Heat Treatment of Titanium and Titanium Alloys

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Because titanium is extremely active, it is easily polluted by carbon, oxygen, hydrogen, and oxygen, which will deteriorate the properties of the alloy. Therefore, vacuum furnace or argon protection should be used for the heat treatment of titanium alloy; if it is heated in an oxidizing atmosphere, strict control And remove the oxide layer on the surface of the workpiece. There are more and more titanium alloy parts that need vacuum heat treatment, in addition to the usual stress relief annealing of titanium alloy sheet metal parts and castings, solution treatment and aging treatment of titanium alloy fastening standard parts, and hydrogen removal annealing of titanium alloy In addition to being carried out in a vacuum furnace, the restraint aging treatment of high-strength titanium alloy sheet metal parts must also be carried out in a vacuum furnace. Vacuum heat treatment is a key process in the manufacturing process of titanium alloy parts, and its final performance depends to a large extent on the correct heat treatment.

One of the main parameters of vacuum heat treatment is the vacuum working pressure. The aviation industry standard stipulates: In the vacuum annealing process, in order to avoid the vacuum surface corrosion due to the vacuum pressure being too low, the vacuum working pressure should generally be controlled at not less than 2×10-3Pa . Some use high-purity argon for partial pressure control.

Inert gas can be used as a heating medium in a protective atmosphere, and can also be used as a quenching medium in vacuum solution treatment. Due to the relatively high chemical activity of titanium, there are also higher requirements for the purity of argon. Pure argon with a purity of not less than 99.99% is generally not suitable for heat treatment of titanium alloys. Foreign standards stipulate that the dew point of inert gases (helium and argon) is not higher than -54°C. The navigation mark stipulates that high-purity argon conforming to GB/T10624 shall be used, with a purity of not less than 99.999%.

Relevant domestic and foreign standards do not specify the temperature of the workpiece after heat treatment in a vacuum furnace. The navigation standard stipulates that the workpiece should be air-cooled below 200 ℃ after vacuum heat treatment. If there is a light yellow oxide film on the surface of the titanium part heated in inert gas or vacuum, it is not necessary to remove it; if there is a light blue, blue or gray oxide film on the surface of the part, it must be removed according to the air furnace regulations.

1.  Solution Treatment Quenching Allowable Delay Time

The cooling after heating of the titanium alloy solution treatment should be carried out strictly in accordance with the quenching requirements. If the cooling rate is too low, the phase components of the alloy will diffuse significantly, which will seriously affect the aging strengthening effect. Generally, the allowable delay time for quenching of titanium alloy workpieces must meet the requirements of Table 1.

Minimum section thickness/mm	Maximum delay times
≤0.6	6
0.6～25	10
2.5～25	15
＞25	30

Table 1 The longest quenching delay time of titanium alloy

2. Titanium Alloy is Heated in the Air Electric Furnace to Remove the Oxide Layer

The heat treatment of titanium alloy semi-finished products, parts, and structural parts can be heated in electric furnaces, but should not be heated in gas flame furnaces or heavy oil furnaces. The oxide layer on the surface of titanium alloy parts must be removed. The removal methods include pickling, chemical milling, sandblasting or machining. After removing the oxide layer on the surface of the titanium alloy finishing parts, a certain thickness of base metal should be removed according to regulations. The specific parameters are shown in Table 2.

Heating Temperature/℃	Heating Time/h
	≤0.2	0.2～0.5	0.5～1	1～2	2～6	6～10	10～20
500～600		8	13	13	13	25	51
600～700	8	13	25	25	51	76	76
700～760	13	25	25	51	76	76	152
760～820	25	25	51	76	142	152
820～930	51	76	142	152	254	—	—
930～980	76	142	152	254	—	—	—
980～1100	152	254	356	—	—	—	—

Table 2  Minimum depth of removal of base metal

Note: When heating in multiple passes, the oxide layer can be eliminated after the last heating, and the heating time is calculated by adding up each time.