Microstructure and Fracture Characteristics of Heat‐Affected Zone in Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

Abstract

To clarify the microstructure and fracture characteristics of heat‐affected zone in shipbuilding steel plates with Mg deoxidation after high heat input welding, two steels with 0.0002 (2Mg) and 0.0049 (49Mg) wt% Mg are prepared and investigated with metallography and crystallography. Compared with 2Mg, TiN particles are incrementally refined in 49Mg, making the prior austenite grain (PAG) size decrease. In the large‐sized grains of 2Mg, there are preferentially formed coarse grain boundary ferrites (GBFs) and ferrite side plates (FSPs), followed by intragranular bainite ferrites (IBFs). The small PAGs of 49Mg make fine‐grained polygonal ferrites (PFs) dominate the microstructure. With Mg addition, the effective grain with θ > 50° refines and the high‐angle grain boundary density increases. The brittle transgranular fracture occurs around little‐deformed coarse GBFs, FSPs, and IBFs with θ < 15° in 2Mg, showing the weak crack resistance. However, in 49Mg, the ductile fracture occurs around fine PFs and IAFs with large plastic deformation. Although there is transgranular fracture in coarse PFs, it can be soon deflected and arrested by small‐sized PFs with θ > 50° in 49Mg. Thus, for 49Mg, the crack initiation and propagation energies increase with the toughness at −40 °C increasing from 20 to 167 J.

Effect of B Segregation at Prior Austenite Grain Boundaries on Microstructures and Toughness in the Heat‐Affected Zone of Mg‐Deoxidized Shipbuilding Steel Plates

“…Consequently, with 0.0022 wt% B addition, GBFs and FSPs reduce, followed by IAFs increasing with the higher deformation ability (KAM maps in Figure 2f) and larger HAGB density (grain boundary distribution maps in Figure 2e), which can significantly improve the ability to barrier the crack nucleation and propagation. [ 15 ] The HAZ toughness at −40 °C is thereby greatly improved from 40 to 141 J (Table 1).…”

“…[13,14] Our previous industrial tests and laboratorial studies revealed that the oxide metallurgy technology with Mg deoxidation can effectively enhance the HAZ toughness of steel plates after HHIW by refining the austenite grains or promoting the ductile microstructures. [13][14][15][16][17][18] The effect of B addition on the mechanical properties of HAZ has been studied by several DOI: 10.1002/srin.202200102…”

“…[ 13,14 ] Our previous industrial tests and laboratorial studies revealed that the oxide metallurgy technology with Mg deoxidation can effectively enhance the HAZ toughness of steel plates after HHIW by refining the austenite grains or promoting the ductile microstructures. [ 13–18 ] The effect of B addition on the mechanical properties of HAZ has been studied by several researchers. [ 19–21 ] Wang et al [ 19 ] considered that B addition accelerates the precipitation of strain‐induced carbonitrides, and trace B can refine the microstructures and improve the mechanical properties of HAZ.…”

Particles, Microstructures, and Impact Toughness of CGHAZ of Ca Deoxidation Shipbuilding Steel Plates with Different Nb Contents

“…Ultimately, the prior-austenite grains (PAGs) of post weld steel are coarsened seriously reducing the low-temperature toughness of CGHAZ. [3][4][5] Considering the application environment of the shipbuilding steel plates, the low-temperature impact toughness plays a crucial role.…”

Effect of Si Content on the Microstructures and Toughness in Heat‐Affected Zone of Offshore Engineering Steels with Mg Deoxidation

“…The oxide metallurgy technology with strong deoxidizers, such as Mg, has been widely reported to improve the HAZ toughness of thick steel plates by refining austenite grain sizes or promoting intragranular acicular ferrites (IAFs). [ 7,8 ] In our previous industrial tests and laboratorial studies, it was shown that the excellent HAZ toughness of shipbuilding steel plates can be obtained with 0.0010–0.0099 wt% Mg addition. [ 9–11 ] The CGHAZ microstructures for high‐strength offshore engineering steels present the bainite feature.…”