PSI - Issue 13

Elena Astafurova et al. / Procedia Structural Integrity 13 (2018) 1129–1134 Elena Astafurova et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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properties of ASS (Lang et al., 2015; Gavrilyuk and Berns, 1999). One of the well-known disadvantages of high nitrogen steels (HNS) is a ductile-to-brittle transition (DBT) (Lang et al., 2015; Gavrilyuk and Berns, 1999), which limits their industrial application. For austenitic steels with high-nitrogen content, the brittle fracture occurs under low-temperature plastic deformation, is associated mainly with cleavage-like cracking along {111} closed-packed planes and is ascribed by different mechanisms – mechanical twinning and deformation- induced martensitic transformations (to ε and α’ - phases), cracking along annealing twin boundaries and slipping-off mechanism (Gavrilyuk and Berns, 1999; Tomota et al., 1998; Chumlyakov et.al., 1999; Wang et al., 2010; Liu et al., 2004). The temperature of DBT is strongly dependent on chemical composition of the steel and, for Cr-Mn HNS, it can be reduced by change in nitrogen content or partial substitution of nitrogen by carbon (e.i. complex C+N solid solution hardening) (Gavrilyuk and Berns, 1999; Hwang, 2012; Tomota et al., 1998), by limitation of the concentration of the alloying elements, which decrease stacking-fault energy (SFE) of the steels (Hwang et al., 2010). At the same time, there are some experimental researches, which show that steels with high SFE, both nitrogen and carbon solid solution strengthened, suffer from DBT (Tomota et al., 1998; Astafurova, 2009). Precipitate-strengthening by V-based particles suppresses brittle fracture of the HNS with 0.9 wt.%N during tensile testing at 77K (Astafurova et al., 2018). But the influence of distribution and volume fraction of precipitates on DBT-characteristics is not study well. The objective of the research was to evaluate the effect of quenching temperature (QT) on temperature dependence of a yield stress, tensile strength, elongation, strain-hardening rate and fracture micromechanism in vanadium-alloyed high-nitrogen steel.

2. Materials and methods

The chemical composition of the high-nitrogen austenitic steel is given in Table 1. Hot-rolled at 1150ºC steel bars were solution-treated for 1 hour at two temperatures, 1100°C and 1200°C, followed by water-quenching.

Table 1. Chemical composition of the steel. Fe – balanced. Notation Cr Mn V Si

Ni

C

N

CrMnVCN steel

18.9

20.6

1.5

0.9

0.1

0.28

0.86

For tensile testing, flat dumb-bell-shaped samples were cut from quenched bars. Uniaxial tension of the samples was carried out in the temperature interval of (77-673)K and with an initial strain rate of 1×10 -4 s -1 . The microstructure of as-quenched specimens was studied by transmission electron microscopy (TEM) including energy-dispersive, diffraction and dark-field analysis. X- ray diffraction (XRD) analysis (in CoKα radiation) was also utilized to characterize the initial (as-quenched) phase composition and a lattice parameter, a , of the steel samples. For characterization of the fracture micromechanisms, a scanning electron microscopy (SEM) was used. The XRD patterns, the lattice parameter variation with extrapolation function ( cosθcotθ ) and TEM characteristic images of as-quenched specimens are shown in Figure 1. Independently on QT, only γ -phase reflections are presented on XRD patterns (Fig.1a), which testifies to the fact that steel possess austenitic structure and volume fraction of other possible phases does not exceed 5% each. Austenite lattice parameter, as it was estimated using XRD data, increases with grow in QT (Fig. 1b): a γ =3.610Å for QT=1100°C and a γ =3.613Å for QT=1200°C. Both values are much higher than that for interstitial- free γ -iron (3.59Å) due to high solid solution hardening effect by nitrogen and carbon. Increase in QT provides variation in lattice parameter Δ a γ =0.003Å, which corresponds to change in nitrogen concentration in austenite solid solution up to ΔC N ≈ 0.1wt.% according to a coefficient 0.029 Å/wt.% found by Santi Srinivas (Santi Srinivas et al., 1997). After quenching, steel has fine- grained austenitic structure with mean grain size less than 10 μm. Large number of anneal twins on TEM images testifies to low stacking-fault energies of the austenite after both solution treatment regimes. Vanadium-alloying provides particle strengthening of the steel. TEM images on Figures 1c and 1d 3. Results and discussion