Heat treatment methods for titanium

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I. Basic heat treatment of titanium: Industrially pure titanium has a single-phase α-type structure. Although there is an α-β polymorphic transformation above 890°C, the characteristics of this phase transition result in relatively weak strengthening effects. Therefore, heat treatments such as quenching and tempering cannot be used to enhance the mechanical strength of industrially pure titanium. The only heat treatment for industrially pure titanium is annealing. Its main annealing methods are three: 1. Recrystallization crystal annealing, 2. Stress-relief annealing, 3. Vacuum annealing. The purpose of the first two is to eliminate stress and work-hardening effects in order to restore plasticity and formability. The work hardening effect has a significant impact on industrial pure titanium during the material production process. Figure 2-26 shows the increase in the yield strength of TA2 after different degrees of cold working. Therefore, during the production of titanium materials, recrystallization annealing is necessary after cold and hot processing, in order to restore plasticity and achieve a stable fine-grained structure as well as uniform mechanical properties. The recrystallization temperature of industrially pure titanium is 550–650°C; therefore, the recrystallization annealing temperature should be higher than this value but lower than the transition temperature between the α- and β-phases. The highest overall mechanical properties are obtained through annealing at 650–700°C (as annealing above 700°C leads to grain growth, resulting in a decrease in mechanical properties). The cold work hardening of annealed materials can generally be eliminated through annealing for 10–20 minutes. This heat treatment is generally carried out at titanium production facilities. To reduce the gas pollution associated with high-temperature heat treatment and to further remove the hydrogen absorbed by titanium materials during heat processing, most titanium manufacturers nowadays require annealing in a vacuum environment. To eliminate the residual stresses in titanium materials during processing (such as welding, explosive bonding, and minor cold deformation during manufacturing), stress-relief heat treatment should be carried out. Stress-relief annealing generally does not require a vacuum or argon atmosphere; it is sufficient to maintain a slightly oxidizing atmosphere inside the furnace. II. Heat treatment of titanium and titanium alloys: To facilitate processing in the machinery industry and to obtain titanium and titanium alloys with certain properties that meet the material requirement of various products, it is necessary to carry out heat treatment on these metals. 1. Heat treatment of industrially pure titanium (TA1, TA2, TA3): When an α-titanium alloy is cooled from high temperatures to room temperature, its microstructure consists almost entirely of the α phase, which does not contribute to strengthening. Therefore, at present, α-titanium only requires stress-relief annealing, recrystallization annealing, and vacuum annealing. The first two are carried out in a micro-oxidation furnace, while the latter should be carried out in a vacuum furnace. (1) Stress-relief annealing: In order to eliminate the internal stresses generated in titanium and titanium alloys during processes such as melting, cold working, machining, and welding, thereby facilitating further processing and preventing cracking and failure due to these internal stresses during use, α-titanium must undergo stress-relief annealing. The stress-relief annealing temperature should not be too high or too low; too high a temperature leads to grain coarsening and unwanted phase transformations that affect mechanical properties, while too low a temperature fails to eliminate stress. Therefore, it is generally chosen to be below the recrystallization temperature. For commercially pure titanium, the heating temperature for stress-relief annealing is 500–600°C. The heating time should be determined based on the thickness of the workpiece and the holding time. To improve economic efficiency and prevent unnecessary oxidation, the shortest time that can eliminate most of the internal stresses should be selected. In the industrial sector, the holding time for stress-relief annealing of pure titanium is 15–60 minutes, with air cooling being the common method of cooling. (II) Recrystallization annealing (complete annealing): Most α-titanium is used in the annealed state; annealing reduces strength and increases plasticity, resulting in better overall properties. To minimize gas contamination of the titanium surface during heat treatment, the heat treatment temperature should be kept as low as possible. The annealing temperature for industrially pure titanium is higher than the recrystallization temperature, but lower than the temperature at which the α phase transforms into the β phase, which is 120–200°C; at this temperature, a fine-grained structure is obtained. The heating time depends on the thickness of the workpiece, and air cooling is generally used for cooling. For industrially pure titanium, the heating temperature for recrystallization annealing is 680–700°C, with a holding time of 30–120 minutes. The selection of parameters should be determined based on actual conditions; generally, when the heating temperature is high, the holding time should be shorter. It should be noted that when the annealing temperature is above 700°C and the holding time is long, grain coarsening occurs, leading to a decline in mechanical properties. Moreover, once the grains have become coarse, it is difficult to refine them using any existing heat treatment methods. To avoid grain coarsening, the following two measures can be taken: 1) Choose the annealing temperature as low as possible, below 700°C. 2) When the annealing temperature is above 700°C, the holding time should be as short as possible; however, under normal circumstances, it must be no less than 3 minutes per mm of thickness, and for all workpieces, it must not be less than 15 minutes. (III) Vacuum annealing: Although hydrogen in titanium does not have a strengthening effect, it is highly harmful as it can cause hydrogen embrittlement. The solubility of hydrogen in α-titanium is very low; it exists mainly in the form of TiH2 compounds, which are stable only below 300°C. If α-titanium is heated in a vacuum, the hydrogen level can be reduced to below 0.1%. When there is an excessive amount of hydrogen in titanium, hydrogen removal is necessary. To remove hydrogen or prevent oxidation, vacuum annealing must be performed. The heating temperature and holding time for vacuum annealing are basically the same as those for recrystallization annealing. The cooling method involves slowly cooling the furnace to an appropriate temperature before it can be reopened; the vacuum level must not be lower than 5×10-4 mmHg. III. Heat treatment of TC4 (Ti-6Al-4V) Among titanium alloys, TC4 is a widely used type, and it is usually employed in its annealed state. TC4 can be subjected to stress-relief annealing, recrystallization annealing, and solution aging. The microstructure after annealing consists of a coexistence of α and β phases, although the β phase constitutes only a small proportion, about 10%. The recrystallization temperature of TC4 is 750°C. The recrystallization annealing temperature is generally set 80–100°C above the recrystallization temperature (although in practical applications it may vary depending on specific conditions, as shown in Table 5-26). After recrystallization annealing, the microstructure of TC4 consists of equiaxed α-phase and β-phase, resulting in good overall properties. However, the annealing treatment of TC4 is merely a phase-stabilization process; to fully unlock its potential for excellent properties, strengthening treatments should be carried out. The α+β/β phase transition temperature of TC4 alloy is 980–990°C. The solution treatment temperature is generally chosen to be 40–100°C below this α+β/β transition temperature (depending on specific conditions, as shown in Table 5-26). This is because while solution treatment in the β phase region results in a coarse Widmanstätten structure that offers high endurance strength and fracture toughness, it also results in very low tensile plasticity and fatigue strength; such disadvantages do not occur when solution treatment is carried out in the α+β phase region. Specification Type Temperature (°C) Time (min) Cooling Method Stress-relief annealing 550–650 30–240 Air cooling Recrystallization annealing 750–800 60–120 Air cooling or cooling in the furnace to 590°C followed by air cooling Vacuum annealing 790–815 Solution treatment 850–950 30–60 Water quenching Aging treatment 480–560 4–8 hours Air cooling The aging treatment involves heating the TC4 after solution treatment to a moderate temperature, holding it there for a certain period of time, and then cooling it in air. The purpose of aging treatment is to eliminate the α’ phase generated by solution treatment, which has an adverse effect on the overall properties. The quenched martensite α’ formed during solution treatment undergoes rapid decomposition during aging (a rather complex phase transformation), which leads to an increase in strength. There are two views on this phenomenon: 1) It is believed that as α’ decomposes into α+β, the dispersion strengthening effect of the decomposition products increases the strength of TC4. 2. It is believed that during aging, the β phase decomposes to form the ω phase, thereby strengthening TC4. As time passes, the strength decreases, and there are two different views on this phenomenon: 1. The aggregation of the β phase leads to a decrease in strength (corresponding to point 1 above). 2. The decomposition of the ω phase is a softening process (corresponding to the above 2). The selection of aging temperature and time should be based on achieving the best overall performance. Within the recommended ranges for solution treatment and aging, it is best to determine the optimal process using the aging hardening curve (as shown in Figure 5-28). This curve represents the age hardening curve of TC4 after solution treatment at 850°C, at different temperatures. Low-temperature aging (480–560°C) is better than high-temperature aging at temperatures above 700°C. This is because, in terms of tensile strength at high temperatures, endurance and creep strength, fracture toughness, and notched tensile properties, low-temperature aging performs better than high-temperature aging. The overall properties of solution-treated TC4 are better than those after annealing at 750–800°C. It should be noted that the original microstructure of the TC4 alloy in its processed state has a significant influence on the microstructure and mechanical properties after heat treatment. For the network-like structure formed through different deformations at temperatures above the phase transition temperature, it cannot be altered by heat treatment; after annealing at 750–800°C, it remains in its original structural state ； For the α and β phase structures obtained by processing below the phase transition temperature, annealing at 750–800°C yields equiaxed primary α phases and transformed β phases. The tensile ductility and area reduction of the former are both lower than those of the latter ； However, it has high high-temperature resistance, fracture toughness, and resistance to hot salt stress corrosion. IV. Heat treatment of Ti-32Mo-2.5Nb Ti-32Mo-2.5Nb is a stable β-phase single-phase solid solution alloy; it only requires stress-relief annealing, with an annealing temperature of 750–800°C and a holding time of one hour. Air cooling or furnace cooling can be used for cooling. V. Several issues in heat treatment (1) Contamination issues Titanium has extremely high chemical reactivity; it can react with almost all elements. At room temperature, it can react with oxygen in the air to form an extremely thin oxide layer, and the oxidation rate is very low. However, at high temperatures, in addition to an increased oxidation rate and diffusion into the metal lattice, titanium also reacts vigorously with hydrogen, nitrogen, carbon, and other elements present in the air; it can also react with gaseous compounds such as CO, CO2, H2O, NH4, and many volatile organic compounds. The heat-treated metal elements react with the titanium on the surface of the workpiece, causing changes in the chemical composition of the titanium surface. Some of these interstitial elements can also penetrate the metal lattice, forming an interstitial solid solution. Moreover, apart from hydrogen, the reactions of other elements with titanium are irreversible. Even hydrogen is not allowed to be removed at high temperatures after the final heat treatment. Interstitial elements not only affect the mechanical properties of titanium and titanium alloys, but also influence the α+β/β transition temperature as well as certain phase transformation processes. Therefore, attention must be paid to interstitial elements, especially gas impurity elements that cause contamination in titanium and titanium alloys, during heat treatment. (II) Selection of heating furnaces: To prevent contamination during the heating process, different measures must be taken for workpieces with varying requirements. If the contaminant layer on the workpiece surface can be removed by grinding or other mechanical processing at the end, it can be heated in any type of heating furnace, with an atmosphere that is neutral or slightly oxidizing. To prevent hydrogen absorption, a reducing atmosphere must be absolutely avoided inside the furnace. When the final processing step for the workpiece is heat treatment, it is necessary to use a vacuum furnace (with a vacuum level of 1×10-4 mmHg) or a heating furnace with an argon atmosphere (where the argon purity is over 99.99% and the gas is dry) for heating. After heat treatment, if necessary, the workpiece is pickled at 50°C using a solution of 30% nitric acid, 3% hydrofluoric acid, and the remainder water, or it is lightly ground in order to remove any surface contamination. (III) Heating method: Before carrying out heat treatment, it is necessary to clean the furnace chamber; no other metals or oxide scales should be present inside the furnace ； For the workpiece, the surface is required to be free of oil, water, and oxide scale. Heating titanium workpieces in a vacuum furnace is an effective method to prevent contamination, but due to current limitations, many factories still use conventional heating furnaces. Heating is carried out in a conventional furnace, and different measures are taken to prevent contamination depending on the requirements. For example: 1. Depending on the size of the workpiece, it can be placed in a sealed low-carbon steel container and heated after vacuuming. If there is no vacuum pump, an inert gas (argon or helium) can be introduced for protection. The protective gas must be introduced and removed repeatedly to completely eliminate air. 2. The use of coatings is also one of the measures in heat treatment to protect titanium from contamination, and certain experience has been gained abroad in this regard. Some domestic factories are also using high-temperature paints and glass coatings as finishes. Some believe that the various protective coatings currently used for titanium can only reduce the depth of contamination, but cannot completely prevent it. For each heat treatment, the permissible contamination depth must be considered, and a suitable and effective coating must be selected, which also includes peeling after heat treatment. 3. When using flame heating, during the heating process, it is crucial to avoid directing the flame directly onto the titanium workpiece; a gas flame is one of the main sources of hydrogen absorption by titanium. When using fuel for heating, care must be taken to avoid excessive oxidation or carbonation of the titanium workpieces. (IV) Cooling: The cooling methods for the heat treatment of titanium and titanium alloys are mainly air cooling or furnace cooling; oil cooling or fan cooling is also used in some cases. The quenching medium can be low-viscosity oil or an aqueous solution containing 3% NaOH; however, the most commonly used quenching medium is water. As long as the requirements of titanium and titanium alloys regarding cooling rate can be met. The cooling devices used for the heat treatment of ordinary steel are also applicable to titanium.