Welding topics.

Welding is a process critical to our present state of civilization and technical advancement, yet little understood and most often taken for granted. Unless exposed to the building, machinery or automotive trades, the average person never realizes how much we depend on the welding process, which is a fundamental part of the process of building most of what we depend on daily, including vehicles, buildings, appliances, bridges and a great deal more. In fact, once you really start to examine the objects around us, it's hard to imagine our world without the welding process. Architecturally speaking, we might all be living in one-room wood or adobe-brick houses if it weren't for welding. Certainly all large commercial and residential structures are built with a considerable "skeleton" of welded structural steel, and even most singlefamily, wood-framed houses are built using some welded components, even down to items like the electrical outlet boxes in the walls. Anyone who has watched the construction progress of a major highway improvement like a bridge or tunnel has seen the helmeted weldors, unsung heroes of the construction process, spraying a shower of sparks from high on a scaffold while they join metals to hold critical loads. Sitting in an airport terminal recently with some time on our hands gave us something to think about. Virtually everything around us involved welding in some way. There was a large rack of telephone books in stainless-steel racks, each carefully TIG-welded and sanded, a post office box that was made of welded steel, the telephone stall had welded components, and the seats we were sitting on were part of a welded-steel structure that held eight seats.

Everywhere you look in the modern world, you'll find examples of how widespread and important is the use of welding techniques and equipment.

Welding had its ancient origins in the fires of blacksmiths, who could forge two white-hot pieces of metal together with hammer blows and patience. It remained for history to bring us to electricity and bottled gasses for welding techniques to develop any further.

Metal cutting in the old days was no easier. A white-hot piece of metal was laid over a hardy (like a wedge or chisel point) in the big anvil, and struck hard with a hammer, chopping the softened piece off. Final shaping of most items was by hand with stones and files. The farrier of today works out of a truck, filled with equipment like a grinder, drill press and welder, all running on AC power when available; otherwise, horseshoes are shaped in a portable forge. Today's blacksmith has the advantage of tools like a MIG welder that speed up making special horseshoes.

Definitions of welding

At one time, the simple definition of welding was "joining metals through heating them to a molten state and fusing them together." As technical progress in welding processes has advanced, the definition has had to change. The term "welding process" means heating metal parts to a temperature high enough to join the metal parts by coalescence. Welding is done with or without the use of pressure, by the pressure alone, and with or without the use of filler metal. Coalescence means the growing together, or growth into one body, of the base metal parts. There are two basic requirements for coalescence: heat and intimacy of contact.

There are now two basic forms of welding: fusion and non-fusion. The former is the most common, and it involves the actual melting of the parent metals being joined. Not all welding today involves melting. Non-fusion welding is most commonly represented by soldering and brazing, two processes of joining metals where the parent metal is heated, but not melted, and a second or "filler" metal is melted between them, forming a strong bond when all are cooled. Pressure and friction alone can weld metals together, such as when a machinist turns down a piece of metal in a lathe. Often, pieces of the metal chips can become welded to the cutting tool, which is a simple example of a process that can be used in production work in joining metals. Other kinds of "cold" welding may today involve sound and light, as in sonic-welding or laserwelding.

Today, the term "welding" has even been applied to the processes of joining non-metallic materials, such as plastic-welding which sometimes involves a fusion of materials as a result of heat or chemical action.

How it works

The most basic principle of the welding process is joining two pieces of metal together (or at least two edges of the same piece, in the case of repairing a crack). This is generally accomplished by heating the metals to be joined until they become liquid or molten and the two edges fuse together. Most often, the complete joining of the two metal edges is accomplished by melting new metal into the joint at the same time. The new metal added to form a fused welding joint is called filler metal, while the original pieces being joined are called the parent metal. Together they form a welded "bead" of filler and parent metal that is usually thicker than the parent metal. Depending on the skill of the weldor and the type of welding, two pieces of metal can be joined in such a way that with a little filing or sanding of the bead, the joint is virtually undetectable, a particularly important aspect when making automotive body repairs.

The welding processes differ depending on the source of heat, the manner in which the heat is applied or generated, and the intensity of the heat. The source of heat may be the combustion of a fuel gas such as acetylene or hydrogen, in air or in oxygen; an electric arc; an electric, gas, or oil furnace; the resistance of metal to the flow of electric current; or a chemical reaction between a metal oxide and finely divided aluminum. The intensity of heat applied or generated at the joint varies according to the welding process used. All welding, processes except brazing use temperatures high enough to melt the base metals. It takes a tremendous amount of localized heat to weld metals together, and heat control is the key to welding properly. Every material has its own specific melting point, and to make a weld you need to heat the material to that point but not beyond it. Visualize an ice cube, which is solid material (when cold). If you heat it to the melting point (above 32 degrees F), the solid becomes a liquid (water), heating it further will vaporize it into steam and for your purposes the material is gone. The same changes happen to metal, although at much higher temperatures.

The heat required to make metal molten enough to fusion-weld can be achieved in several ways, but the most common for home/shop situations will be generated either with a flame or some use of electrical current. The traditional source in welding has been the oxy-acetylene torch, while electricity is now used in most of the other methods, such as arc-welding, MIG-welding, and TIGwelding.

Brazing, the only welding process in which the melting of the base metal is not necessary for coalescence, is similar to soldering, except that higher temperatures are used. The term soldering is used to describe a joining process using nonferrous filler alloys melting below 800°F (427°C).

Soldering is not considered a welding process. Brazing is a welding process using nonferrous filler alloys that have a melting point above 800°F (427°C) but below that of the base metal.

The second basic requirement for coalescence, intimacy of contact, may be divided into two groups: pressure processes and nonpressure processes. In pressure processes, intimacy of contact is achieved by applying pressure while the contact surfaces are at a high enough temperature to allow plastic flow of the metal. In nonpressure processes, a space remains between the surfaces to be joined. This space is then filled, either rogressively or all at once, with molten metal. The molten metal may be obtained from a filler metal (welding rod or electrode) by melting the surfaces to be joined, or by combining a filler metal and melted base metal. Common lead solder such as you might use to solder electrical connections can melt at temperatures from 250-750° F (depending on the alloy), aluminum melts at just below 1250° F, and common mild steel melts at 2750° F.

All nonpressure processes involve fusion, and they are often referred to as fusion processes. However, this term is somewhat misleading since some pressure processes also involve fusion. The various welding processes differ not only in the way coalescence is achieved, but also in their ability to produce a satisfactory joint in a given kind of metal under the conditions in which the weld must be made. Many factors influence the selection of a welding process for a particular application. These factors include the relative cost, the amount of welding required, the location and position of welds, the service conditions the welded structure must withstand, and the qualifications of the person who does the welding. Probably the most important single factor, however, is the weldability of the metal.

Weldability. The term weldability means the capacity of a metal to be fabricated by a welding process into a structure that will perform its purpose satisfactorily. Weldability also means the degree of simplicity or complexity of the procedures and techniques necessary to produce

welds with properties that are equal to or better than the properties of the base material. For example, mild steel can be welded by most welding processes, but the welds produced are not equally satisfactory, and one method may be more complicated or more expensive than another.

While it is possible to weld mild steel through the use of a variety of welding processes, some metals such as aluminum and its alloys can be welded satisfactorily through only a few welding processes. Mild steel does not require elaborate preparations, fluxes, and special techniques because its characteristics are such that the welding operation can be easily performed. Other metals require special preparatory steps, complex welding sequences, skillful use of a specific welding technique, and extensive heat treatments after welding.

Many factors influence the weldability of a metal. Some important ones that must be taken into account and, so far as possible, controlled are:

• (1) the chemical composition of the metals involved (that is, the kind and percentage of elements present) and the effect of radical temperature changes on the various elements;
• (2) the expansion and contraction characteristics of the base metals;
• (3) the filler metal (welding rod or electrode);
• (4) the joint design; and
• (5) the welding procedure.

In steel, carbon is probably the most important element that limits weldability. Carbon gives steel hardenability; that is, when certain carbon steels are heated above a critical temperature and then cooled rapidly, they become much harder. At the same time, they lose ductility. In fact, the metal may become extremely brittle. With few exceptions, the temperatures used in welding exceed the critical temperature of carbon steels. Further, more hardening may occur when the mass of relatively cold metal surrounding the weld area conducts heat away so fast that rapid cooling occurs. Thus, certain steels may become hardened by many of the welding processes.

When the percentage of carbon is less than 0.25 percent, its effect in producing hardness is slight. But when the carbon content exceeds 0.25 percent, or when such elements as manganese, vanadium, chromium, molybdenum, or titanium are present, together with a carbon percentage of less than 0.25 percent, the weldability of the steel is decreased. Special steps should be taken to control preheat, interpass temperature, postheat, and welding sequence. Otherwise a satisfactory weld is likely to crack and to have reduced toughness and less strength than is required. For this reason, tool steels and certain alloys like carbon-molybdenum steel are less weldable than many other steels.

Steels contain certain impurities such as sulfur, phosphorus, hydrogen, nitrogen, and oxygen. If present in large enough quantities, these impurities may decrease weldability. For example, a steel to which about 0.10 percent sulfur has been added to improve machineability is difficult to weld because the weld has a tendency to crack. An excessive amount of phosphorus decreases the ductility of the steel and thus decreases the weldability of the metal. The presence of hydrogen in a steel, filler material, or flux may lead to cracks in the welds.

Stainless steels, high-chromium steels, and other special steels are less weldable than plain low- carbon steels. The elements that give these special steels their desirable properties for specific applications also have the effect of decreasing the weldability of the metals. To make these special steels weldable, the welding procedures, the filler metal, the fluxes, the preheat and postheat temperatures, and the welding sequence must be carefully selected. This is also true for many nickel, copper, and aluminum alloys. In some metals, the heat of the welding process may cause certain elements with low-melting points to vaporize, thus reducing the amounts of those elements present in the weld zone.

Nonferrous alloys containing lead, zinc, and tin are particularly subject to such looses from vaporization. These losses may seriously affect the properties of the joint by causing porosity or oxide inclusions that weaken the weld.

The weldability of a metal is also affected by its thermal conductivity. In general, metals with high thermal conductivity are difficult to weld because they transfer the heat away from the weld so rapidly that the required temperature cannot be maintained at the joint.

Changes in temperature cause a metal to expand or contract and this also affects weldability. When metals expand and contract at different rates, the internal stresses set up by these changes can cause the joint to crack immediately, or to crack later under load. Even when the weld joins identical metals, or metals having approximately the same coefficient of expansion, the expansion and contraction may not be uniform throughout all parts of the metal. These differences lead to internal stresses, distortion, and warping. Metal parts must be free to move or a special weld sequence must be used. When heat is applied or withdrawn, expansion and contraction set up high stresses, which may cause trouble in the weld itself or in the adjacent base metal. In thin materials, uneven expansion and contraction may cause the metal to warp. In heavy material, the stresses set up may exceed the ultimate strength of the metal and cause cracking to occur in the weld, or in the metal next to the weld which is called the heat-affected zone.

Even if the ultimate strength of the material is not exceeded by the stresses developed during welding, the combination of welding stresses plus the stresses developed when the material is placed in service may-be enough to cause failure of-the weld. It is for this reason that many materials are stress-relieved after welding.

Another factor that influences weldability is the filler material used. The wrong electrode or an incorrect welding process will make welding difficult or impossible, and it may lead to failure of the part under service conditions. It is not always essential that the welding rod or electrode be of the same chemical composition as the base metal; the important requirement is that the combination of the filler metal and the base metal will make a satisfactory welded joint.

One thing that is common to all the forms of welding is that the filler material must be compatible with the parent metal, and all efforts must be made to make a "clean" weld free of outside contaminants that could weaken the joint. If you are welding aluminum, the filler rod must be aluminum, a stainless filler rod must be used for welding stainless-steel and steel rods are used on steel. In gas welding, the cleanliness of the weld is controlled by the correct adjustment of the torch flame and the cleanliness of the two edges of the parent metal. In electric welding, an inert gas "cloud" is formed right around the welding area that keeps outside oxygen or impurities from contaminating the weld. The shielding gas is generated in several ways, as you'll see as we further describe the various types of equipment.

In some processes, the flux selected for use with a welding rod has important effects on weldability. Also, the electrode covering influences the weld obtained in certain steels. Molten steels have a tendency to absorb hydrogen from the surrounding atmosphere and to expel it when they solidify. Some types of electrode coverings send a lot of hydrogen into the atmosphere surrounding the arc and the molten puddle. This hydrogen is enough to cause microscopic cracks in the heat-affected zone of some steels. To eliminate this problem, low-hydrogen electrodes have been developed to weld the newer high-tensile steels.

Joint design also influences the weldability of a metal. Several factors must be considered when selecting a joint design. They include the welding process, the thickness of the material to be welded, and the purpose the joint is to serve.

Thin sheets of metal can be butted together without special preparation other than cleaning; but heavy plates must be beveled or grooved to make a satisfactory joint. Again, the design used is related to the purpose; that is, the way the load or stress is applied, the erosive or corrosive conditions it must resist, and the joint efficiency. The term "joint efficiency" is used to indicate the strength of a welded joint as compared with the strength of the unwelded base metal.

Each of the welding processes has a technique or procedure peculiar to that process. Often the technique varies with the kind or size of the filler metal used, or the kind of weld being made.

The incorrect use of a technique, or the use of the wrong technique, may lead to defects that make the joint unsatisfactory.

Metal alloys

The melting point of the metal you work with will vary with the basic nature of the material (iron, steel, aluminum, magnesium, etc.), and the alloy of the metal. Most metals today are not in pure form, they are alloyed or mixed with another metal to give the new material special characteristics. Copper, lead and iron are basic pure metals that have been used by man for tools and other objects for thousands of years. Mixing various metals together can produce a new metal with new uses. Copper mixed with zinc will make brass, which has strength, reduced cost, and better suitability for machining and casting. The same base copper mixed with tin makes bronze, which was alloyed as far back as 2000 years to make weapons. Gold and silver, precious as they are in their pure state are seldom utilized in their natural form which is quite soft in comparison to other metals.

When alloyed with other metals which add strength or other characteristics, gold and silver can be used for jewelry, coins and many other uses. We commonly describe different gold objects by "carats." While pure gold is 24 carat, 12-carat gold is only half gold and half other metals, and the closer the caratnumber is to 24, the more gold is in the object. The other alloys reduce the expense of the pure gold and make it more durable and useful. Were rings and other jewelry to be made of 24-carat pure gold, they would be too soft and not last in normal use.

Types of welding

Most of the metals you will be working with in your welding will be of two kinds, ferrous and non-ferrous. The former includes metals that contain iron, most commonly steel. The most commonly-welded non-ferrous metal is aluminum.

Both steel and aluminum can vary considerably in the welding process depending on the alloy. By changing the alloy of either steel or aluminum, different properties can be obtained, to either make the metal more flexible (ability to bend without breaking), malleable (ability to be formed with a hammer), ductile (ability to be drawn out or hammered thin) or to improve its strength for a specific application. Steel is made from refined iron combined with carbon and other elements.

How much carbon is added determines the properties of the steel alloy.

Most of the steel we might use for projects is relatively low in carbon, called mild steel. With higher levels of carbon, you get medium-carbon steel (used for shafts and axles), high-carbon steel (used for automotive and industrial springs), while very-high-carbon steel is used to make files and sharp-edged cutting tools. The common mild steel we use most often is weldable by virtually all of the techniques available today, while the higher-carbon steels have special requirements.

Other elements commonly alloyed with steel are manganese, tungsten, nickel and chromium. The latter two combine with steel to make stainless-steel, a very useful material that requires somewhat different welding techniques. Anyone familiar with race-car and aircraft construction may have heard of 4130 chrome-moly steel, which is often used in these applications for its high strength relative to its weight. The four-digit number describes the alloy as containing molybdenum, and the amount of carbon. This alloy contains more carbon than mild steel, as well as chromium and molybdenum, both of which add properties of rust-resistance, strength and hardness. Even though this is a higher-carbon steel than mild steel, it really contains only 30/100th of 1% of carbon, which shows how scientific the alloying of metals really is. A tiny change in content can radically affect the properties of the final metal.

In steel and aluminum, not only are there different alloys, but different heattreat processes. In the simplest terms, heat-treating is a scientific process of heating a metal to a specific temperature and then cooling it, either slowly or quickly, and with or without oil. The heat-treating can affect the hardness and other characteristics of the metal. When aluminum is purchased new in sheets or tubes, it is generally marked with its alloy and heat-treat, such as 3003-T3, which is a sheet aluminum that is considered "half-hard" and is commonly used in making race-car bodywork, where it has to be somewhat strong, but also able to be bent, welded and hammered. On the other end of the spectrum, 7075-T6 aluminum is very hard and strong. Called "aerospace aluminum" in the vernacular, it is often used in making machined aluminum parts and applications where very high strength is required. While it is strong and hard, it doesn't bend.

For your purposes as a home weldor, just remember to find out what kind of metal you are welding, and when making a project ask a metals expert to recommend the most suitable material. In general, the higher the carbon content in steel, and the higher the heat-treat on aluminum, the stronger the material will be but tougher to form into a shape, and the tougher metal alloys can be more brittle. If you do decide to purchase a welder, you will eventually get to know the people at your local source for welding equipment, and they should be of considerable help in answering your questions and getting your setup working well.