Study on the corrosion resistance of additively manufactured Inconel 718 and Stainless steel 316L and kinetics involved in the heat treatment of Inconel 718
Additive manufacturing (AM) is a method to fabricate near-net shaped metallic components overcoming the requirement of traditional methods of manufacture like forging, casting, welding and having the advantage of minimum material requirement and ability to produce complex geometries. The most commonly used fusion technique is the powder bed fusion which uses electron beam or laser beam to melt and fuse the powdered material. The former is called electron beam melting (EBM) and the latter is called selective laser melting (SLM). In order to understand the corrosion behavior of these AM manufactured materials we have selectively selected IN718 and SS316. Inconel 718 (IN 718) is a nickel based superalloy that has been widely applied in aerospace, pressure, and energy industries. Stainless steel (SS 316L) is widely applied in the field of biomedical, food processing, petrochemical industries and various other engineering fields. In the present work, SS 316L and IN 718, prepared using SLM technique, is studied and compared for their corrosion resistance. The studies are being conducted for as-printed samples of Inconel 718 and Stainless steel 316L having horizontal and vertical build direction. Corrosion study, conducted in a suitable corrosive medium, is performed using potentiodynamic polarization. The corrosion rates for SLM samples of both alloys are checked. The grain boundary character distribution can be correlated to corrosion rate. Investigation of microstructure of samples has been done by Scanning Electron Microscopy (SEM). Measurement of material hardness was performed with Vickers hardness test. Also, the study on the evolution of precipitation behavior for two different additive manufacturing processes EBM and SLM for Inconel 718 is performed. The precipitation behavior of SLM and EBM Inconel 718 is studied through optimizing heat treatment schedule in order to understand its effect through driving force and kinetics involved.
Keywords: additive manufacturing, selective laser melting, electron beam melting, corrosion resistance, kinetics
|SLM||Selective Laser Melting|
|EBM||Electron Beam Melting|
|IN 718||Inconel 718|
|SS 316L||Stainless Steel 316L|
|SEM||Scanning Electron Microscopy|
Inconel 718 Superalloy
Inconel 718 is a solid-solution or precipitation strengthened nickel-based austenite superalloy which is widely applied in aerospace, petrochemical and nuclear industries due to its good high temperature strength and oxidation and corrosion resistance up to very high temperatures . It can be used in environments from cryogenic all the way up to 704°C. Throughout this entire range it exhibits exceptionally high yield, tensile and creep-rupture properties. It also shows excellent tensile and impact strength .
Inconel 718 accounts for up to 50% of the weight of aircraft turbojet engines, being the main component for discs, blades and casing of the high pressure section of the compressor and discs as well as some blades of the turbine section .
The major phases that precipitate in a face centered cubic (fcc) matrix of the superalloy are metastable γ″ equilibrium γ′ and equilibrium δ phases. The age hardening in this alloy is mainly brought about by the precipitation of the γ″ phase particles that have coherent disc shaped morphology. Some strengthening is brought about by the precipitation of coherent γ′ particles as well. The equilibrium phase corresponding to the metastable γ″ phase is the δ phase . Alloy 718 is susceptible to the δ phase formation that can evolve during processing. On prolonged exposure at intermediate temperatures, γ″ phase gets transformed to the δ phase. At relatively lower temperatures, δ phase nucleates at austenite grain boundaries as well as coherent and incoherent twin boundaries. The δ precipitates are incoherent and their precipitation is known to reduce the strength of γ″ strengthened alloys , but controlled precipitation of δ phase has some beneficial effects like grain stabilization and good stress rupture properties .
Stainless Steel 316L
Stainless steel 316L is a highly corrosion resistant type of stainless steel containing high concentrations of nickel and molybdenum that helps in preventing crevice and pitting corrosion compared to conventional nickel chromium stainless steels such as 302-304 . Stainless steel 316L is very durable and resistant to chemical contaminants and acidic solutions such as bromides, sulfuric acid and chlorides. Its other characteristics include higher creep resistance, excellent formability and rupture and tensile strength at high temperatures .
The chromium, in the presence of air (oxygen), forms a thin film of chromium oxide which covers the surface of the stainless steel. Chromium oxide is inert or “passive” by nature, and chromium in the material gives stainless steel its corrosion-resistant properties.
Objectives of Research
Study and compare the corrosion resistance of as-printed samples of Inconel 718 and Stainless steel 316L. Also, study the precipitation behavior of SLM and EBM Inconel 718 through optimized heat treatment schedule in order to the study the effect of kinetics involved.
Chen et al. studied the effect of heat treatment on microstructure and corrosion behavior of stainless steel 316L made by gas metal arc additive manufacturing technique. It was observed that the heat treatment at 1000°C increased the amount of σ phase in steel while the heat treatment at 1100°C to 1200°C completely eliminates σ phase. The σ phase has more detrimental effect on ductility and it increases the possibility of cracks generation. This is because the σ phase formation leads to a Cr-depleted region at σ phase interface that increases sensitivity of corrosion attack. Therefore it was concluded that heat treatment improves corrosion resistance of steel, and the corrosion resistance increases with the increase of heat treatment temperature and time .
The study on the erosion-corrosion behavior was performed by Laleh et al. on stainless steel 316L produced by selective laser melting and the performance was compared to its conventionally produced counter-part. Though the SLM-produced 316L SS possessed better pitting corrosion resistance compared to conventionally produced one, it showed lower erosion-corrosion resistance compared to the other. This behavior was attributed to weaker repassivation ability of the SLM-produced specimen which might be due to the existence of small pores in the microstructure .
D. Kong et al. studied the corrosion behavior of stainless steel 316L, fabricated by SLM, after heat treatment. It was observed that the passive film thickness and the corrosion potential decreased after the heat treatment due to the compressive stress relief and decrease in dislocation density. It was also observed that the pitting potential decreased sharply after the heat treatment due to the accelerated transition from metastable to steady state pitting which was explained to be caused by the presence of thin passive films at the enlarged pores after heat treatment .
Jia et al. worked on selective laser melting (SLM) processed Inconel 718 alloy and studied the high-temperature oxidation behaviors and mechanisms. The work showed that on increasing the applied volumetric laser energy density, oxidation resistance performance of the alloy was improved. It was confirmed by this work that the uniformly distributed microstructures along with the signiﬁcantly improved relative density of SLM processed Inconel 718 alloy was responsible for the improved high temperature oxidation resistance performance .
B. Zhang et al. investigated the corrosion behavior of SLM Inconel 718 after various post treatments that include heat treatment and electrochemical polishing. Heat treated samples were subjected to 3 different solution treatment temperature- 1040 °C, 1100 °C and 1200 °C. The work showed that as-printed sample and the samples after heat treatments at 1040°C and 1100°C showed relative low corrosion rate and high pitting corrosion resistance due to γ′/γ′′ precipitates generation and residual stress release. But the samples after heat treatment at 1200°C showed a very weak pitting corrosion resistance which was caused by Cr and Mo depletion in the bulk matrix due to carbide formation during solution treatment. Electrochemical polishing process also provided a strong corrosion resistance protection for SLM Inconel 718 sample .
Selective laser melting (SLM) technique
Selective laser melting is an additive manufacturing technique wherein the metal powder is melted using laser beam. SLM is characterized by high cooling rates of about 104 and 106 Ks−1. SLM technique operates under an inert atmosphere, usually Argon or Nitrogen, with a cold powder bed.
The SS316L samples were built horizontally and vertically while IN718 samples were prepared vertically only. The studies were conducted for IN718 and SS316L-horizontal built in along the build direction and SS316L-vertical built in across the built direction.
Parameters for SLM Stainless Steel 316L
The 316 L SS samples was additively manufactured by SLM technique using Intech DMLS Pvt. Ltd, Bangalore, India (EOSINT M 280). The 3D printing was performed using a 195 W power laser with a scan speed of 650 mm/s. The size of the powder particles varied between 25–50 μm.
Electron beam melting (EBM) technique
Electron beam melting is a technique that utilizes a high power electron beam to generates the energy needed for fusing the metal powder to produce metallic components. The electron beam is managed by electromagnetic coils providing extremely fast and accurate beam control. EBM use a hot bed (>870 K) and hence do not produce a fine microstructure like the SLM process.
Polishing of samples
Polishing is used to create a flat and scratches-free surface for examination of the sample’s microstructure. The samples were polished using carborundum emery paper. Coarse polishing was done using 220, 400, 600 and 800 grade papers. Fine polishing was done by using 1000, 2000, 3000 and 4000 grade papers. Paper polishing was followed by cloth polishing. Diamond paste of particle sizes 2, 0.5-1 and 0.25 micrometers were applied on the velvet cloth, one paste at a time, and the samples were polished on the cloth. The orientation of a sample was changed by 90o after polishing with each paper and diamond paste. The paper and cloth polishing was done in the polishing machine to get good surface finish.
To obtain a mirror finish of the sample surface, electropolishing was done in A2 solution (A2 solution contains 78 mL perchloric acid, 70 mL distilled water, 730 mL ethanol, 100 mL butoxy ethanol) and all the residual scratches were removed.
Heat Treatment of Kinetics Samples
Through the optimized heat treatment schedule, the precipitation behavior of SLM and EBM IN718 is studied. It is solution annealed at 1040˚C for 1 hour followed by rapid cooling by water quenching and temperature is brought to 960˚C where it is maintained for 1 hour followed by water quenching. This is followed by precipitation hardening at 760˚C for 8 hours, cooling to 640˚C and holding for 8, 18, 38 and 48 hours respectively for different samples followed by air cooling to room temperature.
Scanning electron microscopy (SEM)
A scanning electron microscope (SEM) is a versatile characterization tool that creates the images of the sample after scanning a focused electron beam over a surface. The electrons in the beam interact with the atoms in the sample, producing various signals that can be used to obtain information about the sample including external morphology (texture), chemical composition, and crystalline structure. SEM provides better resolution and higher magnification compared to optical microscopy.
The electropolished samples were mounted with great care to avoid scratches on the surface. The hardness test is performed using Future Tech Microhardness tester with a diamond intender with an angle of 136o between the opposite faces of the pyramid. The test was conducted by applying a load of 200gf with dwell time of 10 seconds. The average hardness value was calculated after taking 10 intendations in various regions of the sample surface.
The Vickers Hardness Number (VHN) was found using the equation:
VHN = 1.8544P/D2
Potentiodynamic polarization technique is the most commonly used corrosion test wherein the potential of the electrode is varied at a selected rate by application of a current through the electrolyte.
Electrochemical measurements, was carried out a conventional three-electrode cell at room temperature (25 °C), and conducted in a 3.5 wt% NaCl solution with a platinum wire acting as the cathode and a saturated calomel electrode (SCE) as the reference electrode. The sample acts as the anode and the electrical contact of the specimens is made from the rear by soldering a wire on to the back of the specimen and all reported potentials was relative to saturated calomel electrode (SCE).
Extrapolation of linear sections from the anodic and cathodic curves in the Tafel plot is done and the intersection point corresponds to corrosion current density (icorr) or corrosion rate and the corrosion potential (Ecorr).
RESULTS AND DISCUSSION
Kinetics of IN 718
Through the optimized heat treatment schedule, the precipitation behavior of SLM and EBM IN718 is studied. Initially the samples were homogenized to dissolve all the niobium in the matrix which was segregated to inter-dendritic region and it was water quenched to achieve supersaturated solid solution of γ. Three heat treatments are involved further to give sufficient driving force for delta precipitate to come out first. Since the temperature was 960oC very less difference from solvus temperature of delta, the δ would precipitate at the grain boundaries which provides a necessary path for niobium diffusion.
Again is water quenched to achieve to same microstructure. Now, the heat treatment is done at 760oC and 640oC below the solvus temperature of γ' and γ'' in order for the precipitate to come out from the matrix. The driving force involved was very less, so only way to improve the matrix strengthening was by increasing the volume fraction of γ' and γ'' which can be achieved through kinetics (holding the sample at high temperature for prolong times) .
Problem which arises during prolong holding at high temperature is the grain coarsening, which reduces effective grain boundary area that reduce δ precipitates, thereby hardness and strength might decrease even though there is an increases in the fraction of precipitates.
From Fig. 5 a), it was observe that the initial microstructure were columnar and we could see delta along the build direction. As we move on, the microstructure was transformed to equiaxed, the driving force for such transformation was the residual stress induced. In the material, during manufacturing, we could see grains getting coarsened as kinetics increases. We could see around the δ or γ'' region, under BSE mode appears dark due to depletion of niobium surrounding the precipitate. It is never possible to increase δ and γ'' at the same time since the chemical composition is same Ni3Nb, just the structure is different. Thus it is necessary to optimize heat treatment schedule to get the right microstructure to be exploited for high temperature use.
The hardness trends in Fig. 7 show that for both SLM and EBM, the hardness values drop after 48 hrs of heat treatment.
Fig. 6 and Table 1 show the corresponding polarization curves and the measured values of the electrochemical parameters respectively.
|MATERIAL||Ecorr (V)||icorr (A/cm2)|
|SS 316L Vertical build||-0.589||-5.835|
|SS 316L Horizontal build||-0.281||-6.745|
The values of icorr and Ecorr for each set of sample is found out by extrapolating tangents at the inflection points on anodic and cathodic curve.
The low corrosion current density (icorr) and high difference between pitting potential and corrosion potential (∆E) indicate a slow corrosion rate with high corrosion resistance
Based on this icorr, it can be interpreted that SS 316L with horizontal build has higher corrosion resistance as icorr is lower for it and IN 718 has the lowest corrosion resistance
Now looking at the passivating regime, for SS 316L, the cathodic part is much extended which means metal dissolution rate is high, because of oxygen tries to combine with chromium to form the passivating layer at much earlier stage, therefore passivating region begins at much lower negative potential, the passivating regime has serrations which indicates pitting.
Kinetics of IN 718
From the SEM images, the δ precipitates is observed along the grain boundaries and can be seen as bright spots but it needs to confirmed by EPMA. Further tests need to be done for identification of phases and elemental segregation and quantification.
The corrosion resistance is found to be more for horizontal built SS 316L than vertical built and the corrosion resistance is the least for IN 718. Futher microstructural analysis needs to done to confirm this corrosion resistance behaviour.
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It gives me immense pleasure and privilege in thanking everyone who helped me in the successful completion of my internship project.
First and foremost, I’m extremely grateful to the Indian Academy of Sciences, for giving me the opportunity to do my internship in the Department of Materials Engineering in Indian Institute of Science, Bangalore
I express my heartiest thanks and deep sense of gratitude to my guide, Dr. Satyam Suwas, Professor, Department of Materials Engineering, Indian Institute of Science, Bangalore who guided me throughout the work.
I extend my heartfelt gratitude to my mentors Mr. R. J. Vikram, Mr. Supreeth Gaddam, Mr. Deepak Kumar for their constant support and encouragement.
I would like to thank all the research scholars, project assistants, M.Tech students, technical staffs, office staffs and my fellow internship students in IISc for their constant support, care and love to complete this project successfully.
I would also like to express my sincere gratitude to Dr. K. Haribabu from NIT Calicut who had encouraged me in all my endeavours.
Last but not the least I thank my parents and my brother for being my pillar of strength and motivation. I apologize if I missed anyone and I sincerely thank everyone who directly or indirectly helped me finish my project.