Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel

doi:10.1007/s40195-023-01635-3

Abstract

Abstract:

This work investigated the gradient microstructure evolution and tensile property of LPBF fabricated 15-5 precipitation hardening stainless steel in post-process direct ageing (DA) and solution treating & ageing (STA). The varied microstructures for austenite and small-sized oxide inclusions at different sample heights in the as-built (AB) condition was generally preserved after DA treatment. However, austenite was almost disappeared, and oxide particle grew significantly after the STA treatment. As a result, the tensile property differences in sample top and bottom for AB and DA conditions did not occur in the STA samples. For the influence of post-process heat treatment, the STA condition had the highest yield strength due to the highest volume fraction of nano-sized Cu precipitates. However, the DA specimen had the highest ultimate tensile strength and elongation owing to the considerable amount of austenite phase and associated transformation induced plasticity effect.

Key words: 15-5 PH stainless steel, Laser powder bed fusion, Heat treatment, Gradient microstructures, Tensile properties

Sheng Cao, Hongyu Liu, Jin Jiang, Ke He, Binghua Lv, Hao Zhang, Lujie Zhang, Jingrong Meng, Hao Deng, Xiaodong Niu. Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(1): 181-195.

Figures/Tables 16

Fig. 1 a Scanning electron microscopy image showing powder particle morphology; b the particle size distribution of the 15-5 PH stainless steel, c the illustration of tensile specimen extraction heights, d the drawing of M10 tensile specimens

Table 1 Chemical composition (wt%) of the LPBF fabricated 15-5 PH SS sample

Fe	C	Si	Mn	P	Cr	Ni	Cu	Nb	O	N
Bal.	0.02	0.76	0.68	0.03	15.48	4.49	3.36	0.24	0.04	0.07

Table 2 Specimen conditions for LPBF fabricated 15-5 PH SS investigated in this study

Abbreviations	Specimen conditions
AB	As-built condition
DA	AB + 552 ℃ × 4 h/FC
STA	AB + 1040 ℃ × 1 h/AC + 552 ℃ × 4 h/FC

Fig. 2 OM images taken on the vertical cross sections of the as-built a bottom, b top sample heights

Fig. 3 Grain orientation and phase maps on the vertical cross-sections of a top and d bottom of AB specimen; b top and e bottom of DA specimen; c top and f bottom of STA specimen. T and B refer to top and bottom regions. Black boundaries are high angle grain boundaries (HAGB) with a misorientation above 15º. White dashed lines in the grain orientation maps demote the melt pool boundary

Table 3 Grain sizes of martensite and austenite in AB, DA, STA conditions at the sample top and bottom

	Martensite (HAGB) (μm)	Austenite (HAGB) (μm)
AB-T	1.8 ± 1.3	1.3 ± 0.6
AB-B	1.6 ± 1.2	1.1 ± 0.4
DA-T	2.1 ± 1.4	1.6 ± 0.7
DA-B	1.9 ± 1.4	1.2 ± 0.5
STA-T	2.9 ± 2.3	-
STA-B	2.8 ± 2.3	-

Table 4 GND density of AB, DA and STA specimens at the top and bottom heights

	Martensite GND density (10¹⁵ m⁻²)	Austenite GND density (10¹⁵ m⁻²)
AB-T	3.2 ± 1.6	2.1 ± 1.8
AB-B	3.9 ± 2.7	2.1 ± 1.7
DA-T	3.1 ± 2.6	2.1 ± 1.6
DA-B	3.5 ± 3.0	2.1 ± 1.9
STA-T	2.1 ± 2.2	-
STA-B	2.2 ± 1.8	-

Fig. 4 HAADF STEM images and corresponding EDS maps of the LPBF 15-5 PH SS specimen in a AB, b DA, c STA conditions

Fig. 5 HAADF STEM images of 15-5 PH SS specimens showing oxide inclusions in: a top and d bottom heights of AB samples; b top and e bottom heights of DA samples; c top and f bottom heights of STA samples

Fig. 6 HAADF STEM images of 15-5 PH SS showing Cu precipitates in: a top and c bottom heights of DA samples; b top and d bottom heights of STA samples

Fig. 7 Representative engineering tensile stress-strain curves of AB, DA, and STA samples at top and bottom heights

Table 5 Summary of YS, UTS, El. of different specimen conditions

	YS (MPa)	UTS (MPa)	El. (%)
AB-T	631 ± 15	1229 ± 3	18 ± 1
AB-B	735 ± 15	1224 ± 3	19 ± 1
DA-T	829 ± 16	1311 ± 4	23 ± 1
DA-B	919 ± 13	1308 ± 5	22 ± 1
STA-T	1076 ± 4	1102 ± 5	15 ± 1
STA-B	1080 ± 3	1106 ± 4	14 ± 1

Fig. 8 Size distributions and volume fractions of oxide inclusions in different specimen conditions measured from HAADF STEM images: a top and b bottom regions of AB sample; c top and d bottom regions of DA sample; e top and f bottom regions of STA sample

Fig. 9 Size distributions and volume fractions of Cu precipitates measured from HAADF STEM images: a top and b bottom regions of DA specimens; c top and d bottom regions of STA specimens

Table 6 Strengthening constant (βi) and atomic solution concentrations (at.%) of main elements for different specimens. Atomic concentrations in different phases were measured according to the STEM EDS results

β_i	Nb	Si	Cr	Ni	Mn	Cu
	3486	732	434	334	213	164
Martensite phase	AB-T (at.%)	0.21	1.41	13.84	3.33	1.05	0.80
	AB-B (at.%)	0.20	1.17	11.65	2.97	0.92	0.91
	DA-T (at.%)	0.20	1.41	12.02	3.13	0.62	0.53
	DA-B (at.%)	0.19	1.06	11.53	2.41	1.21	0.59
	STA_T (at.%)	0.29	1.06	13.81	3.23	1.21	0.54
	STA_B (at.%)	0.20	1.68	12.01	3.18	1.21	0.32
Austenite phase	AB-T (at.%)	0.21	1.29	13.35	2.98	1.93	2.55
	AB-B (at.%)	0.10	0.89	12.49	3.00	0.96	3.55
	DA-T (at.%)	0.36	1.22	12.57	3.00	0.98	2.72
	DA-B (at.%)	0.20	0.97	10.98	2.26	1.10	2.90

Table 7 Different strengthening contributions, overall estimated and experimental yield strength of different specimens

YS (MPa)	∆σ₀	∆σ_SS	∆σ_GB	∆σ_DIS	∆σ_ODS	∆σ_PPT	σ_YS-estimated	σ_{YS-experimental}
AB-T	40	92	234	243	124	-	639	631 ± 15
AB-B	39	80	248	257	269	-	739	735 ± 15
DA-T	40	84	229	234	130	269	814	829 ± 16
DA-B	39	76	243	242	259	241	913	919 ± 13
STA-T	41	92	181	201	66	642	1050	1076 ± 4
STA-B	41	85	176	206	65	661	1056	1080 ± 3

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