Metal identification is a vital step in any process that requires the usage of metals, regardless if we are talking about using metals in fabrication or simply determining their weldability. The identification step matters because different metal requires different electrodes and methods for welding. Before we use any metal, we need to identify it so we can know which techniques, methods, and materials we need to use in our next steps.
In the following paragraphs, we will discuss how you can identify different types of metals as well as some of their basic characteristics that you may find useful in the process of identification.
General Classifications Of Metals & How To Recognize Them By The Appearance Of Their Surface
Speaking in general, there are two basic, general classifications of metals: ferrous and nonferrous. Those who work with metal are required to be familiar with both types.
1. Ferrous Metals
Ferrous metals are metals that are primarily composed of iron and iron alloys. This is pretty obvious if you know that the word ferrous actually comes from the word ferrum=iron. Ferrous metals are typically magnetic and are primarily used for their tensile strength and durability. As a matter of fact, the tallest skyscrapers you see in the big cities, or the strongest bridges use ferrous metals-specifically mild steel to hold them up. Besides mild steel, other common ferrous metals are:
- Carbon steels – low-carbon steel (contains up to 0.30% carbon, easily formed and welded, doesn’t really respond to any form of heat-treating), medium-carbon steel (contains between 0.30% to 0.60% carbon content, it is stronger, harder which makes it more difficult to be welded and worked with than low-carbon steel), high-carbon steel (contains between 0.45% and 0.75% carbon, can be easily heated, and welded, butt for welding it requires usage of special electrodes and specific process that must include preheating and stress relieving procedures) and very high-carbon steel (also known as carbon tool steel, similar to high-carbon, difficult to cut and bend, with carbon rage between 0.61% and 1.50%);
- Stainless steels – are classified into 2 general series, according to AISI – American Iron and Steel Institute: Stainless Steel 200-300 series (ideal for welding as it is very tough and ductile in the as-welded condition) and Stainless Steel 400 series (further divided into ferritic and martensitic)
- Wrought iron – almost pure iron, made from pig iron and some slag added in the manufacturing process; it’s usage has drastically diminished since the late 19th century as mild steel became more available and replaced it.
- Cast iron – is considered any iron-containing more than 2% carbon alloy; it is considered brittle, except for malleable cast iron. It has a wide range of applications since it is used in the creation of automotive parts, pipes, etc.
- Ingot iron – seen as ‘pure iron as it contains 99.85% iron; it is easily formed and property-wise, it is similar to the lowest carbon steel.
- Alloy steels – their properties primarily come from the alloying elements (other than the carbon) which are added in the manufacturing process to the steel so it can acquire the desired properties. Some of the most common alloy steels are Molybdenum, Manganese Steel, Chromium Steel, Nickel Steel, etc.
2. Non-ferrous Metals
Non-ferrous metals are metals that are primarily composed of elements other than iron, albeit some of them may sometimes contain small traces of iron as an alloying element or an impurity; these amounts are not appreciable. Typically, these metals are not magnetic and are known for their lighter weight, malleability, and corrosion resistance. Some of the most common non-ferrous metals are:
- Precious metals: silver, platinum, gold
- Copper and its alloys bronze (combination of 84% copper and 16% tin) true brass (an alloy of copper and zinc),etc.
- Lead (note that lead dust, vapors, and fumes are highly poisonous so if you are working with it, you need to take proper precautions)
Basic Metal Identification
There are several ways, methods, and tests you can use to identify metals:
1. By Their Physical Properties
Different metals have different physical properties. We categorize them by their:
- Boiling point – refers to the temperature at which the metal will transform from liquid to vapor state.
- Density (mass) – also referred to as specific gravity is expressing the ratio of the mass of a given volume of the metal compared to the mass of the same volume of water at a specified temperature (most commonly it is 4 degrees Celsius or that is 39 degrees Fahrenheit.)
- Melting point – metals have high melting points and these are really important for welding; the melting points are related to the metal’s fusibility.
- Conductivity – refers to the ability of the metal to conduct or transfer heat and electricity, so we distinguish between thermal and electrical conductivity.
- Coefficient of linear thermal expansion – refers to the change in length of a specimen one unit long when its temperature changes by one degree. Typically, solids expand when heated (this type of expansion of metal in a longitudinal direction is called linear expansion; the increase in breadth and thickness is known as volumetric expansion) and contract when cooled.
- Corrosion resistance – their ability to prevent environmental deterioration by chemical or electrochemical reaction. Those containing iron are more prone to corrosion, while stainless steel, for example, is the most corrosion-resistant metal.
- Color – the quality of light that the metal reflects.
2. By The Appearance Of Their Surface
Although it might not be the most reliable method as most sub-categories tend to be similar, the appearance of the surface of the metals can help you identify the class of the metal if it can’t help you with specifications. Bellow, we are showing you a table with information on how the surface of different metals looks lie, and it can help you identify the class of the metal.
Besides these, the distinctive marks which are left from the manufacturing process of the metals can also be useful to you in the identification process:
- The surface of copper, brass, bronze, nickel, and wrought iron is smooth, while the lead is not only smooth but also has a velvet-like appearance;
- When stainless steel is not finished, it is slightly rough
- Malleable and cast iron often can show evidence of the sand mold on their surface, while low-carbon steel can show forging marks; high-carbon steel can also show forging or rolling marks.
3. By Testing
If these are still not enough, then you can do several tests to further help you determine and identify your metals:
- Spark test – The spark test is fast, cheap, and usually quite convenient as it does not require any special equipment. To perform this test, you need to hold a sample of the metal you are trying to identify against an abrasive wheel and try to visually inspect the spark stream. Watch a point roughly one-third of the distance from the tail end of the spark stream and only those who cross your sight; try to save the image in your mind. Then you can compare the sparks (or lack them) with samples of sparks of different metals.
- Chip test – To perform this test, you will need to ‘chip’ a small piece of material from the unknown metal by using a sharp, cold chisel. You will need to note the size and the look of the chip as well as the ease with which you accomplished the chipping and then compare it with a list of chip characteristics for metal identification (you can find it online).
- Magnetic test – Although it is not a 100% accurate test since some stainless steels are nonmagnetic, the magnetic test can help you determine the metal’s general identification most of the time. As you may guess, you need to use a magnet and see whether the metal is magnetic or not, remember, ferrous metals are normally magnetic, while nonferrous is not.
Any work which includes metals requires previous identification of those metals so that proper techniques and equipment/resources are used. Identifying the metals in some cases can be as simple as recognizing their physical properties or identifying them by their appearance; however, in case this is not enough for you to do a proper identification, or you have doubts about the sub-category in which certain metal belongs, then you need to do tests to further determine the type of metal you are dealing with. If you are having any doubts regarding identification, we recommend that you do another test to make sure your identification is correct. We’re eager to read your opinions on this!