11_8

Materials Science and Engineering: An Introduction

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251 2) The alloy is relatively soft. 3) The alloy hardens with increasing time (initially), and becomes more brittle; it may soften with overaging. 4) The alloy may continue to harden or overage at room temperature. 11.14 For precipitation hardening, natural aging is allowing the precipitation process to occur at the ambient temperature; artificial aging is carried out at an elevated temperature. Design Problems 11.D1 A one-inch diameter steel specimen is to be quenched in moderately agitated oil. We are to decide which of five different steels will have surface and center hardnesses of at least 55 and 50 HRC, respectively. In moderately agitated oil, the equivalent distances from the quenched end for a one-inch diameter bar for surface and center positions are 3 mm (1/8 in.) and 8 mm
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Unformatted text preview: (11/32 in.), respectively [Figure 11.8(b)]. The hardnesses at these two positions for the alloys cited (as determined using Figure 11.5) are given below. Surface Center Alloy Hardness (HRC) Hardness (HRC) 1040 50 30 5140 55 47 4340 57 57 4140 56 54 8640 56 52.5 Thus, alloys 4340, 4140, and 8640 will satisfy the criteria for both surface and center hardnesses. 11.D2 (a) This problem calls for us to decide which of 8660, 8640, 8630, and 8620 alloys may be fabricated into a cylindrical piece 75 mm (3 in.) in diameter which, when quenched in mildly agitated water, will produce a minimum hardness of 40 HRC throughout the entire piece....
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