| Title |
Investigation of Tempered Martensite Embrittlement Behavior in Automotive Low- and Medium-Carbon Steel Hardware with Respect to Heat Treatment and Microstructure |
| Authors |
김선진(S. J. Kim) ; 박지니(J. Park) ; 양하영(H. Y. Yang) ; (Raj Narayan Hajra) ; 김정한(J. H. Kim) ; 김지훈(J. Kim) ; 최정묵(J.M. Choi) ; 박준식(J.S. Park) |
| DOI |
https://doi.org/10.3365/KJMM.2026.64.5.387 |
| ISSN |
1738-8228(ISSN), 2288-8241(eISSN) |
| Keywords |
TME; Carbon steel; Microstructure; Hardness; Impact energy; Heat treatment |
| Abstract |
The increasing demand for lightweight automotive structures has intensified the need for advanced
materials and reliable joining technologies for dissimilar hardware components. In this study, four candidate
steels (1021B, 5120B, SCM435, and SWCH45K) were investigated to compare their austenite transformation
behavior during heating and their tempering response after quenching. For hypoeutectoid steels, Ac1 and Ac3
measured during heating represent the start and finish temperatures of austenite formation under a given
heating schedule, whereas Ae1 and Ae3 obtained by Thermo-Calc correspond to equilibrium temperatures. The
transformation temperatures (Ac1 and Ac3) were determined by differential scanning calorimetry (DSC, 3 oC/min)
and compared with the equilibrium temperatures (Ae1 and Ae3) predicted by Thermo-Calc. The measured Ac1
values were 736?755 oC and the measured Ac3 values were 781?803 oC, while the calculated Ae1 and Ae3 values
were 716?736 oC and 770?810 oC, respectively. The differences between measured and calculated temperatures
are discussed in terms of equilibrium versus non-equilibrium transformation, heating rate, initial
microstructure, and solute redistribution during austenitization. For heat treatment, all steels were austenitized
at 910 oC for 1 h, which is sufficiently above the measured Ac3 and calculated Ae3 of all alloys, and then water
quenched and tempered at 200?600 oC. After quenching, all materials exhibited predominantly lath martensitic
microstructures. With increasing tempering temperature, hardness gradually decreased whereas impact energy
generally increased; however, a distinct toughness trough associated with tempered martensite embrittlement
(TME) appeared near 350 oC. Because sub-size Charpy specimens were used, the impact data are interpreted
mainly for relative comparison among the steels rather than for direct comparison with standard Charpy values.
The present work provides an experimentally validated comparison framework linking Ac1/Ac3, Ae1/Ae3, quenchtempered
microstructures, and hardness/impact trends for four industrial automotive hardware steels, and
offers practical guidance for selecting heat-treatment windows that avoid the TME regime. |