Martensite Grains Research Articles

Toughening the grain boundaries by introducing a small amount of the second phase: Ni-Cu-Mn-Ga shape memory alloys as an example

The grain boundary brittleness widely exists in structural and functional materials that possess covalent bonds. Particularly, toughening the grain boundaries and simultaneously retaining the functionalities are of critical importance for functional materials. In this work, we focus on the Heulser-type Ni-Mn-X (X=Ga, In, Sn, Sb, Al, Ti) shape memory alloys, the severe grain boundary brittleness of which have hindered their developments over the past more than two decades. By performing two-step heat treatment for a Cu alloyed Ni30Cu20Mn42.4Ga7.6 alloy, a unique dual-phase configuration with a small amount of thin layer (width less than 20 µm) of single crystalline γ phase just distributing along boundaries of the martensitic grains was formed at room temperature. As a result, the bulk polycrystalline samples could be compressed with the engineering strain up to 81 % without any cracks. More importantly, the tension deformation, which was previously rather difficult for bulk single-phase Ni-Mn-X alloys, was realized in the dual-phase Ni30Cu20Mn42.4Ga7.6 alloy with a tensile strain up to 13 %, exhibiting high plasticity. Simultaneously, remarkable shape memory effect of 3.2 % under the tension condition was achieved via the martensitic transformation. Microstructure and fracture surface analysis revealed the “glue effect” of the γ phase. The high plasticity of the Ni30Cu20Mn42.4Ga7.6 alloy originated from the cooperative deformation of the martensite phase and the γ phase. This work supplies an attractive strategy of grain boundary engineering to toughen brittle functional materials.

Effect of PWHT on metallurgical and mechanical characterization of dissimilar welded joint of P91 and P92 steels

P91 and P92 steels are widely used in nuclear and thermal power plant’s high-temperature (650–700 °C) operating components. The welding procedure, base plate composition, filler material composition, and post-weld heat treatment (PWHT) significantly affect the metallurgical characterization of weldments at room temperature. This work focuses on joining P91 and P92 steel by the TIG welding with filler ER90S-B9. The study analyzes the materialization of coarse-grained HAZ (CGHAZ), inter-critical HAZ (ICHAZ), and fine grained HAZ (FGHAZ) under welded and PWHT. PWHT was conducted at 840 °C for 2, 4 and 6h. An optical microscopic (OM) scanning electron microscope (SEM) was employed to characterize the welded and PWHT samples. It was perceived that the tensile strength of PWHT increased as the heat treatment times increased from 2 to 6 h, while the hardness value decreased accordingly. The maximum tensile strength (594.71 MPa) was observed in PWHT_6 h, while the minimum tensile strength (537.52 MPa) was perceived in the as-welded sample. The PWHT_6h reveals the minimum hardness values (271.12 HV) at the fusion zone among all the conditions. The WFZ consisted of an untampered lath martensitic grain structure, demonstrating an average hardness value of 314.15 HV in the PWHT_2h condition. The hardness value in CGHAZ samples for as-welded, PWHT_2h, PWHT_4h, and PWHT_6h was observed at 456.85, 436.81, 368.63, and 327.41 HV, respectively.

The synergic effects of heat treatment and building direction on the microstructure and anisotropic mechanical properties of laser powder bed fusion Corrax maraging stainless steel

An anisotropic microstructure and mechanical properties are big challenges in the development of various laser powder bed fusion (LPBF) alloys. This study investigated the roles of heat treatment and building direction (BD) on the microstructure and anisotropic mechanical properties of LPBF Corrax maraging stainless steel. The effects of solution treatment (ST) and integrated solution-aging treatment (SAT) were clarified. The results show that the grain size of martensite, amount of austenite, and features of grain boundaries were slightly varied with the building direction due to the thermal history. In the as-built state, the weak <111>α′||BD, <1 1‾ 0>α′||X, and <001>α′||BD textures could be found. After the SAT process, the <1 1‾ 0>α′||X texture was slightly intensified due to the coarsening of large columnar grains. However, the texture of the SAT sample was still weak.Furthermore, the building direction and heat treatment did not lead to obvious anisotropic tensile properties or change the ductile fracture mode. The weak texture and pores in LPBF Corrax did not dominate the tensile properties. Irrespective of sample state, the horizontally-built samples exhibited comparable strengths and slightly higher elongation than the vertically-built ones did. In the as-built condition, this phenomenon can be mainly attributed to the transformation-induced plasticity effect. In the ST and SAT conditions, smaller grain sizes of martensite and higher high-angle grain boundary ratios in the horizontally-built samples provided more resistance to crack propagation. LPBF Corrax maraging stainless steel exhibited superior tensile performances and low anisotropic tensile properties, which are very beneficial to the stability of the material during service.

Study on the Influence of Strengthening Grinding Process on the Surface Integrity of 30CrMnSiA

30CrMnSiA, as a high-strength steel with excellent toughness and superior hardenability, was extensively used in the aerospace industry. At the same time, its lower surface hardness limits its application in impact and vibration environments. The surface of 30CrMnSiA was enhanced using the strengthening grinding process (SGP) technique in this work. The influence of SGP jet pressure on the surface integrity of 30CrMnSiA steel was studied, such as morphology, microstructure, roughness, and hardness. The study indicated that 30CrMnSiA samples treated by SGP reduced surface flatness, primarily due to the formation of plenty of irregular micro-pits by SGP. The greater the jet pressure leads to the more acute surface height fluctuation and the larger surface roughness. Furthermore, SGP notably enhanced the surface hardness and induced the formation of a strengthened layer. This improvement can be attributed to promoting martensite content and grain refinement within the surface layer structure facilitated by the SGP. The harder the surface hardness with the finer grain size, the thicker the strengthening layer with higher jet pressure.

Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity.

Bridging microstructure and crystallography with the crack propagation behavior in Ti-6Al-4V alloy fabricated via dual laser beam bilateral synchronous welding

The microstructure evolution and crack propagation behavior were systematically and thoroughly investigated in α/β Ti-6Al-4V alloy obtained under the inhomogeneous heating effect of a coupled dual laser beam via a combination of postmortem electron backscattering diffraction analyses and numerical simulations. The effect of microstructural attributes (grain size, grain boundary, dislocation) on the fracture property and crack propagation behavior was investigated based on thermodynamic and crystallographic analyses, as well as the examinations of dislocation density and the Burgers vector. By quantifying the degree of variant selection using the degree of variant selection (DVS) equation, it was found that the higher the energy input, the weaker the variant selection, as the secondary α phase suppresses further dislocation-induced variant selection via autocatalytic nucleation and growth. The proportion of high angle grain boundaries (HAGBs) concentrated around 60° gradually increases with improving cooling rate, mainly due to the change in the tendency of martensite transformation to be induced. The grain refinement was the primary mechanism for the enhancement of Ti-6Al-4V alloy T-joints, while the suppression of crack propagation by HAGBs also played a crucial role in improving strength and ductility. The crack propagation direction frequently changes when the order is approximately parallel to the short axis of α′ martensite grains, forming a zigzag propagation path. The above findings should shed light on optimizing dual laser beam bilateral synchronous welding (DLBSW) technology to tailor the microstructure and improve the mechanical properties of Ti-6Al-4V alloy T-joints.

Characterization of microstructural evolution with pre-strain in BG801 bearing steel: Grain, carbides, retained austenite and martensite

The method of multiple cryogenic and tempering treatments significantly limits the microstructure regulation of BG801 bearing steel. In this work, the pre-strain was introduced in the vacuum-melted BG801 steel combing with heat treatment (HT) to adjust the microstructures. The samples were subjected to uniaxial tensile at room temperature with different strains (engineering strain of 0, 0.15, 0.175, 0.2), then heating to 1323 K and water-cooling. The results showed that higher pre-strain was beneficial to obtain finer prior austenite grain (PAG) and martensite grains, and a low fraction of retained austenite (RA) in BG801 steel. With increasing pre-strain, the PAG was elongated along the RD direction (rolling direction) without a change in size. The recrystallized grains with a size of about 5–6 μm nucleated inside the PAG during HT. Meanwhile, the high-density dislocations were introduced in the tensile samples by pre-strain, promoting the growth of micron-M6C by accelerating the diffusion of alloy elements during HT. And the evolution of RA and martensite indicated that the pre-strain accelerates transformation from austenite to martensite by transferring elements from the matrix to M6C and introducing shear bands into the matrix to promote the nucleation of martensite.

An extraordinary improvement in cryogenic toughness of K-TIG welded 9Ni steel joint assisted by alternating axial magnetic field

A keyhole-TIG welding method assisted with alternating axial magnetic field (MF) was proposed to obtain qualified cryogenic toughness joints. The results show that the MF can refine prior austenite grains and establish distinct Ni-rich grain boundaries. Furthermore, the electron back scatter diffraction data demonstrate that the martensite grains have also been refined, and there is a higher density high angle grain boundary (HAGB) in the MF-assisted joint than in the ordinary joint. Under the cryogenic (−196 °C) Charpy V-notch impact test, the MF-assisted joint express a 37 J absorbed energy, which is 300% higher than the 11.7 J of the ordinary joint. This extraordinary increasing in toughness results from the refinement of grains and high density distribution of HAGB, which act as obstacles to crack propagation in cleavage fracture and form small facets on the fracture surface. The MF-assisted welding method has been proven to obtain qualified 9Ni steel joints.

Through-Thickness Inhomogeneity of Microstructures and Mechanical Properties in an As-Quenched Thin Specification High Strength NM450TP Steel Plate

The inhomogeneity of microstructures and mechanical properties in an as-quenched thin specification NM450TP wear-resistant steel plate were quantitatively investigated. The results show that the microstructures exhibit inhomogeneous distribution through the thickness and the area percentage of martensite and ferrite grains varies regularly through the thickness, and the content of ferrite on the top surface of the plate is found to be two times that of ferrite at the core location and more than that of ferrite at the bottom surface. In addition, the steel plate exhibits the obvious anisotropy of tensile properties, the tensile strength paralleling to the rolling direction is lower than that along the transverse direction while the elongation paralleling to the rolling direction is better than that along the transverse direction. The result indicates that the deformation degree of prior austenite grains during hot rolling and the content of martensite after quenching dominate the mechanical properties while the ferrite content is not the main factor affecting the plasticity. The findings provide experimental evidences and lay a theoretical foundation for analyzing the subsequent processing.

Evaluation of hole expansion ratio of ultra-high strength martensitic steels produced with various processing routes

Hole expansion ratio of hot rolled ultra-high strength martensitic steels with tensile strength around 1000 MPa was evaluated. Steels were produced with direct quenching and traditional reheat and quenching processes with final thickness of 3 mm. Achieved yield strength values (Rp0.2) varied between 918 – 1068 MPa depending on the processing route. Mean hole expansion ratios (HER) in direct-quenched (DQ) and direct-quenched and tempered (DQT) conditions were between 22 – 36 %, and no clear improvement was seen after tempering treatment compared to quenched variant. HER values for reheat and quenched (RQ) and reheat, quenched and tempered (RQT) variants were between 31 – 49 %, and the highest values were achieved with RQT steels, which were tempered at 600 °C. Based on the field emission scanning electron microscope with electron backscatter scanning diffraction (FESEM-EBSD) analysis/characterization of martensite grain size, more uniform grain structure was discovered in RQ steels, which could be the reason for improved HER properties. HER values were also compared to tensile test results (uniform elongation, true thickness strain), and formability maps were constructed. However, only minor correlation was found between HER and true thickness strain values.

Comparative Analysis of Minimum Chip Thickness, Surface Quality and Burr Formation in Micro-Milling of Wrought and Selective Laser Melted Ti64.

Selective laser melting (SLM) is a three-dimensional (3D) printing process that can manufacture functional parts with complex geometries as an alternative to using traditional processes, such as machining wrought metal. If precision and a high surface finish are required, particularly for creating miniature channels or geometries smaller than 1 mm, the fabricated parts can be further machined. Therefore, micro milling plays a significant role in the production of such miniscule geometries. This experimental study compares the micro machinability of Ti-6Al-4V (Ti64) parts produced via SLM compared with wrought Ti64. The aim is to investigate the effect of micro milling parameters on the resulting cutting forces (Fx, Fy, and Fz), surface roughness (Ra and Rz), and burr width. In the study, a wide range of feed rates was considered to determine the minimum chip thickness. Additionally, the effects of the depth of cut and spindle speed were observed by taking into account four different parameters. The manufacturing method for the Ti64 alloy does not affect the minimum chip thickness (MCT) and the MCT for both the SLM and wrought is 1 μm/tooth. SLM parts exhibit acicular α martensitic grains, which result in higher hardness and tensile strength. This phenomenon prolongs the transition zone of micro-milling for the formation of minimum chip thickness. Additionally, the average cutting force values for SLM and wrought Ti64 fluctuated between 0.072 N and 1.96 N, depending on the micro milling parameters used. Finally, it is worth noting that micro-milled SLM workpieces exhibit lower areal surface roughness than wrought ones.

Synergistic enhancement of strength, ductility, and toughness in a low carbon micro-alloy steel with an ultrafine-grained heterogeneous lamellar structure

Structural materials with heterostructure design have emerged as an efficient strategy to address the dilemma of balancing strength and ductility/toughness, resulting in increased interest from the scientific community. In this study, a novel ultrafine-grained heterogeneous lamellar (HL) structure has been architected in a common low-carbon micro-alloyed steel. The heterostructured steel exhibits a lamellar microstructure comprising alternating ultrafine/nano-grained lamellae and micrometer-grained lamellae, with a texture gradient along the thickness. Results show that the HL structure has a tensile strength comparable to quenched and tempered martensite (QTM) steel while retaining similar ductility as coarse-grained dual-phase (CG) steel. Furthermore, the HL specimen exhibits enhanced fracture toughness (KQ = 232.8 MPa m1/2), which is 11.3% and 28.8% higher than that in the CG and QTM specimens, respectively. Extrinsically, the macroscopical heterogeneous lamellar structure and microscopical elongated grain shape in ultrafine/nano-grained lamellae could provide substantial lamellar grain boundaries to hinder the connection of voids and make the crack propagation path more tortuous, which effectively increases the energy release rate to contribute to the exceptional fracture toughness. Intrinsically, not only micrometer-sized CG lamellae with high plastic deformation tolerance capacity can develop coarse (deep) dimples for energy consumption, but also the elongated ultrafine martensite grains can undergo significant plastic deformation together with the coordinated deformation of coarse-grained ferrite. We hope the strategy in this study may provide insights for microstructure optimization and synergistic strength-plasticity/toughness enhancement of conventional low-carbon micro-alloyed steels for structural applications.

Novel hybrid laser forging and arc additive repairing process for improving component performances

A new hybrid laser forging and arc additive repairing process was developed to significantly improve the performance of repaired components, in which a leading gas metal arc was adopted to repair the partially damaged component, and a trailing short-pulse laser directly acted on the high-temperature solidified metal without coating (laser forging). Compared with arc additive repairing with post-treatment, this hybrid process performed the arc repair and laser forging synchronously. The laser forging region can be accurately determined using a multi-physical molten pool simulation. The molten metal flow was also studied, indicating that the high sulfur content introduced by the filler metal transfer had a significant influence on the Marangoni stress distribution and thus changed the molten metal flow patterns. The mechanism for laser forging without coating and its related physical effects were investigated. The laser shock pressure was significantly higher than the Hugonoit elastic limit of the high-temperature solidified metal, causing plastic deformation of the repaired layer. The high strain and severe plastic deformation induced by laser forging caused martensite formation and grain refinement, which improved the mechanical properties and electrochemical corrosion performance of the repaired layer.

Study of an Economical and Effective Heat Treatment Method to Improve the Performance of Gear Steels

During heat treatment, gear steels undergo grain coarsening, so an appropriate heat treatment process that can efficiently control the austenite grain growth and ensure a good performance of gear steels is particularly important. Herein, the influence of a requenching treatment on the comprehensive behavior of low‐carbon alloy steel 20Cr2Ni4 is explored. The pristine steel samples with and without Nb microalloying subjected to conventional quenching and tempering (QT) treatment are compared with the requenched steel sample. The results show that different requenching temperatures considerably impact grain refinement and homogenization. The steel requenched at 830 °C exhibits the greatest tensile strength of 1444 MPa and excellent toughness, comparable to the mechanical properties of the steel upon Nb microalloying after the conventional QT treatment. The formation of twin martensite and uniform fine grains with a high dislocation density together with numerous low‐angle and high‐angle grain boundaries is responsible for the simultaneously improved strength and ductility of the requenched specimen; thus, the proposed requenching heat treatment is a promising, efficient, and reliable method to double‐refine prior austenite grains and their packet and block structures and improve the comprehensive properties with a combination of high strength and toughness, yielding high‐strength gear steel.

Achieving low-temperature superplasticity in a cold-rolled medium Mn steel with an equilibrium ultrafine equiaxed dual-phase microstructure

The present study reports an equilibrium ultrafine equiaxed dual-phase microstructure to regulate and achieved the low deformation temperature and high hot ductility balance in a novel 3.6 Al medium Mn steel. The cold-rolled specimens were subjected to high-temperature tensile tests and microstructure characterization with deformation temperature as a variable. The results showed that during the hot tensile process, the initial ultrafine martensite grains were transformed into ferrite + austenite equiaxed dual-phase grains, accompanied by dynamic recovery and recrystallization. At the deformation conditions of 625°C and 5 × 10-4 s-1, the accumulation of dislocations is obvious and the grain refinement is caused significantly. The reason for its failure is the development of the neck. The maximum elongation of 880% was obtained at 675°C and 5 × 10-4 s-1 due to the dynamic balance between ‘the grain refinement and strain hardening caused by dislocation creep and accumulation’ and ‘the grain coarsening and softening caused by dynamic recovery and recrystallization’. When the deformation temperature continues to increase to 775 °C, the elongation sharply decreases up to 491%. The stress concentration at the phase boundary induces cavity behavior, which is the main reason for the premature failure at 775 °C.

Achieving exceptional wear resistance in a crack-free high-carbon tool steel fabricated by laser powder bed fusion without pre-heating

Laser powder bed fusion (LPBF) for the fabrication of dense components used for tooling applications, is highly challenging. Residual stresses, which evolve in the additively manufactured part, are inherent to LPBF processing. An additional stress contribution in high-carbon steels arises from the austenite-to-martensite phase transformation, which may eventually lead to cracking or even delamination. As an alternative to pre-heating the base plate, which is not striven by industry, lowering the martensite content which forms in the part, is essential for the fabrication of dense parts by LPBF of high-carbon tool steels which are then adapted to LPBF. In this study, a successful strategy demonstrates the processing of the Fe85Cr4Mo1V1W8C1 (wt%) high-carbon steel by LPBF into dense parts (99.8%). The hierarchical microstructure consists of austenitic and martensitic grains separated by elemental segregations in which nanoscopic carbide particles form a network. A high density of microsegregation was observed at the molten pool boundary ultimately forming a superstructure. The LPBF-fabricated steel shows a yield strength, ultimate compressive stress, and total strain of 1210 MPa, 3556 MPa, and 27.4%, respectively. The mechanical and wear performance is rated against the industrially employed and highly wear-resistant 1.2379 tool steel taken as the reference. Despite its lower macro-hardness, the LPBF steel (58.6 HRC, 0.0061 mm3 Nm–1) shows a higher wear resistance than the reference steel (62.6 HRC, 0.0078 mm3 Nm–1). This behavior results from the wear-induced formation of martensite in a microscale thick layer directly at the worn surface, as it was proven via high-energy X-ray diffraction mapping.

Damage Evolution and Microstructural Fracture Mechanisms Related to Volume Fraction and Martensite Distribution on Dual‐Phase Steels

The scope of this work is the analysis of the damage evolution and microstructural fracture mechanisms of dual‐phase steels through quasi‐static uniaxial and cyclic tensile tests, considering the influence of the volume fraction and distribution of martensite. The samples are intercritically annealed at different temperatures to obtain three different martensite volume fractions (MVF). Damage evolution is evaluated as a function of stiffness loss. The steels show lower ductility and strength under cyclic loading conditions. The work hardening rate is affected by MVF in different stages of the deformation. A higher MVF is linked to a prompter damage evolution rate. The primary void nucleation mechanisms are ferrite‐martensite and ferrite–ferrite decohesion. Martensite fracture can be activated depending on MVF and martensite distribution along the ferrite grain boundary. The dislocation density on grain boundaries is high due to the austenite–martensite transformation during the quenching process and subsequent plastic deformations. Dislocation density is related to stored strain energy, stress concentration, and the observed fracture mechanisms, particularly near martensite grains. Nanoindentation is conducted to evaluate the ferrite and martensite hardness, which depends on its carbon content. It is observed that the martensite/ferrite hardness ratio affects the uniform elongation of the material.

Mechanical performance and microstructure of the grade 91 stainless steel produced via Directed Energy deposition laser technique

The grade 91 ferritic/martensitic steel is considered a promising structural or cladding material for various nuclear reactor applications. Here, grade 91 was fabricated via the Directed Energy Deposition Laser technique. This alternative manufacturing process potentially enables tailoring of the mechanical properties through increased control of the product’s microstructure.Aimed at linking fabrication to performance via defining the process-structure–property relationships, the current research includes macro and up to nano-scale mechanical testing using microhardness, tensile, and in-situ nanoindentation hardness, coupled with electron diffraction-based microstructure characterization. Mapping of the product structure and properties was conducted by testing miniature-sized samples, parallel to the built direction (‘Z’ direction) and perpendicular (‘X’ direction) at constant distances.We found the majority of the microstructure consists of fine and coarsened-size lath-type martensite grains, with up to 15% δ-phase, preferentially observed at the melt pool boundaries. Most intriguing was the gradual decrease found in observed metallurgical pores alongside softening at farthest distances from the cold build platform. Here, these changes were successfully explained in terms of phase composition, ‘grain-like’ size effects of the lath-type martensite, and geometry necessary dislocation density. In finalizing this work, several competing strengthening mechanisms were addressed, and their activity was considered owing to fabrication-related mechanisms.

Effect of Grain Sizes on the Friction and Wear Behavior of Dual-Phase Microstructures with Similar Macrohardness and Composition

The tribological behavior of dual-phase steels have been studied at the macroscopic scale taking the macrohardness as the main material property to control friction and wear. However, the contribution to the macroscopic behavior of the varying properties of the phase at the microscopic scale are yet to be fully understood. In this study, dual-phase microstructures with various grain sizes and martensite volume fraction are generated. Microhardness of ferrite and martensite are measured by nanoindentation tests while their friction and wear behavior are studied by conducting scratch tests with various conical tips. Results show that for martensite, friction coefficient and wear resistance are proportional to its carbon content, whatever the martensite grain size. Whereas changing the ferrite grain size has two effects on the tribological behavior of the microstructure. First, the friction and wear resistance of ferrite are related to its grain size through a Hall–Petch relationship. Second, at a given martensite volume fraction, the mean wear resistance changes from the Equal Wear mode to the Equal Pressure mode as the ratio of the contact size to the ferrite grain size increases, while the mean friction coefficient always correlates to the Equal Pressure mode.

In-situ micro-cantilever bending studies of a white etching layer thermally induced on rail wheels

A complex and untraceable mechanical and thermal loading situation in rolling-sliding contacts can lead to the formation of white etching layers (WELs), depicting critical crack initiation sites. For our detailed studies, to obtain a holistic view of the microstructural characteristics and micro-mechanical properties, we prepared a WEL on a decommissioned (after 200,000 km service life) rail wheel by laser surface treatments with a defined energy input. This WEL is predominantly martensitic down to a depth of 30–40 μm, after which a transition to the deformed ferritic-pearlitic microstructure of the hypoeutectoid rail wheel steel is present. The martensitic region is with 6.98 ± 0.68 GPa significantly harder than the transition zone (5.17 ± 0.39 GPa) and the deformed ferritic-pearlitic base material (3.30 ± 0.33 GPa). In-situ V-notched micro-cantilever bending experiments of the martensitic (and thus most brittle) region show crack initiation and propagation – mostly along the boundaries of the martensitic grains – besides a plastic behavior. Applying the elastoplastic fracture mechanics allows to derive the local fracture toughness KIQ, which is 16.4 ± 1.2 MPam1/2 for this martensitic region of the WEL. The results outpoint the application of micro-cantilever bending tests in addition to hardness testing as a promising tool to discuss the relationship of microstructural characteristics with its micro-mechanical properties.