Consider this scenario: You are sourcing material for a new project. The project document specifies the need for stainless steel grade 303 round bar. But your search comes up short—with limited availability of this material. It is suggested that grade 304 as a suitable alternative. Now it’s your call on what to do next.
Short of those with a metallurgy degree, your knowledge on the factors that make for a suitable alternative for that 303 round bar is probably limited. The truth of the matter is that in many instances there is an alternate grade that will fit your need. But it’s a decision that involves a few different variables.
This is where having some knowledge on some basic metal characteristics comes into play. Let’s look at five factors that help determine grade selection.
How much stress can your metal withstand? This is the question that we are trying to answer when it comes to strength. This differs from metal stiffness; in fact, all steel has approximately the same stiffness, but differing levels of strengths.
The level of strength is dependent upon the alloying elements of metal. And this is typically measured in two forms:
Yield strength: Here we are looking at the strength of a metal’s shape; measuring the point at which it will begin to deform. In other words, if you apply 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): This measures the level at which metal can be stretched before it breaks. Divide the area of the material being tested by the stress placed on it in order to determine the tensile strength. This ultimately becomes an important measure of a metal’s ability to perform under the stress of its end use.
Factored together, these two measures help determine the strength of your grade. This is an important consideration. Without the required strength, you run the risk that your end product will not perform up to the standards of its intended use, resulting in catastrophe for the case of some applications.
This measures the ease at which the metal can be cut into a desired final shape and size. Metals that possess good machinability can be cut with little power—ultimately reducing the stress and wear on the tooling equipment—and achieve a good finish. The machinability of the metal will depend on its physical properties and the cutting conditions. Typically, machinability will be expressed as either a percentage or normalized value.
How well your metal will hold-up against friction? Hardness measures a metal’s resistance to what is called ‘localized deformation’. This is done by running tests to induce various methods of deformation such as abrasion or indentation. Two widely used tests for hardness are:
Rockwell Hardness: This scale has been in existence since 1914 and 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.
There are multiple versions of the Rockwell Hardness Scale, most notably Rockwell C, which is good for measuring hardened metal, and Rockwell B, which is a good measure for softer metal. 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 Hardness: In existence since around 1900, this is considered the first widely used and standardized test for metals. In this test, a hard steel or carbide ball is used as an indenter to the piece of metal. The value is obtained by dividing the applied load (in kilograms) by the surface area (in square millimeters) of the result force.
It is important to understand the conversion between these two scales. Click here to download the conversion chart.
In addition, you can also refer to ASTM A370, sections 16-19, which describes the hardness testing procedures along with descriptions of the various hardness testing equipment and equations.
How thick should your metal be? Tolerance 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 carbon bar that is 1.5 thick, a tolerance level of +/- 1 means that a piece that comes one inch above or below that measurement would still meet your specs.
As you can imagine, there are multiple reasons for having tight, consistent tolerances. One in which you may not be considering is the ability to ensure that your piece of metal will be compatible with other components in use. In other words, being certain that your piece will fit together nicely with other components. Going too far outside the specified tolerance level could result in added machining work to get the parts to fit together—and ultimately, added waste on a project.
Tolerance level is necessary since metal production can often be an imperfect process. Minor deviations will occur during metal production.
5. Element content:
Do you know the makeup of your metal? All metal is comprised of different elements, all of which impact the material in different ways. This becomes important when you consider things like the environment in which the end application will be used. For example, those that are exposed to a high rate of moisture must exhibit a strong resistance to corrosion.
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.
Among the five families of stainless steels, austenitic is the most resistant to corrosion. This is related to the fact it has high chromium levels. Its corrosion performance can even be adjusted to suit different environments through the adjustment of alloying elements—for example, varying the carbon or molybdenum levels. This means that should you need to make a substitution, it’s advisable to stay within the austenitic stainless family in order to maintain the level of corrosion resistance.
Ultimately, the decision of substituting one grade for another comes down to its end use. If you are unsure you can refer to the mill test report for certain materials, which will reveal the makeup of the metal, among other information. But knowing these five factors can be a good starting point in your decision-making process.
Of course, step one is to find the metal you need. Should you need to explore alternatives, chat with a sales professional. And perhaps even discuss these five factors.