CHAPTER 3
Steel
What Is Steel?
For the beginning welder, steel is an ideal material (Figure 3.1). Steel allows you to weld, reweld, cut, bend, file, grind, drill, and more, all with fairly basic tools. It is strong, readily available, inexpensive, and relatively forgiving when welding and manipulating. For these reasons, it is used in many structural and artistic applications today from demanding feats of engineering such as the Golden Gate Bridge to someone’s backyard garden art.
FIGURE 3.1 Examples of steel.
To understand steel, you must first take a look at its main ingredient—iron. By mass, iron is the most common element forming our planet, mostly owing to its high presence in the earth’s outer crust and inner core. It can be found on and close to the earth’s surface is chunks called ore or by mining. It has been used for thousands of years because of its availability and relatively easy workability through forging. Naturally occurring, iron is fairly soft, although not as soft as another common metallic element—copper. It was discovered that by adding carbon to raw iron, it would get significantly harder and therefore more useful for making tools and weapons. The addition of carbon actually was a by-product of early forms of smelting, where carbon-rich charcoal was burned as the fuel source, thereby contributing to the molecular makeup of the end product. When carbon is combined with iron in amounts between 0.2 and up to 2 percent, the result is steel, which can be up to a thousand times harder and stronger than pure iron. However, most steel contains less than 0.35 percent carbon. An advantage to steels with between 0.35 and 1.86 percent carbon content is that they can be hardened using a technique called quenching, where the material is heated and cooled in a particular way. The type of steel you are going to use in this book is a type of plain-carbon steel (<1.50 percent carbon) called low-carbon steel or mild steel.
Plain-Carbon Steels
Low
Low-carbon steel is the most commonly used type, and it has a carbon content of less than 0.30 percent. Low-carbon steels have relatively easy workability, meaning that they are ductile, can be machined, and weld very well. Note that because of the carbon content, low-carbon steel cannot be hardened by quenching. In fact, rapidly cooling mild steel can cause unwanted stress in the material.
Medium
Medium-carbon steels have a carbon content of between 0.30 and 0.45 percent. While this type of steel is harder than low-carbon steel, it also is more difficult to work with.
High
A carbon content of between 0.45 and 0.75 percent yields high-carbon steel. Even more difficult to work with, many of these types of steel require careful preheating or postheating to weld successfully.
Very High
With up to 1.5 percent carbon content, very high-carbon steels are used primarily for heavy-duty springs and metal-cutting tools. Similar to the workability of highcarbon steels, very high-carbon steels require special heat treating in order to weld.
NOTE: Steel with a carbon content below 0.30 percent is called low-alloy steel, and steel with a carbon content between 1.5 and around 2.0 percent is called high-alloy steel.
When naming types of steels, standards from the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) are used. Table 3.1 shows a portion of the AISI table for the classification of steels.
TABLE 3.1 AISI Carbon-Steel Designations
How to Use Table 3.1
1. The first digit indicates the type of steel (1 for plain carbon).
2. The second digit indicates the alloying elements.
3. The last two digits indicate the average carbon content.
Examples
• AISI/SAE No. 1020: Plain carbon steel, no alloying elements, average 0.20 percent carbon content
• AISI/SAE No. 1518: Plain carbon steel, containing alloying elements (manganese, phosphorus, and sulfur), 0.18 percent carbon content
Of course, mild steel can’t do everything. It can be quite heavy and will need some sort of finish to prevent rust from forming on the surface and eventually eating away the material. Now let’s take a look at some common steel alloys that you may come across or be interested in working with for future projects.
Alloys
Although mild steel may be one of the most common types of steel you encounter, other alloys exist around us in daily life. Stainless steel resists corrosion and is used in kitchens for food preparation and many marine applications (AISI 2XX–5XX). Chromoly, also known as cromo or cro-moly, is a steel alloy containing chromium and molybdenum that has an excellent strength-to-weight ratio, is easily welded, and is stronger than mild steel (AISI 41XX). Chromoly is used frequently for bicycle frames, high-performance automotive roll cages, and other structural tubing.
When steel is combined with other elements, you can make many more steel alloys. Depending on which element or elements are used and how much of each is alloyed, the final physical properties can change drastically. Strength, brittleness, cost, weight, abrasion, elasticity, thermal conductivity, temperature expansion, dimensional stability, toughness, and corrosion resistance are all properties for which there are specific alloys that can be made to fit a particular application. These alloys are also measured by AISI and SAE.
The way in which alloys can be worked or welded also changes with alloy type. Spring steel (AISI 1074), which is used for car springs, antennas, and some rulers, contains 0.74 percent carbon, along with sulfur, manganese, and phosphorus, and these constituents allow the material to return to its original shape if bent. However, when heated and bent, this alloy is very likely to crack or tear, meaning that forging and welding must be done with special heat control and filler material.
Mild-Steel Properties
Strength
Mild steel is very strong when used and welded properly, which is why it is used so frequently today. Like the iron it is made from, it is extremely uniform in makeup, allowing you to engineer with steel and predict how it will perform in demanding applications. This is why a weld, if not performed properly, can cause a carefully engineered structure to fail. For the projects in this book, you’ll generally be overbuilding, meaning that while your coffee table is designed to hold some books, a potted plant, and a few cups of coffee, more than likely you could park a small car or stand an elephant on it without it failing. Overbuilding or overengineering your projects gives you as a beginning welder more leeway when practicing welds.
Mild steel has a good, but not great, strength-to-weight ratio. What it lacks with that ratio, however, it makes up by being very easy to work with and affordable. Fancier steel alloys, while sometimes lighter and stronger, usually cost more and involve more difficult welding and forming processes.
Conductivity
Part of the metal–inert gas (MIG) welding process depends on the ability of steel to conduct an electric current. While clean steel accepts and carries a charge very well, any inhibitor such as rust or paint will seriously inhibit that characteristic.
Expansion and Contraction
Like most solids, when steel is heated, it expands, and when it is cooled, it contracts. Many factors determine the extent of expansion and contraction, so you can pretty much bet that after any workpiece is significantly heated and cooled, it’s going to bend or warp to some degree. Sometimes this is hardly enough to notice, whereas other times it can throw a whole project out of whack. For example, when the first weld is formed between two pieces of steel, the weld area gets heated very rapidly. While this is happening, the two pieces are still noncontinuous until the weld bridges the gap. Now, when the weld is completed and the weld area cools, it’s going to want to contract, but this time as one whole unit. This contraction will always cause some movement to occur over the length of the workpiece.
This effect is more noticeable on thinner materials, especially sheet metal. The heat affects a greater percentage of the material, causing greater warping. In projects later in this book, you’ll learn ways to minimize warping and fix it when it occurs.
Corrosion and Rust
As I mentioned earlier, mild steel is susceptible to corrosion. Specifically, when iron and oxygen react in the presence of moisture, rust is formed. This reaction starts on the surface of the material, and because rust is permeable, it will continue to react and eat away at the underlying steel. Over time, this will cause pitting and flaking to occur, eventually leading to serious loss of material. This is different from how copper oxide, that lovely green color on the Statue of Liberty, actually protects the copper below. Rust can form on steel in hours or years depending on the environment.
Rust inhibits the electrical connection required for MIG welding. Before welding a joint, any rust should be removed from the weld area, as well as an area where the grounding clamp can be attached. There are a few easy methods of rust removal that will be covered shortly in this book. If the rust is light surface rust, you may be able to expose fresh steel using only a stiff wire brush or sandpaper. If the material is pitted or irregularly shaped, sandblasting or a wire wheel on an angle grinder may be your best bet. For large and/or pitted areas, chemical rust removers such as phosphoric acid or organic acids do the heavy lifting for you.
When steel rusts, it forms a reddish orange color. Many artists and gardeners choose to use this color as the final finish of a piece of art. If this is your desired effect, you can leave your work outside after welding or speed up the rusting process by lightly spraying the work with saltwater.
Mill Scale
Steel that’s been prepared at a mill through a hot-rolling process (more on that later) will come with a thin layer of mill scale on its surface, like that on the left side of the steel in Figure 3.2. This scale is formed when working steel above 2,000°F. Scale poses a couple of problems for the welding and finishing of steel. It blocks the flow of electricity from penetrating into the steel, and rather than forming a solid weld, you’ll just be welding to a thin sheet on the surface of the workpiece. Any force to this weld will break the scale right off, causing weld failure. Scale should be removed from all areas to be welded, and any loose and flakey scale should be removed from the whole piece prior to finishing so that it doesn’t contaminate your weld. Scale can start to flake off of new steel relatively quickly and, for this reason, can cause problems if finish is applied over it.
FIGURE 3.2 Mill scale on new steel.
Finishing
To prevent rust on a completed steel project, it must be finished. There are many great ways to finish steel, yet another reason it is such a versatile building material. Prior to finishing, however, and as mentioned earlier, any rust, loose debris, and all surface scale should be cleaned off thoroughly. You’ll learn more specific finishing techniques in Chapter 12.
How to Obtain Steel
Clearly, in order to weld, you’re going to need some steel. There are two principal ways to get it—buy it or salvage it. Buying steel is great. You can get all kinds of shapes and sizes in unlimited amounts, but at over 40 cents a pound, the cost can add up for a project. In today’s world, steel is so common that with a little searching, you can find scrap steel that is cheap or even free. You may be familiar with found-object welded art, which is composed entirely of exactly that—found steel. The two main disadvantages to using found or salvaged steel in your projects are that the piece you found and need more of may be unavailable, and usually salvaged steel requires more cleanup and prep before you are ready to weld it. In the remainder of this section, you’re going to learn how to contact a steel yard and place an order for new steel and how to safely find salvaged steel such as from a scrap recycling dumpster shown in Figure 3.3.
FIGURE 3.3 Steel-scrap dumpster (don’t climb in there!).
Buying Steel
Steel is available from all kinds of hardware and supply stores. The mom-and-pop hardware store nearest you may have short lengths of steel stock, tube, and sheet and most likely will have some steel pipe and fittings for sale. When in a pinch or just browsing for interesting material, such stores can be good places to go. Big-box hardware stores also stock some lengths of steel, usually a slightly larger assortment than small hardware stores but not in full lengths.
The best place to buy steel is from a steel yard (also called a steel supplier) or a scrap yard. A Google search for any of those terms or a look through the Yellow Pages for “steel supply” should return some local results. Also, check the resources at the end of this book to find if any steel suppliers near you have existing relationships with artists. Two of my favorite metal suppliers near Providence, RI, are Mid City Steel and J. Broomfield & Sons Scrap Metal. The men and women at Mid City Steel are always there to help, and they deliver a great product. They seem to understand the needs of artists better than anyone else in the area and typically offer a little more patience and advice. Broomfield has a pretty eclectic collection of salvaged goods, including chain, bumper cars, drum brakes, radiators, and much more.
Steel suppliers generally sell to buyers or construction projects in great quantities and deal with professionals. For artists who deal with a steel supplier, there can be some culture shock. Here are some tips when contacting a steel provider:
• Respect the provider’s time. Explain yourself during your first phone call. Explain that you are a new metalworker and/or welder and are interested in buying steel. You can ask if the provider has a showroom where you can come browse or pick up materials. Maybe the provider has someone in-house who has experience selling to artists. Do your research before calling by reading this chapter and making a list of materials to purchase. If you still have questions for the provider, explain your project, and the provider probably will be able to offer advice.
• Not everybody is going to be eager to help. Many steel suppliers do hundreds of orders a day, and you may get someone on the phone who isn’t interested in selling such a small quantity. Don’t give up on your first try!
• Remember that with most materials, you are responsible for buying the whole length. If you need 2 ft of square tubing, you’re just going to annoy a steel provider if you call and ask for that. Square tubing comes in lengths of around 24 ft, so you’d be responsible for purchasing the whole thing. Typically, plate steel can be cut down to order, and the provider may have already cut sections ready to buy.
• When ordering steel from a steel supplier, you’ll have to consider if you want it delivered or if you have the ability to pick it up. Explain your situation to the salesperson, and he or she can give you advice. Keep in mind there will be a delivery charge, so if you can wait and put in a big order of your own or join your order with that of with friends, you can save money. Having steel delivered is not like a UPS delivery; you need to be there to meet the delivery vehicle, and usually you must help to unload the material. Don’t stand the delivery truck up!
• If you visit a scrap or steel yard, dress appropriately, just like you would when doing metalwork. If the steel or scrap yard allows visitors to view the materials or roam around, the supplier should provide you with a hardhat and have you sign a waiver.
• If you start to build a relationship with a steel provider, some homemade cookies or a six-pack will go a long way!
In general, I try to shop locally and avoid ordering steel online or from big-box stores and would encourage you to do the same. Although some of the characters you find in steel and scrap yards can be a little intimidating at first, I’ve found that many times these people are extremely helpful and are a great resource to have on your side.
Shapes
Steel comes in many shapes and sizes, so when you are shopping for steel, you’ll need to know which shapes and dimensions are available. When choosing the proper shape, or section, for your project, there are many things to keep in mind. In general, solid sections will be heavier by the foot but are more forgiving during welding and are easier to bend. Hollow sections will have a greater strength-to-weight ratio and more torsional stiffness, but with a thin wall size, they may be tricky to weld and will crimp when bending. Sections of channel steel, angle steel, or I-beam are specifically designed to take loads in certain directions but not all of them. For example, they resist bending and shear forces but don’t offer much torsional rigidity. Knowing what dimensions all these sections come in will make designing your project much easier.
Some terms you’ll hear when talking about new steel are sheet, tube, plate, stock, beams, angle, channel, flat, bar, pipe, and rounds. What do they all mean? Take a look at Figure 3.4 to see some common shapes.
FIGURE 3.4 Common steel profiles.
Square Stock. Square stock or square bar is a solid square section of steel that is commonly available starting at ¼ in per side and goes up to at least 3 in square. Smaller sections will be easy to bend by hand, and square stock is a great all-purpose fabricating material. Usually this is available at suppliers in 20-ft lengths.
Square Tube. Square tube is available starting at around a ½ in square and goes up to 12 in or more in various increments. Keep in mind that on top of specifying the dimensions of the square you want, you also must specify what wall thickness you are looking for. The wall thickness is the thickness of the steel that makes up the tube and will be given in gauge or inches. For a gauge chart, see Table 3.3. Square tube can be slightly more difficult to weld than square stock because of the wall thickness, but it has excellent strength-to-weight characteristics, and typically it will be available in 24-ft lengths.
Flat Bar. Flat bar, flat stock, flats, or rectangular bar is a rectangular section of steel that is available in various widths and heights and will come from the steel yard in 20-ft lengths. It is great for bracing or decorative work and is quite strong when force is applied parallel to its long edge.
Rectangular Tube. Rectangular tube, sometimes called box tube, is essentially the tube version of flat bar. Just as with square tube, when ordering, you must specify what wall thickness you require. Rectangular tube also has great strength-to-weight characteristics and is used for a wide variety of applications from racecar chassis to structural building components. Usually it comes in 24-ft lengths.
Round Stock. Round stock or rounds are solid round sections that come in 20-ft lengths. They are great for hand-bending decorative work, stakes, and struts.
Round Tube. Round tube—not pipe—refers to steel that is measured to its actual outside diameter (OD). It is extremely useful for its strength-to-weight ratio and is available in sizes from ¼ in OD to 12 in OD and comes in many different wall thicknesses. Usually it comes in 24-ft lengths.
Steel pipe—yes, steel pipe—or black steel pipe is different from round tube by the way it is measured, but as long as you are buying black steel pipe and not iron pipe, plastic pipe, or copper pipe, it can be welded just the same. The way pipe is measured is by nominal pipe size (NPS). For pipe less than 14 in in diameter, this NPS number is actually different from the outside diameter. The other difference in measuring is how the wall thickness is designated. Rather than inches, wall thickness is indicated by a schedule number (SCH), which refers loosely to the pressure rating of the pipe. The higher the schedule, the thicker the pipe. For this reason, if you order ½-in NPS steel pipe, you’re actually going to get something with nearly a ⅞ in OD with an inside diameter that depends on the specified schedule. Table 3.2 provides some useful NPS sizes from ⅜ to 4 in.
TABLE 3.2 NPS Chart
Pipe measured this way may be used for structural applications or the transport of solids, liquids, and gases and can be made of steel, plastic, cast iron, copper, and other materials. Most of the time when you buy steel NPS pipe, it will come with threaded ends, which will allow you to make removable or temporary connections using fittings and even connect to materials other than steel (that use the NPS system). When purchasing pipe from a steel service center, you frequently can get it uncoated and with the ends unthreaded, all ready to be painted
Another advantage to using steel pipe is that you can find it at your local plumbing store, hardware store, or big-box shop. At most smaller hardware stores, you can buy pieces of this pipe in very easy-to-manage lengths, such as less than 6 ft, so you don’t need to own a pickup truck or roof rack on your car to bring it to your shop. A disadvantage to using black steel pipe is that it has an extra coating that you’ll have to remove before you can safely and effectively weld it.
Hexagon Stock. Hexagon stock or hexagon bar is not as readily available as some other types of stock, but many steel and scrap yards do have some in supply. As the name implies, it is a solid hexagonal section that usually comes in 20-ft lengths.
Channel Stock. The most common type of channel stock you’ll encounter is C channel, also called U channel, for its resemblance to a C or a U depending on which way it’s turned. It’s a solid section but with a hollow area between the legs. It’s very useful for structural applications, and the open channel makes a good area to hide an edge.
Angle Stock. There are two types of angle stock: equal leg, in which both sides of the angle have the same measurement, and unequal leg, in which one side is longer than the other. It comes in many sizes and can be a very strong and lightweight structural component. The open space between the legs also can be used to hide an exposed edge. It comes in 20-ft lengths but also may be supplied in lengths up to 40 ft.
Half-Round Stock. Half rounds are like a solid round section cut in half. They are used mostly for decoration or can be fit with rollers to make a track. They come in 20-ft lengths.
I-Beams. I-beams, H-beams, or just beams are structural-steel sections resembling an uppercase I that have a great load-bearing capacity. The top and bottom of the I are called flanges, and the piece connecting them is called the web. When specifying a section of beam, the dimensions you’ll need to know are the beam height (i.e., flange-to-flange distance), flange width, web thickness, flange thickness, and length. These dimensions greatly affect the structural capacity of the beam and require advance engineering to calculate correctly.
Sheet. Sheet steel, or just sheet metal, is steel formed into a sheet whose thickness is under ¼ in. The thickness of sheet steel is called its gauge. Gauges correspond inversely to their empirical equivalent and only go as thick as 3 (0.2391 in), roughly ¼ in. Steel thicker than this is called plate steel, and the empirical unit is used for its thickness. Thus the next thickest steel plate would be ¼ in thick. Table 3.3 lists decimal equivalents for sheet-steel gauges 30 through 3.
TABLE 3.3 Sheet Metal Gauge Chart
Sheets come as large as 6 by 10 ft, with larger sheets usually available on special request. Commonly, you’ll be able to choose from 3 by 5 ft or 4 by 8 ft. You most likely will be responsible for purchasing the whole sheet, but many suppliers will be able to cut it down to a more manageable size if you prefer. Check with your supplier about its policies relating to cutting down sheet and plate.
Sheet metal is also available in a number of variations called diamond plate, expanded sheet, and perforated sheet that can be fun to experiment with.
Plate. Plate steel is steel formed into sheets with thicknesses over ¼ in. Just like sheet steel, plate comes in large sizes, and the weight can really add up. As with sheet, most suppliers will be able to cut plate down to size using either a shear or torch. When I order sheet and plate, I frequently have it cut into a manageable size of 4 ft or less on a side. Plate may be available from ¼ in thick up and in sizes 8 ft wide and up to 20 ft long. If you are just looking for a small piece of plate, you can ask the steel supplier if it has any randoms or drops in a thickness range that you are looking for. The supplier may have smaller sections of plate already available that will work for you.
Figure 3.5 shows a size and weight chart available from my local steel supplier. Such charts are useful when planning projects because you can see what sizes are commonly available and calculate the total weight of any particular material needed.
FIGURE 3.5 Steel section chart.
Decorative and Ornamental Steel. Decorative and ornamental steel consists of pieces that have been formed by casting or forging to create an interesting detail. Readily available are such pieces as handrails, fence and gate parts, finials, balusters, rosettes, base plates, spheres, hemispheres, and decorative sheet metal. Many of these pieces would be very time-consuming to make and feature metalworking techniques not covered in this book. It is fun to browse one of these decorative metal catalogs to look for inspiration or a small piece that will make a larger project simpler to fabricate. When browsing and ordering decorative metalwork, if you plan to weld the parts, you must make sure that they are steel and not iron or some other material. It won’t be required that you purchase any decorative metals for this book, but for your next projects, have a look around and see what inspires you. Some steel suppliers stock types of ornamental steel. If you have a relationship with a steel supplier, ask what kinds of decorative pieces the supplier has. Some online options for purchasing or browsing decorative steel are as follows:
• Decorative Iron, www.decorativeiron.com
• Kings Architectural Metals, www.kingmetals.com
If looking for something very specific, you could contact a local blacksmith and see if he or she can forge a unique piece for you. Or look for a local blacksmithing course to make the piece yourself.
Hot-Rolled versus Cold-Rolled
Another option you’ll need to consider is whether you want your steel hot- or cold-rolled. When steel is formed, it is pushed through a series of rollers to “extrude” the hot metal to its desired size. Hot rolling involves rolling the steel to its final dimensions while it is hot enough to form mill scale (a hard, dark-gray or blue film that forms on the surface of the steel). Cold rolling involves bringing the steel to its final dimensions after allowing it to cool to below scaling temperature. The difference is that hot-rolled steel has less precise tolerances for the finished product than cold-rolled steel and is considerably less expensive. As I mentioned before, another disadvantage to hot-rolled steel is the mill scale, which should be removed before welding or finishing. For the projects in this book, you are going to be using hot-rolled steel, which usually you do not need to specify when ordering.
Finding Steel
As you learned earlier in this book, steel has been a commonly used structural material for many years. Currently, there is a good amount of scrap material generated that can be used for your artistic or sculptural expression. Like they always say, “One person’s junk is another person’s treasure.” Chances are that if you own a home, you’ve got some scrap metal kicking around in your basement or garage that just needs to be put to a good use. Drive around on a trash or recycling day in most parts of the country and you’ll surely see a number of steel scrap items out by the curb. Old bikes, a busted garden cart, car parts, or the sides from a washing machine all may be made out of steel and can be welded. With a little creativity and ingenuity, you can find interesting scrap metal right under your nose.
Scrap yards exist all over the country and usually have material set aside that customers can browse without getting in the way of daily operations. This material may be sold by weight or a fixed price and is usually cheaper than new steel. Another good place to browse are scrap dumpsters at construction or industrial sites. Of course, you must ask the property manager or owner if he or she minds you poking around a bit and never get inside a scrap metal container.
The drawback to using found material is that you first have to figure out what it really is made of, what it’s covered with (found steel is frequently rusty or painted), and its availability (which may fluctuate). Say that you are looking for another one of those particular Huffy bike frames or a specific folding chair frame … well, good luck with that!
Determining If Something Is Steel
When you find something that you suspect you can weld, you first must determine if it is in fact steel. Here are some simple tests you can perform that should give you a pretty good idea what something is made out of.
Visual Test. This initial test is the least accurate, but after you spend some time in the shop and around metals, it will prove to be your best friend. Also, if you are ever poking around at a scrap yard or scrap dumpster in search of steel, it is very useful to be able to “read” materials at a glance. With some practice, you’ll soon be able to simply look at or pick up most materials and determine the element of which it is composed.
New steel usually has a dark-gray color and satin sheen. The dark-gray color may be interspersed with areas of blue, gold, and black. If the material in question is rusty, it definitely contains some percentage of iron and probably has been outside for a while. Of course, many secondhand materials you’ll find out in the world will have been painted. This makes it more difficult to tell what lies beneath. Look for areas of paint that have chipped or corners that been bent on the material. If you see any rust in these areas, the base material probably contains iron.
If you can’t tell completely by looking at it, you could try to pick up the material. After schlepping around pieces of steel in the shop for some time, you’ll be able to tell right away when something has the density of steel. Aluminum and titanium are both quite a bit lighter than steel and somewhat more rare.
Magnet Test. Most types of steel are magnetic. You can test to see if the material in question is magnetic by placing a magnet of medium strength on the surface. Only a few materials in our world are magnetic enough that you can feel the attraction without precision tools. These ferromagnetic materials are iron, nickel, cobalt, and gadolinium. If the magnet sticks, the base material could have a high percentage of any of these alloys. Since iron is the most common of these elements, chances are it is an iron or steel alloy.
Spark Test. The next test is a little more advanced. You will need to use a bench grinder to cast a shower of sparks from the material in question (Figure 3.6). You will be learning about the bench grinder in Chapter 6. As the grinding wheel contacts the surface, it breaks off small pieces of the base material with great speed. The friction of this action creates heat, which, in turn, causes the small pieces of base metal to become white hot. The color and spark pattern of these small pieces as they burn and fly away from the grinding wheel tell you a lot about the makeup of the parent material. Figure 3.7 shows the pattern of sparks common to some types of steel.
FIGURE 3.6 Spark test.
FIGURE 3.7 Spark patterns.