< WELDING, BRAZING & THERMAL CUTTING INTRODUCTION
OXY-GAS WELDING & BRAZING
The equipment used for gas welding and cutting consists of an independent oxygen and fuel gas supply. The fuel gas is usually ACETYLENE, propane (LPG), or occasionally town gas. These gases are normally contained within high pressure cylinders, with REGULATORS and other safety devices (such as FLASHBACK ARRESTORS), to variously control the gas’s pressure, rate of supply and direction of of flow. The gases are then delivered to a MIXER BLOW PIPE where they are combined, before being ejected through a copper nozzle at the torch head.
Acetylene is usually the fuel gas of choice for welding processes. This gas provides the most intense flame when mixed with oxygen – about 5640°F (3100°C), compared to the oxy-propane flame of 5125°F (2815°C).
The pressure of the two gases delivered to the blowpipe is adjusted and set by regulators on each gas cylinder. The flow of pressurised gas from the blowpipe to the copper nozzle torch outlet is in turn controlled by the on/off valves mounted on the blowpipe housing. Copper nozzles are available in a variety of bore apertures to enable the welding of different thickness materials.
By adjusting the regulators and blowpipe control valves, three types of welding flame can be obtained: NEUTRAL – with a balanced supply of each gas, OXIDISING – with a bias of oxygen, and finally, CARBURISING– with a bias of fuel gas. The equally balanced neutral flame is used for most welding, brazing and cutting applications, although a slightly oxidising flame is usually used for welding copper/zinc alloys (BRASSES). When welding brass, the increasing the oxygen balance helps to create an oxide layer over the weld pool, this oxide layer helps reduce the loss of zinc from the alloy through BURN OFF. The carburising (carbon rich) flame is only used for stainless welding and other specialist work such as stellite (a cobalt/chrome alloy) hardfacing of tooling, not normally a process of relevance to art founding.
When gas welding, the direction of torch travel usually progresses from right to left (LEFTWARDS WELDING) along a horizontally oriented joint, or from the base to top (upwards) when in the vertical position. A rarely used technique of RIGHTWARDS WELDING is sometimes used when welding thick walled casts (> 6mm/ 1/4”) which do not have a grooved ‘V’ joint preparation, though this technique is of passing interest only nowadays as arc welding is generally used for joining heavier materials.
The abutting edges of very thin walled casts and sheet material can be simply melted together with no added filler, however a rod is always used for building up the joints in casts with more than a 3/16” (3mm) wall thickness. Welding requires a preheated area of the parent metal to become molten before introducing the filler rod by dipping into the weld pool – just in advance of the flame. With the filler rod held at an angle of about 30° to the joint, a small amount of filler breaks off the rod, and the molten droplet is drawn into the WETTED pool by capillary action where it freezes with the parent metal. When welding selected metals and metal alloys (including bronze, brass and aluminium), a FLUX is applied both to the general area of the joint, and to the metal filler rod. This flux forms a protective coating, preventing oxidisation of both of the weld, and the surrounding parent metal.
Brazing, bronze welding and soft or hard soldering requires a meticulous preparatory cleaning of the edges of the joint. The application of a matching flux to the cleaned joint area ensures the clean flow of filler rod or solder into the joint. A large area of the joint is heated to a temperature above that of the melting point of the filler rod, but below the melting point of the parent metal. It is important not to allow a weld pool to form when soldering or brazing. When the correct temperature is achieved, the brazing rod or solder can be introduced. Filler material (usually a brass for brazing, bronze for bronze welding, tin or lead alloy for soft soldering, silver alloy for hard soldering), is drawn into the joint by capillary action to adhere the abutted sections as it freezes.
SILVER SOLDERING is a popular hard soldering technique used for joining light gauge or small detailed elements and jewellery scale work. Done with the aid of a neutral gas flame and a fluxed silver alloy solder, silver soldering can produce an exceptionally sound joint. Brazed and hard soldered joints are usually able to withstand the moderate heating of most patination processes, though particular care must be taken if ‘soft soldered’ joints are feature (such as the lead/tin solders used for copper pipe plumbing), not to accidentally re-melt the solder out of the joint.
Most of the cast alloys used to make sculpture casts can be gas welded, brazed or soldered, although electric arc welding (in particular the ‘TIG’ process) has more or less relegated gas to specialist uses only. Gas joining non-ferrous metals always requires careful joint preparation, with the use of an appropriate flux coated filler rod or solder wire – even when welding. The conductivity of the copper alloys used for making most art casts is such that the use of a large bore blowpipe nozzle and a lengthy pre-heating of the joint area may be required to at first initiate, then maintain a satisfactory weld pool. Cast aluminium alloys can be especially problematic to join by a gas welding process owing to the formation of a heat resistant oxide layer over the alloy’s surface. The nature of this metal also makes it difficult to judge by eye the condition of the weld pool, which can be prone to rapid collapse without warning – though with skill and experience satisfactory gas welded joints can be made in most aluminium alloys. The fluxes used for gas welding of aluminium and similat alloys are particularly corrosive, these should be used with care, and the joint area thoroughly washed and cleaned after welding.
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