Additive manufacturing by selective laser melting (SLM) was used to investigate the effect of laser energy density on 316L stainless steel properties. Point distance and exposure time were varied and ...their impact on porosity, surface finish, microstructure, density and hardness, was evaluated. The surface roughness was primarily affected by point distance with increased point distance resulting in increased surface roughness,
R
a
, from 10 to 16 μm. Material hardness reached a maximum of 225 HV at 125 J/mm
3
and was related to the material porosity; with increased porosity leading to decreased material hardness. Different types of particle coalescence leading to convex surface features were observed (sometimes referred to as balling); from small ball features at low laser energy density to a mixture of both small and large ball features at high laser energy density. Laser energy density was shown to affect total porosity. The minimum amount of porosity, 0.38 %, was observed at an energy density of 104.52 J/mm
3
.
Slurry aluminide coatings were elaborated on IN-800HT and HR3C austenitic stainless steels (ASS) and on P92 ferritic-martensitic steels. The thermal treatments conducted in Ar enabled the melting of ...Al and the high temperature synthesis with the substrate elements to result in an aluminium diffusion coating. Whether for the ferritic-martensitic or the austenitic stainless steels, the coatings were formed by the simultaneous Al inward diffusion into the steel matrix and the outward diffusion of Fe (and Ni for the ASS) on both steel substrates. As a result, the coatings exhibited a B2-(Fe,Ni)Al phase for the ASS and B2-FeAl phase for the P92 substrate. A reduction of the grain size after annealing was noticed for the ASS but their microstructures remained mostly austenitic. However, a significant increase of the grain size occurred in the P92 steel with a transformation from the initial tempered martensitic structure to an austenitic structure. The microhardness of the ASS did not change significantly whereas for P92, a large increase occurred.
•Aluminide coatings successfully elaborated on austenitic steels (HR3C, IN-800HT) and on ferritic-martensitic steel (P92).•Diffusion layers were mostly composed by the B2-(Fe,Ni)Al and B2-FeAl for the austenitic and the ferritic steels.•The Al inward and the Fe (and Ni, for the ASS) outward diffusion allowed the formation of a homogeneous diffusion layer.•No significant evolution of the microstructure, grain size and microhardness of the ASS substrates (HR3C and IN-800HT).•The thermal treatment carried out on the P92 substrate had a non-negligible impact on the microstructure of this alloy.
•Understanding influence of processing variables during additive manufacturing of SS 316 to SS 430.•Results show differences in interphase microstructure due to additive manufacturing.•Final products ...show distinct variation in magnetic and non-magnetic properties.
Fabrication of compositionally-graded magnetic-nonmagnetic bimetallic structures was successfully completed using a laser engineered net shaping (LENS™) system. A graded magnetic functionality was implemented by directly transitioning from non-magnetic austenitic stainless steel 316 (SS316) to magnetic ferritic stainless steel 430 (SS430) in a single structure. LENS™ additive manufacturing utilizes a high-powered laser to continuously melt and bond metallic powder in successive layers to create the 3D structure. Microstructures revealed a preferred grain growth direction at the interfaces of the deposited layers. Micro-hardness values across the part’s cross-section exhibited a smooth transition from the highest value of 266 ± 4 HV in the SS430 region to the lowest value at the SS316 substrate of 174 ± 3 HV. Magnetic functionality was observed on the SS430 side of the bimetallic structure, showing how LENS™ can additively combine materials of varying compositions for location-specific functionality.
Selective laser melting (SLM) and laser cladding deposition (LCD) are two typical kinds of laser additive manufacturing techniques that have been developed for many years independently. Although they ...are based on the same principle of laser cladding, there are little comparison on the fundamental studies for metallurgical behavior (including melting and solidification behaviors) and the mechanical properties of these two techniques up to now. In this paper, the single-track formation and the deposition of block sample from 316L stainless steel powders have been carried out by both SLM and LCD techniques. A comparison on pool shape, cooling rate, columnar grain size and mechanical properties under different processing conditions by LCD and SLM respectively has been studied. It is found that, due to the increase of energy input and the decrease of depth-to-width ratio of melting pool (MP) from SLM to LCD, the primary cellular arm spacing (PCAS) of the sample increases from less than 1.0µm to more than 15.0µm, and thus the cooling rate of MP decreases from about 106K/s in SLM to about 102K/s in LCD. Furthermore, due to the decrease of cooling rate from SLM to LCD, the columnar grains of the as forming alloy are getting coarser. Especially, the relationship between gain size (λ) and the reciprocal of square root of cooling rate (Ṫ) in LCD significantly meets the classical linear function of λ=a+b/Ṫ (a and b are constants), while a new relationship of a cubic function is found in SLM, showing the different solidification characteristics between LCD and SLM. Lastly, the samples of 316L stainless steel by SLM have much stronger tensile strength but lower elongation than those by LCD, and the main reason is due to that the solidification behavior of the MPs by SLM can form much finer columnar grains than those by LCD.
Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the ...possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.
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•Individual role of microstructural features on passive film resistance is studied.•Higher donor densities in the sensitized specimen impaired its film resistance.•Greater film ...resistance in coarse-grained specimen is attributed to the lower GBSA.•The defects formed during straining deteriorate the protectiveness of passive film.•Lower defect density in GBE specimen is ascribed to higher fraction of Σ3+Σ9+Σ27.
The individual implication of sensitization, grain size, residual strain, and grain boundary character distribution on semiconducting response of passive film in 304 stainless steel is investigated. Involvement of higher donor densities in the sensitized specimen deteriorated passive film stability. The greater density of random grain boundaries in fine-grained specimen promoted the defects formation in the passive film. In contrast, higher fraction of ‘special’ boundaries and triple junctions, realized through grain boundary engineering, suppressed the formation of defects in the passive film. Interestingly, defects induced via straining accelerated the oxygen vacancy formation at metal/film-interface, which depreciated the protectiveness of passive film.
Ferrite and austenite duplex stainless steel with heterogeneous lamella structure was fabricated by severe cold-rolling and short-time annealing. The processed steel exhibited superior cryogenic ...mechanical properties with a tensile strength of ∼1650 MPa and an ultimate tensile strain of ∼30%. The excellent combination of cryogenic strength and ductility was attributed to fine grain size, nanotwins, back-stress hardening, and deformation-induced martensitic transformation.
•A total of 196 new concrete-filled ferritic stainless steel tubular (CFFSST) joint results are reported.•CIDECT provisions exhibit remarkable scatter predictions for the CFFSST joints.•Existing ...design rules are generally not accurate for the CFFSST joints.•Improved design rules are proposed to predict the capacities of CFFSST X-and T-joints.
The behaviour and resistances of concrete-filled ferritic stainless steel tubular (CFFSST) joints were studied and presented in this paper based upon experimental and numerical investigations. A test program was firstly undertaken on a series of X- and T-joints with square and rectangular hollow section (SHS and RHS) chords. The stainless steel SHS/RHS tubes were cold-rolled from EN 1.4003 ferritic grade sheets, and the specimens were in-filled with either normal or high strength concrete. The experimental study including the detailed material properties, joint test setups, test procedures and results is reported. Numerical models were also developed and verified against the CFFSST joint test results, and followed by a parametric study generating further numerical results over a wider range of key joint parameters. The obtained experimental and numerical CFFSST joint capacities were compared with existing design recommendations as per the CIDECT Design Guide and design rules proposed by previous researchers. The comparison results indicate that the existing provisions are not capable of predicting the capacities of the investigated composite tubular joints in an accurate and reliable manner. Hence, modified design rules are put forward in this paper for the design of CFFSST joints.
In this paper, the in-plane behaviour and design of duplex and ferritic stainless steel welded I-section beam–columns fabricated through the welding of individual hot-rolled stainless steel plates ...are explored. Finite element models able to replicate the structural response of stainless steel I-section members are created and validated against experimental results from the literature. Using the validated finite element models, extensive numerical parametric studies are performed, the results of which are utilised to investigate the accuracy and reliability of existing design provisions for duplex and ferritic stainless steel I-section beam–columns. Scope for improvement is revealed, prompting the development of new beam–column design rules that are compatible with those provided for carbon steel members in EN 1993-1-1. The new proposals are shown to offer improved accuracy and consistency over existing design provisions for duplex and ferritic stainless steel I-section members under combined axial compression and bending, and are recommended for inclusion in the upcoming revision to EN 1993-1-4. The reliability of the proposed design rules, with a partial safety factor γM1=1.1, is demonstrated.
•Behaviour and design of duplex and ferritic stainless steel welded I-section beam–columns are investigated.•Finite element models able to mimic the behaviour of stainless steel beam–columns are developed.•Accuracy of existing beam–column design methods is investigated.•New design rules for duplex and ferritic stainless steel welded I-section beam–columns are proposed.•Proposed beam–column design rules lead to more accurate results relative to the existing design methods.
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