Abbreviations: Ms—martensitic start temperatature; Mf—martensitic finish temperature; Bs—bainite start temperature; Bf—bainite finish tempera- ture; Ps—pearlite start temperature;, and Pf—pearlite finish temperature.
Fig. 1. Representative dilatometry and differentiated dilatometry curves were used to determine transformation temperatures in Eglin steel for (A) 10C/second cooling rate showing martensitic formation; (B) 0.3C/second cooling rate showing mixed martensitic/bainitic transfor- mation; (C) 0.05C/second cooling rate showing fully bainitic transformation; and (D) 0.01C/second cooling rate showing mixed bainitic/ pearlitic transformation.
• Te formation of a martensitic microstructure at these higher cooling rates leads to Vickers microhardness values in excess of 520HV.
• Formation of bainite decreases the martensitic start temperature and the overall hardness of the material.
• A second hardness plateau of about 420HV is formed at cool- ing rates 0.1-0.03C/second that form a bainitic microstructure.
second lead to the formation of a primarily martensitic microstructure.
• The formation of pearlite further decreases the hard- ness, and the pearlite is formed at the prior austenitic grain boundaries in lieu of ferrite because the eutectoid composi-
Fig. 2. Shown is the continuous cooling transformation diagram for Eglin steel. Jan/Feb 2014 | METAL CASTING DESIGN & PURCHASING | 45
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