Tham khảo tài liệu 'volume 08 - mechanical testing and evaluation part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | number is based on the average of these two measurements. Table 5 provides a simple way to convert the indentation diameter to the Brinell hardness number. The indentations produced in Brinell hardness tests may exhibit different surface characteristics. In some instances there is a ridge around the indentation that extends above the surface of the workpiece. In other instances the edge of the indentation is below the original surface. Sometimes there is no difference at all. The first phenomenon called ridging is illustrated in Fig. 13 a . The second phenomenon called sinking is illustrated in Fig. 13 b . An example of no difference is shown in Fig. 13 c . Cold-worked metals and decarburized steels are those most likely to exhibit ridging. Fully annealed metals and light case-hardened steels more often show sinking around the indentation. Fig. 13 Sectional views of Brinell indentations. a Ridging-type Brinell impression. b Sinking-type Brinell impression. c Flat-type Brinell impression The Brinell hardness number is related to the surface area of the indentation. This is obtained by measuring the diameter of the indentation based on the assumption that it is the diameter with which the indenter was in actual contact. However when either ridging or sinking is encountered there is always some doubt as to the exact part of the visible indentation with which the actual contact was made. When ridging is present the apparent diameter of the indentation is greater than the true value whereas the reverse is true when sinking occurs. Because of the above conditions measurements of indentation diameters require experience and some judgment on the part of the operator. Experience can be gained by measuring calibration indents in the standardized test block. Even when all precautions and limitations are observed the Brinell indentations for some materials vary in shape. For example materials that have been subjected to unidirectional cold working often exhibit extreme .
