Mechanism of Post-heat Treatment on Intergranular Corrosion Behavior of SLM-316L Stainless Steel with Different Carbon Contents

doi:10.11902/1005.4537.2022.364

Journal of Chinese Society for Corrosion and protection

2023, Vol. 43

Issue (6): 1273-1283 DOI: 10.11902/1005.4537.2022.364

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Mechanism of Post-heat Treatment on Intergranular Corrosion Behavior of SLM-316L Stainless Steel with Different Carbon Contents

SHANG Qiang¹, MAN Cheng¹(

), PANG Kun¹, CUI Zhongyu¹, DONG Chaofang², CUI Hongzhi¹

^1.School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
^2.Institute for Advanced Materials and Technology, University of Science and Technology of Beijing, Beijing 100083, China

Cite this article:

SHANG Qiang, MAN Cheng, PANG Kun, CUI Zhongyu, DONG Chaofang, CUI Hongzhi. Mechanism of Post-heat Treatment on Intergranular Corrosion Behavior of SLM-316L Stainless Steel with Different Carbon Contents. Journal of Chinese Society for Corrosion and protection, 2023, 43(6): 1273-1283.

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Abstract

Intergranular corrosion is an important form of failure of austenitic stainless steels such as 316L, and the precipitation of M₂₃C₆ and the formation of Cr-poor zones are usually considered to be an important cause of intergranular corrosion. Selected laser melting (SLM) is an emerging metal additive manufacturing technology, and the SLM process of 316L stainless steel has gradually matured in recent years. The rapid condensation of the laser melt pool during SLM processing leads to the existence of sub-grain boundaries, high-density dislocations and other non-equilibrium structures inside the SLM processed 316L stainless steel (later referred to as SLM-316L stainless steel), and the post-heat treatment is used to SLM-316L stainless steel by post-treatment to optimize the organization of the SLM-316L stainless steel can obtain a better overall performance. However, there are few reports on the intergranular corrosion of SLM-316L stainless steel, and the mechanism of the original non-equilibrium structure and post-treatment on the precipitation of M₂₃C₆ and the formation of the Cr-depleted zone is not clear. In this paper, two SLM-316L stainless steels with different carbon contents were selected as the object of study, and the thermodynamic parameters of precipitated phases such as M₂₃C₆ were obtained by using Thermal-calc software with the composition of 316L stainless steel with different carbon contents as input parameters. Based on this, SLM-316L stainless steel was subjected to 900 ºC post-heat treatment and 650 ºC sensitization treatment. Subsequently, SEM, TEM and SKPFM were used to study the characteristics of the organization and precipitation phases of SLM-316L stainless steel, and the intergranular corrosion behavior of SLM-316L stainless steel was studied by DL-EPR and ammonium persulfate electrolysis. The results showed that sub-grain and dislocation started to disappear after heat treatment at 900 ºC, and discontinuous micron-sized M₂₃C₆ precipitated at the grain boundaries of SLM-316L stainless steel with higher carbon content (0.0090%), while no M₂₃C₆ precipitated in the specimen with lower carbon content (0.0063%), the intergranular corrosion resistance of SLM-316L with lower carbon content was higher than that of SLM-316L. The intergranular corrosion resistance of SLM-316L with lower carbon content is higher than that of the specimen with higher carbon content, and the intergranular corrosion resistance of SLM-316L stainless steel with both carbon contents decreases after 900 ºC post heat treatment; the intergranular corrosion of SLM-316L stainless steel after heat treatment at 900 ºC originates mainly in the Cr-poor zone around micron-sized M₂₃C₆, followed by the formation of corrosion pits and gouges around the M₂₃C₆ precipitation phase and along the grain boundary diffusion, respectively.

Key words: selective laser melting 316L stainless steel post-heat treatment intergranular corrosion M₂₃C₆

Received: 21 November 2022 32134.14.1005.4537.2022.364

ZTFLH:

TG172

Fund: National Key Research and Development Program of China(2021YFE0114000);National Natural Science Foundation of China(51901216);National Natural Science Foundation of China(U2106216);National Science and Technology Resource Investigation Program of China(2019FY101400);Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power

Corresponding Authors: MAN Cheng, E-mail: mancheng@ouc.edu.cn

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	SHANG Qiang
	MAN Cheng
	PANG Kun
	CUI Zhongyu
	DONG Chaofang
	CUI Hongzhi

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https://www.jcscp.org/EN/10.11902/1005.4537.2022.364 OR https://www.jcscp.org/EN/Y2023/V43/I6/1273

Fig.1 Thermal-Calc results of the 316L stainless steels with different C contents: (a) thermodynamic equilibrium phase diagram, (b) magnified view of marked region in Fig.1a, (c) thermodynamic curves for M₂₃C₆, (d) relationship between the C content and solution temperature for M₂₃C₆

Table 1 Input parameters for Thermal-Calc (316L stainless steels with different C contents)

Fig.2 Microstructure of SLM 316L stainless steel after different heat treatments: (a) 1# as-received, (b) 2# as-received, (c) 1# 900 ℃×5 h, (d) 2# 900 ℃×5 h

Fig.3 Microstructure of 1# SLM-316L stainless steel sample: (a) SEM image after 900 ℃×5 h heat treatment, (b) M₂₃C₆ precipitated at grain boundaries, (c) TEM images of SLM-316L without overheating treatment, (d) TEM image of SLM-316L after heat treatment at 900 ℃×5 h

Fig.4 SEM images of two kinds of SLM-316L stainless steel under different heat treatment conditions: (a) 1# 900 ℃×2 h, (b) 1# 980 ℃×2 h, (c) 2# 870 ℃×2 h, (d) 2# 900 ℃×2 h

Fig.5 SEM images of intergranular corrosion morphology of SLM 316L stainless steel with 10 s (a) and 30 s (b) of electrolysis in situ observed

Fig.6 SKPFM morphology and potential measurement results of M₂₃C₆ precipitated from 1# SLM-316L stainless steel after heat treatment at 900 ℃: (a) microscopic surface morphology, (b) potential distribution in the corresponding region, (c, d) results of electric potential in line scan area

Fig.7 DL-EPR test results of SLM 316L stainless steel with two carbon contents: (a) 1# SLM-316L stainless steel, (b) 2# SLM-316L stainless steel, (c) statistical diagram of DOS value of sample 1#, (d) statistical diagram of DOS value of sample 2#

Fig.8 Intergranular corrosion morphology of 1# SLM-316l stainless steels (a-d), 2# SLM-316L stainless steel (e-h) after 2 min electrolysis after as-received (a, e), ST/2 h (b, f), 900 ºC/5 h (c, g) and 900 ºC/5 h+ST/24 h (d, h)

Fig. 9 CLSM of SLM-316L stainless steel electrolytically etched from 0 to 300 s observed in-situ: (a) change of intergranular corrosion morphology, (b) corrosion pits measurement area, (c) grain boundary measurement area, (d) curve of loss volume of grain boundary and corrosion pits

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