Last Updated on January 2021
Metalworkers and welders need to know and understand metal composition before welding to yield successful results. See, understanding metal compositions helps the welder determine the proper methods, and the amounts of heat required (if needed).
Ideally, the metalworker should be in a position to examine an engineering drawing and determine whether the metal parts required are forgings, cast iron, and so on.
Metal Composition Tests
Metals are identified based on their physical, mechanical, and chemical properties, which are, in turn, identified through tests.
Here are the common metal composition tests.
The appearance of the metal test involves the determination of aspects such as color, the appearance of machined surfaces, and the appearance of non-machined surfaces.
Color can distinguish many metals, including brass, copper, magnesium, aluminum, and precious metals. See, copper is black, whereas magnesium is whitish – making it easy to tell the two apart. If the surface of the metal in question is oxidized, scrap the oxidation off to reveal the color of the actual metal.
However, rust and oxidation on metal surfaces can also help differentiate corrosion-resisting steels from plain carbon steels.
Also, there are color codes especially in steel metals, where solid colors denote carbon steels, whereas twin colors denote free-cutting and alloys of steel.
Shape and form give definitive clues to identifying a given metal. Shapes are descriptive and include things such as reinforcing rods, automobile bumpers, pipes, I-beams, angle irons, pipe fittings, and cast engine blocks.
Form, on its end, showcases how the metal was rode. For instance, hot rolled wrought materials, cold or extruded material with a smooth surface, and casting, which is obvious thanks to the parting mold lines and surface appearance.
Case example: cast pipes could indicate cast iron or wrought steel.
You can identify some metals by observing the broken area or studying the chips resulting from the fracture. For one, the broken part will show the color of the un-oxidized metal – which is ideal in identifying copper, magnesium, and lead.
In other cases, the texture of the broken surface helps reveal the structure of the base metal – this can give clues to its identity.
Also, the relative ease of fracturing the metal (with a chisel or hammer) indicates the ductility (or lack thereof) of the metal. As such, a fracture test can tell brittle metals from ductile ones.
The spark test is specially designated to identify and classify steels and iron as per their compositions. The test involves the observations of sparks generated by the metal when held against a high-speed grinding wheel.
Although the spark test does not replace any chemical analysis, it is a fast and convenient way of sorting different steels of known spark characteristics.
When held against the high-speed grinding wheel, different steels and iron produce sparks of varying shape, length, and color. For more information on the top rated bench grinders [Click Here]
To get a good stream of the said sparks, the grinding wheel should be ran at 1525m/min (5000ft/min) to produce.
Ensure the grinding wheel is hard enough to wear for a reasonable duration, yet soft enough to maintain a free-cutting edge.
To better observe the color of the sparks, the test should be carried out in subdued light. And for best results, the test should use standard samples of steel and iron, to provide a comparison ground with the test samples.
Spark test is limited as its only useful in telling ferrous from non-ferrous metals. It cannot tell non-ferrous metals apart – the likes of aluminum, coppers, and nickel alloys. The reason being, these non-ferrous metals do not exhibit significant spark streams.
Nonetheless, the test is advantageous as it can be applied to metals in finished parts, machine forgings, bar stock in frames/tracks – every stage of the manufacturing process.
Spark test results
The spark generated should be directed downward and analyzed. Factors such as the shape, length, color, and activity of the sparks can give clues to the identity of the metal in question.
The spark streams possess the following characteristics:
- Straight lines are known as carrier lines, and they are solid and continuous.
- At the end of the line, the streams may divide into three shorter lines known as forks.
- The streams may also divide into more than three shorter lines, in which case they’re called a sprig.
Sprigs can also occur at different places along the carrier line (not just the end) to form fan or starbursts.
At times, the carrier can enlarge slightly over short intermittent lengths. When the enlarged portions appear at the end of a carrier line, they are called buds or spear points. Such scenarios are common with high sulfur.
Cast irons have short streams, whereas most alloy steels and low-carbon steels have long streams.
Steels have white to yellow spark colors, while iron has reddish to straw-yellow colors. Note, the intensity of bursting (of the carrier lines) increases with the amount of carbon present.
A metal worker can use an oxyacetylene torch to identify metals by analyzing the melting speed, as well as the appearance of the molten metal. The welder can also identify the metal by observing the color changes during the heating process in conjunction with the flame color – for flammable metals.
For instance, aluminum barely melts at low temperatures as it’s a good conductor of heat. On its end, zinc melts rather quickly. To read more on the best TIG torches available in 2021 [Read our Full Guide]
A magnetic test serves to tell magnetic metals from non-magnetic metals. And with experience, the welder should judge slightly magnetic metals from strongly magnetic ones.
Strongly magnetic metals include:
- Pure nickel
- Martens tic stainless steel
- Low-carbon steel alloys
- Iron alloys
Slightly magnetic metals include:
- High-nickel alloys
- Stainless steel of 18 chrome 8 nickel stainless
Non-magnetic metals include:
- Zinc-based alloys
- Copper-based alloys
- Aluminum-based alloys
- Annealed 18 chrome 8 nickel stainless
Chisel tests make use of a cold chisel and banner to identify metals. Here, use the chisel to hammer the edge of a given metal. The relative ease of producing metal chips gives a clue to the hardness of the metal. If the chip is continuous the metal is ductile, otherwise, the metal is brittle.
For instance, the chips are both easy to obtain and continuous on malleable iron, mild steel, and aluminum; whereas, the chips from cast iron are easily broken.
On such materials as aluminum, mild steel, and malleable iron, the chips are continuous. They are easily chipped, and the chips do not tend to break apart.
A file test, though less precise, can determine the hardness of steel material. For more accurate results, use a sharp mill file.
Chemical tests are the most accurate methods for identifying metals. The test makes use of acids and bases to determine the chemical composition of the metal. For instance, aluminum can be distinguished from magnesium using silver nitrate. Here, silver nitrate leaves a black deposit on magnesium, which is absent in aluminum.
Welders must understand the basics metal compositions to determine the proper welding methods and the amounts of heat required (if needed). And with the tests above, the metalworker will be in a position to examine an engineering drawing and determine whether the metal parts are forgings, cast iron, and so on.