Martensitic stainless steels are widely used in industries due to their high strength and good corrosion resistance performance. Precipitation-hardened (PH) martensitic stainless steels feature very high strength compared with other stainless steels, around 3-4 times the strength of austenitic stainless steels such as 304 and 316. However, the poor workability due to the high strength and hardness induced by precipitation hardening limits the extensive utilization of PH stainless steels as structural components of complex shapes. Laser powder bed fusion (L-PBF) is an attractive additive manufacturing technology, which not only exhibits the advantages of producing complex and precise parts with a short lead time, but also avoids or reduces the subsequent machining process. In this review, the microstructures of martensitic stainless steels in the as-built state, as well as the effects of process parameters, building atmosphere, and heat treatments on the microstructures, are reviewed. Then, the characteristics of defects in the as-built state and the causes are specifically analyzed. Afterward, the effect of process parameters and heat treatment conditions on mechanical properties are summarized and reviewed. Finally, the remaining issues and suggestions on future research on L-PBF of martensitic precipitation-hardened stainless steels are put forward.Metals 2020, 10, 255 2 of 25 types of microstructures can be obtained with different heat treatment processes [9]. The typical temperature range for aging heat treatment for this alloy is 480-620 • C [1]. Under the H900 condition (aging temperature: 482 • C, time: 1 h), the precipitation in 17-4 PH stainless steel begins with Cu-rich precipitates (bcc, body center cubic) that maintain a coherent relationship with the matrix, which would lead to an increase in tensile strength and toughness [4]. These precipitates can transform into non-coherent Cu-rich particles (fcc, face center cubic) after extended aging at 400 • C [5]. After experiencing over-aging, the precipitates are coarsened. The number of precipitates is reduced, and the coherence relationship is also destroyed [6,10]. These changes together lead to a decrease in mechanical strength, but an increase in ductility and impact toughness.PH stainless steels are widely used in the aerospace industry [11,12], the marine industry [13], nuclear reactor components [14], chemical process equipment [15], and medical apparatus due to their high tensile strength, impact strength, fracture toughness, and corrosion resistance at typical service temperatures below 300 • C [15,16]. Most of these parts are important load-bearing components of heavy machinery in a demanding service environment. However, PH stainless steels have poor workability and machinability due to their high strength and high hardness, which result in a long production cycle and difficulties in obtaining desired shapes through conventional machining and forming processes [17].Due to the high strength and high hardness of PH steels, they are difficult to b...

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Inertia friction welding (IFW) is a solid‐state welding technology that avoids defects associated with molten weld processes; however, this process has rarely been used to weld ultrahigh‐strength steel (UHSS). Herein, 32CrMnSi2Ni6MoV UHSS is joined successfully via IFW. The mechanical properties after welding at different rotational speeds and the microstructure at 2800 rpm are studied in detail. The temperature distributions in the peripheral and central areas are modeled according to the temperature at the weld zone. The microstructural transformation is analyzed for different temperature intervals, and the microstructural characteristics of each interval correspond to the actual microstructure of the welded joint according to the temperature distribution model. High‐strength martensite forms at the weld zone, and the thermomechanical‐affected zone (TMAZ) in the peripheral region is divided into phase transformation zone, partial recrystallization zone, and plastic deformation zone, whereas the TMAZ in the central region includes only the partial recrystallization and pure plastic deformation zones. This difference is due to the higher frictional heat caused by the higher peripheral linear velocity during rotation. A tensile test shows that fracture occurs in the base metal (BM) region, and the yield strength, tensile strength, and elongation are 805 MPa, 1064 MPa, and 13.88%, respectively.