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. 2018 Jun 6;11(6):959.
doi: 10.3390/ma11060959.

Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel

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Free PMC article

Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel

Zhenshun Li et al. Materials (Basel). .
Free PMC article

Abstract

The subzones of the intercritical heat-affected zone (IC HAZ) of low-carbon bainitic steel were simulated by using a Gleeble-3500 simulator to study the impact toughness. The results showed that the IC HAZ is not entirely brittle and can be further divided into three subzones according to the impact toughness or peak welding temperature; the invariant subzone heated between the critical transformation start temperature (Ac1) and 770 °C exhibited unchanged high impact toughness. Furthermore, an extremely low impact toughness was found in the embrittlement subzone, heated between 770 and 830 °C, and the reduction subzone heated between 830 °C and the critical transformation finish temperature (Ac3) exhibited toughness below that of the original metal. The size of the blocky martensite-austenite (M-A) constituents was found to have a remarkable level of influence on the impact toughness when heated below 830 °C. Additionally, it was found that, once the constituent size exceeds a critical value of 3.0 µm at a peak temperature of 770 °C, the IC HAZ becomes brittle regardless of lath or twinned martensite constitution in the M-A constituent. Essentially, embrittlement was observed to occur when the resolved length of initial cracks (in the direction of the overall fracture) formed as a result of the debonding of M-A constituents exceeding the critical Griffith size. Furthermore, when the heating temperature exceeded 830 °C, the M-A constituents formed a slender shape, and the impact toughness increased as the area fraction of the slender M-A constituents decreased.

Keywords: critical size; impact toughness; intercritical heat-affected zone; low-carbon bainitic steel; martensite-austenite constituent.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Effect of peak temperature on the microstructures of the IC HAZ: (a) 760, (b) 765, (c) 770, (d) 800, (e) 830, (f) 860, (g) 890, and (h) 910 °C.
Figure 1
Figure 1
Effect of peak temperature on the microstructures of the IC HAZ: (a) 760, (b) 765, (c) 770, (d) 800, (e) 830, (f) 860, (g) 890, and (h) 910 °C.
Figure 2
Figure 2
Area fraction of the M-A constituents in the IC HAZs as a function of peak temperature.
Figure 3
Figure 3
Size distribution of the blocky M-A constituents in the IC HAZ for different peak temperatures: (a) 760, (b) 765, (c) 770, and (d) 800 °C.
Figure 4
Figure 4
TEM images of M-A constituents in the IC HAZ with varying peak temperatures: (a) 760, (b) 770, (c) 800, and (d) 860 °C.
Figure 4
Figure 4
TEM images of M-A constituents in the IC HAZ with varying peak temperatures: (a) 760, (b) 770, (c) 800, and (d) 860 °C.
Figure 5
Figure 5
Dilatometric curves for the simulated IC HAZ specimens with varying peak temperatures: (a) 770, (b) 800, (c) 830, and (d) 860 °C.
Figure 6
Figure 6
Effect of the peak temperature on the impact toughness of the IC HAZ.
Figure 7
Figure 7
SEM fractographs of the simulated IC HAZ heated to different peak temperatures: (a) 765, (b) 770, and (c) 860 °C.
Figure 7
Figure 7
SEM fractographs of the simulated IC HAZ heated to different peak temperatures: (a) 765, (b) 770, and (c) 860 °C.
Figure 8
Figure 8
Secondary cracks near the fracture surface; peak temperature = 770 °C.

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References

    1. Lis A.K., Lis J., Jeziorski L. Advanced ultra-low carbon bainitic steels with high toughness. J. Mater. Process. Technol. 1997;64:255–266. doi: 10.1016/S0924-0136(96)02575-7. - DOI
    1. Jiang Q.M., Zhang X.Q., Chen L.Q. Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel. J. Iron Steel Res. Int. 2016;23:705–710. doi: 10.1016/S1006-706X(16)30109-1. - DOI
    1. Li X., Ma X., Subramanian S.V., Shang C., Misra R.D.K. Influence of prior austenite grain size on martensite–austenite constituent and toughness in the heat affected zone of 700 MPa high strength linepipe steel. Mater. Sci. Eng. A. 2014;616:141–147. doi: 10.1016/j.msea.2014.07.100. - DOI
    1. Shi Y., Han Z. Effect of weld thermal cycle on microstructure and fracture toughness of simulated heat-affected zone for a 800 MPa grade high strength low alloy steel. J. Mater. Process. Technol. 2008;207:30–39. doi: 10.1016/j.jmatprotec.2007.12.049. - DOI
    1. Haugen V.G., Rogne B.R.S., Akselsen O.M., Thaulow C., Østby E. Local mechanical properties of intercritically reheated coarse grained heat affected zone in low alloy steel. Mater. Des. 2014;59:135–140. doi: 10.1016/j.matdes.2014.02.010. - DOI
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