Manufacturing and Application of Stainless Steels
01 Jan 2020-
About: The article was published on 2020-01-01 and is currently open access. It has received 7 citation(s) till now.
Summary (2 min read)
Jump to: [1. Introduction and Scope] – [2. Contributions] – [1. Introduction] – [L-PBF Produced Components Features] – [3. Stainless Steel Grades Processed in L-PBF Systems] – [3.2. Precipitation Hardening Stainless Steel Grades] and [4. Conclusions]
1. Introduction and Scope
- Stainless steels represent quite an interesting material family, both from a scientific and commercial point of view, owing to their excellent qualities in terms of strength and ductility, combined with corrosion resistance.
- They find application in all fields requiring materials with good corrosion resistance, together with the ability to be worked into complex geometries.
- At the same time, innovations are coming from the manufacturing process of the stainless steel family of materials, including the possibility to manufacture them from metal powder for 3D printing.
- The scope of this Special Issue embraces interdisciplinary work covering physical metallurgy and processes, reporting about experimental and theoretical progress concerning microstructural evolution during processing, microstructure-properties relationships and various applications, including automotive, energy and structural.
- The book collects manuscripts from academic and industrial researchers with stimulating new ideas and original results.
- It consists of four review and ten research papers.
- The study regards the following topics: (i) embrittlement induced by Cr segregation; (ii) the interaction of hydrogen with C-Cr associates; (iii) the nucleation of Cr carbides.
- The inclusions are extracted using electrolytic extraction.
- The compositional characteristics indicated that two types of inclusions with different sizes (from 1 to 30 μm) existed in 304L stainless steel during the refining process, i.e., CaO-SiO2-Al2O3-MgO external inclusions, and CaO-SiO2-Al2O3-MgO-MnO.
- The reverted austenite has been extensively studied on the microstructure and mechanical properties in the literature, but the thermodynamics and kinetics have been paid less attention to the reverted austenite.
- Meanwhile, the volume of the reverted austenite is too little to clarify the transformation mechanism in previous literature.
- The model is used to describe the phase transformation kinetics by the isothermal transformation in this work.
- By reconstructing the kinetic model from the experimental results, a detailed description of the microstructure evolution during tempering is proposed, which provides a basis for explaining the excellent mechanical properties of the steels.
- This work was conducted to evaluate the dependence of experimental kinetic data on the reverted austenite transformation mechanism in the supermartensitic stainless steel.
L-PBF Produced Components Features
- Impurities Relative density is evaluated as a ratio between the density of L-PBF produced material and the density of the same metal alloy processed with conventional technologies (e.g., rolling, forging).
- The molten metal is strictly in contact with the substrate or the previously consolidated layers, determining a steep temperature gradient  that is at the basis of residual stresses phenomenon [91, 92] .
- Scan pattern refers to the path followed by the laser beam in order to accomplish the complete melting of every slice or layer.
- The majority of metal powders used in L-PBF systems are, nowadays, produced through an atomizing process that involves the interaction of a stream of molten metal with a high energy jet, usually gaseous (e.g., nitrogen of argon)  ; this process is called gas atomization (GA).
3. Stainless Steel Grades Processed in L-PBF Systems
- In the following sections, a review of mechanical performances obtained from L-PBF processed alloys is reported.
- For ease of reading, the correlated process parameters are not shown, but can be consulted in referenced papers.
- This observation should aid readers in understanding the primary importance of stress-relief in L-PBF.
3.2. Precipitation Hardening Stainless Steel Grades
- The tensile performances exhibited by L-PBF 17-4 PH specimens, summarized in Table 4 , are characterized by a wide dispersion between different referenced works.
- In particular, the as-built alloy does not reach minimum standard requirements, resulting in the need to apply (and, eventually, develop) specifically tailored heat treatments.
- On the other hand, standard solubilization and aging treatments have been found to lower high cycle fatigue life.
- Mahmoudi et al. in  found that vertically built samples were characterized by lower strength than those built horizontally, claiming that interlayer bonding was insufficient.
- In the latter case, the results would be diminished by surface discontinuities.
- The main goal of this study was the investigation of cold rolling on the weldability of UNS S32750 duplex stainless steel.
- The importance of this problem arises from the fact that the cold rolling before the heat treatment can significantly increase the rate and decreases the starting temperature of the ferrite.
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Abstract: Gradient structures in engineering materials produce an impressive synergy of strength and plasticity, thereafter, have recently attracted extensive attention in the material families. Gradient structured stainless steels (SS) were prepared by surface mechanical attrition treatment (SMAT) with different impacting velocities. The microstructures of the treated samples are characterized by gradient twin fraction and phase constituents. Quantitative relations of gradient microstructure with impacting time and mechanical properties are analyzed according to the observations of SEM, TEM, XRD, and tests of mechanical property. The processed SSs exhibited to be simultaneously stiff, strong, and ductile, which can be attributed to the co-operation of the different spatial distributions of multi-scaled structures. The formation of gradient twinned structure is resolved and the strengthening by gradient structure is explored.
Abstract: In this paper, the best laser powder bed fusion (L-PBF) printing conditions for FeSi steels with two different Si content (3.0% and 6.5%) are defined. Results show very strict processing window parameters, following a lack of fusion porosity at low specific energy values and keyhole porosity in correspondence with high specific energy values. The obtained microstructure consists of grains with epitaxial growth starting from the grains already solidified in the underling layer. This allows the continuous growth of the columnar grains, directed parallel to the built direction of the component. The magnetic behaviour of FeSi6.5 samples, although the performances found do not still fully reach those of the best commercial electrical steels (used to manufacture magnetic cores of electrical machines and other similar magnetic components), appears to be quite promising. An improvement of the printing process to obtain thin sheets with increased Si content, less than 0.5 mm thick, with accurate geometry and robust structures, can result to an interesting technology for specific application where complex geometries and sophisticated shapes are required, avoiding mechanical machining processes for electrical steel with high silicon content.
Abstract: The rapid growth of additive manufacturing techniques requires a parallel tailoring and further development of already existing models applied to industrial solidification processes. Friendly modelling tools can be a valid aid in setting optimal operating parameters ranges for extending those modelling technologies to already existing or innovative alloys. A modelling approach is described simulating the generation of single tracks scanned over the powder bed in a selective laser melting process, attaining track geometry as a function of alloy thermophysical properties, laser speed and power, and powder bed thickness. Post-processing the model results allows for the derivation of the porosity of the printed part, due to lack of fusion, on one hand, and to yield conditions for the formation of porosities due to keyhole formation, on the other hand. The approach followed is based on a simplified representation of the physical aspects. Main simplifying assumptions concern the laser energy input, modelling the formation of the pool cavity, and modelling the powder bed thermophysical properties. In the model, the effective laser absorptivity that increases with rising specific energy is accounted for at the onset of vaporization to show the real trend of pool volume increase, the subsequent pool cavity deepening, and the laser ray’s interceptions. Modelling the effective laser absorption variation has been validated using literature experimental data relating to laser welding tests performed on 316L disks. The model has been adjusted using literature data providing measures of track width and depth and relative density of printed parts relating to different alloys: Ti6Al4V, Inconel625, Al7050, 316L, and pure copper. Few adjusting parameters are employed, namely: liquid pool effective thermal conductivity, slope of the effective laser absorptivity curve vs specific energy, and slope of laser energy application depth vs specific energy. Other checks on different alloys are needed to refine the adjustment; the results show good potential concerning the future possibility of using the model for achieving operating windows for alloys other than the tested ones, avoiding the need to provide experimental data specific for each alloy.
Abstract: Duplex stainless steel (DSSs) is characterized by excellent corrosion resistance with high strength. Twelve single-component fluxes (TiO2, Fe2O3, Cr2O3, ZnO, ZrO2, CaO, Mn2O3, V2O5, MoO3, SrO, MgO, and LaO2) were tested in the initial experiment using activated Tungsten inert gas (ATIG) technic, and then three couples of oxides were selected as binary fluxes (Fe2O3-Cr2O3, ZnOMn2O3, and V2O5-Mn2O3) for the rest of the study. )e results show that the depth weld of binary oxides (Fe2O3-Cr2O3, ZnOMn2O3) was increased by 3.7 times in comparison with tungsten inert gas (TIG) weld bead. )e hardness and the tensile strength of welds carried out with Fe2O3-Cr2O3 and ZnO-Mn2O3 binary fluxes were close to those of the parent metal. Weld bead executed with ZnO-Mn2O3 oxides has more capability to withstand sudden loads. Potentiodynamic polarization tests were performed. The metal welded with flux composed of Fe2O3-Cr2O3 has been found the most resistant to corrosion.
28 Jul 2020
Abstract: A welded stainless steel tube is a component used in several industrial applications. Its manufacturing process needs to follow specific requirements based on reference standards. This calls for a predictive analysis able to face some critical issues affecting the forming process. In this paper, a model was adopted taking into account the tube geometrical parameters that was able to describe the deformation process and define the best industrial practices. In this paper, the effect of different process parameters and geometric constraints on ferritic stainless steel pipe deformation is studied by finite element method (FEM) simulations. The model sensitivity to the input parameters is reported in terms of stress and tube thinning. The feasibility of the simulated process is assessed through the comparison of Forming Limit Diagrams. The comparison between the calculated and experimental results proved this approach to be a useful tool in order to predict and properly design industrial deformation processes.
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