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Metal Casting Technologies : March 2005
36 METAL Casting Technologies March 2005 ardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting. The hardness of ferrous alloys is commonly measured using the Brinell hardness test. The test was first proposed by J. A. Brinell in 1900 and consists of applying a load to a steel or tungsten carbide ball into the surface of the test piece and measuring the diameter of the indentation left in the surface of the test piece. Figure 1 shows a casting with Brinell impression on the surface. The Brinell hardness test is generally considered to be the most satisfactory of the indentation hardness testing methods for materials of variable microstructure such as graphitic cast irons. This is because irons may be microscopically composed of phases of vastly variable hardnesses such as soft graphite in a matrix of varying amounts of ferrite, pearlite and/or tempered martensite. The Brinell test covers a large enough area to average the hardness of the constituents present. A hardened steel ball is used for materials with a Brinell hardness not exceeding 450 HB whilst a tungsten carbide ball is used for materials with a Brinell hardness not exceeding 650 HB. The values obtained using a steel ball or a tungsten carbide ball can be significantly different for hardnesses above 350 HB. When a steel ball is used for the measurement, the hardness value is designated using the symbol HBS, and for cases where a tungsten carbide ball is used the designation is HBW. The diameter of the ball, in millimetres, and the force applied, in kilograms, should also be shown in the test results. For example, a reading of: 250 HBS 10/3000 denotes a Brinell hardness of 250 determined with a steel ball of 10mm diameter and an applied force of 3000 kilograms. TESTING MACHINES Brinell testing machines come in a variety of forms, typically static floor mounted or portable with the load applied hydraulically or mechanically. Some examples are shown below. The bench mounted machine shown in figure 2 uses dead-weights with a pneumatic load system to apply and remove the various standard loads. The portable model shown in figure 3 uses a hydraulic ram to apply loads of up to 3000 kg. and can be operated in any orientation. The model shown in figure 4 is also a portable model Back to Brinell Hardness Testing of Ferrous Alloys By J. F. Meredith, Casting Solutions Pty Ltd BASICS 1. Brinell impression on surface of casting 2. Bench mounted machine 3. Portable C-Clamp hydraulic model 1 2 3 H