Patent Specification No. 1,202,404 and U.S. Patent No. 3,753,700, and the said return to the original form may be effected below, at or above room temperature.

In some commercial applications of heat-recoverable alloys, it is desirable that As be at a higher temperature than Ms, and the reason for this is the following: Many objects made of the alloys are delivered to users in a deformed state, i.e. in the martensitic state. Thus, e.g. couplings for hydraulic components as disclosed in British Patent Specifications Nos. 1,327,441 and 1,327,442, in deformed (i.e. expanded) 3

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state, and users place the expanded coupling over the components, e.g. the ends of hydraulic pipelines to be joined, and then increase the coupling temperature. When the coupling temperature reaches the austenitic conversion region, the coupling will return to or attempt to return to its original shape and shrink around the components to be joined. Since it is necessary for the coupling to remain in its austenitic state during use (e.g., to avoid stress relaxation 10 during martensitic conversion and because the mechanical properties of the austenite are better), the Ms value of the material is chosen so that is below any value it may possibly reach during use, so that the material during use will remain in the austenitic state at all times.

For this reason, after deformation, it must be stored in e.g. liquid nitrogen until used. However, if As, which, as is the case here, means the temperature which marks the beginning of a continuous sigma phase conversion, e.g. as depicted on a voltage-temperature curve, 20 of all the martensite capable of being converted to austenite into the austenitic state could be increased only temporarily, e.g. for one heating cycle, without a corresponding increase in Ms, then the expanded coupling could be stored at a higher and more convenient temperature.

25 in Danish Patent Application No. 418/76, a method is described by which As for certain metal materials can be increased during a heating cycle. In this process, the temperature of the material is first lowered from a temperature at which the material is in the austenitic state to a temperature below its Mf temperature. The material is then heated to a temperature at which the material should normally be completely in the austenitic state, i.e. above the Af temperature. However, the conversion from the martensitic to the austenitic state does not occur if a "slow" heating rate is defined as defined in said application. It must be sufficient

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4 to indicate that it may vary depending on the nature of the metal material, but that it can be readily determined by a person skilled in the art with guidance from the application description.

If, after completion of the slow heating, the metal material is cooled and then reheated at a rapid rate, it does not begin to undergo conversion from the martensitic to the austenitic state until approximately the temperature at which the slow heating is terminated is reached. More importantly, any article made of the material 10 that deforms in the martensitic state, either before or after the slow heating, will not begin to recover to the form in which it existed in the austenitic state before it approximately reaches the temperature at which the slow heating was terminated. This process is kept "thermal preconditioning".

Danish Patent Application No. 5429/87 discloses another method by which the As temperature of metal materials can be increased. In this method, the material is held in a deformed configuration at a temperature above its normal As-Af range for a period sufficient to cause a portion of the deformation to be retained when the clamp is removed. The amount of deformation retained is a function of the temperature at which the material is held and of the duration of the holding time.

The material can be deformed while in its austenitic state. However, it is typical of this austenitic state that such deformation requires considerable force. Accordingly, it is preferable to deform the material while in a more workable state occurring near, within or below the Ms-Mf range, and then to increase the temperature to the desired holding temperature while keeping the material clamped in its deformed state.

35 Analogously to "thermal preconditioning", this method is called "mechanical preconditioning". In one such way 5

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preconditioned material, part of the remaining deformation will be recovered by heating the material at a rapid rate.

As a result of these acknowledgments, it is possible to produce recoverable articles with elevated As temperature. However, it often happens that metal materials that have been converted to the martensitic state exhibit a tendency to lose all or part of their ability to return to the austenitic state upon heating through the As-Af region. and in other cases, the metal materials will not react favorably to either thermal or mechanical preconditioning to raise the As temperature. Therefore, it would clearly be a great advantage if there was a method for preventing loss of these desirable properties.

According to the present invention, there is provided a method by which loss of martensite / austenite reversibility in metal materials can be inhibited and, moreover, make them more responsive to methods for communicating an elevated As temperature.

Accordingly, the invention relates to a method of reducing loss of reversibility between the martensitic and austenitic state of a metal material which can be converted between these states and which has deformation characteristics which would otherwise occur when storing the material at room temperature, and this method is characterized in that the metal material is maintained at a temperature above the Ms temperature while in a metastable austenitic state at a temperature above room temperature for a time sufficient to substantially reduce the loss, the temperature and the holding time is such that there is no significant conversion of the metal material into a phase which does not undergo any reversible austenite-martensite conversion.

35 A further result of this method is an improved ability to be preconditioned. The holding time that is

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6 necessary to achieve these purposes depends on the composition and holding temperature. Normally, the required holding time decreases as the temperature increases. The process of the invention may be termed an "aging" and the products thus treated may be referred to as "aged".

The process of the invention also provides alloys that are aged in this manner and are better suited to being heat-recoverable.

The invention provides a method 10 for reducing loss of reversibility between the martensitic and austenitic states in such metal materials which is capable of undergoing reversible conversions between the martensitic and austenitic states with changes in temperature. When metal materials are subjected to the method of the present invention, their pseudo-elasticity, i.e. their ability to transform from the austenitic to the martensitic state with subsequent deformation when subjected to stresses, and to return to the austenitic state and regain their original form.

The loss of reversibility mentioned above manifests itself in various ways. In some cases, a sample of a metal material which has been cooled to below Mf will fail to fully or partially return 25 to austenite when allowed to warm through its normal As-Af region and which as a result, the recovery of a deformation which is communicated to the sample in its martensitic state may fail at least partially when the sample is heated under conditions where a recovery could otherwise be expected to occur.

In other cases, although it may undergo reversible conversion to austenite after conversion to martensite and subsequent rapid heating, the material may not react to either thermal or mechanical preconditioning when attempting to raise its As. temperature because reversibility is lost in the preconditioning.

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The invention also encompasses a method for improving the response of certain alloys to mechanical or thermal preconditioning (i.e., improving the degree of elevated heat recovery) by carefully controlling the aging so that it takes place within certain time and temperature limits, although total heat recovery can thereby be reduced. The optimum aging conditions can be found in routine experiments, and it is therefore sufficient to state that in these materials, as shown in the following examples, a too short aging time or a too low temperature may provide insufficient reversibility as discussed above. and too long an aging time and too high a temperature may provide insufficiently usable, increased reversibility, although the overall reversibility is improved in the latter case.

The process of the invention is generally applicable to many different metal materials which can undergo reversible austenite-martensite conversions, but it is particularly suitable for metal materials which are alloys, and especially 20 alloys which form electron compounds. Preferred electron compounds are those which correspond to the Hume-Rothery term for structurally analog space-centered cubic phases (e.g., β-brass), or electron compounds having ratios of approx. 3 valence electrons for 2 atoms, see A.S.M.

25 Metals Handbook, Volume 1, 8th ed. (1961) page 4.

Among usable alloys may be mentioned β-phase alloys, typical examples of which are copper / zinc and copper / aluminum alloys forming ^ alloys of the space-centered cubic type characteristic of β-30 measurement. These include copper and zinc alloys or copper and aluminum, in which zinc and aluminum can at least partially replace each other, and may even be partially replaced by other alloying elements, e.g. silicon, tin, manganese or mixtures thereof. Some of the alloys herein are discussed in detail in the aforementioned Danish Patent Application No. 418/76, which describes the thermal

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8 preconditioning method. Preferred alloys of this type include alloys having between about. 60 and approx. 85% by weight copper with varying amounts of zinc and / or aluminum together with silicon, manganese or mixtures thereof, e.g.

5 alloys containing between 0 and approx. 40% by weight of zinc, between 0 and approx. 5% by weight of silicon, between 0 and approx. 14% by weight of aluminum and between 0 and approx. 15% by weight of manganese, forming structures of the space-centered cubic type. Ternary, quaternary and more complex alloys 10 of copper can be used. In the following examples, more specific alloys will be discussed in more detail within these limits, but it should be emphasized that the process of the invention can also be used for alloys falling outside these limits of the preferred embodiments.

Thus, e.g. use within the scope of the invention the process of the invention for alloys based on metals other than copper.

Alloys of the type described above are obtained in β-phase according to well known methods. Generally, the β phase 20 is obtained by quenching the alloy from an elevated temperature at which it essentially exists as a stable / 3 phase to a temperature at which it will exist as a metastable β phase. If the quench rate is too low, too much of a second phase may be formed which does not undergo the reversible austenite-martensite conversion, but an alloy present at least substantially in the / 3 phase and e.g. containing more than 70% J8 phase, however, may still possess substantially the same useful properties as the pure / 3 phase structure.

As indicated above, in the process of the invention, the metal material is kept at a temperature at which it is in the austenitic state for a time sufficient to reduce loss of at least a portion of the reversibility between martensite and austenite.

The time required to reduce the loss of rever-

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9 visibility may vary depending on the composition of the product and the holding temperature. Since the product's response to the aging process is affected by these variables, it is not possible to specify exact time and temperature limits, 5 necessary for obtaining the best results with any composition, but the optimum conditions can easily be determined by experts.

In the case of β-phase alloys, the aging temperature must be a temperature at which there is no appreciable conversion of β-phase into a phase which does not undergo reversible austenite-martensite conversion.

For β-phase alloys of copper such as those described above containing varying amounts of zinc, aluminum, silicon, manganese and combinations thereof and with a Ms temperature below room temperature, an aging at between about 50 and approx. 125 ° C for a period of between approx. 5 minutes and 3 or 4 hours will usually be adequate. However, aging at higher or lower temperatures or for longer or shorter periods can usually be beneficial. For other 20 compositions, the time and temperature can be varied, but the conditions for obtaining optimal results can be easily determined by comparing the degree of reversion between martensite and austenite that occurs in representative samples, e.g. by measuring the amount of deformation recovered as a result of rapid heating of a sample.

It will be clear that aging does not have to be carried out at a single temperature, but that the temperature can be changed one or more times or continuously varied during the aging time.

30 In the drawing, the figures illustrate the relationships discussed in greater detail in Example 1, FIG. 1a and 1b are diagrams showing the effect of aging on Cu-Si-Zn alloys subjected to thermal preconditioning; and FIGS. 2a, 2b and 2c are diagrams showing the effect of aging on Cu-Al-Zn alloys subjected to this preconditioning.

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10

The following examples will serve to illustrate the method of the invention.

Example 1.

A number of experiments were carried out to determine the degree of response of various compositions in the Cu / Zn / Si and Cu / Zn / Al systems to the aging method of the invention and the effect on the thermal preconditioning. Samples of the alloys were cast from melts with 10 different contents of copper, zinc and either silicon or aluminum. The moldings were cold rolled into strips and cut into pieces. 37 x 3 x 0.75 mm and all the specimens were heated until they passed to the high temperature phase, where everything is J8 phase and then quenched in water. Half of the specimens were aged at 100 ° C for 10 minutes, while the other half was not aged, and all specimens were deformed by bending at -79 ° C to produce an outer "fiber" deformation of 6%. After the deformation, the specimens were released and measured to determine how much deformation was maintained. The samples of the aged and the aged group were then heat treated according to one of the following three schemes: (1) heated rapidly by immersion in liquid of 408 C, cooled to room temperature and measured to determine how much deformation there was recovered, and then heated rapidly by immersion in liquid of 200 ° C and again returned to room temperature to determine how much further deformation recovery had occurred, (2) heated slowly at a rate of 0.25eC / min. from -79 ° C to + 40 ° C, cooled to room temperature, measured to determine how much deformation was recovered, then heated rapidly by immersion in liquid of 200 ° C, cooled to room temperature and measured to determine where much additional recovery had occurred, or (3) treated as specified in (2), except that the slow heating occurred at 1 ° C per 24 minutes rather than at 0.25 ° C / min.

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A "value number" for the response of each product tested for the regulation of the temperature range is obtained by the number indicating the percentage recovery that occurs above 40 "C for slowly heated samples, minus 5 the percentage refund above 40" C for rapidly heated. specimens divided by 5 (which is the ideal percentage recovery after the elastic retraction that accompanies the removal of the bending stress), ie:

10 N

/ rest. above 40 ° C - residue. above 40 ° C i) Value = 100 x ^ lanCfsoIIlt up · Tries up quickly. samples /

Examples of metal materials which have proved particularly suitable for treatment by the process of the invention will be described in more detail below with reference to the drawings.

In the drawing, FIG. 1a and 1b are thus diagrams illustrating the effect of aging on Cu / Zn / Si alloys subjected to thermal preconditioning; and FIGS. Figures 2a, 2b 25 and 2c show diagrams illustrating the effect of aging on Cu / Zn / Al alloys subjected to thermal preconditioning.

In FIG. 1a and 1b, the value value defined above is plotted against the composition in topographic format. The long 30 axes for the zones of constant value are generally parallel to conversion isotherms. Alloys with lower conversion temperatures are located on the upper left, while alloys with higher conversion temperatures are located on the bottom right of the diagrams. A clear optimum is seen in the range 35 1.8-2.7% Si, 66.2-67.5% Cu and the balance Zn (29.8-32.0%). A comparison of FIG. 1a and fig. 1b shows that aging for 10 minutes at 100 ° C extends the optimum from the same general central area. The arbitrary choice of 40 ° C as the end of the slow heating obviously disqualifies alloys 40, whose usual conversion range is above or partly above + 40 ° C, ie. those in the lower right part of the fi-

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The 12-hour curve, but it will be clear that a low value number on the curve does not indicate the inadequacy of these alloys for use according to the invention, but merely that as a preconditioning temperature a temperature other than + 40 ° C must be chosen. Similarly, for the alloys in the upper left of the figure, a low value of the curve need not mean that they are not responsive to the process of the invention. In these cases, a low value simply means that the slow-heating rate chosen was not a rate that prevented recovery until 40 ° C was reached. The choice of 40 ° C causes the value-isotherm to approach the high conversion temperature side (bottom right). Alloys in the lower right region are responsive to the slow heating process, as shown by the Cu / Zn / Al data listed below.

A topographic representation of the results with respect to the value number of the Cu / Zn / al system can be found in FIG. 2. Here, too, the zones with constant value numbers lie parallel to the transformation isotherms. The effect of aging is to extend the optimum in the transverse direction of the long narrow area of the diagram.

Five alloy compositions with a normal As temperature at or above 40 ° C were used to test the change in the recovery range at higher temperatures. Here, too, the same general test method was used, but the slow heating continued to + 100 ° C instead of stopping at + 40 ° C. The results for the aged samples are shown in Figs.

2c, from which it is seen that the new optimum is parallel to the optimum of FIG. 2b, but moved, as expected, 30 towards higher conversion compositions.

Although the recovery range is mobile in Cu / Zn / Al alloys, mobility seems more restricted than in Cu / Zn / Si alloys.

As the non-aged Cu / Zn / Al samples lost their memory properties due to the slow heating to 35 100 ° C, while the aged samples did not, it is seen that the aging treatment works to maintain reversibility 13

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of the conversion in the higher temperature range.

The Ib and 2b selected aging periods and conditions result in certain compositions having optimum properties, and other aging times and conditions resulting in 5 other compositions having the same or substantially similar optimum properties. The aged alloys in the regions bounded by lines 40, 60 and 80 of FIG. 1b and line 20 of FIG. 2b is particularly suitable for the method according to the invention.

10

Example 2.

A series of test pieces of an alloy containing 64.5 wt% Cu, 34.5 wt% zinc and 1.0 wt% silicon quenched after 5 minutes at 860 ° C in 20 ° C water and then aged at 50 ° C. C for a period of up to one week. After cooling to below Mf temperature, the specimens were reheated at a rate of 10-20 ° C / min. It was found that only a slight conversion of martensite to β phase (as measured by changes in resistivity) occurred during heating of the sample for 20 minutes, while there was no conversion in the sample for 45 minutes and a complete transformation in the test pieces aged for 90 minutes or more. Other samples of the same alloy were given the same heat treatment and, after aging, deformed 8% 25 under tensile stress at -50 ° C and reheated. The degree of heat recovery was approximately proportional to the amount of martensite found to be converted by the resistivity tests on undeformed specimens. Application of the method of the invention with aging for at least 45 minutes allowed 30 to impart permanent heat-recoverable properties to this alloy.

After aging for 5 minutes at 20 ° C before cooling to -50 ° C, the heat-recoverable deformation was 2.30%, while after 45 minutes aging at + 50 ° C before cooling to -35 50 ° C, 6.20 %. After longer aging, this value increased slowly to 6.50% after 3 hours and to 7.0% after 1 14

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week.

Example 3

A series of samples of an alloy containing 66.50 wt% copper, 31.75 wt% zinc and 1.75 wt% silicon were quenched after 5 minutes at 850 ° C in 20 ° C water and then aged at 50 ° C for varying periods of up to 1 week and deformed 8% at -50 ° C. After 4 minutes at 20 ° C (the least aged sample), the heat-recoverable deformation was 0.1%, after For 45 minutes at 50 ° C it was still 0.1% and after 90 minutes it had only increased to 0.55%. After 3 hours, the heat-recoverable deformation was increased to 0.70%, after 1 day to 1.0% and after 2 days to 3.9%. It is seen that the increase in silicon-15 content seems to necessitate an increase in the aging time to achieve improved recovery.

Example 4

16 test pieces of an alloy of the composition 20 80.8 wt% Cu, 10.5 wt% Al and 8.7 wt% Mn were β-ironed at 800 or 900 ° C for 3 or 6 minutes and then quenched room temperature water. Half of the test pieces were aged for 10 minutes at 100 ° C, while the others were not aged. All the samples were deformed by bending at -79 ° C to an outer fiber deformation of 6%, after which the clamp was removed. Half of the samples were heated to 100 ° C at 0.25 ° C / min, cooled to room temperature and then rapidly heated to 200 ° C. The other half was rapidly heated to 100 ° C, cooled to room temperature and then heated rapidly to 200 ° C. The rate of rapid heating was more than 100eC / min. An analysis of the deformation recovered during the rapid heating to 200 ° C as a function of the controlled variables showed that thermal preconditioning appreciably increases the amount of recovery occurring above 100 ° C. For this particular alloy, a statistical analysis showed that aging did not have 15

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any effect.

Gennemsnitsvirknigner:

Percent deformation recovered above 100 ° C: 5 Rapidly heated 0.39%

Preconditioned 1.89%.

This experiment was repeated with an alloy containing 80.49% by weight Cu, 10.5% by weight Al and 9.01% by weight Mn. Analysis of the deformation recovered under rapid heating to 200 ° C as a function of the controlled variables showed the significance of aging versus no aging and non-preconditioned versus preconditioned.

Average effects:

Percent deformation recovered above 100 ° C: 15 Non-aged 1.00%; rapidly heated 0.15%; aged 0.36%; preconditioned 1.21%.

Example 5

Samples of an alloy containing 79.2 wt% 20 Cu, 10.0 wt% Al and 10.8 wt% Mn were-annealed at 550 ° C for 5 minutes and quenched in water of 20 ° C. As a result of this treatment, the alloy had a Ms temperature of -20 ° C. The test pieces were then either aged for 5 minutes or 1 hour at 50 ° C and then cooled to -30 ° C or cooled to 25 -30 ° C immediately after water quenching without aging. All the samples were deformed 4% under tensile stress at -30 ° C and the clamp was canceled.

Half of the samples were immediately heated at a very high rate by immersion in liquids of 20, 40, 100 and 30 ° C, and the incremental degree of deformation recovered as a result of each immersion was noted.

The other test pieces were initially slowly heated at 6 ° C / min. to 40 ° C, then re-cooled to -30 ° C and heated rapidly as for the first set of samples. The results are given in the table below.

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Table I

Aging _Restitution_ 5

Resul * Deform. Temp. time Heating * at 40 * C above 40 * C

tat (X) almost. (X) (X) 1. 3.8 non-fast 1.4 2.1 10 6 * C / min. to 40 ° C, 0 2. 3.3 non-aged cooling and rapid 0.3 1.2 op.

3. 3.2 50 ° C 5 min. fast 3.1 0 6'C / min. to 40 * 0, 0.3 4. 3.7 50 * C 5 min. cooling and rapid 0.3 2.8 rev.

20 5. 3.6 50 ° C 1 h fast 3.35 0 6 * C / min. to 40 ° C, 2.5 6. 3.4 50 ° C 1 hr. cooling and rapid 0.3 0.1 25 op.

If you first look at the samples that heated quickly immediately after the deformation, the refund was complete at 40 ° C in the samples aged for 5 minutes and 1 hour, but in the non-aged sample most of the refund occurred at over 40 ° C. In the samples initially heated at 6 ° C / min. to 40 ° C, no recovery occurred at 40 ° C in this first heating cycle in the non-aged samples and 35 in the samples aged for 5 minutes at 50 ° C, but after re-cooling and rapid heating again occurred most of the refund is above 40 ° C. The sample, aged for 1 hour at 50 ° C, showed almost complete recovery at the initial heating cycle at 6 ° C / min to 40 ° C.

40 These observations show that aging can decrease the As- temperature, since in non-aged samples there was a noticeable recovery above 40 ° C without preconditioning (see results 1, 3 and 5), but the degree of heat-recoverable deformation obtained , when a sample is thermally preconditioned, 45 is enhanced by aging (see Results 2 and 4). Aging also affects 17

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the rate of thermal preconditioning required for the slow heating. Thus, for a sample aged only for 5 minutes at 50 ° C, it was 68C / min. a "slow" heating rate, since there was little recovery before 5 40 ° C (see result 4), while for a sample aged for 1 hour at 50 ° C, a heating rate of 6 ° C / min. had to be termed a "rapid" heating, as most of the heat-recoverable deformation recovered during the preconditioning attempt. The overall effect of these results is to show that for a given alloy there may be an optimal aging treatment, but a treatment which can be readily determined by one of ordinary skill in the art prior to thermal preconditioning.

Example 6

An alloy containing 64 wt% copper, 35 wt% zinc and 1 wt% silicon was investigated and had a Ms temperature of -40 ° C.

Samples were irrigated for 5 minutes at 860 ° C, quenched in water of 208C and then aged for various 20 periods in the metastable β phase, which was performed at 508C in this series of experiments. After insertion into a tensile loading means (about 5 minutes for setting at ambient temperature), the specimens were cooled to -658C and deformed 8% upon drawing. After the deformation, a clamping member was placed on the pulling device so that no contraction could occur, but that the blanks were free to undergo spontaneous expansion if such occurred. The sample thus loaded was then placed in water of + 208 ° C, which gave a very high heating rate, and kept at this temperature for various periods before cooling to below the Mf temperature. The specimens were released from the clamp during the cooling with a slight expansion compared to the original dimensions after the deformation. The clamping member was removed from the apparatus so that the specimens now in their "preconditioned" state were free to heat-recover when they were recovered.

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ICE

heat "quickly" in an oven set to 600 ° C.

The ash temperatures and the heat-recoverable deformations were measured as a function of the two main variables, namely the aging time at 50 ° C prior to deformation and the holding time below 5 clamping at 40 ° C.

The results of "mechanical preconditioning" are listed in Table II. For each aging time at 50 ° C, some specimens were also rapidly heated directly after deformation at -65 ° C in order to compare the effect of "mechanical preconditioning" on the Asperature temperature.

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TABLE II

Wool- For cond. Refund Total Hold Time Deform. Ag-temp. * C over As No. 2 refund

5 time at 40 ° C X 1. 2. X X

no for- 7.05 -50 - - 6.50 cond.

5 min. 10 sec. 6.90 -43 -4 5.65 6.80 10 am-30 sec. 7.10 -37 31 4.15 5.65 temp. 1 min 6.90 -40 19 4.80 5.90 5 min. 7.65 -37 59 2.90 3.95 10 min. 6.95 -17 23 2.80 3.55 1 hour 7.10 -45 19 3.10 4.00 15 no for- 7.25 -33 - - 6.95 cond.

45 min. 10 sec. 6.75 -49 -9 5.30 6.55 v. 50 * C 30 sec. 6.35 -52 4 4.40 5.85 20 1 min. 7.10 -43 23 4.45 5.70 5 min. 7.35 -40 20 5.60 7.00 10 min. 7.20 -51 19 3.65 5.15 1 hour 7.55 -44 54 2.65 4.20 25 no for- 7.00 -32 - 6.75 cond.

3 hours 10 sec. 7.25 -41 -4 5.75 7.00 v. 50 ° C 30 sec. 7.20 -32 15 4.15 5.65 1 min. 7.05 -30 19 5.65 6.85 30 5 min. 6.85 -47 13 4.80 6.20 10 min. 7.20 -32 29 5.65 6.65 1 hour 7.30 -37 38 4.15 5.25 5 hours 7.15 -44 44 5.60 6.75 16 hours 7.50 -39 80 3, 75 5.25 35 no for- 7.20 -27 - 6.70 cond.

24 hours 10 sec. 7.05 -37 -4 5.85 6.55 v. 50 ° C 30 sec. 7.25 -42 -5 5.80 7.25 40 1 min. 7.45 -43 0. 5.70 6.95 5 min. 7.50 -35 24 5.75 6.70 10 min. 7.50 -42 35 5.85 7.25 1 hour 7.80 -34 29 4.70 5.80 5 hours 7.40 -34 35 5.05 5.95 45 16 hours 7.15 -47 69 2 , 90 4.70 no for- 7.10 -33 - - 6.80 cond.

1 week 10 min. 7.00 -28 33 5.60 6.45 50 v. 50'C 1 hour 7.25 -37 47 5.20 6.20 5 hours 7.45 -37 40 5.15 6.70 16 hours 7, 55 -40 33 5.60 6.70 20

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Table II clearly shows the tendency for As 2 to increase as the holding time at 40 ° C is increased and in many cases the temperature exceeded 40 ° C. to As no.

2) diminished with increasing holding time at 40 ° C, and this loss in restitution occurred mainly in the part of the heat-recoverable deformation which lies between As # 2 and Af. Increasing the aging time at 50 ° C, in metastable β phase, greatly improved the overall heat-recoverable deformations, but had little effect in reducing As temperature # 2.

Claims (3)

A method of reducing loss of reversibility between the martensitic and austenitic state in a metal material which can be converted between these states and which has deformation properties, which loss would otherwise occur when storing the material at room temperature, characterized in that maintaining the metal material at a temperature above the Ms temperature while in a metastable austenitic state 10 at a temperature above room temperature for a time sufficient to substantially reduce the loss, the holding time and temperature being such that no significant transformation of the metal material into a phase which undergoes no reversible austenite-martensite conversion. Process according to claim 1, characterized in that the material is heated to a temperature substantially above room temperature before quenching and then quenched. Process according to claim 2, characterized in that the material is quenched to a temperature at which practically all the material remains in the austenitic state.