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The plastic deformation and martensite evolution of austenitic CrMnNi–N stainless steel with 8.6 wt% Mn and 4 wt% Ni is investigated in a temperature range of −40 to 100 °C. Martensite evolution is determined by in situ magnetic measurements during tensile test. The triggering stress and strain for martensite formation decrease with decreasing temperature. Ex situ volumetric magnetic measurements are used to determine the strain‐induced α′‐martensite volume fractions. The characterization of the microstructure is carried out with scanning electron microscope. Using electron backscatter diffraction, strain‐induced α′‐martensite is detected within deformation bands in the austenite. The strain hardening curve at −40 °C shows a typical progression for metastable austenitic stainless steels with pronounced strain‐induced α′‐martensite formation and can be divided into four hardening stages. At this temperature, the studied steel achieves the highest strain hardening rate. The amount of martensite increases with decreasing test temperature and reaches a maximum volume fraction of 76 vol% at −40 °C. The highest ductility of 83% is achieved at 40 °C, accompanied by a tensile strength of 732 MPa. The in situ magnetic measurement confirms that the inflection point in the strain hardening curve coincides with the maximum martensite formation rate. [ABSTRACT FROM AUTHOR]

Herein, considerable insight is provided into the evolution of strain‐induced martensite and mechanical properties of Fe–19Cr–4Ni–3Mn–0.15N–0.15C wt% austenitic stainless steel (in short Cr19NC15.15) during deformation at room temperature and cryogenic temperatures. In situ magnetic measurements of the martensite evolution during tensile tests at various temperatures are conducted. The triggering stress σtrigg.γ→α′ required for strain‐induced martensite formation is determined at the minimum in the strain hardening curve. It is found that with reducing deformation temperature in the range from 0 to –20 °C σtrigg.γ→α′ slightly decreases, whereas the triggering stress σtrigg.γ→α′ increases with further reduction of deformation temperature to −70 °C. The relation between the formed α′‐martensite fraction and the strain applied during tensile test is established. The results demonstrate that a reduced deformation temperature from room temperature (RT) to −70 °C significantly enhances the transformation rate and the total volume fraction of strain‐induced α′‐martensite. The yield and tensile strength increase whereas elongation continuously decreases due to an increasing α′‐martensite volume fraction with decreasing tensile test temperature. The α′‐martensite formation kinetic follows Olson and Cohen's model and are in good agreement between the in situ experiment and theoretical calculations. [ABSTRACT FROM AUTHOR]

By time-of-flight (TOF) neutron diffraction experiments, the influence of segregation-induced microstructure bands of austenite (γ) and martensite (α′) phases on the partitioning of stress and strain between these phases was investigated. Initially, tensile specimens of a Co-added stainless steel were heat treated by quenching and partitioning (Q&P) processing. Tensile specimens were subsequently loaded at 350 °C parallel to the length of the bands within the apparent elastic limit of the phase mixture. Lattice parameters in both axial and transverse directions were simultaneously measured for both phases. The observation of a lattice expansion for the γ phase in the transverse direction indicated a constraint on the free transverse straining of γ arising from the banded microstructure. The lateral contraction of α′ imposed an interphase tensile microstress in the transverse direction of the γ phase. The multiaxial stress state developed in the γ phase resulted in a large deviation from the level of plastic strain expected for uniaxial loading of single phase γ. Since segregation-induced banded microstructures commonly occur in many engineering alloys, the analysis of stress and strain partitioning with the present Q&P steel can be used to interpret the observations made for further engineering alloys with two-phase microstructures. [ABSTRACT FROM AUTHOR]

Tempering reactions in ternary Fe-2M-0.7C steels (M=Cr, Ni, Mn, Mo, and Si) were studied by correlative dilatometry and magnetic measurements at room temperature. Magnetic measurements were conducted after tempering at progressively higher temperatures. Based on the magnitude of demagnetization in the temperature range associated with the tempering stage I contraction, Mn- and Si-added steels formed the largest and smallest fractions of transition carbides, respectively. Estimation of the magnetization of paraequilibrium cementite indicated that Cr, Mn, and Mo reduced the magnetization while Ni increased it. In the presence of Si, the decomposition of retained austenite and cementite formation were shifted to higher temperatures. At temperatures above approximately 723 K (450 °C), the enrichment of cementite with Mn and Cr significantly reduced the total magnetization. In the Mo-added steel, on the other hand, the magnetization slightly increased implying the formation of ferromagnetic Mo-rich carbides. For the Ni- and Si-added steels, the magnetization remained almost constant indicating minimal redistribution of Ni and Si subsequent to the formation of cementite. The possibility of analyzing the latter redistribution is one of the main advantages of sequential tempering and magnetic measurements at room temperature compared to in situ thermomagnetic measurements. [ABSTRACT FROM AUTHOR]

By means of a simplified quenching and partitioning (Q&P) processing with quench temperature equal to room temperature, austenite fractions of 56, 43, and 24 vol% are stabilized in the microstructures of Q&P-processed Fe-Cr-C, Fe-Cr-C-Co, and Fe-Cr-C-N stainless steels, respectively. The mechanical stability of austenite is quantified by tensile tests at temperatures between 20 and 200 °C. In agreement with the Q&P concept, the stability of austenite is found to be highest for the steel with the highest martensite fraction. The carbon enrichment of austenite in a specimen of the Fe-Cr-C steel processed to have 82 vol% martensite is estimated by X-ray diffraction measurement of the austenite lattice parameter. The results indicate that only about 12% of the carbon contained in the martensite has partitioned into the austenite. The formation of tempering carbides is one of the mechanisms behind the significant deviation of the carbon enrichments from the full partitioning assumption. [ABSTRACT FROM AUTHOR]

In this study, the effect of interstitial contents on the mechanical properties and strain-induced martensite formation in an austenitic stainless steel was investigated. The mechanical properties of solution annealed Fe-15Cr-7Mn-4Ni-0.5Si-(0.01-0.2)N-(0.01-0.2)C concentrations in weight percent stainless steels were studied using room temperature tensile tests. All three alloys used in the present study have a sum content of C + N of about 0.2 wt.%. To verify the influence of C and N on deformation behavior, microstructural investigations are performed using light optical microscopy, scanning electron microscopy, and magnetic and hardness measurements. Moreover, strain-induced α′-martensite nucleation was characterized by scanning electron microscope using EBSD. In the present alloy system, carbon provides a stronger austenite stabilizing effect than nitrogen. Hence, the smallest amount of strain-induced α′-martensite was formed in the steel alloyed with 0.2 wt.% C. It also exhibited the optimal mechanical properties, including the highest ultimate tensile strength (1114 MPa), uniform elongation (63%), and total elongation (68%). Moreover, the interstitial content influences the occurrence of dynamic strain aging (DSA), which was only observed in the steel alloyed with carbon. With increasing C content, the triggering strain for DSA decreases, which can be confirmed by in situ magnetic measurements during tensile testing. [ABSTRACT FROM AUTHOR]

An ultra‐high‐strength austenitic‐martensitic stainless steel is produced by a novel quenching and partitioning (Q&P) process, provoking α'‐martensite formation during a cooling step to cryogenic temperatures. The cooling temperature is varied in order to achieve three different material states with different α'‐martensite volume fractions. Subsequently, all of the steels used are partitioned at 450 °C for 3 min to achieve thermal stabilization of remaining austenite and precipitation strengthening of martensite by M3C‐type carbides. The various states are characterized by tensile and total strain‐controlled fatigue tests (0.25% ≤ Δϵt/2 ≤ 1.2%). Furthermore, the microstructure is investigated by scanning electron microscopy techniques. The state evident after quenching close to the Néel temperature during the Q&P process exhibits both the best quasi‐static and cyclic properties with respect to strength, ductility, and fatigue life. Similar to TRIP steel, the formation of fatigue‐induced α'‐martensite via intermediate ϵ‐martensite (i.e., γ → ϵ → α') is observed and leads to cyclic hardening, the extent of which depends on the applied strain amplitude and the austenite stability of the respective material state. [ABSTRACT FROM AUTHOR]

The deformation-induced formation of α′ martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between −196 °C and 400 °C. The steel did not exhibit spontaneous α′ martensite formation at temperatures as low as −196 °C. Therefore, the critical driving force for the formation of α′ (−2780 J/mol) was obtained by determining the complementary mechanical energy necessary to trigger the deformation-induced α′ martensite at 0 °C. Driving forces for the γ→α′ transformation at other tensile test temperatures associated with the deformation-induced α′ formation were then obtained by subtracting the mechanical energies applied to trigger the martensitic transformation from the critical driving force. The triggering mechanical energies were obtained from in-situ magnetic measurements which enabled to mark the onset of the γ→α′ transformation. The driving forces for the γ→α′ transformation calculated using the preceding method indicated an increase in the stability of austenite which was attributed to changes in the mechanical and physical properties of austenite in the vicinity of the Néel temperature. The method can be used to calculate modified driving forces for the occurrence of the γ→α′ phase transformation. [ABSTRACT FROM AUTHOR]

This paper presents a systematic study of quenching and partitioning (Q&P) processing of a (C + N)-containing austenitic stainless steel with a special focus laid on the influence of partitioning conditions on the mechanical properties and its comparison with C-containing Q&P-treated stainless steels. Similar to C-containing stainless steels, martensite phase in the studied steel exhibited pre-precipitation phenomena such as interstitial segregation to defects below 250 °C and precipitation reactions such as the formation of paraequilibrium M 3 C carbides (M representing Fe and substitutional elements) above 300 °C. The interstitial partitioning between martensite and austenite and the dependence of mechanical properties on the partitioning conditions resembled those of C-containing stainless steels. With increasing partitioning temperature, interstitial enrichment in the austenite and ductility were enhanced while tensile strength decreased. Similar observations were made upon prolonged partitioning at 250 °C. In contrast, the ductility of steels partitioned at 350 °C and 450 °C barely changed with partitioning time. For partitioning at 450 °C, this was because most of the interstitial partitioning at 450 °C took place within the first 3 min of partitioning. For partitioning at 350 °C, the interstitial enrichment of fine austenite regions was completed rapidly as well and the later interstitial partitioning into coarse austenitic regions gave rise to little increase in austenite stability due to their large size. The optimal product of ultimate tensile strength and total elongation, 47 GPa%, was obtained after partitioning at 450 °C for 3 min. The sufficiency of a short partitioning to obtain superior mechanical properties is of great practical significance in the context of growing global concerns about energy demand and climate change. • Quenching and partitioning of a (C + N)-containing austenitic stainless steel. • Reactions competing with interstitial partitioning. • Mechanical properties insensible to partitioning time at specific temperatures provide possibilities for energy efficiency. • Presence of isothermal martensite upon subzero quenching. • Comparison with quenching and partitioning of C-containing stainless steels. [ABSTRACT FROM AUTHOR]

Discontinuous plastic flow due to dynamic strain aging (DSA) in a Fe–13Cr–3.4Mn–0.47C metastable stainless steel was studied by uniaxial tensile tests in the temperature range from 20 °C to 500 °C. A complex picture of the discontinuous flow and its disappearance in an intermediate temperature interval was interpreted in terms of an existing model, which had to be modified. Different mechanisms of DSA were proposed for different temperature intervals. DSA at temperatures up to 200 °C was attributed to deformation-induced martensitic transformation within glide bands. Immobilization of dislocations at martensite/austenite boundaries and emission of fresh dislocations into the adjacent austenite was considered as a major process in this temperature regime. Rapid diffusion of carbon from martensite to the martensite/austenite boundaries was considered as an additional pinning mechanism contributing to discontinuous plastic flow. Serrations on the deformation curves at temperatures between 200 °C and 300 °C were associated with dislocation pinning due to carbon diffusion in austenite. This regime of DSA did not involve high diffusivity martensite. In spite of an increase in the diffusivity of carbon in the temperature range of 350-450 °C, a serration-free regime was observed there. The absence of intermittent dislocation pinning/unpinning events responsible for serrations implied continuous solute drag by carbon atoms according to the Cottrell mechanism. Serrations reappeared at 500 °C and were attributed to dislocation pinning by substitutional chromium and manganese atoms. [Display omitted] [ABSTRACT FROM AUTHOR]