JPS63105954A - Hot-working method for near beta-type titanium alloy - Google Patents

Abstract

PURPOSE:To manufacture a Near beta-type Ti alloy combining high strength with high toughness and having superior quality, by applying hot working under specific conditions in the course of cooling a Ti alloy from a beta-single phase region. CONSTITUTION:In the course of cooling an ingot of Near beta-type Ti alloy (Ti-5 Al-2Sn-2Zr-4Mo-4Cr) of relatively low alpha-phase content from a beta-single phase region, hot working of about 20% draft is applied in an arbitrary temp. range between the initial and finishing temps. of alpha-phase precipitation to form two- phase structure of alpha+beta, followed further by hot working of about 30%. Successively, the about alloy is subjected to heat treatment such as solution heat treatment, ageing treatment, etc., so that Near beta-type Ti alloy combining high strength with high toughness can be manufactured without causing cutoff- direction dependency to mechanical properties, particularly to fracture toughness.

Description

[Detailed Description of the Invention] [Industrial Fields of Application] The present invention is applicable to mechanical properties, especially mechanical properties. The present invention relates to a method for hot working a near β type titanium alloy that can obtain high strength and high toughness without causing dependence of toughness on cutting direction.

[Prior Art] In a conventional method for hot working titanium alloys, in order to obtain a uniform equiaxed structure, a material that has been quenched from the β single phase region is hot worked in the α+β temperature range. This processing method is effective for α+β alloys that have a relatively large amount of α phase, such as the Ti-6AI-4V alloy, but in near β type alloys that have a relatively small amount of α phase, it is difficult to process the β grain boundaries during heating. The α phase precipitated along the surface remains continuously even after processing, and the continuity is maintained even in the subsequent heat treatment, so that an equiaxed structure is not obtained (see Fig. 3).

[Problems to be Solved by the Invention] For this reason, mechanical properties become dependent on the cutting direction, which has a significant effect on the dependence of fracture toughness on the cutting direction. For example, when forging is performed in one direction, if α phase exists continuously along the forging direction, cracks in the forging direction will easily propagate, making it impossible to obtain high toughness in the forging direction.

Therefore, in the present invention, by preventing the continuity of the α phase, which could not be eliminated by conventional processing methods, equiaxed property is imparted, and mechanical properties, especially fracture toughness, do not depend on the cutting direction. The object of the present invention is to provide a method for hot working a near β type titanium alloy that can obtain high strength and high toughness.

[Means for Solving the Problems] The present invention has solved the above problems. During the process of cooling the titanium alloy from the β single phase region, the precipitation of the α phase begins and then the d phase begins to precipitate. The gist of the structure is to apply hot working at any stage until the precipitation is completed, and to further hot work after the α+β two phase region is formed.

[Effect] When a titanium alloy in which a single β phase is maintained quasi-stablely under the condition of no hot working is heated and held in the α + β two phase temperature range, the α phase preferentially precipitates at the β grain boundaries, and the α phase precipitates at the β grain boundaries. The precipitated α phase shows continuity along the β grain boundaries. The conventional technique is to process this state to obtain an equiaxed structure, but the continuity of the α phase that is continuously precipitated at the β grain boundaries cannot be easily eliminated even by this process.

However, in the case of the present invention, the titanium alloy is hot worked in the cooling process from the β single phase region until the precipitation of the α phase is completed, and at this time, shear strain is temporarily introduced into the β grains. , most of the α phase preferentially precipitates in the strained areas (inside the former β grains). Therefore, continuous precipitation of the α phase at the β grain boundaries is prevented, and a strong continuous α phase is no longer formed. Although the amount of hot working in this case is not particularly specified, a sufficient effect can be obtained even with about 20%.

However, with this processing alone, the pro-eutectoid α phase remains plate-like or acicular, and this shape is not only maintained even in the subsequent solution treatment and aging treatment that are normally performed, but also the pro-eutectoid α phase forms at the β grain boundaries. Concentration may occur and continuity may be formed again.

Therefore, hot working is required to further make the pro-eutectoid α phase equiaxed. This is hot working after the α+β two phase region is formed. By hot working in this α+β two phase region, a near β type titanium alloy with a stable equiaxed structure is obtained. There is basically no problem with this hot working temperature if it is in the α + β two phase region, but just below the transformation point, the amount of pro-eutectoid α phase is small, resulting in a non-uniform structure, which is undesirable. This is naturally regulated due to the lack of processing deformability of the alloy. Therefore, hot working in the α+β two phase region is preferably performed at a temperature 15 to 115°C lower than the transformation point.The amount of processing in this case is often determined by the balance with the product shape, but it is recommended that it be approximately 30% or more. From the viewpoint of further promoting equiaxed formation, it is desirable and effective to perform solution treatment in the α+β two-phase region after the initial hot working and before hot working in the α+β two-phase region. .

As described above, the method for processing titanium alloys in the present invention involves first hot working to uniformly disperse and precipitate the α phase within the prior β grains, and then hot working the resulting α+β two-phase region to further convert the α phase to equiaxed This processing method is not influenced in any way by the composition of the titanium alloy. However, it is difficult to apply the present invention to titanium alloys in which the β phase transforms into another phase by solution treatment from the α + β two phase region because other factors affect the structure morphology. Even after solution treatment and aging treatment, the application is limited to titanium alloys consisting of approximately 45% α phase and the remainder mainly β phase, that is, near β type titanium alloys with relatively little α phase.

[Examples and Comparative Examples] Example I N ear β type alloy Ti-5AITi-5AI-
90X90 of 2Sn-2Zr-4 (transformation point: 865°C)
X100 (arm) block from 870℃ to about 75℃
20% was processed during the cooling process to 0°C, and 30% was reprocessed at 830°C. Thereafter, the structure was subjected to solution treatment at 800° C. for 4 hours and aging treatment at 620° C. for 8 hours, and the state of the structure is shown in FIG.

Comparative Example 1 830 blocks of the same material and the same shape as in Example 1 were used.
After 80% processing at 800°C, solution treatment was performed at 800°C for 4 hours and aging treatment at 620°C for 8 hours. The structure of the obtained alloy is shown in FIG.

As is clear from FIGS. 2 and 3, in Example 1, continuous α along the β grain boundary as in Comparative Example 1.
It has an equiaxed structure in which no phase exists and the α phase is uniformly scattered.

In order to evaluate the dependence of the fracture toughness on the cutting direction in the hot-worked samples according to Example 1 and Comparative Example 1, Figure 1 is shown. c(L): Fracture toughness value when the crack progresses in the forging and elongation direction xtc (T): Fracture toughness value when the crack progresses in a direction perpendicular to the forging and elongation direction The Kxc(L) of Example 1, which shows the values of , is greatly improved compared to Comparative Example 1, and the value of K+c(L)/Kxc(T) is also O6, 7 in Comparative Example 1. On the other hand, in Example 1, 0
．． 9 or higher, which is a big improvement. The fracture toughness test was A.
It was carried out according to STME399.

Example 2 N ear β type alloy Ti-10V-2Fe-3
A1 (transformation point 2800℃) 90X90X90 (mm
) was processed by 20% during the cooling process from 820°C to about 730°C, and reprocessed by 30% at 760°C. Thereafter, the structure was subjected to solution treatment at 740° C. for 1 hour and aging treatment at 510° C. for 8 hours, and the state of the structure is shown in FIG.

Comparative Example 2 A block of the same material and shape as in Example 2 was quenched from the β single phase region at 850°C according to the conventional method, processed to 80% at 760°C, and then subjected to solution treatment at 740°C for 1 hour. Aging treatment was performed at 510°C for 8 hours. The obtained alloy structure is shown in FIG.

As is clear from Figures 4 and 5, continuity of the α phase is observed in Comparative Example 2, but the overall equiaxed structure is observed in Example 2, which indicates the effectiveness of this method. shown.

[Effect of the invention] As described above, the near-ear obtained by the method of the present invention
In β-type titanium alloys, the precipitation of the α phase is not continuous along the β grain boundaries and forms an equiaxed structure, so the mechanical properties, especially the fracture toughness, do not depend on the cutting direction and have high strength. It also has high toughness.

[Brief explanation of the drawing]

Figure 1 shows the "fracture toughness value KIC (L) when the crack propagates in the forging direction, and the fracture toughness value KIC (L) when the crack propagates in the direction orthogonal to the forging direction, in an example according to the method of the present invention and a comparative example according to the conventional method. FIG. 2.4 is a diagram showing the value KIC(T) and +c(L)/to!c(T) value, and FIG. 2.4 is an example.
Figure 3.5 is an electron micrograph showing the metal structure of a comparative example. Figure 1 Example Example immediately = ko匈v+tma(x)

Claims (1)

[Claims] α during the process of cooling the titanium alloy from the β single phase region
Nea is characterized in that hot working is applied at any step between the start of phase precipitation and the completion of d-phase precipitation, and further hot working is carried out after the α+β two phase region is formed.
Hot working method for rβ type titanium alloy.