amount of retained austenite. The microstructures of the specimens after quenching are presented in Figure 7. ASTM E562 was followed to quantify the volume fraction of fer- rite and the uncertainty reported is at the 95% confidence level (95%CL). The ferrite volume fraction was statistically
equivalent in the specimens quenched in water and was thus unaffected by subsequent quenching in liquid nitrogen. In contrast, the air-cooled specimen had a higher volume frac- tion of ferrite, which may suggest that the slow cooling al- lowed for ferrite growth as predicted in Figure 3.
(a)
(b)
(c)
Figure 7. Optical micrograph of base heat specimen etched with Marbles reagent. The steel was homogenized at 1200°C for 2 hours, air-cooled to room temperature, and austenitized at 1050°C for 1hour prior to quenching in: (a) air, 7.4% ± 1.3% (95% CL) ferrite (b) water, 5.5% ± 1.6% (95% CL) ferrite and (c) water quenched followed by liquid nitrogen quench, 3.9% ± 1.7% (95% CL) ferrite.
Time (Hours) (a)
Time (Hours) (b)
Time (Hours) (c)
Figure 8. Hardness of age-hardened steel versus time at 482o 64 C and 460o
Time (Hours) (d)
Cbase heat, (b) heat 1, (c) heat 2, and (d) heat 3. International Journal of Metalcasting/Spring 10
Hardness (HRC)
Hardness (HRC)
Hardness (HRC)
Hardness (HRC)
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