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Open AccessArticle

Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel

1
Engineering Training Center, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
2
School of Mechanical and Automotive Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
*
Author to whom correspondence should be addressed.
Materials 2018, 11(6), 959; https://doi.org/10.3390/ma11060959
Received: 6 May 2018 / Revised: 31 May 2018 / Accepted: 4 June 2018 / Published: 6 June 2018
(This article belongs to the Section Structure Analysis and Characterization)
The subzones of the intercritical heat-affected zone (IC HAZ) of low-carbon bainitic steel were simulated by using a Gleeble-3500 simulator to study the impact toughness. The results showed that the IC HAZ is not entirely brittle and can be further divided into three subzones according to the impact toughness or peak welding temperature; the invariant subzone heated between the critical transformation start temperature (Ac1) and 770 °C exhibited unchanged high impact toughness. Furthermore, an extremely low impact toughness was found in the embrittlement subzone, heated between 770 and 830 °C, and the reduction subzone heated between 830 °C and the critical transformation finish temperature (Ac3) exhibited toughness below that of the original metal. The size of the blocky martensite-austenite (M-A) constituents was found to have a remarkable level of influence on the impact toughness when heated below 830 °C. Additionally, it was found that, once the constituent size exceeds a critical value of 3.0 µm at a peak temperature of 770 °C, the IC HAZ becomes brittle regardless of lath or twinned martensite constitution in the M-A constituent. Essentially, embrittlement was observed to occur when the resolved length of initial cracks (in the direction of the overall fracture) formed as a result of the debonding of M-A constituents exceeding the critical Griffith size. Furthermore, when the heating temperature exceeded 830 °C, the M-A constituents formed a slender shape, and the impact toughness increased as the area fraction of the slender M-A constituents decreased. View Full-Text
Keywords: intercritical heat-affected zone; impact toughness; martensite-austenite constituent; critical size; low-carbon bainitic steel intercritical heat-affected zone; impact toughness; martensite-austenite constituent; critical size; low-carbon bainitic steel
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Li, Z.; Zhao, X.; Shan, D. Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel. Materials 2018, 11, 959.

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