JPH09209100A - Postheat treatment for welded member of alpha plus beta titanium alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a postheat treatment method capable of giving sufficiently satisfactory characteristics of weld zone and weld heat-affected zone, required of a structural member, without deteriorating the characteristics of a base material of alpha plus beta titanium alloy. SOLUTION: An alpha plus beta titanium alloy, containing the alloying elements (by weight percentage) in which the value of Moeq, defined by equation Moeq=[Mo]+0.67×[V]+0.44× [W]+0.28 ×[Nb]+0.22×[Ta]+2.9×[Fe] +1.6×[Cr]+1.1×[Ni]+1.4× [Co]+0.77X[Cu]-[Al], is regulated to 2-7wt.%, is welded to a titanium material of the same or a different kind and then heated and held to and at a temp. in the range between (Tβ-230) and (Tβ-80) deg.C when the β-transformation temp. of the alpha plus beta titanium alloy is represented by Tβ.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a post heat treatment method for α + β type titanium alloy welded members having high toughness and high ductility.

[0002]

2. Description of the Related Art Titanium alloys are widely used not only in the aerospace field but also in the civil field due to their characteristics such as high strength and high toughness. In recent years, the demand for mechanical properties of titanium alloys as lightweight structural members has become strict, and particularly good ductility for welded parts of titanium alloys,
Toughness is required. Conventionally, a welded part of a titanium alloy is subjected to post-weld heat treatment for the purpose of removing residual stress.

[0003]

However, although pure titanium and α-type titanium alloys have excellent weldability, α + β-type titanium alloys and β-type titanium alloys, which have higher strength than those, are Technology ”(Company) Titanium Association
As described on page 103, there were cases in which strength and toughness were reduced by welding (prior art 1).

On the other hand, the inventors of the present invention previously disclosed in Japanese Patent Laid-Open No. 3-274.
In Japanese Patent No. 238, an α + β type titanium alloy excellent in strength, toughness, workability, etc. was proposed (prior art 2).
This alloy is a conventional T which is a typical α + β type titanium alloy.
Compared to the i-6Al-4V alloy, it is a β-rich α + β-type titanium alloy with relatively high β-phase stability.
Although it is possible to perform melt welding such as TIG welding and electron beam welding similarly to the -4V alloy, the welded portion during cooling after welding,
In addition, there is a problem that ductility and toughness are reduced due to phase transformation in the heat affected zone of welding. In particular, the β-rich α + β-type titanium alloy disclosed in Prior Art 2 has excellent strength-ductility of the base metal as compared with the conventional titanium alloy, and therefore, even in the welded portion, The strength and ductility characteristics that are superior to those of alloys were required.

The present invention has been made in view of the above circumstances, and the β-rich α + which can sufficiently satisfy the above-mentioned characteristics of the weld and the heat-affected zone as a structural member without deteriorating the characteristics of the base metal. An object of the present invention is to provide a method for post heat treatment of a β-type titanium alloy welded member.

[0006]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that a β-rich α + β type defined by a molybdenum equivalent within a predetermined range (hereinafter referred to as “Mo eq”). After the titanium alloy is welded with the same kind or different kinds of materials, the above object can be achieved by performing a predetermined post heat treatment, and further, the effect obtained by further improving the component composition of the β rich α + β type titanium alloy is improved. Then, they have completed the present invention.

That is, the first invention is the same or different type of α + β type titanium alloy containing alloying elements (each component is wt%) having Mo eq defined by the formula (1) as 2 to 7 wt%. When the β transformation temperature of the α + β type titanium alloy is expressed as Tβ after welding with a titanium material (Tβ−230) ° C.
A post-heat treatment method for an α + β type titanium alloy member, which is characterized by heating and holding at a temperature not higher than (Tβ-80) ° C.

Mo eq = [Mo] + 0.67 × [V] + 0.44 × [W] + 0.28 × [Nb] + 0.22 × [Ta] + 2.9 × [Fe] + 1.6 × [Cr ] + 1.1 × [Ni] + 1.4 × [Co] + 0.77 × [Cu] − [Al] (1)

Further, the second invention is: Al: 3.42 to
5.0%, V: 2.1 to 3.7%, Mo: 0.85
2.37%, Fe: 0.85 to 3.15%, O: 0.0
1 to 0.25%, the remaining unavoidable impurities, and A
After welding an α + β type titanium alloy in which the contents of 1, V, Mo, and Fe are within the range defined by the formula (2) with the same or different titanium materials, (Tβ-230) ° C or higher (Tβ
It is a post-heat treatment method of α + β type titanium alloy member, characterized in that it is heated and maintained at a temperature of −80 ° C. or lower. 2% ≦ [Mo] + 0.67 × [V] + 2.9 × [Fe] − [Al] ≦ 7% (2)

The reason why the component composition of the α + β type titanium alloy in the first invention is limited as described above will be described. Mo eq represented by the formula (1) is a parameter indicating the stability of the β phase of titanium, and the larger the value, the more stable the β phase. If the Mo eq is less than 2%, the volume fraction of the β phase, which is excellent in ductility and workability, becomes too small,
Excellent ductility, which is one of the original purposes, is impaired. On the other hand, when the Mo eq exceeds 7%, the β phase becomes a β type titanium alloy that exists as a metastable phase even at room temperature. β
Although high strength is obtained by age hardening of type-titanium alloy, β crystal grains become coarse in the welded part,
It is known that the ductility of the welded portion is significantly deteriorated.
The coarsened β crystal grains cannot be refined by the post heat treatment, which is a defect of the β type titanium alloy.
Therefore, Mo eq should be limited to the range of 2% or more and 7% or less.

Next, the reason why the component composition is limited as described above in the second invention will be described. Al (aluminum) is one of the α-phase stabilizing elements and is an essential element for α + β type titanium alloys. However, if the Al content is less than 3.42%, it becomes difficult to form an α + β type titanium alloy,
Sufficient strength cannot be obtained. On the other hand, the Al content is 5.0%
If it exceeds, the ductility is deteriorated and the workability, particularly the workability in cold is significantly deteriorated. Therefore, the Al content should be 3.42-5.0%.

The amount of O (oxygen) is preferably the same as that of a normal α + β type titanium alloy, but if the O content is less than 0.01%, the contribution to the increase in strength is not sufficient, while the O content is 0.2.
If it exceeds 5%, the ductility is deteriorated like Al. Therefore, the O content should be limited to the range of 0.01 to 0.25%.

V (vanadium) has a small effect of stabilizing the β phase, but is an important element that greatly lowers the β transformation point. However, if the V content is less than 2.1%, β
The transformation point is not sufficiently lowered, and the effect of stabilizing the β phase cannot be obtained. On the other hand, if the V content exceeds 3.7%, β
The stability of the phase becomes too large and the strength cannot be sufficiently increased. Further, since V is an expensive element, the cost becomes high. Therefore, the V content should be limited to the range of 2.1 to 3.7%.

Mo (molybdenum) has the effect of stabilizing the β phase and suppressing the growth of crystal grains. But M
If the o content is less than 0.85%, the crystal grains become coarse during annealing, and the above-mentioned effect cannot be obtained. On the other hand, when the Mo content exceeds 2.37%, the β phase is too stable and the strength cannot be expected to increase. Therefore, the Mo content should be limited to the range of 0.85 to 2.37%.

Fe stabilizes the β phase, especially β
It strengthens the phase and contributes significantly to the strength increase after solution aging treatment. Further, since the diffusion rate in titanium is high, it has the effect of lowering the deformation resistance during superplastic forming, and also has the effect of improving the diffusion bondability. However, if the Fe content is less than 0.85%, the effect of strengthening is not sufficient, and the effect of lowering the deformation resistance during superplastic forming and the effect of improving diffusion bonding are not sufficient.
On the other hand, when the Fe content exceeds 3.15%, the β phase is excessively stabilized, the superplasticity characteristics are deteriorated, and the strength cannot be expected to increase during the aging treatment. Therefore, the Fe content is 0.8-3.15%
Should be limited to the range.

The expression (2) is obtained by rewriting the expression (1) with the limited components in the second invention, and the reason for the limitation is the same as that of Mo eq.

Next, the reasons for limiting the heat treatment conditions will be described. The weld and the heat-affected zone near the weld are heated to a temperature not lower than the β transformation point during welding and then cooled to room temperature. In a titanium alloy having a Mo eq defined by the formula (1) of 2% or more and 7% or less, fine α in the β phase during cooling.
In some cases, a phase precipitates, the hardness of that portion increases, and as a result, the ductility and toughness decreases. The post-weld heat treatment of such a material is, in addition to the purpose of relaxing and removing residual stress, similar to the post-heat treatment of ordinary titanium alloys. is required. If the post heat treatment temperature is too low, the softening is not sufficient or a very long heat treatment time is required, which is not practical. Also, in some cases, age hardening occurs instead of the post heat treatment. Therefore, the lower limit of post heat treatment temperature is Tβ-230 ℃
Should be. On the other hand, if the post-heat treatment temperature is too high, it is sufficiently softened during heating, but during the subsequent cooling, the fine α phase is precipitated again in the β phase and hardened, so that the effect of the post heat treatment cannot be obtained. Therefore, the upper limit of the post heat treatment temperature is Tβ-8.
Should be 0 ° C.

[0018]

Embodiments of the present invention will be described below. Example 1 Various α + β type titanium alloys having the chemical components shown in Table 1 were electron beam welded in a vacuum chamber. After subjecting these joining materials to post heat treatment at various temperatures, ASTM type flat plate tensile test pieces were sampled so that the weld line was in the longitudinal direction of the tensile test pieces, and the tensile test was performed at room temperature. The post heat treatment time was 1.
5 hours. The results are shown in Table 2 and FIG. FIG.
As is clear from the above, the β-rich α + β-type titanium alloy having Mo eq in the range of 2 to 7% by weight is Tβ of the present invention.
By post heat treatment at a temperature of −230 ° C. or higher and Tβ−80 ° C. or lower, improvement in elongation of 3% or more and room temperature elongation of 10% or more were obtained for the as-welded material.
It can be seen that in the titanium alloy having an eq outside the range of 2 to 7% by weight, the improvement in elongation is as small as less than 3% or is not desired.

[0019]

[Table 1]

[0020]

[Table 2]

Example 2 Al: 4.48%, V: 2.98%, Mo: 1.89
%, Fe: 1.99%, O: 0.083%, C: 0.0
Β rich α + β type titanium alloy containing 1%, N: 0.007%, H: 0.0048% (β transformation point: 905 ° C,
(Mo eq: 5.18%) is heated and forged in the β phase region, heated to the α + β phase region, and hot rolled to a thickness of 3
It was a thin plate of mm. After beveling the material, 2-pass TIG welding was performed in a chamber previously filled with argon gas. This welded material was heat-treated under the conditions shown in Table 3 and then subjected to a tensile test by the same method as in Example 1. The heat treatment time was 1.5 hours in all cases. The results are shown in Table 3 and FIG. From these results,
As a post heat treatment temperature range, a heat treatment at a temperature of Tβ-230 ° C or higher and Tβ-80 ° C or lower resulted in room temperature elongation of 10% or higher.

[0022]

[Table 3]

Example 3 Al used in Example 2 and having a thickness of 3 mm: 4.48%, V:
2.98%, Mo: 1.89%, Fe: 1.99%,
O: 0.083%, C: 0.01%, N: 0.007
%, H: 0.0048% β-rich α + β type titanium alloy (β transformation point: 905 ° C., Mo eq: 5.18)
%) Thin plate was welded to a different titanium alloy, heat treated at various temperatures, and then subjected to a tensile test in the same manner as in Example 1. Welding was performed by TIG welding, and shielded by argon gas (after shield and back shield for torch shield). The chemical composition of the different titanium alloy is Al: 6.47%, V: 4.12%,
Mo: 1.89%, Fe: 0.204%, O: 0.16
%, C: 0.034%, N: 0.012%, H: 0.0
It is 079%. The heat treatment time was 2 hours, and after the heat treatment, it was cooled to room temperature by air cooling. The results are shown in Table 4 and FIG.
Shown in From these results, as the post heat treatment temperature range,
By heat-treating at a temperature of Tβ-230 ° C or higher and Tβ-80 ° C or lower, room temperature elongation of 10% or more was obtained.

[0024]

[Table 4]

[0025]

As described above in detail, according to the method of the present invention, the characteristics of the α + β type titanium alloy welded member can be improved. Therefore, the titanium alloy member produced by the method of the present invention is It can be widely used in aerospace and civilian fields.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of Mo eq on improvement of elongation of a material subjected to heat treatment after welding.

FIG. 2 is a diagram showing the influence of heat treatment temperature on the elongation of the material after the heat treatment after welding.

FIG. 3 is a diagram showing the influence of heat treatment temperature on the elongation of the material after the heat treatment after welding.

Claims (2)

[Claims] 1. Mo eq defined by the equation (1) is 2 to
When the α + β type titanium alloy containing the alloying element of 7% by weight (hereinafter,% by weight) is welded with the same or different titanium material, and the β transformation temperature of the α + β type titanium alloy is expressed as Tβ. (Tβ-230) ° C or higher (Tβ-8
0) α +, characterized by being heated and maintained at a temperature below ℃
Post-heat treatment method for β-type titanium alloy members. Mo eq = [Mo] + 0.67 × [V] + 0.44 × [W] + 0.28 × [N b] + 0.22 × [Ta] + 2.9 × [Fe] + 1.6 × [Cr] +1 .1 × [Ni] + 1.4 × [Co] + 0.77 × [Cu] − [Al] (1) 2. Al: 3.42 to 5.0%, V: 2.1
.About.3.7%, Mo: 0.85 to 2.37%, Fe: 0.
85 to 3.15%, O: 0.01 to 0.25%, remaining unavoidable impurities, and Al, V, Mo, and Fe
After welding the α + β type titanium alloy whose content is within the range defined by the formula (2) with the same or different titanium materials,
A post heat treatment method for an α + β type titanium alloy member, which comprises heating and holding at a temperature of (Tβ-230) ° C or higher and (Tβ-80) ° C or lower. 2% ≦ [Mo] + 0.67 × [V] + 2.9 × [Fe] − [Al] ≦ 7% (2)