Effect of Post-weld Tempering on the Microstructure and Mechanical Properties in the Simulated HAZs of a High-Strength-High-Toughness Combination Marine Engineering Steel

doi:10.1007/s40195-019-00954-8

Abstract

Abstract:

The effects of tempering temperatures on the microstructure and mechanical properties of the simulated coarse-grain heat-affected zone (CGHAZ) and inter-critical heat-affected zone (ICHAZ) were investigated for a high-strength-high-toughness combination marine engineering steel. The results demonstrate that the microstructure of the simulated CGHAZ and ICHAZ after tempering is characterized by tempering sorbites and coarse grain in the simulated CGHAZ. As tempering temperature increases, the tensile strength of the simulated CGHAZ and ICHAZ decreases and the Charpy absorbed energy of the simulated ICHAZ at - 50 °C increases remarkably, but the impact toughness of the simulated CGHAZ is not improved. After tempering at 550 °C, the coarse flake carbides, which distribute at the prior austenite grain and martensite lath boundaries, deteriorate the impact toughness of the simulated CGHAZ. With the increase in tempering temperature, the morphology and the size of the carbides gradually change from coarse flake to fine granular, which is beneficial to the improvement of impact toughness. However, the coarse-grain size of the simulated CGHAZ and the M₂₃C₆-type carbide precipitated along the grain boundaries weakens the enhancing effect of carbides on impact toughness.

Key words: High-strength-high-toughness combination steel, Post-weld heat treatment, Heat-affected zones (HAZs), Carbides, Impact toughness

Wen-Chao Dong, Ming-Yue Wen, Hui-Yong Pang, Shan-Ping Lu. Effect of Post-weld Tempering on the Microstructure and Mechanical Properties in the Simulated HAZs of a High-Strength-High-Toughness Combination Marine Engineering Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(3): 391-402.

Figures/Tables 17

Table 1 Chemical composition of the experimental steel (wt%)

Fig. 1 Welding thermal cycles for simulated CGHAZ a, ICHAZ b

Fig. 2 Dimension figure of nonstandard tensile sample

Fig. 3 Tensile strength of simulated CGHAZ and ICHAZ before and after tempering

Fig. 4 Impact toughness of simulated CGHAZ and ICHAZ before and after tempering

Fig. 5 Optical micrographs of simulated CGHAZ a1 as-welded, a2 PWHT 550 °C, a3 PWHT 600 °C, a4 PWHT 650 °C, and ICHAZ b1 as-welded, b2 PWHT 550 °C, b3 PWHT 600 °C, b4 PWHT 650 °C

Fig. 6 SEM images of simulated CGHAZ a1 as-welded, a2 PWHT 550 °C, a3 PWHT 600 °C, a4 PWHT 650 °C, ICHAZ b1 as-welded, b2 PWHT 550 °C, b3 PWHT 600 °C, b4 PWHT 650 °C

Fig. 7 Mass fraction of phases in the experimental steel under equilibrium state as a function of temperature: a whole diagram, b partial diagram

Fig. 8 XRD spectra of carbides with different tempering temperatures in simulated CGHAZ

Fig. 9 TEM bright-field images and selected area diffraction (SAD) patterns of the precipitated carbides in simulated CGHAZ after tempering for 1.5 h at 550 °C a, 600 °C b, 650 °C c

Table 2 Type, morphology, dimension and chemical composition of carbides in the simulated CGHAZ at different tempering temperatures

Fig. 10 EBSD images of simulated CGHAZ after tempering for 1.5 h at 550 °C a, 650 °C b

Fig. 11 Number fraction with different misorientations for simulated CGHAZ after tempering for 1.5 h at 550 °C and 650 °C

Fig. 12 Schematic diagram in the fracture surface of Charpy impact specimen

Fig. 13 SEM low-magnification fractographs of Charpy impact specimens at - 50 °C in simulated CGHAZ a1 as-welded, a2 PWHT 550 °C, a3 PWHT 600 °C, a4 PWHT 650 °C, and ICHAZ b1 as-welded, b2 PWHT 550 °C, b3 PWHT 600 °C, b4 PWHT 650 °C

Fig. 14 SEM high-magnification fractographs of Charpy impact specimens at - 50 °C in simulated CGHAZ a1 as-welded, a2 PWHT 550 °C, a3 PWHT 600 °C, a4 PWHT 650 °C, and ICHAZ b1 as-welded, b2 PWHT 550 °C, b3 PWHT 600 °C, b4 PWHT 650 °C

Fig. 15 SEM micrographs near the fracture of simulated CGHAZ specimens after tempering for 1.5 h at 550 °C a, 650 °C b

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