Low-carbon Martensitic Steels Research Articles

Characterization of Carbide Precipitation in Low-Carbon Martensitic Steels Using an Ultrawide Field-of-View 3D Atom Probe.

It is important to understand the carbide distribution around high-energy sites such as dislocations and grain boundaries in martensitic steels as they have a major influence on the alloy performance. The aim of this study is to characterize fine ε carbides precipitated in low-carbon lath martensitic steel using the ultrawide field-of-view (FoV) CAMECA Invizo 6000 atom probe. We demonstrate the advantages of the wide FoV and determine the optimum conditions for analysis, by comparing the results such as the background noise and the C++/C+ charge state ratio (CSR) between voltage-pulsed and laser-pulsed modes. Increasing the laser pulse energy decreased the background noise and the CSR, where 70 pJ laser pulse energy produced a comparable mass-to-charge ratio spectrum to that recorded in voltage-pulsed mode, with the bulk compositions of C, Si, and Mn closest to that measured using voltage-pulsed mode. Increasing laser pulse energies to above 300 pJ decreased the bulk carbon content, with a more diffuse distribution of carbon around the carbides. This paper outlines some of the important experimental considerations when performing quantitative study of carbide precipitation in low-carbon martensitic steels using the Invizo 6000, considerations that can also be applied to other ferrous and non-ferrous alloy systems.

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Crystal plasticity modeling of prior austenite orientation effects on deformation behaviors of martensitic steels under different strain paths

The influence of prior austenite grain (PAG) orientation on the deformation behavior of a low-carbon martensitic steel is investigated using crystal plasticity (CP) modeling with hierarchical representative volume elements (RVEs). The Kurdjumov-Sachs (K-S) relationship is refined for accurate PAG reconstruction in martensitic steel. A robust calibration strategy for CP parameters based on nanoindentation tests is developed by integrating analytical calculations with inverse methods. Virtual polycrystalline aggregates are subsequently generated by manipulating initial PAG orientations. The simulations reveal that assigning cube texture to PAGs enhances strain hardening of martensite under plane strain tension. Moreover, the RVEs with brass and copper orientated PAGs exhibit similar deformation behavior, and a more homogeneous strain distribution is realized in the RVE with PAG cube orientation. The influence of PAG orientation on stress and strain fields is correlated with lattice rotation behavior during deformation. Notably, different strain paths elicit distinct lattice rotation trajectories, wherein uniaxial tension and plane strain tension favor grains reorientation toward hard γ-fiber, whilst equi-biaxial tension drives the crystal matrix to concentrate on 〈001〉 and 〈011〉 directions. These findings provide insights into the intricate relationship between microstructural orientation and deformation behavior in martensitic steels, which is crucial for performance optimization.

Development of a Flow Rule Based on a Unified Plasticity Model for 13Cr-4Ni Low-Carbon Martensitic Stainless Steel Subject to Post-Weld Heat Treatment

This study aims to develop a flow rule for evaluating the relaxation and redistribution of residual stresses during the post-weld heat treatment (PWHT) of hydroelectric runners made from low-carbon martensitic stainless steel (13Cr-4Ni composition). During the PWHT, austenite reforms in the filler metal and surrounding areas of the base metal near welded joints. The evolving inelastic strain rate with reformed austenite led to defining two distinct flow rules in the pure martensitic (α′) and austenitic (γ) phases. A linear rule of mixture was then applied to assess global effective stress based on the inelastic strain rate and current austenite fraction during the PWHT. A unified constitutive model incorporating drag stress and back stress, evolving with creep and plastic deformation mechanisms during the PWHT, described the stress–strain behavior. To validate this analysis, a third flow rule was determined in the 18% tempered austenitic microstructure, compared with the rule of mixture’s effective stress contribution from each phase on the inelastic strain rate. Isothermal constant strain rate tests in stabilized crystalline microstructures evaluated constants specific to their respective flow rules. This study demonstrates the stability of reformed austenite at elevated temperatures during slow cooling and its significant influence on the mechanical properties of 13Cr-4Ni steels. The effectiveness of estimating yield stress using the rule of mixture based on individual phase behaviors is also confirmed.

Capacity of hydrogen traps affects H-assisted crack initiation and propagation mechanisms in martensitic steels

Low-carbon martensitic steels are candidate materials for different hydrogen applications. Hydrogen embrittlement (HE) of the steels can be mitigated by designing trap site characteristics. In this study, the different capacities of hydrogen trapping, and reversibility of the trap sites are studied to reveal the extent of HE susceptibility of the steels and H-assisted crack initiation and propagation mechanisms. The trap sites in Ti-contained and Mo-contained martensitic steels were identified and discussed. The trap sites in Ti-contained martensitic steel include basic martensitic microstructure, the interface of the TiC/matrix interface and carbon vacancies inside the TiC carbides. The trap sites in Mo-contained martensitic steel including basic martensitic microstructure and interface of the Mo2C/matrix interface. Ti-contained steel indicates a mitigated HE susceptibility, controlled by carbon vacancies inside TiC acting as strong hydrogen trap sites, and a higher possibility of building up a critical hydrogen content at Mo2C carbide due to lower hydrogen capacity. These results prove that hydrogen trapping capacity at the designated hydrogen trap sites is a determinant factor for H-assisted crack initiation and propagation in martensitic steels. A critical local hydrogen amount should be built up at the trap site to assist the crack to develop. It was also revealed that crack initiation originated from Mo2C carbides accompanied by interfacial decohesion at the interface of martensitic matrix/Mo2C carbides and propagated in quasi-cleavage patterns along {011} planes.

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Evaluation of Dislocation Density and Dislocation Strengthening Mechanism in Ultra Low-Carbon Martensitic Steel

Numerical Simulation of Reversed Austenite Evolution during Intercritical Tempering of Low-Carbon Martensitic Stainless Steel.

Since the formation of reversed austenite during critical tempering treatment is an important factor affecting the mechanical properties of 13Cr4Ni martensitic stainless steel, a detailed study of the content and morphology of reversed austenite in heat treatment is needed. In this study, the variation curves of a reversed austenite volume fraction with holding time at different tempering temperatures were measured by in situ X-ray diffraction (XRD), and the reversed austenite and carbides of each process were evaluated by transmission electron microscopy (TEM). The austenite content shows a parabolic change with the increase in the tempering temperature; the maximum can reach a peak of about 6.8% at 610 °C, and drops to 0% at 660 °C. It also shows a parabolic change with the extension of the holding time, reaching a maximum of about 9.2% at 5 h of holding time, and a decreasing trend at 10 h of holding time, about 6.8%. The results show that the precipitation of carbides in the microstructure causes elemental segregation at grain boundaries and inside, which is one of the main factors affecting the thermal stability of reversed austenite formation. The kinetic process of reversed austenite during the tempering process was simulated using the JMAK model and the KM model, which can describe the trend of reversed austenite content during the tempering process. Combining the two models, a mathematical model for the room-temperature reversed austenite content under different processes was obtained, and this can predict the room-temperature austenite content.

Effect of microstructure on the corrosion-friction behavior of low-carbon martensitic stainless steel

The effect of microstructure on the corrosion-friction behavior of 11Cr4NiMo steel was investigated for three different microstructure states. Quenched martensite and low-temperature tempered martensite with high density dislocations exhibit severe wear under the dual effects of corrosion and abrasion, and their wear mechanism include three-body wear, adhesive wear, and fatigue wear. While high-temperature tempered microstructure mixed structure of tempered martensite, reversed austenite, and M23C6 carbides exhibits weakened wear. Reversed austenite greatly promotes the generation of a dense and stable passivation film with corrosion product of α-FeOOH, and plays a lubricating role and attenuates three-body wear. Utilizing the passivation film induced by corrosion to improve the wear resistance of materials is of great significance in engineering applications.

Effects of prior austenite refinement on microstructure and property of ultra-fine lath martensitic steel

To take advantage of the combination of heavily warm/cold rolling and cyclic salt-bath treatment, ultra-fine martensite with isotropic microstructure was tansformed from equiaxial prior austenite (PA) with grain size approximately 300 ∼ 600 nm. Only ultra-fine martensitic laths with widths less than 50 nm were developed instead of traditional hierarchical martensitic packet-block-lath, which is attributed to ultra-fine austenite stabilization mechanism and limited transition space. The exceptional properties of this low-carbon martensitic steel can be improved to an ultimate tensile strength of ∼ 1976 MPa and a total elongation of ∼ 10.1 %.

Visualization and Location Determination of Pit Initiation Sites in Martensitic Stainless Steel during Corrosion-Fatigue Cyclic Plastic Deformation

Low-carbon martensitic stainless steels, such as 13%Cr4%Ni, are widely used in manufacturing hydraulic turbine runners. High tensile strength steel, such as cast CA6NM or wrought S41500 martensitic stainless steel, has an acceptable resistance to corrosion and cavitation, and reduces the weight and cost while offering a more efficient design. During operation, the combined effect of cyclic loading (fatigue) and corrosion reduces the fatigue strength and can initiate cracks at localized corrosion surface defects.In this study, we determined the bulk and localized corrosion behavior of wrought (S41500) and cast (CA6NM) 13%Cr4%Ni stainless steel before and after plastic deformation.Susceptibility to pitting corrosion was studied using open circuit potential and potentiodynamic polarization measurements in 0.020 M NaCl. Localized corrosion behavior was studied using a scanning droplet cell (SDC) and oil immersed scanning electrochemical cell microscopy (OI-SECCM). The bulk and localized electrochemical measurements after different surface preparation methods (mechanical polishing, followed by etching or electro-etching in different media) were compared.The surface defects including micro-voids and inclusions of different types were characterized. The location of pit initiation was then determined using scanning electron microscopy and profilometry imaging. A three-electrode microcell with a working electrode of 1 mm² was designed and hyphenated with optical microscope to allow in situ tracking of pit initiation and growth. Chronopotentiometry and chronoamperometry controlled pit initiation, while optical microscopy demonstrated dynamic release of corrosion products and measured oxide repassivation kinetics.Finally, the in situ technique was developed further to include a microtensiometer fatigue apparatus to study the synergy effect between corrosion and fatigue. Location of pit initiation was compared with and without plastic deformation. Videos of pit initiation and growth will be presented.

Quantitative analysis of local plasticity accompanying hydrogen-related fracture in low-carbon martensitic steel

In this study, the local plasticity accompanying hydrogen-related fracture in a low-carbon martensitic steel was quantitatively investigated by correlative microscopy analysis combining digital image correlation technique and electron backscatter diffraction. The results revealed that strain localization occurred mainly in the bulk of martensite blocks and at prior austenite grain boundaries, both with and without the presence of hydrogen. Moreover, hydrogen enhanced strain localization during deformation and facilitated crack nucleation and propagation. The quantitative analysis showed that the hydrogen-related quasi-cleavage cracking was closely related to the local plastic deformation, and increasing hydrogen content decreased the local strain level required for quasi-cleavage cracking. It was found that strain localization also occurred around the intergranular cracks, suggesting that hydrogen-induced intergranular cracking was not a classical simple decohesion process and strain localization was involved in the sequence of hydrogen-induced intergranular cracking. These findings shed light on the understanding of the intrinsic nature of hydrogen embrittlement.

Nano-TiC-reinforced 18Ni300 steel manufactured by Powder Bed Fusion-Laser Beam with improved wear resistance for mould inserts

In injection moulding, surfaces can be exposed to harsh working conditions, especially when polymers reinforced with abrasive glass fibres are used. Low carbon martensitic steels, such as maraging steels, are an excellent choice for demanding and complex mould inserts produced by Powder Bed Fusion-Laser Beam (PBF-LB). However, their wear resistance is lower than that of tool steels such as AISI D2 or H13, whose PBF-LB printability is challenging, due to their carbon content that makes them susceptible to cracking. In this study, an alternative material with high printability by PBF-LB was developed, by reinforcing maraging steel 18Ni300 with nano-sized titanium carbide (TiC). TiC-reinforced 18Ni300 nanocomposites showed an improvement in microhardness of up to 23% (730 HV2) after addition of 7.0 vol% TiC. Tribological tests against a polypropylene with 40 wt% glass fibres showed that the specific wear rate decreased by about an order of magnitude (to 0.32 × 10–6 mm3/N m) compared to the unreinforced maraging steel.Graphical

Strength improvement over 2 GPa and austenite grain ultra-refinement in a low carbon martensite steel achieved by ultra-rapid heating and quenching

This study focuses on enhancing the strength of low-carbon martensitic steel through the refinement of the prior austenite grain size. After subjecting the steel to heavy cold deformation, ultra-rapid heating in a salt bath was employed. The steel was then quenched in water. The results showed that the ultra-rapid heating for 5 s achieved an impressive tensile strength exceeding 2 GPa, accompanied by a higher hardness of over 500 kgf/mm2. By introducing more nucleation sites through heavy cold rolling, it was possible to achieve an ultrafine-grain structure in the steel, with the prior austenite grain size ranging from 1∼3 μm. In some regions, even smaller grain sizes of around 1 μm or less were observed. The boundaries between martensite packets or blocks disappeared, replaced by two or three lath variations. The study also investigated the effect of high-temperature holding time on the austenite phase. It was found that increasing the holding time resulted in the growth of the austenite grains and a subsequent decrease in tensile strength. Considering carbide dissolution, the optimal temperature holding time was approximately 5 s at 900 °C. A shorter holding time allowed more carbides to remain, which weakened the strengthening effect of the carbon solid solution. The relationship between strength and grain size was analyzed by fitting experimental data. The Hall-Petch slopes were calculated to be about 862 MPa mm1/2 and 1022 MPa mm1/2 for the prior austenite grain size and the effective grain size, respectively. Moreover, excluding the influence of martensite packet or block boundaries, the strength contributions from the carbon solid solution and dislocation strengthening were determined to be approximately 583 MPa and 268 MPa, respectively.

The Reliability of Single-Step and Double-Step Quench and Partitioning Heat Treatments on an AISI 420A Low Carbon Martensitic Stainless Steel

The microstructural and mechanical effects of various single-step (SS) and double-step (DS) quench and partition (Q&P) heat treatments applied to an AISI 420A low carbon martensitic stainless steel (MSS) has been studied. The goal with this work is to reach a total elongation (E pct) of 12 pct and an ultimate tensile strength (UTS) above 1200/1300 MPa, but ultimately to achieve a superior strength-ductility balance in comparison to its traditional Quench and Temper (Q&T) counterpart. This is being done by retaining austenite within the steel’s martensitic matrix at room temperature (RT) using novel SS and conventional DS Q&P heat treatments. Considerable work has been done to optimize DS Q&P heat treatments, but little has been done to understand the effects of removing a subsequent heating cycle through SS Q&P heat treatments has on MSSs. With that being said, partitioning is performed at the same quench interruption temperature for the SS Q&P heat treatments, and reheated to a higher temperature for the DS Q&P heat treatments. Experimental investigations were carried out on 1 mm thick, sheet samples to increase the number of potential applications for this steel and heat treatment. The microstructure of different SS and DS Q&P heat treatments was investigated through X-ray diffraction (XRD) and transmission electron microscopy (TEM) while mechanical property investigations were carried out using tensile and fracture toughness testing. DS Q&P heat treated samples quenched to 130 °C and partitioned for industrially relevant times of 10 and 30 minutes featured the highest values in terms of total elongation, tensile strength and fracture toughness. The SS Q&P heat treatments, on the other hand, were able to achieve improved mechanical properties to its Q&T counterpart. Overall, this work opens up the possibility of increased MSS usage for reliable, thin-walled component production with improved properties through Q&P heat treatment methods. The best results achieved in this study are a UTS of 1585 MPa, E pct of 22 pct, and a fracture toughness of 77 kJ/m2. Their lower total elongation of 9.6 pct is balanced by high tensile strength of 1812 MPa, ensuring higher toughness compared to traditional Q&T samples.

Effect of Thermocycling Treatment on Physicomechanical Properties of Low-Carbon Martensitic Steel 15Kh2G2NMFB for Parts of Precision Devices

Effect of Thermocycling Treatment on Physicomechanical Properties of Low-Carbon Martensitic Steel 15Kh2G2NMFB for Parts of Precision Devices

Achieving ultrahigh strength by tuning the hierarchical structure of low-carbon martensitic steel

The high strength of martensitic steel can be attributed to its fine hierarchical structure and high dislocation density. To further improve the mechanical properties of low-carbon martensitic steel, a nano-lamellar structure in size of 28 nm was first produced by warm rolling on the dual-phase structure. Subsequent annealing at 870 °C for different times was performed to tune the hierarchical structures of the martensitic steels with different prior austenite grain (PAG) sizes. The WRQ-15 steel sample, which underwent a 15-min annealing process, exhibits an outstanding combination of strength and elongation with a yield strength (YS) of 1.55 GPa, ultimate tensile strength (UTS) of 2.0 GPa, and elongation to failure (ETF) of 8.7%. It shows higher strength than the martensitic sample annealed from undeformed steel (UQ-15 steel), which has a YS of 1.31 GPa and UTS of 1.68 GPa. The WRQ-15 sample has a finer PAG size (7.2 μm) than the UQ-15 sample (9.5 μm). The results reveal that the refined PAGs lead to further refinement of packets, blocks, and laths, as well as increased dislocation density, contributing to the superior mechanical properties of the WRQ-15 sample. Moreover, the refined hierarchical structures provide high-density boundaries, impeding the propagation of cleavage cracks to improve the toughness. However, the excessive refinement of the hierarchical structures, increase of dislocation densities, and the undissolved carbides in the WRQ-1 sample are harmful for the strength-toughness balance.

Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.

Characteristics and formation mechanism of serrated markings on the hydrogen-related quasi-cleavage fracture in as quenched low-carbon martensitic steel

The present paper investigated the characteristics of serrated markings on the hydrogen-related quasi-cleavage fracture in an as quenched low-carbon martensitic steel. The serrated markings corresponded to the ridges between {011} micro-facets. The ridges were formed along lath/block/packet boundaries or nearly parallel to 〈211〉 or 〈011〉 directions within the lath. In addition, the angle between {011} micro-facet and tensile axis was the largest of the six crystallographically equivalent {011} planes in each variant, demonstrating that the resolved normal stress imposed on the {011} plane plays a key role on the quasi-cleavage fracture. The above characteristics are common to martensitic steels and ferritic steels. We propose that the coalescence of vacancies/voids induced by hydrogen-enhanced screw dislocation motion needs to be considered to understand the fundamental mechanism of the hydrogen-related quasi-cleavage fracture along {011} planes in BCC steels.

Effect of cooling rate on the bainite fraction in low carbon martensitic steel: combined analysis of transformation kinetics and crystal curvature

Derived by means of EBSD, statistics of crystal curvature in quenched low carbon steel is analyzed with allowance for its transformation kinetics in terms of dilatometry data. Based on curvature spectra sensitive to the distribution of dislocation density, fractions of martensite and bainite are quantified at high and low cooling rates. Distinction between the resulting microstructures is ascribed to various proportions of their athermal and thermally activated (rate-dependent) constituents.

Effect of Cu on the Microstructure and Mechanical Properties of a Low-Carbon Martensitic Stainless Steel.

Reversed austenite is of vital importance in low-carbon martensitic stainless steel because it improves impact toughness. However, a proper amount of reversed austenite is obtained by tempering at a critical temperature, which reduces the strength of the steel. Therefore, how to improve strength-toughness matching is an important problem. Copper (Cu) is an effective strengthening element in steels. However, there is little in-depth discussion on the role of Cu on the microstructure and mechanical properties of low-carbon martensite steel. In this work, the effect of different Cu content on the reversed austenite formation, tensile strength, and impact toughness of a low-carbon martensitic stainless steel (0Cr13Ni4Mo) was systematically investigated through use of a transmission electron microscope (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT), and other characterization methods and mechanical property tests. The results showed that the addition of Cu decreased the phase transition temperatures of martensite and austenite and increased the volume fraction of the reversed austenite. APT results indicated that Cu-rich clusters first formed with alloying elements such as ferrum (Fe) and nickel (Ni) and then grew to be precipitates through rejection of the alloying elements. The Ni atoms diffused towards the interface between the precipitates and the martensite matrix, which provided heterogeneous nucleation sites for the reversed austenite. Cu precipitations strengthened tensile strength during tempering. However, it generated temper brittleness in the steel at a tempering temperature of 450 °C, resulting in the impact energy of the 3Cu-steel being only 7 J. A good combination with higher tensile strength (863 MPa) and ductility (192 J) was obtained when tempering at 600 °C in the presence of Cu-rich precipitates and a sufficient volume fraction of the reversed austenite. The results provide guidance for the design of steels with reversed austenite and Cu and promote the development of high-strength and high-toughness steels.