Simply put, metalworking is anything you do with metal. Bend it, hammer it, weld it, or drill a hole in it it’s all metalwork. Welding, on the other hand, is more specific. Because metal melts when it is sufficiently heated, two or more pieces of metal can be fused together into one piece. This property of metals has dominated human history and enabled our modern technological world.
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For most of history, iron and its derivatives have been relatively rare and expensive. Many means have been used to mine and refine iron ore, but one of the most common was to simply drag a magnet through a muddy riverbed. Small bits of iron would stick to the magnet and could be easily separated from the surrounding earth. Then a blacksmith would heat the mass of iron fragments and begin to work it, eventually forming it into a single piece of metal.
As you might imagine, this method of mining for iron was timeand labor-intensive. A fine implement such as a sword took hundreds of hours to create. The biblical proverb about beating swords into plowshares reflects this truth: when iron is precious, you must recycle even your sword when you don’t need it any more.
Iron was rare and valuable enough to recycle through most of history, and steel has been even more valuable yet. Pure iron (correctly called wrought iron—the term has only lately come to mean railings and gates) is a comparatively soft and malleable substance compared to steel. Blacksmiths on several continents developed welding techniques to merge iron and steel for maximum benefit well before the common era. In order to make an axe or a sword, the smith would take a small quantity of precious steel and make the cutting edge of the tool, and then make the body of the tool out of plain iron. Then the hard steel edge would be joined to the body through a process known as forge welding. This process (in various forms) was separately developed in Scandinavia, the Middle East, India, China, and Japan. Products like Damascus steel and the legendary Japanese samurai swords are results of forge welding.
The process of forge welding is relatively straightforward. The pieces to be welded are heated in a forge using charcoal or coal. When they are white-hot and on the verge of melting, the smith dusts the parts with a flux material (charcoal ash or borax, typically) and hammers the parts together. The surfaces melt a little bit and weld the separate pieces into one. Before the advent of oxygen- acetylene and electric arc welding in the early 20th century, this is how all welding was accomplished.
These processes used by smiths back to the beginning of recorded history still inform the practice of metalworking today. With all of the exotic alloys available to the modern metalshop, the fundamental properties of ferrous metals have not changed. The hammer and anvil still have a place in the shop next to the TIG welder.
Blacksmiths created the first steel alloys simply by working the raw iron they had. When a smith works iron in a charcoal or coal fire, carbon from the burning fuel is introduced into the iron, making it harder. The amount of carbon put into (or taken out of) a piece of iron depends on the fuel being used and on the temperature to which the material is heated. By manipulating these simple variables, ancient and medieval smiths made tools and developed techniques that are still in use today. That’s enough of a history lesson— we’re lucky to have access to modern equipment and materials, and that’s what this book is about.
The Physical Structure of Steel
Think of a piece of iron or steel as a block of cheese with a melting point at about 2,500 degrees F. Heat that block of cheese and it will get soft and ultimately melt. When the cheese is soft, you can shape it into any form you want. When the cheese melts, you can merge it with other pieces of melted cheese and then let it cool. The result is a single, larger piece of cheese. That’s the basis of welding.
Iron and its alloys have a crystalline structure. You can’t see this structure except under a microscope, but it’s there. You can get a hint of it by looking at the end of a broken steel tool. When you heat up a piece of steel, that crystalline grain begins to melt and you can form the material easily. At a certain point, the crystals melt and become a thick liquid. When they solidify again, new crystals form. Some characteristics of steel result from the composition of the alloy. Add different amounts of carbon, and the hardness of the steel changes. Add in a little nickel, chromium, or other elements and you can make the steel rust-resistant and much harder yet.
But other characteristics of the steel happen because of the way it’s worked. You’ve probably heard of quenching and annealing and relieving (or normalizing) a piece of steel. Those terms all apply to the speed with which you cause the crystal structure of the metal to form. A white-hot piece of steel that is quenched (dunked) into cold water or oil cools quickly and forms many small crystals. This promotes hardness for sharp edges, but the resulting steel is brittle, like glass. On the other hand, if you take a piece of white-hot steel and put it into an oven so it can slowly cool (or anneal) over a period of hours, the resulting steel is made of longer, larger crystals. This promotes toughness—the ability to withstand shocks and bending without breaking. Toughness is a tradeoff against hardness.
Finally, normalizing or relieving may need to take place when you have welded two pieces together without heating all the metal to a soft state. As the welds are heated and then cool, stresses are placed on the surrounding metal that can pull it out of shape. So, you can place an entire piece into an oven and heat it until all the metal relaxes enough to normalize to its new configuration. Then the piece is cooled and the grain forms to hold the new shape. These are the central facts that govern everything we do with steel. We can bend it, pound it, melt it, and cool it to make things we want. The concepts are simple, but getting the execution right takes practice. It’s not magic and almost anyone can learn to do it.
There are a variety of materials and techniques used in automotive metalworking. The materials are collectively called alloys. An alloy is simply a mixture of metallic and other elements. The nature and interaction of the various elements in the mixture gives the alloy its specific attributes and characteristics. For example, the presence of a small amount of carbon in iron makes the material harder than pure iron, and we call the alloy steel. The presence of chromium in addition to carbon in iron makes the material more resistant to rust and corrosion, and we call the alloy stainless steel. There are hundreds of alloys in use, but relatively few are common in automotive work.
The techniques used to work the alloys commonly found in automotive applications are variations on the themes of pouring molten metals into shapes, beating or pressing hot or cold metal into shape, cutting and grinding to size, and melting pieces of metal together to join multiple pieces together. Respectively, these four fundamental techniques are called casting, forging, machining, and welding.
This chapter offers some details on the alloys and the processes used to fabricate these metals for automotive use.
Commonly Used Automotive Alloys
The following metals are frequently used in automotive construction and in automotive projects. These metals are commonly available in a variety of forms such as round tube, box tube, square and flat stock, and in sheets of different thickness.
The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have developed a standard for classifying steel alloys. The AISI-SAE system uses a numeric series to quickly identify commercially produced alloys. The full listing of alloys is very long, but what you need to know for automotive projects is that carbon steel means that the alloy contains up to about 2 percent carbon. Stainless steels contain about 10 percent chromium. Only a few alloys are inexpensive and readily available for automotive projects.
The most common steel used for automotive projects is called mild steel. Mild steel generally has about .1 percent to .2 percent carbon. Carbon steels are given four-digit numeric designations beginning with 1. For example, basic cold-rolled mild steels are commonly available in 1018 or 1020, which have .18 percent and .20 percent carbon content, respectively. There are small amounts of other elements such as silicon and manganese as well.
The American Society for Testing and Materials (ASTM) has a mild steel standard for hot-rolled steel called A36, which is .26 percent carbon and small amounts of other elements. Hot-rolled steel generally has more surface scale present than coldrolled, and should be brushed clean before welding.
Almost all automotive projects are made using some grade of mild steel. This is the universal material for good results. It’s easy to work with, readily available anywhere, and economically priced.
Tool steel is a general term for the higher-grade steel alloys known for hardness and toughness. The name is applied because tool applications require a higher grade of steel than basic structural work. Anyone who has ever watched a cheap wrench gouge or bend on a grade 8 bolt head understands the value of good tool steel. Tool steel has between 1 percent and 1.5 percent carbon, plus traces of other elements, primarily manganese. Chrome vanadium (AISI 6100 series) falls into this category.
There is a special application in the automotive world for spring steel. Springs are made of steel with about 1.5 percent to 2 percent silicon, about 1 percent manganese, about .5 percent carbon and up to .5 percent chromium. The AISI-SAE code for most automotive spring steels is 9255. Anti-sway bars are frequently made of spring steel.
But to make steel into a spring requires more than just the proper alloy. Springs are precisely heattreated to obtain the desired shape retention properties. If you heat a spring to red-hot to make it malleable, you will change those properties— usually a heat cycle makes the metal less springy. You may have heard that you can heat up a spring and quench it in water or oil, or even by blowing compressed air on it, but the fact is that the amateur simply cannot get the precise timing and conditions necessary to make a good, consistent, reliable spring. However, if you decide to make your own custom tools such as chisels or drifts, old coil and leaf springs make good starting material.
Stainless steel has a 3-digit identifier from 200 to 665 for its alloys. To be considered stainless, a steel must generally have at least 10 percent chromium content, and less than .15 percent carbon content. Additionally, stainless steels include other elements such as nickel, phosphorus, sulphur, silicon, and manganese. T-304 stainless steel is the most common formulation used for automotive purposes such as exhaust pipes. Unlike some stainless alloys, T-304 is easily weldable, machinable, and offers good rust resistance. However, T-304 is not recommended for saltwater use. T-316 is a popular saltwater marine-grade alloy.
Another material commonly used for automotive projects is 4130 chromium-molybdenum alloy. This alloy contains between .50 percent and .95 percent chromium and .12 percent to .30 percent of molybdenum, as well as traces of other elements. Chrome-moly is often selected for roll structures because sanctioning bodies allow thinner chrome-moly tubes than the same cage design built from mild steel, and the chromemoly cage is therefore lighter.
Cast iron has been popular for decades in the manufacture of engine blocks, manifolds, older transmission bodies, suspension components, and many other auto parts. Repairing broken cast parts is one of the most common automotive welding tasks. Cast iron contains 2 to 4 percent carbon and 1 to 3 percent silicon. Because most hobbyists do not have access to the equipment necessary to make iron castings, automotive castiron work is generally limited to repair welding.
The term wrought iron is generally taken to refer to decorative railings and fences, but it has a specific meaning when referring to iron alloys. Wrought iron is almost pure iron, with some traces of impurities such as carbon, silicon, and other elements. Wrought iron used to be readily available, and very old brass era antique cars with wooden frames may have some wrought iron fittings. Wrought iron can be welded easily, but be aware that it becomes soft at much lower temperatures than steel.
Aluminum is a metal element extracted from bauxite ore. It’s two thirds lighter by volume than steel, but also less strong than steel. But if you use sheet aluminum that is 50 percent thicker than steel, it will be about 50 percent of the weight of steel and give you the same strength. For example, .090 steel and .125 aluminum have about the same strength, but a .125 aluminum sheet weighs less than half as much as a .090 steel sheet of the same area.
In situations where maximum strength is not needed, aluminum makes a great choice for custom automotive work. You can machine, weld, and braze aluminum, but it’s tricky to bend if it’s thicker than 10 gauge sheetmetal, and aluminum is virtually impossible to forge by hand.
Aluminum alloys are defined according to a series of numbers that specify the additional elements in the alloy. Alloys named with a number between 1000 and 1999 are almost pure aluminum, with just a little bit of a few other elements. Alloys in the 2000-series contain more copper, and have gone by the trade name Duralumin. The 3000- series aluminums are mixed with more manganese and 6000-series metals are mixed with magnesium and other elements for strength.
The 4000-series aluminum alloys are often used in the manufacture of pistons because these alloys contain more silicon. The silicon content helps the metal flow easily into castings. Pistons advertised as hypereutectic have been made with a high-silicon aluminum alloy and a special casting process. Some of the common alloys used in automotive work are shown in the table below.
Automotive Metalworking Techniques
Every metalworking technique is a variation on one of the following processes. If the metal is hot enough to flow into a mold, it’s casting. If the metal is being beaten or pressed into shape, it’s forging, which can be done with hot or cold metal. If part of the metal is being heated to the melting point to join otherwise solid pieces together, it’s welding. And if you’re cutting or grinding metal into shape, it’s machining.
Casting is not typically a process that an amateur or even a professional welding and metalworking shop can perform. The tremendous energies and specialized skills and equipment required to run an industrial foundry mean that making your own cast-iron or steel parts is impractical. However, small-scale sandcasting of aluminum and other metals can be performed, and combined with machining can produce high-quality custom parts for unstressed applications such as trim pieces, shift knobs, and support braces.
Hot forging takes in a number of related techniques. The common factor is that the metal is heated to redhot or greater temperatures, which renders it malleable. The metal is then beaten or pressed into shape and hardens as it cools.
Traditional or hammer forging is done by hand, sometimes using a power hammer in a well-equipped shop, but the defining characteristic is that someone holds the metal and shapes it. This technique is often used in combination with welding to fit flat or bar steel into place. For example, this technique is used when fitting a mounting plate for a roll cage onto an uneven surface. The fabricator welds one edge into position, then heats the whole plate and beats it into shape against the backing structure before welding the remaining edges into place.
Drop forging involves taking a blank piece of heated metal and placing it in a pre-formed set of dies, and then striking the dies together to shape the piece. Excess metal is then trimmed off and the piece is finished with machining. This technique produces a stronger part than casting, so it is often used to create suspension components and other stressed parts.
Hot stamping or pressing is closely related to drop forging, except that it typically uses sheetmetal instead of billet steel, and the stamping/pressing process is more precise and gentle to the material than drop-forging. But the metal is still heated to make it soft and then stamped into shape. As the metal cools, it hardens into its new shape. This method is used to make everything from uni-body chassis to hoods, door skins, and any other stamped sheetmetal part on an automobile. Both sheet steel and aluminum may be hot-stamped.
Cold Bending, Stamping, Pressing and Forging
If you apply enough pressure, you can bend, stamp, or beat steel at room temperature. Performing the shaping while cold has a different effect on the internal structure of the metal than hot-forging, so this technique is not generally used on critical stressed parts such as engine or suspension components. But it is commonly used on many automotive fabrication projects, and many of the projects covered in this book use some form of cold-forming.
Cold-forming steel affects the crystalline structure of the metal—it literally breaks crystals and fibers at the bend. This is why you can tear steel if you bend it back and forth several times. This is called metal fatigue—and if you look at the ragged ends of a torn piece of metal, you can see where the tiny crystalline fibers have stretched and broken. Obviously, you want to avoid fatiguing the metal in any critical stressed part on your project car, but you have some leeway before a bent piece of metal becomes too fatigued to serve your purposes.
Cold-forming techniques include bending tube steel, brake-bending and bead-rolling sheetmetal, hammer- forming steel plate, and bending steel bars. In general, aluminum is not as bendy as iron-based alloys and fatigues sooner. Other materials have different characteristics. Copper is very malleable, but its alloy brass is not.
Welding refers to any technique whereby two or more pieces of metal are joined together by melting the edges into each other. Welding is also used to repair cracks in a single piece of metal. There are several varieties of welding, characterized generally by the equipment used to perform the weld. But whether you use oxyacetylene gas, an electric stick welder, or some form of wire-feed, MIG, or TIG welder, you’re using the machine to create localized heat sufficient to melt the metal being welded, and usually introducing some new metal into the molten area as filler.
The exception to all this is forge welding, which is an advanced blacksmith’s skill. In forge welding, the entire body of metal to be joined is heated to near-molten temperatures and joined by pressing the pieces together with hammer blows.
Machining Machining metal is fundamentally different from other metalworking techniques. Casting, welding, and hot forging all involve heating metal until it is molten or at least ductile and then shaping or joining the metal while it is hot. Cold working metal involves beating or bending it into shape by force. In machining, metal is simply cut away from an existing piece until the desired shape is obtained.
For our purposes, we use the term “machining” to cover all work that involves cutting, grinding, or otherwise removing metal from a piece.
But in the strictest terms, machining is performed on a mill or a lathe. A mill is similar to a large drill press with a very adaptable platform for the part. Milling bits fit into a chuck on the mill and perform a variety of different cuts. A lathe spins the piece of metal being worked, and is used to create perfectly round pieces with cutting bits.
The other common factor with a mill is that the work platform can be adjusted to within 1/1000 inch and moved in the same precise increments during the cutting process. This allows the machinist to cut a part to very precise standards.
Remember that machining can only remove existing material, it cannot be used to add metal to any piece.
Written by Russell Nyberg & Jeffery Zurschmeide and Posted with Permission of CarTechBooks