Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and ...the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.
Metal additive manufacturing (AM), also known as 3D printing, is a disruptive manufacturing technology in which complex engineering parts are produced in a layer-by-layer manner, using a high-energy ...heating source and powder, wire or sheet as feeding material. The current paper aims to review the achievements in AM of steels in its ability to obtain superior properties that cannot be achieved through conventional manufacturing routes, thanks to the unique microstructural evolution in AM. The challenges that AM encounters are also reviewed, and suggestions for overcoming these challenges are provided if applicable. We focus on laser powder bed fusion and directed energy deposition as these two methods are currently the most common AM methods to process steels. The main foci are on austenitic stainless steels and maraging/precipitation-hardened (PH) steels, the two so far most widely used classes of steels in AM, before summarising the state-of-the-art of AM of other classes of steels. Our comprehensive review highlights that a wide range of steels can be processed by AM. The unique microstructural features including hierarchical (sub)grains and fine precipitates induced by AM result in enhancements of strength, wear resistance and corrosion resistance of AM steels when compared to their conventional counterparts. Achieving an acceptable ductility and fatigue performance remains a challenge in AM steels. AM also acts as an intrinsic heat treatment, triggering ‘in situ’ phase transformations including tempering and other precipitation phenomena in different grades of steels such as PH steels and tool steels. A thorough discussion of the performance of AM steels as a function of these unique microstructural features is presented in this review.
Transformation-induced plasticity (TRIP) assisted duplex stainless steels, with three different stabilities of the austenite phase, were investigated by synchrotron x-ray diffraction characterization ...during in situ uniaxial tensile loading. The micromechanics and the deformation-induced martensitic transformation (DIMT) in the bulk of the steels were investigated in situ. Furthermore, scanning electron microscopy supplemented the in situ analysis by providing information about the microstructure of annealed and deformed specimens. The dependence of deformation structure on austenite stability is similar to that of single-phase austenitic steels with shear bands and bcc-martensite (α′) generally observed, and blocky α′ is only frequent when the austenite stability is low. These microstructural features, i.e. defect structure and deformation-induced martensite, are correlated with the micro- and macro-mechanics of the steels with elastoplastic load transfer from the weaker phases to the stronger α′, in particular this occurs close to the point of maximum rate of α′ formation. A clear strain-hardening effect from α′ is seen in the most unstable austenite leading to a pronounced TRIP effect.
The mechanical properties and corrosion resistance of 316 L stainless steel fabricated using the Laser Engineered Net Shaping (LENS) technique have been studied. The crack-free, full density samples ...made using SS316L alloy powder and the LENS technique are characterized by an unusual distinct dual-phase microstructure. STEM analysis revealed a significant increase of Cr and Mo content and a decrease of Ni in the grain boundaries. Based on the Cr and Ni content (austenite stabilizing elements), the Schaeffler diagram and the EBSD results, the existence of intercellular delta ferrite on subgrain boundaries and austenite in the fine-grains are observed. The XRD patterns, in addition to the FCC austenite phase, revealed the second BCC ferrite phase. Moreover, the sigma (FeCr) phases are present in the analyzed 316 L stainless steel. The occurrence of ferrite, which does not occur in regular stainless steel fabricated using conventional metallurgical methods, improves the mechanical and corrosion properties of the LENS-fabricated sample made using 316 L stainless steel powder. The obtained results prove that the microstructure of the SS316L fabricated using LENS is heterogeneous; its impact on the mechanical properties is visible. The analyzed samples are characterized by anisotropic mechanical properties that are favorable. For both the perpendicular and parallel directions of tensile tests, samples had a ductile fracture with many dimples inside of the larger dimples. The corrosion potential of SS316L LENS and classically manufactured steel is similar. The SS316L fabricated using LENS is characterized by a relatively low value of corrosion current density, which translates into much smaller corrosion rates.
Developing ultrahigh strength steels that are ductile, fracture resistant, and cost-effective would be attractive for a variety of structural applications. We show that improved fracture resistance ...in a steel with an ultrahigh yield strength of nearly 2GPa can be achieved by activating delamination toughening coupled with transformation induced plasticity. Delamination toughening associated with intensive but controlled cracking at Mn-enriched prior-austenite grain boundaries normal to the primary fracture surface dramatically improves the overall fracture resistance. As a result, fracture under plane-strain conditions is automatically transformed into a series of fracture processes in "parallel" plane-stress conditions through the thickness. The present "high-strength induced multi-delamination" strategy offers a different pathway to develop engineering materials with ultra-high strength and superior toughness at economical materials cost.
Microstructures and tensile properties of a Ti and Mo micro-alloyed ultrafine-grained medium Mn TWIP-TRIP steel were investigated. The steel exhibits a superior combination of strength and ductility, ...i.e. 46800MPa∙%, achieved by a high phase fraction of room temperature austenite with enhanced stability. Grain refinement is obtained from the formation of nano-sized (Ti, Mo)C precipitates by optimization of the intercritical annealing time and temperature after cold rolling. The intercritical annealing temperature dependence of the deformation mechanisms is investigated in detail. The localized deformation, pronounced when the TRIP effect is dominant, is related to the correlated strain-induced martensitic transformation of austenite islands.
Effect of parameters such as current density, welding time, electrodes force and holding time after welding on tensile-shear strength of dissimilar joint were analyzed. Taguchi method was used to ...design the experiments and Minitab software was used to analyze the effect of parameters and obtain an optimum condition. The microstructure of the optimum condition was analyzed by Scanning Electron Microscope (SEM). The strength of the join was analyzed by tensile-shear test. Fracture mode was assessed by optical microscope. Results showed that current density, holding time after welding, welding time and electrodes force were the most effective parameters hierarchically. Optimum current density of 8 Kamp, holding time after welding of 40 cycle, 16 cycle welding time and 5 kN electrode force were defined as the optimum parameters to join the sheets. Weld nugget (WN) had a martensitic structure containing a relatively high strength and hardness. Its fracture mode was pullout failure.
•Spot welding on DP600 steel and AISI 304 stainless steel sheet were performed.•Optimum conditions was obtained for four weld parameters.•Taguchi method was used to obtain the level of four welding parameters.•Weld nugget Microstructure was martensitic with high mechanical strength & hardness.•The fracture mode of the joint was pullout failure.
•Cr leads to amorphization of the inner rust layer formed on the steels.•Amorphous corrosion products leads to suppression of the oxygen reduction.•Fluid flow results mainly in accelerated cathodic ...reaction.
The corrosion behavior of carbon steel and Cr-containing steels was investigated during flow-accelerated corrosion. The corresponding weight loss and electrochemical properties were determined by using a flow-loop apparatus and a rotating-cylinder apparatus, respectively. The corrosion rate of the Cr-containing steels is lower than that of the carbon steel. Furthermore, the high corrosion resistance of these steels was closely correlated with the properties of the rust layer formed on the steel surface. Cr leads mainly to inhibition of the cathodic reaction. Therefore, Cr is suitable for improving the corrosion resistance of low alloy steel to flow-accelerated corrosion.
Heavy duty (HD) vehicles are projected to be the largest fuel-use subsector in transportation, with current demand for diesel fuel projected to grow 30% by 2040. Historically, a primary strategy for ...increasing diesel engine efficiency has been to increase peak cylinder pressure (PCP). However, increasing PCP imparts greater mechanical and thermal loads on engine components and materials. In recent years, the material property limits for many components have been reached and further increases in PCP above ∼20 MPa have been difficult, while still maintaining the necessary affordability and longevity of on-road HD diesel engines. This paper reviews the historical evolution and major metallurgical advancements of high temperature materials in HD on road diesel engines (10–15 L displacement) up to the current state of the art, focusing on materials in the engine block, cylinder heads, pistons, valves, and exhaust components. These components cover a wide range of material classes, including cast iron, ferritic steel, austenitic steel, titanium alloys, nickel based super-alloys, and high temperature coatings. The microstructural degradation and failure mechanisms of the materials associated with the complex mechanical and thermal loading during service are discussed and key areas for future materials research are suggested that overcome technical barriers.
•Addition of Ni (3.0 wt%) promotes a notable increase in corrosion resistance. To unpainted use:•In ISO C2 atmospheres: all the tested steels.•In ISO C3 atmospheres: the steels alloyed with 0.50 wt% ...Cr or 3.0 wt% Ni.•In ISO > C3 atmospheres: none of the tested steels.
This work considers fourteen weathering steels manufactured by adding Cu, Cr and Ni alloying elements to extra mild steel. The steels were exposed for up to 5 years in different atmospheres. The atmospheric corrosion resistance of the steels was evaluated and the rust layers formed were characterised by X-ray diffraction, optical microscopy and scanning electron microscopy. The addition of Ni, particularly at 3.0 wt%, leads to a notable increase in corrosion resistance. The rusts formed contain lepidocrocite, goethite and spinel phases. X-ray diffraction also detected small amounts of kornelite (industrial atmosphere) and akaganeite (marine atmosphere).
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