Improvement of Strength and Impact Toughness for Cold-Worked Austenitic Stainless Steels Using a Surface-Cracking Technique

metals Article Improvement of Strength and Impact Toughness for Cold-Worked Austenitic Stainless Steels Using a Surface-Cracking Technique Kwangyoon Kim 1 , Minha Park 1 , Jaeho Jang 1 , Hyoung Chan Kim 1 , Hyoung-Seok Moon 1 , Dong-Ha Lim 1 , Jong Bae Jeon 1 , Se-Hun Kwon 2 , Hyunmyung Kim 3 and Byung Jun Kim 1, * 1 2 3 * Energy Plant R&D Group, Korea Institute of Industrial Technology, Busan 46938, Korea; (K.K.); (M.P.); (J.J.); (H.C.K.); (H.-S.M.); (D.-H.L.); (J.B.J.) School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea; Department of Nuclear & Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea; Correspondence: ; Tel.: +82-10-5136-7743 Received: 25 October 2018; Accepted: 9 November 2018; Published: 12 November 2018



 Abstract: For cryogenic applications, materials must be cautiously selected because of a drastic degradation in the mechanical properties of materials when they are exposed to very low temperatures. We have developed a new technique using a cold-working and surface-cracking process to overcome such degradation of mechanical properties at low temperatures. This technique intentionally induced surface-cracks in cold-worked austenitic stainless steels and resulted in a significant increase in both strength and fracture at low temperatures. According to the microstructure observations, dissipation of the crack propagation energy with surface-cracks enhanced the impact toughness, showing a ductile fracture mode in even the cryogenic temperature region. In particular, we obtained the high strength and toughness materials by a surface-cracking technique at 5% cold-worked specimen with surface-cracks. Keywords: cold-working process; surface-cracking process; impact toughness; strength; low temperatures; austenitic stainless steels 1. Introduction The study of very low temperature environments in terms of cryogenics is one field in which the materials play a major part in desirable performances in severe conditions. Cryogenic technologies have various applications in many different fields such as in the power industry, chemistry, electronics, manufacturing, transportation, and food processing by refrigeration [1–7]. For cryogenic applications, materials must be cautiously selected because of a drastic degradation in mechanical properties of the materials when they are exposed to very low temperatures [8]. Generally, austenitic steels with face centered cubic (FCC) structures such as stainless steels [9], Al alloys [10], Ni alloys [11], and Ti alloys [12] are used as low temperature materials because they have high impact toughness at low temperatures [13–15]. Although these alloys with FCC structure have excellent mechanical properties at low temperatures, brittle fracture may occur in welds and rectangular structures due to the stress concentration [16]. A brittle fracture is usually very dangerous because it occurs abruptly with little or no warning, resulting in serious economic losses and potentially a loss of many lives. Therefore, it is Metals 2018, 8, 932; doi:10.3390/met8110932 www.mdpi.com/journal/metals Metals 2018, 8, 932 2 of 15 in high demand to develop new technologies in order to improve the mechanical properties of FCC alloys for use in severe environments such as in the cryogenics field. In particular, materials with good mechanical properties are required for use in severe environments such as the cryogenics field because material characteristics are significantly degraded and changed at low temperatures. Work hardening, also known as strain hardening or cold-working, is a method to strengthen a metal by plastic deformation [17–19]. In detail, the strengthening by cold-working occurs because of a decrease in the mobility of dislocations during plastic deformation of metals. The strain hardening is caused by an increase in the dislocation density within the austenitic structure [20]. Furthermore, the strengthening effect by phase transformation in austenitic stainless steels where the strained-induced martensitic transformation from austenitic phase shows a substantial strengthening effect [21–25]. It has also been found that the yield and tensile strength of austenite stainless steels is gradually improved by increasing the cold-working level. Therefore, a cold-working process is typically considered to be an important technique for increasing the strength of steels [26,27]. However, negative effects such as the unfavorable material embrittlement can be caused by reductions in ductility and impact toughness after a cold-working process [28]. Generally, impact toughness is reduced with an increase in work hardening and precipitates, whereas it typically shows a positive correlation with increases in ductility [29]. In recent research, the grain size refinement was an important technique for improving the impact toughness for high-strength steels [30–34]. Thermal heat treatments affected enhanced impact toughness by an ultra-fine grained structure [29]. Another approach to enhancing impact toughness is to provide multiple pathways for crack propagation, for example, by adding ultra-fine particles within the fine granular structure. Delamination, or splitting, resulting from anisotropic microstructures such as crystalline grains and secondary phases is well known to improve the toughness of metals at low temperatures [23,28,29]. Toughening mechanisms are for distributing the stress near the crack tips by allowing the delamination fracture of the grains from one another, instead of brittle fractures in bulk materials [35–37]. Most engineering designs require materials with high strength and impact toughness to avoid dangerous failure due to brittle fracture at low temperatures [38]. However, it is difficult to obtain tougher and stronger steels because materials typically show opposing characteristics for ductility and brittleness. Therefore, the achievement of a simultaneous enhancement of strength and toughness is a challenge [38,39]. Higher strength and toughness are key requirements for steels used in various structural applications, such as for aircraft, buildings, and heavy machinery including cryogenics applications, in order to satisfy the increasing demands for reliability, durability, and safety. Toughness and strength do not always have opposite characteristics [40,41]. Increasing the toughness of metals without sacrificing other properties is critical for their economic competitiveness [31,38,42,43]. It is true that for intrinsically ductile materials, such as metals, the improvement in strength usually comes at the expense of toughness [44,45]. However, the present results show the possibility and potential of improving toughness and strength at the same time. In this study, we have developed a new technique using a cold-working and surface-cracking process to increase the strength and toughness at the same time. O (...truncated)