RU2049622C1 - Method of fusion welding of refractory metals with ferrum-base alloys - Google Patents

Abstract

FIELD: metal welding. SUBSTANCE: method comprises steps of fusion welding of refractory metals with ferrum base alloys with use of an intermediate insert, containing copper and niobium with ratio of a copper layer thickness to a thickness of a niobium layer, equal to (0.8-1.4); before welding process soldering a niobium gasket by a copper solder to the refractory metal, the copper solder being a component of a barrier layer. In the result the barrier layer, including niobium and copper, is being formed on a surface of the refractory metal. EFFECT: enhanced quality of welded seams of different-type metals one with another. 1 cl, 4 dwg, 2 tbl

Description

 The invention relates to fusion welding of dissimilar metals, for example, molybdenum with steel, molybdenum with an iron-nickel alloy, molybdenum with treacherous, and can be used in the production of heat exchange elements.

A known method of fusion welding of refractory metals with steel through an intermediate gasket of copper, in which a 0.1 mm thick copper foil is introduced between the metals being welded and only the edge of the steel is melted [1]
The disadvantages of this method include the poor quality of the welded joint due to the formation of brittle intermetallic compounds of refractory metal with steel components, since in this combination copper cannot be a barrier due to its intense evaporation from the joint zone and penetration into steel.

The closest in technical essence and the achieved results to the invention is a fusion welding method using an intermediate barrier gasket of an alloy containing 35% nickel and 65% copper, in which only the steel edge and the intermediate gasket are melted [2]
The disadvantages of the prototype are the low quality of the welded joint, the limited choice of the range of welding modes, a significant change in the composition of the weld, due to the insufficient stability of the barrier gasket due to the evaporation of its components, intensive dissolution of the refractory metal and gasket in the weld metal due to their direct contact at a high heating temperature (1440 -1500 ^о С) with molten steel, which leads to the formation of a brittle layer of intermetallic compounds in the weld and a decrease in mechanical properties (strength and ductility).

 The aim of the invention is to increase the strength and ductility of the welded joint.

 This is achieved by the fact that in the method of fusion welding of a refractory metal with iron-based alloys, the process is carried out using an intermediate barrier insert consisting of niobium and copper of certain thicknesses (niobium 0.07-0.1 mm), and the thickness of the layer from copper to layer from niobium refers as 0.8-1.4. Before welding, the niobium gasket is pre-soldered to the refractory metal with copper solder, which is a component of the barrier layer. As a result of this, a barrier layer consisting of niobium and copper is formed on the surface of the refractory metal.

Selection of niobium as the main barrier sublayer due to the high temperature of its melting point (2400 ° ^C), which prevents the contact of molten steel with copper, and the choice of niobium 0.07-0.1 mm thickness is defined by conditions of its interaction with the steel in order to avoid the formation of intermetallic phases niobium-iron and for a smooth transition of the coefficient of linear expansion from an alloy based on iron to a refractory metal. When the thickness of the niobium gasket is less than 0.07 mm during the welding process, niobium rapidly dissolves in liquid steel, copper evaporates and the steel interacts with a refractory metal. The quality of the weld is reduced. Fracture during mechanical testing occurs at the joint. An increase in the thickness of the niobium pad over 0.01 mm can lead to an excess of the solubility limit of niobium in molten steel during welding and the formation of intermetallic layers that degrade the properties of the joint.

 The thickness of the copper layer is in the range of 0.06-0.14 mm. The presence of copper in the barrier layer prevents the interaction of the refractory metal with niobium, since copper is a persistent barrier to the refractory metal due to its insolubility in copper. If the thickness of the copper in the barrier layer is less than 0.06 mm, then the barrier layer overheats during welding, which leads to the dissolution of the refractory metal in the weld with the formation of intermetallic compounds of the type refractory metal, steel components and carbide phases of the type iron-chromium-refractory metal-carbon . The connection quality in this case is poor. Fracture during mechanical testing occurs at the joint. An increase in copper over a thickness of more than 0.14 mm leads to a sharp decrease in the temperature of the solder joint.

 Figure 1 shows the assembly diagram of parts to obtain a soldered connection of molybdenum with a niobium gasket through copper; on figa, b diagram of the welding of molybdenum with steel through the barrier layer; on figa, b diagram of the influence of the thickness of the barrier layer of copper-niobium on the mechanical properties of the welded joint; figure 4 illustrates the influence of the thickness of the copper in the barrier layer on the temperature of the decomposition of the compound molybdenum-steel.

 The method is as follows.

The design (sheet, tubular) of refractory metal and a barrier insert (niobium-copper) is soldered in a vacuum furnace by melting a copper foil. At this heating temperature is 1100-1150 ^{° C,} residence time 300 sec. After that, parts from a refractory metal with a soldered barrier insert and an iron-based alloy are assembled and assembled to perform welding by a defocused electron beam in vacuum with the melting of a more fusible metal. The values of welding parameters are selected so that the heating temperature of the compound was equal to or above the melting temperature of the alloy based on iron at 70-100 ^C, the residence time of the liquid welding bath to 10 seconds. The electron beam is arranged with a displacement of 1/3 of the beam diameter, a refractory metal with a soldered barrier sublayer and 2/3 of the beam are an alloy based on iron. Welding modes, in turn, depend on the structural features of the product, the materials being joined, their thermophysical and mechanical properties, the operating conditions of the product, and are found experimentally for each case.

 PRI me R 1. A prefabricated structure was made consisting of a molybdenum plate (MChVP grade) 100 mm long, 40 mm wide, 1.0 mm thick and a plate made of austenitic chromium-nickel steel (12X18H10T grade) 100 mm long, 60 mm wide , 1.0 mm thick. As a barrier layer, a gasket made of oxygen-free technically pure copper (MOB grade) 100 mm long, 4 mm wide and 0.03-0.18 mm thick and a gasket made of technically pure niobium (LF brand) 100 mm long, 4 mm wide, 0.05-0.12 mm thick. The barrier layer of copper and niobium was applied to the molybdenum plate in a vacuum furnace according to the scheme depicted in figure 1, in an assembly-welding device. Layers of copper 2 and niobium 3 were applied to the molybdenum plate 1 and screwed through ceramics with a force of the order of 10-20 KPa, after which they were brazed.

Soldering modes: the heating temperature of ¹¹⁰⁰ C, the holding time of 300 s, the residual pressure in the chamber 3 × 10 ^-4 mmHg

 A flange 5 mm high was made along the length of the steel plate and assembled according to the scheme shown in FIG. 2. The assembled structure was welded with an electron beam in vacuum with the beam positioned relative to the joint axis as 2/3 on the steel side and 1/3 on the molybdenum side with the barrier sublayer. Welding was carried out from a U-250 high-voltage source on an ELUM-1 installation with an electron beam gun CEP-4.

 Welding modes: accelerating voltage 17 kV, beam current 18-55 mA, welding speed 4-12 m / h, focusing current 137 mA, residual pressure in the chamber 10-5 mm Hg

The resulting welded structure was visually checked (magnification 10 times) for the absence of cracks and pores and for tightness (helium leak detector). Mechanical tests at room temperature of 20 ^{° C} to determine the bond strength were carried out at deformation rate of 2 mm / min, a bending test bending angle at a rate of 4 mm / min. The test results are presented in table. 1 and 3. Tests to determine the temperature of the decomposing molybdenum-steel compounds depending on the thickness of the copper in the barrier layer were carried out according to GOST 20487-75 and are presented in figure 4.

 Of the samples welded in various modes, and samples welded according to the prototype [2], thin sections were made, metallographic studies and X-ray microanalysis of welds were carried out. Metallographic studies were carried out on a NEOFOT microscope and on a PMT-3 microhardness measuring instrument (GOST 9450-76). X-ray microanalysis was performed on a CAMEBAX MICROBEAM microanalyzer. For quantitative analysis, diffraction spectrometers with PET, LIF, and TAP monochromators were used. Quantitative analysis was performed in increments of 0.001 mm using high purity metal standards. The results of the studies are shown in table. 2.

 As can be seen from the table. 1 and 2, the claimed ratio of thicknesses is optimal, since a smaller value of the thickness of copper and niobium leads to dissolution of the barrier layer and a decrease in the properties of the welded joint, if the thicknesses of copper and niobium are greater than stated, the deterioration of the properties of the welded joint occurs due to a decrease in the strength of the solder joint and reduction of ductility in the fusion zone.

Claims (1)

 METHOD FOR MELTING OF REFRIGERANT METALS WITH IRON-BASED ALLOYS using an intermediate insert, including assembling and melting the flanges from an iron-based alloy, characterized in that a niobium layer is soldered to the refractory metal through a copper layer, and the thickness of the copper layer is 0.8 1.4 layers of niobium.

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