GB2135229A - A method and an apparatus for controlled-atmosphere brazing of high-alloy Cr-Ni steel components - Google Patents

Abstract

It is the object of the invention to render possible the easy and reliable joining of high-alloy Cr-Ni steel components under site conditions without the need of any separate chemical or mechanical pre-treatment of the oxide layer adhering to the metal. According to the invention, this is brought about in that the brazing joint is heated according to the direct electric resistance-heating principle whilst surrounded by a protective chamber. There will then flow through this arrangement during the heating phase and the subsequent cooling phase an inert gas having a maximum contamination of 100 ppm. The chamber volume must not exceed a maximum of 200 cm<3> but must be 20 to 30 times larger than the component volume involved in the brazing process.

Description

SPECIFICATION A method and an apparatus for controlledatmosphere furnace brazing of high-alloy Cr-Ni steel components The invention relates to a method and an apparatus for brazing high-alloy Cr-Ni steel components under assembly conditions, more especially for joining small-diameter tubes.

The main problem during the brazing of high-alloy Cr-Ni steels is the elimination of the layer of oxide adhering to the metal. In known brazing methods, this layer is removed by salt fluxes. The disadvantage of these methods is the fact that there are left, following brazing, aggressive flux residues which, combined with moisture, lead to corrosion of the base material. The mechanical removal of these flux residues is very expensive and the performance thereof may even become impossible in practice.

If an expensive chemical treatment of the brazed joint is carried out and, even then, there are left flux residues, then these may lead to the material failing at a later point in time, resulting in serious consequences.

On account of the mentioned problems, recourse is frequently had to furnace brazing methods which bring about the removal of the oxide layer by a vacuum or by a protective gas. As protective gases there are used either reducing gases (e.g. dry hydrogen or cracked ammonia gas) or inert gases (e.g. high-purity argon, the total of all contaminants being less than 1 Oppm). Since Cr-Ni steels frequently contain other metals which have a great affinity for oxygen, such as titanium, an extremely dry protectie gas (dew point - 700 C) has to be employed if use is made of an inert gas.

In order to bring about wetting of the base material, the surface has to be expensively dressed off mechanically or chemically so that only a thin oxide film is present. If the brazing temperature is high (10000 C), this oxide film breaks and thus enables the solder to come into contact with the base material, as required. At the points where the oxide film has broken, the solder moves therebeneath, lifts it off and incorporates it in the solder structure (Zimmerman, K.G. "Mechanisiertes Hart listen auf Lötvorrichtungen und Lötmaschinen Ausfuhrungsbeispiele fUr Of enlöteinrichtungenOfenloteinrichtungen mit und ohne Schutzgasatmosphäre", Technik die verbindet, Berichte aus Forschung und Praxis No.

25).

If use is made of reducing protective gases, the oxide layers are removed by the reducing constituents of the protective gas - substantiallly hydrogen. The level of the temperature and the composition of the protective gas are decisive for its reducing effect. Metal oxides which are difficult to reduce (e.g. chromium oxide) necessitate a protective gas which is virtually free of oxygen and oxygen compounds and is extremely dry. For this reason, dry hydrogen or dry cracked ammonia gas is frequently used as the protective gas.

Another method to remove the disturbing oxide layers during brazing is the vacuum application, Here, the removal of the oxide layers of carbo naceous materials is based on the reducing effect of the carbon itself. The conditions for a quick course of this reaction become more favourable with an increasing carbon content of the steel, a low stability of the oxide to be reduced and a falling pressure in the vacuum furnace. Since the stability of the oxide depends on the temperature, the CO reaction for the oxides is temperature- and pressure- dependent. For example, there is a reduction of carbon in chronium oxide at as low a temperature as 6800C if the CO partial pressure is below 10-6 at.The same reaction is still possible at 10000C if the pressure has risen to 10-2 at. (Ogiermann, G.; Zimmermann, K.G.; "Mechanisiertes Hartlöten auf Lötvorrichtungen und Lotmaschinen ..." Technik die verbindet - Berichte aus Forschung und Praxis No. 26).

In the case of non-carbonaceous materials, the oxides are separated by the hydrogen or the oxide layers burst when heated and the molten solder then moves therebeneath, lifts them off and incorporates them in the solder structure. (Mahler, W.; Zimmermann, K.F.; "Bindungsverhalten einiger Hartlote unter Wasserstoff aufverschiedenen Grundwerkstoffen", Schweissen und Schneiden 21 (1969) No.6, p.249-254).

However, these methods described herein have substantially the following disadvantages: -Because of the high expenditure pertaining to apparatus, brazing under vacuum can only be used for the field of stationary production.

-Reducing gases containing mainly hydrogen as the reducing agent are eliminated for safety reasons because the danger of an oxyhydrogen gas explosion occurring under site assembly conditions is too great.

Another known method of joining Cr-Ni steels is induction brazing using high-purity argon (Kaiser, L.; Induktionslöten von Hochdruckrohrleitungen im Flug- und Raumfahrzeugbau, Schweissen und Schneiden 24(1972) No. 5, p. 161-165). Because of the high expenditure pertaining to apparatus for the heating unit, this method is also only suitable for stationary production, the high expenditure for the mechanical dressing-off of the oxide layers and the high protective-gas costs (high-purity argon) additionally have a negative effect on the economy of this joining technology.

It is the object of the invention to provide a method and an apparatus for joining components made of high-alloyed Cr-Ni steels which render possible an easy and reliable connection of such components, more especially under assembly conditions.

The problem underlying the invention is to provide a method and an apparatus for brazing components made of high-alloyed Cr-Ni steels in an inert-gas atmosphere dispensing with a separate chemical or mechanical pre-treatment of the oxide layer adhering to the metal.

According to the invention, this problem is solved in that the surfaces to be joined by brazing according to the principle of direct electric resistance heating are surrounded by a protective chamber. There then flows through this arrangement an inert gas during the heating phase and the subsequent cooling phase. The maximum technical contamination may be 100 vpm, the moisture content not exceeding 35 ppm, the oxygen content not exceeding 8 ppm, the methane content not exceeding 10 ppm, the hydrogen content not exceeding 1 ppm and the carbon dioxide content not exceeding 1 ppm.

The connection for brazing high-alloy Cr-Ni steel components consists of a protective chamber which is arranged around the surfaces to be joined by brazing and which has a maximum chamber volume of 200 cm3. However, this chamber volume has to be 20 to 30 times larger than the component volume involved in the brazing process. The inlet or the inert gas into the protective chamber is at a maximum distance of 2 to 3 cm from the surfaces to be joined.

The invention will hereinafter be explained in more detail with the aid of an exemplified embodiment.

High-alloyed steel tubes (X8CrNiTi 18, 10) dimensioned 12 x 2 mm are to be brazed to one another in a tube line. For this purpose, the surfaces to be joined are cleaned with a fat-dissolving solvent and are prepared as follows: First, solder rings, consisting for example of Cu87%Mn10% & Co3% are placed on the two tube ends and are aligned with a gauge.

Then a soldering sleeve is placed on one tube end and the second tube is also moved into this soldering sleeve so that the abutting tube surfaces touch each other. Around the thus prepared surfaces to be joined there is positioned a controlled-atmosphere chamber which is provided with a clear-vision screen at the top. The electric contacts of a resistance-soldering appliance are clamped to the tubes to the right and left of the chamber and to one side of the tubes to be brazed together there is fastened a connection for introducing an inert protective gas into the tube interior. Then the protective gas flows through the tube (10 1/min) so that there is no longer any air present in the tube interior. Now the other tube end is sealed and the introduced protective gas is pressed via the tube joint through the close-joint space into the controlled-atmosphere chamber.After the same flow of protective gas as that of the tubes to be brazed together has been passed via a separate connection through the controlledatmosphere chamber for approximately 30 seconds, and the air present therein has escaped, the brazing process can be initiated.

The oxide layers, which obstruct the flow of solder during the soldering process, burst at temperatures above 10000 C. The flowing of the solder is only possible when a re-oxidation of the base material is prevented. This is brought about by the inert-gas atmosphere, which may consist of argon or nitrogen with technical contaminants of up to 100 vpm (argon: moisture 35 ppm, oxygen 8 ppm, methane 1 vpm; nitrogen: moisture 30 ppm, oxygen 1 ppm, carbon dioxide 1 ppm, hydrogen 10 ppm).

Under these conditions, the molten solder moves beneath the burst oxide layers, lifts them off and incorporates them into the solder structure. The pre-condition for brazing with the described gases is the use of a controlled-atmosphere chamber. The dimensional variations of the controlled-atmosphere chamber must be such that its volume does not exceed 200 cm3,the chamber volume having to be in a concrete case that it is 20 to 30 times, preferably 25 times larger than the component volume involved in the brazing process. The inlet for the protective gas into the controlled-atmosphere chamber should be only 2 to 3 cm from the surfaces to be joined by brazing.Furthermore, the surfaces to be joined by brazing must be heated according to the direct electric resistance-heating principle so as to ensure rapid heating of the close-joint space and so as to allow the solder to rush into the close-joint space.

When the solder rings have melted around the entire tube circumference, heating can be stopped. The brazed joint now has to be cooled in the controlled atmosphere for approximately 30 seconds so as to prevent any scaling of the base material. Then the brazing cable and the controlled-atmosphere chamber are removed and any tempering colours that may be present are removed with an emery cloth.

High-alloy steel components are often brazed with nickel-based solders in a controlled-atmosphere furnace. In order to reduce the melting temperature of the solder to 8350 C - 11350 C, these solders contain non-metellic alloying elements of Si, B or P.

This results in these alloying elements, in combination with iron, forming brittle intermediate layers which, following brazing, can only be removed by subsequent expensive homogenising. The homogenising time is one to five hours, depending on the proportion of Si, B or P. For this reason, such nickel-based solders are unsuitable for use under site assembly conditions.

The method according to the invention, along with the arrangement forming part thereof, now allows the use of solders whose melting point is only a little less than that of the base material since a temperature control can occur directly at the brazing point, so that the constituents reducing the melting temperature can be dispensed with. A solder meeting these requirements consists of the following consti tuents: 0.1 % C, 1 % Si, 0.60% Mn, 0.02% P, 0.015% S, 19% Cr, 29% Ni, remainder Fe.

Due to the application of this high-temperature solder, there is no need for the expensive homogenising. Since the base material and the solder have approximately identical compositions, the joint has a corrosion behaviour which is similar to that of the base material. (ppm = parts per million units by volume).

Claims (5)

1. A method for controlled-atmosphere brazing of high-alloy Cr-Ni steel components under site conditions, characterised in that the brazing joint to be heated according to the direct electric resistanceheating principle is surrounded by a controlledatmosphere chamber, and in that there flows through this arrangement an inert gas with technical contaminants of up to a maximum of 100 vpm, the moisture content not exceeding 35 vpm, the oxygen content not exceeding 8 vpm, the methane content not exceeding 10 vpm, the hydrogen content not exceeding 10 vpm and the carbon dioxide content not exceeding 1 vpm.

2. A method for controlled-atmosphere brazing of high-alloy Cr-Ni steel components under site assembly conditions as claimed in Claim 1, characterised in that there is used a solder with the composi tionofOtoO.1%C,1to1.5%Si,OtoO.6%Mn,Oto 0.02%P,0to0.15%S, 15to25%Cr,20to80%Ni, the remainder being Fe.

3. An apparatus for controlled-atmosphere brazing of high-alloy Cr-Ni steel components under site conditions, characterised in that the brazing joint is surrounded by a controlled-atmosphere chamber having a maximum chamber volume of 200 cm3, this chamber volume having to be 20 to 30 times in excess of the component volume involved in the brazing process, and in that the maximum distance of the inlet for the inert gas into the protective chamber from the brazing joint is 2 to 3 cm.

4. An apparatus for controlled-atmosphere brazing of high-alloy Cr-Ni steel components under site conditions as claimed in Claim 3, characterised in that the top of the protective chamber is provided with a clear-vision screen.

5. A method as claimed in Claim 1 and substantially as described herein.