Metal Properties: Hardness
“Hardness” is a concept we use every day to compare objects: fresh bread vs. stale bread, a baseball vs. a softball, or a living room sofa vs. a park bench. In physics (and manufacturing) the definition is a bit more specific: hardness is a measure of a material’s ability to resist localized scratching or deformation by indentation at a specific loading location.
Our understanding of hardness and relative hardness between materials is key to manufacturing metal products. The correlation of hardness with other mechanical properties also makes it possible for us to predict, with a high degree of accuracy, the strength and ductility of a material without submitting it to destructive testing.
In the world of manufacturing, there are numerous hardness types and measures. In this blog, we’ll talk about how they differ and what role they play in producing metal products. Of course, material hardness isn’t exclusive to metals, but in both metalcasting and CNC machining it is a key metric in deciding the best alloy for a product.
Hardness Compared With Strength
Similar to strength as a mechanical property, hardness is a measure of how well a material resists deformation. Whereas the deformations we measure with strength are the result of a tensile test, with hardness the deformations we measure are from localized, mechanical indentations or abrasions. Hardness is a measure of how well a material resists deformation when you indent, pierce, hammer, scratch or drop a heavy ball on the exact same spot, repeatedly. The emphasis is on ‘the same spot’: hardness is all about measuring resistance to deformation at a very specific location on the material. As a performance indicator, hardness is often a measure of how well a part will withstand environments with significant localized pressure or abrasion.
The Importance of Hardness in Metalcasting and CNC Machining
Most metals are relatively hard. Even some of the softer metals, like lead, gold, or silver, are still considered hard compared to many materials we handle on a regular basis like wood, plastic or rubber.
With metals, there’s something special, and therefore useful, about hardness: it’s not only an important metric in itself, but it’s also a repeatable and easy-to-obtain measurement that can tell us a lot about a part’s other mechanical properties – namely strength and ductility. As such, there is often a direct correlation between hardness and tensile strength, and an inverse correlation between hardness and ductility.
Because of these consistent relationships, we can closely approximate a metal’s strength profile by conducting a hardness test. With the hardness reading, along with the mechanical specifications we expect from the chosen material grade, we can infer a great deal about a material’s resistance to external forces. The convenience of hardness testing makes it a reliable method to validate the material response to the designated heat treatment cycles. In other words, hardness is a useful tool in manufacturing environments to quickly qualify material properties because some materials, like carbon and alloyed steels, have a known direct correlation between tensile strength and hardness. At Eagle Alloy, a casting foundry specializing in shell molding, hardness testing is commonly performed after casting and heat treatment in order to validate that the prescribed heat treatment was effective to produce material within a specified hardness range.
Hardness is also an important measure for CNC machining. When a customer orders a part to be CNC machined, we want to make sure the job gets done as quickly and efficiently as possible. The tools machinists use to cut and shape parts wear down over time, and they wear down more quickly as material hardness increases. At Eagle CNC, knowing the material hardness allows our team to predict how long a tool will last and what type of tool to use. Knowing a material’s hardness also avoids excessive downtime between tool changes and speeds up order delivery.
The Eagle Group’s “Casting to Completion” service motto means that we take into account every step of the manufacturing process when advising on the optimal alloy, heat treatment, and manufacturing process for each part. We utilize Design for Manufacturing (DFM) principles to maximize both quality and production efficiency. For example, a product that requires a high degree of machining may benefit from a redesign that brings the cast part closer to net shape. And, conversely, a product that requires less machining can be cast in a harder alloy without resulting in high machining costs.
Types of Hardness and How We Measure Them
There are three different types of hardness that are important to metal manufacturing: scratch hardness, indentation hardness, and rebound hardness. Materials can have different values across these types of hardness. Depending on the operating environment, any one of them can be a more or less important factor in the ultimate part design.
Scratch hardness is exactly what it sounds like: how well a material maintains its integrity when subjected to repeated surface scratching from a sharp object. Think of a scratch-off lottery ticket: depending on the softness of the material or how hard you rub, the coated layer will come off more or less quickly. Scratch hardness answers the question: if there is a constant and subtle surface abrasion happening, how long will the part hold out? The Mohs scale is the definitive test for benchmarking scratch hardness.
Rebound hardness is somewhat analogous to the strength measurement known as yield strength. They both draw on a material’s elasticity. Also known as dynamic hardness, rebound hardness measures a metal’s ability to deform elastically (i.e., non-permanently), before it kicks the indenter (or any other measurement tool) back up to where it came from. It’s like a trampoline: how much energy from a pounding or piercing motion can the material absorb without changing shape, and then how well does it return the indenter to the original dropping height. Often, a diamond-tipped hammer is used for measuring rebound or dynamic hardness. This method, called the Leeb Method, is typically dependent on surface preparation and other factors due to the small size of the testing tip. For that reason, the Leeb method is recommended for reference only, and if a more accurate hardness reading is needed the indentation method must be used for most metals.
Indentation hardness is the measure we use most at the Eagle Group. Specifically, we use the Brinell hardness test - it is the test to quickly and accurately determine a material’s resistance to plastic deformation. The Brinell hardness test is what's known as an indentation test, whereby a known load is applied to the surface of a material with an indenter of standard size and for a specific amount of time. After performing a Brinell test, we combine the force used in the test, the diameter of the indenter, and the diameter of the indentation to calculate a Brinell Hardness Number (BHN). The BHN is commonly used for benchmarking cast materials in the foundry industry, and it’s the method most common for our cast parts. There are also other hardness methods, like Rockwell and Vickers, that use different sized indenters. These are sometimes better suited for smaller castings, like some of the investment parts manufactured at Eagle Precision.
Understanding Different Hardness Tests
The Brinell test is the most common indentation hardness test for us at the Eagle Group, but a number of other tests have been developed over the years. The usual variables are the size of the indenter used, how much force is applied, and the type of material. The most common indentation tests are Vickers, Knoop, Janka, Mohs, Meyer, Rockwell, and, of course, Brinell. Knoop and Vickers can also be used to measure materials that deform under ‘low applied loads’, e.g. materials on the softer end of the spectrum (what we described above as microindentation). We use the Brinell test at Eagle Alloy because its measurement spectrum coincides well with our part sizes and surface finishes.
Results from different test types don’t necessarily correlate to results from others, so it’s important to understand which test your foundry is using, and to make sure they are using the same test consistently.
Clockwise from top-left: Brinell testing machine; closeup on indenter; reference plate for machine calibration; monitor allowing precise measurement of indentations.
Determining Mechanical Hardness for New Cast Parts
If you come to us with a part that isn’t performing properly, often the first thing we do is conduct a hardness test. Because of the established relationship between hardness and strength, a simple conversion from a hardness reading will give us a very good estimate of a material’s tensile strength. Often when a part isn’t performing well, it is because the material strength isn’t what it’s supposed to be.
But if you’re coming to us with an idea for a new part, you might be asking: do I have to specify hardness characteristics when I come to the Eagle Group for a metalcasting job? At the Eagle Group we carry out an extensive APQP process for every new part, during which time we ask a number of questions about a part’s intended use, service environment and mechanical properties. However, specifying hardness isn’t usually necessary: if you tell us how strong or ductile a part must be, we can determine the target measure of hardness.
It’s hard to imagine running a metal manufacturing facility without hardness testing. Being able to easily assess a part’s hardness helps us avoid material waste from destructive tensile testing and minimize downtime due to tool wear in CNC machining – both of which help us keep deliveries on schedule. For new products, bringing hardness into the conversation is important if the intended operating environment exposes the part to frequent rubbing, scratching, abrading, or puncturing. Whether your parts have specific hardness requirements or simply strength and ductility requirements, the Eagle Group will put our knowledge of hardness to work in order to produce cast parts that match your specifications.
To learn more about metals' mechanical and physical properties, check out our Metal Properties blog series:
Tags: Metallurgy, Metal Properties, Measurement, Mechanical Properties
Written by Jason Bergman
Jason Bergman is Senior Quality Engineer and Metallurgist at Eagle Alloy. He has been with the company since 2013.