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
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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)