Heat treatment of titanium alloy
Heat treatment characteristics of titanium alloy
- (1) Martensitic transformation does not change the properties of titanium alloy significantly. This characteristic is different from the martensitic transformation of steel. The heat treatment and strengthening of titanium alloy can only rely on the aging decomposition of metastable phase (including martensitic phase) formed by quenching. Moreover, the heat treatment method of pure A-type titanium alloy is basically not effective, that is, the heat treatment of titanium alloy is mainly used for α + β type titanium alloy.
- (2) The formation of ω phase should be avoided in heat treatment. The formation of ω phase will make the titanium alloy brittle. The proper selection of aging process (for example, using a higher aging temperature) can make the ω phase decompose.
- (3) It is difficult to refine titanium alloy grain by repeated transformation. This is also different from that of steel materials. Most steels can control the nucleation and growth of new phase by means of the repeated transformation of austenite and pearlite (or ferrite and cementite), so as to achieve the goal of grain refinement. However, there is no such phenomenon in titanium alloy.
- (4) Poor thermal conductivity. The poor thermal conductivity can lead to the poor hardenability of titanium alloy, especially α + β titanium alloy. The quenching thermal stress is large and the parts are easy to warp during quenching. Due to the poor thermal conductivity, the deformation of titanium alloy is easy to cause the local temperature rise to be too high, so that the local temperature may exceed the β transition point and form widmanstatten structure.
- (5) Chemically active. During heat treatment, titanium alloy is easy to react with oxygen and water vapor, forming a certain depth of oxygen rich layer or oxide skin on the surface of the workpiece, which reduces the performance of the alloy. At the same time, titanium alloy is easy to absorb hydrogen and cause hydrogen embrittlement during heat treatment.
- (6) The β transition point is different. Even if it is the same composition, but due to different smelting furnace times, its β transformation
- The temperature is sometimes very different.
- (7) When heated in the β phase region, β grains tend to grow. It is suggested that the temperature and time of heating should be strictly controlled, and the heat treatment in β phase area should be used with caution.
Heat treatment types of titanium alloy
The transformation of titanium alloy is the basis of heat treatment of titanium alloy. In order to improve the properties of titanium alloy, in addition to reasonable alloying, proper heat treatment is necessary. There are many kinds of heat treatment for titanium alloy, such as annealing treatment, aging treatment, deformation heat treatment and chemical heat treatment.
The main purpose of annealing is to eliminate the stress, improve the plasticity and stabilize the structure of titanium alloy. Annealing forms include stress relief annealing, recrystallization annealing, double annealing, isothermal annealing and vacuum annealing
Stress relief annealing
In order to eliminate the internal stress in the process of casting, cold deformation and welding, stress relief annealing can be used. The temperature of stress relief annealing should be lower than the recrystallization temperature, generally 450 ~ 650 ℃, and the time required depends on the cross-section size of the workpiece, processing history and the degree of stress relief required.
General annealing. Its purpose is to eliminate the basic stress of titanium alloy semi-finished products, and to have high strength and meet the requirements of technical conditions of plasticity. The annealing temperature is generally equal to or slightly lower than the recrystallization starting temperature. This kind of annealing process is generally used when metallurgical products leave the factory, so it can also be called factory annealing.
The aim is to eliminate work hardening completely, stabilize the structure and improve the plasticity. This process mainly occurs recrystallization, so it is also called recrystallization annealing. Annealing temperature is better between recrystallization temperature and transformation temperature. If the temperature is higher than transformation temperature, widmanstatten structure will be formed and the properties of the alloy will deteriorate. For different kinds of titanium alloy, the annealing type, temperature and cooling method are different.
In order to improve the plasticity, fracture toughness and stable structure of the alloy, double annealing can be used. The microstructure of the annealed alloy is more uniform and close to equilibrium. In order to ensure the stability of structure and properties under high temperature and long-term stress, this kind of annealing is often used in heat-resistant titanium alloy. Double annealing is to heat and air cool the alloy twice. The heating temperature of the first high temperature annealing is higher than or close to the end temperature of recrystallization, so that recrystallization can be fully carried out without obvious grain growth, and the volume fraction of AP phase can be controlled. After air cooling, the structure is not stable enough, so the second low-temperature annealing is needed. The annealing temperature is lower than the recrystallization temperature, and the heat preservation time is longer, so that the metastable β phase obtained by high-temperature annealing can be fully decomposed.
The best plasticity and thermal stability can be obtained by isothermal annealing. This kind of annealing is suitable for two-phase titanium alloy with high content of β stable elements. The isothermal annealing adopts the method of staged cooling, that is, after heating to above the recrystallization temperature for heat preservation, it is immediately transferred to another lower temperature furnace (generally 600 ~ 650 ℃) for heat preservation, and then air cooled to room temperature.
Quenching and aging is the main way of heat treatment and strengthening of titanium alloy. It uses phase transformation to produce strengthening effect, so it is also called strengthening heat treatment. The strengthening effect of heat treatment of titanium alloy depends on the properties, concentration and heat treatment specification of alloy elements, because these factors affect the type, composition, quantity and distribution of metastable phases obtained by quenching, as well as the nature, structure and dispersion degree of precipitates in the decomposition process of metastable phases, which are related to the composition, heat treatment process specification and original structure of the alloy.
For the alloy with certain composition, the effect of aging strengthening depends on the selected heat treatment process. The higher the quenching temperature is, the more obvious the effect of aging strengthening is. However, the higher the quenching temperature is, the brittleness will be caused by too coarse grains. For the two-phase titanium alloy with lower concentration, higher temperature quenching can be used to obtain more martensite, while for the two-phase titanium alloy with higher concentration, lower temperature quenching can be used to obtain more metastable β phase, so as to obtain the maximum aging strengthening effect. Generally, the cooling mode is water-cooling or oil cooling, and the quenching process should be rapid to prevent the decomposition of β phase in the transfer process and reduce the aging strengthening effect. The selection of aging temperature and time should be based on the best comprehensive performance. Generally, the aging temperature of α + β titanium alloy is 500-600 ℃, and the time is 4-12h; while the aging temperature of β titanium alloy is 450-550 ℃, and the time is 8-24h, and the cooling mode is air cooling.
Deformation heat treatment is an effective combination of pressure processing (forging, rolling, etc.) and heat treatment process, which can play the role of deformation strengthening and heat treatment strengthening at the same time, and obtain the structure and comprehensive properties that can not be obtained with a single strengthening method. The common thermomechanical treatment process is shown in Figure 1. Different types of thermomechanical treatment are classified according to the relationship between deformation temperature, recrystallization temperature and phase transition temperature.
Fig. 2 Thermomechanical treatment process of titanium alloy
1-heating; 2-water cooling; 3-aging; 4-high or low temperature deformation; tβ – β phase transition point; T再 – recrystallization temperature
- (1) High temperature thermomechanical treatment. After heating to the recrystallization temperature, the deformation is 40% ~ 85%, and then quenching is carried out quickly, and then conventional aging heat treatment is carried out.
- (2) Low temperature thermomechanical treatment. The deformation is about 50% under the recrystallization temperature, and then the conventional aging treatment is carried out.
- (3) Composite thermomechanical treatment. A process combining high-temperature thermomechanical treatment and low-temperature thermomechanical treatment.
Chemical heat treatment
The friction coefficient of titanium alloy is large, and its wear resistance is poor (generally 40% lower than that of steel). It is easy to bond on the contact surface and cause friction corrosion. The corrosion resistance of titanium alloy is strong in oxidation medium, but poor in reduction medium (hydrochloric acid, sulfuric acid, etc.). In order to improve these properties, electroplating, spraying and chemical heat treatment (nitriding, oxygen infiltration, etc.) can be used. The hardness of nitriding layer after nitriding is 2-4 times higher than that of the surface layer without nitriding, so the wear resistance of the alloy is obviously improved, and the corrosion resistance of the alloy in reducing medium is also improved; the corrosion resistance of the alloy can be increased by 7-9 times by oxygen infiltration, but the plasticity and fatigue strength of the alloy will be damaged in different degrees.
Microstructure of titanium alloy
Various structures can be observed in titanium alloy, especially in α + β biphasic titanium alloy. These structures are different in morphology, grain size and intragranular structure, which mainly depends on the alloy composition, deformation process and heat treatment process. There are two basic phases in titanium alloy, namely, α phase and β phase. The mechanical properties of titanium alloy depend on the proportion, shape, size and distribution of the two phases to a great extent. The structure types of titanium alloy can be basically divided into four categories: widmanstatten structure (lamellar structure), basket structure, bimodal structure and isometric structure. Fig. 2 shows the typical microstructure characteristics of titanium alloy. It can be seen that the performance of TC4 titanium alloy varies greatly in different microstructures.
Figure.2 Typical structure in titanium alloy
(a) lamellar structure; (b) basket structure; (c) bimodal structure; (d) isometric structure
It is characterized by coarse primary β – grains and complete grain boundary α – phases, forming larger “bunches” in the primary β – grains, and more in the same “bunches”. The sheets are parallel to each other in the same orientation, as shown in Fig. 3 (a). This microstructure is formed when the alloy is cooled from the β phase region slowly without deformation or small deformation after heating. When the alloy has this structure, its fracture toughness, endurance and creep strength are good, but its plasticity, fatigue strength, notch sensitivity, thermal stability and thermal stress corrosion resistance are poor.
Net basket organization
It is characterized by that the original β grain boundary is destroyed in the deformation process, and there is no or only a small amount of scattered granular grain boundary α. The α sheet in the original β grain becomes shorter, the size of α “bundle” is smaller, and the pieces are staggered, just like a woven basket, as shown in Figure 3 (b). When the alloy is heated or deformed in the β phase region, or the deformation amount in the (α + β) dual phase region is not large enough, this kind of microstructure is generally formed. The fine basket structure not only has better plasticity, impact toughness, fracture toughness and high cycle fatigue strength, but also has better thermal strength.
It is characterized by the distribution of unconnected primary α on the matrix of p-transformed structure, but the total content is not more than 50%, as shown in Fig. 3 (c). When the heating temperature of thermal deformation or heat treatment of titanium alloy is less than β transition temperature, the bimorphic structure can be obtained generally. Bimorphic tissue means that there are two forms of α phase in tissue, one is primary α phase with equal axis shape, the other is lamellar α phase in β transformed tissue, which corresponds to primary α phase. Phase is also called secondary alpha phase or secondary alpha phase. This kind of structure will be formed when the temperature and deformation of the alloy are higher in the (α + β) two-phase region.
It is characterized by a certain amount of transformation β structure distributed on the primary α phase matrix with more than 50% uniformly distributed content, as shown in Fig. 3 (d). The deformation processing and heat treatment of titanium alloy are all carried out in (α + β) two-phase region or α phase region, and the equiaxed structure can be obtained when the heating temperature is much lower than β transition temperature. Compared with other structures, these structures have better plasticity, fatigue strength, notch resistance sensitivity and thermal stability, but lower fracture toughness, durability and creep strength. At present, this kind of structure is widely used because of its good comprehensive properties.
Effect of heat treatment process on microstructure evolution of titanium alloy
The heat treatment process of titanium alloy is shown in Figure 4. The main control parameters are solution temperature, solution time, cooling mode [including water quench (WQ), oil quench (OQ), air cooling (AC) and furnace cooling (FC)], aging temperature and aging time.
Fig.3 Typical heat treatment process
Effect of solution temperature on Microstructure of TC21 alloy
Figure 5 shows the microstructure of TC21 alloy at different solution temperatures. It can be seen from Figure 5 that with the increase of solution temperature, the volume fraction of α P phase decreases, and when the solution temperature is higher than t β, α P phase disappears. After solution treatment at 940 ℃, the grain boundary of β grain is bent out due to the obstruction of equiaxed α P phase, as shown by the arrow in Fig. 5 (c). After solution treatment (> t β) at 1000 ℃, α P phase disappears, and β grain grows rapidly due to the obstruction of β grain boundary movement, with an average diameter of about 300 μ m, as shown in Fig. 5 (d).
It can be seen that the solution temperature has a significant effect on the microstructure of TC21 alloy. The size, morphology and distribution of α P phase will directly affect the size of β grain in (α + β) two-phase solution. Titanium alloy. The α P phase and β grain size play an important role in the mechanical properties of the alloy. In order to avoid the rapid growth of β – grains, the solution temperature of TC21 alloy should be lower than t β, which can obtain the bimorphic structure with proper grain size and mixed primary and secondary phases.
Fig.4 Effect of solution temperature on Microstructure of TC21 alloy
(a) 850℃/AC；(b) 910℃/AC；(c) 940℃/AC；(d) 1000℃/AC
Effect of solution time on Microstructure of TC21 alloy
Figure 5 shows the microstructure of tciz alloy after solution treatment and air cooling for 4H. It can be seen from Fig. 6 and Fig. 5 (a) and (b) that with the increase of solution time, the volume fraction and distribution of AP phase in TC21 alloy have not changed significantly. It can be seen that the microstructure of TC21 alloy is not sensitive to the solution treatment time after solution treatment for a certain time, but the solution treatment temperature plays a decisive role in the solution structure of the alloy.
Fig.5 Effect of solution time on Microstructure of TC21 alloy
(a) 850℃/4h，AC；(b) 910℃/4h，AC
Effect of cooling mode on Microstructure of TC21 alloy
Fig. 6 shows the effect of cooling mode on the microstructure of TC21 alloy. It can be seen from Fig. 6 that the cooling mode has obvious influence on the microstructure of TC21 alloy after solution treatment. Under WQ and OQ conditions, due to the fast cooling speed, only metastable β is formed without β t formation, while under AC conditions, a certain amount of β t is formed; the size of α P phase obtained under WQ and OQ conditions is slightly smaller than that obtained under AC conditions. The reason for this difference is that the cooling rate of AC is slow, and the α P phase in the alloy can grow sufficiently in the cooling process. At high temperature, the β phase can be fully transformed to form β t in the slow cooling process.
Fig.6 Effect of cooling mode on Microstructure of TC21 alloy
Effect of aging temperature on Microstructure of TC21 alloy
Fig. 7 shows the microstructure of TC21 alloy aged at 500 ℃ and 600 ℃. It can be seen from Fig. 7 that the microstructure of the alloy after aging is α P phase + β t phase. With the aging process, the secondary α phase grows and merges; with the aging temperature increasing, the secondary α phase gradually increases. As shown in Fig. 7 (a), (b) and (c), when aging at 500 ℃, due to the low aging temperature, the metastable β obtained by solution treatment lacks the driving force of decomposition in the aging process, resulting in less secondary phase.
Fig.7 Effect of aging temperature on Microstructure of TC21 alloy
(a) 910℃/1h，WQ+500℃/6h,AC；(b) 910℃/1h，OQ+500℃/6h,AC
(c) 910℃/1h，AC+500℃/6h,AC；(d) 910℃/1h，WQ+600℃/6h,AC
(e) 910℃/1h，OQ+600℃/6h,AC；(f) 910℃/1h，AC+600℃/6h,AC
Effect of aging time on Microstructure of TC21 alloy
Fig. 8 shows the microstructure of TC12 alloy aged at 550 ℃ for different times. It can be seen from Fig. 8 that with the extension of aging time, β t increases continuously, while the size of α P phase has no obvious change, but there is a phenomenon of merging and growing up, and the secondary strip α phase with larger size also has a phenomenon of merging and growing up.
Fig.8 Effect of aging time on Microstructure of TC21 alloy
(a) 910℃/1h，WQ+500℃/2h,AC；(b) 910℃/1h，WQ+550℃/12h,AC
(c) 910℃/1h，AC+500℃/2h,AC；(d) 910℃/1h，OQ+550℃/12h,AC
(e) 910℃/1h，OQ+600℃/2h,AC；(f) 910℃/1h，AC+550℃/12h,AC
Effect of heat treatment on Microstructure of typical titanium alloy
By controlling the heat treatment process conditions of TC12 alloy and Ti60 Alloy, LM and BM are obtained, as shown in Figure 9.
Fig.9 Effect of heat treatment on Microstructure of typical titanium alloy
(a) TC21 970℃/1h，FC；(b)TC21 910℃/1h，AC+550℃/6h,AC
(c) TC21 910℃/1h，FC+550℃/6h,AC；(d)Ti600 1020℃/2h，AC+650℃/8h,AC
(e)Ti600 1005℃/2h，AC+650℃/8h,AC；(f)Ti600 AC+600℃/100h,AC
It can be seen from Fig. 9 (d) and (E) that LM and BM structures can be obtained respectively when the solution temperature of Ti600 alloy is selected above and below TB (l010 ℃). The lamellar thickness of LM is 2-3 μ m, the volume fraction of α P phase in BM is about 20%, and the average diameter is about 15 μ M.
Fig. 9 (f) shows the microstructure of Ti600 alloy with BM structure after 100 h of thermal exposure (TE) at 600 ℃. The difference between BM and BM + te can not be distinguished only from the microstructure shown in Fig. 9 (E) and (f). In the long-term aging or heat exposure process of high temperature titanium alloy, α 2 (Ti3Al) phase is easy to precipitate from its al rich α P phase. It can be seen from TEM that α 2 phase is found in α P phase of Ti 600 alloy with BM structure after heat exposure, as shown in FIG. 11.
Figure.10 TEM morphology and selected area electron diffraction pattern of α 2 phase in Ti600 alloy after heat exposure
(a) TEM morphology; (b) selected area electron diffraction pattern
Source: China Titanium Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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