Processing of a Martensitic Tool Steel by Wire-Arc Additive Manufacturing.

This work investigates the processability of hot-work tool steels by wire-arc additive manufacturing (DED-Arc) from metal-cored wires. The investigations were carried out with the hot-work tool steel X36CrMoWVTi10-3-2. It is shown that a crack-free processing from metal-cored wire is possible, resulting from a low martensite start (Ms) temperature, high amounts of retained austenite (RA) in combination with increased interpass temperatures during deposition. Overall mechanical properties are similar over the built-up height of 110 mm. High alloying leads to pronounced segregation during processing by DED-Arc, achieving a shift of the secondary hardness maximum towards higher temperatures and higher hardness in as-built + tempered condition in contrast to hardened + tempered condition, which appears to be beneficial for applications of DED-Arc processed material at elevated temperatures.

Validation of the Powder Metallurgical Processing of Duplex Stainless Steels through Hot Isostatic Pressing with Integrated Heat Treatment.

Duplex stainless steels exhibit an excellent combination of corrosion resistance and strength and are increasingly being manufactured through powder metallurgy (PM) to produce large, near-net-shaped components, such as those used for offshore applications. Hot isostatic pressing (HIP) is often used for PM production, in which pre-alloyed powders are compacted under high pressures and temperatures. Recent developments in HIP technology enable fast cooling as part of the process cycle, reaching cooling rates comparable to oil quenching or even faster. This enables the integrated solution annealing of duplex stainless steels directly after compaction. In contrast to the conventional HIP route, which requires another separate solution annealing step after compaction, the integrated heat treatment within the HIP process saves both energy and time. Due to this potential gain, HIP compaction at a high pressure of 170 MPa and 1150 °C with integrated solution annealing for the production of duplex stainless steels was investigated in this work. Firstly, the focus was to investigate the influence of pressure on the phase stability during the integrated solution annealing of the steel X2CrNiMoN22-5-3. Secondly, the steel X2CrNiMoCuWN25-7-4, which is highly susceptible to sigma phase embrittlement, was used to investigate whether the cooling rates used in the HIP are sufficient for preventing the formation of this brittle microstructural constituent. This work shows that the high pressure used during the solution heat treatment stabilizes the austenite. In addition, it was verified that the cooling rates during quenching stage in HIP are sufficient for preventing the formation of the sigma phase in the X2CrNiMoCuWN25-7-4 duplex stainless steel.

Development and Validation of SCFE Percutaneous Pinning Surgical Simulation.

BACKGROUND: In situ screw fixation with a single percutaneously placed femoral screw remains widely accepted for femoral head fixation in adolescent patients with slipped capital femoral epiphysis (SCFE). Given the potential risks involved with this procedure, a simulation whereby surgical skills could be refined before entering the operating room may be of benefit to orthopaedic trainees. METHODS: We developed a synthetic model for the simulated treatment of SCFE. Five orthopaedic attendings and twenty trainees were recorded performing an in situ percutaneous fixation on the SCFE model. Time, radiation exposure, and final anteroposterior and lateral radiographs of the SCFE model were recorded. After completion, the attendings and trainees answered a Likert-based questionnaire regarding the realism and utility of the simulation, respectively. Two blinded orthopaedic surgeons rated each participant's skill level based on previously described assessment tools, including a Global Rating Scale (GRS) of technical proficiency and radiographic grading index for screw placement. Performance metrics and survey responses were evaluated for construct validity, face validity, and interrater reliability. RESULTS: The attendings demonstrated superior technical proficiency compared with trainees in terms of higher GRS scores (27.9±1.9 vs. 14.7±5.0, P<0.001) and better radiographic grading of screw placement on lateral views (P=0.019). Similarly, compared with the trainees, the orthopaedic attendings demonstrated shorter operative times (11.0±4.1 vs. 14.7±6.2 min, P=0.035) and less radiation exposure (3.7±1.7 vs. 9.5±5.7 mGy, P=0.037). The interrater reliability was excellent for both the GRS scoring (intraclass correlation coefficient=0.973) and radiographic grading (weighted κ=1.000). The attendings and trainees rated the realism and teaching utility of the simulation as "very good," respectively. CONCLUSION: Our surgical simulation for in situ percutaneous fixation of SCFE represents a valid and reliable measure of technical competency and demonstrates much promise for potential use as a formative educational tool for orthopaedic residency programs. LEVEL OF EVIDENCE: Level II.

Impact of the Allowed Compositional Range of Additively Manufactured 316L Stainless Steel on Processability and Material Properties.

This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties.