Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior |
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Authors: | V H Baltazar Hernandez S S Nayak Y Zhou |
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Affiliation: | (1) Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada;(2) Present address: MpyM-EPMM Academic Unit of Engineering, Autonomous University of Zacatecas, C.P. 98000 Zacatecas, Mexico;(3) Centre for Advanced Materials Joining, University of Waterloo, Waterloo, ON, N2L 3G1, Canada;; |
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Abstract: | The isothermal and nonisothermal tempering of martensite in dual-phase (DP) steels was investigated mainly by analytical transmission
electron microscopy, and the effect on softening behavior was studied. The isothermal tempering resulted in coarsening and
spheroidization of cementite and complete recovery of laths. However, nonisothermal tempering manifested fine quasi-spherical
intralath and platelike interlath cementite, decomposition of retained austenite, and partial recovery of laths. The distinct
characteristic of nonisothermal tempering was primarily attributed to the synergistic effect of delay in cementite precipitation
and insufficient time for diffusion of carbon due to rapid heating that delays the third stage of tempering. The finer size
and platelike morphology of cementite coupled with partial recovery of lath resulted in reduced softening in nonisothermal
tempering compared to severe softening in isothermal tempering due to large spheroidized cementite and complete recovery of
lath substructure. The substitutional content of precipitated cementite in nonisothermal tempering was correlated to the richness
of particular steel chemistry. Softening resistance during nonisothermal tempering was related to DP steel chemistry, i.e., Cr and Mn content. Fine cementite and less decomposed martensite in rich chemistry confer high resistance to softening compared
to leaner chemistries, which indicated severe decomposition of martensite with coarser cementite. |
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