The phase transitions in selective laser-melted 18-NI (300-grade) maraging steel
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The phase transitions in selective laser‑melted 18‑NI (300‑grade) maraging steel Mariusz Król1 · Przemysław Snopiński1 · Adam Czech2 Received: 16 August 2019 / Accepted: 10 January 2020 © The Author(s) 2020
Abstract Dilatometric studies in 18-Ni steel components fabricated by selective laser melting technique were carried out to determine the influence of heating rate on transitions occurring during the heating cycle. SLM components have been examined in controlled heating and cooling cycles. For analysis, heating of the analysed materials was carried out at heating rates of 10, 15, 20, 30 and 60 °C min−1. During the heating process, two solid-state reactions were identified—i.e. precipitation of intermetallic phases and the reversion of martensite to austenite. A simplified procedure based on the Kissinger equation was used to determine the activation energy of individual reactions. For precipitation of intermetallic phases, the activation energy was estimated 301 kJ mol−1, while the martensite to austenite reversion was determined at the activation energy 478 kJ mol−1. Keywords Maraging steels · Precipitation · Martensite reversion · Phase transitions
Introduction Maraging steel contains an extremely low amount of carbon (0.03% maximum) and a high amount of nickel (17–19%) together with lesser amounts of cobalt (8–12%), molybdenum (3–5%), titanium (0.2–1.8%) and aluminium (0.1–0.15%). This type of iron alloys belongs to the group of materials that are characterised as a martensitic crystal structure and strengthened by ageing heat treatment at approximately 500 °C, hence the name ‘maraging’. This ultra-low-carbon alloy has very high-strength high-toughness alloy that gain their exceptional mechanical properties derived from precipitation of intermetallic compounds and a martensitic matrix. The martensitic microstructure is not obtained by a high carbon content but by addition of nickel as one of the main element in chemical composition. The ultra-high-strength maraging steel is attracted by material engineers and structural designers of aerospace, machining * Mariusz Król [email protected] 1
Department of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18a St, 44‑100 Gliwice, Poland
Department of Lightweight Structures and Polymer Technology, Chemnitz University of Technology, Chemnitz, Germany
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areas, nuclear and defence industries. They are classified into M200, M250, M300 and M350 grades according to their 0.2% proof stress or yield strength levels, namely 200, 250, 300 and 350 ksi [1, 2]. The maraging steels are usually subjected to heat treatment—hardening and then ageing at a temperature of 450–510 °C, which causes a significant increase in hardness and strength in the effect of precipitations [3, 4] of γ-Ni3Mo, η-Ni3Ti, Fe-2Mo, σ-FeMo, µ-Fe-7Mo6, FeTi, F e2Ti [4–6]. This causes nickel enrichment in the matrix, stabilising the austenite and reducing the initial temperature of the martensite-to-austenite transitio
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