killed steel smelted by traditional method and the TiN particles could pin austenite ... steel smelted by Oxide Metallurgy technology and the IAF divided the prior ...
The 8th Pacific Rim International Congress on Advanced Materials and Processing Edited by: Fernand Marquis Marquis TMS (The Minerals, Metals & Materials Society), 2013 706 7KH0LQHUDOV0HWDOV 0DWHULDOV6RFLHW\
IN SITU OBSERVATION OF THE MICROSTRUCTURE EVOLUTION IN HAZ AND ANALYSIS BY EBSD Zhihui Xiong, Xuemin Wang, Xinlai He, Chengjia Shang, Shilong Liu and Guanghua Yu 1
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083 China Keywords: heat-affected-zone, intragranular acicular ferrite, effective grain size Abstract
The microstructure evolution of pressure vessel steels by different smelting technology was observed in situ by the laser scanning confocal microscopy the second phase particles of the steels were observed by TEM, The results showed that there are many TiN particles in the Al killed steel smelted by traditional method and the TiN particles could pin austenite grain boundary and refine the austenite grain size. While there were many IAF formed in the HAZ of steel smelted by Oxide Metallurgy technology and the IAF divided the prior austenite grains into smaller and separated regions and refine the microstructure of the HAZ, and the analysis of the effective grain size of two steels carried out by electron back scattering diffraction(EBSD)show that Refining Effect of the steel smelted by Oxide Metallurgy technology is better. Introduction In an attempt to understand the grain refinement and toughening mechanisms in the HAZ of the pressure vessel steel smelted by Traditional method and Oxides metallurgy technology. The formation and growth of austenite and intragranular acicular ferrite was observed in situ by the laser scanning confocal microscopy ((LSCM). since The EBSD in SEM make it possible to identify the phase and crystallographic orientation of the sample in different regions by recording diffraction patterns along the sample surface in steps of defined size [1-4].the heat-affected zone (HAZ) of the samples was studied by EBSD in terms of microstructure and effective grain size. 1 Experimental Procedure Two low carbon steels were prepared, one was Al-killed steel smelted by Traditional method, marked as steel No. 1; and another was Ti-killed steel smelted by Oxides metallurgy technology, marked as steel No. 2. The chemical compositions of these steels were listed in Table 1. The steel plates were machined intoΦ7×3.5 mm samples for in situ observation and 11×11×90 mm for HAZ simulation. The phase transformation process of samples was observed by VL2000DXLSCM , specimens for HAZ simulation were simulated on a Gleeble 1500 thermo simulator, the samples were heated to 1320℃ at a rate of 200 ℃/s and held for 1s, the preheat temperature was 100℃, and cooled to room temperature under the conditions of t8/5 of 20, 80and 160s. After thermal simulation, standard V-notch Charpy impact test specimens were machined and tested at −20°C.
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Table1 Chemical composition of experiment steels (wt%)
1 2
C 0.1-0.2 0.1-0.2
Si 0.2-0.3 0.2-0.3
Mn 1.4 1.4
P 0.012 0.008
S 0.055 0.004
Ti 0.01~0.02 3°, >15°and>45°. It is found that the grain boundary between IAF and the surrounding bainite is high angle(>45°) in picture (d),and the high angle boundary plays an important role in stopping cleavage crack propagation[6-7].
Fig. 8. The EBSD analysis of the steel No. 1and 2, (a) and (b) IPF of steel No. 1and 2, (c)and(d) grain boundary distribution of steel No. 1and 2, (e) inverse pole map.
The effective grain size of two steels is summarized in Table 2 for 5 fields of view for each specimen with grain angles set as 15°, the austenite grain size is also shown in Table 2. the steel No.2 has a bigger austenite grain size but smaller effective grain size. it is indicate that the austenite grain size is not only the influencing fact in grain refinement, but also the microstructure formed in HAZ. since there are more IAF formed in HAZ and the IAF divided the
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prior austenite grains into smaller and separated regions, and have high-angle boundary with the surrounding bainite transformed at lower temperatures. and hence the steel No. 2 has a better effect on grain refinement in HAZ. Table2 the austenite grain size and the effective grain size in two steels steel No. 1
steel No. 2
austenite grain size /μm
49
82
the effective grain size /μm
11
9.6
4 Conclusions (1)The second-phase particle on both steel can hinder the austenite grain growth. The TiN particle in the steel No. 1 smelted by traditional method has smaller size and higher density, can pin austenite grain boundary strongly. so that the austenite grain can be refined. (2) The IAF formed in the HAZ of steel No. 2 divided the prior austenite grains into smaller and separated regions, and have high-angle boundary with the surrounding bainite, plays an important role in the refinement of HAZ and is helpful to improve toughness in HAZ. (3)By comparing the austenite grain size and effective grain size of steel No. 1and 2, it is confirmed that both austenite grain and acicular ferrite make a contribution to the steel refinement. Although the austenitic grain size of sample 2 is bigger than sample 1, but there are more acicular ferrites in steel No. 2, which improve toughness greatly.
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