Effect of Heat Treatment on Microstructure and Mechanical Properties of 18Ni300 Maraging Steel Fabricated by Selective Laser Melting

doi:10.11900/0412.1961.2024.00009

Acta Metall Sin

2025, Vol. 61

Issue (10): 1515-1530 DOI: 10.11900/0412.1961.2024.00009

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Effect of Heat Treatment on Microstructure and Mechanical Properties of 18Ni300 Maraging Steel Fabricated by Selective Laser Melting

WU Wenwei¹^,², XIANG Chao²(

), ZHANG Tao²(

), ZOU Zhihang², SUN Yongfei¹^,², LIU Jinpeng², ZHANG Tao²(

), HAN En-Hou²^,³

1 School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China
2 Institute of Corrosion Science and Technology, Guangzhou 510530, China
3 School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China

Cite this article:

WU Wenwei, XIANG Chao, ZHANG Tao, ZOU Zhihang, SUN Yongfei, LIU Jinpeng, ZHANG Tao, HAN En-Hou. Effect of Heat Treatment on Microstructure and Mechanical Properties of 18Ni300 Maraging Steel Fabricated by Selective Laser Melting. Acta Metall Sin, 2025, 61(10): 1515-1530.

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Abstract

Recently, 18Ni300 maraging steel has been widely used for preparing conformal cooling molds via additive manufacturing. The requirements pertaining to the service life of these molds have become more stringent, but whether the microstructures and properties of these molds can meet the service requirements largely depends on the applied heat treatment. This paper studies the effects of two typical heat treatment processes—direct aging and solution aging—on the microstructure and tensile properties of 18Ni300 maraging steel fabricated via selective laser melting. In all prepared specimens, austenite was present and the classical Nishiyama-Wassermann orientation relationship was observed between austenite and the martensitic matrix. Elements in the as-prepared samples were evenly distributed, with obvious molten-pool and cell structures composed mainly of dislocation entanglements. In addition, a small number of long austenite strips appeared at the grain boundaries. Direct aging partially dissolved the cell and molten-pool structures and enriched Ni at some grain boundaries. The direct-aging sample exhibited relatively high austenite content. Meanwhile, the solution-aging sample exhibited a nearly complete martensite structure with evenly distributed elements. In addition, cell and molten-pool structures were almost completely removed and Ni was enriched at some grain boundaries. Further, trace amounts of austenite remained. Austenite retained in the as-prepared samples showed no obvious chemical composition segregation. Austenite present in the direct-aging and solution-aging samples was Ni enriched and confirmed to be of the reverted type. Ni at certain grain boundaries and cell walls was enriched due to cell-wall dissolution during the direct- and solution-aging treatments. Ni enrichment promoted the formation and stability of reverted austenite. Numerous round rod-shaped Ni₃Ti intermetallic compounds precipitated from the matrix after both the treatments, greatly increasing the yield strength from (1090 ± 1.5) MPa of the untreated sample to (1854 ± 13.2) MPa and (2059 ± 9.9) MPa of the direct-aging and solution-aging samples, respectively. The strength of the as-prepared samples was mainly contributed by austenite-to-martensitic phase transformation and solid-solution strengthening, while those of the direct- and solution-aging samples were mainly contributed by austenite-to-martensitic phase transformation, solid-solution strengthening, and precipitation strengthening. Moreover, the solution-aging samples exhibited greater precipitation strengthening than the direct-aging samples, mainly owing to the high density and large length-diameter ratio of their precipitates.

Key words: additive manufacturing maraging steel microstructure heat treatment austenite

Received: 12 January 2024

ZTFLH:

TG142.1+5

Fund: Youth Innovation Fund of Institute of Corrosion Science and Technology(E1551601)

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	Articles by authors
	WU Wenwei
	XIANG Chao
	ZHANG Tao
	ZOU Zhihang
	SUN Yongfei
	LIU Jinpeng
	ZHANG Tao
	HAN En-Hou

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00009 OR https://www.ams.org.cn/EN/Y2025/V61/I10/1515

Fig.1 Morphology of 18Ni300 maraging steel powder (a), powder particle size distribution (D₁₀, D₅₀, and D₉₀ are the particle sizes of the powder at 10%, 50%, and 90% cumulative distribution, respectively) (b), and schematics of tensile specimen dimensions, impact specimen dimensions, and the scanning strategy employed during selective laser melting (SLM) process (unit: mm) (c)

Fig.2 XRD spectra of SLM 18Ni300 samples after different heat treatments (V_A—volume fraction of austenite; AB—as-built, DA—direct aging, SA—solution aging; A—austenite, M—martensite)

Fig.3 OM images of SLM 18Ni300 samples after different heat treatments (a-c) XOZ surface images of AB (a), DA (b), and SA (c) samples (d-f) XOY surface images of AB (d), DA (e), and SA (f) samples

Fig.4 Low (a-c) and high (d-f) magnified SEM images of SLM 18Ni300 samples after different heat treatments (GB—grain boundary) (a, d) AB (b, e) DA (c, f) SA

Fig.5 Inverse pole figures (IPFs) (a-c) and phase distribution maps (d-f) of SLM 18Ni300 samples after different heat treatments (a, d) AB (b, e) DA (c, f) SA

Fig.6 TEM characteristics of AB sample
(a) bright-field (BF) image (Inset is the selected area electron diffraction (SAED) pattern)
(b) retained austenite
(c) TEM images and EDS maps (HAADF—high-angle annular dark field)
(d) detailed view of the cell structure
(e) detailed view of the retained austenite (Inset is the SAED pattern)
(f) EDS line scan result of Fig.6e

Fig.7 TEM characteristics of DA sample
(a) BF image (Inset is the SAED pattern)
(b) reverted austenite
(c) TEM images and EDS maps
(d) detailed view of the cell structure
(e) detailed view of the reverted austenite (Inset is the SAED pattern)
(f) EDS-line scan result of Fig.7e

Fig.8 TEM characteristics of SA sample
(a) BF image (Inset is SAED pattern) (b) detailed view of the cell structure (c) TEM images and EDS maps

Fig.9 TEM characteristics of the reverted austenite of SA sample, including BF image (a), TEM image and EDS maps (b), HRTEM image of Fig.9a (c), fast Fourie transform (FFT) of regions d (d) and e (e) in Fig.9c, and FFT of Fig.9c (f)

Fig.10 BF images, HAADF images and EDS maps of the precipitated phase, and the EDS-line scan results of a nanoparticle in the DA (a) and SA (b) samples

Fig.11 HRTEM images (a, c) and corresponding FFT (b, d) of the precipitated phase in DA (a, b) and SA (c, d) samples (The precipitated phases of DA and SA samples are η-Ni₃Ti)

Fig.12 Engineering stress-strain curves of SLM 18Ni300 samples after different heat treatments

Table 1 Mechanical properties of SLM 18Ni300 samples after different heat treatments

Fig.13 Low (a-c) and high (d-f) magnified SEM images of tensile fractures of SLM 18Ni300 samples after different heat treatments (a, d) AB (b, e) DA (c, f) SA

Fig.14 Schematic of microstructure evolution of SLM 18Ni300 samples after different heat treatments

Fig.15 Kernel average misorientation (KAM) statistics of SLM 18Ni300 samples after different heat treatments
(a) AB (b) DA (c) SA

Fig.16 Estimated and experimental results of yield strength of SLM 18Ni300 samples after heat treatments (Δσ_Mart—phase transformation streng-thening, Δσ_ss—solid solution strengthening, Δσ_p—precipitation strengthening)

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