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Braze-weldingwith low heatSolutions with Effective, Powerful Advice
Braze Welding, oxyacetylene joining, copper alloy filler metal, with standard portable equipment, soft and ductile weld deposit layers, used for production assembly, for repair of tools or cast iron items, for buildup. Low cost substitute for brazing. Lower temperature alternative to welding. Good strength joints. These and other highlights are presented here for information of the interested reader. What is in here for me? Knowing all the alternatives can help you find the best and most economic solution for a production or repair problem. What are the characteristics of the Braze-welding process? When is it convenient to consider? Why is it recommended for certain jobs? These and other questions are going to be discussed in the following. In a previous page on Brazing, the process for capillary joints was introduced, listing advantages and limitations, and especially those requirements to be fulfilled for obtaining economically acceptable permanently brazed connections. UPDATED PAGE
Here we are presenting Braze-welding, a similar but different process, more forgiving in joint requirements (larger gaps permitted) and suitable for different cases where a regular brazing driven by capillarity is not feasible. Braze-welding: productive and economical The manual Braze-welding process makes use of the oxyacetylene flame which is known and generally available in many shops for production and repair jobs of welding and brazing. By employing copper base filler rods and special fluxes it permits, with some readily acquired skills, to perform useful joining for production or repair. Braze-welding of cast iron is an attractive alternative to welding as it requires less heat (less time, less distortion, less cracks). The edges to be joined should be cleaned to the highest degree, possibly by using a cleaning salt bath. If the cast iron was ground for removing gross contamination, the graphite may be smeared on the surface, impairing wetting and brazing. In this case one should first remove the smear by fast heating the surface with the oxyacetylene flame until dark red in color, and then, after cooling, wire brushing the joint area. Preheating of cast iron for Braze-welding, although not always necessary, may be used for improving results. Depending on the size of weld relative to the casting size, there may be a need of thorough preheating at dull red heat, but not over that, because wetting and tinning might become impaired. This characteristic presents a self adjusting feature. The welder will easily find the most suitable preheating range for Braze-welding, because either at lower or at higher temperature the wetting is found to be more difficult. Slow cooling after Braze-welding is usually recommended by employing suitable insulating materials. Post heat is not required. The main advantage of Braze-welding is the fact that it requires less heat and by consequence it is faster (better productivity) than gas welding and also less prone to producing distortion. Strength at room temperature is usually adequate for most purposes. Overheat of the base metal should always be avoided. Disadvantages of Braze-welding are mainly the no-match color of the filler metal alloy, and a possible increased sensitivity to corrosive conditions, because of general lower corrosion resistance of the filler alloy and of galvanic couple. Strength is found to fall off rapidly with increasing temperatures, so that service temperature of items fabricated or repaired by Braze-welding should be limited to 500 oF (260 oC). Filler metals usually employed are types of brasses (copper alloys with zinc and other metals) classified in specifications:
ANSI/AWS A5.7-84(R1991) ANSI/AWS A5.8/A5.8M:2004 ISO 3677:1992 BS EN 13347:2002 AWS A5.31-92(R2003)
Crack repair by Braze-welding. For the repair of cracks in cast iron by Braze-welding, the crack should first be dressed to present an included angle of 60o, with a small radius at the bottom. It is customary and recommended to drill stop holes at the real ends of existing cracks: they will be filled up in the process of Braze-welding the crack. If oil remnants from machining or service are present in the cast iron body, they should be removed by burning at 1400 oF (760 oC) with an oxyacetylene torch. Then, after cooling down, the groove should be wire brushed to clean all remaining surface contamination. The proper type selection and the correct application of flux is critical to the success of the application. Preheat should be applied for the purpose of maximizing wetting of the cast iron surface by the filler metal. A root pass is usually deposited, and then the Braze-welding filler is melted in place to build up all the thickness. An Article reporting on precautions needed when one wishes to perform braze-welding for joining Aluminum to nodular Cast Iron was published in Issue 28 of Practical Welding Letter for December 2005. To read the article (in Section 11) click on PWL#028. An Article on Filler Metal for Braze-Welding was published in section 4 of the issue 30 of Practical Welding Letter. To see the article click on PWL#030.
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