Info

Mill Anneal

Recrystallization Anneal

Duplex Anneal

Beta Anneal

Ultimate Tensile Strength

High

Low

Low

Low

Ductility

High

High

High

Lower

Fatigue Strength

Intermediate

Intermediate

Lower

Lower

Fracture Toughness

Lowest

High

Intermediate

Highest

Fatigue Crack Growth Rate

Lowest

Intermediate

Intermediate

Highest

Creep Resistance

Lowest

Lowest

Intermediate

Highest

Mill Anneal - Roughly 300-450° F below beta transus, air cool. Recrystallization Anneal - Roughly 50-100° F below beta transus, slow cool. Duplex Anneal - Roughly 50-100° F below beta transus, air cool followed by mill anneal. Beta Anneal - Usually 50-100° F above beta transus, air cool.

Mill Anneal - Roughly 300-450° F below beta transus, air cool. Recrystallization Anneal - Roughly 50-100° F below beta transus, slow cool. Duplex Anneal - Roughly 50-100° F below beta transus, air cool followed by mill anneal. Beta Anneal - Usually 50-100° F above beta transus, air cool.

the beta transus. To obtain the maximum strength with adequate ductility, it is necessary to solution treat within about 50-150° F of the beta transus. When an alpha-beta alloy is solution treated, the ratio of beta phase to alpha phase increases and is maintained during quenching. The effect of solution treating temperature on the strength and ductility of Ti-6-4 sheet is shown in Fig. 4.24. During aging, the unstable retained beta transforms into fine alpha phase which increases the strength.

The cooling rate after solution heat treating has an important effect on the strength of alpha-beta alloys. For most alpha-beta alloys, quenching in water or an equivalent quenchant is required to develop the desired strength levels. The time between removing from the furnace and the initiation of the quench is usually about 7 s for alpha-beta alloys and as long as 20 s for beta alloys. For alloys with appreciable beta stabilizing elements and moderate section thickness, air or fan cooling is usually adequate. Essentially, the amount and type of beta stabilizers in the alloy will determine the depth of hardening. Unless an alloy contains appreciable amounts of beta stabilizers, it will not harden through thick sections and will exhibit lower properties in the center where the cooling rates are lower.

Aging consists of reheating the solution treated part in the range of 800-1200° F. A typical STA cycle for Ti-6-4 would be to solution treat at 1660-1700° F followed by water quenching. Aging would then be conducted at 1000° F for 4 h followed by air cooling. Ti-6-4 is sometimes solution treated and overaged (STOA) to achieve modest decreases in strength while obtaining improved fracture toughness and good dimensional stability.

The solution treatment for beta alloys is carried out above the beta transus. Commercial beta alloys are usually supplied in the solution treated condition with a 100% beta structure to provide maximum formability and only need to be aged to achieve high strength levels. After forming, the part is aged to provide

Fig. 4.24. Effect of Solution Temperature on Ti-6Al-4V Sheet

Fig. 4.24. Effect of Solution Temperature on Ti-6Al-4V Sheet maximum strength. Beta processed alloys have improved fracture toughness, better creep strength, and more resistance to stress corrosion cracking; however, there is a considerable loss in ductility and fatigue strength. Beta alloys are usually air cooled from the solution treating temperature.

Although there are coatings that can be used to protect titanium alloys during heat treatment from oxygen, hydrogen, and nitrogen, the best practice is to conduct the heat treatments in a vacuum furnace. Prior to heat treating, it is important that the surfaces are clean and free of all organic contaminants, including finger prints. After heat treatment, any alpha case must be removed from the surface either by machining or chemical milling.

Was this article helpful?

0 0

Post a comment