Chen Yunxia, Wang Honghong, Cai Huan, Li Junhui, Chen Yongqing
School of Materials Science and Engineering, Shanghai Dianji University, Shanghai 201306, China.
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China.
Materials (Basel). 2018 Oct 29;11(11):2127. doi: 10.3390/ma11112127.
Reversed austenite transformation behavior plays a significant role in determining the microstructure and mechanical properties of heat affected zones of steels, involving the nucleation and growth of reversed austenite. Confocal Laser Scanning Microscope (CLSM) was used in the present work to in situ observe the reversed austenite transformation by simulating welding thermal cycles for advance 5Mn steels. No thermal inertia was found on cooling process after temperature reached the peak temperature of 1320 °C. Therefore, too large grain was not generated in coarse-grained heat-affected zone (CGHAZ). The pre-existing film retained austenite in base metal and acted as additional favorable nucleation sites for reversed austenite during the thermal cycle. A much great nucleation number led to the finer grain in the fine-grained heat-affected zone (FGHAZ). The continuous cooling transformation for CGHAZ and FGHAZ revealed that the martensite was the main transformed product. Martensite transformation temperature (Tm) was higher in FGHAZ than in CGHAZ. Martensite transformation rate was higher in FGHAZ than in CGHAZ, which is due to the different grain size and assumed atom (Mn and C) segregation. Consequently, the softer martensite was measured in CGHAZ than in FGHAZ. Although 10~11% austenite retained in FGHAZ, the possible Transformation Induced Plasticity (TRIP) effect at -60 °C test temperature may lower the impact toughness to some degree. Therefore, the mean absorbed energy of 31, 39 and 42 J in CGHAZ and 56, 45 and 36 J in FGHAZ were exhibited at the same welding heat input. The more stable retained austenite was speculated to improve impact toughness in heat-affected zone (HAZ). For these 5Mn steels, reversed austenite plays a significant role in affecting impact toughness of heat-affected zones more than grain size.
逆奥氏体转变行为在决定钢的热影响区的微观结构和力学性能方面起着重要作用，这涉及逆奥氏体的形核与长大。在本研究中，采用共聚焦激光扫描显微镜（CLSM）通过模拟5Mn高级钢的焊接热循环来原位观察逆奥氏体转变。在温度达到1320℃的峰值温度后，冷却过程中未发现热惯性。因此，在粗晶热影响区（CGHAZ）未产生过大的晶粒。母材中预先存在的薄膜状残余奥氏体在热循环过程中充当了逆奥氏体额外的有利形核位置。大量的形核数量导致细晶热影响区（FGHAZ）中的晶粒更细小。CGHAZ和FGHAZ的连续冷却转变表明，马氏体是主要的转变产物。FGHAZ中的马氏体转变温度（Tm）高于CGHAZ。FGHAZ中的马氏体转变速率高于CGHAZ，这是由于晶粒尺寸不同以及假定的原子（Mn和C）偏析所致。因此，CGHAZ中测得的马氏体比FGHAZ中的更软。尽管FGHAZ中保留了10% - 11%的奥氏体，但在-60℃试验温度下可能的相变诱发塑性（TRIP）效应可能会在一定程度上降低冲击韧性。因此，在相同的焊接热输入下，CGHAZ的平均吸收能量分别为31、39和42J，FGHAZ的平均吸收能量分别为56、45和36J。推测更稳定的残余奥氏体可提高热影响区（HAZ）的冲击韧性。对于这些5Mn钢，逆奥氏体在影响热影响区的冲击韧性方面比晶粒尺寸起着更重要的作用。
