The evolution of the microstructure and its effect on mechanical properties of a 10% Cr steel under long-term aging at 650 °C have been investigated. During short-term aging (<6000h), the reduction of room temperature hardness and yield strength are mainly caused by the decline of dislocation density. However, under long-term aging (from 6000h to 38200h), they are directly attributed to the coarsening of subgrains. The coarsening kinetics of Laves phase (≥18000h), M23C6 carbides and MX carbonitrides are well consistent with the ripening model in multicomponent alloys. Before 18000h, the coarsening of Laves phase is strongly affected by the swallowing growth mechanism. The plasticity in terms of fracture elongation at room temperature shows a non-monotonic dependence on the aging time, the effect factors such as dislocation density, strain level and distribution, subgrain width, precipitates size and ratio of brittle phase interfaces have been studied in detail. During tensile deformation, the large and irregular Laves phase particles induce severe strain localization and tend to form large strain concentration block. It is found that deterioration of plasticity is dominated by severely inhomogeneous and large size of Laves phase caused by two different nucleation and growth mechanisms and high coarsening rate (∼32.0 nm/h1/3), rather than the increase in the ratio of brittle phase interfaces. The swallowing growth mechanism for Laves phase, growing by swallowing the adjacent M23C6 carbides, has been emphatically discussed.The swallowing growth mechanism for Laves phase: (a) 650°C/6000h/TEM [16], (b) 650°C/12000h/TEM, (c) schematic illustration of swallowing growth mechanism. The other mechanism, swallowing growth mechanism, based on the nucleation of Laves phase next to M23C6, Laves phase tends to grow by directly swallowing the adjacent M23C6, as shown in Fig. 5. Moreover, the swallowing process preferentially initiates along M23C6/ferrite interfaces and then gradually extends to the center of M23C6 (Fig. 5), which is mianly because a strong segregation of Si and P at M23C6/ferrite interfaces promotes the formation of Laves phase [17,18].Download high-res image (278KB)Download full-size image

The mechanical properties and designed microstructure evolution have been investigated in ultra-low carbon medium Mn steels for enhanced strength, plasticity and toughness, after an innovative quenching-partitioning-tempering (QPT) treatment with different initial conditions, cold rolling (CR) and hot rolling (HR). The transmission electron microscopy (TEM) combined with 3D atom probe tomography (APT) observed plenty of block austenite (30%) with dispersed precipitation in CR-QPT steels, while less amounts of film austenite (18%) with a higher density of nanoprecipitates were found in HR-QPT steels. The controlled multiphase microstructure evolution strongly depends on the Mn diffusion and segregation process. The overall strength-ductility combinations of two QPT-steels from the contribution of combined nanoprecipitation hardening and transformation-induced plasticity (TRIP) effects, are strongly influenced by the varying austenite mechanical stability connected with the volume fraction, grain size, morphologh and dislocation density of CR and HR-QPT samples. The unloading-reloading tests reveal the respective roles of precipitation hardening and TRIP effect in the overall mechanical properties according to the Baushinger effect (BE): the nanoprecipitation results in a higher back stress strengthening, while the deformation-induced martensite transformation in a wide strain regime degrades the large stress concentration in grain boundaries (GB), leading to a back stress softening but effective stress hardening in the later deformation stage. In addition, CR-QPT samples show a significant higher value of impact toughness than HR-QPT samples. The QPT treatment of CR-QPT steels can not only eliminate the susceptible prior austenite grain boundaries, but also drive Mn enrichment at the phase boundaries diffusing into the pre-existing austenite interior due to a low migration rate of austenite/ferrite interfaces impeded by the nanoprecipitations in ferrite, contributing to a homogeneous Mn distribution and removing the grain boundary embrittlement.Schematic sketch of the microstructure evolution process during the multistage QPT treatment. In the HR-QPT steels, the reverse austenite easily transforms to martensite again due to a low partitioning level during the high temperature aging stage. The CR-QPT steels show a higher transformation rate due to high density of dislocations as nucleation sites. In the following tempering process, the secondary austenite reversion occurs in the HR-QPT steels while the Mn enrichment in the grain boundary can diffuse into the austenite interior in the CR-QPT steels.Download high-res image (178KB)Download full-size image

•Compressive stress can retard the interstitial carbon diffusion.•Diffusion coefficients of carbon are concentration dependent.•The activation energy of carbon diffusion is increased by applying compressive stress.In this study, hydrostatic compressive stresses are applied on the prior-carburized samples of 316 ASS to discover the relationship between interstitial diffusion process and the applied stresses. Carbon diffusion profiles are compared and concentration dependent diffusion coefficients are discussed to further advance scientific understanding of diffusion process in supersaturated S-phase. The experimental results demonstrate that the applied compressive stress can retard the fast diffusion process comparing to the unstressed one. Based on theoretical modelling, the effect of the applied hydrostatic compressive stresses on the diffusion of carbon in carbon supersaturated 316 austenitic stainless steel is discussed.

We investigated the effect of deformation temperature and gain size on the tensile behaviors of a new medium Mn-TRIP steel processed by batch and continuous annealing. It was found that the tensile strength decreased when the testing temperature increased from −20 to 300 °C. The total elongation firstly increased, reaching the maximum at 25–100 °C, and then decreased when the temperature was higher than 100 °C. The results can be explained by (i) the influence of temperature on chemical driving force for martensitic transformation and (ii) the grain size effect on the stability of austenite. To achieve optimal mechanical properties, the stability of austenite should be tailored so that the transformation-induced plasticity effect occurs under continuous deformation. Also, ferrite should have appropriate grain sizes so that work hardening of ferrite can coordinate the deformation of austenite.

•The thermoelectric power (TEP) was employed to investigate the low temperature tempering of a medium carbon alloyed steel.•Evolution of carbon dissolution was investigated for different tempering conditions.•Carbon concentration variation was quantified from 0.33 wt.% in quenching sample to 0.15 wt.% after long time tempering.Low temperature tempering is important in improving the mechanical properties of steels. In this study, the thermoelectric power method was employed to investigate carbon segregation during low temperature tempering ranging from 110 °C to 170 °C of a medium carbon alloyed steel, combined with micro-hardness, transmission electron microscopy and atom probe tomography. Evolution of carbon dissolution from martensite and segregation to grain boundaries/interfaces and dislocations were investigated for different tempering conditions. Carbon concentration variation was quantified from 0.33 wt.% in quenching sample to 0.15 wt.% after long time tempering. The kinetic of carbon diffusion during tempering process was discussed through Johnson-Mehl-Avrami equation.

We studied the relationship between locations of retained austenite (RA) and the deformation behaviors of quenching and partitioning treated steels. By using a variety of quantitative characterization techniques and comparing ferrite and martensite dual-phase counterparts, we confirmed that the stability of RA within different matrix phases is largely dependent on the plastic incompatibility between ferrite and martensite phases. If there exists significant difference in strength between ferrite and martensite, the transformation of RA is discontinuous. The RA inside ferrite and martensite transforms under low and high strain, respectively. As a result, deformation behaviors of the matrix phases, especially the ferrite phase at low strain, can be substantially affected by the change of RA. Our analysis offers insight into the interactions among different phases, the stability of RA, and the strain hardening behavior of multi-phase steels.

The effect of retained austenite (RA) stability and morphology on the hydrogen embrittlement (HE) susceptibility were investigated in a high strength steel subjected to three different heat treatments, i.e., the intercritical annealing quenching and partitioning (IAQP), quenching and partitioning (QP) and quenching and tempering (QT). IAQP treatment results in the coexistence of blocky and filmy morphologies and both QP and QT treatments lead to only filmy RA. No martensitic transformation occurs in QT steel during deformation, while the QP and IAQP undergo the transformation with the same extent. It is shown that the HE susceptibility increases in the following order: QT, QP and IAQP. Despite of the highest strength level and the highest hydrogen diffusion rate, the QT steel is relative immune to HE, suggesting that the metastable RA which transforms to martensite during deformation is detrimental to the HE resistance. The improved resistance to HE by QP treatment compared with IAQP steel is mainly attributed to the morphology effect of RA. Massive hydrogen-induced cracking (HIC) cracks are found to initiate in the blocky RA of IAQP steel, while only isolate voids are observed in QP steel. It is thus deduced that filmy RA is less susceptible to HE than the blocky RA.

Two types of multiphase steels containing blocky or fine martensite have been used to study the phase interaction and the TRIP effect. These steels were obtained by step-quenching and partitioning (S-QP820) or intercritical-quenching and partitioning (I-QP800 & I-QP820). The retained austenite (RA) in S-QP820 specimen containing blocky martensite transformed too early to prevent the local failure at high strain due to the local strain concentration. In contrast, plentiful RA in I-QP800 specimen containing finely dispersed martensite transformed uniformly at high strain, which led to optimized strength and elongation. By applying a coordinate conversion method to the microhardness test, the load partitioning between ferrite and partitioned martensite was proved to follow the linear mixture law. The mechanical behavior of multiphase S-QP820 steel can be modeled based on the Mecking–Kocks theory, Bouquerel’s spherical assumption, and Gladman-type mixture law. Finally, the transformation-induced martensite hardening effect has been studied on a bake-hardened specimen.

The effect of ε-carbide on hydrogen embrittlement (HE) susceptibility was evaluated in a quenching–partitioning–tempering (Q-P-T) treated steel. Total elongation loss (1 min hydrogen charging) drops from 42.7% to 0.6% after the tempering treatment. A significant improvement to HE is associated with the trapping capacity of ε-carbide, which is revealed by thermal desorption spectroscopy analysis and three-dimensional atom probe. A possible mechanism is discussed to explain the improved resistance to HE.

In this work, the influence of sub-zero Celsius treatment and tempering on the mechanical and thermal stability of retained austenite in bearing steel were assessed by tensile test and DSC. Compared with traditional quenched and tempered treatment, sub-zero Celsius treatment obviously decreases the volume fraction of retained austenite. Moreover, the mechanical stability of retained austenite was enhanced due to the accumulation of compressive stresses in retained austenite after sub-zero Celsius treatment and tempering. Meanwhile, the morphology of retained austenite changed from film-like to blocky with austenitization temperature increasing, and the mechanical stability of film-like retained austenite is higher than that of blocky one. The DSC results showed that the activation energy of retained austenite decomposition slightly increased through sub-zero Celsius treatment and tempering. This result may probably be ascribed to partitioning of carbon during tempering. However, the temperature at which retained austenite starts to decompose is unchanged.

•Hydrogen dramatically causes deterioration in Q&P treated steel.•Hydrogen trapping sites are directly observed by atom probe tomography.•Hydrogen is three times more soluble in austenite than that in martensite.•Hydrogen-induced cracks initiate mainly at martensite/austenite interfaces.The effect of hydrogen on the tensile properties and fracture characteristics was investigated in the quenching & partitioning (Q&P) treated high strength steel with a considerable amount of retained austenite. Slow strain-rate tensile (SSRT) tests and fractographic analysis on cathodically charged specimens were performed to evaluate the hydrogen embrittlement (HE) susceptibility. Total elongation was dramatically deteriorated from 19.5% to 2.5% by introducing 1.5 ppmw hydrogen. Meanwhile, hydrogen caused a transition from ductile microvoid coalescence to a mixed morphology of dimples, “quasi-cleavage” regions and intergranular facets. Moreover, hydrogen trapping sites were directly observed by means of three-dimensional atom probe tomography (3DAPT). Results have shown that hydrogen in austenite (33.9 ppmw) is 3 times more soluble than that in martensite (10.7 ppmw). By using DENT specimen, hydrogen-induced cracking (HIC) cracks were found to initiate at martensite/austenite interfaces and then propagate through retained austenite and martensite. No crack was observed to be initiating from ferrite phase.

•The LTS-SK internal friction peak was found as the steel aged at temperatures below 373 K.•The evolution of the LTS-SK peak is accompanied by the decrease of solid dissolved carbon content.•The origin is attributed to the interaction between carbon atoms and twin boundaries in martensite.A distinct internal friction peak located at the low-temperature shoulder of Snoek-Köster peak (LTS-SK) was found in Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel and its evolution with respect to various aging treatments was investigated. The LTS-SK internal friction peak was found to occur when aged below 373 K. TEM observation confirmed that the ε-carbide precipitated beyond 373 K, providing an evidence that the LTS-SK peak cannot be caused by ε-carbide precipitation. The corresponding evolution on the S-K peak and thermoelectric power (TEP) illustrated that the carbon content in the solid solution decreases due to carbon atoms segregation on the surrounding dislocations during low-temperature aging. The origin of the LTS-SK peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite.

•The microscopic strain evolution is revealed by in-situ synchrotron XRD with EBSD.•The γ → α′ transformation shows a shift of strain localization from austenite to ferrite.•The γ → ε → α′ transformation shows a self-compatible strain evolution in multiphase.•The compatible strain evolution can prevent the nucleation and growth of HICs.Two duplex TRIP-assisted stainless steels have been investigated showing distinct γ (face centered cubic) → ε (hexagonal close-packed) → α′ (body centered cubic) transformation sequences and a single γ → α′ martensite transformation. The microscopic strain evolution in two phases was revealed by in-situ high energy synchrotron X-ray diffraction combined with electron backscattering diffraction. The direct γ → α′ transformation route experiences a shift of strain localization from austenite to ferrite during deformation, which induces high back stress with massive dislocation accumulation in ferrite. The multistage γ → ε → α′ transformation sequences contribute to a good combination of strength and ductility. It is demonstrated that the compatible strain evolution in austenite and ferrite is achieved due to γ → ε transformation, leading to a mild dislocation multiplication in each phase. The following α′-martensite transformation supplies an extra work hardening capacity related to the piling up of dislocations. The obtained compatible strain evolution due to ε-martensite can not only prevent the nucleation of hydrogen-induced micro-cracks but also alleviate the localized plastic deformation in ferrite, which ensures a higher total elongation (TEL) and resistance to hydrogen embrittlement (HE).