Introduction of quenching and aging process in heat treatment of titanium alloy forgings

1. Quenching

Quenching is a preparatory process before the aging process, the purpose of which is to obtain some kind of unstable tissue through quenching, this unstable tissue in the subsequent aging process of decomposition or precipitation, forming precipitation hardening, to improve the strength of the alloy.

The quenching of titanium alloy forgings should be divided into two types: no phase change quenching and phase change quenching.

In essence, the process of phase-change quenching is to keep the metal at a high temperature and to form a saturated solid solution. The phase-change quenching of titanium alloys can be performed either by β-zone (β-alloy) or by (β) zone.

The phase transformation quenching or martensite quenching of titanium alloy forgings can also be carried out by β region or (β) region, and the main characteristic is that the titanium alloy can be changed and formed α′ and Α″.

The microstructure of the room temperature after quenching mainly depends on quenching heating temperature and cooling temperature. When the (β) alloy is heated and quenched in the upper part of (β), the martensite phase is obtained, and the unstable β phase is obtained from the lower part of the (β) region.

The temperature of the alloy is higher than that of critical TB in order to improve the process plasticity of the alloy, so as to obtain a single metastable β-phase structure after quenching treatment. In addition, in order to ensure that after the aging to achieve higher strength also need to use high-temperature quenching. Taking into account the high alloying levels of beta alloys, low critical points (such as the Tb=750℃ of TB1 and TB2 alloys, and (β) TC4 alloy TB up to 980~1000℃), heating in beta zones slightly above critical points does not lead to severe brittleness. In view of the above reasons, both TB1 and TB2 of β-type alloys were quenched at a higher temperature than TB.

(β) alloy hardenability difference, such as TC4 for the 25mm,tc6 40mm, it is only suitable for small size parts. β-type alloy TB1 and TB2 hardenability is higher, can reach 150~200mm, the general size of the parts in the air cooling conditions can also be obtained single-phase β-tissue.

2. Limitation

For the (β) type and near β-type titanium alloy, the microstructure of the β is in equilibrium condition. The difference between the different alloys is only the proportion of α and β, which varies with the aging heating temperature and the length of heat preservation time. For example, the content of β phase in bt3-1 alloy strengthened by heat treatment is 19%, and after a long time (over 15000h) the content of β phase is 8%.

The metastable phase formed by forging quenching, whether martensite Α′,α″ or ω phase and metastable beta phase, is decomposed or precipitated in the aging process, and the final product is (β) phase, except the change mechanism and degree are different.

In order to make the titanium alloy (β) have satisfactory comprehensive properties (both in strength and plasticity) after quenching and aging, the alloy preferably has an isometric or basket-like structure before hardening. 1~6-level original microstructure can ensure satisfactory performance, 7~9-grade original needle-like tissue, there is a primary beta phase boundary, hardening heat treatment will reduce plasticity. Therefore, the quenching heating temperature of (β) alloy should not exceed the homogeneous transformation point TB. The heating time before quenching is different from the section thickness of forgings, generally for 10~60min.