Although the mechanical strength of titanium at normal temperature is quite high, but with increasing temperature up to 250 °C strength is lost by almost half. The situation is saved by titanium alloys, in which this drawback is leveled. Titanium is notable for its remarkable corrosion resistance. Corrosion resistance is evaluated by the amount of loss from 1 square metre of surface.
|Corrosion Resistance||Loss weight per square metre||Rating in points|
|Extremely resistant||Less than 0,001 g||1|
|Highly persistent||0,001 - 0,005||2|
|0,005 - 0,01 г||3|
|Persistent||0,01 - 0,05 г||4|
|0,05 - 0,1 г||5|
|Satisfactorily persistent||0,1 - 0,3 г||6|
|Slightly resistant||0,3 - 1,0 г||7|
|1 - 5,0 г||8|
|Unstable||More than 5 g.||9|
Comparative tests of corrosion resistance in industrial and marine atmospheres revealed that aluminum alloys, stainless steels, copper-nickel alloys and Inconel alloy showed visible signs of corrosion over a five-year period, while the titanium plate did not lose its original luster. This corrosion resistance is due to the presence of a passive oxide film on the metal surface that protects the metal from contact with an aggressive agent.
Titanium is particularly resistant to corrosion in the presence of oxygen. For example, under conditions of air aeration, titanium practically does not corrode in formic acid in any concentration up to a temperature of 100 °C, whereas without aeration it quickly corrodes in a 25% solution of formic acid.
Titanium alloys are characterized by their precise chemical composition, careful manufacturing, and absence of impurities. Titanium alloys are classified as:
|α-alloys||Pseudo-α-alloys||α +β alloys||Pseudo-β alloys||β-alloys|
TITAN alloys annealing modes
|Titanium grade||Temperatures of annealing of sheet products||Annealing temperature of bars||Annealing mode|
|VT1-00||520 - 540||670 - 690||---|
|VT1-0||520 - 540||670 - 690||---|
|VT5||---||800 - 850||---|
|VT5-1||700 - 750||800 - 850||---|
|OT4-0||590 - 610||690 - 710||---|
|OT4-1||640 - 660||740 - 760||---|
|OT4||660 - 680||700 - 760||---|
|OT4-2||700 - 730||840 - 860||---|
|VT20||700 - 800||700 - 800||---|
|AT2||600 - 650||650 - 700||---|
|AT3||800 - 850||800 - 850||---|
|AT4||850 - 870||850 - 870||---|
|TS5||760 - 780||760 - 780||---|
|VT6C||750 - 800||750 - 800||---|
|VT6||750 - 800||750 - 800||---|
870 - 920
600 - 650
|Isothermal annealing: heating to 870-920°C, holding time, cooling to 600-650°C Transfer to another furnace, holding time 2 hours, cooling in air|
870 - 920
650 - 600
|Double annealing, soaking at 550 - 600°C for 2-5 hours. For power parts - annealing at 650 °C air cooling.|
|VT14||740-760 790-810 640-600||740-760 790-810 640-660||Isothermal annealing: heating to 790-810°C, holding time, cooling with furnace, transfer to another furnace to 640-660°C, holding time ½ hour, air cooling|
|VT16||730 - 770||770 - 790||Cooling with furnace at a rate of 2 to 4°C per minute to 550°C, air cooled|
|VT22||740 - 760||680 - 800||Furnace batch cooling at 2 - 4 °C/ minute, up to 350 °C, air cooled|
|VT15||790 - 810||790 - 810||---|
|TS6||790 - 810||790 - 810||---|
|BT23||740 - 750||740 - 750||---|
TITAN hardening alphasing, nitriding, carbideization
When heat treating titanium, a so called alphiated layer is formed on the surface of semi-finished products because of active interaction of hot titanium with oxygen and nitrogen
|Alloy grade||Oxygen content in the altered layer mg/cm2||Depth of alphinated layer (mm)|
|Alloy grade||Nitriding depth||Surface hardness kgf/mm2|
|WT-1||0,06 - 0,08||750 - 850|
|VT3-1||0,04 - 0,05||700 - 750|
|VT-5||0,08 - 0,1||750 - 800|
|VT-8||0,12 - 0,14||700 - 750|
|VT-14||0,14 - 0,16||750 - 850|
Nitriding of titanium alloys
Nitriding increases heat and especially wear resistance and resistance to abrasion. The most widely employed method of nitriding is the heating of semi-finished products in an atmosphere of pure nitrogen to a temperature of 850 to 950°C. If the alloy contains aluminum or zirconium, speed and depth of nitriding increase, depth - up to 0.2 mm.
Carbidization (cementation) is an effective method of hardening titanium alloys. It is carried out in a mixture with activated carbon in a vacuum or in an atmosphere of a mixture of argon 5% carbon monoxide, methane or propane... Processing in an atmosphere of a gas mixture leads to saturation with hydrogen, which reduces the plasticity of titanium. Processing with charcoal takes 24 to 48 hours but leads to deeper carbideization
Chemical composition of sponge titanium obtained by reduction from Ti Cl4 by various methods
|Element||Vacuum separation||Magnesium with salt leaching||Sodium with salt leaching|
It is characterized by the accuracy of composition, thoroughness of manufacturing and processing, the absence of impurities.
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