Getting help from hardness testing

In a world where material fabrication and metal heat treatment are outsourced, how can engineers be sure the proper procedures have been performed, short of doing the work themselves? The answer is hardness testing ­— quick, minimally destructive testing that can provide peace of mind and eliminate costly rework and warranty issues.

Many steel, aluminum, and copper parts must be heat treated before or during assembly. Heat treatments alter the metals’ strength, toughness, and corrosion resistance. And designers increasingly rely on mechanical properties of nonmetallic materials like plastics and elastomers for less-expensive, lighter, and more-aesthetically pleasing parts.

So, engineers may need to specify material hardnesses for outside vendors. They may also need to verify heat treatments were performed correctly and check material composition.

What’s the worst that could happen if parts aren’t correctly heat treated? Let’s take AISI 4130 steel as an example. When water quenched and tempered at 400°F, the metal’s tensile strength reaches 236 ksi. Yield strength, the value most designs are based on, is 212 ksi after this procedure. If it were instead annealed at 1,585°F, however, tensile strength drops 66% to 81.3 ksi and yield strength drops 75% to 52.3 ksi.

Hardness testing measures a material’s surface ability to withstand distortion. Generally, hardness testing is considered a destructive test because it leaves dents in the materials. On parts ready for final assembly, such dents are made on nonstructural areas of the part.

Engineers performing hardness tests must choose the right tests and hardness scales based on the materials, their thickness, and heat-treatment processes. Traditional tests include Rockwell, Brinell, Shore, Scleroscope, Vickers, Knoop, and Mohns. Shore and Scleroscope tests are considered nondestructive.

Brinell hardness
Brinell hardness number (HB or formerly BHN) is most commonly used with steel and aluminum castings, iron, forgings, heat-treated billets, and heavy plates. Of the tests listed, it is the least affected by nonhomogeneities or irregular surfaces. However, it’s inappropriate for surface-hardened parts like those with hard coatings or case hardening.

Brinell testers press a ball into the material with a specified force for a specified time. Balls are 1 to 10 mm in diameter and made of steel or tungsten carbide. Forces range from 1 to 3,000 kg. Test results are assigned codes. For example a 75 HB 10/500/30 represents a Brinell hardness number of 75 determined using 500 kg applied via a 10-mm ball for 30 sec.

The Brinell number is calculated by dividing the applied force by the surface area of the spherical indentation:

HB = F ÷[(π × D/2) × (D – √(D² – D_ii²))]

where F = force, kg; D = indenter diameter, mm; and D_i = impression diameter, mm. When using tungsten-carbide indenters, multiply the Brinell number by 0.102, and refer to the number as HBW (W is the chemical symbol for tungsten.). For convenience, engineers can refer to tables that correlate indent diameter and Brinell number.