13 Tutorial01-callister7e_Chapter11

# 13 Tutorial01-callister7e_Chapter11 - 11.24 Hardness is a...

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11.24 Hardness is a measure of a material's resistance to localized surface deformation, whereas hardenability is a measure of the depth to which a ferrous alloy may be hardened by the formation of martensite. Hardenability is determined from hardness tests. 11.25 The presence of alloying elements (other than carbon) causes a much more gradual decrease in hardness with position from the quenched end for a hardenability curve. The reason for this effect is that alloying elements retard the formation of pearlitic and bainitic structures which are not as hard as martensite. 11.26 A decrease of austenite grain size will decrease the hardenability. Pearlite normally nucleates at grain boundaries, and the smaller the grain size, the greater the grain boundary area, and, consequently, the easier it is for pearlite to form. 11.27 The two thermal properties of a liquid medium that influence its quenching effectiveness are thermal conductivity and heat capacity. 11.28 (a) This part of the problem calls for us to construct a radial hardness profile for a 75 mm (3 in.) diameter cylindrical specimen of an 8640 steel that has been quenched in moderately agitated oil. In the manner of Example Problem 11.1, the equivalent distances and hardnesses tabulated below were determined from Figures 11.15 and 11.17(b). Radial Position Equivalent Distance Hardness (mm) (HRC) Surface 13 48 3/4 R 18 41 M i d r a d i u s 2 2 3 8 Center 26 36 The resulting hardness profile is plotted below.

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(b) The radial hardness profile for a 50 mm (2 in.) diameter specimen of a 5140 steel that has been quenched in moderately agitated oil is desired. The equivalent distances and hardnesses tabulated below were determined using Figures 11.14 and 11.17(b). Radial Position Equivalent Distance Hardness (mm) (HRC) Surface 7 51 3/4 R 12 43 Midradius 14 40 Center 16 38.5 11.D3 This question provides us with a list of several metal alloys, and then asks us to pick those that may be strengthened by heat treatment, by cold work, or both. Those alloys that may be heat treated are either those noted as "heat treatable" (Tables 11.6 through 11.9), or as martensitic stainless steels (Table 11.4). Alloys that may be strengthened by cold working must not be exceptionally brittle (which may be the case for cast irons, Table 11.5), and, furthermore, must have recrystallization temperatures above room temperature (which immediately eliminates zinc). The alloys that fall within the three classifications are as follows:
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13 Tutorial01-callister7e_Chapter11 - 11.24 Hardness is a...

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