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 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 – Dii2))]
where F = force, kg; D = indenter diameter, mm; and Di = 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.
Rockwell hardness (HR) is another test typically used with ferrous metals and certain heat-treated nonferrous metals. For accurate results, the test surface must be smooth, uniform, and dirt free. The test machine first presses a carbide ball or conical diamond-dipped indenter into the sample with an initial load, called the minor load, using weights or springs. Next, it adds force up to the major load, which is held for a predetermined time before dialing back to the minor load.
The first load sets the indenter into the sample to ensure linear deformation once the major load is brought to bear. As in the Brinell test, the hardness number is calculated by measuring how deeply the indenter penetrated the sample.
Engineers use different Rockwell scales, indicating different indenters and loads, depending on material and thickness. Rockwell hardness numbers have codes that detail the indenters and loads used. (See table, Rockwell hardness testing, for a list of scales and uses.)
There are two main types of Rockwell tests: regular and superficial. The superficial tests give engineers an idea of the hardness of thin materials, shallow case hardening, and coatings. Regular tests use 10-kg minor loads and 60, 100 or 150-kg major loads; superficial tests use 3-kg minor loads and 15, 30, or 45-kg major loads.
Microhardness testing, appropriate for brittle materials and small or thin samples, quantifies extremely small or narrow features like coatings, bonding layers, case hardening, bimetallic couples, and metallic grains. Technicians use microhardness tests to monitor nitriding and carburizing, find unfavorable surface conditions like decarburization and grinding burns, and measure hardness close to edges.
Vickers microhardness tests (HV) use diamond indenter in the shape of a square pyramid with 136° apex angles. The tester’s indenter presses into the sample with 5, 10, 20, 30, 50, or 120 kg of force for 30 sec. Technicians measure the lengths of the impression diagonals using a microscope to determine the impressions’ size and the hardness numbers:
HV = [2F sin(136°/2)] ÷ [(d1 + d2)/2]2
where F = applied force, kg, and d1 and d2 = diagonal lengths, mm. In practice, tables correlate the average diagonal length with Vickers hardness numbers for a given test force.
Some engineers contend that square Vickers indents are easier to read and measure than circular ones left by Brinell testers. Other advantages of the Vickers test include a uniform scale that applies to all testing loads and a single indenter for all materials. However, Vickers equipment may be more expensive than Brinell or Rockwell devices.
Another type of microhardness test is Knoop microhardness (HK). This test also uses a diamond indenter with a pyramid shape. However the Knoop pyramid has a rectangular base and two apical angles, 130° and 172°. The result is a diamond-shaped impression where one diagonal is seven times longer than the other. The indenter gets pressed into samples with loads ranging from 25 gm to 5 kg. As in the Vickers test, a microscope is needed to measure the longer diagonal. The hardness number is then calculated using:
HK = 14.229 × (F/D2)
where F = applied load, kg, and D = length of the longer diagonal, mm. The constant corrects for the indenter’s shape.
Knoop tests are often used for materials too thin or brittle to be tested by other methods because very little load is required. As with the Vickers test, a single scale and indenter are used over the entire hardness range.
Shore hardness, also sometimes called durometer after the test equipment, measures hardness in terms of an indentation’s depth caused by a predetermined force. Durometers don’t leave permanent indentations in samples, so the test is considered nondestructive. Shore hardness is typically associated with rubbers, polymers, elastomers, and foams.
Durometers require flat, smooth, clean, horizontal surfaces and materials at least 0.25-in. thick. There are 12 different scales created by varying indenter type and applied load, but the load is always applied for 15 sec. (See table, Shore hardness testing) Each scale ranges from 100 (a lack of indentation) to 0 (a maximum indentation that varies by scale). Most scales are based on 0.1 in. maximum indentation.
The most commonly used scales are Shore A and Shore D. Shore A tests use a pointed indenter and 0.8 kg of force. Shore D tests also use a pointed indenter but increase the force to 4.6 kg to measure hard and extrahard polyurethanes and plastics.
Other hardness tests
The Scleroscope test measures the rebound of a 0.0914-oz, diamond-tipped hammer released from 10 in. above a sample. Scleroscope Type C tests use glass tubes marked with a scale from 0 to 140 to determine rebounds. Type D tests rely on dial gages marked from 0 to 120. The test, developed in the early 1900s, has been largely superseded by Rockwell and Brinell testing, but is still used on large metal components like rolls, gears, and dies.
The Mohs scale measures the hardness of minerals with nonmetallic properties, so it is seldom used for structural engineering.
Some engineers also classify an informal file test as a hardness test, despite the fact it doesn’t generate a numerical hardness value. Technicians hold a sample in one hand and run a metal file back and forth over it with some force. If the file cuts into the material with this hand pressure, the metal most likely hasn’t been heat treated.