Production of welded structures of heat-hardenable titanium alloys presents numerous technical difficulties. When hardening of large parts or non-rigid structures are considerable leashes, which are very difficult and in some cases can not be eliminated; lengthy welded structures is generally difficult to transfer to the quenching medium, in the process of heating for quenching and transfer to a quenching medium thin sheet structure is oxidized, Even if it be carried out in the heating furnace with protective atmosphere, formed by heating the dross must be removed, which presents some difficulties, and sometimes impossible (eg, etching of structures with lap joints in spot or seam welding), descaling chemical method accompanied by the hydrogenation of thermally hardened alloys, which contain a glycol * ^-tional to the stabilizing elements, the assembly structure by welding of hardened elements, followed by the aging of the whole structure, as a rule, does not provide optimal mechanical and service properties of welded joints because the thermal cycle of welding creates a rather vague state of the seam and the transition zone, which depends on a number of difficult factors considered.
Circumstances above are the main factors hindering the development of thermally hardened alloy sheet in welded structures.
In connection with the above, we were sought special treatment hardening heat treatment of titanium alloys,
allowing to process large welded design in a protective atmosphere. For this purpose are the most promising high-alloy R-stabilizing elements of titanium alloys with a p-phase with increased stability.
Thus, the alloy VT16 with a Ka = 0.8 was found as a hardening effect of the base metal and weld at a certain rate of cooling from the annealing temperature.
With increasing cooling rate up to 4 - 8o C / min and above is observed a continuous increase in tensile strength and lower ductility characteristics of the alloy VT16. When the cooling rates within the 12-17O C / min can increase the strength of alloy VT16 with 85 kgf/mm2 annealed to 105 and above kgf/mm2 after hardening heat treatment of this type. The proposed method of hardening heat treatment was effective in the manufacture of honeycomb structures by welding, brazing and diffusion joining. It was enough to perform the operation of soldering or diffusion annealing, carried out usually at temperatures around 900 ° C and cooled honeycomb in an oven or container at a rate of ~ 15 ° C / min, as its strength (as the base metal and weld) increased to 105 kgf/mm2 and above. With this technological process is easy to protect from oxidation ponds with inert gases or vacuum.
Ratio of strength and ductility in this type of hardening heat treatment is about the same as during quenching and aging.
Hardening mechanism for this kind of heat treatment consists in the fact that in titanium alloys with a + p-structure is determined lennogo composition (with a certain amount of p-phase) at some cooling rates of decay occurs metastabnlnyh phases with the formation of dispersed particles of a-and p-sostavlyayushey, which leads to hardening of the alloy. In this case, the cooling rate of the alloy is such that there is no fixed metastabnlnyh phases, and at the same time is so small that it does not allow to pass to the equilibrium conversion of a + p-states.
Even more interesting way of hardening heat treatment can be applied for processing of alloys supercritical composition. It was found that titanium alloys of supercritical with further increase in the content of p-stabilized-reducing elements of the metastable P-phase can be fixed at very low cooling rates commensurate with the rate of cooling of large industrial furnaces, together with the SADC (4 - 10 ° C / min .) Further isothermal heating of a "hardened" alloy at temperatures of aging leads to the decay of the metastable P-phase and the formation of a dispersed-and P-components, ie, leads to a significant hardening. Obviously, the maximum capacity for hardening heat treatment of this type have titanium alloys with K = 1.6-2.2. At the least, and other titanium alloys can be heat-hardened to perceive this type. Na.etoy basis have developed a way of hardening heat treatment of large welded structures from titanium alloys of supercritical free transfer in the quenching medium. The proposed method is deprived of all the shortcomings of the hardening heat treatment associated with the transfer of cages in a quenching medium. It allows the hardening heat treatment in vacuum furnaces and industrial furnaces with protective atmosphere. So, for example, welded parts or structures of titanium alloy VT32 (Ti-2, 5% Al-8, 5% Mo-8, 5% V-1, 2% Fe-1,2% Cr), treated in a vacuum furnace under the regime: heating at 750 ° C for 1 h, cooling in the furnace at a rate of> = 4 ° C / min to 500 ° C, holding at 500 ° C for 4 h, provided for in the *> = 120 kgf / mm2, bb> = 7%, while in the annealed condition the alloy has s = 82 kgf/mm2 and 65 = 16%. Widespread use of the above-described method of heat treatment found in the manufacture of welded components and structures made of alloy VT22. welded construction of this alloy require stabilizing annealing at 850 ° C, ie, at temperatures limits a + p = p-transformation. After annealing, the alloy has a tensile strength of about 100 kgf/mm2. Hardening heat treatment of the regime: heating at 850 ° C for 1 h, cooling in the oven to 750 ° C, holding 2 h, cooling with furnace to 500 ° C, exposure at 600 ° C for 4 h yields on welded constructions from alloy VT22
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
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