Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

doi:10.1016/j.jmst.2013.03.022

Journal of Materials Science & Technology

Volume 29, Issue 5, May 2013, Pages 446-450

https://doi.org/10.1016/j.jmst.2013.03.022 Get rights and content

Continuous cooling transformation of a low carbon microalloyed steel was investigated after it was subjected to the simulation welding thermal cycle process and the interrupted cooling test. Microstructure observation was performed by optical microscopy and transmission electron microscopy. On the basis of the dilatometric data and microstructure observation, the continuous cooling transformation (CCT) diagram was determined, which showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to full granular bainite with the increase in the cooling time t_8/5 from 10 to 600 s, accompanied with a decrease in the microhardness. The interrupted cooling test confirmed that the bainitic ferrite can form attached to grain boundaries at the beginning of transformation even if the final microstructure contains a mixture of granular bainite and bainitic ferrite.

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 P_cm 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 t_8/5 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 t_8/5 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)

L. Lan et al.
Mater. Sci. Eng. A
(2011)
Y.Q. Zhang et al.
Mater. Sci. Eng. A
(2009)
A. Lambert-Perlade et al.
Acta. Mater.
(2004)
H.K.D.H. Bhadeshia
Mater. Sci. Eng. A
(1999)
K. Poorhaydari et al.
Mater. Sci. Eng. A
(2006)
W.B. Morrison
Mater. Sci. Technol.
(2009)
F. Xiao et al.
J. Mater. Sci. Technol.
(2004)
D.M. Viano et al.
Sci. Technol. Weld. Join.
(2000)
D.P. Fairchild et al.
Metall. Mater. Trans. A
(2000)

There are more references available in the full text version of this article.

Cited by (28)

Local microstructure evolution of a V-containing Fe–Cr–Ni–Mo weld metal subjected to post-weld heat treatment
2023, Materials Characterization
The high-strength steels are widely used in modern shipbuilding and marine facilities. However, welding of >900 MPa grade steels is particularly challenging owing to the lack of understanding of fine microstructure for the weld metal (WM). The present investigation is mainly focused on a V-containing Fe–Cr–Ni–Mo WM fabricated by gas metal arc welding (GMAW) in aspects of localized microstructure evolution during post-weld heat treatment (PWHT). Microanalysis of the localized microstructure of inter-dendritic regions (IDRs) and dendrite core region (DCRs) for WMs was conducted by transmission electron microscopy (TEM), transmission kikuchi diffraction (TKD) and atom probe tomography (APT), complimented by nanoindentation hardness testing. It is found that the microstructure of WM in as-welded (AW) and PWHT condition was characterized by elongated martensitic laths with high-density dislocations in IDRs, while that in DCRs was coarse bainitic microstructure with relatively fewer dislocations. Most of retained austenite (RA) formed during cooling after welding was decorated martensite lath boundaries within IDRs owing to the segregation of austenite stabilizing elements after solidification. For PWHT condition, more numerous and smaller nano-scale MC carbides were observed in IDRs compared to DCRs. Furthermore, V atoms were mainly responsible for the nucleation of nano-scale MC carbides, and the growth and coarsening process of them were mainly controlled by diffusion of Mo atoms. Besides, the RA could also decompose into nano-scale MC carbides during PWHT. The IDRs of the PWHT specimens showed the average nanohardness value of 7.4 ± 2.3 GPa, which was higher than that of the DCRs (4.0 ± 0.2 GPa).
Effect of crystallographic features on low-temperature fatigue ductile-to-brittle transition for simulated coarse-grained heat-affected zone of bainite steel weld
2023, International Journal of Fatigue
The coarse-grained heat-affected zone (CGHAZ) of bainite steel welds is prone to lose toughness due to welding thermal history. This study aims to reveal the correlation between crystallographic characteristics and fatigue ductile-to-brittle transition (FDBT) for typical microstructures (lath bainite and granular bainite) in the CGHAZ at low temperatures. Compared to the granular bainite, the lath bainite presents retarded growth of fatigue cracking and significantly lowered FDBT temperature (-20℃ vs −40℃). The superior low-temperature fatigue performances of lath bainite are ascribed to the high-fraction high-angle grain boundary formed by its unique variant selection.
Developing 1000 MPa grade LCLA steels through continuous cooling: Effects of Cr and cooling rate on bainitic and martensitic transformations
2022, Materials Characterization
Citation 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].
In this study, the effects of Cr addition and cooling rate on bainitic and martensitic transformations of 1000 MPa grade low carbon low alloy (LCLA) steels during continuous cooling were investigated, and the formation mechanism of C-rich martensite (CRM) was discussed. The appearance of CRM was closely related to the high concentration of C enrichment because of the incomplete formation of bainitic ferrite (BF). Most of CRM formed in the low temperature zone was distributed near the prior austenite grain boundaries (PAGB), with the maximum lattice imperfections, and the twinned substructure of high C martensite could be seen in it. With the same composition, the volume fraction and size of CRM were decreased as the cooling rate was increased. With the same cooling rate, the addition of Cr increased the volume fraction of CRM and reduced its size. Moreover, in the medium temperature zone, the addition of Cr and the increase in cooling rate lowered the transformation start temperature (TST), made the transformation after the formation of BF gradually transition to single martensitic transformation.
An overview on pipeline steel development for cold climate applications
2022, Journal of Pipeline Science and Engineering
For some decades, the resources within the northern hemisphere have been studied for possible exploration. The need for reliable infrastructures in such extreme cold climatic condition is constantly on the rise. There is an imminent need to develop pipeline steels that can retain good characteristics under extremely low temperature. The focus of this review is to evaluate the basic requirements for producing steels designated for application in extreme cold polar regions. This study includes construction steels and the high strength pipeline steel grades used in sub-zero temperature applications. The emphasis is on the role of mechanical properties, chemical composition, and microstructure in designing steels for cold region. How these factors influence failure is critical, especially in terms of cracking behavior. Therefore, additional details about the synergy between low temperature and corrosive degradation are also discussed.
Study of the heat-affected zone metal of reactor pressure vessel welded joints in the initial state
2022, International Journal of Pressure Vessels and Piping
The metal of the heat-affected zone (HAZ) of reactor pressure vessel of nuclear power plant VVER-1000 was studied. Based on hardness measurement, HAZ is divided into two sub-zones: a sub-zone of high hardness and a sub-zone of low hardness. Measurement of the distribution of the chemical elements in the area of fusion line (at ∼5 mm from the fusion line) showed that the area with the changed chemical composition of the welded joint corresponds to the weld metal. The tendency to brittle fracture of HAZ metal was estimated by the value of ductile-to-brittle transition temperature according to the results of mini-Charpy specimens (5 × 5 × 27.5 mm) tests. The parameters of the grain structure of welded joint metal were determined, including the weld metal, fusion line, two sub-zones of HAZ, and the base metal. According to the results, the high hardness sub-zone (HAZ) is characterized by a smaller primary austenite grain compared to base metal and T_K is 60–65 °C a lower than for base metal. The low hardness HAZ zone does not differ from the base metal in grain size. The T_K value in this area is 24–29 °C lower than for base metal.
Failure analysis and the cold crack formation mechanism for the QP1180 steel welded joint
2021, Engineering Failure Analysis
Gas metal arc welding (GMAW) is applied to fabricate the QP1180 steel welded joint. The formation of the Widmanstatten ferrite (WF) in weld metal (WM) and its effect on the cold crack generation for QP1180 welded joint was systematically discussed. It is found that the inhomogeneous distribution of the WF can result in the lowest microhardness (∼275 HV) in the WM. High cooling rate, large grain size, low content of element C and high content of element Mn in the region near the WM together trigger the formation of WF, and their epitaxial growth can lead to their extension into the heat affected zone (HAZ) along the prior austenite grain boundaries. As a result, the micro-cracks are formed at the interface between the WF and martensite. Subsequently, the harder martensite with large size further promotes the propagation of the micro-cracks along the grain boundaries in the HAZ, forming the intergranular fracture eventually.

View all citing articles on Scopus

View full text

Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

Introduction

Section snippets

Experimental

Microstructure morphology and CCT diagram

Conclusions

Acknowledgments

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta. Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Technol.

J. Mater. Sci. Technol.

Sci. Technol. Weld. Join.

Metall. Mater. Trans. A