It is essential to identify the base metal to be welded, brazed, or soldered. If metals have become mixed during storage and identifying marks have been lost, it is necessary that some means be taken to sort out the mixed metals and identify each item. Obviously, the best and most reliable method is to perform a spectrographic or quantitative chemical analysis. This is not always possible or practical. There are some relatively quick and fairly reliable tests to identify metals in a shop or construction environment.
Carbon and Structural Steels
Carbon and structural steels may be satisfactorily identified by one or more of the following tests:
Spark Test. The most common test to sort grades of structural carbon steel and tool steels is the spark test. The piece to be identified is touched against a grinding wheel; this results in a definite pattern of sparks. These sparks can then be compared to steels of known composition either by (1) using a comparison chart showing sparks from known steel compositions or (2) by sparking a steel specimen of known composition and comparing it with the unknown steel. Many shops that use a variety of steels will keep sample specimens of steels of known composition to compare with unknown specimens. A technician experienced with this technique can make rapid identification of steel
specimens.
Chip Test. In this relatively simple test the metal to be identified is chipped with a cold chisel. Identification is made by comparing the size of chips, color of metal, hardness, and surface condition of chipped metal with a known metal. Additional tests such as
magnetic tests, hardness tests, or specific gravity may be necessary.
Hardness Test. The hardness can be approximated with a file test. It is done by comparing the resistance of the metal to the cutting action of the file. Again, an experienced welder, machinist, or technician can approximate the Rockwell or Brinell hardness number.
Magnetic Test. Magnetic properties are determined using a bar magnet. All structural and carbon steels are magnetic, as are most tool steels.
Stainless Steels
Some quick tests can be used to separate stainless steels from other metals and also to identify the grade of stainless steel.
Copper Sulfate Spot Test. This is one of the simplest tests to differentiate between carbon steels and all types of stainless steel. A solution of 5 to 10% copper- sulfate (blue vitriol) in water is used. Before performing the spot test the areas to be tested should be thoroughly cleaned and roughened with a mild abrasive. A drop of the test solution is then released on the cleaned and prepared area. Carbon steel or iron will become
coated with metallic copper in a few seconds; stainless steel will show no deposit or copper color.
Magnet Test. This test is used to distinguish between austenitic stainless steel (300 series) and ferritic stainless steels (400 series). Annealed austenitic stainless steel types are nonmagnetic; if heavily cold-worked, they exhibit a slight attraction to a magnet. Ferritic
stainless steels are always strongly attracted to a magnet.
Nitric Acid Spot Test. Stainless steels are noted for their resistance to nitric acid attack. This property makes it easy to separate them from other metals and alloys. Only high-carbon stainless steel alloys (420 and 440) may show signs of a slight attack by nitric
acid. Carbon and structural steels are vigorously attacked by dilute nitric acid.
Spark Test. This test has somewhat limited value for separating stainless steels, although an experienced technician can classify stainless steels into four groups but usually cannot identify individual classifications. The four groups with their characteristic spark appearance follow:
Group 1: Types 302, 303, 304, and 316 produce a short reddish spark with few forks.
Group 2: Types 308,309,310, and 446 produce few short red sparks with few forks.
Group 3: Types 410, 414, 416, 430, and 431 produce long white streams with few forks.
Group 4: Types 420,42OF, and 440 A, B, C, and F produce long white to reddish sparks with pronounced bursts.