Consider the effect of different types of welding on the chemical composition, properties and structure of welded joints of different types of titanium alloys. Each specific type of welding alters the structure and the degree of alloying of the weld metal due to different base metal dilution of weld and the additional introduction of the metal electrode, whose composition may differ from that of base metal, as well as due to changes in gas content of the weld metal. Thus, the impact of type of welding on the properties of the weld metal of titanium alloys is determined by several factors: the chemical composition of the metal electrode, which provides the optimum combination of strength and ductility, the ratio percentage of the ground and the electrode metal in various forms of cutting edges, typical for this type of welding structure and phase composition, depending on the thermal cycle of welding and subsequent heat treatment, the possibility of formation of various defects inherent in this type of welding, and influence them (especially the poor penetration and porosity) on the physico-mechanical and operational characteristics, the additional gas saturation of the molten metal gas (nitrogen, oxygen and hydrogen) and the influence of these gases on the various properties and especially the tendency to brittle fracture and the detainee. The influence of these factors increases with increasing strength of the alloys. The choice of a particular type of welding is mainly dependent on the chemical composition of the alloy and the weld thickness.
Welds of a-and pseudo-alloys. In the single-phase and pseudo-alloys, and alloys, for almost the entire range of thicknesses can be welded using different methods of welding in which the formation of the molten weld metal occurs as a direct penetration of the base metal, as well as an additional input electrode filler metal. At the same basic meaning as in welding and pseudo-alloys are the interaction of the molten weld metal with protective environments and the ability to provide the minimum gas saturation of the weld metal. Mechanical properties of cast metal, regardless of thickness similar to those of base metal, and reduction of ductility due to the peculiarities of formation of the cast structure. For these alloys is characterized by a significant amount of die-cast grain, which depends on the value of this type of heat input welding. For all the a-and pseudo-alloys can be used electrode metal of technical titanium VT1-00, BT2 alloy (Ti-ZA1) or an alloy of similar composition to the base metal. In all cases, regardless of changes in the weld cooling rate and fixed-or a'-phase. The nature of the formation of a-or a'-phase in the joints mainly depends on the cooling rate in the range of p wo-MH or a pre-rotation.
At low cooling rates produced a wide and long plate, high cooling rates, and formed melkoigolchataya or a 'phase. By varying the cooling rate can be controlled within certain limits, structural transformations in the weld metal. Thus, the characteristic of the electroslag welding heat input increases, slowing the rate of cooling in the martensite range, leads to the transformation of thin needles of a-phase alloy VT1-0 in the large grains with serrated boundaries. Dispersity of the structural components significantly affect the plastic properties of the weld metal. For these alloys, the change of plasticity, depending on the cooling rate is on a curve with a peak and is associated with a grain size of cast structure and dispersion of a-or a'-phase. Slow cooling of a decrease in ductility of cast metal as a result of increasing the size of the grains. The high cooling rate leads to a decrease in ductility due to the formation or a 'phase.
For the alloys of this group is characterized by the fact that the mechanical properties and structure of the weld metal during welding is only marginally affected the entire range of thicknesses and different methods of welding. Annealing after welding stabilizes the structure but does not affect the mechanical properties of the weld, and therefore the heat treatment of welded joints of these types of alloys, including titanium and maintenance is carried out only to reduce the magnitude of internal stresses from the welding process, as well as to reduce the peak concentration of hydrogen in different zones of the welded joint. This is confirmed by the structures of welds that are shown in Fig. 46. Influence of different technological factors on the mechanical properties of weld metal, excluding the effect of alloying elements is convenient to consider the example of alloy BT1-0.
For all types of welding alloy BT1-0 strength of the weld metal is determined by the initial strength of the base-metal and filler wire. If welding is a significant increase in the grain, as well as additional gas saturation of the weld metal, its strength may exceed the original while reducing ductility. The research results presented in this study showed that the welding of alloy VT1-0 non-consumable electrode without the additive increase in the hydrogen content in the main influence on the toughness and the bending angle of the weld metal without altering its hardness. The increase in oxygen and nitrogen strongly influences the strength and ductility when tested in the bending angle. Especially dangerous gas saturation of the surface layers of the seam. Increasing the oxygen content from 0.15 to 0.38% (while 0,02% N) reduces the bending angle of the weld metal in the alloy VT1-0 1.5 mm from 180 to 100 ° C. With increasing thickness of the weld metal and the increase heat input welding depth of the layer with high hardness (contaminated with oxygen and nitrogen) increases, while the hydrogen content in weld metal during welding without the introduction of the filler wire is lower than the base metal due to its desorption and diffusion from the melt in the weld zone . Reduced ductility of the weld metal, associated with oxygen and nitrogen, increases its sensitivity to the influence of the level of hydrogen and a tendency to brittle fracture and the detainee. The data confirm the influence of oxygen and nitrogen on the tendency of the weld metal of alloy VT1-1 to the formation of cracks with increasing hydrogen content.
Thus, the type and technology of welding depends directly on the gas content in the weld metal and, consequently, its mechanical and performance properties. Research has shown that the condition of the protective gas atmosphere in a strong influence on the intensity of the absorption and gas content in the weld metal, depending on the type and mode of welding is the change in the effective thermal capacity of the arc, which leads to a change in the amount of hydrogen supplied to the gas phase, the protective atmosphere.
the source of the saturation of the weld metal hydrogen may be adsorbed, and the moisture, which is at the electrode edges and welded wire. The solubility of hydrogen in the weld pool depends largely on the content of alloying elements in the weld metal. Experimental data on the magnitude of the hydrogen content in weld metal obtained in the studies show that when all the major types of fusion welding in shielding gases argon, the use of the first composition and the electrode wire, held vacuum annealing isoderzhaschey up to 0.0006% N, allows to obtain the weld metal with a lower hydrogen content than the base metal due to its obezvodorazhivaniya and transition of hydrogen in the gas phase. When the automatic submerged arc welding (without additional protection with argon), as well as for ESW, there is additional gas saturation of the weld metal oxygen and hydrogen.
At the same time the implementation of automatic welding in vacuum drastically reduces the gas saturation of the weld metal due to its additional degassing, thus there is decrease in strength and increase ductility weld. Similar processes occur during the degassing electron-beam welding, and the value of reducing the concentration of gases in the weld metal depends primarily on the depth of depression, as well as the heat input welding process and the cooling rate of weld metal. Increasing the concentration of oxygen or nitrogen in the weld metal depends on the amount and the age of the weld pool, which depends on the type and welding conditions and the partial pressures of these gases in a protective atmosphere.
Regardless of the type of alloys for welding of structures is recommended to conduct the process in the cells with a protective atmosphere or to provide a thorough , steady defense throughout the weld zone. For example, in electroslag welding of thick metal of titanium alloys is impossible to completely prevent the molten metal from the effects of atmospheric gases using slag alone, so the additional gas is created by filing a protective atmosphere of argon above the slag bath [ON]. Violation of these conditions leads to significant gas saturation of the weld metal, particularly its surface layers. When welding in a controlled atmosphere, increasing the total gas content in the weld metal slightly, and the hydrogen content in weld metal due to its desorption and diffusion of heat-affected zone, even in decline. Results of the study of the microhardness and gas saturation of welds show that only electron-beam welding in vacuum does not increase the hardness of the seam. All other welding methods lead to an increase in hardness by increasing the amount of gas in the surface layers of the seam.
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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