Shop Tests to Identify Steel
Simple tests can be performed in the shop to identify metals if the identification color or SAE number has been cut off or removed. To state the obvious, spending the time to mark your materials is much quicker than performing these tests! Since the ability to judge metals can be developed only through personal experience, practice these tests with known metals until familiar with the reactions of each metal to each type of test.
This test includes such things as the color and appearance of machined as well as unmachined surfaces. For example heat scale or black mill scale is found on all hot-rolled steels. Other things visual inspection can detect might be the sherardized (zinc vapor inoculated inot the surface of iron or steel), plated, case-hardened, or nitrided surfaces.
Some metals can be quickly identified by looking at the surface of the broken part or by studying the chips produced with a hammer and chisel.
All ferrous metals such as iron and steel are magnetic. With the exception of nickel, cobalt and a few rare earth metals, nonferrous metals are generally not magnetic. This test would be useful in identifying Austenitic (300 series) stainless steel that has not been work hardened since this type of steel is not attracted to a magnet.
This is a simple identification test used to observe the color, spacing, and quantity of sparks produced by grinding. It is a fast and convenient method of sorting mixed steels with known spark characteristics. This test is best conducted by holding the steel stationary and touching a high-speed portable grinder to the steel with sufficient pressure to throw a spark stream about 12 inches long. These spark patterns provide general information about the type of steel, cast iron, or alloy steel. In all cases, it is best to use standard samples of metal when comparing their sparks with that of the test sample. Below is a chart indicating various results from this test:
- Wrought iron sparks flow out in straight lines. The tails of the sparks widen out near the end, similar to a leaf.
- Mild steel sparks are similar to wrought iron’s, except they will have tiny forks and their lengths will vary more. The sparks will be white in color.
- Medium-carbon steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel.
- High-carbon steel has a bushy spark pattern (lots of forking) that starts at the grinding wheel. The sparks are not so bright as the medium-carbon steel ones.
- Manganese steel steel has medium length sparks that fork twice before ending.
- High-speed steel has a faint red spark that sparks at the tip.
- 300-series stainless steels emit sparks which are not so dense as the carbon steel sparks, do not fork, and are orange to straw in color.
- 310-series stainless steels emit sparks which are much shorter and thinner than the 300-series sparks. They are red to orange in color and do not fork.
- 400-series stainless steels emit sparks which are similar to 300-series sparks, but are slightly longer and have forks at the ends of the sparks.
- Cast iron has very short sparks that begin at the grinding wheel.
- Nickel and cobalt high-temperature alloys emit sparks which are thin and very short, they are dark red in color, and do not fork.
One simple way to check for hardness in a piece of metal is to file a small portion of it. If it is soft enough to be machined with regular tooling, the file will cut it. If it is too hard to machine, the file will not cut it. This method will indicate whether the material being tested is softer or harder than the file, but it will not tell exactly how soft or hard it is. The file can also be used to determine the harder of two pieces of metal; the file will cut the softer metal faster and easier. The file method should only be used in situations when the exact hardness is not required. This test has the added advantage of needing very little in the way of time, equipment, and experience.
This is the poor man’s hardness test. Simply scratch one material on the other and the softer sample will be marked.
Rockwell Hardness Test
This test determines the hardness of metals by measuring the depth of impression which can be made by a hard test point under a known load. The softer the metal, the deeper the impression. Generally speaking, the more carbon (up to 2 percent) and other elements that steel contains, the harder, stronger and less ductile it becomes, even if in an annealed state. This test then is helpful in separating low-carbon steels from higher carbon steels. Soft metals will be indicated by low hardness numbers. Harder metals permit less of an impression to be made, resulting in higher hardness numbers. Rockwell hardness testing is accomplished by using the Rockwell hardness testing machine
Brinell Hardness Test
Brinell hardness testing operates on almost the same principle as the Rockwell test. The difference between the two is that the Rockwell hardness number is determined by the depth of the impression while the Brinell hardness number is determined by the area of the impression. This test forces a hardened ball, 10 mm (0.3937 in) in diameter, into the surface of the metal being tested, under a load of 3,000 kilograms (approximately 6,600 lb). The area of this
impression determines the Brinell hardness number of the metal being tested. Softer metals result in larger impressions but have lower hardness numbers.