Ultra-high Strength Martensitic Steels Research Articles

Very high cycle fatigue properties of ultrahigh strength steels in the presence of hydrogen

The very high cycle fatigue (VHCF) properties of 2000 MPa ultrahigh strength martensitic steels after hydrogen pre-charging were investigated by means of ultrasonic fatigue tests. Results showed that the VHCF lives of the experimental steels were drastically decreased by about two orders after pre-charged with a hydrogen concentration of CH = 1.23 wppm. The experimental steel tempered at a higher temperature of 300 °C with some epsilon-carbides and a lower density of dislocations showed higher VHCF lives in the presence of hydrogen. It could be attributed to the lower hydrogen diffusion coefficient since hydrogen might accelerate fatigue crack propagation in the granular bright facet (GBF) area.

Industrially produced 2.4 GPa ultra-strong steel via nanoscale dual-precipitates co-configuration

An ultra-high strength martensitic steel, strengthened by the dual-precipitation of NiAl and M2C nano-particles, has been successfully developed and produced on an industrial scale. It has achieved a remarkable tensile strength of 2467 MPa and exhibited ductility approaching 10%. The precipitation configurations of NiAl and M2C nano-particles during ageing at 510 °C for durations ranging from 0.5 to 10 h were characterized using atom probe tomography. The results indicate that the preferential precipitation of NiAl, facilitated by minimal lattice misfit and low interfacial energy, promotes the uniform nucleation of M2C particles within the matrix while restricting their growth. As a result, both types of precipitates maintain a high number density. With increasing ageing time, the distribution density of the precipitates decreases, their size increases and the increase in yield strength gradually decreases. Furthermore, the addition of element cobalt effectively reduces the diffusivity of other major alloying elements, synergistically inhibiting the growth of NiAl and M2C particles. The dual precipitates co-configuration provides a substantial hardening effect (1046 MPa at 510 °C for 220 min) compared to the individual hardening effect of M2C observed in AerMet100 steel.

Towards the understanding of fracture resistance of an ultrahigh-strength martensitic press-hardened steel

Identifying the fracture resistance capabilities and gaining a deeper understanding of the controlling damage mechanisms are important to the development of press-hardened steels (PHS) for automotive applications. In this study, the fracture properties of a novel PHS alloyed with Cr and Si (CrSi-PHS) were assessed by using uniaxial tensile and double-edge notched tensile (DENT) tests. Under uniaxial tension, the fracture resistance is characterized by the fracture strain and work of fracture, and the CrSi-PHS presents a desirable compromise with strength when comparing with other grades of advance high-strength steels (AHSS). The large fracture strain is attributed to the high resistance to damage nucleation even with a high density of grain boundaries. Fracture toughness is characterized by the DENT tests. The fracture toughness parameters of CrSi-PHS are lower than that of the commercial PHS 22MnB5 but comparable to the multiphase AHSSs with lower strength levels. The stress triaxiality near the fatigue pre-crack in DENT tests leads to a change of damage mechanism in CrSi-PHS. The crack propagates discontinuously by joining the micro-cracks and subsequently by the ductile fracture of micro-ligaments. The local plasticity at the crack tip region is suggested to contribute significantly to the energy dissipation. The observations demonstrate a strong dependence of the fracture mode and fracture resistance on the loading condition in the ultrahigh-strength martensitic steels.

Improvement of plasticity and toughness of ultra-high strength martensitic steels via tailoring trace ferrite

The susceptibility of ultra-high strength martensitic steels to cracking during processing and service has hindered their extensive application. This paper presents a novel strategy to improve plasticity and toughness of ultra-high strength martensitic steels via tailoring trace ferrite. Given this thought, we have successfully developed an ultra-high strength martensitic steel with modified ferrite. The modified ferrite is evenly distributed in the martensitic matrix, with a small size, low aspect ratio and high dislocation density. The strength class of this steel is comparable to conventional ultra-high strength martensitic steels at 1500 MPa, but the total elongation and impact toughness have been increased by 13.0% and 10.1%, respectively. The improved performance can be attributed to the strong synergistic relationship between modified ferrite and martensite, which effectively prevents the formation of microcracks at the interface between the two phases.

Temperature Dependence of Cleavage Fracture Toughness for an Ultrahigh Strength Martensitic Steel

Abstract This work conducts an exploratory evaluation of the brittle fracture behavior for an ultrahigh strength martensitic steel using conventional three-point bend SE(B) and precracked Charpy V-notch (PCVN) specimens. A primary purpose of this study is to verify the effectiveness of the Master Curve methodology in providing a reliable estimate of the reference temperature (T0) derived from fracture toughness data sets measured in the ductile-to-brittle transition (DBT) region of an ultrahigh strength, low alloy martensitic steel. Fracture toughness testing conducted on three-point bend SE(B) specimens and PCVN configurations at different test temperatures in the DBT region provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc, and its corresponding KJc-values for the tested material. Although this class of ultrahigh strength steel having a martensitic microstructure is currently beyond the reach of ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, the analyses described here show that the predicted normalized curves of median fracture toughness versus temperature are in good agreement with the experimental measurements.

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.

A thermo-metallurgical-mechanical model for microstructure evolution in laser-assisted robotic roller forming of ultrahigh strength martensitic steel

Laser-assisted robotic roller forming (LRRF) apparatus and process were developed to bend a plate to form a straight channel for ultrahigh strength steel MS1300. Since the thermal processing during the roller forming impacts the microstructure and mechanical behavior of the steel, an integrated thermo-metallurgical-mechanical finite element simulation considering the heat source, phase transformation and material constitutive models was established. A rectangular laser source was devised to homogenize the temperature around the bending corner and a new surface heat source model was proposed and validated. The phase transformation model accounting for the austenitization process, austenite decomposition and tempering was embedded in the finite element model through self-developed user subroutines. The predicted microstructure evolution and the phase distribution were consistent with experimental microstructure characterization. More specifically, it is found that tempering dominates at the inner layer of the bend, resulting in two different phases, i.e., the original and tempered martensitic phases after the LRRF process. The outer layer of the bend, however, goes through austenitization, quenching, and tempering processes, resulting in a combination of fresh martensite, a small amount of tempered martensite and retained austenite phases.

Role of retained austenite in improving the mechanical properties of 1.5 GPa-grade high-strength martensitic steels

This study investigated the role of retained austenite (γ) in improving the mechanical properties of 1.5 GPa-grade ultra-high strength (UHS) martensitic steels from in situ neutron diffraction experiments during tensile deformation. The UHS martensitic steel with retained γ of about 10% had a tensile strength (TS) of 1550 MPa and uniform elongation (U.El) of 6%. The 0.2% proof strength (YS) was smaller but better U.El was obtained with the same TS compared to the UHS martensite single microstructure steel. γ phase was judged from the phase strains to have higher strength than the tempered martensite of the matrix phase (α). However, from a detailed investigation using lattice strain, the yield behavior of γ and α phases were different due to the heat treatment at 573 K. The mechanical stability of γ varied widely and the carbon content of γ increased with increasing strain. Because the difference in phase stress between γ and deformation-induced martensite (α′) phases is small, the strength of α′ alone hardly affects the work hardening of UHS martensitic steels. The more γ transforms to α′, the more effective it is for high TS and large U.El as a transformation-induced plasticity (TRIP) effect.

Effect of coating on hydrogen embrittlement of hot stamping 22MnB5 high strength steel during electrochemical charging

This study investigated the effect of Al–Si coating and its service condition on hydrogen embrittlement (HE) of hot-stamped ultra-high strength martensitic steels (22MnB5) through electrochemical hydrogen charging. The HE behaviour of coated and uncoated specimens under different charging conditions was investigated. The deterioration degree of mechanical properties of charged specimens with charging current is monotonous. At low-current charging, the more severe deterioration of coated specimens due to the excess hydrogen which was provided by corroded coatings. Contrary, with high-current charging, the integral coatings provided effective protection and retarded HE behaviour. Furthermore, the scanning electron microscopy revealed a similar transition of fractography, i.e., from ductile (uncharged) to quasi-cleavage (0.001A-2h) and a mixture of quasi-cleavage and intergranular fracture (0.1A-2h). Additionally, the coated tailor rolled blank steel was selected to investigate the effect of coating condition on HE. It revealed that the deformation-induced defects on coatings would impair the resistance to HE. Thus, we demonstrated that the Al–Si coating which effectively alleviate HE, was dependent on its integrality.

The optimization of martensitic stainless steel composition to reduce hot tearing after laser powder bed fusion

Ultra-high strength martensitic stainless steel plays an essential role in aerospace, nuclear and other fields. This paper addresses laser-based powder bed fusion of martensitic stainless steel, based on thermodynamic software and computer languages for high-throughput calculation in combination with the hot tear sensitivity index criterion, Optimization of the alloy composed of Fe11Cr8Ni5Co3Mo martensitic stainless steel for resistance to hot tearing. The martensitic stainless steel composition with the lowest crack sensitivity was selected from hundreds of millions of data sets, reducing the hot tearing sensitivity index from 6841 to 1861. The alloy with the lowest hot tearing sensitivity was Fe11.3Cr8.18Ni2.0Mo2.8Co0.24Mn0.02V0.02Si0.01C0.17Mg. The hot tearing sensitivity has been experimentally verified in conjunction with additive manufacturing, and a comparative study of the building state organization and mechanical properties has been made.

Investigation of the Microstructure and Mechanical Properties of an Ultrahigh Strength Martensitic Steel Fabricated Using Laser Metal Deposition Additive Manufacturing

Ultrahigh strength steels were additively manufactured (AM) using different batches of powders by means of the laser metal deposition (LMD) technique. After quenching and tempering treatments, the microstructures, mechanical properties, and fracture modes of ultrahigh strength steels were investigated by several testing methods. The results demonstrate that martensite and Fe3C cementite were found in the three specimens after quenching and tempering treatments, and the tempered martensite microstructure had a lamellar structure in all specimens. The widths of these martensite lathes were observed to be different for the APHT-1, APHT-2, and APHT-3 samples, and their sizes were 1.92 ± 0.90 μm, 1.87 ± 1.09 μm, and 1.82 ± 0.85 μm, respectively. The martensitic steel exhibited excellent mechanical properties (tensile strength and impact toughness). The yield strength and the ultimate tensile strength of the APHT-3 sample reached 1582 MPa and 1779 MPa, respectively. Moreover, the value of the impact energy for the APHT-1 sample was 46.4 J. In addition, with the changes in the batches of ultrahigh strength steel powders, the fracture mode changed from ductile fracture to brittle fracture under tensile force and impact loads.

Laser-assisted robotic roller forming of an ultrahigh strength martensitic steel

Laser-assisted robotic roller forming (LRRF) was developed as a means for bending ultrahigh strength martensitic steel (MS1300) strip without fracture. The strip is bent through application of a thermal load from a fiber laser followed by a mechanical load from a steel roller: the translational motion of both is controlled via an industrial robot. A combined experimental and computational methodology was developed using a lab-scale LRRF cell and a finite element (FE) model that accounts for thermo-elastic-plastic bending from laser irradiation and the roller contact. The FE model was calibrated according to the experimental temperature data to reconstruct the spatiotemporal temperature field. The simulated temperature field was further applied to analyze MS1300 microstructure evolution and microhardness distribution. It is shown that the material under the laser beam was austenitized and then quickly cooled down. The microstructure in the bend subsequently consisted of new martensite near the tension (irradiated) surface, a through-thickness region consisting of both martensite and tempered martensite, and tempered martensite at the compression side of the bend.

Very high cycle fatigue properties of 2000 MPa ultrahigh-strength steels

The microstructure, mechanical properties and very high cycle fatigue (VHCF) properties of two 2000 MPa ultrahigh-strength martensitic steels were investigated. Results show that the newly developed steel possesses lower ultimate tensile strength but higher yield strength compared with the conventional steel, as well as a higher impact toughness and fracture toughness due to its better cleanliness and tempering at a higher temperature. Both steels experienced a typical VHCF fracture process. The newly developed steel had higher fatigue strength and longer fatigue life at the same stress amplitude compared with the conventional one, which could be related to its lower crack growth rate and better toughness.

Correction: Zheng et al. Correlating Prior Austenite Grain Microstructure, Microscale Deformation and Fracture of Ultra-High Strength Martensitic Steels. Metals 2021, 11, 1013

In the original article [...]

Evaluation of the T0 reference temperature for an ultra high strength martensitic steel

This work conducts an exploratory evaluation of the brittle fracture behavior for a high-strength martensitic steel using conventional three-point bend SE(B) specimens. A primary purpose of this study is to verify the effectiveness of the Master Curve methodology in providing a reliable estimate of the reference temperature (T0) derived from fracture toughness data sets measured in the ductile-to-brittle transition region (DBT) of an ultra high strength, low alloy martensitic steel. Fracture toughness testing conducted on three-point bend SE(B) specimens at different test temperatures in the DBT region provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc, and its corresponding KJc-values for the tested material. While this class of ultra high strength steel having a martensitic microstructure is currently beyond the reach of ASTM E1921, the analyses described here show that the predicted normalized curves of median fracture toughness vs. temperature are in good agreement with the experimental measurements.

The metadynamic recrystallization behavior of ultrahigh-strength stainless steel: effects of precipitates and shear bands

The metadynamic recrystallization behavior of Cr-Co-Ni-Mo ultrahigh-strength martensitic stainless steel was studied in a double-pass isothermal compression test, and a metadynamic recrystallization kinetics model for softening was established. The results showed that the metadynamic recrystallization softening rate of the steel not only depended on the deformation temperature and strain rate but was also related to the dynamic precipitation and the local shear bands in the steel. When the deformation temperature was below 1050 °C, the dynamically precipitated M6C carbides pinned the grain boundaries and hindered metadynamic recrystallization. When the steel was deformed at a deformation temperature of 1000∼1050 °C and a strain rate of 1.0∼5.0 s−1, a large number of local shear bands were generated. The local shear bands increased the number of nucleation sites for dynamic recrystallization and enhanced the softening rate of metadynamic recrystallization.

Effect of heat treatments on the microstructure and mechanical properties of an ultra-high strength martensitic steel fabricated via laser powder bed fusion additive manufacturing

A newly developed ultra-high strength martensitic steel, AF9628, has generated academic and industrial attention due to its combination of high yield strength and ductility at a low cost. Several studies have demonstrated that this steel can be successfully additively manufactured (AM) using laser powder bed fusion (LPBF). However, as-printed parts have displayed microstructural inhomogeneity, anisotropy, and a discrepancy in mechanical properties as compared to the traditionally processed material. There is currently no work detailing the effects of post processing heat treatments on the microstructure or mechanical properties of as-printed AF9628. In the interest of producing robust near-net-shape parts with complex geometries, this work explores the effects of LPBF and post processing heat treatments on the microstructure and mechanical properties of AF9628. Parts fabricated by LPBF were subjected to three different quench and temper heat treatment schedules. Quench and temper treatments displayed refined equiaxed prior-austenitic grain structures, reducing grain size by up to 45%. Significant variability was observed in Vickers hardness measurements of as-printed samples due to tempered and untempered bands of martensite alternating throughout the microstructure caused by thermal cycling during printing. Heat treatments were observed to increase the overall hardness of the material and reduce variability. As-printed samples displayed a tensile strength of 1.41 GPa, 10% elongation, and a Charpy impact toughness of 28 J. Quench and temper heat treatments were observed to increase tensile strengths to 1.66 GPa, but reduce the elongation to 7.6% and impact toughness to 24 J. Understanding the effects of these post-processing treatments is expected to allow for property optimization of AF9628 additively manufactured parts.

Mitigation of hydrogen embrittlement in ultra-high strength lath martensitic steel via Ta microalloying

Hydrogen embrittlement (HE) is a key challenge limiting the utilization of ultra-high strength martensitic steel. In this work, we reported a novel method for dramatically improving HE resistance by Ta microalloying, and the significant effect of Ta on the HE susceptibility of lath martensitic steel was elucidated from the perspectives of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). As the Ta content increased, numerous dispersed nano-sized TaC precipitates were generated and the effective areas of martensite/prior austenite grain boundaries also increased, which increased both the irreversible/reversible H trap densities, impeded localized H aggregation at defects and weakened HEDE. A quantitative analysis regarding each type of H trap revealed that, compared with reversible traps provided by microstructural refinement, TaC precipitate-induced irreversible traps exhibited a dominant role in weakening the HEDE process. Additionally, in Ta-bearing steels, the resistance to hydrogen-assisted crack propagation was enhanced through the increased Σ11 boundary, increased low-angle grain boundary fraction and the reduced Σ3 boundary fraction, which combined with the suppressing role of TaC precipitates on H-dislocation interaction, impeded the HELP process. This study provided new, deep insights into the impact of Ta on HE, which has important implications for developing steels with high HE resistance.

Hot Deformation Behavior and Dynamic Recrystallization of Ultra High Strength Steel

In this paper, in order to improve the microstructure uniformity of an ultra-high strength martensitic steel with a strength greater than 2500 Mpa developed by multi-directional forging in the laboratory, a single-pass hot compression experiment with the strain rate of 0.01 to 1 s−1 and a temperature of 800 to 1150 °C was conducted. Based on the experimental data, the material parameters were determined, the constitutive model considering the influence of work hardening, the recrystallization softening on the dislocation density, and the recrystallized grain size model were established. After introducing the model into the finite element software DEFORM-3D, the thermal compression experiment was simulated, and the results were consistent with the experimental results. The rule for obtaining forging stock with a uniform and refinement microstructure was acquired by comparing the simulation and the experimental results, which are helpful to formulate an appropriate forging process.

Correlating Prior Austenite Grain Microstructure, Microscale Deformation and Fracture of Ultra-High Strength Martensitic Steels

Herein, we correlate the prior austenite grain (PAG) microstructure to deformation and fracture mechanisms of an ultra-high strength martensitic steel. To this end, a low-carbon martensitic steel is subjected to five heat-treatments and the PAG microstructure in the material is reconstructed from the EBSD inverse pole figure maps of the martensitic microstructure. The deformation and fracture response of all heat-treated materials are characterized by in situ tension tests of dog-bone and single-edge notch specimens that allow us to capture both the macroscopic mechanical response and the evolution of microscopic strains via microscale digital image correlation. The experimental results, together with microstructure-based finite element analysis, are then used to elucidate the effect of the PAG microstructure on the mechanical response of the material. Our results show that the interaction between the heterogeneous deformation fields induced by the notch and the bimodal PAG size distribution leads to an increase in the propensity of shear deformation and degradation in the fracture response of the material with increasing heat-treatment temperature and time. Our results also suggest that achieving a unform distribution of fine grains is an effective way to enhance both the strength and fracture properties of this class of materials.