Effect of Induction Heat Treatment on Microstructure, Mechanical and Corrosion Properties of Stainless Steel 308L Fabricated using Wire Arc Additive Manufacturing

doi:10.21203/rs.3.rs-4836306/v1

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Effect of Induction Heat Treatment on Microstructure, Mechanical and Corrosion Properties of Stainless Steel 308L Fabricated using Wire Arc Additive Manufacturing

https://doi.org/10.21203/rs.3.rs-4836306/v1

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Induction solution heat treatment can change the mechanical characteristics and corrosion resistance properties of 308L manufactured via wire arc additive manufacturing (WAAM). Moreover, compare with traditional heat treatment methods, this method can reduce heat treatment time and achieve in-situ local heat treatment. By applying induction solution heat treatment to 308L fabricated using WAAM, ferrite and austenite phase proportions were changed. Heat treatment at 1100 ℃ effectively reduced the δ-Fe and σ-Fe content, resulting in a slight decrease in UTS, while YS, EL, and microhardness have a certain degree of increase. σ-Fe exhibits a more pronounced strengthening effect than austenite, albeit at the potential expense of steel's elasticity. At the same time, induction heat treatment alters the ferrite to austenite ratio, which also enhances the anti-corrosion properties of the stainless steel. However, the presence of σ-Fe will cause a worsening of the corrosion resistance of the steel. In addition, as the heat treatment progress advances, there is a notable transformation in the ferrite's microstructure in the deposition direction, from continuous dendrites to a small number of equiaxed grains.

Physical sciences/Engineering/Mechanical engineering

Physical sciences/Materials science/Structural materials/Mechanical properties

Wire arc additive manufacturing (WAAM)

Induction heat treatment

Mechanical properties

Corrosion resistance

Wire arc additive manufacturing (WAAM) can enable the direct fabrication or restoration of metal parts.(Ref 1) Compared with conventional manufacturing methods like casting and forging, WAAM offers technical and economic advantages such as high material utilization rate, low manufacturing cost, and manufacturing of complex structural parts.(Ref 2) The technique uses an electric arc to melt the wire and stack the metal layer by layer to obtain a production part, guided by a layered approach from a computer-aided design model.(Ref 3) However, due to the extensive heat input, high-temperature gradient, and complex thermal history in the manufacturing process,(Ref 4) the obtained part materials are quite different from those obtained by traditional manufacturing methods in terms of microstructure and mechanical properties, and uncontrollable changes in material properties will have an impact on the performance of the parts.(Ref 5)

Austenitic stainless steel is a prevalent metallic material extensively employed in contemporary industries such as shipbuilding, aerospace, and nuclear reactors because of its favorable corrosion resistance, mechanical properties, and processibility.(Ref 6,7) Austenitic stainless steel is mainly composed of austenite, ferrite, and minor quantity of carbide, among which ferrite can be divided into δ-Fe and σ-Fe. δ-Fe is a high-temperature ferrite produced during solid solution phase transformation, and σ-Fe is generated from δ-Fe when the material is exposed to 450 ℃~850 ℃ for a long time.(Ref 8,9) Studies have shown that in austenitic stainless steel obtained by welding or other processes, the σ phase in ferrite can play a specific role in strengthening. (Ref 10) However, at the same time, due to its hard and brittle characteristics, if it exists in large amounts, it will reduce the plasticity and corrosion resistance of the steel.(Ref 11) The temperature history during the WAAM process is complex and uncontrollable, and there are many phase transition processes. It is easy to cause more ferrite phases to exist.(Ref 12) In addition, the microstructure of stainless steel shows that σ-Fe appears in a dendritic form, with grain growth in the direction opposite to the direction of maximum cooling, which is usually consistent with the deposition direction.(Ref 13) The dendrite structure is also different at different deposition positions, leading to disordered microstructure distribution and poor homogeneity, differences in mechanical properties at different locations, and poor homogeneity of part performance.(Ref 14)

In order to improve the material properties of WAAM austenitic stainless steel, it is necessary to take specific performance optimization methods. Heat treatment is a common material modification method, and some researchers have applied it to WAAM austenitic stainless steel. Chen et al.(Ref 15) conducted solution heat treatment of WAAM 316L stainless steel and found that water quenching after heat treatment at a temperature of 1100 ℃~1200 ℃ could eliminate the ferrite in the steel, reduces strength, and increases ductility and corrosion resistance. This heat treatment method can also eliminate the effect of work hardening, leading to a reduction in the material’s surface hardness. In addition, regulating both the temperature and duration of the heat treatment process is essential to prevent sensitization and preserve corrosion resistance. Rodrigues et al.(Ref 16) established a synchrotron X-ray diffraction analysis method to measure ferrite and austenite in the induction heat treatment of austenitic stainless steel. The higher dissolution temperature can effectively dissolve δ-ferrite dendrites distributed in the austenite matrix. However, the researchers did not describe changes in the material properties of the steel after induction heat treatment, such as mechanical and corrosion resistance properties. Induction heating has a high heating rate and uniform and high-temperature control accuracy. When applied in WAAM, it can realize in-situ local heat treatment, improve heat treatment efficiency, and customize the performance of different parts. This study focuses on the phase transformation and microstructure evolution of WAAM austenitic stainless steel during induction heat treatment and the effects on mechanical properties and corrosion resistance were characterized.

2.1 Material fabrication

This study uses 100 mm×150 mm×4 mm thin-walled parts made by WAAM using 1.2 mm diameter 308L stainless steel wire. The specific manufacturing process and process parameters are shown in Fig. 1 and Table 1.

In-situ induction heat treatment was performed on thin-walled parts made by WAAM, and the induction heat treatment equipment was shown in Fig. 2. In the heat treatment experiment, three control groups were set as with the holding time as the variable, heat treatment was performed at 1100℃ for 2 min, 4 min and 6 min, using water as the quenching cooling medium, as listed in Table 2.

Table 1

Material and process parameters
Parameters	Value
Wire material Wire diameter	ER308L 1.2 mm
Wire feeding speed	3.0 m/min
Welding speed	0.010 m/s
Layer height	1.0 mm
Ar gas	22 L/min
Cooling	20 s natural cooling per layer

Table 2

Heat treatment of 308L fabricated using WAAM
No.	Heating rate (℃/s)	Induction heat treatment
1#	20	1100 ℃/2 min, WQ
2#	20	1100 ℃/4 min, WQ
3#	20	1100 ℃/6 min, WQ

2.2 Microstructure analysis

Metallographic specimens were prepared through wire-electrode cutting from both as-deposited and heat-treated parts, and then a series of polishing steps were performed on specimens involving 180#, 240#, 800#, 1200#, 1500#, and 2000# grit sandpaper, and W0.5 grit polishing agent in turn for grinding and polishing. Subsequently, the specimens were etched with a solution comprising 20 ml of HCl, 20 ml of H2O, and 4 g of CuSO4. The microstructural characterization of the 308L fabricated using WAAM was conducted utilizing the ZEISS optical microscope (OM) and the Gemini-SEM 300 scanning electron microscope (SEM). To ensure the precision of the results, the volume fraction (vol%) values of austenite and ferrite phases in 308L specimens were determined from metallographic photographs using Image-Pro Plus software under deposition and heat treatment conditions. Additionally, these values were cross-verified with data obtained through X-ray Diffraction (XRD) analysis using Jade software.

2.3 Mechanical testing

Tensile testing was conducted according to the ASTM E8 standard using the SASTest CMT5205 electronic universal testing machine. Tensile testing was carried out at room temperature with the crosshead speed set at 0.5 mm/minute and the strain during the test was recorded by an electronic tensiometer. The dimension of the plate specimen prepared for tensile tests was 25 mm in gauge length, 100 mm in total length and 4 mm in thickness. The direction of tensile specimens was parallel to the deposition direction as depicted in Fig. 3. To mitigate measurement discrepancies, at least three samples were tested under identical conditions and their average values were calculated. Microhardness was measured with an MH-5 Vickers hardness tester. The test load was set as 200g and the pressure holding time was set as 10 s.

2.4 Electrochemical testing

Samples used for the electrochemical testing were 5×5×3 mm³ in size and put in a 0.5 cm² sample support box as a working electrode. The surface of each sample was firstly ground to 2000#, followed by a polishing step employing 0.5 µm diamond paste. Electrochemical tests were performed using the VersaSTAT 3 electrochemical workstation with a corrosion solution of 3.5wt % NaCl solution at room temperature (25℃). A three-electrode system consisting of the counter electrode, the reference electrode, and the working electrode was used for the electrochemical testing. For these experiments, the counter electrode was composed of platinum foil, while the reference electrode used a saturated calomel electrode (SCE). In order to establish steady-state conditions, firstly record the open circuit potential (E_o) for 40 min. Set the sweep rate of dynamic polarization curve test to 30 mV/min, starting at 50 mV below the steady open circuit potential and ending when the current density surpassed 10^− 4 A/cm². In the polarization curve, the pitting potential (E_p) is the potential at which the current density increased significantly. According to the polarization curves of samples, the corrosion current density (i_corr) was determined by using Tafel extrapolation method.

3.1 Microstructure

In as-deposited 308L stainless steel, the ferrite exhibits a dendritic morphology within the austenite matrix. The dendrites are more continuous in the deposition direction, and the growth direction is consistent with the deposition direction. The morphology is more dispersed in the printing direction, as can be seen from Fig. 4. After induction heat treatment at 1100 ℃, the ferrite will gradually dissolve in the austenite matrix. As shown in Fig. 5, the remaining ferrite will spheroid. The volume fractions of ferrite and austenite in both as-deposited and heat-treated steel specimens were determined by XRD results and metallographic analysis, as shown in Fig. 6 and Fig. 7. The volume fractions of ferrite and austenite phases in the as-deposited state are about 89 vol% and 11 vol%, respectively. As listed in Table 3, there is a gradual reduction in the volume fraction of ferrite as the time of the heat treatment increases. The ferrite gradually decreases from the initial 11% to about 1%. In addition, the morphology of the grains changed from columnar grains in the as-deposited state to equiaxed ones after heat treatment.

Table 3

Phase composition ratio analysis results
	XRD		Metallographic Analysis
	Austenite proportion(%)	Ferrite proportion(%)	Austenite proportion(%)	Ferrite proportion(%)
As-deposited	89.8	10.2	88.0	12.0
1100 ℃/2 min, WQ	93.4	6.6	94.2	5.8
1100 ℃/4 min, WQ	98.8	1.2	97.4	2.6
1100 ℃/6 min, WQ	99.4	0.6	98.4	1.6

3.2 Mechanical properties

As presented in Fig. 8 and Table 4, the ultimate tensile strength (UTS) and the yield strength (YS) were obtained from stress-strain curves of as-deposited and heat-treated steel specimens, while the elongation (EL) was obtained by measuring the sample after breaking. The UTS and YS of 308L stainless steels in the as-deposited state are 576 MPa and 318 MPa, respectively. After the induction solution heat treatment at 1100 ℃ for 2 min, UTS decreased from 576 MPa to 554 MPa, YS increased from 318 MPa to 343 MPa, and EL increased from 44.8–46%. With the increase of heat treatment time, after induction solution heat treatment at 1100 ℃ for 4 min, UTS decreased to 534 MPa, YS increased to 336 MPa, EL increased to 48.4%; after solution heat treatment for 6 min, UTS decreased to 538 MPa, YS became 336 MPa, the EL rises to 49.6%, as shown in Table 4.

Table 4

UTS, YS and EL variation results
	UTS/MPa	YS/MPa	EL/%
As-deposited	576 ± 10	318 ± 6	44.8 ± 1
1100 ℃/2 min, WQ	554 ± 4	343 ± 2	46 ± 0.5
1100 ℃/4 min, WQ	534 ± 12	336 ± 11	48.4 ± 0.4
1100 ℃/6 min, WQ	538 ± 5	336 ± 4	49.6 ± 0.4

The microhardness results are shown in Fig. 9. The as-deposited hardness is 210.6 HV, and the hardness is reduced after induction heat treatment. As the heat treatment time increased, it decreased from 210.6 HV to 194.9 HV, 189.2 HV, and 173.3 HV.

The result shows that the induction solution heat treatment leads to a decrease in the strength and the hardness of WAAM 308L stainless steel but improves its ductility.

3.3 Corrosion resistance

The corrosion resistance of both as-deposited and heat-treated 308L stainless steel specimens was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5%wt NaCl solution. The polarization curves and electrochemical parameter measurements are presented in Fig. 10 and Table 5. The larger the difference between pitting potential and corrosion potential (ΔE) and the lower the corrosion current density (i_corr), the slower the corrosion rate and the better the corrosion resistance. As delineated in Fig. 10 and Table 5, with the time of induction solution heat treatment increasing, the i_corr value gradually decreases, and the ΔE value gradually increases. It means that induction solution heat treatment is an effective means of enhancing the corrosion resistance of stainless steel. The same results are obtained by polarization impedance spectroscopy and equivalent circuit simulation, as shown in Fig. 11. and Table 6

Table 5

Polarization curve linear fitting results
	i_corr (10^− 8A/cm²)	E_corr (V) SCE	E_p (V) SCE	ΔE = Ep – E_corr (V) SCE
As-deposited	4.05	-0.230	0.02123	0.25169
1100℃/2min, WQ	2.40	-0.224	0.36585	0.58993
1100℃/4min, WQ	2.34	-0.221	0.39882	0.61939
1100℃/6min, WQ	1.87	-0.224	0.58489	0.80757

Table 6

Values for equivalent circuit elements derived from electrochemical impedance spectroscopy fitting following a 40 min immersion in 3.5%wt NaCl solution
	R_s (Ω·cm²)	R_ct (kΩ·cm²)	CPE-T (F·s^φ−1/ cm²)	CPE-T-φ
As-deposited	25.13	283.18	0.165	0.91313
1100 ℃/2 min, WQ	30.41	379.02	0.133	0.90134
1100 ℃/4 min, WQ	32.36	604.04	0.133	0.91292
1100 ℃/6 min, WQ	32.18	767.80	0.124	0.90231

When the steel is heat treated at 1100 ℃, the ferrite phase will transform into the austenite phase, as shown in Fig. 12, and the increase in the heating rate can significantly accelerate the austenitization rate. The preset temperature of 1100 ℃ can be reached in the 50 s by induction heating. Increasing the heating rate of induction heating can accelerate the rate of austenitization during heat treatment of austenitic stainless steel, thus significantly reducing the overall heat treatment time. In terms of grain morphology, the as-deposited steel exhibits a distinct dendritic structure in the deposition direction, while the grains are transformed to equiaxed by heat treatment.

The mechanical characteristics and corrosion resistances of stainless steel are interlinked with the constituent phases and their morphology. Through the induction solution heat treatment, the decrease in the ferrite proportion resulted in a reduction of σ-Fe, which, in turn, contributed to a decline in the UTS of the stainless steel from 576 MPa to 534 MPa after heat treatment, while the YS and EL have a slight increase. The result means that ferrite plays a certain degree of strengthening effect in austenitic stainless steel, but it will adversely affect the ductility of the steel. The materials examined in this study, both before and after heat treatment, fulfill the industry's minimum standards for forging 316L stainless steel, as stipulated by UTS at 450 MPa, YS at 170 MPa, and EL at 40%.(Ref 17) For microhardness, the hardness value through heat treatment decreases gradually with the reduction of the ferrite phase and the coarsening of grains.

The potentiodynamic polarization results show a gradual reduction in the i_corr of the steel and an incremental increase in the ΔE with extended heat treatment duration. These trends signify that induction solution heat treatment can effectively enhance the corrosion resistance of WAAM stainless steel. It's a well-established fact that the formation of Cr-rich carbides along grain boundaries leads to Cr-depletion in the regions neighboring these boundaries. The σ-Fe phase, which forms and grows by consuming chromium, is notably Cr-rich. Consequently, there exists a Cr-depleted zone at the interface between the σ-Fe phase and the substrate. The passive oxide film within this chromium-depleted area is susceptible to corrosion, resulting in diminished corrosion resistance. Through the reduction of σ-Fe content, the heat-treated steel shows better corrosion resistance.

Induction heat treatment can reduce heat treatment time. Due to the high heating rate of induction heating, the austenitization rate during heat treatment of austenitic stainless steel can be accelerated so that the overall heat treatment time is significantly reduced.
The induction solution heat treatment at 1100 ℃ is effective in diminishing the presence of the ferrite in austenitic stainless steel. Upon subjecting to heat treatment at 1100 ℃, the ferrite gradually dissolves into the austenite matrix, leading to a decrease in the residual ferrite content as the time of the heat treatment progresses. Furthermore, the morphology of the remaining ferrite undergoes a transformation from the original dendritic to equiaxed.
The reduction of the ferrite leads to a decrease in UTS and microhardness of austenitic stainless steel after heat treatment, while a slight increase in YS and EL. It is because σ-Fe in the ferrite phase can strengthen austenitic stainless steel, but it will reduce the plasticity of the steel.
Induction solution heat treatment proves to be an effective method to improve the corrosion resistance of 308L stainless steel. The σ-Fe phase formed in the ferrite phase results in regions depleted of chromium at their interfaces, thereby increasing the susceptibility to corrosion attack. The elimination of the ferrite phase has been shown to contribute to the effective enhancement of the corrosion resistance of austenitic stainless steels.

Author Contribution

Sun and Li wrote the main manuscript text,Xu and Liu were mainly responsible for the experiments. Shen made suggestions on the overall idea.All authors reviewed the manuscript.

Acknowledgements

This work was financially supported by The Key Research and Development Plan of Zhejiang Province(2023C01169),Natural Science Foundation of Zhejiang Province for Distinguished Young Scholars(LR22E050002), Natural Science Foundation of Zhejiang Province(LD24E050011).

Data Availability

The datasets analyzed during the current study available from the corresponding author on reasonable request.

B. Baufeld, O.V.d. Biest, R. Gault, Additive manufacturing of Ti-6Al-4V components by shaped metal deposition: Microstructure and mechanical properties, Mater. Des., 2010, 31, p S106-S111. https://doi.org/10.1016/j.matdes.2009.11.032
X. Yan, P. Gu, A review of rapid prototyping technologies and systems, Comput Aided Design, 1996, 28(4), p 307-318. https://doi.org/10.1016/0010-4485(95)00035-6
D. Ding, Z. Pan, C. Dominic, H. Li, Wire-feed additive manufacturing of metal components: technologies, developments and future interests, Int. J. Adv. Manuf. Technol., 2015, 81(1-4), p 465-481. https://doi.org/10.1007/s00170-015-7077-3
V. Fachinotti, A. Cardona, B. Baufeld, O. VanderBiest, Finite-element modelling of heat transfer in shaped metal deposition and experimental validation, Acta Mater., 2012, 60(19), p 6621-6630. https://doi.org/10.1016/j.actamat.2012.08.031
X. Chen, J. Li, X. Cheng, H. B, W. HM, H. Z, Microstructure and mechanical properties of the austenitic stainless steel 316L fabricated by gas metal arc additive manufacturing, Mater. Sci. Eng. A, 2017, 703, p 567-577. https://doi.org/10.1016/j.msea.2017.05.024
M. Ma, Z. Wang, D. Wang, X. Zeng, Control of shape and performance for direct laser fabrication of precision large-scale metal parts with 316L Stainless Steel, Opt. Laser Technol., 2013, 45, p 209-216. https://doi.org/10.1016/j.optlastec.2012.07.002
B. Song, J. Xiang, D. Hou, Characteristics of Mechanical Properties and Microstructure for 316L Austenitic Stainless Steel, J. Iron Steel Res. Int., 2011, 18(11), p 53-59. https://doi.org/10.1016/S1006-706x(11)60117-9
D. Chastell, P. Flewitt, Formation of the Sigma-Phase during Long-Term High-Temperature Creep of Type-316 Austenitic Stainless-Steel, Materials Science & Engineering, 1979, 38(2), p 153-162. https://doi.org/10.1016/0025-5416(79)90091-0
S. Katayama, T. Fujimoto, A. Matsunawa, Correlation among Solidification Process, Microstructure, Microsegregation and Solidification Cracking Susceptibility in Stainless Steel Weld Metals(Materials, Metallurgy & Weldability), Transactions of JWRI, 1985, 14(1), p 123-138. https://doi.org/10.18910/10942
W. Delong, Ferrite in Austenitic Stainless-Steel Weld Metal, Weld. J., 1974, 53(7), p S273-S286. https://doi.org/10.1016/0024-3205(74)90266-5
E. Chun, H. Baba, K. Terashima, K. Nishimoto, K. Saida, Prediction of σ Phase Precipitation in Type 316FR Stainless Steel Weld Metal, Quarterly Journal of the Japan Welding Society, 2013, 31(4), p 168s-172s. https://doi.org/10.2207/qjjws.31.168s
D. Olson, Prediction of austenitic weld metal microstructure and properties, Weld. J., 1985, 64, p 281s-295s. https://doi.org/10.1016/0036-9748(86)90115-8
L. Wang, J. Xue, Q. Wang, Correlation between arc mode, microstructure, and mechanical properties during wire arc additive manufacturing of 316L stainless steel, Mater. Sci. Eng. A, 2019, 751, p 183-190. https://doi.org/10.1016/j.msea.2019.02.078
T. Skiba, B. Baufeld, O. vanderBiest, Microstructure and Mechanical Properties of Stainless Steel Component Manufactured by Shaped Metal Deposition, ISIJ Int., 2009, 49(10), p 1588-1591. https://doi.org/10.2355/isijinternational.49.1588
X. Chen, J. Li, X. Cheng, W. HM, H. Z, Effect of heat treatment on microstructure, mechanical and corrosion properties of austenitic stainless steel 316L using arc additive manufacturing, Mater. Sci. Eng. A, 2018, 715, p 307-314. https://doi.org/10.1016/j.msea.2017.10.002
T. Rodrigues, J. Escobar, J. Shen, V.R. Duarte, G.G. Ribamar, J.A. Avila, E. Maawad, N. Schell, T.G. Santos, J.P. Oliveira, Effect of heat treatments on 316 stainless steel parts fabricated by wire and arc additive manufacturing: Microstructure and synchrotron X-ray diffraction analysis, Addit. Manuf., 2021, (Pt.B), p 48. https://doi.org/10.1016/J.ADDMA.2021.102428
B. Huang, C. Li, L. Shi, Q. GZ, Z. TY, China materials engineering canon, first ed., Chemical Industry Press, 2006, p 646-650.

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You are reading this latest preprint version

Effect of Induction Heat Treatment on Microstructure, Mechanical and Corrosion Properties of Stainless Steel 308L Fabricated using Wire Arc Additive Manufacturing

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Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Material fabrication

2.2 Microstructure analysis

2.3 Mechanical testing

2.4 Electrochemical testing

3 Results

3.1 Microstructure

3.2 Mechanical properties

3.3 Corrosion resistance

4 Discussion

5 Conclusions

Declarations

Author Contribution

Acknowledgements

Data Availability

References

Additional Declarations

Status:

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