Vol.:(0123456789) 1 3 Metals and Materials International https://doi.org/10.1007/s12540-018-0100-0 Change of Precipitation Behavior and Impact Toughness with Depths in Quenched Thick SAF 2507 Super Duplex Stainless Steel Godwin Kwame Ahiale 1  · Doo‑Hyun Kim 2  · Won‑Jon Yang 2  · Jong‑Hoon Lee 2  · Yong‑Jun Oh 1 Received: 10 November 2017 / Accepted: 9 January 2018 © The Korean Institute of Metals and Materials 2018 Abstract We investigated the change of precipitation behavior and impact resistance as a function of depth from the surface of thick block of SAF 2507 super duplex stainless steel with the thickness (T) of 200 mm after water quenching from 1050 °C. The amount of detrimental sigma phase increased smoothly until the depth of 0.25T, followed by a rapid increase from 0.25T to the center. However, the impact strength decreased signifcantly with only 1.3% of area fraction of sigma phase as the depth increased past 0.1T. Based on fractography analysis for the samples at such small depth ranges, the distance between the sigma phase particles afected the relative amount of initiating brittle cracks in front of the notch and was one of the crucial factors that dramatically reduced impact resistance with depth. Keywords Impact test · Precipitation · Alloys · Fracture · Super duplex stainless steel 1 Introduction Super duplex stainless steel (SDSS) is the class of duplex stainless steels with a pitting resistance equivalent number (PREN) greater than 40 that is determined by the contents of chromium, molybdenum and nitrogen [1]. SDSS exhibits higher strength and toughness than ferritic and austenitic stainless steels, respectively, and has a high corrosion resist‑ ance, which make these steels suitable for several structural applications such as in marine industries, petroleum, chemi‑ cal, paper, and pulp plants [2–7]. These properties arise from their microstructures, which are composed of approximately equal amounts of ferrite (α) and austenite (γ) phases [1, 8, 9]. However, the SDSS microstructure are sensitive to exposure to elevated temperatures during their fabrication, welding, heat treatment or operation, and such exposure could alter their mechanical properties. The alteration in the mechani‑ cal properties is attributed to the precipitation of undesired deleterious secondary phases, such as sigma (σ), chi (χ), nitrides and carbides in the microstructure, that mostly form in the 600–1000 °C temperature range [1, 10–13]. Among these precipitates, the sigma phase exhibits the most adverse efect on impact resistance and has attracted much atten‑ tion due to its very brittle nature, even though it has been reported in a small number of studies that the χ phase and M 23 C 6 may also reduce the toughness of duplex stainless steels [14, 15]. The temperature range and dwelling times for sigma phase formation were determined in several paramet‑ ric studies for isothermally annealed SDSS, and the efect of the phase on the toughness was also confrmed [16–18]. In particular, the efect of small sigma phase amounts on tough‑ ness was very dramatic, which may signifcantly limit the industrial application of SDSS: only 2–4% of sigma phase led to a sharp decrease in the toughness by more than 80% [18]. However, the origin of this phenomenon is still unclear. Although some reports have indicated that R phase may be responsible for this efect, this claim was based only on the data for annealing at 600 °C, and no direct evidence regard‑ ing the degradation of impact toughness was presented [19]. To date, the correlation between precipitation behavior and impact toughness in SDSS was mainly examined under iso‑ thermal aging conditions. Few studies [20, 21] have considered diferent cooling rates. Conventionally, since SDSS is used in large structures and exposures to diferent cooling rates with depths, the investigation of the thickness efect on precipita‑ tion and toughness in a thick material is highly required. The present study systematically investigates the precipitation at * Yong‑Jun Oh yjoh@hanbat.ac.kr 1 Department of Materials Science and Engineering, Hanbat National University, Daejeon 34158, Korea 2 Industrial Technology Support and Safety, Korea Institute of Materials Science, Changwon 51508, Korea