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.

Steels in additive manufacturing: A review of their microstructure and properties

Metal matrix composites are an attractive choice for aerospace and automotive applications because of their high stiffness-to-weight ratio. Composites with aluminum and magnesium matrices have been investigated extensively, while less work has been carried out on steel matrix composites. In the present study the hot isostatic pressing (HIP) process of steel matrix composites is described, and the factors influencing the reinforcement distribution, interface processes, as well as the mechanical and corrosion properties, are revealed. Both stainless steels and tool steels were used as the matrix material, and the particulate reinforcements were Al 2 O 3 , TiC, Cr 3 C 2 , or TiN. The results are compared with those of the corresponding unreinforced alloys and also with those of aluminum and magnesium matrix composites. It was found that the incorporation of a relatively low volume fraction of ceramic particulate reinforcements significantly increases the wear resistance of the steel matrices, without deteriorating the corrosion properties. On the other hand, reductions in the tensile strength, ductility and toughness were observed. The superaustenitic stainless steel–TiN and hot work tool steel–Cr 3 C 2 composites may offer the best combination of properties.

Processing and properties of particulate reinforced steel matrix composites

The influence of nitrogen and molybdenum on passive films formed on the austenoferritic stainless steel 2205 studied by AES and XPS

A time-temperature-transformation (TTT) diagram, with respect to the formation of intermetallic phase in the range from 700 °C to 1000 °C, has been assessed by point counting for a recently developed 29Cr-6Ni-2Mo-0.38N superduplex stainless steel (SDSS). Using a computer program previously developed by the authors, a continuous cooling-transformation (CCT) diagram was calculated from the TTT diagram, assuming that the transformation can be described by an Avrami-type equation. Controlled cooling experiments were made to confirm the validity of this computer model. Mechanical testing showed that the minimum acceptable impact toughness of 27 J corresponded to 5 vol pct of intermetallic phase. A comparison of impact toughness to hardness showed that toughness was a very sensitive measure of intermetallic phase formation, while hardness was insensitive and showed no significant increase until the material was catastrophically brittle. The thermodynamic computer program Thermo-Calc was used to predict microstructures and compare microstructures obtained after aging for 72 hours. It was found that this program could be used in a qualitative manner for predicting microstructural changes at various temperatures, but was unable to predict variables such as dissolution temperature and volume percentage with precision. The intermetallic phase was found to consist entirely of tetragonal σ phase with a P42/mnm crystal structure. The absence of χ phase as a precursor of σ phase provided an explanation for the delayed formation of intermetallic phase, as compared to that of more molybdenum-rich SDSSs.

Mechanical properties, microstructural stability and kinetics of σ-phase formation in 29Cr-6Ni-2Mo-0.38N superduplex stainless steel

Effect of annealing temperature on the pitting corrosion resistance of super duplex stainless steel UNS S32750

This paper presents an overview of duplex stainless steels (DSS) with particular emphasis on super DSS, i.e. steels containing sufficient amounts of chromium, molybdenum, and nitrogen to produce a pitting resistance equivalent greater than 40. Duplex stainless steels have an attractive combination of mechanical and corrosion properties and are thus suitable for many marine and petrochemical applications, particularly where chlorides are present. The paper covers the following aspects of DSS: physical metallurgy, mechanical properties, corrosion properties, metallurgy of welding, machinability, and applications. A large number of references to the literature are given to aid the reader who is interested in acquiring a deeper understanding of the behaviour of this family of steels.MST/1685

J.-O. Nilsson

Papers

Super duplex stainless steels