Cryogenic toughness in a low-cost austenitic steel

ARTICLE https://doi.org/10.1038/s43246-021-00149-8 OPEN Cryogenic toughness in a low-cost austenitic steel 1234567890():,; Yuhui Wang1, Yubin Zhang2, Andrew Godfrey3 ✉, Jianmei Kang1, Yan Peng1, Tiansheng Wang1,4, Niels Hansen5 & Xiaoxu Huang2,6 ✉ At low temperatures most metals show reduced ductility and impact toughness. Here, we report a compositionally lean, fine-grained Fe-30Mn-0.11C austenitic steel that breaks this rule, exhibiting an increase in strength, elongation and Charpy impact toughness with decreasing temperature. A Charpy impact energy of 453 J is achieved at liquid nitrogen temperatures, which is about four to five times that of conventional cryogenic austenitic steels. The high toughness is attributed to manganese and carbon austenite stabilizing elements, coupled with a reduction in grain size to the near-micrometer scale. Under these conditions dislocation slip and deformation twinning are the main deformation mechanisms, while embrittlement by α′- and ε-martensite transformations are inhibited. This reduces local stress and strain concentration, thereby retarding crack nucleation and prolonging workhardening. The alloy is low-cost and can be processed by conventional production processes, making it suitable for low-temperature applications in industry. 1 National Engineering Research Center for Equipment and Technology of Cold Rolled Strip, Yanshan University, Qinhuangdao, China. 2 Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark. 3 Laboratory of Advanced Materials (MoE), School of Material Science and Engineering, Tsinghua University, Beijing, China. 4 State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, China. 5 Risø Campus, Technical University of Denmark, Roskilde, Denmark. 6 International Joint Laboratory for Light Alloys (MOE), College of Materials Science and Engineering, Chongqing University, Chongqing, China. ✉email: ; COMMUNICATIONS MATERIALS | (2021)2:44 | https://doi.org/10.1038/s43246-021-00149-8 | www.nature.com/commsmat 1 ARTICLE L COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-021-00149-8 ow-temperature metals and alloys are widely used in industry, covering a broad range of areas including liquefied natural gas (LNG) tanks, ice-breakers, cryogenic superconductivity, and outer space exploration1. For such applications, high cryogenic impact toughness is desired to prevent catastrophic failure during impact loading2. A general challenge here is that impact toughness in metals and alloys almost invariably decreases with decreasing temperature. As such, the conventional metals and alloys that have been developed for low-temperature applications, including steels3,4, Ti5, and Al6, have relatively low impact toughness at cryogenic temperatures, limiting their application at such service temperatures. Solutions to this problem have been explored, though all with significant drawbacks in terms of cost. For example, a mature class of alloys based on 9% Ni has been developed over the last 60 years and is used widely for the construction of LNG tankers1. The fracture toughness of these alloys is still, however, limited only to moderate values (<100 J). Moreover, to achieve the required properties in these alloys, ultrapure smelting technology and complex heat treatments are required7,8, resulting in high production costs. It remains therefore a dream in material science to develop cheap, industrially practical alloys with toughness that remains unchanged or even increases, at low temperatures. An inverse temperature dependence of toughness has, in fact, been reported recently for a ferritic steel with an ultrafine-grain structure3, where the enhanced toughness was attributed to fine scale delamination resulting from crack branching on aligned crystallographic cleavage planes. The inverse temperature dependence of toughness only holds, however, down to a moderately low temperature of −50 °C, with a corresponding maximum impact energy of about 300 J. At lower temperatures the toughness decreases dramatically in the usual manner. More recently, an increase in impact toughness with decreasing temperature has also been reported for some high- and mediumentropy alloys (HEAs/MEAs), e.g., CrMnFeCoNi, where an exceptionally high cryogenic impact toughness of ~400 J has been achieved9,10. Despite this high cryogenic impact toughness, such alloys are likely to have only limited industrial application, as a result both of the high cost associated with the required amounts of expensive alloying elements (especially Co), and due to the complexity of large-scale production of components using HEAs and MEAs11. The sustainability of such alloys is also a challenge, due to difficulties in recyclability associated with the large alloying fractions used in HEAs and MEAs. Two lessons that can be learned from the studies on HEAs are, however, important. These are: (i) that an ultrahigh impact energy can be achieved by enabling a transition with decreasing temperature from planar-slip dislocation activity at room temperature (RT) to deformation by mechanical nanotwinning at lower temperatures, and (ii) that to achieve an inverted toughness–temperature relationship, both strength and ductility should simultaneously increase with decreasing test temperature8,12. The first criterion can be fulfilled in some cost-effective high-Mn austenitic steels13,14. However, in these alloys HCP ε- and BCC α′-martensitic transformation can take place during plastic deformation at low temperatures, or even in some cases during cooling15, which leads to embrittlement and hence premature fracture and low fracture toughness16,17. The martensitic transformation can be suppressed to a certain extent by increasing the Mn, Al, Si, or C content18–23 or via grain refinement24–27. By use of these approaches, the highest cryogenic toughness value for a high-Mn steel presently achievable is about 220 J15,16,22. Regarding the second criterion, a survey of the literature reveals one Fe30Mn steel28 and some FeMnAl alloys29 that exhibit an increase in both strength and ductility with decreasing temperature, though whether an inverse temperature dependence of toughness holds for these steels is not reported. 2 Inspired by these studies, here we report data for a fine-grained (mean grain size of 5.6 µm) Fe-30Mn-0.11C austenitic steel. The choice of composition is based on previously reported promising results for a Fe30Mn steel28, with the addition of a small amount of C as an additional austenite stabilizing element. In combination with appropriate grain refinement the resulting steel is resistant to the formation of ε martensite20,24–27 during lowtemperature deformation20,24–27. The results show that this steel can satisfy both of the criteria listed above, and also exhibit an unusual increase in toughness with decrease of temperature to the cryogenic regime, leading to an ultrahigh Charpy impact (...truncated)