Gas Welding and Brazing

Joining in the workshop involves many risks due to the use of high currents, temperatures and flammable gasses.

Arc/Mig/Tig Welding

During these welding types high currents are being used so it is important to isolate the process from anything that may catch fire or cause the operator any electrical shock (oil, water, rags etc). High intensities of visible and UV light are emitted so it is mandatory that the operator uses a welding mask that protects the user from harmful rays and spatter.

It is also important that the skin of the user is fully covered since the UV rays damage the skin and can lead to skin cancer.

Gas Welding & Brazing

The safety precautions in welding with gas include: a number of one-way safety valves fitted to regulators to prevent gasses which might cause the cylinders to explode from re-entering the pipes. The pipes are colour coded to prevent any confusion between the oxygen (blue) and acetylene (red) pipes.

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During both processes gloves and a boiler suit should be worn and tongs used to prevent burning of the skin due to extreme temperatures of the metal and from spatter. Goggles should also be worn to stop hot pieces and infrared radiation entering the eyes during the processes using gas.

Process Theory Brazing

Brazing is a non-fusion type of welding used to fuse dissimilar types of metal together. It joins them by creating a metallurgical bond between the filler metal and the surfaces of the two metals being joined.

It uses the combustion of a mixture of oxygen and acetylene (oxyacetylene) = (), which burns at around 2000 ? C. The flame must be correctly set to ensure accuracy and enough heat (too much oxygen - blows flame out, too little - dirty flame). Flux is used to prevent the oxidation of the metals and acts as a barrier between the air and molten metals.

In the example discussed, two flat rectangular pieces are joined together by heating the two plates then melting the brass filler rod (brass alloy - melting point 900 ? C - flux coated to help the flow run between the plates) along the edge of the adjoining plates. The liquid metal runs between the plates then solidifying to bond the two plates. The remaining flux must later be removed to prevent corrosion (due to the porosity associated with the use of flux). Gas Welding (Oxyacetylene Welding) This type of welding is similar to the brazing process but it does not use flux and the filler rod in this case consisted of copper.

The two ends were tacked together (to hold them in place) using the torch then the two plates and filler rod are all fused together in a linear fashion similar to the brazing. SMAW (Shielded Metal Arc Welding) The SMAW process consists of a metal electrode, which is connected to a power supply through the electrode holder. The work piece is connected to the opposite terminal so a circuit can be created as the rod strikes the work piece. The rod is coated in a flux, which vaporises into a shielding gas when the arc is created; this protects the molten metal in an inert gaseous shroud from the oxygen in the air.

The arc set up heats the end of the rod and the surface of the work piece to a temperature of 3800? C. This heat initiates spray transfer between the two components. Another factor in the Arc Welding process is the polarity of the terminals when using DC. In our case the electrode is positive and the spray is forced onto the work piece. This set-up is for welding onto a metal surface and is by far the most common. On rare occasions it may become necessary to reverse the polarity. Shown on the bottom right is an example of where multiple welds are done on top of one another to fix large pipes together.

Sometimes it may become necessary, on the first run only, to reverse the polarity (usually when welding from above). Changing the polarity, so that the work piece is positive means that the weld pool has a tendency to hug the work piece rather than blasting threw it and thus increasing the gap. This gives a nice finish on the underside of the initial weld. The general technique for doing a straight weld is to do small arcs side-to-side, pausing for a fraction of a second at the sides (shown bottom right). Another technique is to follow figure of 8 patterns all the way down (this gives slightly longer time at the sides).

This pausing gives the weld pool a better shape. The most important factor in Arc Welding is the set up also the correct size and type of rod has to be used to fit the job in hand. Other factors include: Arc length, current setting and moisture. These will be discussed in more detail in "Welding Effects on Microstructure". Process Applications Brazing Brazing is performed at low temperature at which the brazing alloy melts but not the base material. Brazing can be performed on different types of metal due to the nature of the bonding.

It is more suited to joining different metals when the parent metals need to be kept intact since it does not melt them. The negative aspects of brazing are that brazing alloys are relatively expensive in comparison to Arc rods, which already have flux on them. The different metals can cause sacrificial protection to occur thus corroding the more reactive metal. The process is time consuming and involves more than one stage in its process. Gas Welding Gas Welding is probably one of the earliest of modern welding history and also one of the most widely used processes.

Of all the combustible gases Acetylene is the most widely used gas for welding mainly because the process is versatile, adapted to many different jobs, ease of production and transportation and that the equipment is relatively of low cost. SMAW (Shielded Metal Arc Welding) SMAW welding is widely used in the fabricating industries for the construction and repair of plain carbon and alloy steels. It is also used to weld surfacing type electrodes to make the base material wear resistant. SMAW welding has a lower equipment cost than other types of Arc welding like TIG/MIG as you do not need equipment like gas hose etc.

SMAW can be used to weld in tight spaces with less difficulty. This process also deposits the filler at a faster rate than TIG/MIG. The equipment of the SMAW welding is easy to move and can be used out doors I conditions of up to medium wind. The disadvantages of SMAW are that the cost of filler per weld is greater as one electrode deposits a small quantity of filler metal. You also need skin and eye protection from UV radiation emitted and rate of production is slow because you have to change the electrode and scrape the slag. Procedure of SMAW

Aim

Using 6 mm thick low carbon steel plate as the parent material. Deposit a weld bead on each side of the parent using the shielded metal arc process. Vary the size of electrode in each case. Equipment used DC voltage/current power supply, 2. 5 mm and 4 mm flux covered electrodes, 10mm thick low carbon steel plate, metal work table (earthed), gloves, protection helmet, electrode holder, pliers and stop clock. Procedure  Two welds are deposited one on each side of the plate. A stop clock is used to record the time taken to weld each side.

This is done in pairs as one does the welding and the other records the time and then they swap places for the second weld. Different size of electrode is used during each welding procedure.  The first weld is carried out using a 4mm electrode. The transformer is set at DC positive, 25V and 175A.  Check the circuit to see that the voltage/current settings are correct and that the power supply is ON. Ensure the welding area is curtained off, to protect those who may be nearby. Check all your skin is covered. Check the worktable to see that it is earthed. Check Equipment for damage.

Check that the electrode is secure in holder. Ensure that the electrode does not touch the worktable until the protection helmets are put on. Warn the person holding the timer when about to start.  Scratch the rod on the bench to heat it, so it is easier to start.  Check starting position  Ensure that the angle of the electrode is about perpendicular to the plate surface and penetration or travel angle is about 45i??.  Begin the weld and start the stop clock.  Adjust arc length and speed accordingly to get neatest weld. Once the weld is complete raise the electrode from the plate and stop the clock.

Remove the helmets, and use a pair of pliers to take the steel plate in a sink to cool it down under cold water and then dry off the plate. This procedure is repeated by the other person using the 2. 5 mm electrode with the transformer settings at 25V and 75A.  Both times recorded in a table.

RESULTS

Weld 1 Weld 2 Current (A) 175 75 Electrode diameter (mm) 4 2. 5 Time (s) 48 31 Length (mm) 146 142 Width (mm) 14 7 Using these results we can calculate the rate of heat input. The heat input rate is one of the most important variables in fusion welding as its variation affects the microstructure.