Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process
Introduction
In recent decades, the hot rolled low carbon microalloyed steels have been widely used in building, bridge, pipeline and offshore structures because of their excellent balance of high strength and good toughness[1], [2]. However, these excellent mechanical properties can be disturbed by the welding thermal cycles characterized by rapid heating and uneven cooling rate with high peak temperature, especially for large heat input welding process[3], [4], [5]. Therefore, how to predict the microstructure evolution in the heat affected zone (HAZ) of the weldment is a necessity to optimize the mechanical properties and to ensure the safety of the weldable steels.
Normally, continuous cooling transformation (CCT) diagram is regarded as the effective method to predict microstructural evolution during the welding thermal cycle because the composition, cooling rate, and austenite grain size of the material are related to the phase transformation temperature and resultant microstructure[6]. Although quite a number of CCT diagrams have been provided for the traditional steels according to the previous literature[6], [7], [8], continued development of the chemical composition design of the modern steels requires an understanding of their transformation behavior in detail under the welding thermal cycle conditions. Furthermore, because the cooling rate is very uneven at the whole weld cooling stage[9], the microstructure evolution may exhibit different features at the different transformation stages. For example, the product phase may be controlled by a shear mechanism at the beginning stage of the transformation due to the relatively high cooling rate. On the contrary, the carbon diffusion rate may be mainly responsible for the transformation rate as the cooling rate decreases at the finish stage of the transformation. Therefore, understanding of the change in microstructure morphology at different stages of the transformation is also very important to completely exploit the transformation behavior during the welding. However, rare research focused on the microstructure morphology at different stages of the transformation[10].
In this study, the simulation welding thermal cycle technique was employed to conduct the continuous cooling transformation test of the coarse grained HAZ (CGHAZ) of a modern low carbon microalloyed steel. The corresponding CCT diagram was determined according to the dilatometric data and microstructure observation. Meanwhile, the interrupted cooling process by water quench was used to investigate in detail the change of microstructure morphology with the increase in volume fraction transformation.
Section snippets
Experimental
The low carbon bainitic steel for the study was a 20-mm thick plate produced by thermo mechanical controlled process (TMCP). Its chemical composition and mechanical properties are shown in Table 1. The carbon content and welding crack susceptibility index Pcm are 0.053% and 0.183%, respectively, in order to improve the steel resistance to the welding cold crack in the HAZ.
The specimens cut from the steel plate were machined into long cylindrical shape with the dimension of Ф6 × 50 mm. The
Microstructure morphology and CCT diagram
Fig. 2 shows the microstructural variation with the increase of the cooling time after the specimens were subjected to the simulation welding process with the peak temperature of 1350 °C. The main microstructure is bainitic ferrite as the cooling time t8/5 is 30 s (Fig. 2(a)). It is obvious that the prior austenite grain boundaries are present in the final microstructure because the bainitic ferrite forms by a shear mechanism reported by Bhadeshia[11], who considered that this kind of
Conclusions
- (1)
The main microstructure of the low carbon microalloyed steel changes from a mixture of lath martensite and bainitic ferrite to full granular ferrite with the increase in the cooling time t8/5 when the specimens are subjected to the simulated CGHAZ welding thermal cycle process. The maximum microhardness is about 295 HV, indicating that this steel has an excellent ability of resistance to welding cold crack.
- (2)
The interrupted cooling test shows that the bainitic ferrite can nucleate firstly along
Acknowledgments
The authors gratefully acknowledge the financial support of Shenyang Key Laboratory of Construction Project (Grant No. F12-256-1-00) and Science Foundation for the Excellent Youth Scholars of Ministry of Education of China (Grant No. 90403006).
References (18)
- et al.
Mater. Sci. Eng. A
(2011) - et al.
Mater. Sci. Eng. A
(2009) - et al.
Acta. Mater.
(2004) Mater. Sci. Eng. A
(1999)- et al.
Mater. Sci. Eng. A
(2006) Mater. Sci. Technol.
(2009)- et al.
J. Mater. Sci. Technol.
(2004) - et al.
Sci. Technol. Weld. Join.
(2000) - et al.
Metall. Mater. Trans. A
(2000)
Cited by (28)
Local microstructure evolution of a V-containing Fe–Cr–Ni–Mo weld metal subjected to post-weld heat treatment
2023, Materials CharacterizationDeveloping 1000 MPa grade LCLA steels through continuous cooling: Effects of Cr and cooling rate on bainitic and martensitic transformations
2022, Materials CharacterizationCitation Excerpt :Low C high strength steel is widely used in mechanical engineering, pressure vessels and shipbuilding due to its high strength, good toughness and excellent weldability [1–4].
An overview on pipeline steel development for cold climate applications
2022, Journal of Pipeline Science and EngineeringStudy of the heat-affected zone metal of reactor pressure vessel welded joints in the initial state
2022, International Journal of Pressure Vessels and PipingFailure analysis and the cold crack formation mechanism for the QP1180 steel welded joint
2021, Engineering Failure Analysis