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Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

The development of ultrastrong maraging stainless steels (MSSs) is always in high demand. However, traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic (BCC) lath martensitic matrix. Here, we design a novel ultrahigh strength MSS (Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B, wt.%) using a cluster formula approach. A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-NiAl nanoprecipitates (3−5 nm) in BCC martensitic matrix was successfully obtained. This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa, but also a decent uniform elongation of 4.2%−5.1%, which is almost triple of the value observed in existing MSSs. We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit high-performance MSSs.

A series of TA15 specimens with different process parameters were built by electron beam melting in this paper to systematically investigate the relationship of material-process-microstructure-mechanical properties. A quantitative mathematical model of yield strength and Vickers hardness with respect to α lath thickness was established. Microstructure analysis results showed that the microstructure of TA15 titanium alloy were basketweave microstructure, lamellar microstructure and bimodal microstructure depending on process. In general, the yield strength, Vickers hardness and α plate thickness are correlated with the Hall-Petch equation, but the mathematical model established in this paper has a better fitting effect. Process parameters affect the mechanical properties of the TA15 titanium alloy by affecting α lath thickness, and speed factor has the greatest influence on α lath thickness. The tensile properties of TA15 titanium alloy are as well as, if not better than, that of TA15 titanium alloys with tri-modal microstructure.

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.

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.

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450-460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

In order to explore the effect of different the welding heat input on microstructure and impact properties in coarse grain zone in heat affected zone (CGHAZ) of ultra-low carbon bainitic steel welded joint. In this paper, Gleeble-3500 thermal simulation testing machine was used to simulate different heat input, to study the effect of linear energy on the microstructure and impact toughness of the CGHAZ of Q420qEN steel joint. The scanning electron microscope (SEM), oscillographic impact microscope and transmission electron microscope (TEM) were used. The results show that the microstructure of CGHAZ is mainly the lath bainite and granular bainite. When the welding heat input is 30kJ•cm-1, the CGHAZ grains have obvious growth phenomenon. With the increase of the welding heat input, the grain size also grows. Meanwhile, as the welding heat input increases from 18kJ•cm-1 to 30kJ•cm-1, the impact toughness of the CGHAZ of joint increases first and then decreases at -20°C.

Research has been carried out on the Measurement of the Outer Thread Pitch Distance from the Tongtai Tcs 1500[M] Cnc Lathe Using a Projector Profile. The use of a screw system to unite two components, is almost always found in all industrial technology results from the form of industrial technology with a low level of accuracy (rough) to a very high level of precision (precision). Therefore the screw system has become one of the important factors in industrial progress in all types of production. The higher the level of a component made means the higher the level of accuracy of the thread system. In this study, calculations were carried out to determine the level of precision of a thread made through the machining process using a TONGTAI TCS 1500 [M] CNC lathe. The precision measurement was carried out by measuring using a dimension measuring instrument, namely Profile Projector type HST-CPJ-3000/3000Z. To find out the average value and the value of the screw deviation, a research and measurement process was carried out with several product samples or test objects (sample A, sample B, sample C) where each sample was the same size, namely M15 × 50 P 1.5. The first measurements were made on the major and minor diameters. Because this measurement is more focused on the level of precision of the thread pitch distance, the second measurement is carried out by measuring the dimensions of the distance between the pitches as much as N = 10. From the 10 measuring points of the thread pitch distance, different average values and deviation values will be produced although in the same size.. The purpose of this study was to determine the level of pressure of a thread made through a machining process which in this study used a CNC TONGTAI TCS 1500 [M] lathe. it is known that the overall average value of the three samples is with a major diameter = 14,949 mm, a minor diameter = 13,219 mm, and a thread pitch distance = 1.5309 mm.

In the hierarchically-arranged crystallographic structure of reduced-activation ferritic/martensitic (RAFM) steel, the smallest but the most abundant microstructural unit, i.e., the lath, plays an important role in dislocation plasticity. Due to the multi-scale complexity in the lath-martensitic microstructure, miniaturized mechanical characterization at very fine scale is required to understand the deformation mechanism associated with laths. In this study, uniaxial micro-compression tests combined with rigorous crystallographic analysis were performed to figure out the plastic deformation mechanism of lath boundary sliding in RAFM steels. These experimental results were further interpreted via molecular dynamics simulations to discover the underlying dislocation mechanisms. We found that the amount of lath boundary sliding is controlled by the crystallographic orientation of the lath boundary plane, the direction of Burgers vectors of the interfacial dislocations, and the magnitude of resolved shear stress on the lath boundary plane. Also, the effect of normal stress on the lath boundary plane was investigated.

Dissimilar metal welded joints were fabricated between SA508 low alloy and 304LN stainless steel using IN82, IN182, and IN152 as buttering alloys. In-situ tensile testing of the welds demonstrated that the crack initiation and propagation were away from the fusion boundary between low alloy steel and buttering material. The heterogeneities like clustered lath martensite with high dislocation density, Type-I boundary, and Type-II boundary near the interface have a meager role in influencing the tensile properties of the welds. On the contrary, grain structure and second phase distribution in re-solidified buttering alloy played a dominant role in determining the tensile properties of welds. This understanding may pave the way to modify the welding parameters, select appropriate welding consumables, and explore the failure-prone location for such kind of transition joints.

The present work aims to study the variation of microstructure, microhardness and wear properties of the laser powder bed fusion (LPBF) processed Ti6Al4V thin-walled sample along the build direction. The results revealed that a harder α′ and Ti3Al phases in the Ti6Al4V’s microstructure. The lath thickness was thinner at the top and bottom than the middle region. The microhardness value variation was found to be around ∼ 5 % along the build direction. A linear relationship between microhardness and wear loss was observed. The presence of the Ti3Al phase resulted in inferior sliding wear of the thin-walled part along the build direction. However, the coefficient of friction values was influenced by variable lath thickness along the build direction. The outcome of the present study is technically captivating and crucial for the design and development of advanced industry-specific thin-walled Ti6Al4V end-use parts.

The martensitic transformation in dilute Fe-Ni alloys was firstly investigated by using a composition gradient in the range from 0 to 31 mass% Ni. The current study exhibited the transformation of austenite to equiaxed or acicular ferrite at lower Ni contents can be suppressed and lath martensite can form. The electron backscattered diffraction results showed Ni has important effect on misorientation distribution in martensite. With the increase of Ni content up to ∼27.5 mass%, the misorientation distributions above 15° increased, and a peak at around 7° gradually disappeared. With the further increase of Ni content, two peaks between about 50° and 60° at low Ni contents changed into three peaks at high Ni contents and a new peak at around 17° arose due to formation of plate martensite. Moreover, it was observed that both block and sub-block was refined with increasing Ni content. Moreover, the micro-hardness/strength of martensitic Fe-Ni alloys between previous works and this work and the micro-hardness of martensite between Fe-C and Fe-Ni alloys were compared and discussed.

Orientation induced anisotropic hardness and modulus of Ti-6Al-4V alloy has been minimized through microstructural engineering. This involves the formation of martensite upon quenching and further its decomposition facilitated by cold deformation and low temperature heat treatment. The martensite ( α ′ ) formed from the β phase has nearly equal distribution of multiple variants in the form of fine laths. After decomposition, this transforms to fine equiaxed β along the lath boundaries of α , instead of a thin lamellar β . The variations in the nano-indentation derived hardness and modulus are much smaller compared to the reported literature and is attributed to the fine nature and morphology of the constituent phases.

The microstructural heredity of as-rolled lath martensite with different prior austenite grain morphologies during inter-critical quenching has been studied by using a quasi in-situ EBSD technique in a 6.5%Ni-containing cryogenic steel, to reveal the effect of as-rolled microstructure on final microstructure and toughness at 77 K. It is found that the refinement of final microstructure can be achieved by refining the as-rolled lath martensite through both refinement and elongation of prior austenite grains, which is attributed to two aspects of heredity effect. On the one hand, the fine as-rolled martensitic structure in the interiors of prior austenite grains is inherited during inter-critical quenching. On the other hand, the refinement of as-rolled lath martensite promotes the formation of newly oriented grains along the prior austenite grain boundaries to further refine the microstructure. The cryogenic toughness of Ni-containing cryogenic steels is significantly improved through the heredity effect of as-rolled lath martensitic refinement.

A new (α + β) Ti-alloy, Ti-6Al-2V-1Cr-1Fe (wt%), with fine grain sizes, fine precipitates, together with the high strength and excellent ductility in its as-cast state is developed. As compared to the as-cast commercial Ti-6Al-4V alloy, the grain size and α lath thickness of the new alloy are substantially refined by 50∼75%, and the yield strength and ductility are increased by 19.7% and 51.8%, respectively. The grain size refinement is achieved by tuning the supercooling capacity of the alloy through alloying with the aid of CALPHAD calculations. In addition, the alloying of Cr and Fe substantially reduces the α lath thickness. This low-cost Ti alloy with the enhanced properties is anticipated to be highly suitable for various structural applications in its as-cast or as-printed state.

In the present study, a modified H13 hot-work tool steel (M-H13) was fabricated by laser beam powder bed fusion (LB-PBF). The effect of two types of post processing, direct tempering from as-built condition (DT) and conventional quenching followed by tempering (QT), on the microstructure and mechanical properties was evaluated. The typical microstructure in QT condition was tempered martensite with carbides along lath boundaries. In DT condition, melt pool boundaries and cellular structure from as-built condition were still observed. While comparable tensile properties and hardness were obtained, DT samples exhibited significantly lower impact toughness compared to QT samples. This was attributed to the difference in work hardening ability and strain rate sensitivity originating from different microstructures obtained under these two heat treatment conditions. The study was also focused on the softening behavior and the correlation with the microstructure of the two post treatments at the elevated temperatures. It was found that the DT samples showed lower thermal softening compared to QT samples. The evolution of carbides was discussed based on the microanalysis results and the JMatPro simulation.

The influence of martensite morphology and volume fraction on low-temperature impact toughness of dual-phase (DP) steels was investigated by impact tests, nanoindentation tests, atom probe tomography (APT) and microstructural examination. The APT results of medium-C martensite show that many small-sized C clusters are formed at the twin grain boundaries inside the lath, which increase nanohardness and decrease impact toughness. The impact test results indicate that the low-temperature impact toughness of lath martensite is significantly superior to ferrite. Refining the lath martensite substructure can improve the low-temperature impact toughness of DP steels. However, martensite twins obviously reduce the low-temperature impact toughness of DP steels. It is further found that the impact toughness of DP steel is affected by the toughness of ferrite and martensite, and conforms to rule of mixtures. Finally, the low-temperature impact fracture mechanism of DP steels was observed, and it is found that the large-size ferrite grains in DP steels preferentially occur cleavage fracture. As the impact temperature decreases, the fracture mode of DP steel changes from dimple plus quasi-cleavage fracture to brittle cleavage fracture.

Two wear-resistant steel plates with different Ni content were studied to reveal the significant role of Ni on hardenability from the perspective of crystallography. From the surface to the quarter, and then to the center of the plates, the 0.44Ni steel exhibited higher hardness compared with the 0.02Ni steel, indicating that the 0.44Ni steel was of higher hardenability. The microstructure at the center changed from granular bainite to lath bainite with increased Ni content, together with an enhanced density of block boundaries. Ni improves the hardenability by weakening the variant selection through inducing larger transformation driving force and more self-accommodation of transformation strain to obtain a high density of block boundaries.

In this study, we have investigated the effects of heat treatment and loading direction on anisotropic compressive behaviour of additively manufactured (AM) Ti–6Al–4V ELI (extra low interstitials). AM samples were fabricated by powder bed fusion (PBF) processing, heat-treated between 800 °C and 950 °C, and compressed along two directions (i.e. parallel and perpendicular to the building direction) with a strain rate of 10−3 s−1 at room temperature. The heat treatment effectively leads to increasing ductility of as-built by decomposing the α′ phase to α and β phases. The heat-treated samples show a significantly lower yield strength than the as-built sample, which has a higher pre-existing dislocation density. As the annealing temperature increases, the yield strength decreased with a broadening of α lath thickness. Furthermore, perpendicular loaded samples have significantly higher strain hardening rate and ductility than parallel loaded samples. However, most of these anisotropic responses disappeared with indistinct morphologies. Anisotropic properties can be ascribed to the directional features of the microstructure resulting from the additive manufacturing process.

Plasma arc melted 17Cr-2Ni steel containing boron was studied under conditions of as-deposited and heat-treated state in this paper. The deposits showed high hardness and strength. However, it contained carbides network and martensite lath which are known as a deleterious phase to both mechanical and corrosion properties of steels. Various double austenitizing treatments were performed to improve the mechanical properties of deposits. At first, to reduce the degradation caused by carbides network, the deposits were subjected to different austenitizing temperatures of 1000, 1050 and 1140 °C during the homogenizing treatment. Homogenizing at 1140 °C was selected due to significant dissolution of carbides network and intragranular large size carbides (>2 μm). Second, to reduce the influence of martensite lath on the mechanical properties, the samples were subjected to isothermal quenching (IQ) and tempering at 420 °C for 60 min following the double austenitizing treatment, which were referred to as IQ1140 and T1140, respectively. Different from the microstructure of tempered martensite lath structure of the T1140, the microstructure of pearlite with carbide particles size less than 500 nm was found in IQ1140, which was verified to improve the tensile strength and ductility of steels from 1486 MPa and 0.5% to 1708 MPa and 3.8%, respectively. In addition, MX-type nanoprecipitate were found in the ferrite matrix of heat-treated steels. The deposits heat-treated by IQ1140 process was proved to have comprehensive high hardness, high strength and a certain ductility, with an improvement of 11%, 15% and 660% to the ones of deposits, respectively.

Axially loaded cyclic tests of a precipitation-hardened martensitic steel at different stress ratios in 3% NaCl solution and steam environment were conducted up to very high cycle fatigue regime. In-depth fracture surface observation, quantitative characterization of microstructural damage, and theoretical modeling were carried out to illustrate the physics and mechanics of micro-defect induced interior cracking. Results showed apparent environmental effect on fatigue strength and crack initiation morphology. The fine granular area was observed for the first time in environmental media, heterogeneously distributed around micro-defect, and was found dependent on local fracture mode with a less probability in the case of faceted failure. The formation of fine granular area was confirmed as a result of microstructural damage with significant contribution from cyclic stress amplitude, and could be assisted by hydrogen speeding up lath martensites breakdown. A chemo-mechanical model of interior cracking was finally established based on the concept of interaction of inclusion, matrix, and environment induced plasticity. All these underpin a general fatigue damage law in micro-defects assessment for long-life structural integrity.

Welding costs can be reduced by increasing the interpass temperature (IT). However, this can adversely impact the mechanical properties of the heat-affected zone (HAZ) of the welded joint; therefore, it is necessary to evaluate the influence of higher ITs on the HAZ. The influence of the IT on coarse-grained HAZ (CGHAZ) and intercritically reheated coarse-grained HAZ (ICCGHAZ) of an API 5 L X65 steel pipe joint welded with gas metal arc was investigated. A microstructure-predicting method (MPM) was built from numerical and thermodynamic simulations to examine the CGHAZ and ICCGHAZ. The results were validated using optical and scanning electron microscopy as well as dilatometry. Vickers microhardness and Charpy V-notch (CVN) impact tests were performed on physically simulated specimens. For all ITs considered herein, granular and lath bainite were observed in the CGHAZ; and bainite, ferrite, and Martensite–Austenite–Carbides (M-A-C) were observed in the ICCGHAZ. Increasing the IT did not significantly change the hardness of the CGHAZ and ICCGHAZ, but it decreased the CVN impact energy of these zones owing to presence of granular bainite and massive M–A. Contrary to that of the ICCGHAZ, the impact energy of the CGHAZ was greater than the DNVGL-ST-F101 standard specified limit (45 J). Based on the MPM and experimental results, it can be concluded that increasing the IT leads to embrittlement of the HAZ.

P92 steel (10Cr9MoW2VNbBN) is widely used in high-temperature steam piping and high-temperature header fabrications of ultra-supercritical units. In this paper, a follow-up study on the microstructure, mechanical properties and life of P92 steel steam pipes which are virgin, 31,000 h in service, 49,000 h in service and 71,000 h in service was carried out. The results show that the mechanical properties of the P92 steel drop significantly after 31,000 h of service, then decrease slowly with the increase of the service time. When the service time reaches 71,000 h, the high temperature yield strength, percentage elongation, and impact absorbed energy are all close to the lower limit regulated by the ASME standard. Moreover, the morphology of lath martensite becomes fragmentary, the width of the martensitic lath increases and carbide particles cluster and grow at the grain boundary. M23C6 phase coarsens and local Laves phase precipitates. Meanwhile, the dislocation density decreases to some degree. Furthermore, there is a good corresponding relationship between the room temperature hardness and the high temperature tensile strength of the P92 steel after a long period of service. The prediction model of the remaining endurance life of P92 components is proposed on the basis of the room temperature hardness.

The congruently melting, single phase intermetallic Ni5Ge3 has been subject to rapid solidification via drop-tube processing wherein powders, with diameters between> 850 and 150 µm, are produced. At low cooling rates (> 850 µm diameter particles, 850 µm fraction showing random grain orientation. However, grain orientation in plate & lath structure from 300 to 212 µm fraction displaying the distribution predominance of low angle grain boundaries.

Ultra-supercritical (USC) power plants with enhanced thermal efficiency have gained significant attention for reducing carbon dioxide (CO2) emissions. A new tempered martensitic heat-resistant steel, so-called the G115 steel, shows great potential as the candidate for next-generation USC power plant components. However, so far, the intrinsic relationship between microstructure and creep response of this steel still remains ambiguous. In this study, we examined the microstructure-creep behavior relationship of the G115 steel by utilizing a combined experimental and micromechanical modeling approach. More specifically, the modified Orowan's equation was applied to evaluate the creep strain since dislocation motion was a basis of creep deformation. Three kinds of dislocation configurations were incorporated to model the evolution of the substructures, and meanwhile, the kinetic equations were also employed here to describe the damage evolution. The creep curves of G115 steel simulated by this model are reasonably good for up to thousands of hours compared with our experimental data. Moreover, the model outputs, i.e., dislocation densities and width of tempered laths, also agree well with the microstructural parameters achieved from the interrupted creep tests at 923 K and 140 MPa. Regarding the microstructural evolution during the creep process, internal dislocation density (mobile + dipole dislocation densities) first decreases rapidly along the steep slope, and then the rate of decline becomes slower with increasing creep time. In contrast, boundary dislocation density first increases whereas subsequently decreases due to the dominance of tempered lath growth in the later stage of creep. Our results suggest that the micromechanical creep model can fairly elucidate the relationship of microstructural parameters and creep response for G115 steel, which greatly enhances the fundamental understanding of its microstructural features for governing the creep responses.

The fully martensitic microstructures of a manganese bearing precipitation hardenable (PH) stainless steel and a commercial 17-4 PH steel were compared to investigate the influence of chemical composition on the grain boundary network of the lath martensitic microstructure. The martensitic transformation in both steels led to the bimodal misorientation angle distribution having multiple maxima at misorientations in the range of 10–15° and 50–60°. This closely matched with the misorientations between variants associated with the Kurdjumov-Sachs orientation relationship (K-S OR). However, the Mn addition appeared to relatively reduce the area of boundaries with misorientations near 60°. This was a result of the reduction in the area of 60°/[011] intervariant boundaries in the Mn bearing steel. The phenomenological theory of martensite revealed that the Mn addition altered the 3-variant clustering (i.e., V 1 V 3 V 5 ) in 17-4PH steel to the 4-varaint clustering (i.e., V 1 V 2 V 3 V 5 ) arrangement to minimize the strain associated with the displacive martensitic transformation, promoting the population of 60°/[111] intervariant boundaries at the expense of 60°/[011] intervariant boundary. The changes in local variant selection affected the connectivity of the grain boundary network, suggesting that grain boundary network characteristics (i.e., population and connectivity) of the lath martensite microstructure can be manipulated by altering the chemical composition of the steel. The five-parameter grain boundary analysis for both martensitic steels, however, revealed similar grain boundary plane distributions for all boundaries associated with the K-S OR, being terminated on {011} planes due to the constraint that results from the displacive phase transformation.

This study investigates the effect of tool rotation speed on the microstructure evolution of the friction stir welded 1Cr11Ni2W2MoV martensitic stainless steel. The microstructure changes in the different weld zones were examined using optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD). The microstructure differences suggest that the stir zone (SZ) and thermo-mechanical affected zone (TMAZ) have been stirred in the austenite phase region, resulting in a smaller martensite lath, prior austenite grain size and higher hardness than those in the base metal (BM). Grain refinement in the SZ and TMAZ occurred at all welding conditions, with the finest microstructure was produced at lowest tool rotation rate. The heat-affected zone (HAZ) has been divided into three subzones. The HAZs close to the BM consist mainly of tempered martensite lath with lower average hardness values than that in the BM, while a newly hardened martensite lath has been observed in local regions in the HAZ close to the TMAZ, which has an average hardness equal to that in the BM. The microstructure changes in the HAZ have led to the suggestion that the HAZ close to the TMAZ reached a temperature between Ac1 and Ac3, while the peak temperature in the HAZs close to the BM did not exceed the Ac1.

In general, stress-induced martensite (SIM) is frequently employed to enhance the strain hardening behavior of Ti alloys through a so-called transformation-induced plasticity (TRIP) effect. However, the SIM does not necessarily lead to the improved mechanical performance of Ti alloys with the underlying mechanism unclear yet. The present work investigates the damage behavior of a typical model metastable Ti-1023 alloy with dominant SIM using uniaxial tensile test and nano-indentation measurement. The full beta microstructure with varied grain size (70 μm-350 μm) is obtained in the present Ti-1023 alloy through careful heat treatments. The integrated mechanical and microstructural characterizations indicate that the SIM displays a dual impact on the damage behavior of the present Ti-1023 alloy. In particular, the SIM could either facilitate the nucleation of cracks or inhibit the propagation of cracks depending on the martensitic lath spacing. The above dual-impact of SIM on the damage of Ti-1023 alloy can be rationalized based on the competing role of martensitic lath spacing on the dislocation pile-up and damage nucleation at lath boundaries. The dislocation-based plasticity could additionally assist the crack blunting. The effect of the beta stability on the damage behavior is found to be indirectly determined by the martensitic lath spacing. Since the martensitic lath spacing is mainly governed by the beta grain size, the present work suggests that the grain boundary engineering should be harnessed to fully explore the potential of SIM in developing strong and ductile/tough Ti alloys for broad industrial applications.

The aim of this study was to evaluate the tensile properties and microstructures of dissimilar welds between 11Cr-ferritic/martensitic steel and 316 stainless steel after thermal aging at temperatures between 400 and 600 °C up to 30,000 h. Characterization of microstructure was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural analysis showed that the microstructure in the weld metals (WMs) consisted of lath martensite containing a small amount of residual austenite. Thermal aging hardening of WMs occurred at 400 and 450 °C due to the effects of both α-α’ phase separation and G-phase precipitation. However, there was no significant change in the total elongation, and fracture surfaces indicated that very fine dimpled rupture was predominant rather than the cleavage rupture. It was suggested that lath martensite phases enhanced the tensile strength due to phase separation, while residual austenite played a role in keeping elongation as a soft phase.

In this paper, Ni-Cr-Fe, 17-4PH and 18Ni300 alloy coating were deposited on the rail surface by laser-induction hybrid cladding (LIHC), respectively. The microstructure, microhardness of different coatings, and their influences on the wear properties and contact fatigue behaviors of rails were studied systematically. Results indicate that the as-deposited Ni-Cr-Fe coating is constituted of the dendrite structure, and the as-deposited 17-4PH and 18Ni300 coating mainly consist of the lath martensite structure. The microstructure and microhardness of Ni-Cr-Fe coating are little influenced by the in-situ thermal cycling in LIHC process, whereas the grain size and precipitates’ content in 17-4PH and 18Ni300 coating are affected significantly, making the microhardness therein increased along the depth direction. Compared with the untreated rail, the wear rate and plastic deformation (PD) depth of rails with 17-4PH and 18Ni300 coating are both decreased significantly, while the contact surface damage is alleviated simultaneously. Especially for that with 18Ni300 coating, the decreased proportions are as high as 40.2% (mass wear rate), 60.1–81.2% (dimensional wear rate) and 78.7% (plastic deformation depth), respectively. By contrast, the wear rate of rail with Ni-Cr-Fe coating is increased conversely than that of the untreated rail although its microhardness is 32% higher. Therefore, the LIHC deposited 18Ni300 coating is a competitive choice for repairing the rail in engineering.

Hydrogen permeation tests were conducted in 1.2 MPa H2 on welding thermal simulated specimens to determine the hydrogen permeation parameters in each subzone of the girth welded joints. Optical microscope, electron back-scattered diffraction, and X-ray diffraction were applied to analyze the corresponding microstructure changes. The results show that the hydrogen diffusion coefficient D is mainly determined by the final heat-treated peak temperature, and the key microstructural affecting factors of hydrogen permeation will change with this temperature. The order of D from high to low is acicular ferrite, granular bainite, lath bainite, polygonal ferrite, and M-A constituent.

Grain-scale dwell fatigue behavior in Ti-Fe-O alloy is elucidated, with attention focused on the stress redistribution phenomenon that occurs within the soft-hard grain combination. Elastoplastic properties of the β phase are determined using nanoindentation and crystal plasticity modeling. On this basis, the role of intergranular β lath on stress redistribution is evaluated. Redistribution of stress from the soft to hard α grain is inhibited in the presence of intergranular β lath during dwell period. Meanwhile, the peak stress in the β lath is enhanced. This may result in the competition of two dwell fatigue failure modes at high stress state.

Directed energy deposition (DED) can be used to repair damaged components. However, interfacial defects can occur between the repaired zone and substrate. Avoiding defects in repair processes using DED is critical because these defects significantly degrade the mechanical properties of the repaired parts. In this study, we investigated the occurrence of defects based on the laser power and powder feed rate. When experimenting with different powder feed rates (PFR), no defects such as lack of fusion and interfacial crack occurred under PFR 3.5 g/min conditions. However, repair deposition with a higher laser power resulted in cracks at the interface between the repaired region and the substrate. Microstructure observations showed that a mixture of lath martensite and retained austenite was formed in the repaired region. In the substrate, lath martensite was dominant due to solution annealing. Electron backscatter diffraction and X-ray diffraction analysis results confirmed that a large amount of the face-centered cubic (FCC) phase was formed in the deposited zone, and the heat-affected zone (HAZ) and substrate were mostly composed of the body-centered cubic (BCC) phase. Owing to these microstructural characteristics, the hardness of the deposited zone was lower than that of the substrate. Meanwhile, in the HAZ, the hardness increased significantly, which was attributed to the aging effect caused by continuous heat input during the DED process. The results of a tensile test indicated that the strength of a specimen with interfacial defects on the slope was 80.8% of that of the wrought material. However, compared with the cracked specimen, the repaired specimen exhibited improved tensile properties, even when there was a defect at the interface. Meanwhile, the strength of the specimen without interfacial defects was 97% of that of the wrought material. These results can provide guidelines for selecting the appropriate laser power and powder feed rate for defect-free repair using DED.