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Brazing-stainless,Easy Joining for Tough Requirements.SOLUTIONS with Effective, Practical Advice
Brazing-stainless steel is considered easy to perform if a basic understanding of the materials is obtained and if the applicable processes are mastered. In particular it is advantageous for joining dissimilar materials that cannot be welded together. When properly applied Brazing-stainless permits to build strong and leak tight joints in thin wall items with minimum deformation. Delicate assemblies can be performed even in inaccessible joint locations. Suitable methods are available for mass production.
It should be remarked that Stainless Steel is not a well defined material. This general name indicates all those iron base materials containing at least 10% Cr. The five main classes are briefly described in our page on Stainless Steel Welding. When Brazing-stainless Austenitic (non hardenable) attention is required to avoid carbide precipitation (see a.m. reference).
Ferritic types, non hardenable, (405,430, 446) may be subject to interfacial corrosion. BAg-21 is suitable for brazing type 430. High temperatures promote grain growth and fragilization. Self fixturing required. Martensitic types, hardenable, low carbon (403, 410 and 416) and high carbon (440A, B or C) must have brazing combined with hardening thermal cycle. Precipitation hardening types have brazing combined with hardening thermal cycle. Nickel plating or vacuum brazing is needed. Duplex types need special attention to avoid embrittlement. BNi-7 was used successfully in hydrogen atmosphere. The chromium content that gives to stainless steels their most important properties, mainly protection from rusting, produces on the surface a chromium oxide film that must be removed before wetting (by the molten filler metal) and Brazing-stainless can occur. The same applies to titanium oxide that forms in stainless steel titanium stabilized grades (321). These oxides are tenacious and refractory, and must be prevented from forming again while heating, either by a suitable flux or by the presence of adequately protective atmosphere. Once a brazing process has been developed with successful results, rigid adherence to process control measures must be assured.
Filler Metals The selection of suitable filler metal for Brazing-stainless is based on the mechanical properties of the joints, on their corrosion resistance, on the service conditions including operating temperature, stresses and environment, on heat treatment requirements, on the Brazing-stainless temperature and on the heating method. Furthermore joint geometry and thickness, brazing process, production required per unit time, cost, provisions against sensitization of unstabilized alloys and safety considerations must be taken into account. Many types are available based on different material families. Most of them are included in ANSI/AWS A5.8M/A5.8:2011
Additional proprietary filler metals may be found in suppliers' catalogs. If the Brazing-stainless temperature is very near to that of sensitization (See Stainless Steel Welding), there is the risk to impair corrosion resistance of the stainless steel involved unless the time at temperature is very short. In case of concern either the base metal or the filler would better be changed. Additional filler metals were developed for use in vacuum service conditions. The following Table lists some of the important Silver Base Filler Metals.
Nickel Base Brazing-stainless filler metals, suitable for higher service temperatures, are also quite frequently used, generally in furnace brazing. These materials are generally supplied in the form of powder or paste (powder with binder). Paste containers are generally kept under refrigeration before use, because the binder has an expiry date beyond which the paste should not be used.
Notes: Boron containing BNi-1 to 5 tend to corrode thin sheets. Additional filler metals based on copper, gold and precious metals, and cobalt are also in use for special applications. Fluxes Fluxes are necessary for Brazing-stainless steels in air, by torch or by induction. Manufacturers' recommendations as to their temperature of activity should be followed, depending on the filler metal used. ANSI/AWS A5.31-92 Atmospheres The protective atmospheres used for Furnace Brazing-stainless steels are, besides vacuum, dry hydrogen and dissociated ammonia. The dew point should be under control and kept low for best results. Occasionally an inert atmosphere of dry argon can be used if hydrogen is prohibited because of its reactivity to some metals. Quality The usual quality requirements are applied, depending on the application. Once the Brazing-stainless Procedure Specification has been approved, process control must be applied in all stages of production from preparation to final delivery. Safety Safety should always be a concern, but especially for the danger of inhalation of zinc and cadmium by operators in torch brazing, and for the danger of explosion if hydrogen atmosphere is used. In this case the furnace must have facilities for suitable management of emergencies like sudden loss of power. Training and refreshing of instructions should be repeated at regular intervals.
Resources Modern Brazing of Stainless Steel Brazing-stainless Steels Brazing-stainless steel using Amorphous Brazing Foil Brazing & Soldering Materials The European Association for Brazing and Soldering (EABS) Fundamentals of Brazing [...] An Article on New Silver-Free Brazing Filler Metals was published (4) in Issue 140 of Practical Welding Letter for April 2015. An Article on Filler Metal for superconducting radiofrequency cavities was published (4) in Issue 145 of Practical Welding Letter for September 2015. An Article on Aim for Small, Concave Braze Fillets was published (3) in Issue 148 of Practical Welding Letter for December 2015. An Article on A new method of brazing stainless steel parts was published (7) in Issue 151 of Practical Welding Letter for March 2016. Commercial Sources Publications Hot-Zone Design Meets Tight Brazing Temperature Requirements High Temperature Brazing Alloys (16 pages) Frequently Asked Questions Filler Metal Technical and Safety MSDS Documents
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