Properties of 25Cr7Ni stainless steel fabricated through laser-powder bed fusion.

Stainless steel is a low carbon high alloyed system with higher concentrations of Cr& Ni, which impart high corrosion resistance to them. Alloys with approximately 25% Cr & 7% Ni in their chemical composition are commercially referred to as ‘Super Duplex Stainless Steel’. They have a unique phase composition of approximately 50% ferrite & 50% austenite, yielding a robust combination of high mechanical strength & corrosion resistance. They find extensive interest & application in the fields which demand a longer service life under intense mechanical / corrosive environment such as offshore oil rigs & pipelines in nuclear power plants. Traditional thermal processing and fabrication of super duplex stainless steel are fraught with limitations and shortcomings in terms of detrimental phase formation. Laser-Powder Bed Fusion is a form of additive manufacturing that involves layer wise addition and consolidation of metal powders in near net shape parts. The process is characterized by high cooling rates to the tune of 107 k/s. This unique characteristic allows for the suppression of formation of detrimental phases and is leveraged in processing of super duplex stainless steels. The available literature on L-PBF fabrication of super duplex stainless steel in comparison to conventional stainless steel alloys is quite lacking. This study quantitively established the influence of the Laser-Powder Bed Fusion (L-PBF) process parameters, starting powder attributes, chemical composition, inert atmosphere & Hot Isostatic Pressing (HIP) on the as-printed properties of the fabricated super duplex stainless steel samples. As-printed samples of a gas atomized super duplex stainless steel yielded the highest UTS, yield strength and comparable corrosion resistance to wrought-annealed, MIM, PM, L-PBF literature super duplex stainless steel. Economical water atomized super duplex stainless steel powder was used to fabricate samples which had higher UTS, yield strength & comparable corrosion resistance to wrought-annealed stainless steel

Effects of Hot Isostatic Pressing on the Properties of Laser-Powder Bed Fusion Fabricated Water Atomized 25Cr7Ni Stainless Steel

25Cr7Ni stainless steel (super duplex stainless steels) exhibits a duplex microstructure of ferrite and austenite, resulting in an excellent combination of high strength and corrosion resistance. However, Laser-Powder Bed Fusion fabrication of a water-atomized 25Cr7Ni stainless steel of novel chemical composition resulted in a purely ferritic microstructure and over 5% porosity. The current study investigated the effects of two hot isostatic pressing parameters on the physical, mechanical, and corrosion properties as well as microstructures of water-atomized 25Cr7Ni stainless steel of novel composition fabricated by L-PBF for the first time in the literature. The corrosion behaviour was studied using linear sweep voltammetry in a 3.5% NaCl solution. The Hot Isostatic Pressing-treated sample achieved over 98% densification with a corresponding reduction in porosity to less than 0.1% and about 3 similar to 4% in annihilation of dislocation density. A duplex microstructure of ferrite 60% and austenite 40%was observed in the X-Ray Diffraction and etched metallography of the HIP-treated samples from a purely ferritic microstructure prior to the HIP treatment. With the evolution of austenite phase, the HIP-treated samples recorded a decrease in Ultimate Tensile Strength, yield strength, and hardness in comparison with as-printed samples. The variation in the morphology of the evolved austenite grains in the HIP-treated samples was observed to have a significant effect on the elongation. With a reduction in porosity and the evolution of the austenite phase, the HIP-treated samples showed a higher corrosion resistance in comparison with the as-printed samples

Effects of Hot Isostatic Pressing on the Properties of Laser-Powder Bed Fusion Fabricated Water Atomized 25Cr7Ni Stainless Steel

25Cr7Ni stainless steel (super duplex stainless steels) exhibits a duplex microstructure of ferrite and austenite, resulting in an excellent combination of high strength and corrosion resistance. However, Laser-Powder Bed Fusion fabrication of a water-atomized 25Cr7Ni stainless steel of novel chemical composition resulted in a purely ferritic microstructure and over 5% porosity. The current study investigated the effects of two hot isostatic pressing parameters on the physical, mechanical, and corrosion properties as well as microstructures of water-atomized 25Cr7Ni stainless steel of novel composition fabricated by L-PBF for the first time in the literature. The corrosion behaviour was studied using linear sweep voltammetry in a 3.5% NaCl solution. The Hot Isostatic Pressing-treated sample achieved over 98% densification with a corresponding reduction in porosity to less than 0.1% and about 3~4% in annihilation of dislocation density. A duplex microstructure of ferrite 60% and austenite 40%was observed in the X-Ray Diffraction and etched metallography of the HIP-treated samples from a purely ferritic microstructure prior to the HIP treatment. With the evolution of austenite phase, the HIP-treated samples recorded a decrease in Ultimate Tensile Strength, yield strength, and hardness in comparison with as-printed samples. The variation in the morphology of the evolved austenite grains in the HIP-treated samples was observed to have a significant effect on the elongation. With a reduction in porosity and the evolution of the austenite phase, the HIP-treated samples showed a higher corrosion resistance in comparison with the as-printed samples

Effects of Powder Characteristics and Chemical Composition on the Properties of 25Cr7Ni Stainless Steel Fabricated by Laser-Powder Bed Fusion and Evaluation of Process Simulation

The 25Cr7Ni stainless steel alloy system is gaining increasing interest in the oil and gas industry because of its combination of high strength and corrosion resistance properties. However, very few studies on the effects of starting powder attributes and chemical composition on the as-printed properties of 25Cr7Ni stainless steel fabricated through laser-powder bed fusion (L-PBF) exist in the literature. This study examined the influence of powder attributes and chemical composition on the samples from gas atomized and water atomized 25Cr7Ni stainless steel powders, fabricated through L-PBF, on their as-printed microstructure and properties. The mechanical properties that were examined included ultimate tensile strength (UTS), elongation (%), and hardness. The corrosion behavior was also studied using linear sweep voltammetry in 3.5 wt.% NaCl solution. The evolved phases were characterized using optical and scanning electron microscopy, as well as through X-ray diffraction. The gas atomized powders, with their spherical and uniform morphology, yielded as-printed parts of higher relative densities when compared to water atomized powders, with irregular morphology due to better powder bed compaction. The higher densification obtained in the L-PBF samples from gas atomized powders translated into the highest UTS, hardness, and yield strength among the L-PBF samples from water atomized powders and wrought–annealed 25Cr7Ni stainless steel. The presence of higher amounts of N and Mn in the chemical composition of the gas atomized powders over water atomized powders promoted the presence of retained austenite in the corresponding L-PBF samples. Higher amounts of Mo, combined with austenite content, yielded a higher corrosion resistance in the L-PBF samples from the gas atomized powder than in the L-PBF samples from the water atomized powders. The latter part of the work is focused on the evaluation of simulation parameters for analyzing the fabrication procedure for the L-PBF process using Simufact software. For a given set of process parameters, Simufact provides the distortion and internal stresses developed in the printed parts as output. The present study sought to evaluate the process simulation by comparing the experimental observations in terms of the part distortion achieved in a stainless steel cube fabricated through L-PBF with Simufact process simulation obtained using the same set of process parameters

Laser powder bed fusion of in-situ composites using dry-mixed Ti6Al4V and Si3N4 powder

Herein, we report laser powder bed fusion (L-PBF) of dry-mixed Ti6Al4V + Si3N4 powder to create in-situ titanium matrix composites. The dry-mixed Ti6Al4V powder with 5 wt.% Si3N4 was processed using L-PBF at varying laser energy densities, between 44 and 133 J/mm(3), by changing the laser scan speed (400-1200 mm/s) at constant laser power of 96 W, layer thickness of 20 mu m and scan spacing of 90 mu m. The selected samples were examined for microstructural evolution, in-situ reaction products and hardness. The results showed that the in situ reaction between liquid titanium and Si3N4 forms fine TiN and Ti5Si3 reinforcements in these L-PBF processed samples. However, the irregular shape and fine size of Si3N4 reduced the feedstock flowability, and the composites could not be processed with laser energy density (E) < 89 J/mm(3). The amount, distribution and size of the reinforcements were found to depend on the laser energy density. These in-situ composites exhibited high hardness of 860 +/- 49 KHN, which is 110 % higher than that of Ti6Al4V and ex-situ processed Ti-TiN and Ti-TiC composites. Our results show that the dry-mixed Ti6Al4V-Si3N4 feedstock can be processed using L-PBF but further improvement is required through adjusting Si3N4 powder attributes (size, shape) and concentration

Properties of Water Atomized 25Cr7Ni Stainless Steel Processed by Laser-Powder Bed Fusion

The 25Cr7Ni stainless steel is characterized by its two-phase microstructure consisting of ferrite and austenite, contributing to an excellent combination of mechanical and corrosion properties. The present study examined the effects of laser energy density and laser powder bed fusion (L-PBF) process parameters on the physical, mechanical and corrosion properties of a water atomized 25Cr7Ni stainless steel powder processed through L-PBF. The results from the study saw that a combination of L-PBF process parameters (laser scan speed and laser scan spacing at a constant layer thickness) as critical factors affecting the mechanical and corrosion properties of the printed samples. The Archimedes density, mechanical and corrosion properties of samples improved with increase in energy density. The as-printed samples displayed single-phase ferritic microstructure and higher mechanical strength (1050 MPa) compared to wrought, metal injection molded (MIM), powder metallurgically sintered (PM) 25Cr7Ni stainless steel (super duplex stainless steel) alloys. The samples exhibited comparable corrosion resistance to that of a wrought 25Cr7Ni stainless steel despite the presence of only ferritic microstructure