Brazing Section
Brazing is a popular form of welding. It is an excellent way to join two metals. Fillers are brought to high temperatures – usually above 800 degrees Fahrenheit – and join workpieces together by flowing into the spaces between them and cooling. A flux is often used to protect atmosphere around the work area. There are many techniques for brazing that a welder can choose from.
Torch Brazing
Brazing, often discussed with welding, actually is a fundamentally different process. It’s more like heavy-duty soldering using bronze rods instead of solder. The underlying metals are not melted, so it requires substantially less heat than welding processes, which do melt and fuse underlying metals. Mild (low carbon) steel and cast iron include a large percentage of iron, and their composition makes them ideal candidates for brazing.
If you’re brand new to metalwork, then brazing is a good way to gain experience without spending a lot of money. It’s useful for repairing lightweight machinery parts, thin metal railings and gates, or sheet-metal items such as wheelbarrow pans or steel lawn mower decks.
The technique is simple: Hold a torch in one hand and a bronze rod in the other. Heat the metal parts you’re joining to red-hot. Touch the bronze rod to the heated metal so it melts and flows between the parts, forming a strong bond as it cools. Brazing is ideal for metals up to about a quarter-inch thick.
Standard propane torches don’t generate enough heat for brazing, but an inexpensive oxygen/propane or oxygen/MAPP gas torch will do the job nicely. These torches cost less than $150 and work quite well for ferrous metals less than an eighth of an inch thick. Either is great starter equipment and easily portable. You’ll find them rather expensive to operate for larger jobs, especially for the oxygen. It’s the more expensive of the two gases, and the torch uses it much more quickly than the combustible gas. You’ll get approximately 20 minutes of brazing time from a disposable cylinder of oxygen that costs about $15.
Soldering, Brazing, and Welding Tips
Soldering with a propane torch is the easiest way to join copper and brass. You can even use solder to join copper or brass to stainless steel, you just need the proper flux. But there are a couple tips to keep in mind to make it work right the first time:
1. Use a liquid flux instead of a paste flux. The paste flux tends to leave tacky residue that is difficult to clean off. If you must use a paste flux, use it sparingly.
2. Use plumbing (silver) solder only. Do not use electrical or jeweler's solder because these often contain lead or cadmium. These are toxic metals.
3. Apply solder separately to each of your parts before joining them. This practice is known as "tinning" and makes joining the parts easier.
4. Heat the parts, not the solder. Play the flame all around the joint to get it good and hot before you apply the solder. This allows the solder to flow evenly over the joint. Brazing is like soldering but it is done at higher temperatures and is applicable to more metals. It can readily join stainless steel to itself, and is an alternative to welding. The recommended filler rod for brewing service is AWS type BAg-5, and its temperature range 1370-1550°F (743-843°C). While brazing can provide a stronger joint, the high brazing temperatures can be bad for stainless steel. At those temperatures, carbon in the stainless steel can form chromium carbides which takes the chromium out of solution, making the steel non-stainless near the joint. This area is prone to rust and cracking after it is in service. The problem cannot be fixed by re-passivation so it is best to avoid excessively heating the parts during the braze and keep the total time at temperature to four minutes or less. Propane torches are usually not adequate for brazing. You will need to use MAPP gas or acetylene.
Types of Brazing
Torch brazing is the most common form of mechanized brazing. In some countries it is the method used for the majority of the brazing that is done. Specialized operations or small production volumes are often where this method is used. The three categories of torch brazing are machine, manual, and automatic.
Manual torch brazing has heat applied with a gas flame near or on the joint. It can be a hand held torch or held in a fixed position, depending on the method. Usually it is used where other methods are impossible or for small production volumes. Flux is required. Machine torch brazing is used when the operation is repetitive. It is a mix of manual and automated methods and uses flux, reduces the cost and works for small to medium production projects. Automatic torch brazing has a high production rate, reduced costs, and a uniform braze quality. A worker is needed just for unloading and loading the machine
Furnace Brazing
This method is semi-automatic and is used in industrial operations. It can produce large numbers of small parts, has a controlled heat cycle, and post braze cleaning is not needed. To prevent oxidation, inert, vacuum, or reducing atmospheres are used. It is cost efficient to run but uses a lot of power compared to other methods, is more difficult to design, and the equipment is expensive. The kinds of furnaces used are batch type, vacuum, retort with controlled atmosphere, and continuous.
Batch type furnaces have relatively low costs and heat each load separately. It can be turned on and off easily, has a large degree of flexibility, and is suited to medium or large productions. Flux or a controlled atmosphere can be used. Vacuum furnaces are fairly economical, work well with such oxides as aluminum or titanium, and is often used with refractory materials or alloys that can't be brazed in atmosphere furnaces. It is vital to clean because there is no protective atmosphere. Continuous type furnaces work by making a steady flow of parts go through the furnace on a conveyor. These are good for large productions. Retort-type furnaces have a sealed lining where the atmosphere can be completely changed inside and is best for semi-continuous or batch productions and alloys that resist oxidation.
Silver Brazing
This method uses silver alloy based filler for brazing. It is also known as hard soldering or silver soldering. The silver alloys have a lot of variety and different percentages of silver and other metals in them, such as cadmium, zinc, and copper. A special method of silver brazing is pin brazing (pinbrazing). It is used especially for cathodic protection installations or for connecting cables to railway track. It can be used in the tool industry to do such jobs as fasten hardmetal (like carbide) to such tools as saw blades.
Braze Welding
This method uses a brass or bronze filler rod that is coated with flux in order to join steel workpieces. It requires more heat than basic brazing and acetylene or methylacetylene- propadiene (MPS) gas fuel is often used. The name comes from the fact that this method does not have capillary action. Dissimilar metals are able to be joined with this method, there is a reduced need for pre-heating, and minimal heat distortion. However, there is a loss of strength when the work is under high temperatures and it cannot withstand high stress.
Cast Iron
"Welding" Welding cast iron is actually a type of brazing. Filler rods that are mainly nickel are used but there are cast iron rods available. Cast iron is a difficult metal to work with and many find the skill extremely difficult to learn. It is used often in repairs.
Vacuum Brazing
In this technique the brazing process is done inside of a vacuum. There are many advantages such as flux-free joints that are very strong and have high integrity, are superior to other joins and are extremely clean. It can be an expensive process. Residual stresses are greatly reduced because of the slow heating and cooling cycles in this process. The material's thermal and mechanical properties are improved and things such as heat treating or age hardening can be done during the metal joining process. The process is done in a furnace and heat is transferred with radiation.
Dip Brazing
This particular technique is very suited for brazing aluminum because there is no air and therefore no oxidation. The brazing compound is usually applied in slurry form and the assembly dropped into a molten salt bath that will work as both a flux and heat transfer.
You have to wait 30 seconds here.
You can weld in several ways, but all the processes boil down to the same basic premise: Apply enough heat to melt metal parts so they fuse together, and then let the weld area cool. The heat can come from a burning gas, such as acetylene, or from a high-voltage electric spark that jumps (arcs) from the welding tool to the metal you’re welding. Regardless of the heat source, good welds are stronger than the surrounding metal, in part because metal is added to the weld from a rod or wire that melts into it. With the proper equipment and materials, you can weld cast iron, stainless steel, aluminum and other metals.
When it comes to acquiring welding gear, you’ve got three main options to consider: oxygen/acetylene torches, stick-type electric arc welders and wire-feed electric arc welders. Each option varies in cost, size and effectiveness. The best system for you depends on your expected usage and your budget.
Gas welding is like brazing in that it’s done with a torch, but instead of using molten bronze to “glue” metal parts together; welding generates enough heat to actually melt the metal on each side of the joint.
To weld, you use a hand-held steel welding rod that melts, adding metal to the weld pool. This added steel fills gaps and boosts joint strength.
You hold the torch in one hand and the welding rod in the other, then heat the metal parts at the joint line until a pool of molten steel develops.
Move the torch and rod in half-inch diameter circles to keep the weld pool moving along the joint.
You need a really hot flame for gas welding, and that’s where the oxygen/acetylene torch comes in. Two cylinders of compressed gas feed this welding system to create an extremely hot, blue flame that’s suitable for work with all kinds of ferrous metals.
Oxygen/acetylene torch systems are available for less than $175 and include gas regulators, hoses and different torch heads for brazing, welding and cutting. The torch heads used for both brazing and welding are relatively simple.
Oxygen/acetylene torches and hoses include standardized fittings. This means the same set of torches can hook up to a variety of tank sizes. You can get sets ranging from small, portable tanks that fit into a carrying caddy to large, semi-stationary tanks meant for use in a workshop. All but the smallest tanks are available as refillable rental units. You pay an annual leasing fee for the tank, plus the cost of whatever gas you use.