High-Temperature Stability of Acicular Ferrite in a Low-Carbon Low-Alloy Steel Weld Metal

doi:10.11900/0412.1961.2024.00135

Acta Metall Sin

2026, Vol. 62

Issue (4): 599-610 DOI: 10.11900/0412.1961.2024.00135

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High-Temperature Stability of Acicular Ferrite in a Low-Carbon Low-Alloy Steel Weld Metal

HU Fusheng¹^,²^,³, CHENG Lin¹^,²^,³, HOU Tingping¹^,²^,³, CHENG Shi¹^,²^,³, SONG Fengyu⁴, WU Kaiming¹^,²^,³(

)

^1.State Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, China
^2.Hubei Province Key Laboratory of Systems Science on Metallurgical Processing, Wuhan University of Science and Technology, Wuhan 430081, China
^3.International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China
^4.College of Physics, Mechanical and Electrical Engineering, Longyan University, Longyan 364012, China

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HU Fusheng, CHENG Lin, HOU Tingping, CHENG Shi, SONG Fengyu, WU Kaiming. High-Temperature Stability of Acicular Ferrite in a Low-Carbon Low-Alloy Steel Weld Metal. Acta Metall Sin, 2026, 62(4): 599-610.

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Abstract

Maintaining steel’s high mechanical properties after a long period of tempering (aging) is a worldwide challenge. This study investigates the microstructure and mechanical properties of a low-carbon, low-alloy steel weld metal consisting of acicular ferrite (AF) using OM, SEM, EBSD, TEM, and impact and tensile tests. The precipitation kinetics is used to analyze and discuss the experimental results to study the microstructure evolution and its effect on the mechanical properties of the low-carbon, low-alloy steel weld metal consisting of AF after tempering for a long time in a high-temperature environment. The results show that after tempering at 580-700 ^oC for 1-12 h, the dislocation density and size of the AF showed no noticeable change, indicating that the microstructure of the AF was very stable after high-temperature, long-time tempering. Precipitates increase along with the increase in tempering temperature, pin grain boundaries and dislocations, hinder the coarsening of AF, and thus increasing the precipitation strengthening effect. The tensile strength increased at 640-700 ^oC and peaked at about 700 ^oC. The kinetics curves and calculations indicate that the “nose” temperatures of both the nucleation rate-temperature (NrT) and precipitation-temperature-time (PPT) curves are about 700 ^oC; precipitation occurs in large amounts at about 640-730 ℃ and thus provides a strong precipitation strengthening effect. High-density dislocation and Mo addition increase the nucleation rate and refine the precipitates of TiC. Adding Mo increases the “nose” temperature of the NrT and PTT curves and brings forward the precipitation start time, thus improving the high-temperature tempering mechanical properties of AF.

Key words: weld metal acicular ferrite tempering precipitation kinetics

Received: 07 May 2024

ZTFLH:

TG142.1

Fund: National Natural Science Foundation of China(U20A20279);Key Research and Development Project of China(2022YFB4201500);Shandong Taishan Industrial Leading Talent Project Blue Talent Special Foundation Project(2020007)

Corresponding Authors: WU Kaiming, professor, Tel: 13100610041, E-mail: wukaiming@wust.edu.cn

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	HU Fusheng
	CHENG Lin
	HOU Tingping
	CHENG Shi
	SONG Fengyu
	WU Kaiming

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00135 OR https://www.ams.org.cn/EN/Y2026/V62/I4/599

Fig.1 Schematic of specimen dimension for micro-tensile testing (unit: mm)

Fig.2 OM images of weld metal specimens
(a) as-welded
(b-d) tempering at 580 ^oC (b), 610 ^oC (c), and 640 ^oC (d) for 12 h
(e-g) tempering at 670 ^oC (e), 700 ^oC (f), and 730 ^oC (g) for 2 h

Fig.3 SEM images of weld metal specimens at as-welded state (a) and tempering at 580 ^oC (b), 610 ^oC (c), and 640 ^oC (d) for 12 h

Fig.4 TEM images of weld metal specimens at as-welded state (a) and tempering at different temperatures and time (b-e)
(b) 580 ^oC for 12 h (c) 610 ^oC for 12 h (d) 640 ^oC for 2 h (e) 700 ^oC for 2 h

Fig.5 TEM images (a-c) and selected area electron diffraction (SAED) patterns (d-f) of weld metal specimens (a, d) as-welded (b, e) tempering at 610 ^oC for 12 h (c, f) tempering at 640 ^oC for 2 h

Fig.6 TEM (a, d) and HRTEM (b, e) images, and EDS analysis results (c, f) of precipitate of weld metal specimens tempering at 640 ^oC (a-c) and 700 ^oC (d-f) for 2 h

Fig.7 Grain orientation (a1-d1), grain boundary misorientation (a2-d2), and kernel average misorientation (KAM) (a3-d3) maps of weld metal specimens at as-welded state (a1-a3) and tempering at 580 ^oC (b1-b3), 610 ^oC (c1-c3), and 640 ^oC (d1-d3) for 12 h

Fig.8 Phase volume fractions (a), average grain sizes (b), low angle grain boundary distributions (c), and kernel average misorientations (d) of weld metal specimens at as-welded state and tempering at 580, 610, and 640 ^oC for 12 h

Fig.9 Hardnesses (a), tensile strengths (b), elongations (c), and impact absorbing energies (d) of weld metal specimens at as-welded state and tempering at different temperatures and time

Fig.10 OM images (a, b) and EDS analysis result (c) of inclusions of weld metal specimen at as-welded state (a) and tempering at 700 ^oC for 2 h (b, c)

Table 1 Numbers, sizes, and volume fractions of inclusions of weld metal specimens at as-welded state and tempering at different temperatures and time

Fig.11 NrT (a) and PTT (b) curves of TiC and (Ti, Mo)C in acicular ferrite at different tempering temper-atures (NrT—nucleation rate-temperature, PPT—precipitation-temperature-time, I—nucleation rate in a stable state, K and t₀—constants, t_0.05—time for 5% precipitation phase transformation)

Fig.12 Kinetics curve comparisons of TiC and (Ti, Mo)C in acicular ferrite at different tempering temperatures
(a) critical size (d^*) (b) precipitation size (d) after tempering for 2 h (c) NrT curves (d) PTT curves

Fig.13 Comparisons of hardness (a) and calculated yield strength (b) of weld metal specimens tempering at different temperatures and time

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