Hardness tests are used to evaluate welds, either alone or to complement information from other test results. The Rockwell, Brinell, Vickers, and Knoop tests are indentation hardness tests that measure the area or depth of indentation under load to determine the hardness. The indentations are made with testing machines selected on the basis of specimen size, form, and purpose of the hardness measurement. Indentation hardness testing is a complex measurement because of the different degrees of work hardening that occur in metals and the influence of the indenter used.
In the Brinell, Vickers, and hoop tests, the area of the indentation is measured to determine hardness. Rockwell hardness testing relates hardness to the depth of indentation under load.
Rockwell Hardness (HR). The Rockwell hardness test has become the most widely used method for determining hardness because it provides scales that can accommodate specimens of a wide variety of metals in a wide variety of sizes and shapes. The Rockwell hardness test is simple to perform; the hardness number is conveniently read directly on the testing machine, and the testing can be automated if required.
The procedure involves initial application of a minor seating load to the indenter to establish a zero datum position. A diamond-tipped indenter with a sphero-conical shape is used for hard metals, and a small hardened steel ball of prescribed size is used for softer metals. Both the minor load and the major load can be selected, depending on specimen requirements. More than a dozen scales of hardness numbers have been tabulated; each is designated by a letter of the alphabet. These basic scales are supplemented by additional scales that provide modified conditions to compensate for specimen form (eg, curvature) and approximate level of hardness. Rockwell hardness
numbers should always be quoted with a scale symbol, which indicates the kind of indenter, major load, and other testing conditions.
Three Rockwell scales are most commonly used for measuring the hardness of steels:
(1) C Scale, which uses a sphero-conical indenter which applies a 150kg major load
(2) B Scale, which uses a ball indenter (usually 1.588 mm [1/16 in.] diameter) and a major load of 100 kg (these conditions can be adjusted by an established correction factor)
(3) N Scale, which encompasses many established conditions for superficial hardness testing.
Brinell Hardness (HB). The Brinell method for testing hardness, like the Rockwell scale, has a long history of applications and is commonly used in many metal working plants. The Brinell test is used to monitor mechanical properties in metal articles of substantial size, such as bars, beams, or plates. The Brinell scale is based on the impression made in a flat surface by a hardened steel ball 10 mm (.39 in.) in diameter,
when driven into the metal at a force of 3000 kg (6600 lb.). The 30-second test time ensures that plastic flow of the metal surrounding the indentation has ceased. A standard procedure is used to measure the diameter of the indentation and to compute the Brinell
hardness (HB) number, using an equation that relates load applied, ball diameter, and indentation diameter to the hardness number. (Computation is seldom needed, since most test results are available in tabular form).
Standards for testing are set forth in ASTM E10, Brinell Hardness of Metallic Materials, and ASTM E 370, Mechanical Testing of Steel Products.
Vickers Hardness Test (HV). The indenter, a square-based diamond pyramid with a 136″ included angle, is used with a variety of loads from 1 kg (2.2 lb) to 120kg (264 lb). In this microhardness test, impressions can be closely spaced and depth of penetration can be very small.
A standard method for this test is provided in ASTM E92, Vickers Hardness of Metallic Materials.
Knoop Hardness Test (HK). A very small indenter, a rhombohedral-based diamond with edge angles of 172″30 and 130″, is used with a variety of loads, usually under 1 kg (2.2 lb). The impression has one long and one short diagonal. Impressions can be very
closely spaced and the depth of penetration can be extremely small.
Microhardness Testing
Microhardness tests can be performed with a number of instruments that use a very small indenter and a very light, precise load to make an indentation in a polished surface. The resulting indentation is measured by microscope. A polished and etched metallographic specimen is frequently used to allow hardness determinations on individual phases or constituents in the microstructure. By using an indenter with a test
load in the range of 1 to 1000 g, the indentation can be confined to a single grain in the microstructure. Standards for microhardness testing using the Knoop and the Vickers instruments are covered in ASTM E384, Microhardness of Materials.
Scleroscope Testing Equipment
The Shore Scleroscope is a hardness testing machine which consists of a vertical glass tube in which a small cylinder, or hammer, with a very hard point slides freely. This hammer weighs 2.5 grams, and is allowed to fall on the sample to be tested from a height of 25 cm. The distance which it rebounds, measured on a scale on the glass tube, constitutes the hardness.
The scale is divided into 140 parts, each part representing a degree of hardness. As examples of this scale, the hardness of glass is 130; the hardest steel is 110; mild steel is from 26 to 30, and cast gray iron is 39.
Comparison of Scales
The relationship among the several hardness scales is presented in Table H-6, showing the appropriate equivalent hardness values for steels.
Brinell Tensile Strength
The Brinell hardness of steel will give a fairly accurate indication of the tensile strength of the material. It has been found that by correlating the Brinell hardness numbers and the tensile strength of various steels in lb/ in., the tensile strength of a given steel is approximately 500 times its Brinell hardness number. In determining the tensile strength by the use of this rough check it has been found that as a rule, the tensile strength will be slightly low for hardness below 200 HB and above 400 HB. Between the two figures the indicated tensile strength is slightly above the actual strength.