RU2311274C1 - Bimetallic material producing method - Google Patents

Abstract

FIELD: manufacture of bimetallic blanks of different kind metals, mainly large-size steel-titanium blanks.

SUBSTANCE: method comprises steps of placing thrown plate with gap over fixed plate; initiating arranged over it charge of explosion matter. Surfaces of one or both plates are treated before joining them by means of cathode spots of vacuum arc excited between surfaces of plate used as cathode and anode. Prepared material is subjected to heat treatment at temperature 500 - 600°C.

EFFECT: stable level of strength along the whole area of joined surfaces due to preparing highly uniform surfaces before joining and creation of optimal combination of residual stresses after cohesion.

2 cl, 5 dwg

Description

The proposal relates to metallurgy and can be used in the manufacture of bimetallic billets from dissimilar metals.

Traditionally used metallic and non-metallic materials have largely reached the limit of structural strength. However, the development of modern technology requires the creation of materials that work reliably in a complex combination of force and temperature fields when exposed to aggressive media, radiation, deep vacuum and high pressures. Often the requirements for materials can be controversial. These requirements can be met through the use of composite materials.

Composite material is called a bulk heterogeneous system, consisting of mutually insoluble components that differ greatly in properties, the structure of which allows one to take advantage of each of them.

The most effective way to obtain blanks and parts with such properties is to create them in the form of two- or multilayer compositions. In metallurgy and in mechanical engineering, both in domestic and in foreign practice, a number of methods for producing bimetallic materials and billets are used. The most important of them are electric arc and electroslag surfacing, casting and batch rolling, explosion welding.

However, they all have significant drawbacks.

The disadvantages of surfacing include high complexity; insufficiently smooth surfacing surface, requiring cleaning and grinding; the need to apply several layers to suppress the effect of dilution of the metal overlaying the base metal; the impossibility of surfacing in cases where brittle intermetallic layers occur between the surfacing metal and the base.

These disadvantages are deprived of methods for producing bimetallic materials by batch rolling.

A known method of obtaining a workpiece from a sheet of two-layer steel clad with chromium corrosion-resistant steels [1].

To do this, the surface of the pearlite steel billet is thoroughly cleaned of scale and covered with a thin layer of nickel, after which a clad sheet of high alloy steel is laid on it. The nickel layer prevents the intense diffusion of carbon from the base layer into a clad layer with a high chromium content. When heated, a thin film of oxides forms on the surface of high-chromium steel, however, it crumbles during rolling, exposing juvenile surfaces, between which a strong welded joint is formed as a result of significant plastic deformation at high temperature.

The disadvantages of batch rolling include the high complexity of preparatory operations.

An important feature of explosion cladding is the possibility of joining together metals whose welding by other methods is difficult or practically impossible. This applies in particular to metals and alloys forming very hard and brittle intermetallic compounds, for example, steel with aluminum or titanium. Explosion welding allows you to get bimetallic materials of almost any composition.

The term "explosion welding" refers to the phenomenon of the connection of metal plates that occurred during their collision as a result of throwing detonation products of explosives.

However, to obtain a high-quality connection in explosion welding, preliminary cleaning of the surfaces to be connected is necessary.

A known method of producing bimetallic material by explosion welding, including mechanical cleaning of the surfaces to be welded to a metallic luster [2].

The disadvantage of this method is that any mechanical processing of metal is accompanied by significant mechanical hardening of the surface layers. This leads, during their plastic flow in the physical contact zone under conditions of unequal all-round compression, to excessive heat generation with the formation of fused sections or intermetallic inclusions that adversely affect the strength of the joint. This is explained by the fact that significant plastic deformations in the wave-forming zone accompanying the explosion welding process in this case do not occur due to the accumulation and interaction of defects in the crystal structure, but due to the annihilation of the defects accumulated during machining with the release of their energy in the form heat.

It should be noted that there are no accessible and reliable methods for controlling the distribution of such hidden surface defects, which explains the significant heterogeneity in the distribution of the bond strength of the layers along the welding area.

There is also a method of producing a multilayer tape from a package of pre-prepared strips by cleaning them, assembling into a package and co-rolling [3].

In the framework of this method, the cleaning of the original tapes is carried out by etching them in sulfuric acid, drying and processing of the connected surfaces with metal brushes. This technology is time-consuming, expensive, dangerous for personnel and the environment and does not provide high quality cleaning, guaranteeing the necessary strength of the connection of dissimilar metals.

The closest in technical essence and the achieved result to the claimed solution is a method for producing large-sized bimetallic steel-titanium sheets by explosion welding [4].

The disadvantages of this method is that when connecting by explosion welding of titanium alloys with steels of various classes, it is impossible to obtain high-quality material due to the formation of brittle melts at the joint boundary due to the instability of the welding process itself and the heterogeneity of the surface properties of the materials being joined. Fragile melts of variable composition are formed in the zone of maximum hardening with reduced ductility, in the zone of action of high residual tensile stresses. This significantly reduces the stability of the mechanical characteristics of the joint and determines the grain-boundary nature of fracture along the joint zone.

From the above analysis of the prior art it is seen that the development of a simple and economical method for producing high-quality bimetallic material with a strong connection of dissimilar surfaces is an urgent task, the difficulty of solving which increases when receiving large-sized blanks, that is, with an increase in the area of connected surfaces.

Moreover, all existing mechanical and electrochemical methods of surface preparation before joining dissimilar materials do not give a positive result. With carefully selected parameters of the explosion welding process, the number of brittle inclusions in the resulting diffusion layer can be reduced to 8%. The thickness of this layer, as a rule, is 10-40 microns.

It should be noted that with further thermal exposure, the initial strength, starting from temperatures of the order of 450 ° C, decreases due to the growth of brittle diffusion layers. As a result, for titanium alloys with steels during explosion welding due to optimally selected modes, the formation of durable welded joints is possible, however, such billets cannot be subjected to thermal action above 450 ° C without reducing the strength characteristics of the joint.

The technical result of the proposed solution is to achieve a stable level of strength over the entire area of the connected surfaces, mainly large workpieces, by increasing the uniformity of the surfaces when preparing them before joining and creating the optimal combination of residual stresses after joining.

To achieve the specified technical result in a method for producing a steel-titanium bimetallic material, mainly for large-sized billets, comprising connecting at least two pre-processed plates, in which a throwable plate is mounted over a fixed plate with a gap and an explosive charge located above it is initiated, according to the proposal, the surface treatment of one of the plates before joining is carried out by cathode spots of a vacuum arc excited between ited plate used as a cathode and anode, and then heat-treated compound obtained material at a temperature of 500-600 ° C. Also, according to the proposal, the cathode spots of the vacuum arc process the surface of another plate.

The presence of distinctive features, namely pre-treatment of the surface of one and / or another plate before connecting the cathode spots of a vacuum arc excited between the surface of the plate and the anode, as well as the subsequent heat treatment of the material obtained by explosion welding at a temperature of 500-600 ° C, indicates the conformity of the claimed technical solutions to the patentability criterion of “novelty”.

From the patent literature there is known a method of manufacturing a multilayer metal strip, including surface treatment of the source sheets in vacuum, assembly into a bag and rolling, and the surface treatment of the main and clad sheets is carried out using cathode spots of a vacuum-arc discharge excited between the tape and a graphite anode [5] .

The cathode spots of the electric arc move along the surfaces of the sheets under the influence of an external electric field at a speed of the order of hundreds of meters per second. An energy density of about 10 ¹¹ W / m ² is released in the cathode spot, and the temperature reaches 5 × 10 ³ 10 × 10 ³ K, and all oxides, adsorbed films, and foreign non-metallic inclusions (e.g., scale) dissociate, ionize, or go into a gaseous state. Moreover, positively charged metal ions under the influence of a cathodic voltage drop are accelerated and carry out the cathodic bombardment of the treated surface. Thus refined, the thin surface layer of the sheet is a layer of chemically pure (distilled) metal that does not contain carbon, sulfur, phosphorus, nitrogen (can be left behind) and other impurities. As a result, a thin film of a metal juvenile in chemical purity, free of any foreign inclusions, forms on the surface of the sheet. The surface layer thus transformed possesses 100% mechanical, physical, and chemical homogeneity, which is unattainable with other methods of surface preparation, and also has high adhesion, which ensures high mechanical properties and uniformity over the entire surface of the subsequent compound.

The consequence of the indicated chemical homogeneity of the layer is the absence on its surface of microregions with different electrochemical potentials, that is, the transformed surface is equipotential, which ensures the complete absence of the zone of brittle melts, significantly increases the temperature and the incubation period of the decomposition of zones of variable composition during temperature exposure.

Depending on the regimes of the vacuum-arc action, including the realized temperature, structural changes occur in the surface layer of the metal in the region of the cathode spot and thin interlayers with an ultrafine-grained structure are formed, as shown by the results of X-ray diffraction analysis. Diffusion processes are also possible with a change in the chemical composition of the base metal in these layers. So in steels of pearlite class sublimation of carbon with a depth of about 100-150 microns is noted.

At the same time, this process of converting metal surfaces makes it possible to deposit thin layers of other surface-modifying metals introduced into the plasma using additional devices.

The joint use of the above transformation of the surfaces of the plates or sheets with the subsequent connection of these sheets by explosion welding and additional heat treatment without limiting the temperature regime allows us to give the surfaces interacting during the connection fundamentally new properties and, as a result, increase the strength of the obtained bimetallic materials.

The resulting welded bimetallic material has maximum performance, namely, maintaining the bond strength throughout the entire service life by creating the optimal combination of residual stresses after the explosion and the mechanical properties of the bimetallic compound, i.e., the narrowing stresses of the heterogeneous components in such material tend to zero.

It is known that residual tensile stresses after an explosion reach 1000 MPa and are not uniformly distributed over the thickness [6].

For example, figure 1 shows the residual stresses in the titanium-steel joint in the state after explosion welding. To remove residual stresses after explosion welding, heat treatment is used, during which stress relaxation occurs, that is, a return to an unloaded, initial state. In this case, maximum relaxation occurs in the temperature range 350-500 ° C (Fig.2, 3). However, when heated to a temperature of 500 ° C, a decrease in the yield strength (Fig. 4) is observed due to the onset of the formation of a brittle intermetallic layer. A similar character of the dependence of the strength of the bimetallic welded joint on the heat treatment temperature is characteristic of all titanium-steel compositions.

The proposed preparation of the surfaces of joined metals before explosion welding provides the formation of a thin layer of ultrafine structure with a grain size of 100-150 nm, which provides a stable connection of surfaces and prevents premature precipitation of intermetallic compounds at temperatures up to 600 ° C. At the same time, stable mechanical characteristics of the bimetal, stable cross-sectional properties are achieved, which allow further alteration of the semi-finished product, including stamping, rolling, hot dressing, which significantly expands the scope of the bimetal.

The experimental work on explosion welding of steel sheet grade 09G2S and titanium sheet grade VT1-0 showed extremely high and stable results in the tensile stress and shear of the layers of steel and titanium, which is not achieved with conventional methods of preparing the surface for explosion welding (Fig. 5 )

From the foregoing, it follows that the technical result of the invention is achieved by a new set of essential features, both newly introduced and known, therefore, the claimed method meets the patentability criterion of "inventive step".

The method is illustrated by drawings, where:

figure 1 presents a graph of the residual stresses in the titanium-steel joint in the state after explosion welding;

figure 2 presents a graph of the residual stresses in the titanium-steel joint in the state after explosion welding and subsequent heating to a temperature of 350 ° C;

figure 3 presents graphs of the dependence of the strength of the compound (1) and the thickness of the diffusion layer (2) in the connection of a titanium alloy and steel on the heating temperature;

figure 4 presents a graph of the residual stresses in the titanium-steel joint in the state after explosion welding and subsequent heating to a temperature of 500 ° C;

figure 5 presents a graph of the dependence of the strength of the connection (lines 1, 3) and the thickness of the diffusion layer (lines 2, 4) in the connection of the titanium alloy and steel on the heating temperature, where lines 1 and 2 correspond to the standard manufacturing technology of the material, and line 3 and 4 characterize the properties of the material after a vacuum-arc (plasma) surface treatment.

The method is as follows.

The surfaces to be joined are subjected to vacuum arc treatment.

The process of applying a cladding metal layer (throwing plate) on the base metal (fixed plate) is carried out due to the detonation energy of the explosive placed above the surface of the clad metal. The direct source of energy for welding the interacting surfaces of the plates is the energy of their kinetic interaction, directed and distributed over a limited area. The plates are placed parallel to each other on special spacers, which are removed in the process of connecting the surfaces with a gas stream in the gap.

The combined kinetic properties of the clad layer and the flow of detonation products directed from the surface of the clad metal determine the required characteristics of the impact. For this reason, the size of the gap and the thickness of the explosive layer are different for different thicknesses of the connected plates and various types of metals.

As a result of a specially selected effect of detonation products immediately following the front of the detonation wave, an “instant” plastic kinetic displacement (deposition) of the metal of the propelled plate on the surface of the base metal occurs.

Stress waves propagate from the collision point, the speed of which corresponds to the speed of sound propagation in this type of metal. In this case, the detonation wave and, accordingly, the collision point advance along the surface of the base with a subsonic (relative to the metal) speed. Optimal for reliable welding are speeds in the range of 2000-2400 m / s. The alternation of zones of elastic and plastic deformation of the base leads to the formation of a wavy surface of the interface of the plates.

The basis is taken of a massive plate made by hot rolling or forging, which has high mechanical properties and low cost in relation to the plate being thrown.

A material having a high corrosion resistance, for example a cold-rolled material, initially having satisfactory geometric surface properties and low roughness, is taken as a throwing plate.

The base is mounted horizontally on a previously prepared pedestal. Depending on the place of initiation of the explosion and the geometry of the base, spacers are installed on its surface to determine the guaranteed amount of clearance. A frame is mounted around the perimeter of the throwing plate, for example, of thin wooden planks to accommodate a layer of explosive (BB) of the required thickness in it. A booster (a certain number of cylindrical explosives with a high detonation speed) and an electric fuse are installed at the detonation point.

Optimal for achieving the desired result is the detonation velocity, which is in the range of 2000-2400 m / s. The angle of impact is usually in the range of 5-10 °. The collision velocity in this case is about 500 m / s, while at the contact point of the base and the missile plate, a pressure of up to 60 GPa develops.

The method is illustrated by the following example.

They take two semi-finished sheets: steel grade 09G2S 40 mm thick and titanium grade VT1-0 8 mm thick. The connected surfaces are subjected to vacuum-arc treatment with an energy of 10 ¹⁰ -10 ¹¹ W / m ² . Moreover, not only oxide films are removed from each surface to be treated, a thin layer is formed on it with a depth of 100-150 nm of an ultrafine-grained structure of a chemically distilled metal. The surface roughness is R _a ≤3.6 μm.

Further, the sheets thus transformed are placed parallel to each other at a distance of 8 mm using specially prepared spacers that must be removed in the event of an explosion. Explosion welding is performed under the following conditions: detonation speed - 2400 m / s, collision speed - 500 m / s, collision angle - 5-7 °.

To remove residual stresses, heat treatment is carried out at a temperature of 500 ° C. The result is the maximum stress relief over the cross section while maintaining the strength of the welded joint at the level of the initial state (figure 5). In this case, no intermetallic inclusions were found in the connection zone, and the thickness of the obtained diffusion layer was less than 1.0 μm (Fig. 5).

It follows from the foregoing that the proposed method provides a technical result, does not cause difficulties, involves the use of developed materials and standard equipment, which indicates the compliance of the claimed technical solution with the patentability criterion of "industrial applicability".

Information sources:

1. Cladding became an explosion. Edited by A.S. Gelman. M .: Mechanical Engineering, 1978, p. 4-5.

2. Krupin A.V. et al. Deformation of metals by explosion. M .: Metallurgy, 1975, p. 153-155.

3. Copyright certificate SU 1269951, V23K 20/04, 1983.

4. Patent RU No. 2174458, V23K 20/08, 2001.

5. Patent RU No. 2180365, C23C 14/36, 2002.

6. Gurakov V.M., Pokataev E.P. The effect of heating on the residual stresses, strength and plastic properties of an explosion-welded joint of a VT6S titanium alloy with 08Kh18N10T steel.

Claims (2)

1. A method of producing a steel-titanium bimetallic material mainly for large-sized billets, comprising connecting at least two pre-processed plates, in which a throwable plate is mounted above the fixed plate with a gap and initiating an explosive charge located above it, characterized in that the processing the surface of one of the plates before connection is carried out by cathode spots of a vacuum arc excited between the surface of the plate used as a cathode, and house, and after the compound obtained heat-treated material at a temperature of 500-600 ° C. 2. The method according to claim 1, characterized in that the cathode spots of the vacuum arc process the surface of another plate.

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