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The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10)., A. Kozłowska acknowledges the financial support of the Silesian University of Technology, Faculty of Mechanical Engineering through the 10/010/BKM_22/1104 project.

Three composite bronzes reinforced with steel dendrites from steels of different classes, which perform the function of a bearing surface in sliding friction, are studied. The chemical compositions of the dendrites and of the matrix are determined as a function of the heat treatment of bronze BrZhNOA 12-7-2-1. The fine structure of dendrites in bronze BrZhNKoA 9-4-1-1 is studied and the effect of the aging temperature in the intermetallic reinforcement of the dendrite with composition corresponding to maraging steel N23Yu1 is determined. The type of the crystal lattice of the dendrite is shown to affect substantially the tribological properties of bronze BrZhNKhA 12-9-3-1. © 2022, Springer Science+Business Media, LLC, part of Springer Nature.

Abstract Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite–martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is implemented to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress–strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars conducted previously, and then used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bi-crystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries. Graphical abstract Micromechanics models provide insights on grain boundary elastic and phase transformation strain incompatibility in shape memory zirconia

This study aims to compare the microstructure of 17–4 PH stainless steel (SS) manufactured via laser powder bed fusion (L-PBF) and laser powder directed energy deposition (LP-DED) in non-heat treated (NHT) and heat treated conditions. In addition, the room-temperature tensile behavior of heat-treated L-PBF and LP-DED 17–4 PH SS has been investigated and compared with that of the wrought counterpart with the same heat treatment conditions. The results show that the L-PBF specimens have a finer microstructure (ferrite + lath martensite) than the LP-DED ones (massive ferrite + Widmanstatten ferrite) in NHT condition. Electron backscatter diffraction analysis shows that the L-PBF and LP-DED specimens have twin-based substructure lath martensite after heat treatment. Despite the lower tensile strength of the LP-DED specimens compared with the L-PBF ones, the ductility of peak-aged LP-DED specimens was reduced due to the presence of the δ-ferrite phase having a significant plastic deformation incompatibility with the martensite.

55Cr17Mo1VN high nitrogen martensitic stainless steel is usually applied to the high-quality mold, which is largely produced by the pressurized electro slag remelting process. The microstructure evolution of quenching and tempering heat treatment were investigated and an optimal heat treatment process to achieve excellent mechanical properties was found out. The main precipitates in the steel included carbon-rich type M23C6 and nitrogen-rich type M2N. With increasing austenitizing temperature, the equivalent diameter of the precipitates got fined, and retained austenite content increased significantly when the austenitizing temperature exceeded 1020 °C. The fracture mode gradually changed from brittle fracture to ductile fracture with increasing tempering temperature from 200 to 550 °C. The experimental steel tempered at 350 °C achieved a good combination of hardness (60.6 HRC) and strength (2299.2 MPa) to meet service requirements. Flake M23C6 precipitated along martensite lath boundaries and the secondary hardening phenomenon occurred when the tempering temperature was 450 °C. Due to the high nitrogen content, M2N precipitated from the inside of laths and matrix when tempered at 550 °C.

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate e carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.

To take advantage of the excellent mechanical properties of ultra-high-strength steels, welding processes must be properly controlled to maintain the mechanical properties in welded structures. Electron beam welding (EBW) provides high energy density and thus a relatively low heat input compared to arc welding. However, the narrow fusion zone (FZ) and the heat-affected zone (HAZ) can have insufficient toughness values due to the rapid cooling of the joint. In the present study, S1100MC welded by EBW without filler material was investigated with respect of microstructure, toughness properties, strength of the joint, hardness, and residual stresses close to the top surface. The microstructure of the FZ generally consisted of martensite and tempered martensite with inhomogeneous prior austenite grain (PAG) size between root and face FZ. The martensite phase with smaller PAG sizes caused a strong increase in hardness value in fine-grained HAZ. Tensile tests fractured only in the base material since welds show higher strength than the base material. Evaluated impact toughness levels are moderate, and fracture path deviations only occurred for a particular notch type. The residual stresses in the transverse and longitudinal direction reached up to 79% of the yield strength.

The influence of heat input and Ti addition on microstructure evolution, mechanical property, and corrosion behavior of welded joint was investigated by scanning electron microscopy, auto micro-Vickers hardness tester and scanning Kelvin probe technique. The experimental results indicate that the “wine cup like” shape becomes unobvious with arc power increasing when the welding materials remain constant, and the area of fusion zone (FZ) has a trend of rising. The main constituent of FZ changes from a mixture of acicular ferrite and upper bainite or acicular ferrite to lath martensite or granular bainite with addition of Ti. Therefore, the hardness of FZ in samples without Ti addition is lower than that in samples with Ti addition for the same heat input. With the increase of heat input, the upper bainite content of FZ gradually decreases in samples welded with only wire, while the martensite content of FZ gradually decreases in samples with Ti addition. Consequently, the hardness of FZ has a trend of drop with the increase of heat input keeping the welding materials invariant. The corrosion resistance of FZ is related to the grain size and volume fraction of phases rich in cementite.

Carbon diffusion at the solid–liquid interface is of fundamental importance in scrap melting. Herein, the scrap microstructure at the melt interface and the carbon and silicon distributions are described using optical microscopy and electron microprobe analysis (EPMA). The microstructural path from the surface to the interior of the scrap was primary carbide → acicular martensite → dislocation martensite (original structure). The corresponding carbon concentration gradient was > 4 wt.% → 1–1.5 wt.% → 0.2 wt.%. This was consistent with the observed microstructural changes. Furthermore, the depth of the carbon diffusion layer was 200 μm and 220 μm at 1300°C and 1350°C, respectively. The silicon-enriched layer may be a retarding factor for carbon dissolution. The area of the austenite phase region in the Fe-Fe3C phase diagram was reduced owing to the presence of silicon. Therefore, acicular martensite formed after water quenching decreased, which reduced the thickness of the carburized layer.

Hydrogen embrittlement can occur in steels with metastable phases, due to activation of the hydrogen-enhanced decohesion mechanism upon transformation. Meanwhile, recent investigations suggest that alloys undergoing e-martensite transformation may exhibit resistance to hydrogen embrittlement. To better understand hydrogen effects in these alloys, we investigate the hydrogen-induced microstructural transformations in a metastable Fe45Mn35Co10Cr10 alloy. To this end, we electrochemically charge unstrained samples, quantify the hydrogen evolution by thermal desorption spectroscopy, and observe microstructural transformations by scanning electron microscopy techniques. Through these analyses, we find that the hydrogen-induced e-martensite formation is dependent on the crystallographic orientation of the austenite grains, and takes place preferentially along Σ3 boundaries. Further charging of hydrogen induces extension twinning within the martensite. We examine the microstructural factors influencing these transformations to better understand the hydrogen-microstructure interactions.

In this study, rapidly solidified Cu-Al-Mn ribbons containing 13% Al and 1-14% Mn alloying element by weight were produced by melt spinning method. The structural, thermal, and magnetic properties of produced ribbons were investigated. The parent phase was β1’ martensite for melt-spun ribbons containing Mn amount 1-4%, whereas Cu2AlMn cubic phase for melt-spun ribbons containing Mn amount 7-14%. The microstructures of produced Cu-13Al-1Mn, Cu-13Al-2Mn, Cu-13Al-3Mn, and Cu-13Al-4Mn melt-spun ribbons were consisted of a martensite plate and grain. In addition to this, there were coaxial and equaxial shaped Cu2AlMn phase grains in Cu-13Al-7Mn, Cu-13Al-9Mn, Cu-13Al-11Mn, and Cu-13Al-14Mn melt-spun ribbons. It was observed homogeneous elemental distribution in all ribbons containing different Mn amount. The phase transformation temperatures changed with Mn amount. Austenite-martensite and martensite-austenite phase transformation temperatures decreased with increasing Mn content, while Curie temperatures increased very little with increasing Mn content. Two-way shape memory property was observed for ribbons containing 4 wt.% Mn or less. The magnetic memory effect occurred in the ribbons contains high Mn. The saturation value increased with increasing Mn amount in the produced ribbons.

Direct energy deposition (DED) is an additive manufacturing method that allows repairing the broken parts and building the meter-scale samples. However, the printing of large parts is associated with huge residual stresses and martensite phase formation, which can change the geometry of final samples or initiate the crack. The last factor is especially important for titanium alloys. In this work, we investigated the effect of DED thermal history on the obtained structural and mechanical properties of Ti-6Al-4V using a thermocouple. It was demonstrated that printing with long pauses leads to α′ phase formation, which embrittles the material. Continuous printing with small pauses between tracks leads to the formation of the dual α+β structure. The effect of the texture on the material properties is also discussed. As a result of the study, the specific DED process parameters allow the same mechanical characteristics for as-built titanium alloy and the alloy after heat treatment.

Severe abrasive wear of drill pipe and casing often occurs in exploration drilling operations. To achieve the surface enhancement of the drilling components in terms of abrasive wear resistance, various flux-cored wires (FCWs) containing multiple alloying elements were developed, and the Fe-based hardfacing layers were developed by metal active-gas (MAG) welding process. The macro morphology, microstructure, and phase composition of the alloys were characterized by using OM, SEM, XRD, and EDS methods, while the wear resistance performance was measured by using an abrasion tester. The results showed that the hardfacing layer prepared by Fe-Cr-C FCW is hypoeutectic that mainly consisted of austenite matrix and (Fe,Cr)7C3-type carbides. On this basis, a higher C content in combination with Nb, Ti, and B additives resulted in the microstructure comprised of martensite-based matrix and the precipitations of (Nb,Ti)C and Fe2B hard phases. With the removal of Cr, the further addition of Nb, Ti, and B, the hard phases change from particles to bulk and lath shapes. The hard phases with higher microhardness values than the matrix contributed to an improved Rockwell hardness and a highly decreased wear weight loss of hardfacing layer. Accordingly, the wear mechanism changed from severe plastic fracture to micro-cutting going with a slight brittle micro-peeling. The interlaced distribution of (Nb,Ti)C and Fe2B provided a stable skeleton that enables an further enhanced abrasive wear resistance.

In this study, room temperature tensile properties of type 304, a metastable austenitic stainless steel, prior rolled above Md temperature (200 °C and 300 °C) are compared with mill-annealed and material prior rolled at room temperature (25 °C). Strain-induced martensite that formed during the tensile tests, followed using in-situ using magnetic measurements, display kinetics that vary with prior deformation temperature and strain. Scanning Electron Microscopy and Electron Backscattered Diffraction studies provided direct evidence for shear band formation during prior deformation. Kinetics of strain-induced martensite is found to fit the Olson–Cohen equation modified to include the prior deformation strain and martensite formed during prior deformation. The systematic changes observed in the model parameters of the modified Olson–Cohen equation are explained. The strain hardening in the material is analyzed and correlated with changes in the rate of formation of strain-induced martensite with respect to strain.

The hole expansion characteristics of two steel grades with equivalent tensile strengths: a microalloyed steel, HS-700, and a dual-phase steel, DP-600, was examined. The hole expansion ratio (HER) was 63% for HS-700 and 41% for DP-600. The HS-700 steel gave superior HER due to more homogeneity in microstructure and hardening phases. The DP-600 had inferior HER attributed to the microcrack generation & propagation at the interface ferrite/martensite interface. While the microstructure of DP-600 shows 22% hard martensite in a very soft ferrite, the HS-700 steel hardening is mainly due to thermomechanical processing, which gave fine grain structure along with finer carbides and nitrides associated with Nb and Ti. The YS/UTS ratio of HS-700 was 0.92 against 0.61 for DP-600, which favoured a more homogeneous strain distribution in ferrite that enhanced its HER. Although the DP-600 had higher elongation (26% against 22% for HS-700) and higher n value (0.21 for DP-600 against 0.12 for HS-700), the stretch formability was lower.

A technology has been developed for laser welding of dissimilar materials based on the Ni-Fe-Cr and Fe-C-Mn-Si systems, which can replace the technology of brazing joints between a diamond unit and a steel tube in crown drill bit manufacturing. The optimal parameters for obtaining welded seams without internal defects using laser radiation were determined. The amount of heat input as well as the energy per unit volume of the molten material were also calculated. It was found that the use of optimal energy modes of laser treatment made it possible to avoid the destruction of diamonds during the welding process. Welding led to the formation of martensitic structures in and around the weld. Despite this, preliminary tests of the crown drill bit showed the advantage of a laser-welded joint over a brazed one.

In this research, the effect of solidification pressure on phase transformation temperature and sequence, amount of proeutectoid ferrite, characteristics of pearlite and carbonitrides have been investigated by microstructure observation, crystal structure and component analysis, and thermodynamic calculation. The microstructure of 30Cr15Mo1N ingot is mainly composed of martensite, austenite, pearlite, ferrite, M23(C,N)6 and M2(C,N). With increasing solidification pressure, M23(C,N)6 precipitation temperature, austenite (γ) and ferrite (δ) formation temperature increase, and M2(C,N) precipitation temperature and ferrite (α) formation temperature decrease. Under 0.5 MPa, the precipitation sequence during solidification process of 30Cr15Mo1N ingot is “L → δ → γ → M2(C,N) → M23(C,N)6 → α + martensite/retained austenite → pearlite”. There is a negligible change in phase transformation sequence with increasing solidification pressure from 0.5 to 2 MPa. In addition, the increment in solidification pressure is beneficial to significantly increase the amount of proeutectoid ferrite and reduce pearlite lamellar spacing by enhancing cooling of 30Cr15Mo1N ingot. Meanwhile, with increasing solidification pressure from 0.5 to 2 MPa, the size of M2(C,N) and the amount of precipitated phases (pearlite and carbonitrides) have a decreasing trend, and the change in the size of M23(C,N)6 can be neglected.

This study investigates the microstructure and the hardness of thermomechanically processed joints of ultra-high-strength steel (UHSS) S1100MC, which were butt-welded by means of metal active gas (MAG) welding and electron beam welding (EBW). In MAG welding, the microstructure of the fusion zone (FZ) consisted predominantly of fine ferrite grains. Due to the formation of martensite/austenite (M/A) constituents, the hardness of the reheated weld metal (WM) was higher for vertical-up (PF) than for horizontal flat (PA) welding position. The microstructure of the heat-affected zone (HAZ) of the last welding pass consisted mainly of hard martensite in consequence of fast cooling. However, necklace-type M/A constituents and bainite phases of lower hardness formed when the HAZ of prior welding passes were reheated by subsequent welding passes. The microstructure of the EBW fusion zone was dominated by martensite. Therefore, the fusion zone had considerable higher hardness than the base material (BM). The width of the HAZ in EBW weldments is less than in MAG weldments.

A superior combination of yield strength (1001 MPa) and − 20 °C impact toughness (166 J) was obtained in Nb–V–Ti-microalloyed Ni–Cr–Mo–Cu steel treated by direct quenching and tempering route (DQT). The tested steels treated by DQT route and re-austenitization and tempering route (QT) were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy, transmission electron microscopy, X-ray diffraction, electron back-scattered diffraction method and so on. Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature (400–600 °C), shown as under-aged state, peak-aged state and over-aged state. All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition. Furthermore, the largest difference of yield strength between DQT and QT specimens was shown in DQT600 and QT600 specimens. DQT600 or QT600 specimens refers to direct quenched (DQ) or quenched (Q) specimens isothermally aged at 600 °C. The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains, dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix, which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens. Higher dislocation density of matrix and finer average diameter of both MC (M is any combination of Nb, Mo and V) and Cu-rich particles were shown in DQT600 specimens than in QT600 specimens. Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT600 and QT600 specimens. In addition, strong dislocation strengthening and precipitation strengthening in DQT600 specimen also elevated its ductile-to-brittle-transition-temperature, compared with QT600 specimen.

It was observed that the laser welding at low speed or medium speed did not deteriorate the fatigue property significantly, with a drop of about 9–12% in fatigue limit, compared to the parent material. Further, at a high stress amplitude, all samples survived almost similar numbers of fatigue cycles because of similar type of cyclic hardening–softening effect. However, at low stress amplitude, the cyclic softening of martensite was dependent on the magnitude of residual stress. For the joints fabricated at higher welding speed of 0.1 m/s, a decline of 28% in fatigue limit was observed in comparison with that of the parent material. Additionally, faster crack propagation was visible for high speed laser welded sample at both high and low stress amplitudes. It was proposed that the presence of detrimental {001} cleavage plane and high residual stress might have restricted the cyclic softening phenomena for any stress amplitude.

This paper presents the results of studies on texture formation in ferritic/martensitic steel 12Cr-2W subjected to equal channel angular pressing, warm rolling, and additional heat treatment. X-ray diffraction analysis, transmission electron microscopy, and electron backscatter diffraction were used to study the structure. An unusual texture transition H {001} → L {0 $$\overline{1}$$ 1} was observed for the first time during additional heat treatment of rolled ferritic/martensitic steel. A possibility of such texture transition is due to activation of twinning processes and increase of a number of boundaries with the coincidence site lattice relationships (the CSL boundaries).

Considering the thermophysical properties of materials and the latent heat of phase change, the temperature field, HAZ microstructure evolution, residual stress and post-weld deformation of S355JR-316L dissimilar metals were numerically simulated using SYSWELD software and verified by experiments. After welding, the phase transition only occurs on the HAZ side of S355JR, in which the maximum content of martensite is 9.8%, the maximum content of residual austenite is 0.3%, and the remaining is bainite. Along the weld direction, the longitudinal and transverse residual stresses of S355JR and 316L appear near the weld centerline. The maximum longitudinal residual stress is 410 MPa, and 310 MPa, respectively, and the maximum transverse residual stress is 204 MPa, and 188 MPa, respectively. The overall deformation of the welded joint is V-shaped symmetrical with respect to the weld centerline. The deformation of S355JR is smaller than that of 316L, and the maximum edge deformation is 3.1 mm. The result of angular deformation is 4.1°. The overall deformation and angular deformation of welded joints welded with dissimilar steel are greater than those welded with the same steel.

Ever-leaner compositions are targeted for nominal type 16–8-2 weld metals in attempts to limit sigma-phase formation during elevated temperature operation. Three variants of type 16–8-2 weld metals were exposed to aging at 750 °C for up to 3500 h. Evaluation of the resultant structures by magnetic measurements, neutron diffraction (at ambient and cryogenic temperatures), and electron backscatter diffraction established that the leaner variants were susceptible to forming martensite after aging. This is ascribed to a local increase in the martensite start temperature due to sensitization taking place during elevated temperature aging. The extent of martensite formation diminished during aging treatments exceeding 1000 h owing to diffusion of solute elements from the austenitic matrix to the sensitized regions (recovery). Subsequent elevated temperature aging of the microstructures containing martensite also resulted in a decrease in martensite content: the mechanism, however, differs from that for single-cycle exposure. This observation is explained by martensite reversion to austenite. Martensite formation was completely absent from the higher-alloyed variant. This variant experienced intergranular carbide precipitation and delta ferrite decomposition into secondary austenite and carbides. This work demonstrates significantly different aging responses for composition variants within the allowed ranges for type 16–8-2 weld metals.

Dissimilar SAE 213 T11 and SAE 213 T91 boiler graded steel tubes are welded using magnetically impelled arc butt welding (MIAB) process, a hybrid welding process that combines the advantage of solid state and pressure welding technique. In this research, dissimilar tubes with an outer diameter of 44.5 mm and thickness of 4 mm with a length of 250 mm were joined by using the MIAB welding process. The microstructural behavior and changes of MIAB welded T11 and T91 tubes were analyzed at different zones of the butt-welded tubes using the metallurgical microscope and scanning electron microscope. Micro-Vickers hardness and potentiodynamic polarization test and radiographic testing were conducted. The hardness was taken at different zones along the transverse direction of the join. In the weld interface, the formation of a 2-3 µm martensitic structure leads to the hardness value of 281HV1, and at the base metal of T11 is 217HV1, T91 is 254HV1. The bead width at the welded region is approximately 10-12 µm with the narrow heat-affected zone. The metallurgical bonding between the T11 and T91 dissimilar tubes shows sound joint, which is confirmed with the radiography test.

In high-carbon unalloyed steels produced by the powder metallurgy method, the effect of different spheroidizing heat treatments on microstructure, mechanical properties, and wear performance was studied. For this purpose, 1.2 wt% graphite powders were added to Fe powders, and a powder composition was formed. The prepared powder mixtures were pressed at room temperature under 850 MPa and then sintered at 1180 °C for 20 min. After sintering, sample phase compositions were determined by the XRD analysis. Then, three different spheroidizing heat treatments were applied. First, the conventional spheroidizing was used for 5–10–20–40 h at a constant temperature of 700 °C. Secondly, for 5–10–20–40 h, cyclical spheroidizing was involved in the temperature range of 700 °C and 740 °C. A third heat treatment (post-quenching spheroidizing), which represented the study's original aspect and was an alternative to these known spheroidizing methods, was applied. Fully austenitized work-pieces were quenched in water at room temperature. Consequently, the martensitic structure was formed. These samples were over-annealed for 5–10–20 h at 700 °C. Even after a short period, such as 5 h, homogeneous and fine spherical cementite was formed in the microstructure. Depending on the spherical cementite morphology and distribution, impact toughness and hardness values were evaluated. Besides, steels' wear characteristics were determined using the block-on disc device, and three-dimensional worn surface morphologies were examined with the profilometer device. With the scanning electron microscope (SEM), microstructures formed due to different heat treatments were displayed. Also, the fractured and worn surfaces of the samples were examined. It was determined that with the post-quenching spheroidizing treatment, which may be an alternative to known and widely used spheroidizing treatments, the desired microstructure and properties such as machinability and ductility could be achieved in a much shorter period.

This paper describes the development and characterisation of bainitic steel for rail applications based on carbide-free, low-alloy steel. The results show that after rolling and subsequently cooling, the designed carbide-free bainitic steel exhibits better mechanical performance than standard pearlitic steel. This is because of its fine, carbide-free bainitic microstructure, which consists of bainitic ferrite and retained austenite laths. Microstructural and mechanical property analysis was carried out using scanning and transmission electron microscopy, X-ray diffraction, hardness measurements, tensile and low-cycle fatigue tests. The obtained results demonstrate that during low cyclic deformation, a partial transformation of the retained austenite into deformed martensite α′ takes place, and strain-induced martensitic transformation occurs. The initial strengthening of the material during low-cycle fatigue was caused by the transformation of austenite into martensite and the increase in the dislocation density of the steel. In addition, an optimal amount of retained austenite in the form of thin layers and islands (dimensions not exceeding 1 µm) made it possible to obtain a high yield while maintaining the high plasticity of the steel. These microstructural features also contributed to the high crack resistance of the tested carbide-free bainitic steel.

Instead of conventional quenching and tempering, fast austenitization from an initial microstructure of lamellar pearlite followed by quenching and tempering was carried out, leading to the formation of inhomogeneous microstructure. It comprised different morphologies of lath martensite and retained austenite (RA). The effect of tempering temperature on microstructure evolution and tensile properties was systematically investigated. With increasing tempering temperature from 150 to 250 °C, transition carbides gradually coarsened and their amount increased, the dislocation density in martensitic laths gradually decreased, and RA fraction decreased from 10.9% to 2.2%. The precipitation and dislocation strengthening can ensure a high strength, while RA can ensure a good ductility, leading to a simultaneous increase in the strength and ductility when decreasing tempering temperature. Specifically, the best combination of tensile properties (ultimate tensile strength of 2133 ± 41 MPa and total elongation of 11.1% ± 1.3%) was achieved after tempering at 150 °C.

This work concurrently investigates the microstructure evolution and stress state during continuous cooling of 1045 steel gear parts. A finite element algorithm was developed by two-way coupling of microstructural and thermal fields. Thermal coefficients were correlated to microstructural evolution, and a phase transformation kinetics model was considered to be both temperature and time dependent. Magee's rule was chosen for martensitic transformation modeling to incorporate the effect of stress state on microstructural field. The dilation curves for the benchmark sample show that martensite starting temperature increases when the stress state is considered in microstructure modeling. To determine the validity of the presented model, a cylindrical specimen was quenched in water and oil media, and their predicted microstructures were compared with experimental results. A reasonable harmony between simulated and experimental results was observed. The simulation was then performed for internal gear parts quenched in oil and water media. The simultaneous monitoring of microstructure and stress evolution during continuous cooling of an internal gear part was conducted. It was shown that simultaneous and separate transformations occurring between the tooth and root region have a determining role in the sign of stress for internal gear parts. Lastly, based on monitoring of different phase transformation scenarios occurring during water and oil quenching, a hybrid quenching was proposed to increase martensite volume fraction at the tooth region of an oil-quenched sample without altering the microstructure at root region.

This work investigates the effect of increasing both martensite phase and ferrite phase strength on tensile properties and fracture behavior of dual-phase (DP) steels. The martensite phase strength is varied by changing its carbon contents and tempering process, whereas the ferrite phase strength is varied by dispersion of nano-sized vanadium carbides (VC) precipitates in the ferrite phase and also by changing its total fraction in DP samples. It is found that strengthening of the martensite phase improved the tensile strength with almost no loss of the tensile strength–uniform elongation balance. This is attributed to the enhancement of strain partitioning between the ferrite and martensite phase so that the work hardening rate increased. On the other hand, strengthening of ferrite phase effectively improved both the tensile strength and the tensile strength–post-uniform elongation balance due to suppression of strain partitioning which likely promoted homogenous deformation after necking. More importantly, a simultaneous increase in the strength of martensite phase and ferrite phase leads to a moderate phase strength difference, which is revealed as a promising strategy to achieve high strength in addition to a good balance of tensile strength, uniform elongation, and post-uniform elongation in low-carbon DP steels. Furthermore, the quantitative analysis of void formation reveals that decohesion of the interface between the ferrite phase and martensite phase is the dominant fracture mechanism, which is less likely to be affected by phase strength difference.

The influence of severe plastic deformation induced by ultrasonic shot peening (USP) on evolution of microstructure and the resulting properties of 316 austenitic stainless steel is evaluated. Microstructural characterization was carried out by using optical, scanning, and transmission electron microscopies in addition to electron backscattered diffraction, x-ray diffraction and x-ray photoelectron spectroscopy. Mechanical properties were evaluated using hardness and sliding wear tests. USP resulted in high dislocation density and refined microstructure that led to significant increase in hardness. Although no strain induced martensite formed after USP at room temperature, USP carried at cryogenic temperatures led to martensite formation indicating the importance of temperature in USP on strain induced martensite development. Contrary to this, sliding wear at room temperature led to martensite development suggesting dominant role of deformation mode on martensite development during sliding wear. Sliding wear mechanism involved abrasion of steel accompanied by plastic deformation. Localized corrosion resistance improved due to nanostructured surface produced after USP enhancing the diffusional transport of chromium to surface. XPS analysis revealed higher Cr: Fe ratio in the oxide film after USP. Simultaneous enhancement of wear and corrosion resistance of stainless steels by the application of USP is discussed.

Laser welding of Al-Si coated 22MnB5 hot stamping steel/galvanized steel with different laser power, welding speed, and coating conditions were presented. The weld appearance, microstructure, and mechanical properties of the joints were investigated. With the increase of heat input (raising the laser power or lowering the welding speed), the welding seam changed from Y-shape to X-shape gradually, and the widths of the weld seam increased. The fusion zone (FZ) consisted of lath martensite (M). On the side of 22MnB5, the heat-affected zone (HAZ) was divided into coarse grain zone, refined zone, intercritical zone, and tempered zone along the direction from FZ to the base material (BM). Coarse grain zone and refined zone comprised martensite, intercritical zone comprised martensite and a little ferrite, and tempered zone comprised tempered martensite, which expressed obvious softening. On the side of galvanized steel, HAZ was mainly composed of ferrite and pearlite. Significantly, band-like δ-ferrite (a low ductility and low strength phase) was formed along the fusion boundary owing to the accumulation of the mixing of Al-Si coating and base metals. The maximum tensile shear strength of the joint was only 484 MPa in coated condition but increased to 700 MPa in de-coated condition because of the existence of δ-ferrite along the fusion boundary, which deteriorated the mechanical properties of the joints. The failure occurred at the fusion boundary (FB) in the coated conditions with a mixture of cleavage, quasi-cleavage, and dimpled fracture. However, the failure occurred at the interface in the de-coated condition with a dimpled fracture. In addition, for the coated condition, higher laser power or lower welding speeds led to wider bead width and more intense weld pool flow, which enhanced the diffusion of Al, resulted in alleviating the accumulation of Al at the fusion boundary, which increased the mechanical properties.

A novel Fe-Cr-B-C-Al-Si-Mn composite coating was designed and synthesized on 2Cr13 steel substrate using laser cladding technique in this study. The microstructure evolution, microhardness and wear resistance of the composite coating were investigated. The results showed that the microstructure of the coating was mainly composed of α-(Fe,Cr,Al) martensite, retained γ-Fe, and the eutectic comprised of CrFeB, M3(C,B)2, M2(B,C) and M23(C,B)6. The average microhardness of the coating (742.1 HV0.5) was 3.95 times compared with the 2Cr13 steel substrate, and the wear resistance was enhanced by 12.5 times compared with the 2Cr13 steel substrate. The wear mechanisms were dominated by abrasive wear and oxidative wear.

The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100)B2||(100)B19’and (100) B2 || (011)B19’; (011)B2|| (001)B19’and$${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$(011)B2||111¯B19′. Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11$$\bar{1 }$$1¯) invariant twin boundaries was observed at higher degrees of deformation.

Compact components are prone to failure at brazed joints. A comprehensive understanding of the effect of brazing temperature on the metallurgical reactions and microstructure evolution of 316L/BNi-2 brazed joints will provide support for optimization of the brazing process and improving joints strength. In this paper, the brazing process was carried out at 966°C 991°C, 1033°C, 1065°C, 1100°C, 1137°C and 1170°C in a vaccum furnace. The morphology and crystal structure of different zones of the brazed joints including athermal solidification zone (ASZ), isothermal solidification zone, and diffusion-affected zone (DAZ) were investigated. Results show that the DAZ begins to form, and the Ni3B, Ni3Si, CrB, and γ-Ni in the ASZ gradually coarsen below 1033°C. When the temperature reaches 1065°C, the filler alloy completely liquidizes. The boron in the filler alloy diffuses into the base metal, resulting in isothermal solidification. The residual liquid phase generates binary and ternary eutectic reactions. According to TEM patterns analysis in the DAZ, borides, and carbon-boride are mainly distributed on grain boundaries and within grains, respectively. In addition, M5B3 boride in the DAZ gradually transforms into M2B boride due to its better thermodynamic stability. Above 1170°C, the borides in the DAZ partly dissolve, and meanwhile, the matrix transforms from martensite to austenite, forming the alternating distribution of austenite and boride. During the heating process, with increasing brazing temperature, the hardness and elastic modulus of brazed joint microregions gradually decrease.

The numerical study on the complex failure mechanism in dual phase steels is significant because of their intricate microstructure. Their mechanical behavior depends on the volume fraction of martensite/ferrite and microstructural morphology, such as size, aspect ratio, interconnectivity, and mechanical behavior of individual phases. Like all multiphase materials, the connectivity between different phases and possible changes in mechanical properties near the interface (interphases) play significant roles in microstructural damage initiation and propagation and, therefore, affect the overall mechanical response. This study focused on the stress–strain behavior of DP-800 steel using finite element analysis of three-dimensional representative volume elements. Based on experimental results, local hardening was considered in the ferrite matrix near the martensite islands. Also, various volume fractions and material properties for different hardening zones were implemented to study the stress–strain behavior of DP steels. Interface elements based on cohesive zone modeling were considered to simulate the damage on the ferrite and martensite interface numerically. Furthermore, hydrostatic stress distribution which is known as an indicator to show the void initiation was used to predict damage zones and the stress distribution is compared with the results obtained from the cohesive zone method. The mechanical behaviors of the finite element models by the proposed methodology, considering cohesive elements and hardening zones in three-dimensional representative volume elements, indicated a better adaptation to the experimental results.

This paper aims at analyzing the effect of heat treatment using Concentrated Solar Energy (CSE) on the martensitic microstructure evolution of Ti-6Al-4V alloy manufactured by Selective Laser Melting (SLM) technology. The application of CSE, a renewable energy sources, in post-processing of SLMed Ti-6Al-4V alloy is discussed based on heat treatments performed at Plataforma Solar de Almeria, Spain, using solar furnace SF40. In SLM process of Ti-6Al-4V powder because of local high heating followed by a rapid cooling the microstructure consists of an acicular martensite (α′ phase). Decomposition of α′ martensite of SLMed Ti-6Al-4V after heat treatment in solar furnace, using CSE, by SEM, EDX, TEM, SAED, and XRD analyses, was investigated. It has been found that for treatment below β-transus at 705 °C, because of low temperature, the α′ martensite was not fully converted into α + β and the β phase was not identified by XRD analysis. Heat treatments below (940 °C followed by 650 °C) and above (1050 °C) β-transus have revealed that α′ martensite was converted into a lamellar α + β mixture and also β phase was identified by XRD and TEM analysis. In the analyzed areas, middle areas of the samples contain an area fraction of β phase greater than the top areas. Area fraction of β phase, after heat treatment above β-transus, at 1050 °C, is greater than that obtained after treatment below β-transus.

Austempered Ductile Iron (ADI) is a candidate material to replace case-hardened steel in many applications. The mechanical properties of ADI can be tailored by the chemical composition and heat treatment conditions. In this study, different nickel content Ductile Irons (DI) were cast and heat treated. In order to design the austempering process, time–temperature–transformation diagrams were generated with the Calculation of Phase Diagrams method. For each chemical composition, one group was tested in its as-cast state while the second and third groups were austempered. For austempering, the casting is reheated to austenitization temperature (900 °C) and then quenched in a salt bath at a temperature of 300 °C and held at this temperature for 2 h. The third group was cryo-treated (−196 °C for 6 h) and tempered (200 °C for 2 h) after the austempering process. Microstructural examination was performed using an optical microscope and X-ray diffraction technique. The effect of heat treatment on the hardness, toughness, and tribological behaviors of samples was investigated. The results showed that austempering with correct parameters significantly improved the hardness, toughness, and wear resistance of DI. The nickel content of DI plays a significant role in determining the properties of the alloy, and the optimum Ni amount among the tested compositions was found to be 1.64%. It was observed that cryogenic treatment facilitates some of the austenite to martensite transformations and improves wear resistance (20%); however, it has a limited effect on hardness (2–3 HRc) and toughness (±3 J).

To elucidate the dynamic strain aging (DSA) mechanism that causes serrated flow in the tensile curves of an intercritical annealed steel with retained austenite (RA) and intercritical ferrite, the multi-aspect microstructural characterization and micromechanical deformation behavior were systematically investigated in this study. Our results demonstrate that the DSA phenomenon is dependent on the strain rate, deformation temperature, and strain. This is accompanied by deformation-induced martensitic transformation (DIMT) and the generation of crystallographic defects in nucleating and propagating Portevin-Le Chatelier bands. An increase in the deformation temperature and strain rate leads to an increase in the stacking fault energy of RA. This inhibits the growth of martensitic embryos and retards the kinetics of martensitic transformation of RA, consequently decreasing or suppressing the DSA phenomenon. Furthermore, neither the long-range diffusion model of carbon atoms nor the short-range diffusion model on the reorientation of C-Mn complexes could reasonably explain the DSA phenomenon in this study. Based on the in situ synchrotron X-ray diffraction results and established mathematical models stemming from the dislocation multiplication theory, the DSA could be attributed to the periodic instantaneous dislocation multiplication in constituent phases caused by burst DIMT.

This article investigated the solidification structure evolution for a typical high-carbon steel billet under strong cooling intensity during the process of continuous casting. The results suggested that a regular solidification phase transformation route of ‘austenite → lamellar pearlite’ was obtained in the center surface of billet; however, an abnormal microstructure evolution route of ‘austenite → martensite → tempered sorbite (fine spheroidized cementite in the ferrite matrix)’ appeared around the corner of billet. Besides, the secondary dendrite arm spacing for the center surface was 90.92 μm, and it reduced to 49.01 μm for the corner part because of different cooling conditions. In addition, the billet corner with a major tempered sorbite phase shows higher values of tensile strength, elongation and hardness, indicating sorbite phase could improve high carbon steel mechanical properties more significantly than the phase of lamellar pearlite because of its unique homogeneously distributed spheroidized structure. Furthermore, a laboratory experiment was conducted to verify the formation mechanism of tempered sorbite, and the results suggested that martensite phase would be formed around the billet corner under strong cooling intensity during the second cooling zone, and then the quenched martensite would be transferred to tempered sorbite, as the billet was out of the second cooling zone and reheated by its inner solidification heat.

The sliding wear property of high-carbon nanostructured bainitic bearing steel with the equal initial hardness and different microstructures was investigated, and the reasons for the difference of wear resistance between the cementite-bearing (CB) and cementite-free (CF) specimens were analyzed. The results show that CF specimens have lower mass loss and surface roughness and shallower wear depth than CB specimens during wear process. Compared with CB specimen, CF specimen presents superior wear resistance. This is due to two reasons: (1) a lot of retained austenite in CF specimen is easy to produce TRIP effect and be transformed into martensite during wear process, which notably increased the surface hardness of worn specimen; (2) there is a nondestructive oxide layer in the surface of cementite-free worn specimen, which can protect the surface of worn specimen from destruction. Under the combined effect of retained austenite and oxide layer, the loss of matrix is reduced. Thus, CF specimen exhibits high wear resistance. It reveals that the wear mechanism of high-carbon nanostructured bainitic bearing steel with different microstructures can provide a reference for improving the wear resistance in high-carbon nanostructured bainitic bearing steel in future.

A lean alloyed low carbon Si and Mn steel was austenitized at three different temperatures in the inter-critical zone (790, 810 and 830 °C) followed by austempering in salt bath in the temperature range of 425 and 525 °C to develop an attractive combination of strength and ductility with a TRIP assisted bainitic ferrite microstructure. The microstructure revealed a combination of ferrite, bainite along with martensite/retained austenite. Superior tensile strength and ductility were found in the steel austempered at about 450 °C due to the presence of higher bainitic, lower ferrite and moderate martensite/retained austenite. The product of tensile strength and percentage of elongation obtained were greater than 20 GPa% at some of the heat-treated conditions qualifying for third generation AHSS. The microstructural evolutions were correlated with the theory of bainite formation.

The reinforcing bar (rebar) produced by Tempcore process had tempered martensite structure on its surface, owing to quenching and tempering, and ferrite–pearlite structure at the core owing to slow cooling rate. Bainite, a low-temperature transformation phase, was observed in the transition area. Mechanical properties of the rebar are significantly influenced by area ratio and hardness of the tempered martensite. In this study, microstructure and area ratio of the tempered martensite of the rebar produced by the Tempcore process were predicted using finite volume method and a continuous cooling transformation diagram. The area ratio of the tempered martensite was predicted with error range of ± 6%. Furthermore, association between the microstructure and hardness was determined by a non-isothermal tempering experiment. The hardness of the tempered martensite was successfully predicted using the tempering temperature with R-squared 98%. The proposed methodology can be effectively used to control the mechanical properties of the rebars.

The objective of this paper is to study the surface texture behavior of low and medium-carbon steels machine elements in acidic environment using Abbott Firestone curve depending upon MATLAB software. The chemical composition of low-carbon steel is 0.16C-0.27Si-1.47Mn-0.02Al while medium-carbon steel is 0.49C-0.30Si-0.91Mn-0.03Al. They were hot-rolled at 1200°C for 30 min followed by air cooling. Two different heat treatment processes were applied on the hot-rolled samples. The first treatment was quenching after martensite finish (QAMf) and the second one was quenching and partitioning (Q&P). For 0.16C, the microstructure gives bands of ferrite and pearlite after hot-rolled steel. However, 0.49C produces coarse pearlite islands surrounded by ferrite phase. While, the microstructure after QAMf and Q&P processes contain ferrite, bainite, lath martensite, and retained austenite for 0.16C steel, and polygonal ferrite, lath martensite, and retained austenite for 0.49C steel, respectively. In low-carbon steel (0.16C), after hot-rolling, steel suffers from low exploitation zone (44%) of surface texture. However, after QAMf, low-carbon steel (0.16C) gives lower surface texture (36%). In addition, hot-rolled steel (0.16C) produces 40% voids zone while after QAMf, voids zone increases to 52% of surface texture. In medium-carbon steel (0.49C), after hot-rolling, steel produces 78% exploitation zone of surface texture. Furthermore, Q&P slightly enhances surface texture (exploitation zone) to 82%. At the same time, hot-rolled and Q&P of 0.49C steels exhibit very small number of voids for surface texture (1–4%). Therefore, 0.16C steel (hot-rolled and QAMf) is highly subjected to severe failure in etchy environment. However, 0.49C steel (hot-rolled and Q&P) is highly recommended in case of moderate safety factor.

Electron beam welding was used to fabricate dissimilar butt weld joints of AISI 321 and AISI 409 stainless steel 18-mm-thick plates. On both sides, i.e., fusion boundary of AISI 321 and fusion boundary of AISI 409 and along the thickness of the joint, the microstructure evolutions were studied using optical microscopy and scanning electron microscopy. The results revealed that martensite was the major phase formed in the WM. The thickness of HAZ was more on side of AISI 409 as compared to AISI 321. In comparison with the WM, the HAZ was softer. The mechanical testing revealed that the root region was more ductile in comparison with the weld’s top region. The brittle fracture was demonstrated when the notch was formed at the HAZ of AISI 409 SS and the impact toughness reduces to 2 J. However, when the notch was formed in the HAZ of AISI 321 SS, the impact toughness improved to 32 J.

The effect of ferrite and martensite hardening variation on the mechanism of void formation in a low-carbon low-alloy steel with different ferrite–martensite dual-phase (DP) microstructures has been investigated. Accordingly, several ferrite–martensite DP microstructures containing a wide range of martensite volume fractions (Vm) from 23% to 87% were developed by application of intercritical annealing heat treatment processes. Optical and field-emission scanning electron microscopy were supplemented by hardness measurements and tensile testing to follow the microstructural changes and correlate them with the mechanism of void formation. The experimental results indicate that the mechanism of void formation changes from ferrite/martensite (F/M) interface decohesion to martensite cracking as Vm increases in the microstructures. For DP specimens containing low Vm, F/M interface decohesion is the dominant mechanism of void formation, while in DP samples with moderate level of Vm, both mechanisms of F/M interface decohesion and martensite cracking are involved in the void formation. In DP samples containing high Vm, void nucleation and growth due to martensite cracking is the dominant damage mechanism. This change in the void formation mechanism with Vm can be rationalized based on the change in the plastic behavior of ferrite and martensite microphases developed as a consequence of ferrite and martensite hardening variation. The martensite hardening response decreases monotonically with increasing Vm from 23% to 87%, while that of ferrite shows an increasing trend up to 73% then decreases with further increase up to 87%.

In the frame of a quasi-viscous mechanism of plastic deformation via the flows of nonequilibrium point defects in the fields of local pressure gradients, peculiarities of propagation of the front of nanobands of crystal lattice reorientation are studied. A theoretical analysis of the velocities of point defects and the carriers of the rotational deformation mode – nanodipoles of partial disclinations, is made in terms of the type of point defects (vacancies and interstitial atoms), deformation temperature, disclination structure peculiarities and elastically stressed state at the nanoband propagation front. A possibility of simultaneous occurrence of the martensitic and quasi-viscous deformation modes at this front is shown. The effective viscosity coefficient is estimated for different energies of migration activation and temperatures. An important role of high local moments or stress field gradients is revealed during activation of the rotational deformation mode. It is shown that under the conditions of high local stress fields the material viscosity can reach the values close to that of liquid glass.

The grain-size-dependent microstructure evolution and overall mechanical behavior of polycrystalline NiTi thin films have not been widely studied under thermomechanical loading. We propose a non-isothermal phase field model with an additional grain boundary energy term introduced into the local energy density to take into account the contribution from the grain boundary in the polycrystalline system. We investigate the grain-size effects in the process of temperature-induced martensitic transformation, detwinning and reorientation of martensitic variants, and superelastic deformation. This multi-variant two-dimensional phase field study showed that, as the average grain size of the thin film drops from 1.5 μm to 10 nm, the temperature-induced martensitic transformation tends to be inhibited. The polytwinned martensitic microstructure tends to be replaced by single-variant martensite microstructure in the grains. The martensitic transformation will not even take place once the grain size is around 10 nm. Meanwhile, the stress-induced martensitic detwinning and reorientation are less active for all types of martensite variants. The shape memory functionality may be weakened with decreasing grain size. Regarding the superelastic behavior, the stress-induced martensitic transformation is delayed with decreasing average grain size. The stress plateau tends to be higher and less noticeable as the grain size drops. The underlying mechanism of this grain-size effect is associated with the inhibiting effect of the grain boundary, the fraction of which will noticeably increase as the average grain size drops, and thus the phase transformation area will significantly reduce in the polycrystalline thin film.

Fatigue crack growth (FCG) behavior of 9Cr/CrMoV dissimilar welded joint at elevated temperature and different stress ratios was investigated. Attention was paid to the region near the fusion line of 9Cr where carbon-enriched zone (CEZ) and carbon-depleted zone (CDZ) formed due to carbon migration during the welding process. Hard and brittle tempered martensite dominated the stress ratio-insensitive FCG behavior in the coarse grain zone (CGZ) of 9Cr-HAZ. For crack near the CGZ–CEZ interface, crack deflection through the CEZ and into the CDZ was observed, accompanied by an accelerating FCG rate. Compared with the severe plastic deformation near the secondary crack in 9Cr-CGZ, the electron back-scattered diffraction analysis showed less deformation and lower resistance in the direction toward the brittle CEZ, which resulted in the transverse deflection. In spite of the plastic feature in CDZ revealed by fracture morphology, the less carbides due to carbon migration led to lower strength and weaker FCG resistance property in this region. In conclusion, the plasticity deterioration in CEZ and strength loss in CDZ accounted for the FCG path deflection and FCG rate acceleration, respectively, which aggravated the worst FCG resistance property of 9Cr-HAZ in the dissimilar welded joint.