1600 Question 4. An austenitized 4340 standard steel bar is cooled at different rates (see Figure 2). Predict what will be the constituents in the 0.41 C, 0.87 Ma, 0.28 Si, 1.8 0.72 Cr. 0.20 Mo N ----- -- 4340 12.7 mm 25 mm S1 mm 19 mm N Austenitized at I5SOF C Grain size: ASTM No.7 1400 A 1390'F (754' Ae 1330'F (721'C in. mm 63 mm microstructure of the bar at 200°C for the 1200- 9.5 mm Cooling curve from 1550 (843'C) at indicated distances fror quenched en following cooling rates: (a) 5°C/s (51-mm from the quenched end of a Jominy bar) (b) 8°C/s (19.0-mm from the quenched end of a Jominy bar) (c) 50°C/s (9.5-mm from he quenched end of a Jominy bar) E 1000 800 Transfo 600 400- Temperature (F) 1---
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
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