JPH05112856A - Method for improving fatigue property of alpha + beta type titanium alloy subjected to beta treatment - Google Patents

Abstract

PURPOSE:To provide the method for improving the fatigue properties of an alpha+ beta type titanium alloy subjected to beta treatment without deteriorating its ductility. CONSTITUTION:An alpha + beta type titanium alloy subjected to beta treatment is subjected to shot peening and is subjected to heat treatment of holding it under heating to the temp. range from the beta transformation point -150 deg.C to the betatransformation point -20 deg.C for 5min to 8hr. Or, cooling after the above heat treatment is executed at a cooling rate of that of air cooling or above and is furthermore subjected to stabilizing annealing of holding it under heating to 450 to 820 deg.C for 30min to 24hr. Or after the above-mentioned heat treatment or stabilizing annealing, shot peening is moreover executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an α + β type titanium alloy, and more particularly to a method for improving the fatigue characteristics of an α + β type titanium alloy which has been subjected to β treatment.

[0002]

2. Description of the Related Art α + β type titanium alloy is the most general purpose among titanium alloys, and in order to take advantage of the characteristics of lightweight and high strength,
It has been applied to aircraft, automobiles, steam turbines, etc.
Many of them use equiaxed α-phase microstructures that are excellent in strength, ductility and fatigue properties, but recently, they have been heated and cooled to a temperature range above the β transformation point, that is, β-treated, or after β-treated. Further, it is subjected to stabilizing annealing to generate an acicular α-phase structure, which is particularly used for applications requiring creep characteristics and fracture toughness. However, these needle-like structures are inferior in ductility and fatigue crack initiation properties to equiaxed structures, and their use is limited especially for applications requiring fatigue properties.

As a method for improving fatigue characteristics, there is, for example, shot peening described in Japanese Patent Laid-Open No. 50-105515. This is a method aimed at suppressing the occurrence of fatigue cracks by hardening the surface layer portion where fatigue cracks are likely to occur by shot peening or applying residual stress. However, α that has been β-processed
The needle-like structure characteristic of + β type titanium alloy is less ductile than the equiaxed structure, and the ductility is further deteriorated when shot peening is performed. was there.

[0004]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and in an α + β type titanium alloy which has been subjected to β treatment, it has excellent properties such as creep properties that the acicular structure has. It is an object of the present invention to provide a method for improving fatigue properties without impairing the ductility or damaging the ductility.

[0005]

Means for Solving the Problems As a result of repeated research efforts for improving the fatigue characteristics of α + β type titanium alloys subjected to β treatment, the present inventors carry out appropriate heat treatment after performing shot peening. Then, it was found that an equiaxed structure excellent in fatigue characteristics and ductility is generated in the surface layer portion. That is, the present invention includes (1) shot-peening an α + β type titanium alloy that has been subjected to β treatment, and then heating it to a temperature of β transformation point −150 ° C. or higher and β transformation point −20 ° C. or lower for 5 minutes or more and 8 hours or less. It is characterized by holding. (2) After performing the heat treatment of the present invention (1), it is cooled at a cooling rate of air cooling or more, and then at 450 ° C to 820 ° C for 30 minutes.
It is characterized in that the material is heated and held for at least a minute and no more than 24 hours and then subjected to stabilization annealing. (3) After carrying out the step of the present invention (1) or (2),
Further, it is characterized by performing shot peening. In the present invention, the β treatment is a process consisting of heating, holding, and cooling to a temperature range above the β transformation point, and the β transformation point is a β phase single phase at the temperature above the equilibrium state. Temperature, generally 850 for α + β titanium alloys
C to 1050C. Also, α + β type titanium alloy means that when quenched from the β transformation point or higher, all or part of the β phase transforms to α phase or martensite, and both α and β phases are in equilibrium at room temperature. A type of titanium alloy that exists, such as Ti-6Al-4V.

[0006]

The present invention will be described in detail below. In the present invention (1), first, shot peening is performed on the α + β type titanium alloy that has been subjected to β treatment. This is intended to give plastic strain to the surface layer portion. Here, when the β treatment is performed in the atmosphere, an oxygen-enriched layer and scale are generated in the surface layer portion, and plastic strain due to shot peening cannot be applied to a sufficient depth of the base material, so prior to shot peening. It is desirable to remove the scale and the oxygen-enriched layer by shot blasting, pickling, grinding or the like.

Next, this material is heated and held at a temperature of β transformation point of −150 ° C. or higher and β transformation point of −20 ° C. or lower for 5 minutes or more and 8 hours or less. This is because the acicular α-phase structure is refined by recrystallizing the surface layer portion to which plastic strain is applied, and the β phase is generated at the boundary of the refined α phase to divide the α phase,
The purpose is to create an equiaxed structure with excellent fatigue properties. At this time, since the plastic strain due to the shot peening which is the previous step is not applied inside the material, the needle-like structure is retained as it is. Where the heating and holding temperature is β
The reason why the transformation point is set to −150 ° C. or higher is that the recrystallization and the amount of β phase are insufficient at a temperature lower than this temperature, and the effect of the present invention is not sufficiently achieved. Further, the heating and holding temperature is set to the β transformation point of −20 ° C. or lower because the ratio of β phase becomes extremely large at a temperature exceeding this temperature, and the number of equiaxed α phases excellent in fatigue characteristics and ductility is reduced. This is because the characteristics cannot be improved sufficiently. Further, the holding time of 5 minutes or more is because the holding time of less than 5 minutes causes recrystallization and β
The reason is that the amount of phases produced is insufficient and the effects of the present invention are not sufficiently achieved. Further, the holding time is set to 8 hours or less because the α phase is coarsened excessively and the fatigue characteristics are deteriorated if the holding time is exceeded. Further, when the main heat treatment is performed in the atmosphere, the effect of oxygen reaches the depth of the equiaxed surface layer portion or more after 8 hours, so that the effect of the present invention is lost.

The present invention (2) is a method for further improving fatigue characteristics by combining the effect of the present invention (1) with a strengthening heat treatment. First, after performing the heat treatment of the present invention (1), it is cooled at a cooling rate higher than air cooling. This is because the β phase existing between the equiaxed α phases is transformed into a fine acicular structure such as martensite. The reason why cooling is performed at a cooling rate higher than that of air cooling is that fine needle-like structures are not generated at a cooling rate slower than that of air cooling.

Next, at 450 to 820 ° C. for 30 minutes or more 2
Stabilization annealing is performed by heating and holding for 4 hours or less.
By this step, the fine needle-like structure is tempered to form a stable equiaxed + fine needle-like mixed structure and the strength is increased. Here, the heating and holding temperature is set to 450 ° C. or higher because
This is because if the temperature is lower than this, diffusion of the element is insufficient and a stable structure is difficult to be generated. The reason why the temperature is set to 820 ° C. or lower is that at a temperature exceeding this temperature, most of the fine needle-like structure generated during cooling is again transformed into the β phase and the purpose of strengthening cannot be achieved. The heating and holding time is set to 30 minutes or more because the element diffusion is insufficient and a stable structure and material properties cannot be obtained in a time less than 30 minutes. The reason why the heating and holding time is set to 24 hours or less is that a stable microstructure is obtained within 24 hours, and performing heat treatment for a time longer than this is wasteful in terms of energy.

In the present invention (3), shot peening is carried out again after carrying out the present invention (1) or (2). This is achieved by carrying out the present invention (1) or (2).
The purpose is to further improve the fatigue properties of a material having an equiaxed structure in which only the surface layer has excellent fatigue properties.
Here, when shot peening is performed on the needle-shaped structure, the low ductility value is further reduced.
In the samples subjected to (2) and (2), the surface layer has an equiaxed structure rich in ductility or an equiaxed + fine needle-shaped mixed structure, so sufficient ductility is still secured even if ductility decreases due to shot peening. it can.

[0011]

EXAMPLE Ti-6Al-4V (β transformation point: 1000
The present invention will be described in more detail by taking the case of applying the present invention to (° C.) As an example. Ti-6Al-4V ingot melted by vacuum arc melting is forged at 1100 ° C., 9
A 12 mm plate was manufactured by hot rolling at 30 ° C. A round bar test piece having a parallel portion of 35 mm length and 8.05 mm diameter was cut out from this plate and subjected to furnace cooling β treatment at 1050 ° C for 15 minutes,
Further, the treatments shown in Tables 1 to 4 were performed. In order to remove the unevenness caused by shot peening, the test piece after each treatment was polished to remove about 0.025 mm of the surface layer and subjected to a rotary bending fatigue test at room temperature under the condition of 60 Hz to obtain the fatigue limit. In addition, some samples were subjected to a tensile test to determine tensile strength and elongation. The results are shown in Tables 5 to 8. All heat treatments are performed in Ar atmosphere, and shot peening is 1
mm diameter glass beads were applied at about 40 m / s with about 150% coverage, followed by 100 μm diameter glass beads at about 30 m / s with about 150% coverage.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

[Table 3]

[0015]

[Table 4]

[0016]

[Table 5]

[0017]

[Table 6]

[0018]

[Table 7]

[0019]

[Table 8]

As shown in Tables 1 and 5, the as-treated β material (test number 1) has a fatigue limit of 400 MPa or less. β
When shot peening is applied after the treatment (Test No. 2), the fatigue limit rises to 460 MPa, but the elongation is 4.4% and 5%.
It drops to below.

In Table 2, test numbers 3-5 and 7,1
Reference numeral 0 is an embodiment of the present invention (1). As shown in Table 6,
By applying the present invention (1), the fatigue limit is 430 MPa or more in all cases. In addition, in Test Nos. 3 to 5, the elongation is 7% or more, which is 5 as in Test No. 2.
%, No remarkable decrease in ductility is observed. This is because the surface layer portion in which plastic strain was introduced by shot peening was recrystallized by heat treatment, and the β phase was divided into α phase, so that the equiaxed structure was excellent in fatigue characteristics and ductility. Is.

On the other hand, in Test Nos. 6, 8, 9, and 11, which are comparative examples of the present invention (1), as shown in Table 6, the fatigue limit was significantly less than 430 MPa. The reason for this is as follows. In Test No. 6, the heat treatment temperature after shot peening was 990 ° C., which was higher than the upper limit value of 980 ° C. of the present invention (1), so that the proportion of β phase was extremely large, and α phase excellent in fatigue characteristics was used. The number was small and the fatigue properties were not sufficiently improved. In test number 8,
Since the heat treatment time after shot peening was less than the lower limit of 5 minutes of the present invention (1), the amount of recrystallization and β phase produced was insufficient, and the effects of the present invention were not sufficiently achieved. In Test No. 9, the heat treatment temperature after shot peening was less than 850 ° C., which is the lower limit of the present invention (1), the recrystallization and β phase production was insufficient, and the effect of the present invention was sufficiently exerted. Not achieved. Further, in Test No. 11, the heat treatment time after shot peening exceeded the upper limit value of 8 hours of the present invention (1), so that the α phase was significantly coarsened and the fatigue characteristics were not sufficiently improved.

In Table 3, test numbers 12, 13, 1
Reference numerals 5 and 18 are examples of the present invention (2). As shown in Table 7, the fatigue limit exceeded 450 MPa in all cases, and the fatigue limit was further increased as compared with the case where the present invention (1) was applied. In addition, in Test Nos. 12 and 13, the elongation was 6% or more, and no remarkable decrease in ductility was observed. This is because the β phase transforms into a fine needle-like structure during cooling after heat treatment after shot peening, and further stable annealing provides stable equiaxed + fine needles with excellent fatigue properties and ductility. This is because a mixed tissue having a shape is generated in the surface layer portion.

On the other hand, in Test Nos. 14, 17, 19 and 20 which are comparative examples of the present invention (2), as shown in Table 7, the fatigue limit was 450 MPa or less, and the present invention (1)
With a value equal to or less than that of Test No. 3 in which the test No. 2 was performed, the sufficient effect of the present invention (2) was not obtained. The reason is as follows. In Test No. 14, the cooling rate after the heat treatment after shot peening was slower than the air cooling, which is the lower limit of the cooling rate in the present invention (2), and therefore the β phase was not transformed into a fine acicular structure. is there.
In Test No. 17, the stabilization annealing temperature was less than 450 ° C., which is the lower limit value of the present invention (2), so that the diffusion of elements was insufficient and a stable structure was not generated, and the fatigue limit was not improved. It was In Test No. 19, since the stabilizing annealing temperature exceeded the upper limit of 820 ° C. in the present invention (2), most of the fine needle-like structure was reverse-transformed into β-phase again, and the improvement of fatigue limit was not achieved. It was In Test No. 20, the stabilization annealing time was less than the lower limit of 30 minutes in the present invention (2), so element diffusion was insufficient and stable microstructure and material properties could not be obtained. Further, in Test No. 16, good characteristics of 450 MPa or more were obtained, but in Test No. 15 with a short stabilization annealing time, a sufficient value had already been reached, and the stabilization annealing time was unnecessarily long Is useless.

In Table 4, test No. 21 is an example of the present invention (3) in which shot peening was further performed on test No. 3 which carried out the present invention (1). Test number 2
No. 2 is an example of the present invention (3) in which shot peening was further performed on the test number 12 for which the present invention (2) was performed.

As shown in Table 8, the fatigue limit was further increased and the elongation was 5 as compared with Test No. 3 or Test No. 12.
% Or more is secured. This is because the surface layer had an equiaxed structure that was excellent not only in fatigue characteristics but also in ductility.

[0027]

As described above, α subjected to β processing
By applying the present invention to a + β type titanium alloy, β
It is possible to improve the fatigue characteristics of the treated material and ensure sufficient ductility.

Claims (3)

[Claims] 1. An α + β type titanium alloy that has been subjected to β treatment is subjected to shot peening, and then heated and held at a temperature of β transformation point −150 ° C. or higher and β transformation point −20 ° C. or lower for 5 minutes or more and 8 hours or less. Characteristic, β + processed α + β
Method for improving fatigue characteristics of type titanium alloy. 2. After performing the heat treatment according to claim 1,
Cool at a cooling rate of air cooling or higher, and then 450 ° C to 820
A method for improving fatigue characteristics of a β-treated α + β titanium alloy, which comprises heating and holding at 30 ° C. for 30 minutes or more and 24 hours or less. 3. After carrying out the process of claim 1 or 2,
Furthermore, β is characterized by performing shot peening.
A method for improving fatigue properties of treated α + β type titanium alloy.