In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

International Journal of Minerals, Metallurgy and Materials, Sep 2014

The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γ transformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles decreased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.

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In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

Int. J. Miner. Metall. Mater. In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels Xiang-liang Wan Kai-ming Wu Gang Huang Ran Wei Lin Cheng The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the transformation. After the transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles decreased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains. Corresponding author: Kai-ming Wu E-mail: University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2014 alloy steel; austenite; grain growth; heat-affected zone; coarsening; titanium nitride - The State Key Laboratory of Refractories and Metallurgy, Hubei Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, China (Received: 19 January 2014; revised: 5 March 2014; accepted: 12 March 2014) 1. Introduction High-strength low-alloy (HSLA) steels are important structural materials. They have good mechanical properties, including high strength, resistance to brittle fracture, cold formability, and good weldability. When HSLA steels are welded with large heat input, the grains of the coarse-grained heat-affected zone (CGHAZ) are coarsened, and the mechanical properties deteriorate. The austenite grains grow via grain boundary migration [1], which can be inhibited by the presence of particles of a second phase, thereby producing a grain boundary pinning effect [2]. This grain growth inhibition has different effects, depending on the size and fraction of precipitates [3]. A large volume fraction of fine particles is most effective in inhibiting grain growth. Previous studies have shown that the existing austenite grains in the CGHAZ can be refined by the addition of Ti as the microalloying element [46]. The addition of a small amount of Ti can lead to the dispersion of small-sized nanoscale TiN precipitates and effectively inhibit the austenite grain growth [7]. However, the level of Ti addition must be carefully controlled, otherwise TiN particles will be coarse and their density will be reduced such that the austenite grains in the CGHAZ are coarsened [7]. Austenite nucleation and growth are known to be important phenomena in steel and to occur at high temperatures. The changes in the microstructure at high temperatures cannot be observed. Furthermore, the commencement of phase transformation cannot be identified and the mechanism by which the grain migration is hindered cannot be monitored. In recent years, the in situ observation of changes in the microstructure at high temperatures has been demonstrated to be a useful approach to investigate the phase transformation, grain growth, and precipitation phenomena in steels [812]. In the present work, in situ observation was utilized to observe the austenite grain growth during high heat input welding thermal cycles. The objective of the present study was to investigate the austenite grain growth behavior in the simulated weld CGHAZ of Ti-microalloyed steels. 2. Experimental Three experimental steels microalloyed with different levels of Ti (0.012wt%, 0.040wt%, and 0.061wt%) were prepared in a 10-kg vacuum melt induction furnace. The chemical compositions of the steels are listed in Table 1. The ingots were forged into plates, machined into cylindrical specimens of 5 mm in diameter and 5 mm in length, and mounted in an alumina crucible of 0.5 mm in thickness. The in situ observation was conducted using a high-temperature laser scanning confocal microscope and an infrared image furnace. A thermocouple with a precision of 0.1C was positioned under the crucible and used to measure the temperature in the furnace. The specimens were heated to 13501400C at a rate of 5C/s, maintained at 13501400C for 530 s for austenitization, and then cooled at a rate of 5C/s, as shown in Fig. 1. Photographs were taken at a rate of 1 image per second during the simulated thermal cycling to observe the growth behavior of austenite grains. The size of the austenite grains was measured using a (...truncated)


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Xiang-liang Wan, Kai-ming Wu, Gang Huang. In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels, International Journal of Minerals, Metallurgy and Materials, 2014, pp. 878-885, Volume 21, Issue 9, DOI: 10.1007/s12613-014-0984-8