Hardening heat treatment of welded joints of titanium alloys, consisting of quenching and aging (vacation), serves as an additional reserve for increasing the strength of welded structures. However, it should be noted that the use of hardening heat treatment on the welded joint is more limited compared with the base metal. This is due to the fact that coarse acicular structure is poorly perceived hardening heat treatment, ie does not provide sufficiently good combination of strength and ductility after quenching and aging. Therefore, for the use of welded joints "soft" hardening heat treatment, which increases the strength by 10-20% compared with the strength of the annealed condition. In this case, moderate strength is possible to obtain satisfactory characteristics of plasticity of the welded joint. In recent years several new methods of hardening heat treatment of welded joints of titanium alloys, which allow multiple applications to empower the hardening heat treatment of welded structures, which we will also be considered in this section. All the more widespread hardening heat treatment of welded structures, when the base metal hardened by quenching and aging to the required level and weld is thickened and contained in the annealed condition by the local heat treatment. This method allows to obtain equal strength as the design of the base metal and weld on a high capacity for work.
Here are some questions hardening heat treatment of welded joints of titanium alloys.
Hardening heat treatment consisting of quenching and aging, is applicable to welded joints with the two-phase + p titanium alloys, since the martensitic alloys and ending with the pseudo-Riemannian alloys.
The principle of hardening heat treatment of welded joints, as well as the base metal is the fact that the accelerated cooling of metastable retained Me-p, a '(' L-phase and the subsequent artificial aging is an allocation of dispersed particles of a-and p- phases. The effect of the hardening heat treatment depends on the type, quantity and composition of the metastable phases, as well as the dispersion of the particles formed after the aging of a-and p-phases.
The peculiarity of hardening heat treatment of welded joints of titanium alloy is used in some cases, the thermal cycle of welding as a hardening heat treatment for hardening. Welded joints with single-pass weld metal of small thickness can be regarded as a tempered temperature p-type region.
Metastable components of p-and a'-phase in titanium alloys are prone to degradation during isothermal heating at low temperatures with the formation of equilibrium a + p-structure. On initial stage of aging and the formation of dispersed allocation-and p-phase is accompanied by a significant hardening of the alloys.
decay of a metastable g-phase is on the way:
rnestab Trieste-a-b + k + p.
For isothermal heating, a '(a') phase falls under the scheme a '(a') - + a '(a') {0botgtts-a-+ a-{'Rnestab * a + p.
collapse a '(a') phase is accompanied by the first stage of the formation of ct phase and a '(a')-enriched phase of p-stabilizing elements.
These schemes transformation of metastable phases during isothermal heating are valid for the processes taking place at temperatures above 450-500 ° C. At lower temperatures, the isothermal heated wa decay processes can be take place with the formation of an intermediate phase. In practice, the hardening heat treatment of welded joints as well as the base metal heat treatment regimes preclude the formation of ©-phase.
Depending on the mode of hardening heat treatment - quenching temperature, aging temperature and time (vacation) - Mechanical properties of welded joints will vary widely. With the increase of quenching temperature in the welded joint is preserved more and more (by volume) of the metastable phases. In alloys of subcritical composition is first raised the number of metastable g-phase, and then a '(a') phase. In alloys of supercritical composition is a continuous increase in the number of metastable g-phase with increasing quenching temperature to the point of complete polymorphic transformation in the alloy. The volume of metastable phases in the alloy determines the effect of hardening, which can be obtained during the subsequent aging of the quenched alloy. As a rule, with increasing temperature quenching of thermally hardened alloy with a + p-structure increases its strength and reduced ductility. With continued aging time with increasing aging temperature increases the volume and rate of decay of metastable phases and highlights the dispersed particles. This is accompanied by an increase of strength and, as a rule, reduction in ductility. At a certain temperature aging resistance reaches a maximum at higher temperatures the strength is gradually reduced to the level of the strength of the annealed metal. This portion of the curve is characterized by a coagulation of dispersed-and p-particles to an equilibrium state with increasing aging temperature. duration of aging factor expressed in the fact that the maximum strength on the curve is shifted to higher temperatures, and the maximum value decreases as the duration of aging. Such a character of strength change depending on the mode of aging due to the fact that with increasing duration of aging can provide more complete decomposition of metastable phases, while maintaining a high dispersion of the reinforcing particles. Practical use of modes of aging, as a rule, the strength of the downward branch of the curve, i.e, the modes. This allows us to provide the best combination of strength and ductility in thermally hardened metal at a satisfactory stability of mechanical properties. This is even more true of the welded joints of titanium alloys with a + p-structure, which is used for hardening plan with even more profound structures. In these modes, the hardening heat treatment at a loss of strength can get some gain in ductility, which is very necessary for welded joints with a cast structure, which is worse than the structure of deformed metal.
See also:
Heat Treatment of Welded Joints of Titanium Alloys
Annealing of Titanium Welded Joints
Hardening Heat Treatment of Titanium Welded Joints
Special Modes of Hardening Heat Treatment of Welded Structures
Effect of Welding on the Structure and Properties of Different Zones of the Welded Joint
Structure and Properties of Heat Affected Zone
Structure and Properties of Welded Joints
Properties and Structure of Welded Joints of Industrial Titanium Alloys
Welding of Titanium
No comments:
Post a Comment