METHOD OF PRODUCING STRENGTHENED ALLOY

To provide a method of producing a strengthened alloy capable of shortening the time necessary for aging treatment and obtaining a strengthened alloy with enhanced tensile strength. The method of producing a strengthened alloy comprises a solution treatment step S1 of immersing an alloy material into molten lithium held at a solution treatment temperature higher than solution temperature of solute metal of the alloy material, a solution stop step S2 of immersing the alloy material into molten lithium held at a cooling temperature lower than the solution treatment temperature after the solution treatment step S1, an aging treatment step S3 of immersing the alloy material into molten lithium held at an aging treatment temperature lower than the solution temperature after the solution stop step S2, and an aging stop step S4 of immersing the alloy material into molten lithium held at an aging stop temperature lower than the aging treatment temperature after the aging treatment step S3.

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Description

This application is based on and claims the benefit of priority from Japanese Patent Application No. 2009-103733, filed on 22 Apr. 2009, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing a strengthened alloy.

2. Related Art

Intended mechanical properties of carbon steels and other alloys have been heretofore obtained by solution treatments such as quenching and then aging treatments such as tempering.

Among others, titanium alloys are lightweight, highly strong, highly heat-resistant, and highly corrosion-resistant, therefore, applications thereof have been rapidly growing in recent years. The basic crystal structure of metal titanium is close-packed hexagonal (α-phase) at room temperature; however, body-centered cubic crystal (β-phase) is formed at higher temperatures than the β-transformation temperature (about 885° C. in pure titanium) and α-phase is formed again when being cooled further. With regard to titanium alloys containing various alloy elements such as aluminum (Al) and vanadium (V) in metal titanium, various metal structures are formed by changing their constituent ratios, heating temperature, heating rate, or cooling rate, and titanium alloys exhibit various charcteristics after heating.

When an α+β type alloy among titanium alloys, in which α-phase and β-phase coexist at ambient temperature, undergoes a solution treatment by heating to a temperature that is higher than that at which alloy elements disperse into solute metal to form a solid solution (solution temperature), a β-phase is formed in the metal structure and then α′-martensite phase is formed inside the β-phase upon rapid cooling. When the solution-treated alloy is further heated at a temperature that is less than the solution temperature (aging treatment), a fine α-phase is formed inside the β-phase. Structural ratio of the phases formed in the metal structure and crystal size of the phases change in this stage; therefore, alloys are formed that have a strength or elongation different from that before the heat treatment.

Here, means is publicly known in which a solution-treated titanium alloy is heated to a predetermined temperature in an induction heating apparatus and aging-treated by holding the titanium alloy at a constant temperature for a predetermined period followed by air cooling (see Non-Patent Documents 1 and 2). Furthermore, means is publicly known in which a solution-treated titanium alloy is aging-treated by immersing into molten lithium at a predetermined temperature followed by natural cooling (see Patent Document 1).

Japanese Unexamined Patent Application, First Publication No. 2006-016691

Tatsuro Morita and three researchers, “Strengthening of Ti-6Al-4V alloy by Short-Time 2-Stage Induction Heat Treatment”, J. Japan Inst. Metals, The Japan Institute of Metals, October 2002, Vol. 68, No. 10, pp. 862-867.

Tatsuro Morita and three researchers, “Influence of Short-Time Duplex Heat Treatment on Fatigue Strength of Ti-6Al-4V Alloy”, Journal of the Society of Materials Science, Japan, The Society of Materials Science, Japan, April 2007, Vol. 56, No. 4, pp. 345-351.

SUMMARY OF THE INVENTION

However, when titanium alloy is heat-treated by rapidly heating to a predetermined temperature (e.g. by 8 seconds) in an induction heating apparatus as described in Non-Patent Document 1, the surface temperature of the induction-heated titanium alloy is unlikely uniform and tends to be heated higher than the predetermined temperature due to overshoot of the heating apparatus. Therefore, in Non-Patent Document 2 published later, the induction heating is improved so that the time required to reach the predetermined temperature is longer (e.g. by 360 seconds) when the heat treatment is carried out in an induction heating apparatus, this however requires a longer time for the heat treatment. Furthermore, although the alloys obtained in accordance with the means of Non-Patent Documents 1 and 2 show an increase in 0.2% proof stress and tensile strength, ductility tends to decrease against an increase in strength.

Another means may be exemplified in which heat treatment is carried out by immersing a solution-treated titanium alloy into molten lithium followed by natural cooling as described in Patent Document 1. As a result of treatment of titanium alloy using the means of the present inventors, all of 0.2% proof stress, tensile strength, and ductility were enhanced, but these 0.2% proof stress, tensile strength, and ductility are desired to be enhanced further.

Accordingly, it is an object of the present invention to provide a method of producing a strengthened alloy that can shorten the time necessary for heat treatment and obtain the strengthened alloy of which 0.2% proof stress, tensile strength, and ductility are enhanced further.

The present inventors have discovered that when a solution-treated alloy material undergoes an aging treatment step of immersing into molten lithium held at an aging treatment temperature and then an aging stop step of immersing into molten lithium held at an aging stop temperature lower than the aging treatment temperature, excessive growth of α-phase with larger tensile strength is suppressed and transformation of β-phase with larger ductility, formed between α-phases, into α-phase is suppressed, thereby achieving the present invention.

In a first aspect of the present invention, a method of producing a strengthened alloy, the method comprising: a solution treatment step of immersing an alloy material into molten lithium held at a solution treatment temperature higher than solution temperature of solute metal of the alloy material; a solution stop step of immersing, after the solution treatment step, the alloy material into molten lithium held at a cooling temperature lower than the solution treatment temperature; an aging treatment step of immersing, after the solution stop step, the alloy material into molten lithium held at an aging treatment temperature lower than the solution temperature; and an aging stop step of immersing, after the aging treatment step, the alloy material into molten lithium held at an aging stop temperature lower than the aging treatment temperature.

In a second aspect of the method according to the first aspect of the present invention, the alloy material is immersed into molten lithium at not more than 350° C. in the aging stop step.

In a third aspect of the method according to the first or second aspect of the present invention, a titanium alloy is used as the alloy material.

According to the present invention, when a solution-treated alloy material undergoes an aging treatment step of immersing into molten lithium held at an aging treatment temperature and then an aging stop step of immersing into molten lithium held at an aging stop temperature lower than the aging treatment temperature, excessive growth of α-phase with larger tensile strength is suppressed and transformation of β-phase with larger ductility, formed between α-phases, into α-phase is suppressed. Therefore, the method of producing a strengthened alloy, that can shorten the time necessary for heat treatment and obtain the strengthened alloy of which 0.2% proof stress, tensile strength, and ductility are enhanced further, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing an embodiment of the method of producing a strengthened alloy according to the present invention;

FIG. 2 is a graph showing a change in surface temperature of an alloy material in the method of producing a strengthened alloy according to the present invention; and

FIG. 3 is a cross-sectional view showing a production device preferably used in the method of producing a strengthened alloy according to the present invention.

 DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the method of producing a strengthened alloy according to the present invention is explained with reference to figures. FIG. 1 is a flow chart showing an embodiment of the method of producing a strengthened alloy according to the present invention. FIG. 2 is a graph showing a change in surface temperature of an alloy material in the method of producing a strengthened alloy according to the present invention. FIG. 3 is a cross-sectional view showing a production device preferably used in the method of producing a strengthened alloy according to the present invention.

As shown in FIG. 1, the method of producing a strengthened alloy of this embodiment includes a solution treatment step S1, a solution stop step S2, an aging treatment step S3, and an aging stop step S4.

Alloy Material

The alloy material 50 used in this production method is an alloy of which the solution temperature is higher than the melting point (181° C.) of molten lithium and lower than the boiling point (1342° C.) of molten lithium among the materials of which the tensile strength of α-phase is larger than that of β-phase and the breaking elongation of β-phase is larger than that of α-phase. The alloy material 50 satisfying these requirements may be exemplified by titanium alloy. More specifically, Ti-6Al-4V alloy may be exemplified. When the Ti-6Al-4V alloy is used, a part of metal structure thereof transforms the phase from α-phase of hexagonal close-packed crystal to β-phase of body-centered cubic crystal having a large breaking elongation by the solution treatment step S1 to heat rapidly the surface temperature of the alloy material to a solution treatment temperature higher than 850° C.

The alloy material 50 may be shaped by rolling or cutting work prior to the heat treatment. Thereby, processing strains accumulate inside the alloy material 50, therefore, crystal nuclei can easily form when the alloy material 50 undergoes heating and cooling and a finer metal structure can be achieved after the heat treatment.

Heat Treatment Device

The heat treatment device used in the production method may be exemplified by the heat treatment device 1 equipped with a solution treatment bath 12, a solution stop bath 13, an aging heating bath 14, and an aging stop bath 15 which are inside a heat treatment room 11 as shown in FIG. 3, for example. The heat treatment device 1 is equipped with a transport mechanism 27 to move and sequentially immerse the alloy material 50 into the molten lithium L1 to L4 respectively contained in the solution treatment bath 12, the solution stop bath 13, the aging heating bath 14, and the aging stop bath 15.

Here, the heat treatment device 1 may be provided with a sealable entrance room 30 at one side of the heat treatment room 11 and a sealable exit room 40 at another side. In this stage, the entrance room 30 is provided with a door 31 to separate the inside of the entrance room 30 and ambient air, and a door 32 to separate the inside of the entrance room 30 and the inside of the heat treatment room 11. On the other hand, the exit room 40 is provided with a door 41 to separate the inside of the exit room 40 and the inside of the heat treatment room 11, and a door 42 to separate the inside of the exit room 40 and ambient air. The heat treatment room 11 is provided with the entrance room 30 and the exit room 40 and an inactive gas such as argon gas non-reactive with the molten lithium L1 to L4 is filled inside the heat treatment room 11, thereby the inactive gas is unlikely to leak outside the heat treatment room 11, therefore, reaction between the molten lithium and air or moisture can be inhibited.

(S1) Solution Treatment Step

The alloy material 50 is heated and the surface temperature of the alloy material 50 is held at a solution treatment temperature T1 higher than the solution temperature of the alloy material 50. Thereby, a part of alloy elements of the alloy material 50 disperses into solute metal to form a solid solution of β-phase of the solute metal.

A means to immerse the alloy material 50 into the molten lithium L1, heated at the solution treatment temperature T1, is used as the means to heat the alloy material 50. Thereby, temperature overshoot of the alloy material 50 is avoided. Consequently, the decrease of tensile strength, breaking elongation, and 0.2% proof stress of the alloy material 50 due to abnormal heating of the alloy material 50 can be suppressed. Furthermore, heating can be rapidly conducted to the alloy material 50 through the molten lithium with a higher thermal conductivity, therefore, the alloy material 50 can be heated more promptly and the solid solution of β-phase of the solute metal can be formed quickly. Therefore, inclusion of hydrogen and/or oxygen into the metal structure can be reduced during the solution treatment of the alloy material 50 while avoiding the growth and enlargement of crystals in the metal structure, thus the decrease of tensile strength, breaking elongation, and 0.2% proof stress due to crystal enlargement or void formation in the metal structure can be suppressed.

The heating means may be specifically exemplified by the means of containing the molten lithium L1, heated at the solution treatment temperature T1, in the solution treatment bath 12 and immersing the alloy material 50, conveyed from the entrance room 30 to the heat treatment room 11 of the heat treatment device 1, into the molten lithium L1, as shown in FIG. 3. Here, a heater 21 to heat the molten lithium L1 is used to control the temperature of the molten lithium L1 contained in the solution treatment bath 12, for example.

The solution treatment temperature T1 in the solution treatment step S1 is higher than the solution temperature of the alloy material 50. Here, the solution temperature of the alloy material 50, which depends on composition etc. of the alloy material 50, is often in the range of 700° C. to 1100° C., for example. More specifically, the solution temperature for the Ti-6Al-4V alloy is in the range of 850° C. to 1000° C.

The solving time, to hold the surface temperature of the alloy material 50 heated in the solution treatment step S1 at the solution treatment temperature T1, is appropriately set depending on the temperature of the heated alloy material 50, thickness of the alloy material 50, and application of the strengthened alloy, and the time may be 10 to 300 seconds. Particularly, when the solving time is at least 10 seconds, the temperature of the inner portion of the alloy material 50 can be sufficiently increased before the end of the solving time, therefore, the solid solution of β-phase of the solute metal and alloy constituents can also be formed in the inner portion of the alloy material 50. On the other hand, when the solving time is not more than 300 seconds, growth and enlargement of β-crystal formed by heating can be suppressed, therefore, miniaturization of the metal structure of the alloy material 50 can be promoted. Here, the means to hold the surface temperature of the alloy material 50 may be exemplified by holding the temperature of molten lithium L1, into which the alloy material 50 is immersed, at the solution treatment temperature T1 by adjusting with the heater 21, for example.

(S2) Solution Stop Step

After the solution treatment step S1, the alloy material 50 is cooled to the solution stop temperature T2 that is lower than the solution temperature (step S21 of FIG. 2). The phase transition from β-phase to α-phase is suppressed by cooling the alloy material 50 to the solution stop temperature T2 and thus a part of β-phase remains in the metal structure under a condition of solid solution with alloy constituents. When the alloy material 50 is rapidly cooled at this stage, α′-martensite phase is formed in the metal structure of the alloy material 50. Accordingly, a microstructure of solid solution of alloy elements consisting of the α′-martensite phase and the remaining β-phase can be formed in the alloy material 50 by rapidly cooling to the solution stop temperature T2. The solution stop temperature T2, which is appropriately selected depending on the composition of the alloy material 50, is preferably lower than 500° C. and more preferably lower than 300° C. when the Ti-6Al-4V alloy is used as the alloy material 50, for example.

A means of immersing the alloy material 50 into the molten lithium L2 held at the solution stop temperature T2 is used as the means of rapidly cooling the alloy material 50 to the solution stop temperature T2. Thereby, heat is rapidly removed from the alloy material 50 by the molten lithium L2 with a higher thermal conductivity, thus the alloy material 50 can be cooled rapidly still further.

The cooling means may be specifically exemplified by the means of immersing the alloy material 50 into the molten lithium L2, held at a cooling temperature (solution stop temperature T2) and contained in the solution stop bath 13, within the heat treatment room 11 of the heat treatment device 1, as shown in FIG. 3. Here, a heater 22 to heat the molten lithium L2 and a fan 23 to cool the molten lithium L2 are used in order to control the temperature of the molten lithium L2 contained in the solution stop bath 13, for example.

After cooling the alloy material 50 by immersing into the molten lithium L2 of the solution stop bath 13, the alloy material 50 may be further cooled to a temperature lower than the melting point of lithium (181° C.), more specifically to room temperature (ambient temperature) (step S22 of FIG. 2). Thereby, deposition of α-phase inside the alloy material 50 is reduced further, therefore, decrease of α′-martensite phase and the remaining β-phase due to the passage of time can be suppressed. In addition, the cooling means for the alloy material 50 after removal from the solution stop bath 13 may be exemplified by natural cooling under an inert gas atmosphere within the heat treatment room 11 of the heat treatment device 1 as shown in FIG. 3, for example.

Here, after the solution stop step S2 the alloy material 50 may undergo mechanical processing such as rolling. Thereby, processing strains are formed in the internal structure of the alloy material 50. Therefore, when it is heated again, recrystallization of internal structure can progress simultaneously from these processing strains, and miniaturization of the crystals formed during re-heating can be designed.

(S3) Aging Treatment Step

The alloy material 50 is heated to the aging treatment temperature T3. In this stage, the alloy material 50 includes a solid solution in which alloy elements dissolve into the solute metals of α-phase, α′-martensite phase, and β-phase. The β-phase including the alloy elements tends to be decomposed into compounds including fine α-phase and alloy elements by heating the alloy material 50 to the aging treatment temperature T3. Therefore, with respect to the strengthened alloy obtained after the aging treatment of the alloy material 50, the ratio of the α-phase crystal can be increased, and the tensile strength and 0.2% proof stress of the alloy material 50 can be increased.

Here, the aging treatment temperature T3 during the aging treatment step S3 is lower than the β-transformation temperature at which solute metal transforms to β-phase (Ti-6Al-4V alloy: 885° C.) and is preferably in the range of 400° C. to 650° C. Particularly, when the Ti-6Al-4V alloy is used as the alloy material 50, it is more preferable that the surface temperature of the alloy material 50 during the aging treatment step S3 is maintained at not more than 600° C. Because when the temperature of the alloy material 50 becomes higher than the temperature range mentioned above, the α-phase formed in the alloy material 50 is likely to transform into β-phase with lower tensile strength, and eventually, tensile strength and 0.2% proof stress at the portions suddenly decrease.

The heating of the alloy material 50 to the aging treatment temperature T3 is carried out for a short time in the aging treatment step S3. Thereby, a part of β-phase of the solute metal decomposes to form α-phase with larger tensile strength and lower particle diameter; on the other hand, compounds are produced between the solute metal and the alloy elements and β-phase with high ductility remains between these products. Therefore, with respect to the strengthened alloy after the aging treatment step S3, the tensile strength and/or 0.2% proof stress and also breaking elongation can be increased. Accordingly, strengthened alloy having hardness as well as toughness is obtainable.

The means of heating the alloy material 50 to the aging treatment temperature T3 for a short time may be exemplified by the means of immersing the alloy material 50 into molten-state lithium (molten lithium L3) heated to the aging treatment temperature T3. Thereby, heat is rapidly conducted to the alloy material 50 by molten lithium with a thermal conductivity (41.4 W/mK) higher than that of air (2.41×10−2 W/mK). Therefore, a great number of α-phase crystal nuclei can deposit from β-phase of the solute metal within a short time and α-phase fine texture of the solute metal can be formed in these crystal nuclei. Accordingly, the particle diameter of α-phase crystals contained in the strengthened alloy can be lowered, and the breaking elongation can be increased by allowing the β-phase crystals to form in the α-phase crystals. In this stage, the surface temperature of the alloy material 50 becomes approximately the same as the temperature of the molten lithium L3, for about 1 to 2 seconds after immersing the alloy material 50 into the molten lithium L3.

The heating means may be specifically exemplified by the means of immersing the alloy material 50 into the molten lithium L3 heated to the aging treatment temperature T3 in the aging heating bath 14 which is contained in the heat treatment room 11 of the heat treatment device 1 shown in FIG. 3. A heater 24 to heat liquid lithium L3 is used to control the temperature of the molten lithium L3 in the aging heating bath 14, for example.

In the aging treatment step S3, the aging treatment time of immersing the alloy material 50 into the molten lithium L3 heated to the aging treatment temperature T3, to be decided depending on the size and/or shape of the alloy material 50, is preferably 30 seconds to 30 minutes, for example. The inner portion of the alloy material 50 can also be easily heated to the aging treatment temperature T3 for not less than 30 minutes, and therefore, the tensile strength and/or 0.2% proof stress of the alloy material 50 can be enhanced further. On the other hand, α-phase of solute metal of the alloy material 50 is unlikely to grow and enlarge unnecessarily when the aging treatment time is not more than 30 minutes, and the lowering of breaking elongation due to decease of β-phase of solute metal can be suppressed.

(S4) Aging Stop Step

After the aging treatment step S3 the alloy material 50 is cooled down for a short time until the surface temperature reaches the aging stop temperature T4 (step S41 of FIG. 2). Thereby, the α-phase crystal of solute metal formed in the aging treatment step S3 is suppressed from additional growth on the surface of the alloy material 50, and transforming from β-phase remaining in the alloy material 50 into α-phase is suppressed. Furthermore, the alloy metal is unlikely to become supersaturated within the solute metal due to excessive formation of α-phase and thus compounds of alloy metal are unlikely formed. Therefore, the average particle diameter of α-phase of the strengthened alloy can be lowered after the aging stop, and the breaking elongation of the resulting strengthened alloy can be increased while maintaining the fine metal structure consisting of α-phase crystal and remaining β-phase. The fact that 0.2% proof stress, tensile strength, and ductility of strengthened alloy can be increased by the cooling after the aging treatment step S3 is newly found.

A means of immersing the alloy material 50 into the molten lithium L4 held at the aging stop temperature T4 is used as the means of cooling the alloy material 50 for a short time. Thereby, heat is rapidly removed from the alloy material 50 by molten lithium with a thermal conductivity (41.4 W/mK) higher than that of air (2.41×10−2 W/mK). Therefore, the alloy material 50 can be rapidly cooled. Here, the aging stop temperature T4 is set within a temperature range of higher than the melting point of lithium (181° C.) and lower than the aging treatment temperature T3; for example, when the temperature is set at not more than 350° C., preferably at not more than 300° C., the growth of α-phase of the solute elements can be suppressed. In addition, the aging stop temperature T4 may be the same as the solution stop temperature T2 in the solution stop step S2.

The cooling means may be specifically exemplified by the means of immersing the alloy material 50 into the aging stop bath 15 containing the molten lithium L4 in the heat treatment room 11 of the heat treatment device 1 as shown in FIG. 3. Here, a heater 25 to heat the molten lithium L4 and a fan 26 to cool the molten lithium L4 are used in order to control the temperature of the molten lithium L4 contained in the aging stop bath 15. In this stage, the surface temperature of the alloy material 50 immersed into molten lithium L4 is cooled down to the aging stop temperature T4 in about 1 to 2 seconds.

In the aging stop step S4, the time that the alloy material 50 is immersed into the molten lithium L4 held at the aging stop temperature T4, is chosen depending on the size and/or shape of the alloy material 50, and is preferably not less than 10 seconds. Particularly, when the immersing time is not less than 10 seconds, the growth of α-phase crystal can be suppressed in the inner portion of the alloy material 50, therefore, the tensile strength and/or 0.2% proof stress of the alloy material 50 can be enhanced further.

After cooling down the alloy material 50 by immersing into the molten lithium L4 of the aging stop bath 15, the alloy material 50 is further cooled to a temperature lower than the melting point of lithium (181° C.), specifically to room temperature (ambient temperature) (step S42 of FIG. 2). In addition, the cooling means for the alloy material 50 after removal from the aging stop bath 15 may be exemplified by natural cooling under an inert gas atmosphere inside the heat treatment room 11 of the heat treatment device 1 as shown in FIG. 3, for example.

Lithium attached to the surface of the strengthened alloy obtained by the aging stop step S4 is removed with a washing means. Thereby, heat generation etc. due to contact of the lithium with air and/or water can be reduced. The washing means for the strengthened alloy may be exemplified by a means of immersing into a large amount of water and ultrasonically cleaning.

As described above, the method of producing a strengthened alloy according to the present invention is explained by one embodiment; however, the present invention should not be limited to the embodiment.

For example, the heat treatment device 1 is not limited to this embodiment, the solution stop bath 13 and the aging stop bath 15 may be constructed as an identical molten lithium bath and the solution treatment bath 12 and the aging heating bath 14 may be constructed as an identical molten lithium bath. Thereby, the number of molten lithium baths used for producing the strengthened alloy is decreased, therefore, the heat treatment device 1 can be simplified and the amount of lithium used for the heat treatment device 1 can be reduced. In this regard, if the solution treatment bath 12 and the aging heating bath 14 are constructed as an identical molten lithium bath, the temperature of molten lithium is set to the solution treatment temperature T1 when used as the solution treatment bath 12 and the temperature of molten lithium is set to the aging treatment temperature T3 when used as the aging heating bath 14 by way of controlling the output of the heater 21 (24).

EXAMPLES

The present invention is explained more specifically with reference to examples hereinafter; however, the present invention is not limited to the examples.

Example 1

Ti-6Al-4V alloy (β-transformation temperature: 885° C.) was used as the alloy material. A thermocouple was attached to the surface of the alloy material in order to measure the surface temperature.

A heat treatment device, having a heat treatment room sealed and filled with argon gas, was used in order to heat and cool the alloy material. An entrance room sealable by a door was provided at one side of the heat treatment room and an exit room sealable by a door was provided at another side. Furthermore, a solution treatment bath, a solution stop bath, an aging heating bath, and an aging stop bath containing respectively molten lithium were provided inside the heat treatment room.

Among these, the temperature of molten lithium contained in the solution treatment bath was fixed at the solution treatment temperature of 980° C. using a heater, and the alloy material, conveyed from the entrance room of the heat treatment device to the heat treatment room, was immersed into the molten lithium of the solution treatment bath. Thereafter, the temperature of molten lithium immersing the alloy material was held for a solving time of 60 seconds.

Then the alloy material was transferred into the solution stop bath and the alloy material was cooled rapidly. When the alloy material was immersed into the solution stop bath, the temperature of molten lithium contained in the solution stop bath was controlled to a solution stop temperature of 200° C. using a heater to heat the molten lithium and a fan to cool the molten lithium. After immersing into the molten lithium of the solution stop bath, the alloy material was further cooled to ambient temperature by exposing to an inert gas atmosphere in the heat treatment room.

Then the alloy material was immersed into the aging heating bath. When the alloy material was immersed into the aging heating bath, the temperature of molten lithium contained in the aging heating bath was controlled to an aging treatment temperature of 550° C. using a heater. After immersing the alloy material into the aging heating bath, the temperature of molten lithium immersing the alloy material was held for an aging treatment time of 10 minutes to hold the surface temperature of the alloy material at the aging treatment temperature.

Then the alloy material was transferred to the aging stop bath to cool the alloy material rapidly. When the alloy material was immersed into the aging stop bath, the temperature of molten lithium contained in the aging stop bath was controlled to an aging stop temperature of 200° C. using a heater and a fan. In this stage, the surface temperature of the alloy material was lowered to the aging treatment temperature in 2 seconds. After immersing into the molten lithium of the aging stop bath, the alloy material was further cooled to ambient temperature by exposing to an inert gas atmosphere in the heat treatment room.

The strengthened alloy after heat treatment was ultrasonically cleaned with a large amount of water to remove the lithium attached to the surface and then tensile test in accordance with JIS Z 2241 was carried out to measure 0.2% proof stress, tensile strength, and breaking elongation.

Comparative Example 1

Comparing to Example 1, the alloy material was cooled without immersing into molten lithium of the aging stop bath in the aging stop step S4. That is, the alloy material taken out of the aging treatment bath similarly as Example 1 was cooled to ambient temperature by exposing to an inert gas atmosphere in the heat treatment room without immersing into the aging stop bath. In this stage, the time when the surface temperature of the alloy material became lower than 200° C., same as the aging stop temperature, was one hour after removing the alloy material from the molten lithium. The others were similar as those of Example 1.

Comparative Example 2

Comparing to Example 1, the aging treatment time was prolonged in the aging treatment step S3 and the alloy material was cooled without immersing into molten lithium of the aging stop bath in the aging stop step S4. That is, the alloy material taken out of the solution treatment bath and cooled to ambient temperature similarly as Example 1 was heated to an aging treatment temperature of 550° C. using a heater without immersing into the aging heating bath. In this stage, the alloy material was heated to the aging treatment temperature in about 2 seconds. After the surface temperature of the alloy material had reached the aging treatment temperature, the surface temperature of the alloy material was held at the aging treatment temperature for 2 hours.

Then heating of the alloy material by a heater was stopped, and the alloy material was exposed to an inert gas atmosphere in the heat treatment room to cool to ambient temperature. In this stage, the time that the surface temperature of the alloy material became lower than 200° C., same as the aging stop temperature, was one hour after removing the alloy material from the molten lithium. The others were similar as those of Example 1.

Measured values of mechanical strength based on tensile test in accordance with JIS Z 2241 are shown in Table 1 below with respect to the alloy materials of Example 1, Comparative Examples 1 and 2 of before the solution treatment step S1 (before test) and after the aging stop step S4 (after test).

TABLE 1 0.2% proof Breaking stress Tensile strength elongation (N/mm2) (N/mm2) (%) Example 1 Before test 879 991 16.3 After test 1098 1266 19.1 Difference +219 +275 +2.8 Comparative Before test 879 991 16.3 Example 1 After test 1051 1213 17.5 Difference +172 +222 +1.2 Comparative Before test 1003 1022 15.7 Example 2 After test 1322 1429 4.3 Difference +319 +407 −11.4

From the results of Example and Comparative Examples described above, the following can be verified.

With regard to Example 1 in which the alloy material was immersed into molten lithium at the aging stop temperature, the cooling time to the aging stop temperature was shortened and also all of 0.2% proof stress, tensile strength, and breaking elongation of the aging strengthened alloy were enhanced, compared to Comparative Examples 1 and 2 in which the alloy material was not immersed into molten lithium at the aging stop temperature.

Claims

1. A method of producing a strengthened alloy, the method comprising: a solution treatment step of immersing an alloy material into molten lithium held at a solution treatment temperature higher than solution temperature of solute metal of the alloy material;

a solution stop step of immersing, after the solution treatment step, the alloy material into molten lithium held at a cooling temperature lower than the solution treatment temperature;
an aging treatment step of immersing, after the solution stop step, the alloy material into molten lithium held at an aging treatment temperature lower than the solution temperature; and
an aging stop step of immersing, after the aging treatment step, the alloy material into molten lithium held at an aging stop temperature lower than the aging treatment temperature.

2. The method according to claim 1, wherein the alloy material is immersed into molten lithium at not more than 350° C. in the aging stop step.

3. The method according to claim 1, wherein a titanium alloy is used as the alloy material.

Patent History
Publication number: 20100276040
Type: Application
Filed: Apr 22, 2010
Publication Date: Nov 4, 2010
Applicant: JAPAN THERMOTECH CO., LTD. (Kanagawa)
Inventor: Susumu Ninomiya (Kanagawa)
Application Number: 12/765,085
Classifications
Current U.S. Class: Titanium(ti) Or Titanium Base Alloy (148/669); Heating Or Cooling Of Solid Metal (148/559)
International Classification: C22F 1/18 (20060101); C21D 9/00 (20060101);