Refractory and chemically active metals,
consider only the refractory and reactive metals, which can be used as structural materials such as zirconium, hafnium, niobium, tantalum, molybdenum. Materials such as vanadium, tungsten, chromium, used as construction is much less and only in the combined welded joints.
Welding materials considered difficult heat of fusion, high affinities for gases: oxygen, nitrogen and hydrogen, which leads to the formation of pores in the weld and reduce its plastic properties, susceptibility to grain growth during heating. In terms of the behavior of individual metals in welding should be noted.
Zirconium has two allotropic modifications: alfa-phase with the hexagonal close-packed lattice, existing prior to the transformation temperature 865 degrees. C, and beta-phase with a cubic, body-centered lattice, the existing above 865 deg. C. Due to the fact that the metal has a polymorphism in the heat affected zone and weld hardening occurs with the formation of beta-phase. The formation of mixed-phase structure can lead to the formation of makrogalvanopar and cause localized corrosion of the alloy. To eliminate this danger weldment is subjected to vacuum annealing at a temperature of 575 deg. to align the structure. When the temperature rises above 800 degrees. With zirconium reacts vigorously with nitrogen to form nitrides (ZrN), and at temperatures of 300-1000 degrees. Since it absorbs hydrogen to form hydrides (ZrH2). are used zirconium alloys with tin, iron, nickel and chromium have strength of 44-54 kgf/mm2 and high corrosion resistance.
Hafnium - polymorphic transformation temperature of the metal to 1760 degrees. C. Before this temperature is hexagonal close-packed hafnium lattice alfa-phase at higher temperatures - a body-centered lattice of beta-phase. When heated, hafnium reacts with the atmosphere of air, forming NfO2 dioxide, nitrides.
Niobium and tantalum - metals that are close in their physical properties, polymorphic transformations do not have. They have a very high resistance under the action of the most corrosive substances (liquid metal coolants,) and are used in some reactor designs. Tantalum is also used in medicine, surgery, made from a rod, brackets and similar items may be in for a long time in the tissues of the human body without causing it significant inflammation. The technique also used in alloys of niobium with a small amount of alloying elements (molybdenum, tungsten, zirconium, vanadium, titanium) and alloys, tantalum, tungsten, with the addition of vanadium, niobium (up to 10%). When heated, these metals are strongly absorbing gases of the atmosphere: at temperatures above 300 degrees. C - oxygen, above 350 degrees. C - hydrogen, more than 400 degrees. N - nitrogen. As a result, the metal oxides are formed, nitrides, hydrides, the metal is strengthened, while the ductility decreases sharply. When welding these metals in the weld metal and heat affected zone are also possible grain growth and embrittlement of the metal, which can be enhanced by the formation of grain boundary carbides (Nb2C, Ta2S) if there are impurities in the metal carbon.
Molybdenum, which has a high melting point, high values mechanical properties and modulus of elasticity, is used as a sheet of small thickness for the individual elements of the combustion chambers, turbochargers, etc. In some environments, it has a high corrosion resistance. Metal has no polymorphic transformations. welding difficulties associated with its increased tendency to the formation of crystallization cracks due to the formation of various low-melting eutectics (MoO3 + M0O2 - Mo; mp = 780 deg. C), as well as ohrupchpvaniem weld metal and heat affected zone due to the possible ingress of atmospheric gases or other pollutants.
Molybdenum is sensitive to pollution of various kinds, depending on the content of oxygen, nitrogen and carbon changes the critical temperature of the transition metal in a fragile state. The most dramatically affected by oxygen, 0.0002% of O 2 Ta increases up to 200 degrees. C. Effects of thermal cycle of welding leads to grain growth in the weld zone, with a thickening of intergranular layers, the enrichment of impurities and a sharp embrittlement of the base metal in the area. In the presence of carbon in the metal, the formation of pores (due to the reaction of MoO3 + Mo + ES = SBV), which are located along the axis of the weld and fusion line. Therefore, along with a thorough cleaning of the base metal and welding materials are presented particularly stringent requirements for cleanliness of the base metal ( oxygen <0.0002%, N <0.0001%, carbon <0.003%) in the weld pool is administered active deoxidizers 0 5-1% Ti; to 0.1% Ce, up to 0,25% Zr, welding is carried out at the minimum heat input . on weldability of the materials in question can be divided into two groups. The first group metals (zirconium, hafnium, and tantalum dioby), with the welding process conditions have good weldability. Welding Group II metals (molybdenum, tungsten) is in serious difficulties because of their high sensitivity to impurities, embrittle the metal. Heated to a temperature of 200-315 molybdenum deg. C and removal of residual stresses after welding (when heated to 980 deg. C) reduces the probability of formation of cold cracks. main ways of obtaining welds with satisfactory properties: reduction of harmful contaminants in the ground and filler metal, reducing the time (temperature) and the residual stresses in welded joints, preventing contamination of the weld metal and heat affected zone during welding, especially gases of the atmosphere. Therefore, the metals welding is performed mainly by the electron beam in vacuum or in chambers with controlled atmosphere. In the latter case, the use of argon and helium are high frequency, which further drained from the gas by passing it through silica gel, alumina gels, and heated to 900 - 1000 deg. With titanium shavings. Performed TIG welding with direct current straight polarity. To reduce contamination of the weld metal welding is usually performed without filler metal. In some cases, the welding performed on the air, but use the burner with extra nozzles for supplying a protective gas, and blowing on the back side. Strength and ductility of welded compounds are at 80-95% of the base metal properties. When the electron beam welding in vacuum chambers with a 10 ~ 4 pm. of Art. contains hundreds of times less impurities than in the most pure argon premium. In this method of welding it is possible to clean the base metal from the edges of the gases heated defocused electron beam. In the most successful degassing of hydrogen is removed, to a lesser extent oxygen and nitrogen, and only the surface layers. most common method is to weld the materials in question arc and gas shielded electron beam. Arc welding of zirconium alloys most successfully sealed in chambers with a controlled environment, filled with helium, after preliminary evacuation of air to 0.03 mm Hg. of Art. Material of filler wire corresponds to the composition of the alloy: Zircaloy-2 and Zircaloy-3 (Table 109). There is information about the modes of welding of niobium and tantalum thickness of small tungsten electrode direct current straight polarity with the use of blast protection with an additional supply of gas through the nozzle and the back side seam. Welding molybdenum using ink-jet protection can successfully run using helium of high purity tungsten and consumable electrode. Molybdenum thickness of 3 mm welded tungsten electrode diameter of 3 mm at a constant current of direct polarity mode: I = 425 A; U = 18 V ; v = 18 m / h The diameter of the burner nozzle 15 mm helium flow through the burner, and a prefix of 20 l / min, with the back of the 5 l / min. Welding of molybdenum greater thickness can be carried out consumable electrode diameter of 1-1.2 mm dostoyannom current of reversed polarity on the mode: I = 400 - 500 A; U = 32 V; v St = 30 - 40 m / h; v = pp 600 - 900 m / h, the flow of helium through the burner and the prefix 140 l / min, with the back of 20 l / min. The electrode wire is pre-activated cesium chloride-coated it. known welding technique hafnium in a chamber filled with helium or argon, tungsten electrode diameter of 3.2 mm on the regime: I = 125-135 A; U = 14-18 B; v = 10 m / h, a constant current, the polarity of the line. When the electron-beam welding connection is made by remelting the base metal. Along with welding butt welding is possible with overlapping seams and slash type of cork. For refractory and reactive metals is of great importance to the possibility of pre-treatment in vacuum degassing. In principle, electron beam welding in two passes allows the weld metal thickness up to 100 mm.
See also:
Welding of Aluminum
Titanium
Welding Magnesium
Welding of Titanium
Copper
Welding of Copper
Refractory Metals
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