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Abstract
The hot deformation of 18Cr-5Ni-4Cu nitrogen-alloyed austenitic stainless steel was tested with a Gleeble-1500D simulator in the temperature range of 1273–1473 K and in the strain rate range of 0.01–10 s−1. The Zener-Hollomon parameter method was used to construct a constitutive equation for high-temperature plastic deformation. The energy dissipation diagram of the material was calculated based on dynamic material modelling (DMM). The microstructural variations were characterized via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The thermodynamic calculation results showed that the addition of nitrogen to 18Cr-5Ni-4Cu steel promoted the formation of Cr2N and gas phases and expanded the austenite phase region; these results were consistent with the XRD test results of the solid solution sample. The hot deformation activation energy after nitrogen addition was 556.46 kJ∙mol−1. The processing map predicted that the optimum hot working regimes were in the temperature range of 1416–1461 K, where ln ε ˙ was 0.75–1 on the power dissipation map. At high temperatures and a small strain rate, dynamic recrystallization easily occurred. The TEM analyses showed that nano-scale M23C6 and Cr2N precipitated at the grain boundary, and NbC with a diameter of approximately 150 nm appeared along the grain boundary, resulting in grain boundary strengthening. The phase precipitation results were consistent with the Thermo-Calc calculation results. The nitrogen solid solution in the steel promoted the precipitation of nitrides, which caused grain boundary strengthening. Thus, the grain boundary stress increased and wedge-shaped grain boundary cracks formed.
Highlights
Many objects, including medical materials and kitchenware, are often composed of Cr-Ni-Cu-N austenitic stainless steel because of its non-magnetic and corrosion-resistant properties and its low nickel content.In recent years, research on Cu-bearing nitrogen alloy steel has mainly focused on the solid solution strengthening of nitrogen, the nano-precipitation strengthening of Cu and the antibacterial effect of Cu
23C66 and Cr22N precipitated at the grain boundary, and NbC with a diameter of approximately 150 nm appeared along the grain boundary, resulting in grain boundary strengthening
The constitutive equation is mainly used to describe the quantitative relationship of the thermodynamic parameters during hot rolling at high temperatures
Summary
Many objects, including medical materials and kitchenware, are often composed of Cr-Ni-Cu-N austenitic stainless steel because of its non-magnetic and corrosion-resistant properties and its low nickel content. Research on Cu-bearing nitrogen alloy steel has mainly focused on the solid solution strengthening of nitrogen, the nano-precipitation strengthening of Cu and the antibacterial effect of Cu. Tan et al [1] studied the precipitation characteristics of the Cu-rich phase and its effect on the strengthening mechanism of Super 304H steel with 2.2 wt.%, 4 wt.% and 5 wt.% Cu content during 2000. Tan et al [11] studied the thermal processing map of Super 304 H steel based on the dynamic material model theory and the Prasad instability criterion in hot compression experiments. A Gleebe-1500D simulator was used to study the effect of high N content on the deformation of 18Cr-5Ni-4Cu steel at different strain rates and deformation temperatures based on DMM. The microstructural evolution during hot working was analysed to discuss the thermal processing optimization and microstructure of 18Cr-5Ni-4Cu-N steel
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