Selecting Metal Grade: 5 Factors to Consider

Aluminum, carbon, and stainless steel: you’ve narrowed it down to the type of metal you need for your project. Now comes the big question: Which grade is the right fit?

When selecting metal grades, here are five factors to consider:

1. Alloying elements: All metal is comprised of various elements (e.g., chromium, copper, or molybdenum) all of which impact the material in different ways, such as their ability to withstand various conditions. A good rule of thumb is to determine the type of environment in which your application will be used.

   In Texas, for example, where humidity levels are high, choosing a grade exhibits strong resistance to corrosion is a must. Let’s look at stainless steel, for example. A basic rule of thumb says that the higher the chromium levels contained within the stainless steel, the higher the corrosion resistance. All stainless steels are iron-based alloys containing at least 10.5% chromium. The rest of the makeup is defined by various alloying elements, which control the microstructure of the alloy.

   Two of the more commonly used grades of stainless steel are 304 and 316. Both are highly resistant to corrosion, but 316 exhibits a slightly higher level of resistance.

   As another example, the alloying elements impact 3003 aluminum in various ways. This grade has excellent workability, weldability and corrosion resistance, and conforms to AMS QQ-A-250/2 and ASTM B209.

2. Strength: The alloying elements will also impact the strength of the metal. This is measured in two forms, yield strength and tensile strength, that must be factored together. Let’s take a closer look at each:

   Yield strength: Measuring the point at which deformation will begin to occur. When applying a level of stress that is less than its yield strength, the metal will be able to return to its original shape once the stress has been eliminated. Applying stress beyond the yield strength means permanent deformation of the metal.

   Tensile strength (also referred to as ultimate strength): Measuring the level at which metal can be stretched breaking. This ultimately becomes an important measure of a metal’s ability to perform under the stress of its end use.

3. Machinability: The ease at which metal can be cut into a desired final shape and size makes up its machinability. Metal grades that exhibit good machinability can be cut with little power, which will reduce the stress and wear on the tooling equipment, ultimately achieving a better finish.

4. Hardness: The measure of how well your metal will hold-up against friction determines its hardness. This is often referred to as a measurement of resistance to ‘localized deformation’. This is determined by running tests to induce various methods of deformation such as abrasion or indentation. Two widely used tests for hardness are:

   • Rockwell Hardness Scale: This scale is based on the indentation hardness of the metal. It measures the depth of penetration by an indenter into the surface. The hardness is inversely proportional to the depth of the penetration. Multiple versions of the Rockwell Hardness Scale exist, most notably Rockwell C, which is good for measuring hardened steel, and Rockwell B, which is a good measure for softer steel. These measures will be indicated by a number, accompanied by a set of letters. For example, 65 HRC (Rockwell C) or 75 HRB (Rockwell B).

   • Brinell: The typical Brinell test uses a 10 millimeters diameter steel ball as an indenter with a 6,614 pounds of force (with a smaller force used for softer metals). The indentation is measured, and the hardness is calculated.

5. Tolerance: This is a measure of metal thickness. Tolerance is the level of approved deviation of a piece from the target measurement. For example, if you need a part that is 100 mm long, a tolerance level of +/- 1 means that the piece could still meet your specs at 99 mm or 101 mm long.

These five factors can help in your decision-making process when it comes to selecting metal grades. Of course, if you need further information, contact us to discuss your metal needs.