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.

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Additive manufacturing of steels: a review of achievements and challenges

Effect of tempering temperature on the toughness of 9Cr–3W–3Co martensitic heat resistant steel

Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function of the involved elements is often lacking. It is one of the two main goals of this review to summarize the knowledge for some selected respective topics with a certain focus on non-stoichiometric, i.e., non-ideal Laves phases. The second, central goal of the review is to give a systematic overview about the role of Laves phases in all kinds of materials for functional and structural applications. There is a surprisingly broad range of successful utilization of Laves phases in functional applications comprising Laves phases as hydrogen storage material (Hydraloy), as magneto-mechanical sensors and actuators (Terfenol), or for wear- and corrosion-resistant coatings in corrosive atmospheres and at high temperatures (Tribaloy), to name but a few. Regarding structural applications, there is a renewed interest in using Laves phases for creep-strengthening of high-temperature steels and new respective alloy design concepts were developed and successfully tested. Apart from steels, Laves phases also occur in various other kinds of structural materials sometimes effectively improving properties, but often also acting in a detrimental way.

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Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

Degradation mechanisms in martensitic stainless steels: Wear, corrosion and tribocorrosion appraisal

Schiff bases of 2-({-1-methyl-3-[(2-sulfanylphenyl)imino]butylidene}-amino)-1-benzenethiol and 2-({-1,2-diphenyl-2-[(2-sulfanylphenyl)imino]ethylidene}amino)-1-benzenthiol are investigated as corrosion inhibitors in acid solution. Polarization, electrochemical impedance spectroscopy, and weight loss measurements were performed on mild steel in 15% HCl with and without the inhibitors. A significant decrease in the corrosion rate of mild steel was observed in the presence of investigated inhibitors. Polarization curves indicate that both compounds are mixed inhibitors, affecting both cathodic and anodic corrosion currents. The adsorption of inhibitors on mild steel surface in 15% HCl was found to follow Langmuir adsorption isotherm. Thermodynamic adsorption parameters (Kads, ∆Gads) of studied Schiff bases were calculated using the Langmuir adsorption isotherm. Activation parameters of the corrosion process such as activation energies, Ea, activation enthalpies, ∆H*, and activation entropies, ∆S*, were calculated by the obtained corrosion currents at different temperatures.

The inhibition of mild steel corrosion in hydrochloric acid media by two Schiff base compounds

The effect of tempering on the microstructure and mechanical properties of a non-quenched (NQ) bainitic steel was investigated by optical microscopy, X-ray diffractometry, scanning electronic microscope (SEM) and transmission electronic microscopy (TEM). The results show that the NQ steel which has been investigated is granular bainite composed of bainitic ferrite lath, retained austenite film and island of austenite and martensite (M/A island) before tempering. The amount of retained austenite decreases with the rise of tempering, slowly before 400 °C, sharply at 450 °C, and it is close to 0 at 600 °C. When tempered at 350 °C, this kind of NQ steel has the optimum mechanical properties because of M/A islands partly decomposition, especially for the martensite in the M/A islands. However, when tempered at 450 °C, it appears brittleness, which results from carbides distributing along prior austenite grains because M/A island is largely decomposed.

Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel

Influence of deep cryogenic treatment on microstructure and evaluation by internal friction of a tool steel

This paper is devoted to the internal friction behaviors in cold work die steel after deep cryogenic treatment (DCT). The experimental results include the Snoek relaxation and their dependence on carbon concentration with the soaking time in liquid nitrogen as well as the Snoek–Ke–Koster (SKK) relaxation and explaining by coupling model which is based on the considering of the cooperative migration of the foreign interstitial atoms C, N, O (FIAs) caused by two kinds of interactions, including the FIAs themselves and between the FIAs with time-dependent strain field of dislocations. The decreasing of the Snoek peak height is indicated that the solute C amount in matrix is reduced after DCT treating. Moreover, the carbon atoms segregated to nearby dislocations producing a strong Snoek–Ke–Koster relaxation.

Effect of deep cryogenic treatment on internal friction behaviors of cold work die steel and their experimental explanation by coupling model

A comparative study on non-quenched and quenched prehardened steel for large section plastic mould

This paper investigated the influence of deep cryogenic treatment on the internal friction behaviors in the process of tempering. The internal friction indicates the carbon atoms segregated to nearby dislocations and produced strong interactions, including interstitial carbon atoms themselves and between the interstitial carbon atoms with time-dependent strain field of dislocations. It is made clear that the cluster of carbon atoms nearby the dislocations in the DCT either acts as or grows into nuclei for the formation of carbide on subsequent during tempering. Thus the improvement of wear resistance due to more carbides precipitation after DCT treating has been observed indirectly by using internal friction.

Hongbin Wang

Papers

Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel

Influence of deep cryogenic treatment on microstructure and evaluation by internal friction of a tool steel

Effect of deep cryogenic treatment on internal friction behaviors of cold work die steel and their experimental explanation by coupling model

A comparative study on non-quenched and quenched prehardened steel for large section plastic mould

Influence of deep cryogenic treatment on internal friction behavior in the process of tempering