The toughness of martensitic steels is strongly related to their fine and complex morphologies. To control the toughness of martensitic steels, the effect of carbon content on the morphology of lath martensite must be studied. However, these morphological changes are difficult to clarify using conventional two-dimensional observations because their morphologies are complex and tangled. Previously, we analysed and reported the three-dimensional microstructures of ultra-low-carbon lath martensite. Here we look further at three-dimensional microstructures of low-carbon lath martensite. Lath martensite in both specimens contains a few coarse packets in a prior austenite grain. The coarse packets in low-carbon lath martensite contain plate-like blocks stacked from end to end of the coarse packets and many fine blocks embedded in the coarse packets. The fine and included blocks are much smaller than the plate-like blocks in size. A part of the fine blocks belongs to the crystallographic packets different from the surrounding coarse packet, which should be regarded as ‘fine’ packets. The fine packets are also observed in ultra-low-carbon martensite. On the other hand, low-carbon martensite contains fine blocks that belong to the same crystallographic packet as the surrounding coarse blocks. These results suggest that the block structure in the coarse packet of low-carbon martensite is more entangled with each other than that of ultra-low-carbon martensite.

This paper aims to present new observations and relate these to other recent findings concerning the influence of local micro-stresses (Type II residual stresses) on the behaviour of martensitic steels. A major source of these stresses is the shearing that accompanies phase transformation in individual austenite grains and which is blocked by constraint from the surrounding matrix. Residual stresses are the principal reason for early plasticity and gradual yielding during tensile testing of martensite.

Diffraction techniques are commonly used to measure residual stresses, where peak profiles are influenced by the micro-strains and also by dislocations. Most publications have considered only dislocations and ignored the role of micro-strains. We demonstrate experimentally that this assumption is untenable and must lead to incorrect values of dislocation parameters. The unusual behaviour whereby diffraction peaks from martensite become narrower during plastic deformation is explained by the progressive relaxation of the micro-strains.

We hypothesise that freshly formed martensite is always tetragonal but that it decomposes spontaneously to a cubic structure by auto-tempering in most low carbon lath martensites where the Ms temperature is sufficiently high. This transformation is examined in detail in higher carbon steels which reveals another surprising effect, namely that diffraction peaks can become broader during annealing, resulting from a newly recognised source of internal micro-stresses. These arise when the contraction of crystals along the c-axis during tempering is inhibited by restraint from their surroundings, so preventing equilibrium atomic spacings from being achieved.

Two mechanisms inconsistent each other, a relaxation of type II internal stress and a presence of mobile dislocation, were previously proposed to describe the low elastic limit of as-quenched lath martensite steels. In this study, neutron diffraction experiments were performed to revisit the deformation behavior of lath martensite steel. Dislocations with very dense in the order of 10¹⁵ m⁻² were observed in the as-quenched martensite single phase condition of a Fe–18Ni alloy. The diffraction profiles had good symmetry, showing that the inhomogeneous type II internal stresses which might be introduced in individual blocks or packets during martensitic transformation were well balanced in a bulky specimen size, and the influence to the yield stress would be very small. In a 0.22C steel, dislocations with very dense in the order of 10¹⁵ m⁻² and random arrangement were also observed in the as-tempered condition. The diffraction profiles had also good symmetry. The symmetry collapsed by tensile deformation, displaying the occurrence of load sharing between the packets with the active slip systems of in- and out-of-lath-plane. The highly dense random arrangement dislocations easily moved at the beginning of deformation, then accumulated, annihilated and changed the arrangement differently depending on the orientation of the packet with respect to the deformation direction. The movement of highly dense random arrangement dislocations played an important role as a mechanism at the beginning of deformation, and can be a true feature of mobile dislocations.

Stress relaxation tests were conducted in the elastic region of an ultralow carbon martensitic steel (Fe–18%Ni alloy) to quantitatively analyze the effect of mobile dislocations on the low elastic limit of the steel. The elastic limit of the as-quenched material was measured at 255 MPa, although its tensile strength was as high as 720 MPa. The stress relaxation tests, which were performed at 255 MPa, revealed a remarkable stress reduction due to the movement of the mobile dislocations present in the as-quenched material. The total dislocation density barely changed during the test, while the distribution parameter (M-value) decreased significantly, indicating that the mobile dislocations exhibited stable arrangements. The 5% cold rolling before the relaxation tests suppressed the relaxation and simultaneously increased the elastic limit to a maximum, 435 MPa. By estimating the mobile dislocation density by relating the stress reduction in the stress relaxation tests to the distance of the dislocation movement evaluated via transmission electron microscopy (TEM) observations, it was estimated that the mobile dislocation density of the 5%-cold-rolled material was lowered to ~1/10 of that of the as-quenched material.

Quenched martensitic steels are known to show the characteristic feature of stress–strain relations, with extremely low elastic limits and very large work-hardening. The continuum composite approach is one way to express this characteristic feature of stress–strain curves. Although the overall stress–strain curves, as a function of alloy chemistries of steels, were well represented by this approach, the relationship between the macroscopic deformation behaviors and microstructural information is yet to be clarified. A high-spatial-resolution digital image correlation analysis was conducted to demonstrate the possible unit size corresponding to the microstructure. The continuum composite approach model was also modified to consider the size effect of the microstructure on the stress–strain curves of the as-quenched martensitic steels. Strain concentrations were observed at various boundaries, including lath boundaries, and the characteristic microstructural size was also predicted by the present model, which is smaller than the reported spacing between adjacent strain-concentrated regions.

The precipitation of nano-sized alloy carbides in steels with a large amount of strengthening can be obtained by conventional tempering of martensite or interphase precipitation occurring during isothermal ferrite transformation. In this study, a vanadium-microalloyed low carbon steel with a composition of Fe-0.1C-0.4V-1.5Mn-0.05Si (mass%) was either isothermally transformed or quenched and tempered at 923 K for various periods, to comparatively investigate the precipitation behaviors of vanadium carbide and the resultant strengthening effects in ferrite and tempered martensite. When compared under the same conditions, tempered martensite is characterized by a higher yield strength and a smaller uniform elongation than that of ferrite. The quantification of microstructural features via multiple characterization techniques demonstrates a finer crystallographic grain size, a higher dislocation density of tempered martensite compared with that of ferrite, together with a relatively coarser dispersion of nano-sized precipitates due to its lower nucleation rate and higher growth rate than interphase precipitation. The strengthening amounts of all these factors are estimated using the theoretical models, the summation of which can well reproduce the yield strength of both ferrite and tempered martensite in the microalloyed low carbon steel.

The deformation and fracture behavior in spheroidized pearlitic steels were investigated using digital image correlation and the replica methods, and the origin of the inhomogeneous strain distribution and its effect on fracture were discussed. The cementite roundness increased while the area fraction within a colony decreased with increasing spheroidization time. Many coarse cementites were observed on the colony and block boundaries, which explains the decrease in the cementite area fraction within a colony. The strength–ductility balance deteriorated with cementite spheroidization. The inhomogeneous strain distribution in a unit of the colony was introduced by the tensile deformation in the spheroidized pearlitic steels. The numerous voids or cracks detected at low- and high-angle boundaries inside the cementite and in ferrite at the ferrite/cementite boundary tended to nucleate from the high-strain region. The deformability of the colony depended on the progress of cementite spheroidization, and the crystallographic orientation relationship between ferrite and cementite could affect the ease of cementite spheroidization in a colony. The strain gradient between the soft ferrite and hard cementite phases induced void or crack nucleation around the coarse cementite at the colony or block boundary; hence, voids or cracks tended to nucleate from the high-strain region. It can be concluded that inhomogeneous cementite spheroidization results in an inhomogeneous strain distribution, which causes the preferential nucleation of voids or cracks in the high-strain colony.

The local plasticity and associated microstructure evolution in Fe-5Mn-0.1C medium-Mn steel (wt.%) were investigated in this study. Specifically, the micro-deformation mechanism during Lüders banding was characterized based on multi-scale electron backscatter diffraction measurements and electron channeling contrast imaging. Similar to other medium-Mn steels, the Fe-5Mn-0.1C steel showed discontinuous macroscopic deformation, preferential plastic deformation in austenite, and deformation-induced martensitic transformation during Lüders deformation. Hexagonal close-packed martensite was also observed as an intermediate phase. Furthermore, an in-situ neutron diffraction experiment revealed that the pre-existing body-centered cubic phase, which was mainly ferrite, was a minor deformation path, although ferrite was the major constituent phase.

In-situ deformation experiments with cold-rolled and intercritically annealed Fe-5Mn-0.1C steel were carried out at ambient temperature to characterize the deformation heterogeneity during Lüders band propagation. Deformation band formation, which is a precursor phenomenon of Lüders band propagation, occurred even in the macroscopically elastic deformation stage. The deformation bands in the Lüders front grew from both the side edges to the center of the specimen. After macroscopic yielding, the thin deformation bands grew via band branching, thickening, multiple band initiation, and their coalescence, the behavior of which was heterogeneous. Thick deformation bands formed irregularly in front of the region where the thin deformation bands were densified. The thin deformation bands were not further densified when the spacing of the bands was below ~10 µm. Instead, the regions between the deformation bands showed a homogeneous plasticity evolution. The growth of the thin deformation bands was discontinuous, which may be due to the presence of ferrite groups in the propagation path of the deformation bands. Based on these observations, a model for discontinuous Lüders band propagation has been proposed.

The present study investigated the high-speed deformation behavior of 6Ni–0.2N–0.1C steel. 0.2% proof stress (0.2% PS) of the 6Ni–0.2N–0.1C steel increased with an increase in strain rate () but tensile strength (TS) indicated almost the same value at above 10⁻¹ s⁻¹. Uniform elongation (U.El) largely decreased with an increase in . TS and U.El of the 6Ni–0.2N–0.1C steel at 10³ s⁻¹ were almost the same as those of SUS304 steel. When the effect of on mechanical properties was compared between the 6Ni–0.2N–0.1C and SUS304 steels, the strain rate dependence on 0.2% PS was larger in the 6Ni–0.2N–0.1C steel and that on TS was different at above 10⁰ s⁻¹. And the decrease of U.El with an increase in was larger in the 6Ni–0.2N–0.1C steel. The decrease of U.El at 10³ s⁻¹ was discussed from the viewpoint of change of flow stress at the maximum load point with an increase in . The estimated results proposed that austenite phase hardly transformed into deformation-induced martensite (α′) up to the maximum load point at 10³ s⁻¹ in the 6Ni–0.2N–0.1C steel. The tensile properties of the 6Ni–0.2N–0.1C steel are largely influenced by the higher strength of α′. It is difficult for the 6Ni–0.2N–0.1C steel to produce TRIP effect at high strain rates because the deformation-induced martensitic transformation is suppressed.

The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width-gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could be quantitatively converted into strain values through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the “strain gradient” as well as strain, which would depend on the adhesion and surface condition of the specimen.

In this study, the strain distribution in grains with a preferred orientation in a cold-rolled steel plate was investigated. This was done by measuring the amount of strain in a region of the material using nanoscale fine markers applied by a focused ion beam (FIB). We obtained the crystal orientations and strain distributions in the same region using scanning electron microscopy-electron backscatter diffraction and the markers made by the FIB during rolling to a 60% to 70% thickness reduction. The method revealed the strain distributions in the grains with the major preferred orientations (cube:{100}<001>; α-fiber:{100}<011>, {211}<011>; γ-fiber:{111}<011>, {111}<211>). The average strains that accumulated in the grains with different major preferred orientations during cold-rolling were almost the same at thickness reduction in the range of 60%–70%. However, the strain distribution width of the γ-fiber grains was approximately twice that of grains with other orientations. These results suggest that the deformation inhomogeneity during rolling is more pronounced for γ-fiber-oriented grains than for grains with other preferred orientations.

Effect of crack-tip shielding by dislocations is the most fundamental mechanism governing the fracture toughness of crystalline materials. Brittle-to-ductile transition (BDT) caused by increasing temperature is a general phenomenon observed not only in metals and alloys but also in various crystalline materials such as ionic crystals or semiconductors. The increase of fracture toughness in BDT is closely related to the shielding effect due to dislocations multiplied around a crack-tip. The present paper reviews the fundamental theory of crack-tip shielding and its experimental evidence, and also shows the reason why the nature of interatomic bonding has a remarkable influence on macroscopic fracture toughness, based on the shielding theory.

Hydrogen embrittlement has attracted much attention in the fields of materials science and mechanical engineering although there still remain many arguments on its mechanism. In this paper, the phenomena being characteristic to hydrogen embrittlement are reviewed, and its mechanism is also discussed from the viewpoint of dislocation shielding.

A typical hydrogen-related transgranular fracture, namely quasi-cleavage fracture, is usually accompanied by serrated markings on the resultant fracture surfaces in steels with body-centered cubic phases. The present paper investigated the microscopic three-dimensional morphology and crystallographic feature of serrated markings in a 2Mn-0.1C steel mainly composed of ferrite microstructure. The serrated markings corresponded to the corners of the step-like morphologies which consisted of microscopic {011} facets whose longitudinal directions were almost parallel to <110> or <112> direction. In addition, the microscopic {011} quasi-cleavage facets had the largest inclination angle from tensile axis among six crystallographically equivalent {011} planes, suggesting that resolved normal stress imposed on the {011} plane is an important factor for the hydrogen-related quasi-cleavage fracture. We propose that not only the slip deformation enhanced by hydrogen but also the coalescence of vacancies / voids induced by hydrogen-enhanced plastic deformation should be considered for understanding the mechanism of the hydrogen-related quasi-cleavage fracture along the {011} planes.

The present study investigated the hydrogen-related fatigue fracture under various test frequencies in low-carbon martensitic steel. In the hydrogen-charged specimen, although the number of cycles to failure decreased with decreasing test frequency, the time to failure was almost the same regardless of the test frequency. Observation of fracture surface revealed that the transgranular surface was a main component in the uncharged specimen, while the intergranular surface was often observed especially at the lower test frequency in the hydrogen-charged specimen. In addition, for the transgranular fracture, cracks often propagated across the laths regardless of test conditions. The high-strained region was observed over a relatively wide area in the uncharged specimen. On the other hand, the hydrogen-related fatigue-crack propagation was accompanied by intense localized plastic deformation, which could accelerate crack growth. The intergranular cracking and high localization of plastic deformation could be the possible reasons for decreasing the fatigue life by the presence of hydrogen.

Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H₂S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04–0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.

It is well known that the presence of hydrogen causes deterioration of the mechanical properties of steel, which appears in the forms of reduced fracture toughness, shorter fatigue life, etc., and these phenomena are recognized as hydrogen embrittlement. Here, the effect of hydrogen on crack initiation in fracture toughness tests was investigated using a combination of experimental and computational approaches. Tempered lath martensitic steel was subjected to fracture toughness tests with a monotonically rising load in air and high-pressure hydrogen gas environments. While cracking propagated continuously within grains in the air environment, cracking in the hydrogen environment grew by linking of isolated interfacial failures ahead of the main crack tip. To understand the nucleation mechanism of isolated failure in the presence of hydrogen, tensile simulations of twist grain boundaries (TGBs) rotated around the <110> axis at various misorientation angles were conducted using molecular dynamics (MD) simulations. While dislocation emission from TGB rotated 70° is the dominant deformation mode in the absence of hydrogen, rupture along TGB rotated 110° and 170° without stress relaxation due to dislocation emission is the dominant deformation mode in the presence of hydrogen. As a consequence, it is indicated that the origin of hydrogen-induced isolated crack initiation in the vicinity of a fatigue precrack is rupture along the block boundaries within the martensitic structure due to hydrogen-induced inhibition of dislocation emission from grain boundaries (GBs).

In this study, the cleavage fracture of the C14 Fe₂W Laves phase was investigated by first-principles calculations and crystal orientation analysis using scanning electron microscopy. Trace analysis of the orientations of cleavage planes revealed that cleavage fracture occurred in five types of crystal planes: (0001), {1100}, {1120}, {1101}, and {1122}. Among these fractures, the fracture at (0001) is the most preferable. From, the first-principle calculations of the surface energy for fracture, Young’s modulus, and Poisson’s ratio, the minimum fracture toughness value of 1.62 MPa·m^1/2 was obtained at (0001). The tendency of the calculated fracture toughness to become larger with high indexed planes is almost the same as the frequency of the types of cleavage planes in the trace analysis. It was concluded that the fracture toughness of the C14 Fe₂W Laves phase is controlled by the surface energy for fracture and Young’s modulus.

A multiscale model based on micromechanics of short crack growth has been proposed and proved to be able to predict the fatigue strength of steels via experiments with thin specimens in our previous work. The present study aims to validate the applicability of our model in predicting fatigue strength in long crack problems. A camera with high resolution was set up and successfully captured the crack growth process during the fatigue test. Very good agreements for both fatigue life and crack growth process were demonstrated by comparing the experimental results and the predictions from the proposed model, indicating that our model can be served as a generalised foundation to accurately predict the fatigue behaviours of ferrite-pearlite steel for both the short and long crack growth problems.

Effects of vanadium (V) addition on strain distribution, crack initiation and propagation behavior during low-cycle fatigue test in ferrite and martensite dual phase (DP) steel were revealed. V addition effectively extended the low-cycle fatigue life, even though the tensile properties were approximately identical between V-added DP and DP steels. The stress amplitude suddenly decreased just before fracture in the V-added DP steel. The fatigue crack initiated from the surface and propagated inside, and the brittle fracture occurred in the crack propagation region in the V-added DP steel. The strain distribution introduced during the low-cycle fatigue test was more inhomogeneous for the V-added DP steel than that for the DP steel, and the fatigue crack was generated from the high-strain region. Considering that the number of cycles for crack initiation in the V-added DP steel was larger than that in DP steel, the inhomogeneous strain distribution in the V-added steel promoted crack nucleation but suppressed crack growth in the crack initiation stage. Cracks propagated independent of the strain distribution in the V-added DP steel in the crack propagation stage. The crack propagation rate in the V-added DP steel was remarkably higher than that in DP steel in the crack propagation stage. Thus, V addition effectively extended the number of cycles for crack initiation but caused brittle fracture and faster crack propagation.

The influence of Mn addition on fatigue properties of ferritic steel containing solute carbon was examined using rotating bending fatigue tests on water-quenched Fe–0.016C–1.9Mn ferrite–pearlite steel containing 0.0035 mass% solute carbon in comparison with water-quenched Fe–C ferritic steels containing 0.0063–0.017 mass% solute carbon. The fatigue tests were carried out at ambient temperature around 300 K and a frequency of 50 Hz with a stress ratio of −1. The Fe–C–Mn steel exhibited a comparable hardness and fatigue limit to the water-quenched Fe–0.017C steel which contains about three times the amount of solute carbon than the Fe–C–Mn steel. In addition, the Fe–C–Mn steel exhibited a significant coaxing effect in comparison to the Fe–C steels, when the test was started from a stress amplitude just below the fatigue limit. Crack initiation sites were changed by stress amplitude unlike in the Fe–C steels. Specifically, intergranular cracks were observed at the high stress amplitudes and transgranular cracks were observed at the low stress amplitudes near the fatigue limit. It was concluded that the Mn addition suppresses intergranular cracking at the low stress amplitudes.

This study developed a deformation process to form a uniform nano-crystalline layer with relatively high thermal stability that can retain even after an induction heating and quenching process on a surface of cylinder-shaped sample. The effect of the surface nano-crystalline layer on rolling contact fatigue life of carbon steels (JIS S45C and S55C) was investigated. The sample with the surface nano-crystalline layer showed lower friction coefficient under cylindrical rolling contact condition comparing to that without the layer. The rolling contact fatigue life was extended to 4 times higher cycles by forming the nano-crystalline layer. It is presumable that the improvement of a rolling contact fatigue is owing to not only the high hardness but also the reduction of friction coefficient during the test followed by a suppression of dynamic tempering softening and variation of stress distribution.