The Art of Spark Testing Steels
“STEEL is steel,” some may say. “So, why test it?” But have they never heard of nickel steel or tungsten steel or cobalt steel? There are some very important differences.
To illustrate: High-speed trains roll along on rails that can really take punishment. These must be able to withstand extreme wear. That calls for a special steel containing 12 to 15 percent of manganese.
Why, even housewives are choosy about steel! They do not want just any kind. That shiny cookware containing up to 12 percent of chromium, and perhaps some nickel, is not just “steel.” It is stainless steel. And there are many kinds of stainless steel—some, for example, are modified to have more resistance to corrosion than others.
Thousands who work in machine shops and fabricating plants know that each product made requires a certain kind of steel. Several types may be on hand, so how can the right one be identified? Merely looking at the metal is generally not enough to tell. Should the workman send a sample off to a laboratory for chemical analysis? That would be very costly and time consuming. Surely there must be a more practical way.
A Color Coding System
Steel manufacturers color code the ends of the steel rods produced in their mills. Corresponding color charts are provided, and these tell the user exactly what alloy steel he is selecting from the stock bin.
As you may know, an alloy is made when two or more metals are melted together. For instance, copper and zinc combine to make the alloy called brass. Similarly, alloy steels contain various elements besides iron. These may be added to make the steel harder or more resistant to corrosion or more flexible, and so forth. Remember that it takes a combination of steel and chromium (possibly with some nickel) to make bright stainless-steel cookware.
What if we have various types of steel rods on hand and the color-coded ends are cut off, or the color chart cannot be found? Take courage, all is not lost.
Spark Testing Reveals the Unknown
There is a way to determine the kind of steel you have on hand. This can be done by spark testing, an ancient art recently developed into a modern industrial science. This art enables one to determine not only the metal’s chemical composition, but also the characteristics taken on by steel due to decarburization and heat treatment.
Sparks are emitted when a piece of metal is touched to a grindstone. Fast, nondestructive and accurate, spark testing detects the presence of most alloying elements in the metal. A high-speed portable hand grinder is used. Its two-and-a-half-inch (6.3-centimeter) abrasive wheel is capable of reaching 15,000 to 20,000 revolutions per minute without breaking up. The grinding wheel should be dressed clean before a person begins spark testing. Otherwise, there may already be steel particles on it that will give misleading results.
Constant pressure between the grinding wheel and the steel sample is very important. This pressure should be sufficient to produce a spark stream about two feet long. “But,” you ask, “what good does it do to create sparks?”
Well, those sparks reveal the unknown to the trained eye. In the stream of sparks each element produces its own particular pattern and individual color. The amount of an element present in the steel is determined by observing the frequency of its characteristic spark. So it is possible to detect the presence of such elements as carbon, manganese, silicon, nickel, chromium, molybdenum, tungsten, copper, aluminum, titanium, vanadium and columbium. Thus, an experienced spark tester can pick out of an assortment the manganese steel needed to make those long-lasting rails for the trains.
There are two broad categories of steel: The plain carbon steels and the alloyed steels.
Plain Carbon Steels
Let us watch as a piece of carbon steel is placed firmly against a high-speed grinding wheel. The overall appearance of the spark stream helps to determine whether it is a low-carbon steel or a high-carbon steel. The low carbons produce long narrow streams, whereas the high-carbon steels have short, broad streams.
We must also pay attention to the carrier lines, that is, the individual lines of trajectory making up the spark stream. These are composed of minute steel particles that are torn loose from the steel bar by the friction of the grinding wheel. The heat generated in tearing a particle away, and the friction produced as it travels through the air, cause the particle to glow. Why, each tiny particle of steel resembles a minute meteor traveling through space!
Since low-carbon steel is soft, larger pieces are torn off by the grinding wheel. So, the individual carrier lines are broader and, because of the mass effect, they glow longer than the small particles of the harder, more brittle high-carbon steels.
The type and shape of the burst near the end of the carrier lines will enable one to determine more accurately the steel’s carbon content. Bursts vary in size, shape, form, intensity and distance from the grinding wheel. Low carbon results in a few fork bursts, whereas high-carbon content produces many starlike bursts. A glance at the accompanying illustration will show you some of the differences just mentioned.
As the carbon content increases, so does the density of the spark stream. This is observed at the center of the stream. These differences are evident in the accompanying illustration, but remember, correct pressure of the steel sample against the grinding wheel is extremely important in judging density.
Spark Testing Alloy Steels
Special factors must be observed when spark testing alloy steels, for each element and the percentage of each element present in the steel produce individual spark characteristics and color effects.
A factor that might be called the individual trademark of each element is its characteristic. The higher the content of the alloying element in the steel, the more pronounced will be the element’s characteristic within the spark stream. Suppose in testing an unknown steel you observe that an ‘arrowhead’ is detached from the end of each main carrier line. That means that the steel contains molybdenum. The presence of vanadium is indicated by what looks like an inverted umbrella at the end of each carrier line.
Color is another factor. The general rule is that elements easily oxidized make the spark stream brighter, whereas those that resist oxidation tend to make the stream darker. Most carbon steels have a straw color. Alloy steels, for the most part, produce a dirty-yellow color. Tungsten-bearing high-speed steels result in a red or dark-orange spark stream.
From this brief discussion we see that spark testing is an important industrial art and one that is very helpful in identifying mixed and unknown grades of steel. You may never become such a spark artist, but it’s good to know that others have cultivated such abilities. Think about this the next time you use that stainless steel cookware or ride a high-speed train over smooth ribbons of steel.—Contributed.
[Picture on page 25]
[Picture on page 26]
LOW CARBON (sparse stream)
HIGH CARBON (dense stream)