JPS61194163A - Heat treatment of (alpha+beta) type titanium alloy - Google Patents

Abstract

PURPOSE:To easily attain the increase of the toughness of the titled alloy by performing the proper heat treatment for (alpha+beta) type titanium alloy wherein the thermal working is performed. CONSTITUTION:After an (alpha+beta) type titanium alloy wherein the thermal working is performed is heated and held in the temp. range of 10-60 deg.C desirably about 20-50 deg.C lower than beta-transus, it is cooled up to the temp. of <=500 deg.C in 0.1-5 deg.C/sec cooling velocity. By the above-mentioned simple operation, the toughness of the (alpha+beta) type titanium alloy is improved without necessitating the annealing treatment or the aging treatment and without the large decrease of the strength and the ductility.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method of heat treating titanium alloys, particularly to a method of heat treating α+β type titanium alloys for the purpose of improving toughness.

(Prior Art) Titanium alloys have very low toughness compared to steel, and while titanium alloys have many advantages, this low toughness is one of the few drawbacks. Among the many uses of titanium alloys, these shortcomings are particularly problematic in pressure vessels and shells used in the deep sea where there are no other suitable substitutes: In the case of α+β type alloys, solution treatment is the typical heat treatment method. There is STA treatment, which combines treatment and aging treatment, but this method provides considerably better strength than simply annealed material, but has inferior toughness. Furthermore, so-called transform II led β structures such as acicular α structures and plate-like α structures that are generated when heated or processed in the β region have high toughness but extremely low strength and ductility.

Therefore, a number of methods have been proposed in the past for the purpose of improving the toughness of titanium alloys, particularly α+β type titanium alloys, without significantly reducing their strength.

For example, M. Peters et al.
-6AQ Effect of microstructure on fatigue of 4V J
, Titanium'80 (1980), pp.
In 1777-1786, the equiaxed α+β structure was rapidly cooled from 900°C, which is the two-phase V4 region, to transform the β phase into martensite, and then tempered at 800°C to precipitate and grow the α phase from this martensite. Cool quickly. A method is disclosed in which the last remaining martensite is aged at 500° C. for 24 hours. According to this so-called three-step heat treatment method, the resulting bi-moda1 structure consists of the original pro-eutectoid α grains and three tempered parts of finer plate-like α and β phases, and has high fatigue strength and fatigue resistance. Crack extension rate is slow.

In addition, as a heat treatment method for equiaxed α+β structure,
No. 37004 discloses a heat treatment method in which cooling is performed at a faster rate than air cooling from an α+β temperature range 150 to 60° C. lower than the β transus, and then stabilization annealing is performed to improve notch placha strength.

However, all of these methods require a three-step heat treatment or a stabilizing annealing treatment, which takes a long time, or the resulting structure is a tempered structure, resulting in insufficient improvement in toughness. do not have.

(Problems to be Solved by the Invention) Thus, the purpose of the present invention is to provide a method for improving the toughness of titanium alloys by heat treatment, since the mechanical properties of titanium alloys change significantly when heat treated. The purpose is

(Means for solving the problem) As mentioned above, in the past, for the purpose of improving the toughness of titanium alloys, a bi-moda1 structure or an α-β structure was used.
Toughness has been improved by refining the pro-eutectoid α grains through transformation and generating fine acicular α phases.

However, as a result of extensive studies by the present inventors, we found that a mixed phase of fine spherical α phase and plate-like α phase is more effective in improving toughness while maintaining appropriate strength. They discovered this and completed the present invention.

Therefore, the gist of the present invention is to prepare a hot worked α+β type titanium alloy with a β transus or less.
This is a method of heat treating an α+β type titanium alloy for the purpose of improving toughness, characterized in that after heating and holding in a temperature range of 0 to 60°C, the toughness is 0.1 to b.

Thus, in the present invention, the target alloy is Ti-6A
(24V, α of Ti 6AQ 6V 2Sn etc.
+β-type titanium alloy, the above-mentioned alloy is heated and held at a temperature below the β transus of 10 to 60°C, preferably 20 to 50°C, for an appropriate period of time, and then heated to 0.1 to b or O62 to 3°C/ It cools down at a speed of seconds.

Here, "β transus" is the β-α+β transformation point,
For example, Ti-6AQ-4V titanium alloys generally have 9
It is within the range of 70 to 1000°C.

The structure thus obtained consists of pro-eutectoid α grains and a plate-shaped α phase, with plate-shaped α precipitated at the prior β grain boundaries, thereby connecting the pro-eutectoid α grains. 3i
Although similar to the -modal structure, the structure of the present invention differs in that there is no part where martensite is tempered.

The heating temperature greatly affects the spheroidization of pro-eutectoid α grains and the ratio of α and β phases. By controlling the cooling rate, the dimensions of the pro-eutectoid α, the formation of three grain boundaries, and the growth of the plate-like α phase are affected, so here the cooling rate is controlled so as not to coarsen the pro-eutectoid α.

Therefore, according to the present invention, the above-mentioned structure can be obtained by simply heating and cooling, and as a result, this structure prevents crack extension and improves toughness. Moreover, the entire structure, which was traditionally considered to have high toughness, is transformed
Unexpectedly higher toughness is obtained than in the case of the ro+edβ structure, and it can be seen that the effects of the present invention are significant.

(Function) Next, the present invention will be explained with reference to the accompanying drawings. FIG. 1 is an explanatory diagram schematically showing the method according to the present invention.
Heat to a temperature range of 60°C (indicated by diagonal lines in the figure). At this time, it is necessary that the α phase and β phase ratios be approximately equal at a temperature close to the β transus. The final quantitative ratio of the α phase and the β phase at room temperature is influenced by the heating temperature and cooling rate, but the heating rate is not limited.

Thus, in the present invention, the heating temperature is limited to 10 to 60 degrees Celsius below the β transus, preferably 20 to 50 degrees Celsius, because at temperatures below 10 degrees Celsius, most of the material becomes the β phase during heating, and the cooling rate cannot be controlled. This is because, even after cooling, the microstructure after cooling has an excessive amount of plate-like α phase, and the toughness slightly decreases. On the other hand, if the temperature exceeds 60° C., that is, if the temperature is low, the required amount of plate-like α phase cannot be secured.

In other words, this heating temperature almost determines the quantitative ratio of the pro-eutectoid α grains and the plate-like α phase in the final structure, and an appropriate quantitative ratio is necessary to obtain the effects of the present invention. It is limited to the range mentioned above.

Note that the heating time at this time is not limited. Generally, crystal grains grow at high temperatures, but in the temperature range of the present invention, the growth of α grains is suppressed because the amount ratio of β phase is around 50%, so grain growth is hardly affected by heating time. This is so that it will not happen.

Next, the α+β type titanium alloy heated to the above-mentioned temperature range is cooled at a cooling rate of 0.1 to b. If the cooling rate at this time is less than 0.1°C/sec, the pro-eutectoid α phase will grow during cooling, and a structure consisting of pro-eutectoid α grains and plate-shaped α phase will not be obtained. The toughness decreases, and the strength also decreases due to the growth of pro-eutectoid α grains. On the other hand, when the cooling rate exceeds 5°C/sec, the plate-like α
The thickness of the phase becomes thinner, the desired structure cannot be obtained, and the toughness decreases.

The titanium alloy thus treated according to the present invention does not require annealing treatment or aging treatment as in the conventional methods, and this combined with the simple treatment method has a great practical effect.

Next, the present invention will be described in more detail with reference to examples.

Urasen ■ As a soil sample material, β transus whose chemical composition is shown in Table 1 is 9.
Ti-6AQ 4V (ELI) alloy (E
LI-JztrB l□HInterstitial)
Width 70mm as a test piece for heat treatment from 25+U+ thick plate of
A piece having dimensions of x length 150mm - x thickness 20+sm was cut out and heat treated according to the present invention. A tensile test piece and a Charpy impact test piece (JIS No. 4) were taken from this plate along the rolling direction (L) and the direction (T) perpendicular thereto. Two types of impact test specimens were prepared: one in which the notches were provided in a direction parallel to the plate thickness and one in a direction perpendicular to the plate thickness. The procedure for collecting this Charpy impact test piece is schematically shown in FIG. In the figure, the symbol rT
J indicates that the notch direction is perpendicular to the rolling direction, rLJ indicates that it is parallel to the rolling direction, and the symbol "N" indicates that the notch direction is perpendicular to the plate surface, and rPJ indicates that it is parallel. show.

The results of the Charpy impact test are summarized in Tables 2 and 3 together with each heat treatment condition.

As can be seen from these data, as long as the present invention is used, excellent toughness can be obtained in any case.

In addition, Fig. 3 (al to Fig. 3 (dl) of the attached drawings are respectively before heat treatment and 0.01'C/ after heating at 940°C
Microscopic structure photographs are shown when cooling at 1° C./second, 1° C./second, and 30° C./second, respectively.

The structure obtained according to the present invention is shown in FIG. 3 (C1), and it can be seen that the plate-like α phase is sufficiently developed.

Figure 3 (In the case of dl, this plate-like α phase is not generated,
That part has a very fine needle-like structure.

The results shown in Table 3 also show that the toughness is considerably degraded in that case.

Table 2 QD *: 31yMM! yJ"c, the cooling rate is 1°C to 1°C Table 3) However, as a comparative example, a sample subjected to heat treatment to obtain a conventional bi-modal structure is also shown.The heat treatment conditions and the results of the Charpy impact test are summarized in Table 4.

Table 4 (Note) Test temperature: 20°C Also, regarding the sample material of the invention example obtained as described above, L
Mechanical properties were investigated in each of the directions and the T direction. The results are summarized in Table 5.

Table 5 (Effects) As described above, according to the present invention, with only one heat treatment,
Compared to conventional products that required complicated processing operations, the toughness is significantly improved without any significant decrease in strength, elongation, etc., and the effects of the present invention are significant in practice.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram schematically showing the heat treatment method of the present invention; FIG. 2 is a schematic diagram showing the procedure for collecting Charpy impact test pieces; and FIG. These are microscopic structures and photographs of each titanium alloy in. Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Akira Hirose - Figure 1 Figure 3 Figure 1

Claims (1)

[Claims] After heating and holding a hot-worked α+β type titanium alloy in a temperature range of 10 to 60°C below the β transus, 0.1
A method for heat treating an α+β type titanium alloy for the purpose of improving toughness, the method comprising cooling to 500°C or less at a rate of ~5°C/sec.